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doi:10.14379/iodp.pr.350.2014

Scientific results

New descriptive scheme for volcaniclastic rocks

The lithologic classification of sedimentary, volcaniclastic, and igneous rocks recovered by Expedition 350 uses a new scheme for describing volcaniclastic and nonvolcaniclastic sediments but uses generally established (International Union of Geological Sciences) schemes for igneous rocks (Fig. F16; Tables T1, T2, T3, T4). The new scheme was devised to improve description of volcaniclastic sediments and their mixtures with nonvolcanic (siliciclastic, chemical, and biogenic) sediments but maintaining the usefulness of prior schemes for describing nonvolcanic sediments (Fig. F16). The new scheme follows the recommendations of a dedicated core description workshop held in January 2014 in College Station, Texas (USA), prior to the cruise, that was attended by participants of all 2014 IODP expeditions (349, 350, 351, and 352) and was modified and tested during Expedition 350. The new scheme was devised for use in a spreadsheet-based descriptive information capture program designed by IODP (DESClogik), and the spreadsheet configurations were modified to use this scheme. This scheme was devised to facilitate the understanding of volcano-sedimentary processes by making reproducible and quantifiable observations of volcanic input to the sedimentary record. Also during Expedition 350, the new scheme shown in Figure F16 was applied to microscopic description of core samples, and the DESClogik microscope spreadsheet configurations were modified to use this scheme.

Rationale

During Expedition 350, sediments and sedimentary rocks were classified using a rigorously nongenetic approach, which integrates volcanic particles into the sedimentary descriptive scheme typically used by IODP (Fig. F16). This is necessary because volcanic particles are the most abundant particle type in arc settings like those drilled during the IBM expeditions. The methodology developed allows, for the first time, a comprehensive description of volcanogenic and nonvolcanogenic sediments and sedimentary rocks and integrates with descriptions of coherent volcanic and igneous rocks (i.e., lava and intrusions) and the coarse clastic material derived from them. This classification allows expansion to bioclastic and nonvolcanogenic detrital realms.

The purpose of the new classification scheme (Fig. F16) is to include volcanic particles in the assessment of sediments and rocks recovered in cores, be accessible to scientists with diverse research backgrounds and experiences, allow relatively quick and smooth data entry, and display data seamlessly in graphical presentations. The new classification scheme is based entirely on observations that can be made by any scientist at the macroscopic and microscopic level, with no genetic inferences, making the data more reproducible from user to user.

Classification and nomenclature of deposits with volcanogenic clasts has varied considerably throughout the last 50 y (Fisher, 1961; Fisher and Schmincke, 1984; Cas and Wright, 1987; McPhie et al., 1993; White and Houghton, 2006), and no consensus has yet been reached. Moreover, even the most basic descriptions and characterizations of mixed volcanogenic and nonvolcanogenic sediments are fraught with competing philosophies and imperfectly applied terminology. Volcaniclastic classification schemes are all too often overly based on inferred modes of genesis, including inferred fragmentation processes or inferred transport and depositional processes and environments. However, submarine-erupted and deposited volcanic sediments are typically much more difficult to interpret than their subaerial counterparts, partly because of more complex density-settling patterns through water relative to air (e.g., the ability of pumice to float is proportional to the volume of a clast) and the ease with which very fine grained sediment is reworked by water. Soft-sediment deformation, bioturbation, and low-temperature alteration are also more significant in the marine realm relative to the terrestrial realm.

The goal of the new classification scheme is to make the description of volcanic-sedimentary mixtures more accessible to nonspecialists and also more “database ready” so that volcanic inputs can be evaluated quantitatively.

Units

Sediments and sedimentary rocks, including volcaniclastic, siliciclastic, and bioclastic, were described at the level of (1) the descriptive interval (a single descriptive line in the DESClogik spreadsheet) and (2) the lithostratigraphic unit (Table T1). A descriptive interval typically consists of a single lithofacies distinct from those immediately above and below it (e.g., an ash interval intercalated between mudstone intervals); it is typically analogous to beds, with thicknesses classified in the same way (e.g., Ingram, 1954). In the case of closely intercalated, monotonous, repetitive successions (e.g., alternating thin sand and mud beds), lithofacies were grouped within the descriptive interval; this was done by using the lithology prefix “closely intercalated,” followed by the principal name, which represents the most abundant facies, followed by suffixes for the subordinate facies, in order of abundance (Fig. F16). Lithostratigraphic units (Table T1) are assemblages of multiple descriptive intervals containing similar facies that are typically tens to hundreds of meters thick. Lithostratigraphic units should be clearly distinguishable from each other by several characteristics, such as composition, bed thickness, grain size class, and internal homogeneity. Following IODP tradition, they are numbered sequentially Unit I, Unit II, and so on, from the top to the bottom of the core.

Sedimentary lithologic classes

The new descriptive scheme used during Expedition 350 defines four sedimentary lithologic classes, based on types of particles (Fig. F16; Table T2):

1. Volcanic lithologic class, defined as >75% volcanic particles.
2. Tuffaceous lithologic class, containing 25%–75% volcanic-derived particles mixed with nonvolcanic particles. The definition of the term “tuffaceous” (25%–75% volcanic particles) is modified from Fisher and Schmincke (1984).
3. Nonvolcanic siliciclastic lithologic class, containing <25% volcanic siliciclastic particles, where nonvolcanic siliciclastic particles dominate chemical and biogenic particles.
4. Biogenic lithologic class, containing <25% volcanic siliciclastic particles, where nonvolcanic siliciclastic particles are subordinate to chemical and biogenic particles.

Principal names

In our new scheme, the principal name for sediments and sedimentary rocks is based on grain size (Fig. F16) and is purely descriptive; it does not depend on interpretations of fragmentation, transport, or depositional or alteration processes. The sedimentary grain size classes of Wentworth (1922) are used for the nonvolcanic siliciclastic and tuffaceous lithologic classes, whereas the grain size classes of Fisher and Schmincke (1984) are used for the volcanic lithologic class.

We use the general term “particles” to refer to the fragments that make up volcanic, tuffaceous, and nonvolcanic siliciclastic sediments and sedimentary rocks, regardless of the size of the fragments. However, we adopt a strict definition of the terms “grains” and “clasts,” using clasts to describe particles >2 mm in size and grains to describe particles <2 mm in size. This size cut-off (2 mm) corresponds to the sand–granule grain size division of Wentworth (1922) and the ash–lapilli grain size divisions of Fisher (1961), Fisher and Schmincke (1984), Cas and Wright (1987), and McPhie et al. (1993) (Table T3). In addition, we include important information on the angularity of clasts by using the term “conglomerate” for a deposit where the clasts are exclusively (>95 vol%) rounded and subrounded, “breccia-conglomerate” where clasts are predominantly (>50 vol%) rounded and/or subrounded, and “breccia” where clasts are predominantly (>50 vol%) angular.

Prefixes

During Expedition 350, the prefix “monomict” was applied where clast compositions were restricted to a single type and “polymict” was applied where clast compositions of multiple types were present (Fig. F16). Where not obscured by alteration, an estimate of the compositional range of volcanic clasts was represented on macroscopic descriptions by three entries: “mafic,” “bimodal,” and “evolved,” with intervals described as mafic inferred to be basalt and basaltic andesite and intervals described as evolved inferred to be intermediate and silicic in composition. In macroscopic analysis, mafic versus evolved intervals are defined by the grayscale index of the main particle component, with mafic grains and clasts usually ranging from black to dark gray and evolved grains and clasts ranging from dark gray to white. Microscopic examination further aided in assigning the prefix mafic or evolved using glass shard color and mineralogy; however, precise determination of bulk composition requires chemical analysis. During Expedition 350, the prefix “matrix-supported” was used where smaller particles visibly envelop each of the clasts and “clast-supported” was used where clasts (defined as >2 mm in diameter, above) form the sediment framework.

Suffixes and other parameters

The suffix is used for a subordinate component that deserves to be highlighted. It is restricted to a single term (e.g., “with foraminifer,” “with crystals,” etc.). Bed thicknesses followed the terminology of Ingram (1954), and for simplicity, sorting values were restricted to three terms (well, moderately, and poor) and clast roundness values were also restricted to three terms (rounded, subrounded, and angular). Volcanic components included three major types (vitric, crystal, and lithic) sorted by their abundance into “dominant,” “2nd order,” and “3rd order.” Vitric clasts and grains include pumice and scoria (light colored and dark colored, respectively) as well as dense glass, and “fiamme” was used to describe flat wispy volcanic particles with no implication for origin (e.g., welding versus burial/diagenetic compaction). Crystals are described as grains because they are <2 mm in size, and their shape (euhedral, subhedral, or anhedral) and type (e.g., quartz or feldspar) is noted where possible. Lithic grains and clasts were described as angular, subrounded, or rounded with types (e.g., volcanic evolved or volcanic mafic) noted. In macroscopic descriptions, matrix can be well, moderately, or poorly sorted with types (e.g., vitric, crystal, or lithic) noted.

Summary of volcaniclastic rock descriptive scheme

The new volcaniclastic descriptive scheme applied during Expedition 350 uses a more nongenetic approach than proposed by previous authors because the sediments and rocks are named based on materials that are visible macroscopically and microscopically and not on the basis of inferred fragmentation, transport, and depositional processes (i.e., pyroclasts, autoclasts, hydroclasts, epiclasts, and reworked volcanic clasts [Fisher and Schmincke, 1984; Cas and Wright, 1987; McPhie et al., 1993]). Nonetheless, process interpretations can be entered as comments in the database; these may include inferences regarding fragmentation processes, eruptive environments, mixing processes, transport processes, alteration, and so on. The new descriptive scheme allows more quantitative evaluation of volcanic input and its nature, as demonstrated in the various lithostratigraphy, geochemistry, and interpretation sections in this report.

Site U1436: Izu fore-arc record of arc-front explosive volcanism

Site U1436 is located at 32°23.88′N, 140°21.93′E (1776 m water depth), in the western part of the Izu-Bonin fore-arc basin ~60 km east of the arc-front volcano Aogashima and ~170 km west of the axis of the Izu-Bonin Trench, 1.5 km west of Site 792 (Figs. F1, F3, F17).

The plan for Site U1436 was to carry out a 1 day operation to core 150 meters below seafloor (mbsf) and collect geotechnical samples in preparation for potential future drilling at proposed Site IBM-4. Here, the Chikyu is proposed to drill 5.5 km below seafloor to reach arc middle crust, which is inferred to represent juvenile continental crust. The origin of continental crust remains one of the biggest unsolved problems in Earth science because continental crust, though volumetrically insignificant on a planetary scale, is an important reservoir for many trace and minor elements. The “andesite model” proposes that subduction zone magmatism produces the nuclei of new continental crust. However, the processes involved (e.g., partial melting of mantle sources, crystallization differentiation, or other processes) remain poorly understood. Ultra-deep drilling into arc crust is the best way to sample unprocessed juvenile continental-type crust as it is first generated at intraoceanic arcs, before its primary features are overprinted by accretion or incorporation into a larger or more mature continent. Geotechnical Site U1436 is important for assessing the suitability of near surface conditions for this ambitious undertaking.

Although Site U1436 was scheduled as a geotechnical hole for proposed Site IBM-4, it yielded a rich, relatively complete record of Late Pleistocene fore-arc sedimentation, strongly influenced by frontal arc explosive volcanism. This is highly complementary to the main objective of Expedition 350, the rear-arc subduction factory (Site U1437), in terms of understanding the Izu arc system as a whole. In addition, Site U1436 gave scientists the opportunity to “test drive” the newly devised descriptive scheme for volcaniclastic rocks, described above.

The Izu fore arc is a repository of ash/tuff erupted in the Izu-Bonin frontal arc because the wind prevailingly blows from west to east. Mafic effusive eruptive products are better preserved on the frontal arc islands, whereas more silicic materials from explosive volcanism are preserved in adjacent deep ocean basins. The major objective of Site U1436 (besides collecting geotechnical samples) is to characterize the chemistry, age, provenance, and textural characteristics of mafic and silicic explosive volcanic products from the arc front in order to better understand outputs that are not preserved on land. At Site U1436, this stratigraphic record consists of tuffaceous mud interstratified with mafic and evolved ash and lapilli ash.

