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doi:10.14379/iodp.pr.350.2014

Site summaries

Site U1436

Background and objectives

Site U1436 is located at 32°23.88′N, 140°21.93′E (1776 m water depth), in the western half 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.

The plan for Site U1436 was to carry out a 1 day operation to core 150 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 oceanic arcs produce 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. This 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” a newly devised descriptive scheme for volcaniclastic rocks drafted at an IODP workshop in January and elaborated during the early part of Expedition 350. 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.

The Izu fore arc is a repository of ash/tuff intervals erupted in the Izu-Bonin frontal arc because the wind 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. Rhyolites are also abundant in other oceanic arcs; thus, models for the crustal evolution of oceanic arcs must explain their bimodal volcanism. Basalts and rhyolites of the Izu arc front share radiogenic isotope characteristics that make them clearly distinguishable from active rift lava immediately behind the arc front, as well as from rear-arc lava behind (west of) the active rifts. However, a puzzling feature of the arc front is that basalt volcanoes lie on thick middle crust and rhyolite volcanoes lie on thin middle crust, in apparent contrast with continental margin arcs. A major objective of Site U1436, therefore, is to characterize the chemistry, age, provenance, and textural characteristics of mafic and silicic explosive volcanic products that are sampled in the fore arc to better understand outputs that are not preserved on land.

Science results

Operational overview

The transit from Keelung, Taiwan, started on 4 April 2014 and ended at 1142 h on 8 April, marking the beginning of operations at Site U1436. A seafloor survey was conducted first and verified that no subsea communications cables exist in the drilling area. Hole U1436A (9–10 April) cored from 0 to 150 mbsf using the advanced piston corer (APC), half-length APC (HLAPC), and extended core barrel (XCB), recovering 71.64 m (48% recovery) of tephra and mud