Age model, biostratigraphy, paleomagnetism, and physical properties

A 131.72 m succession of Pleistocene to Pliocene sediments was recovered over a cored interval of 150 m at Site U1436 (Fig. F18). Thirteen out of sixteen biostratigraphic datums and one magnetostratigraphic datum were selected to construct an age-depth relationship for this site and to estimate the linear sedimentation rates (LSRs) and mass accumulation rates (MARs). Biostratigraphic and magnetic reversal data are in agreement for the Late–Middle Pleistocene section, and the age model was constructed using all datum types. Both the biostratigraphic and magnetostratigraphic datums indicate a possible hiatus between Cores 350-U1436C-17F and 350-U1436A-14X (66–74 mbsf), with linear segments above (0–66 mbsf) and below (74–127 mbsf) the inferred hiatus; LSRs are 71 m/My above the inferred hiatus and 45 m/My below it. At nearby Site 792, a hiatus was identified at 87 mbsf. A comparison of LSRs between Sites U1436 and 792 over the same interval shows broadly similar values (81–120 m/My at Site 792) in the Late–Middle Pleistocene sequence. In the late Pliocene to early Pleistocene interval, however, Site U1436 shows a low LSR (46 m/My) compared to that of Site 792 (122 m/My). This difference could be due to underestimation of the LSR below the hiatus at Site U1436 because the tie points are not well constrained as a result of the rarity of marker species. The higher MAR in the younger part of the succession could explain the higher LSR, which could be related to greater volcanic input.

The biochronology at Site U1436 was primarily based on planktonic foraminifers and calcareous nannofossils. All the core catcher samples from Holes U1436A–U1436C were examined, and studies of both fossil groups showed that the upper 132 m of the cored interval in Hole U1436A (the deepest hole) spans the last 2.7 My (late Pliocene–Holocene). The timing of bioevents in the undisturbed part of the succession agrees with paleomagnetic data down to the Brunhes/Matuyama reversal (0.781 Ma) in Section 350-U1436A-9H-3. Deeper than 66 mbsf, several nannofossil and foraminifer bioevents were missing, indicating the presence of a hiatus. Below this potential hiatus, the rarity of markers and the occurrence of reworked specimens made identifying bioevents difficult. Planktonic foraminifers and calcareous nannofossils were generally well preserved and abundant, except in some layers where the concentration of biogenic constituents was reduced via dilution by volcaniclastics. The Pleistocene/Pliocene boundary was placed between Cores 350-U1436A-18X and 20X based on the presence of the top of Globorotalia pseudomioceanica (2.39 Ma) in Sample 350-U1436A-18X-CC and the top of Globoturborotalita decoraperta (2.75 Ma) in Sample 350-U1436A-20X-CC. Benthic foraminifer assemblages (i.e., extinction of Stilostomelidae) corroborate this biochronology.

Paleomagnetic analysis in Hole U1436A comprised archive-half demagnetization and remanence measurement at 10 mT steps to 40 mT. Severe core disturbance resulted in complete destruction of the depositional remanence in many intervals, and discontinuous recovery (Fig. F18) compromised the recognition of magnetostratigraphy in the lower half of the hole. Nevertheless, intervals of continuous mud recovery yielded a good paleomagnetic record, with the drill string overprint largely removed. The base of normal Chron C1n (the Brunhes/Matuyama boundary; 0.781 Ma) was recorded in one of the last continuous mud intervals, at Sample 350-U1436A-9H-3, 25 cm (56.8 mbsf). The reversal appears sharp because it occurred in the time interval between two successive depositional events.

One discrete paleomagnetic cube was sampled per section in undisturbed mud and silt. Discrete samples in the discontinuous record below the hiatus allowed us to recognize two additional datums: top of normal Chron C1r.1n (0.988 Ma) between Samples 350-U1436A-9H-4, 66–68 cm (reversed), and 10F-2, 64–66 cm (normal), and base of normal Chron C1r.1n (1.072 Ma) between Samples 10F-2, 64–66 cm, and 16X-2, 53–55 cm (reversed). Discrete samples and a patchy record in the archive half superconducting rock magnetometer (SRM) measurements indicate that Core 17X is all reversed polarity and probably still lies in the Matuyama interval (<1.778 Ma), which is however, not consistent with biochronology and thus has not been included in the age model.

Physical properties show an abrupt change at the ~50 mbsf inferred hiatus, below which recovery rates decrease. Shear strength, natural gamma radiation, and color reflectance L* parameter all decrease in the interval deeper than 50 mbsf, which is likely related to an increase in the relative abundance of mafic ash layers in that interval. Mafic ash layers have an average magnetic susceptibility value that is more than twice as high as the average value for evolved layers.

Lithostratigraphy

Out of the 71.6 m of core recovered in Hole U1436A, a total of 5.5 m of whole-round samples were removed for shore-based geotechnical testing and shipboard paleontological and interstitial water analysis. Of the remaining recovery, 65.7 m was measured and described as a single lithostratigraphic unit (Unit I) (Fig. F19). Unit I consists of mud with dispersed ash (referred to as tuffaceous mud) with intercalated intervals of ash and lapilli ash (~150 intervals total). Of these, ~80 intervals are mafic ash and scoria lapilli ash and ~70 intervals are evolved ash and pumice lapilli ash. In total, Unit I comprises ~40 m of tuffaceous mud and ~26 m of ash and lapilli ash. The mafic intervals are 50% thicker than the evolved beds, for a total mafic to evolved thickness ratio of 1.5:1. Representative images of the tuffaceous mud and mafic and evolved tephra are shown in Figure F20A–F20C.

Mud intervals average 25 cm in thickness, are up to 420 cm thick, and are massive and bioturbated. They consistently contain abundant ash, mainly vitric with rare crystals, and foraminifers. The mud is light gray to dark gray brown, commonly with a greenish hue. Rare glauconite (1–2 cm thick) occurs in the mud near the top contacts with evolved ash intervals.

The mafic ash and scoria lapilli ash intervals average 14 cm in thickness and are up to 230 cm thick, but this maximum thickness was probably greatly expanded by core disturbance (Core 350-U1436A-8H, the distinctive black glassy mafic ash discussed further below). The evolved ash and pumice lapilli ash intervals average 9 cm in thickness and are up to 60 cm thick. Pumice and scoria lapilli clasts are commonly <1 cm in size and angular; larger clasts are up to ~3 cm in size and subrounded. Most of the mafic lapilli ash intervals have subordinate pumice, and most of the evolved lapilli ash intervals have subordinate scoria. Most mafic and evolved intervals are normally graded with sharp bottom contacts and diffuse/gradational tops showing upcore increase in mud content.

The tuffaceous mud is interpreted to record hemipelagic background sedimentation with substantial ash contribution from explosive eruptions and/or resedimentation products, presumably originating chiefly from the Izu-Bonin arc front. Mafic and evolved ash and lapilli ash intervals may record distinct explosive events, also from the Izu-Bonin arc front, although evolved ash may be from more distal sources. The mode of transport and deposition of the mafic and evolved ash and lapilli ash intervals is not clear but involves some combination of subaqueous fallout and vertical settling through the water column and/or sediment gravity flows.

A very distinctive ash facies occurs in two intervals in Core 350-U1436A-8H (intervals 8H-1, 0 cm, to 8H-2, 110 cm, and 8H-3, 49–64 cm), referred to as the black glassy mafic ash (Fig. F20C, F20D). The lower ash is 15 cm thick and is underlain and overlain by tuffaceous mud (Fig. F20C). The upper ash is 220 cm thick, lies at the top of a core, and shows evidence for thickening by core disturbance; therefore, its original thickness is not known. Both layers contain macroscopically visible foraminifers. The two distinctive black glassy mafic ash intervals are the most mafic deposits analyzed shipboard at Site U1436 (basaltic andesite). Under the microscope the glass is brown to greenish brown (polarized light) with few microlites. Glass particles are flat, blocky, curviplanar and/or plastically deformed, and contain vesicles (Fig. F20D). The distinctive black glassy ash facies appears unusually homogeneous in componentry and texture, suggesting an eruption-fed origin. The overwhelmingly glassy nature of the ash further suggests subaqueous explosive eruption, and its sorting may suggest deposition by vertical settling through the water column from an ash plume. These two black glassy mafic ash layers attracted a great deal of interest in the science party because they could possibly be the product of large-volume mafic explosive eruptions. For this reason, we returned to Site U1436 to drill three more holes (U1436B, U1436C, and U1436D) in hopes of recovering undisturbed cores containing these layers and were finally successful in Hole U1436D.

Geochemistry

Inorganic and organic geochemistry measurements at Site U1436 aimed to characterize the interstitial water chemistry and elemental composition of igneous rocks and sediment samples as well as to determine the hydrocarbon gas concentrations within the sediments.

Headspace samples were analyzed routinely from every core in Hole U1436A as part of the shipboard hydrocarbon safety program. No hydrocarbon gases other than methane were detected in the cored sequences. Methane was either present in very low concentrations near or below the detection limit with an average concentration of 2.5 parts per million by volume (ppmv). Downhole interstitial water compositions obtained from selected whole-round samples (~1 per core) from Hole U1436A are generally in good concordance with previous observations at nearby Site 792 (Leg 126). Pore water Na/Cl scatters around an average of 0.86 throughout the sampled interval, which is equivalent to modern seawater. Variations in pore water compositions may be controlled by stronger seawater infiltration into porous ash-rich sections compared to muddy deposits. The most prominent deviations occur in pore waters from mud collected at ~13–27 mbsf, where sulfate concentrations subtly decrease (minimum = 25.8 mM) compared to seawater (28.9 mM) with concomitant subtle increases in pH and alkalinity and decreases in calcium. Magnesium concentrations are slightly elevated compared to Site 792 values at the same depths and show no evidence for magnesium sequestration because of volcanic glass alteration and clay mineral precipitation observed in the deeper sections of Site 792.

Concentrations of major elements and several trace elements in solid samples from Hole U1436A were measured by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and additional portable X-ray fluorescence (pXRF) analysis (Fig. F21). Reconnaissance pXRF and ICP-AES results show excellent agreement for K₂O and CaO; ICP-AES data include light elements (SiO₂, Al₂O₃, and Na₂O) not reliably obtained by pXRF, and pXRF data appear more reliable for zirconium. Composite lapilli (n = 11) and bulk ash samples (n = 3) were analyzed for a complete set of major elements by ICP-AES. Loss on ignition (LOI) values range between 0.54 and 3.8 wt% (9.8 wt% for impure mud with ash) and are generally higher in evolved ash and lapilli samples compared to mafic samples. These LOI values are elevated compared to those from regional fresh subaerial volcanic rocks and are indicative of secondary hydration of volcanic glass.

All but one of the Hole U1436A tephra samples define broadly linear trends in Harker diagrams (SiO₂ = 54.4–70.1 wt%). They classify as low-K rocks and are indistinguishable from intermediate to acidic rocks from basalt-dominant island volcanoes of the arc front (e.g., Aogashima Volcano ~56 km to the west), although basalt (with <53 wt% SiO₂; present in arc-front volcanoes) has not been found in the tephra at Site U1436. The most mafic samples are from the black glassy ash lithofacies and are basaltic andesite (55 wt% SiO₂). One interval of evolved lapilli ash (at ~45 mbsf) contains pumice with elevated K₂O, indicating that it did not come from Aogashima or the East Aogashima Caldera ~45 km to the west, which have lower K₂O. This pumice is similar in composition to those from submarine calderas including three within 65 km of the drill site (Myojin Knoll, Myojinsho, and South Hachijo), which exist south and north of Aogashima. Rhyolite-dominant submarine caldera volcanoes with elevated K₂O are common in the arc front (Tamura et al., 2009).

Turbidites (younger than 1 Ma) from Site 792, which is 1.5 km east of Site U1436, have compositions similar to tephra collected at Site U1436. However, they are a little lower in TiO₂ and higher in K₂O at the same SiO₂ content compared with the main trend of Site U1436 tephra, except for the high K₂O pumice. Turbidites, by their nature, show a mixing trend between mafic and felsic end-members. Thus, the differences between turbidites and tephra at Sites 792 and U1436, respectively, are consistent with the existence of two types of pumice at Site U1436, which results in the mixing line being different from fractionation trends shown in Site U1436 tephra.

Site U1437: Izu rear arc

Site U1437 is located in the Izu rear arc and is ~330 km west of the axis of the Izu-Bonin Trench (Figs. F1, F3) and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll (Fig. F22A) at 2117 meters below sea level (mbsl).

The preliminary results of seismic surveys for Site U1437 are summarized briefly here from Tamura et al. (2013); full results of the seismic surveys will be presented in a full paper at a later time after drilling results are integrated (M. Yamashita, pers. comm., 2014). Numerous lines were shot in two different campaigns; parts of three seismic sections that cross at Site U1437 are plotted on Figure F22B and described here (Figs. F15A, F15B, F23). Line IBr5 is the longest seismic line, running east–west from the Manji rear-arc seamount chain across the Enpo seamount chain to the arc front; it was shot both by wide-angle ocean-bottom seismometer (OBS) and by multichannel seismic (MCS) (Fig. F23). The wide-angle OBS survey shows the velocity structure of the upper ~10 km, and the MCS line shows the upper ~5 km. Generally, the velocity transition to >5 km/s is thought to represent the transition to igneous rocks, perhaps representing arc upper crust lava or crystalline rocks, and the velocity transition to 6 km/s is generally thought to represent the transition to middle crust (e.g., see boundaries picked in Fig. F23). Tamura et al. (2013) estimated the 5 km/s iso-velocity contour to lie at ~2100 mbsf at Site U1437 and suggested that these rocks could be Oligocene–Eocene “igneous basement,” consisting of lava and/or intrusions. Line IBM3-NW5 (Fig. F15A) clearly shows that Site U1437 lies in a volcano-bounded basin between the Enpo and Manji rear-arc seamount chains.

Shipboard age model

At Site U1437 it was possible to identify a Pleistocene to upper Miocene succession (Fig. F24). Fourteen biostratigraphic and 29 magnetostratigraphic datums obtained in the upper 1303 mbsf of the succession were selected to construct the age-depth model. The age model has not been extended deeper than 1303 mbsf because no biostratigraphic or magnetostratigraphic datums are detectable from 1303 to 1806 mbsf. Also, no biostratigraphic datums are recognized deeper than 867 mbsf; thus, the age model for 867–1303 mbsf was constructed using only magnetic reversal datums.

Seven intervals were selected to calculate the LSR, assuming constant sedimentation rates within those intervals. The LSRs range from a minimum of 98 m/My to a maximum of 259 m/My. The highest LSR (259 m/My) is found from the top of lithostratigraphic Unit II through the upper part of Unit III, to ~825 mbsf. A minimum LSR of 98 m/My is recorded from 825 to 844 mbsf. Lithostratigraphic Units IV and V record an increase in LSR with values of 157 and 105 m/My.

An offset in the LSR between intervals 868–1056 and 1122–1302 mbsf corresponds to the change from Hole U1437D to U1437E and also to a missing interval in the magnetostratigraphy. The probable explanation is a normal fault between the two holes, resulting in a partial loss of section within lithostratigraphic Unit IV. If the LSRs within lithostratigraphic Unit V are extrapolated to Unit VI, ages are about 0.6 Ma less than the minimum age constraint given by the nannofossil assemblage in Sample 350-U1437E-36R-CC (10.97–11.85 Ma at 1403 mbsf); a hiatus or erosion is the most likely explanation for this discrepancy, given that sedimentation rates would be expected to be high in the coarse-grained Unit VI.

One additional age control point was added postcruise before publication of this report. Sample 350-U1437E-35R-1, 76 cm, to 35R-2, 55 cm (1388.86–1390.07 mbsf) from igneous Unit 1 (see below) has a preliminary U-Pb zircon concordia intercept age of 13.6 +1.6/–1.7 Ma (2σ error; mean square of weighted deviation [MSWD] = 2.12; number of analyses n = 9) (Schmitt, pers. comm., 2014).

Lithostratigraphy and physical properties

Site U1437 was drilled in three holes (U1437B, U1437D, and U1437E), which we divide into seven lithostratigraphic units (Fig. F25; Table T4), described in this section. Physical properties for Site U1437 (Fig. F25B, F25C) are discussed with reference to the lithostratigraphic units below. Summary lithostratigraphic logs for Holes U1437B, U1437D, and U1437E are shown in Figure F26A, F26B, and F26C, respectively. The downhole evolution in proportion of mud and volcaniclastics is shown in Figure F27, dividing the volcaniclastics further into relative proportion of mafic and evolved materials (Holes U1437B and U1437D) or composition of volcanic clasts (Hole U1437E).

Lithostratigraphic Units I–VII are distinguished from each other based on the proportion and characteristics of tuffaceous mud/mudstone and interbedded tuff, lapilli tuff, and tuff breccia. The tuffaceous mud/mudstone is strongly to intensely bioturbated. Alteration becomes more pervasive and increases in intensity downhole in both holes; it is initially predominantly glauconitic–smectitic and eventually becomes more chloritic. Iron sulfides are pervasive throughout both holes, especially as replacements of worm burrows, and according to rock magnetic properties, greigite is progressively replaced by pyrite downhole in Hole U1437D. Compaction of sediment at Site U1437 increases linearly from ~0% shallower than ~410 mbsf to ~36% at the base of Hole U1437D (~1100 m). The transition from unconsolidated to lithified rocks occurred progressively; however, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downward.

Lithostratigraphic Unit I

Unit I (interval U1437B to 350-U1437D-28R-2, 112 cm; 0–682.12 mbsf) is 0–4.3 Ma in age, 682.12 m thick, and consists largely (88%) of mud or mudstone with >25% dispersed ash, referred to as tuffaceous mud/tuffaceous mudstone (Fig. F3). For fine-grained deep marine sediment, it has a high average sedimentation rate (~165 m/My, described above). Unit I has minor (12%) tephra, consisting almost entirely of ash (unlithified) or tuff (lithified) intervals (n = 649); only 11 of these intervals contain lapilli, for a total thickness of 59 cm, or just 1.2% of the tephra. Furthermore, the lapilli are small, averaging ~1 cm in size. Ash or tuff beds have a median thickness of just 8 cm, whereas tuffaceous mud/mudstone intervals have a median thickness of 26 cm. The sparseness, thinness, and fine grain size of discrete tephra layers in lithostratigraphic Unit I is enigmatic, given the fact that it accumulated in close proximity to volcanoes of the active rift and back-arc knolls extensional zone (<3 Ma) and rear-arc seamount chains (>3 Ma), in addition to lying within 90 km of the arc front (Fig. F6).

The tuffaceous mudstone of Unit I contain abundant fine colorless glass shards and rare crystals, plus carbonate materials such as foraminifers. They are typically bioturbated and commonly have green horizons (possibly glauconite) where they overlie evolved ash/tuff. Pyrite clots and greigite are associated with bioturbation in the mudstone.

The ash/tuff intervals of Unit I include both evolved (white to dark gray) and mafic (black) intervals (Fig. F28); however, the evolved ash/tuff intervals (n = 480) are four times as common as the black mafic ash/tuff intervals. The evolved ash/tuff intervals are on average only 7 cm thick but individual intervals can reach a maximum of 70 cm, with the thickest intervals occurring mainly between 100–140 and 530–600 mbsf. The evolved ash/tuff intervals are mainly vitric with sharp bases and tops that grade upward into mudstone, some with a crystal-rich base dominated by plagioclase with lesser pyroxene. Some ash/tuff intervals are laminated with darker layers richer in crystals and pumice grains and lighter layers richer in glass shards. Hornblende is found in only 7% of the evolved ash/tuff intervals; these are mainly in the lowermost part of lithostratigraphic Unit I, where hornblende-bearing ash has elevated K₂O contents relative to most of the other evolved ash intervals, suggesting that these record rear-arc seamount volcanism rather than arc-front or rift volcanism (see “Geochemistry”). A subset of ash/tuff intervals (n = 47) is described as bimodal because microscopic inspection reveals that these intervals contain >25% of both colorless and colored mafic glass, but colorless glass predominates in most cases. There are only 11 lapilli ash/lapilli tuff and lapillistone intervals in lithostratigraphic Unit I, bearing subrounded mafic scoria and/or evolved pumice and subordinate lithic clasts.

Unit I is divided into three units on the basis of physical properties (PP Units 1, 2, and 3, Fig. F25B). PP Unit 1 (0–430 mbsf) is characterized by a downhole increase in bulk density (from 1.5 to 1.6 g/cm³) and P-wave velocity (from 1501 to 1848 m/s) and a corresponding downhole decrease in porosity (from 70 to 65 vol%). The top of PP Unit 2 (430–550 mbsf) is marked by an initial increase in porosity (from 61 to 67 vol%) then a downhole continuation of the decrease observed through PP Unit 1. The top of PP Unit 3 (550–682 mbsf) is characterized by a sharp increase in porosity (from 52 to 65 vol%) and a corresponding decrease in bulk density (from 1.8 to 1.6 g/cm³), followed by more normal downhole trends similar to those seen in PP Unit 2. The color reflectance ratio of a*/b* displays a significant decrease in the scatter of the data, indicating that color is less variable through PP Unit 3 than in PP Unit 2. The base of PP Unit 3 corresponds to the base of lithostratigraphic Unit I (Fig. F25B).

Lithostratigraphic Unit II

Unit II (interval 350-U1437D-28R-3, 0 cm, to 32R-CC, 28 cm; 682.12–728.1 mbsf) is 44.38 m thick, with much more abundant tephra (~75%) and much less tuffaceous mudstone (~25%) than is present in Units I or III (Figs. F26, F27). Additionally, tephra in Unit II is coarser in grain size than that of Units I and III (Fig. F26), with pumice lapilli tuff and pumice lapillistone forming slightly more than half of the thickness of the tephra and tuff forming slightly less than half. Tephra in Unit II also differs from that of Units I and III by being entirely evolved (no mafic tephra present; Figs. F27, F28). Tephra has pumice clasts and planar bedding, cross bedding, and normally or reversely graded intervals (Fig. F29) and is on average 16 cm thick with a maximum of 95 cm.

Unit II has three lithofacies types. The first lithofacies, evolved tuff, pumice lapilli tuff, and pumice lapillistone, are monomictic and contain plagioclase, clinopyroxene, orthopyroxene, and amphibole crystals in variable proportions. The second lithofacies, dark gray evolved tuff, is similar to the darker colored evolved tuff in Unit I. They are commonly graded with a sharp base overlain by a crystal-rich layer that grades upward into a vitric tuff, commonly bioturbated where overlain by tuffaceous mudstone. The third lithofacies, tuffaceous mudstone, is similar to the tuffaceous mudstone of Unit I but is more strongly lithified and altered to green clay minerals (likely glauconite and smectite) plus pyrite. Pumice is commonly devitrified or replaced by palagonite, zeolites, or clay minerals.

PP Unit 4 (682–728 mbsf) corresponds to lithostratigraphic Unit II (Fig. F25B) and is characterized by an increase in the scatter of density and porosity, a significant increase in P-wave velocity values and range, an abrupt increase in magnetic susceptibility (MS) (average of 476 IU versus 138 IU for PP Unit 3), and an abrupt decrease in natural gamma radiation (NGR) from ~20 to ~5 cps.

Lithostratigraphic Unit III

Unit III (interval 350-U1437D-33R-1, 0 cm, to 63R-2, 26 cm; 728.1–1017.88 mbsf) is 289.7 m thick and is composed of tuffaceous mudstone (~64%) and lesser tuff (~35%) (Figs. F26, F27). It contains only ~1% lapilli tuff (3.1 m in total). All the tuff intervals and the rare lapilli tuff intervals are evolved (Fig. F27). Unit III also contains one distinctive interval (1.91 cm thick) that consists of deformed mudstone intraclasts (up to ~20 cm in size) and clasts of scoria and pumice (up to 5 cm), supported in a tuffaceous mudstone matrix; this is interpreted to represent a submarine debris flow deposit. Unit III shows an increase in fine-grained tuff (relative to tuffaceous mudstone) in its basal ~80 m (Fig. F27); above that, Unit III is similar to Unit I, except that it lacks the mafic tuff that makes up ~20% of the tuff in Unit I.

The tuffaceous mudstone intervals in Unit III have abundant bioturbation (Fig. F30) and, in the lower part of the core, dark laminae that may reflect higher clay content. The evolved tuff intervals generally have sharp basal contacts, some with a crystal-rich basal layer, and bioturbated tops that grade into tuffaceous mudstone. The evolved tuff is composed of glass and pumice or fiamme grains and crystals, including feldspar, clinopyroxene, orthopyroxene, and occasional hornblende.

Two main facies are recognized in the evolved tuff of Unit III (Fig. F30):

• Dark gray evolved tuff, which is medium-grained to coarse-grained, and owes its dark color to the presence of crystals and pumice (Fig. F30A–F30D). It is identical to the dark gray tuff of Units I and II and not described further here.
• White and green fine-grained tuff, which is much finer grained and better sorted than other evolved tuff at Site U1437, in places appearing chert-like (Fig. F30E). The fine-grained tuff has laminations produced by alternation of glass shard–rich layers (white) and layers of mixed shards, pumice, and crystal fragments (gray-green), repeated over intervals up to several meters thick, with no bioturbation or mudstone interbeds. Thus, the intervals seem to record fairly continuous but pulsating sedimentation. The laminations commonly show soft-sediment deformation, supporting the interpretation that the white and green fine-grained tuff intervals were deposited rapidly (see below). White to gray-green fine-grained tuff intervals form much of the tephra in the lower part of Unit III, where the tephra content is highest for this unit (except for its uppermost part; Fig. F27A). The white and green fine-grained tuff lithofacies also occurs in Unit IV (described below).

The top of lithostratigraphic Unit III corresponds to the upper boundary of PP Unit 5 (728–794 mbsf) (Fig. F25B), defined by an increase in NGR values to ~16 counts/s and a decrease in the scatter of P-wave velocity, followed downhole by the continuation of the P-wave trend observed in PP Unit 3. PP Unit 6 is contained within lithostratigraphic Unit III. The top of PP Unit 6 (794–846 mbsf) is defined by an increase in the scatter of the P-wave velocity and color reflectance (a*/b*) values. The top of PP Unit 7 (846–1018 mbsf) is marked by an initial ~0.3 g/cm³ decrease in bulk density and a corresponding increase in porosity (846 to ~860 mbsf), followed by regular trends downhole and marked by significant scatter. The top of PP Unit 7 is also characterized by an abrupt decrease in the average MS value from 491 IU (PP Unit 6) to 167 IU. The base of PP Unit 7 coincides with boundaries between lithostratigraphic Units III and IV, described below.

Lithostratigraphic Unit IV

Lithostratigraphic Unit IV (interval 350-U1437D-64R-1, 8 cm, to 350-U1437E-6R-3, 122 cm; 1017.88–1120.11 mbsf) is 102.23 m thick and is dominated by polymictic tuff and lapilli tuff, in contrast to Unit III (dominated by tuffaceous mudstone) and Unit V (dominated by monomictic lapilli tuff). Unit IV consists of coarse-grained tuff and polymictic lapilli tuff and lapillistone (78%), with minor tuffaceous mudstone (22%) becoming more frequent toward the base.

Unit IV consists of four lithofacies, in order of abundance:

• Normal-graded polymictic lapilli tuff and lapillistone. Lapilli are small (average 3–5 mm, up to 1.5 cm). Lithic clasts dominate over vitric (mainly pumice) clasts, and lithic clasts are light to dark in color, but nearly all are plagioclase-pyroxene andesites; hornblende is rare.
• White and green fine-grained tuff. This is identical to the white and green fine-grained tuff described under Unit III (above), characterized by laminae of very fine grained vitric tuff (white) alternating with fine- to medium-grained pumice and crystal tuff (green). Similarly, it forms thick, unbioturbated intervals up to 5.54 m thick with planar lamination or soft-sediment deformation. The glass shards are too altered in thin section to allow determination of their original morphology.
• Dark gray evolved tuff. Also like those described under Units I, II, and III (above), this is a dominantly medium- to coarse-grained evolved tuff that owes its dark color to the presence of crystals (plagioclase with minor clinopyroxene) and pumice.
• Tuffaceous mudstone. This is also like that described in Units I, II, and III, except perhaps more bioturbated and with pumice lapilli in two intervals.

The upper boundary of PP Unit 8 (1018–1140 mbsf) corresponds to the upper boundary of lithostratigraphic Unit IV. PP Unit 8 is characterized by an increase in the average MS value to 916 IU and a shift to very low (~1 W[m·K]) thermal conductivity.

Lithostratigraphic Unit V

Lithostratigraphic Unit V (interval 350-U1437E-6R-3, 122 cm, to 27R-CC, 15 cm; 1120.11–1312.21 mbsf) is 192.1 m thick and is distinguished largely on the basis of its intervals of monomictic, reversely graded pumice lapilli tuff (Fig. F31A); these distinctive beds contrast with the polymictic, dominantly lithic lapilli tuff of the overlying and underlying units (IV and VI). Like Units I, III, and IV, Unit V also has tuffaceous mudstone (69%) and evolved tuff intervals (15%). Lapilli tuff accounts for only 16% of Unit V, but the monomictic reversely graded pumice lapilli tuff recurs throughout Unit V.

Each monomictic reversely graded pumice lapilli tuff interval in Unit V has (Fig. F31A) (1) a sharp base, typically eroded into the underlying tuffaceous mudstone, overlain by (2) evolved tuff with abundant glass shards and grains of pumice, in turn grading upward into (3) pumice lapilli tuff with flattened or unflattened pumice that become progressively larger upward (i.e., reverse graded). This passes upward into (4) tuffaceous calcareous mudstone. This lithofacies is thus composed almost entirely of vitric material (glass shards and pumice).

The tuff in Unit V varies in color from light green to dark gray to brown and has variable vitric-crystal contents (Fig. F31B). The lapilli tuff in Unit V has volcanic lithic lapilli and pumice with crystals of plagioclase, pyroxene, and opaque minerals (Fig. F31C). Soft-sediment faults are present in tuffaceous mudstone and tuff, with small offsets of 1–10 cm (Fig. F30D).

The upper boundary of PP Unit 9 (1140–1315 mbsf) is 20 m below the upper boundary of lithostratigraphic Unit V and is defined by a decreased scatter in density, porosity, P-wave velocity, and NGR counts, as well as a downhole decrease in the thermal conductivity.

Lithostratigraphic Unit VI

Lithostratigraphic Unit VI (interval 350-U1437E-28R-1, 0 cm, to 42R-3, 60 cm; 1320.00–1459.80 mbsf) is 139.80 m thick and is characterized by an abundance of polymictic lithic and pumice lapilli tuff, although it also contains monomictic pumice lapilli tuff (Fig. F32). The top of lithostratigraphic Unit VI is marked by the first appearance of multiple intervals of polymictic lapilli tuff, and its base is marked by the top of the very distinctive black monomictic glassy lapillistone and lapilli tuff in the upper part of Unit VII. Unit VI is dominated by lapilli tuff and lapillistone (~57%) with lesser tuff (32%) and minor tuffaceous mudstone (11%). The polymictic lapilli tuff and lapillistone form very thick beds (>1.5 m, the length of a core section, or up to 2.8 m thick assuming complete recovery between core sections). Tuff averages 0.24 m in thickness, to a maximum of 1.43 m. Tuff and tuffaceous mudstone is interbedded.

Polymictic lithic and pumice lapilli tuff are four times more abundant than monomictic pumice lapilli tuff in Unit VI; monomictic varieties contain only pumice (Fig. F32A), whereas polymict varieties have evolved and lesser mafic volcanic lithic clast types as well as pumice (Fig. F32B). The polymictic lithic and pumice lapilli tuff show a complete gradation from clast-supported to matrix-supported (Fig. F32C), whereas the monomictic pumice lapilli tuff is all matrix-supported (Fig. F32A).

Lithic lapilli clasts in Unit VI are dominantly

• Porphyritic andesite with plagioclase and clinopyroxene. These range from nonvesicular to highly vesicular (up to ~50% vesicles), locally filled with zeolites, clay minerals, and/or chalcedony. Clasts of this type also occur as scattered small blocks (>6.4 cm in size) in the polymictic lapilli tuff.
• Rhyolite-dacite, which becomes more common near the rhyolite intrusive sheet with peperitic boundaries, described as Igneous Unit 1 below. These include crystal poor and porphyritic varieties, with amphibole, plagioclase, and quartz.

Pumice lapilli clasts in Unit VI are light to dark green and commonly flattened by compaction and lithification into fiamme; less flattened pumice clasts have vesicles filled with chlorite (Fig. F32B, F32D). Red to brown mudstone clasts are also present in Unit VI.

Physical properties do not show distinct changes at the top of Unit VI but are gradational into the unit from the base of Unit V. NGR declines from ~10–12 counts/s at the base of Unit V to a sustained ~5–6 counts/s in Unit VI. MS increases into Unit VI and becomes less variable as compared with Unit V. Porosity and P-wave velocity show no appreciable change from Units V to VI. The base of Unit VI (Section 350-U1437E-42R-3, 60 cm) coincides with a sustained one order of magnitude reduction in MS and a sustained but minor reduction in P-wave velocity. Porosity increases slightly into Unit VII but becomes much less variable. Conversely, NGR ceases to be stable and becomes highly variable in the upper part of Unit VII.

Igneous Unit 1

The only igneous unit observed at Site U1437 consists of a single rhyolite intrusion at 1389 mbsf, which lies within lithostratigraphic Unit VI (Fig. F33). Core recovery was much lower in igneous Unit 1 (45%) than it was in the host volcaniclastic rock (~94%), and the recovered igneous Unit 1 core material was affected by fragmented core disturbance, probably due to greater competency of rhyolite intrusion than the surrounding volcaniclastic host. Therefore, although only 1.2 m thickness was described for igneous Unit 1, its maximum thickness is estimated at 6.5 m assuming all the material not recovered from this interval was part of igneous Unit 1. Its true thickness probably lies somewhere between 1.2 and 6.5 m.

Igneous Unit 1 was described by the general term rhyolite-dacite, but shipboard geochemical analysis (discussed below) shows that it is a rhyolite with 74.5% SiO₂. It has sieve-textured subhedral plagioclase (up to 4 mm, ~7%), euhedral hornblende (up to 0.5 mm, ~3%), large anhedral to subhedral quartz (up to 8 mm, ~1%) with fresh glassy melt inclusions, some opaque minerals, and rare zircon (20 µm in size). Flow banding is observed across the entire unit in various orientations (Fig. F33B). The groundmass varies from cryptocrystalline near the upper and lower contacts to fine grained in the center of the unit. The upper margin is chilled, and the overlying lapilli tuff is baked, indicating that igneous Unit 1 is an intrusion rather than a clast or lava. The presence of xenoliths of the host unit supports the interpretation that it is an intrusion. The lower contact of igneous Unit 1 is a peperite, defined as a magma-wet sediment mixture (Busby-Spera and White, 1987); the contact shows complex mingling between the intrusion and the host, including crenulated lobate margins on the intrusion and dispersal of the magma into the host on the microscopic scale (Fig. F33C).

Lithostratigraphic Unit VII

The top of Unit VII is at 1459.8 mbsf (Sample 350-U1437E-42R-3, 60 cm), and Hole U1437E ended in Unit VII, which was described to a depth of 1800.3 mbsf (79R-3, 83 cm). Unit VII is 340.5 m thick, and ~90% is extremely thick bedded, nongraded, nonstratified, poorly sorted, coarse-grained angular andesitic lapilli tuff, in places with blocks tens of centimeters in size. Some clasts have quenched margins, jigsaw-fit textures, intricate fluidal margins, or peperitic margins; as discussed below, these indicate hot emplacement of clasts, or of margins of lava or intrusions, or both. Thus Unit VII is interpreted to be a vent-proximal deposit. Unit VII is divided into upper and lower parts (above and below 1643.73 mbsf).

The upper part (183.93 m thick) of Unit VII is dominated by a black, glassy, homogeneous, nonstratified deposit of unaltered angular lapillistone and lapilli tuff with abundant large clinopyroxene glomerocrysts and plagioclase glomerocrysts. The glass is isotropic and nonvesicular, and bubble-wall shards or broken crystals are absent (Fig. F34A, F34B). Only a few sparsely phyric volcanic lithic clasts are present, some with quenched margins, and a few red oxidized clasts are present. The black glassy lapilli tuff and lapillistone lack stratification completely, except for one thin (~25 cm thick) interval of crudely stratified ash. The black glassy lapillistone and lapilli tuff are interpreted to be hyaloclastite formed by autobrecciation and quenching of lava in a submarine environment (discussed further below).

The lower part (156.57 m thick) of Unit VII is dominated by green (more altered) angular andesite lithic lapilli tuff with blocks up to 53 cm in size. Like the black glassy lapillistone and lapilli tuff unit that forms the upper part of lithostratigraphic Unit VII, these lithic lapilli tuff and tuff breccia are massive, but they are intercalated with stratified lithic lapilli tuff and tuff (also altered green). The clasts are also more heterogeneous, with variable plagioclase and pyroxene contents, and they range from nonvesicular to moderately vesicular, with vesicles mostly filled with secondary minerals. Some of the clasts show evidence for hot emplacement (Fig. F34C), including intricate fluidal margins, quenched margins and breadcrust texture, jigsaw-fit brecciated margins, and in at least one case, a clast appears to be surrounded by sediment with a baked margin. Additionally, clasts with broken chilled margins are absent; these would be expected if the clasts were transported and deposited after they cooled. In core, it is not possible to determine whether all of these features formed on clasts that were emplaced at high temperatures or if some of these features formed on the complexly embayed margins of small intrusive bodies or lava bodies. Further support of hot emplacement is provided by paleomagnetic inclinations from two of the “clasts” (discussed further below).

The green lower part of Unit VII also has minor intervals of tuff, averaging ~20 cm thick, with variable color (brown, dark gray, red-brown, and green). These thin tuff intervals are stratified or massive, nongraded or normally graded (with rare reverse grading), and commonly contain minor lapilli.

PP Units 11–13 occur within lithostratigraphic Unit VII, with the top of PP Unit 11 (1460–1580 mbsf) corresponding with the top of lithostratigraphic Unit VII. PP Unit 11 is characterized by an abrupt decrease in the MS values and a decreased scatter in density, porosity, and P-wave velocity values. NGR counts from the upper part of PP Unit 11 are higher than in PP Unit 10 and then decrease again in the lower part of PP Unit 11. The upper boundary of PP Unit 12 (1580–1742 mbsf) is defined by an abrupt increase in the MS relative to PP Unit 11. Density and P-wave velocity increase downhole, whereas porosity decreases. The top of PP Unit 13 (1742–1800.3 mbsf) is marked by a decrease in P-wave velocity and thermal conductivity, which then increase downhole toward the bottom of the hole.

Geochemistry

Hydrocarbon gases

Samples for hydrocarbon gas analysis from headspace (n = 184) were collected and analyzed for every core at Site U1437 in compliance with the shipboard hydrocarbon safety program. Methane was the only hydrocarbon gas above detection limits in sediment headspace samples in Holes U1437B and U1437D. Methane abundances gradually increase with depth, with the highest abundances at ~750–1459 mbsf (Fig. F35), and reach maximum values of 638 ppmv at 920 mbsf, which is still well below critical safety thresholds. The zone of methanogenesis is unusually deep because of a release of sulfate below the sulfate reduction zone (27–83 mbsf; see “Pore fluid analysis” and “Rock magnetism”), which may be buffering the methanogenesis by anaerobic methanogens. Deeper than 1459 mbsf, methane concentrations decrease again and are <10 ppmv in the lowermost sections of the cored sequence. Coincidently upon starting drilling in Hole U1437E, ethane was also detected with maximum abundances of 13 ppmv at 1275 mbsf (Fig. F35). In the horizons where both methane (C₁) and ethane (C₂) were detected (Sections 350-U1437E-4R-2 through 42R-3; 1105–1450 mbsf), C₁/C₂ values are <100, possibly indicating the organic matter is mature and the hydrocarbon gases are thermogenic. However, C₁/C₂ never reached the critical threshold to cause drilling operations to be halted according to the shipboard safety program.

Pore fluid analysis

Interstitial water (IW) samples (n = 59) were squeezed from whole-round core sections between 5 and 10 cm long that targeted muddy intervals in cores between 8.3 and 693 mbsf. The interstitial water salinity and chlorinity profiles are characterized by moderate increases above seawater values with depth. The chloride profile is nearly constant in Hole U1437B between 18 and 401 mbsf. Bromide shows a gradual decrease with depth but in contrast to Hole U1436A is poorly correlated with Cl. Higher chlorinity with depth indicates hydration of volcanic ash and the formation of hydrous alteration products such as clays and zeolites. These reactions consume H₂O and increase pore water salinity and chlorinity. Variations in Ca, Mg, and B abundances with depth are consistent with this interpretation.

Depth profiles for alkalinity, pH, ammonium, sulfate, and phosphate (Fig. F36) are highly complementary and characterized by prominent deviations from seawater compositions over the uppermost 50 m of the profile, followed by a more gradual reversal to near seawater compositions at depth. The sharp decrease in sulfate between the surface (~24 mM) and ~50 m depth (~5 mM) is mirrored by an approximately equimolar increase in alkalinity, which is consistent with microbial sulfate reduction and the release of HCO₃^– into solution according to the simplified net reaction

Microbial activity could also be responsible for production of phosphate, which peaks at nearly 100 mM, or ~100× seawater abundance, at 55.5 mbsf, and ammonium, which peaks at 2094 µM at 82.5 mbsf.

The overall shape of the sulfate profile, strong depletion in a shallow region of microbially mediated sulfate reduction, and a gradual return to seawater compositions is peculiar but has been previously documented in several holes (ODP Leg 170 Site 1039 and Integrated Ocean Drilling Program Expeditions 334 and 344 Site U1381). In these cases it was attributed to the infiltration and upward diffusion of sulfate-rich fluids from below the zone of sulfate reduction (Expedition 334 Scientists, 2012; Harris et al., 2013). We hypothesize that this region of fluid influx coincides with a zone of poor core recovery, grain-size increase, and strong seismic attenuation at ~300 mbsf in Hole U1437B. This zone is also characterized by a broad peak in pore water Li that would be consistent with vertical diffusion from a zone of horizontal fluid infiltration (see below).

Variations in IW major elements (Na, Ca, and Mg) (Fig. F37) are best evaluated by separating the trends observed over a shallow depth range (0–100 mbsf) attributed to biologic processes from those at greater depth (>100 mbsf), which are more strongly controlled by processes such as distal fluid migration and diagenesis. At ~450–500 mbsf, the trends for Na, Ca, and Mg show prominent breaks: Na and Ca increase more strongly with depth, with a Ca concentration versus depth gradient of 0.14 mM/m, and Mg strongly decreases at –0.11 mM/m to a minimum of 9.6 mM at 693 mbsf. Lithification increases at about the same depth producing dominantly tuffaceous mudstone from tuffaceous mud. Moreover, the abundance of mafic and evolved tuff increases deeper than ~450 mbsf, and therefore the increase in Ca with concomitant depletion in Mg is reasonably attributed to alteration of volcanic glass and the formation of smectite (e.g., Riedel et al., 2006).

Depth discontinuities of IW minor components (B, Ba, Fe, Li, Mn, Si, and Sr; Li and Si shown in Fig. F37) often correlate with the changes described above. B depth variations mirror the hockey stick profile for pH by very gradually decreasing from near seawater compositions to ~600 mbsf followed by a sharp drop to the terminal depth of IW sampling at 693 mbsf. The correlation of this decrease with strong depletions of deep pore waters in Mg and concomitant increases in pH and Ca suggests absorption of B in clays formed from alteration of volcanic glass. Ba and Fe concentrations show little variability with depth and often scatter widely over small depth intervals.

Li displays a hump-shaped profile with a maximum (70 µM) between 283 and 343 mbsf (Fig. F37). Li declines deeper than 343 mbsf to a local minimum at 459 mbsf, coincident with the sulfate minimum (Fig. F36). The main hump of the Li profile is consistent with the ingression of a high-Li fluid at this depth, which corresponds to a zone of poor core recovery that also matches a major seismic reflector at ~300 mbsf. This situation is similar to observations for Integrated Ocean Drilling Program Expedition 344 Site U1380, where a horizon of elevated Li concentrations correlated with a shear zone interpreted to act as a conduit for fluids with elevated source temperatures (Harris et al., 2013). The upward decrease in Li in the Site U1437 profile suggests progressive dilution, whereas the downward decrease indicates uptake of Li by clay minerals formed during alteration of volcanic glass.

The Mn depth profile mimics the hockey stick profile for chlorinity, except for an excursion to high Mn concentrations in the near-surface IW sample from Core 350-U1437D-2H (8.4 mbsf). Dissolved Si parallels the hump-shaped Li depth profile.

At shallow depth, the progressive dissolution of diatoms could be responsible for Si increases, whereas transformation of biogenic opal-A to opal-CT could act as a sink for Si (Fig. F37) (Littke et al., 1991). The peak in IW Si (~1250 µM) qualitatively agrees with the decline in diatom preservation with depth (they are last observed in Core 350-U1437D-6R at 471 mbsf), and the decrease of Si below the putative opal-A to opal-CT transition zone correlates with accelerated B uptake (Brumsack and Zuleger, 1992). Furthermore, downhole logging data indicate a temperature of ~50°C at the transition; coupled with an age of ~3.5 Ma, these conditions appear optimal for forming opal-CT (Hein et al., 1979). Alternatively, the Si maximum at intermediate depth could also be attributed to fluid influx, analogous to Li. Sr defines a complex downhole profile, displaying a minimum of 52 µM at 37 mbsf, consistent with the zone of sulfate reduction and CaCO₃ precipitation, which shows a broad maximum at this depth. Although Sr concentrations are depleted at shallow depth, the Sr/Ca ratio peaks at ~45 µM/mM at ~100 mbsf. This is consistent with diagenetic carbonate recrystallization, which releases Sr into the pore waters (Baker et al., 1982). Coincidentally, the depth interval with near-seawater Sr/Ca at ~370 mbsf corresponds to the Li maximum and might indicate the influx of modified seawater along fast pathways.

Mud and mudstone bulk geochemistry

A total of 229 sediment samples were collected at Site U1437 and analyzed for concentrations of CaCO₃, total carbon (TC), total organic carbon (TOC), and total nitrogen (TN) (Fig. F38) using coulometry and elemental analyses, respectively. The atomic ratios of TOC and TN (TOC/TN_at) were calculated to determine the source of the sedimentary organic matter. Total carbon contents are highly variable over the whole cored sequence and range between 0.26 wt% at 17 mbsf (Section 350-U1437B-3H-2W) and 6.89 wt% at 913 mbsf (Section 350-U1437D-53R-1W) with an average value of 3.2 wt%. CaCO₃ shows a highly variable profile with average concentrations of 21.9 wt%. Minimum (0.46 wt%) and maximum (57.1 wt%) CaCO₃ concentrations occur at the same depths as minimum and maximum TC values, respectively. Plotting the five-point average values of CaCO₃ contents reveals two intervals with comparatively high CaCO₃ contents in the uppermost ~170 m of the cored sequence (Fig. F38). Another interval with elevated CaCO₃ contents is observed at ~900 mbsf. Thereafter, CaCO₃ contents gradually decrease with depth.

The observed strong and short-termed variations in sedimentary CaCO₃ weight percentages may not be a result of highly variable inputs of CaCO₃ reaching the seafloor but a result of varying inputs of detrital material such as clay and ash, which may act as diluents.

TOC contents range from below detection to a maximum of 3.64 wt% at 508 mbsf (average = 0.45 wt%) throughout the cored sequence and are comparatively higher in the upper ~230 mbsf, where the TOC average is 0.61 wt%, although there are several peaks with elevated TOC contents thereafter. Despite these peaks, TOC contents remain at a low level of 0.26 wt% deeper than ~230 mbsf.

TN contents are generally low throughout Site U1437. Values range up to a maximum of 0.1 wt% with an average of 0.024 wt% and highest TN contents in the uppermost ~230 m of the cored sequence. Deeper than that depth, TN contents decrease continuously, probably as a result of nitrogen loss during diagenesis.

TOC/TN_at ratios vary from 2.33 to 213 with an average of 18.6, suggesting a mixed input of both marine- and terrestrial-derived organic matter. Typical values for marine sources of organic matter range between 4 and 10, whereas terrestrial-derived organic matter has TOC/TN_at ratios >20. However, the TOC and TN values at Site U1437 show only a very weak correlation (R² = 0.18), possibly suggesting an important contribution of inorganic nitrogen sources. Ammonium adsorbed to clay particles can make up a significant TN fraction in organic-poor marine sediments (Müller, 1977). The admixtures of this inorganic nitrogen species and organic nitrogen may lead to overestimation of marine-derived organic matter, which is typically enriched in nitrogen (Meyers, 1994).

Major and trace elements

Major and trace element compositions of mud indicate mixing between at least three major components. Based on variations in CaO and K₂O (Fig. F39), these are characterized as (1) high CaO/low K₂O, (2) low CaO/low K₂O, and (3) low CaO/high K₂O. Sr and Rb display similar compositional patterns (Fig. F39) and can be interpreted as the trace element equivalents of CaO and K₂O, respectively, with CaO and Sr being dominantly contributed by marine carbonate (with stoichiometric CaO = 56 wt% for pure CaCO₃, and average Sr = 1100 ppm; Morse and Mackenzie, 1990), whereas K₂O or Rb are essentially nil in marine carbonates but comparatively enriched in volcanic ash or terrigenous clays (e.g., Plank, 2014). A significant amount of carbonate in mud is supported by large LOI values (average = 16 wt%), although this value may overestimate CaCO₃ because of the presence of H₂O and minor volatile components such as N or S. Direct determination of CaCO₃ in a different set of samples (see above) yielded an average of 21.9 wt% and a maximum of 57.1 wt%. The identification of Components 2 and 3 is tentative because of the limitations of shipboard data in frequency and number of elements analyzed, but binary variation diagrams for K₂O versus CaO and Rb versus Sr are consistent with the low K₂O-low Rb component (2) being Izu arc front and rear arc. Izu arc-front volcanism includes basalt-dominant island volcanoes with compositionally related rhyolites and compositionally distinct rhyolites from submarine calderas (e.g., Tamura et al., 2009). Rhyolites from submarine calderas (R2 type in Tamura et al., 2009) are higher in K₂O and Rb compared to rhyolites from basalt-dominant island volcanoes (R1 type), but R2 rhyolites still fall short of the high K₂O and Rb abundances at low CaO and K₂O in some of the mud samples. The same holds for the average composition of rear-arc tephra determined from Site U1437 data (Fig. F39). We therefore invoke another component with K₂O and Rb higher than the arc-front or rear-arc compositions. A suitable match for this component (3) is distal Ryukyu arc ash (Scudder et al., 2009), whereas terrigenous sediment such as Chinese loess (Chen et al., 2001) is insufficiently enriched in K₂O and Rb to match end-member (3) in mixing triangles (Fig. F39). This explanation does not rule out the presence of terrigenous clays, but their presence would be geochemically cryptic because it would plot close to a binary join between arc-front and rear-arc ash (2) and distal Ryukyu ash (3) in Figure F39. Dilution of ash and clay components by siliceous oozes is also possible but hard to quantify with the existing geochemical data.

Downhole variations in mud geochemistry are monitored by minor and trace elements (Rb, Zr, and TiO₂; Fig. F40). These elements are only contributed in significant amounts by ash or terrigenous sediment, and Zr/Y is invariant with dilution from carbonates and siliceous oozes and alteration. Zr/Y from ICP-AES (n = 10) and pXRF (n = 38) closely agree and show systematic decreases with depth that correlate with decreasing Rb and Zr. TiO₂, by contrast, increases especially below the transition from lithostratigraphic Unit III to IV at 1018 mbsf. This indicates a larger contribution from ash and clay relative to carbonate, which mirrors the increase in TiO₂ (Fig. F40). The overall average for Site U1437 mud is Zr/Y = 3.1, which is slightly higher than in mud from fore-arc Site U1436 (Zr/Y = 2.7) but not as elevated as the average subducting sediment composition for the Izu-Bonin trench at 31°20′N (ODP Leg 185 Site 1149; Zr/Y = 3.3; Plank and Langmuir, 1998). Significant variations in Zr/Y with depth are detected by both pXRF and ICP-AES analyses of mud: maximum Zr/Y in mud from Unit I is 4.4, whereas values as low as 1.6 are detected in Units III–VI (no mudstone was recovered from Unit VII). Importantly, the decrease in Zr/Y is correlated with downhole decreases in Rb (Fig. F40). It thus appears unlikely that the downhole trends in Zr/Y reflect variations in ash provenance from local sources (i.e., arc front versus rear arc), which are both extremely depleted in Rb, but rather a declining influence of terrigenous clay or distal ash, especially from sources with high Zr/Y and Rb such as the Ryukyu arc. This agrees with findings from Site 1149 (Scudder et al., 2009) and suggests a remarkable far-field influence of <3.5 Ma volcanic activity in the Ryukyu arc on the chemistry in fine-grained sediment in the northwest Pacific Ocean.

Rock magnetism

Rock magnetic measurements at Site U1437 comprised saturation isothermal remanent magnetization (SIRM) acquisition, backfield demagnetization of SIRM by a 300 mT field to yield the S_–0.3T ratio, stepwise acquisition of partial anhysteretic remanent magnetization (pARM), thermal demagnetization analysis, and anisotropy of magnetic susceptibility (AMS).

Through Holes U1437B and U1437D, a number of significant features in the downhole distribution of rock magnetic properties correlated with features of the physical properties, pore fluid geochemistry, and hydrocarbon geochemistry records. Overall, the rock magnetic properties show a downhole trend suggesting that after initial biologically mediated reduction in the sulfate reduction zone to produce greigite, the proportion of greigite gradually decreased, presumably though slow completion of the iron sulfide reduction chain to convert this ferrimagnetic sulfide to paramagnetic pyrite. Superimposed on this trend is a break at ~400 mbsf, where SIRM/k, S_–0.3T, and magnetic foliation all show step changes. This represents a sudden downhole increase in the proportion of magnetic sulfides, followed by gradual returns to trend over the 300 m below. The return to trend in SIRM and S_–0.3T occurs in the lower part of lithostratigraphic Unit I and appears to be complete by the Unit I/II boundary at ~680 mbsf, corresponding also to the top of PP Unit 2 (Fig. F41). After initial sulfate reduction, sulfate recovers over a diffusion zone extending to ~275 mbsf, where there appears to be a fluid source (see “Pore fluid analysis”) and plateaus from there to ~400 mbsf; over this interval S_–0.3T values drop to below 0.97, suggesting renewed and enhanced production of greigite. Deeper than ~400 mbsf, sulfate increases again downhole to near-seawater concentrations at ~460 mbsf, corresponding to the highest value of SIRM/k in all samples measured at Site U1437, and remains high to the deepest IW sample taken at 700 mbsf. Greigite concentration inferred from the rock magnetic parameters decreases downhole below 460 mbsf, returning to the background trend at ~680 mbsf, corresponding to the Unit I/II boundary and the top of PP Unit 2. A second source of fluid at ~460 mbsf appears to be driving both sulfate concentration and greigite genesis.

Surprisingly, given the apparent continuity of broadly similar lithologies through lithostratigraphic Units I–V across Holes U1437B, U1437D, and U1437E, rock magnetic properties in Hole U1437E do not follow the trends seen through Holes U1437B and U1437D. The background log-linear downhole decrease in SIRM/k that persists throughout Units I–IV is absent from Units V and VI. Instead, SIRM/k values appear to be randomly scattered over a wide range. Coercivity spectra from pARM analysis also show a wide range, from very magnetically soft (peak coercivity < 20 mT) to harder (broad coercivity peak from 20 to 30 mT) without any systematic downhole trends. Magnetite in samples from Hole U1437E appears to be dominated by large multidomain (MD) grains (presumably because of complete dissolution of finer magnetite grains), prompting our use of liquid nitrogen pretreatment to remove MD-carried overprints.

The break between Holes U1437D and U1437E manifests as an apparently instantaneous change in polarity and a break in magnetostratigraphy. Normal polarity of Subchron C3An.2n persisted to the bottom of Hole U1437D, but cores in Hole U1437E, which started at the same subbottom depth as the base of Hole U1437D, commenced immediately in reversed polarity. Pattern matching of the polarity record in Hole U1437E indicated that the reversed polarity interval began immediately above normal Subchron C4n.1n. This apparent loss of section can be most easily explained by the presence of a normal fault intercepting at a depth range near the break between the two holes; this fault may have been responsible for the poor hole conditions that terminated Hole U1437D. Differences in fluid circulation systems between the fault footwall and hanging wall could be the cause of both the sudden appearance of ethane at the top of Hole U1437E (see “Hydrocarbon gases”) and the contrasting rock-magnetic properties of the two holes.

Igneous geochemistry

Sampling, data acquisition, and data treatment

Analyses were carried out by pXRF and ICP-AES, and samples included ash and tuff (pXRF n = 88; ICP-AES n = 26), lapilli tuff and lapillistone (pXRF n = 7; ICP-AES n = 14), igneous clasts (pXRF n = 39; ICP-AES n = 9), and a rhyolite sheet (pXRF n = 1; ICP-AES n = 1). Reconnaissance pXRF and ICP-AES analyses generally agree within <20% (relative) for elements that can be analyzed by both techniques. ICP-AES data reported here include only samples with analytical totals of 100% ± 5% (after ignition).

Identifying tephra sources at Site U1437 requires accounting for the detrimental effects of pervasive alteration, which causes element mobility. These effects are most severe for alkali elements and Ba (Gill et al., 1994) and less so for elements that are insoluble in seawater (Ti, Al, and Fe; e.g., Stroncik and Schmincke, 2002). High–field strength element ratios such as Zr/Y remain largely unaffected by alteration (Gill et al., 1994). Another concern is contamination with pelagic sediment, especially in fine-grained ash or tuff samples, which can impact major and trace elements such as CaO and Sr that are enriched in calcareous materials. HFSE ratios are reliable indicators here because they are invariant with regard to carbonate abundances (estimated from CaO). All major elements are plotted normalized to 100 wt% volatile-free.

Tephra compositions and provenance

Rear-arc seamount and arc-front volcanoes are geochemically distinct in several major and trace elemental as well as isotopic parameters. Here, we compare shipboard analyses with a compilation of literature data, which we grouped into two major categories, and these fields are shown in Figures F42A, F42B, and F43:

1. Izu arc front includes basalt-dominant island volcanoes and rhyolite-dominant submarine calderas (Tamura et al., 2009), as well as tephra samples collected in the fore-arc region (Jordan et al., 2012; Gill et al., 1994; Bryant et al., 2003; Straub et al., 2003, 2010). The basalt-dominant island volcanoes also produced minor amounts of rhyolite (termed R1 type by Tamura et al., 2009), which have lower Zr/Y (1.1–3.2) than rhyolite erupted from submarine calderas (R2 type; Zr/Y = 2.7–5.5; Tamura et al., 2009).
2. Rear-arc volcanic rocks include all the rear-arc seamount volcanoes from 3 to 17 Ma (Hochstaedter et al., 2001; Ishizuka et al., 2002, 2003a, 2003b, 2006a, 2006b; Machida et al., 2003, 2008; Tollstrup et al., 2010), which are generally higher in Zr/Y (1.4–6.7) compared to arc-front rocks at comparable SiO₂, but there is some overlap between R2 arc-front rhyolites and rear-arc seamount rocks.

Volcanism younger than ~3 Ma immediately to the west of the arc front also comprises bimodal eruptions in active rift basins, ridges, and seamounts, collectively termed back-arc knolls (Tollstrup et al., 2010). For simplicity, we excluded these compositions from plotting but we note that they are often transitional between Category 1 and 2 rocks. This limits unambiguous source assignments for samples younger than 3 Ma from Site U1437.

Lava from rear-arc volcanoes has higher K₂O, Sr, and Zr/Y ratios at the same SiO₂ content than that of arc-front volcanoes (Figs. F42B, F43, F44). Thus, K₂O, Sr, and Zr/Y ratios at the same SiO₂ of tephra suggest they came from either arc-front or rear-arc volcanoes. This discrimination is only applicable to the Neogene tephra.

Unit I

Unit I samples from 0 to 440 mbsf (<3 Ma) include 6 ICP-AES and 13 pXRF analyses of mostly fine-grained ash and rare lapilli. They range from basaltic andesite to rhyolite and have relatively low K₂O abundances (0.35–1.48 wt%), generally overlapping with compositions of arc-front and active rift volcanic rocks (Fig. F42). Unit I samples from 440 to 682 mbsf (3–4.2 Ma) include 2 ICP-AES and 19 pXRF analyses of tuff that range in composition from andesite to rhyolite. These have slightly higher K₂O contents (0.50–2.43 wt%) compared to the <3 Ma ash and have low Zr/Y (2.2–2.3) similar to arc-front lava and tephra (Fig. F43). With the possible exception of high-SiO₂ Samples 350-U1437B-1H-3W, 13–14 cm (3.1 mbsf; SiO₂ = 71.6 wt%), 7H-3W, 12–13 cm (55.7 mbsf; SiO₂ = 70.2 wt%), and 12R-6W, 98–100 cm (531.7 mbsf; SiO₂ = 75.1 wt%), most ash is a mixture between mafic and evolved end-members, which may not be representative of the magma compositions. Reconnaissance pXRF analyses for shallow Unit I (<3 Ma) yielded Zr/Y from 2.1 to 8.4 with a median value of 3.2 (n = 13). The high Zr/Y Sample 35X-2W, 6–7 cm (244 mbsf), is a lapilli tuff and is thus likely proximal, although its Zr/Y exceeds that of known rear-arc eruptions. Low Zr/Y samples (e.g., Sample 5H-5W, 32–33 cm, at 40 mbsf) are fine-grained mafic ash, consistent with a distal origin from the arc front. Unit I (>3 Ma) samples show a similar range in pXRF Zr/Y (1.7–6.0) and a median Zr/Y (2.9) that is indistinguishable from those of shallower tephra from Unit I, consistent with ICP-AES analyses (Fig. F43). Low Zr/Y (2.3) of the high-silica tuff Sample 12R-6W, 98–100 cm (531.7 mbsf), suggests a possible origin as a R1 rhyolite associated with a basalt-dominant island volcano in the arc front (Tamura et al. 2009).

Unit II

Unit II (682.12–726.50 mbsf; age model ~4.2–4.4 Ma) is marked by a shift in grain size and style of deposition containing the first lapilli tuff and lapillistone deposits. Unit II samples include 3 ICP-AES and 4 pXRF analyses of tuff and lapilli tuff ranging from andesite to dacite with K₂O contents from 0.39 to 2.22 wt% (Fig. F42). Sample 350-U1437D-29R-2W, 4–5 cm (691 mbsf), represents an individual lithified pumice clast that is dacitic in composition (SiO₂ = 63.6 wt%) with high LOI (6.9 wt%) suggesting some alteration. This is supported by elevated MgO concentrations (6.48 wt%) relative to SiO₂. However, Zr/Y (6.2) is elevated to a level that is only found in rear-arc lava (Fig. F43). Bulk lapilli tuff Sample 30R-2W, 122–125 cm (701.2 mbsf), comprises intricate layers with variable proportions of glass and crystals (feldspar, amphibole, clinopyroxene, biotite, and minor quartz), whereas Sample 30R-6W, 58–61 cm (706.2 mbsf), consists primarily of fresh and devitrified glass with plagioclase and amphibole of uniform grain size. Both are dacites (68.3 and 67.6 wt% SiO₂) with moderate K₂O concentrations of 2.12 and 1.89 wt%, respectively, consistent with a rear-arc origin (Fig. F42).

Unit III

Unit III (726.50–1017.88 mbsf; age model ~4.3–6 Ma) consists primarily of altered green tuffaceous mudstone with occasional interbedded tuff. Fourteen tephra samples analyzed by ICP-AES have intermediate compositions ranging from basaltic andesite to dacite and are mostly similar to the arc-front field in K₂O versus SiO₂ (Fig. F42). There are four exceptions: Samples 350-U1437D-40R-7W, 44–46 cm (805.2 mbsf), 41R-1W, 0–2 cm (805.8 mbsf), 42R-5W, 40–43 cm (820.5 mbsf), and 43R-2W, 31–33 cm (826.9 mbsf), which have elevated K₂O. Sr concentrations vary greatly (up to 804 ppm) and are elevated compared to Unit I and II tephra as well as lava from the volcanic front and typically higher than rear-arc lava. Two basaltic andesite Samples 40R-7W, 44–46 cm (805.2 mbsf), and 41R-1W, 0–2 cm (805.8 mbsf), contain exceptionally low Sr concentrations (45 and 139 ppm), for which the closest equivalent are Manji Seamount volcanic rocks with potassic alteration (Sr = 107 ppm; Ishizuka et al., 2002).

All but three tephra samples display elevated Zr/Y compared to rocks from basalt-dominant volcanic islands in the Izu arc front, which generally have Zr/Y peaks at 2.5. This includes several tephra samples that are coarse-grained tuff to minor lapilli tuff. Low Zr/Y (<2.5) tephra in Unit III comprises andesitic tuff Samples 350-U1437D-40R-7W, 44–46 cm (805.2 mbsf), 55R-6W, 95–97 cm (940 mbsf), and 56R-5W, 0–4 cm (947.55 mbsf; Zr/Y = 1.8), in which glass shards are often altered (Fig. F43). Despite the presence of some low Zr/Y tephra likely derived from basalt dominant volcanoes in the arc front, high Zr/Y tephra is predominant in Unit III, as indicated by the overall median of Zr/Y (3.1) (pXRF n = 43; ICP-AES n = 14), which is elevated above the average for arc-front basalts, intermediates, and R1 rhyolites.

Unit IV

Samples analyzed by ICP-AES comprise a single andesite clast (Sample 350-U1437D-68R-2W, 43–46 cm; 1049.2 mbsf) and three bulk lapilli tuff samples (69R-1W, 37–39 cm, 1056.5 mbsf; 72R-1W, 44–46 cm, 1085.7 mbsf; and 350-U1437E-6R-3W, 106–109 cm, 1120.0 mbsf). All samples show evidence for alteration. Samples 350-U1437D-69R-1W, 37–39 cm (1056.5 mbsf), and 72R-1W, 44–46 cm (1085.7 mbsf), also have low CaO relative to SiO₂ and anomalously high K₂O (4.78 and 4.88 wt%, respectively) and are characterized by high abundances of submicroscopic alteration phases (~40 and ~50%, respectively). Sample 350-U1437E-6R-3W, 106–109 cm (1120.0 mbsf), has high FeO* (Fig. F42), consistent with pervasive chlorite alteration visible in thin section. With this caveat, we interpret the elevated Zr/Y (median Zr/Y from pXRF = 4.1; n = 41) and ICP-AES (average Zr/Y = 3.6; n = 4) as supporting evidence for the comparatively large grain size indicating proximal sources in the rear arc, most likely Manji Seamount (Fig. F43).

Unit V

Nine tephra samples (4 lapilli tuff, 3 tuff, 1 lapilli stone, and 1 tuff breccia) analyzed by ICP-AES from Unit V (1120.11–1312.21 mbsf) show large composition variations from basaltic andesite to rhyolite and are chemically more depleted than Unit IV tephra. Samples 350-U1437E-7R-7W, 71–74 cm (1130.9 mbsf), 16R-6W, 114–117 cm (1211.6 mbsf), 19R-1W, 104–105 cm (1233.7 mbsf), and 27R-1W, 99–102 cm (1311.3 mbsf), have slightly higher K₂O (0.91–1.70 wt%) than arc-front rocks (Fig. F44). The other five ICP-AES samples in Unit V have low K₂O (0.34–0.69 wt%). This includes a rhyolite tuff with fiamme (Sample 20R-2W, 25–26 cm; 1244.0 mbsf) that displays low K₂O (0.69 wt%) and high Ba (620 ppm) and Sr (566 ppm) concentrations, suggesting the influence of alteration on the chemical composition (Fig. F42). Four tephra samples (tuff breccia, lapillistone, lapilli tuff, and tuff) display elevated Zr/Y (>3) that are higher than compositions for basalt-dominant island volcanoes in the arc front, including Samples 16R-6W, 114–117 cm (1211.6 mbsf), 17R-2W, 114–117 cm (1211.6 mbsf), 19R-1W, 104–105 cm (1233.7 mbsf), and 20R-2W, 25–26 cm (1244.0 mbsf). The lack of systematic variations between K₂O and Zr/Y (both potential discriminants between rear-arc and arc-front sources) is interpreted to result from intense alteration. Sample 7R-7W, 71–74 cm (1130.9 mbsf), for example, is an andesitic tuff that has rear arc-like K₂O (0.98 wt%) but low Zr/Y (1.6) indicative of an arc-front magma composition. On the contrary, Sample 17R-2W, 114–117 cm (1216.0 mbsf), a lapillistone with andesite clasts, has low K₂O (0.53 wt%) but high Zr/Y (3.9).

Unit VI

Six samples from Unit VI (1320.00–1459.80 mbsf) analyzed by ICP-AES are four lapilli tuff samples, one andesite clast, and one rhyolite intrusion (igneous Unit 1). Three lapilli tuff samples and a clast are basaltic andesite in compositions. Single andesite clast Sample 350-U1437E-41R-2W, 1–3 cm (1448.0 mbsf), has slightly higher K₂O (0.90 wt%) relative to SiO₂ than expected if it evolved along the arc-front trend (Fig. F42), yet its Zr/Y (2.4) is well within the range of basalt-dominant arc-front lava and tephra (Fig. F43). Three lapilli tuff Samples 28R-4W, 42–44 cm (1324.5 mbsf), 31R-4W, 63–65 cm (1353.8 mbsf), and 34R-4W, 31–32 cm (1382.1 mbsf), also have low Zr/Y (1.8–2.1), consistent with low K₂O (0.21–0.58 wt%) except for 34R-4W, 31–32 cm (1382.1 mbsf), which is moderately enriched in K₂O (1.15 wt%). Nineteen pXRF analyses on Unit VI samples yielded a median Zr/Y of 2.5, consistent with low Zr/Y in ICP-AES analyses (2.2; n = 8) and confirm the generally depleted nature of Unit VI tephra. One unusual composition is that of Sample 34R-5W, 50–53 cm (1383.5 mbsf), which has high SiO₂ = 85.6 wt% indicating silicification, which is consistent with the presence of chalcedony observed petrographically.

The rhyolite intrusion (igneous Unit 1; Sample 350-U1437E-35R-2W, 42–44 cm; 1390.0 mbsf) is a highly evolved rhyolite with extremely low Sr (8 ppm) and Ba (16 ppm), unusually low K₂O (0.48 wt%), but high Zr/Y (5.8) (Figs. F42, F43). Its origin remains elusive with limited shipboard analyses, but high-silica rhyolites with extremely low abundances of feldspar-compatible trace elements such as Sr and Ba are interpreted to be products of protracted fractional crystallization rather than direct products of crustal melting (Mahood and Halliday, 1988).

Unit VII

Six volcanic lithic clasts from volcanic breccia in Unit VII (1459.8–1806.5 mbsf) were analyzed by ICP-AES comprising five andesites and one dacite. All but one sample are highly depleted in K₂O (0.11–0.77 wt%) similar to arc-front rocks (Fig. F42). Sample 350-U1437E-70R-2W, 95–97 cm (1721.4 mbsf), displays slightly higher K₂O (1.05 wt%) compared to other Unit VII samples. This sample also has significantly higher Ba and lower Cr and Cu compared to the other three andesite samples at equivalent SiO₂ (Samples 43R-2W, 99–101 cm, at 1468.2 mbsf; 44R-2W, 36–38 cm, at 1477.1 mbsf; and 51R-1W, 31–34 cm, at 1544.1 mbsf), suggesting different volcanic sources. All Unit VII ICP-AES samples have comparatively low Zr/Y (average = 2.6; n = 3), overlapping with the range of pXRF analyses of clasts from this unit (average = 3.0; n = 45). These values generally fall between the peak in Zr/Y for basalt-dominant island volcanoes in the arc front and Zr/Y in rear-arc lava (Fig. F43).

Possible effects of alteration and grain size on chemical compositions of tephra

The presence of low-temperature alteration (indicated by the presence of biotite) and high-temperature alteration (indicated by the presence of epidote) has been observed from dredged and submersible samples on the Manji Seamount (Ishizuka et al., 2002). These alterations can greatly affect major and trace element compositions evident in steep depletions in CaO relative to increasing SiO₂ (Fig. F42). We focus on Zr and Y because they are relatively fluid immobile and remain robust during alteration (Gill et al., 1994). This is supported by published Manji Seamount data that have near-constant Zr/Y even when highly altered (Ishizuka et al., 2002). Consequently, we use primarily shipboard Zr/Y analyses as an indicator for magmatic provenance and to distinguish between rear-arc and arc-front sources.

Tephra provenance and in situ magma evolution

Downhole geochemical variations in Units I–V generally reflect the relative proportions of distal arc-front and proximal rear-arc volcanic sources (Fig. F44). As a general trend, coarse-grained tephra from Units II and IV show stronger rear-arc affinity compared to fine-grained tuff from Units I, III, and V. Complications for provenance arise from mixing evident by mafic and evolved glass shards in fine-grained tephra samples and pervasive green alteration in Units III and V.

The predominance of ash layers from Unit I containing low K₂O relative to SiO₂ indicates a likely volcanic front or active rift (ash <2 Ma) provenance. Some <3 Ma tephra in Unit I with high Zr/Y could thus be mixtures of mafic and evolved (high Zr/Y rhyolites of the R2 type of Tamura et al., 2009) ash derived from arc-front volcanoes, but this cannot be confirmed without onshore in situ analyses of glass. The coarse-grained deposits from Units II and IV indicate proximal sources. Active Manji Seamount chain volcanoes around the time of deposition of Unit II (4.2–4.3 Ma) were the Meireki Seamount (3.76 Ma; Ishizuka et al., 1998) ~20 km to the north and the Daigo-Higashi Aogashima Knoll (5.05 Ma; Ishizuka et al., 2003b) ~40 km to the northeast of Site U1437. Both are rhyolite volcanoes with similar SiO₂ contents (72–76 wt%), whereas Meireki Seamount volcanic rocks have higher K₂O (~3 wt%) but lower Zr/Y (~2.8) compared to the Daigo-Higashi Aogashima Knoll (K₂O = ~1.5 wt%; Zr/Y = ~4.4) (Hochstaedter et al., 2001). Although the limited data (five analyses in total) available for both seamounts preclude reliable geochemical matching with Unit II tephra, they are potential sources for coarse-grained lapilli from Unit II considering their geographic locations, ages, and chemical composition. Similarly, single clast compositions of Unit IV can be tentatively matched to available data for Manji Seamount volcanic rocks (~6.5–6.9 Ma; Ishizuka et al., 2002). Two high-K₂O tephra resemble the high-K Manji Seamount rocks with potassic alteration, whereas most tephra from Unit IV follow the trend for altered Manji Seamount rocks leading to depletions in CaO with increasing SiO₂ (Fig. F42) (Ishizuka et al., 2002).

Unit V is primarily mud, and given the pervasive alteration throughout this interval it is difficult to provide an accurate provenance for tephra. Based on ICP-AES Zr/Y data and the only moderately elevated median Zr/Y from pXRF (2.7; n = 39) (Fig. F43), we tentatively interpret Unit V to be geochemically transitional, with tephra and clast compositions that include both rear-arc and arc-front sources. The presence of volcanic breccia (with clast sizes up to 40 cm) in Units VI and VII supports proximal volcanic deposition. However, the Zr/Y values of blocks within breccia vary widely between 0.4 and 4.0. Similar compositional heterogeneity exists in the late Miocene Shirahama group in the Izu Peninsula of southern Japan, which represents accreted rocks of rear-arc origin (Tamura, 1994, 1995; Tamura and Nakamura, 1996; Tani et al., 2011). The Shirahama lava is divided into tholeiitic and calc-alkaline series, which have low Zr/Y and high Zr/Y, respectively. However, the late Miocene Shirahama lava does not display the same degree of depletion seen in the K₂O contents of Unit VI and VII blocks.

Summary and interpretation of volcaniclastic deposits

The 1800 m thick section drilled at Site U1437 is dominantly fine grained, with tuffaceous mudstone making up 58% of the section. The discrete thin ash/tuff layers that make up 21% of the described core are the products of some combination of subaqueous suspension fallout and dilute density currents, as discussed further below. Only 20% of the described core contains particles coarser than 2 mm (lapilli), and those are almost entirely restricted to fine-grained lapilli (<2 cm); only the basal two lithostratigraphic units (VI and VII) have coarser lapilli and scattered blocks, and those two units make up only ~25% of the 1800 m thick section.

The mud/mudstone that dominates Site U1436 is featureless except where ash tuff layers grade up into mudstone and bioturbation is marked by mixing of the ash into the mud. Therefore, it seems likely that virtually all primary sedimentary structures in the mud/mudstone were obliterated by bioturbation. That makes it impossible to determine whether the mud was deposited from hemipelagic rain, dilute turbid flows, bottom currents (e.g., related to the Kuroshio Current), or some combination thereof. Future geochemical work is expected to contribute to an understanding of the provenance of the mud/mudstone. Any hypothesis for the origin of the mud/mudstone must take into the consideration the fact that the dominantly mud/mudstone section drilled at Site U1437 accumulated at unusually high rates for such fine-grained material (Fig. F44).

Interpretation of eruption and depositional processes

Despite the dominance of featureless mudstone in the section drilled at Site U1437, macroscopic and microscopic shipboard analysis of the discrete ash/tuff and lapilli ash/lapilli tuff beds at Site U1436 allows us to make preliminary interpretations about the eruption, transport, and depositional processes that produced them. This analysis deals with deposits from all of the lithostratigraphic units described above (Units I–VII).

Evolved ash/tuff and mafic ash/tuff

The sharp basal contacts, good sorting, and normal grading in this lithofacies could indicate deposition by suspension settling through water, seafloor hugging density currents, or some combination (e.g., vertical density currents that transition into lateral density currents when they reach the seafloor in a manner envisioned by Carey [1997] and Manville and Wilson [2004]). These are the only volcaniclastic deposits in Units I and III besides the tuffaceous mudstone.

Intercalated white to green tuff

This lithofacies is fine grained, with laminations produced by alternation of glass shard–rich layers (white) and layers of mixed shards, pumice, and crystal fragments (gray-green), repeated over intervals up to several meters thick, with no bioturbation or mudstone interbeds. Thus, the intervals seem to record fairly continuous but pulsating sedimentation, probably from unsteady density currents, over a relatively short period of time for each interval (possibly days or weeks). The very large quantity of very fine glass shards in this facies suggests either phreatomagmatic eruption with extremely efficient glass fragmentation or extreme sorting of products from dry pyroclastic eruptions. This lithofacies occurs in Units III and IV.

Monomictic pumice lapilli tuff and lapillistone

This lithofacies is relatively well sorted with abundant interstratified well-sorted crystal and vitric tuff, and is stratified, with planar and cross lamination, sharp bases, and grading bioturbated tops. This lithofacies is interpreted to represent sediment gravity flow deposits, and the monomictic composition may indicate that at least some were eruption fed. This lithofacies dominates Unit II.

Monomictic tuff with pumice and fiamme

This lithofacies consists of poorly sorted beds with basal scour, composed largely of ash-sized material, with lesser pumice lapilli or fiamme that become more abundant upward in each bed, indicating density grading. The monomict composition and the presence of abundant evolved glass shards, pumice, and broken crystals suggest that these were fed from pyroclastic eruptions. This lithofacies occurs in Unit V.

Polymictic, evolved lapilli tuff and lapillistone

This lithofacies occurs as very thick (multimeter) relatively well sorted intervals with no internal stratification, composed of volcanic clasts of a variety of evolved types. These characteristics suggest deposition from high-concentration density currents, probably by mass wasting or resedimentation from one or more silicic volcanoes; alternatively, this facies could be products from pyroclastic eruptions that remobilized large volumes of lithic clasts. This lithofacies is most abundant in Units IV and VI.

Black glassy lapillistone and lapilli tuff

This lithofacies occurs as one massive ~184 mm thick deposit of nonvesiculated glass lapilli clasts, which together with the lack of bubble-wall shard or broken crystals, indicates fragmentation by autobrecciation and quenching of lava in a submarine environment (i.e., hyaloclastite). The few nonglassy clasts in the deposit suggest minor accidental incorporation of clasts during transport, but most of the unit is monomict and nonstratified, indicating minimal resedimentation. A lack of mudstone interbeds indicates rapid accumulation. This lithofacies forms the upper half of Unit VII.

Coarse-grained massive lapilli tuff with in situ quench-fragmented blocks

This lithofacies consists of extremely thick intervals of nonstratified, very poorly sorted monomict andesite lapilli tuff with blocks, with intercalated stratified lithic lapilli tuff and tuff. Clasts in the extremely thick nonstratified intervals are angular or have very irregular shapes, indicating very minimal transport. Many intervals contain blocks and coarse lapilli that are glassy and some have glassy rims and poorly inflated breadcrust textures/cauliflower texture, indicating that clasts came to rest at high temperatures. Other intervals have a small percentage of other clast types, suggesting at least some resedimentation. In some intervals, very angular, jigsaw-fit hyaloclasts (formed of quenched glass) indicate in situ mixing of hot clasts and/or intrusions with the host hyaloclastic tuff breccia, all of the same andesitic composition. The lack of clasts with broken glassy rims indicates minimal resedimentation. Like the black glassy lapilli tuff, a lack of mudstone interbeds indicates rapid accumulation, although these show more evidence of more episodic deposition because they are intercalated with stratified lithic lapilli tuff and tuff. This lithofacies forms the lower half of Unit VII.

Interpretation of depositional environment at Site U1437: deepwater basinal succession

As noted above, the section drilled at Site U1437 accumulated in a deepwater volcano-bounded basin between the Manji and Enpo seamount chains. In this section we argue that the section is best described as a deepwater basinal succession and that the term “volcaniclastic apron” is inappropriate, expect perhaps for the basal 25% of the section.

Although some workers use the term volcaniclastic apron to loosely refer to any accumulation of sediment around a volcano or chain of volcanoes, the term has been used in a much more rigorous sense by sedimentologists over the past 40 y (Karig and Moore, 1975b; Sample and Karig, 1982; Carey and Sigurdsson, 1984; Farquharson et al., 1984; Fisher, 1984; Busby-Spera, 1985, 1988; Cas and Wright, 1987; Smith, 1987; White and Busby-Spera, 1987; Houghton and Landis, 1989; Palmer and Walton, 1990; Fisher and Smith, 1991; Fisher and Schmincke, 1994; Smith and Landis, 1995; Orton, 1996; Wright, 1996; Mitchell, 2000; Carey, 2000; Gamberi, 2001; Karátson and Németh, 2001; Allen et al., 2006; Casalbore et al., 2010; Carey and Schneider, 2011). In these papers, a volcaniclastic apron is defined as a thick accumulation of coarse volcanic debris that fringes a volcano or a chain of volcanoes and builds outward from them; volcaniclastic aprons are fan shaped or are composed of coalescing fans that form a wedge. They are steep in their proximal reaches, with abundant large lithic blocks and slumps, passing smoothly into medial to distal reaches that have gentler slopes, formed of debris flow and coarse-grained pyroclastic density current deposits. For example, the “volcanic apron” of Gran Canaria (Shipboard Scientific Party, 1995) is a volcaniclastic apron (Funck et al., 1996), consisting of volcano-flank seismically chaotic pillow breccia and hylaoclastite and poorly stratified debris flow deposits, which pass basinward into crudely stratified slump, debris flow, and turbidity current deposits. Volcaniclastic aprons form in both nonmarine and marine environments, and they commonly prograde into basins with time, producing an overall upward-coarsening sequence.

The section drilled at Site U1437 differs from volcaniclastic aprons in the following ways:

• It is mostly mudstone (~60% of the section as a whole) deposited from hemipelagic rain, dilute turbid flows, bottom currents, or some combination thereof; the section is not mainly composed of volcanic debris.
• The grain size of volcanic clasts in discrete tephra layers is much smaller than that reported from volcaniclastic aprons (20% ash and ~20% fine-grained lapilli, for the section as a whole).
• There is no geomorphic or seismic stratigraphic evidence for fan- or wedge-shaped sediment bodies or of chaotic facies; instead, the upper 75% of the section (above Units VI and VII) is extremely well stratified, consistent with its fine grained character.
• Little firm evidence for sediment gravity flow deposits exists in the upper 75% of the section, except for Unit II, which is thin; most of the tephra could represent suspension fallout.
• Volcanic blocks are very rare and sparsely scattered through the lowermost 25% of the section (Units VI and VII).

The submarine fans and aprons of siliciclastic deepwater systems are relatively coarse grained constructional features, whereas the basin plain beyond is flat and fine grained with laterally continuous deposits (Reading and Richards, 1994; Stow et al., 1996; Richards, 2009). The upper 75% of the section drilled at Site U1437 is more analogous to the basin plain; it is a fine-grained, well-stratified sequence that is best referred to as a deepwater basinal succession, not a volcaniclastic apron. The lower 25% of the section, in contrast, contains vent-proximal deposits with geochemistry that differs from the upper 75% of the section; therefore, the lower 25% of the section could represent the proximal part of a volcaniclastic apron, although it may instead represent a localized deposit within the deepwater basinal succession.

Interpretation of event periodicity using tephra

Tephra beds within marine sediments have long attracted attention for their potential to provide a time-precise, high-resolution record of volcanic activity (e.g., Kennett and Thunell, 1975; Cambray and Cadet, 1994). In this section we present a preliminary assessment of volcanic event periodicity for the part of the section at Site U1436 that has good age control (Units I–III), which extends to 1097 mbsf and to 6.9 Ma (Fig. F45). This is possible because combined Holes U1437B and U1437D cored a 1097 m thick section of mudstone with abundant centimeter- to decimeter-thick tephra beds with high recovery (average = 80% ± 23%). The excellent biostratigraphic and magnetostratigraphic data in these holes demonstrate that the tephra represents a time series of instantaneous volcanic events that were preserved within rapidly accumulating tuffaceous mud that show no evidence for hiatuses (see Fig. F24).

Each tephra bed >1 cm thick entered in DESClogik was considered to be a single volcanic event, and for each bed the thickness and depositional age were obtained using sedimentation rates calculated in the age model. Tephra bed abundances were calculated in time slices of 100 ky, and a linear correction for core recovery was applied to the number of tephra beds per time interval; for example, if 3 tephra beds were recorded at 50% recovery, the corrected number of tephra beds would be 6.

The corrected curve (Fig. F45) shows a tephra maximum at 4.3–4.7 Ma (lithostratigraphic Unit II) and two additional maxima at ~2.0–2.2 Ma and ~3.1–3.2 Ma; a minor peak lies at ~0.35 Ma.

The 2.0–2.2 Ma tephra abundance maximum coincides with the maximum in sedimentation rate at ~1.9–2.5 Ma (230–360 mbsf), which is consistent with the interpretation that high sedimentation rates are linked to high influx of tephra. The age of the most pronounced tephra maximum (4.3–4.7 Ma) is permissive of a source in the ~16–3 Ma rear-arc seamount chain, which is consistent with the relatively coarse grain size of Unit II (lapilli tuff) relative to tephra in Units I and II (mainly ash/tuff). The abundance maximum at ~2.0–2.2 Ma is clearly too young to represent rear-arc seamount chain volcanism.

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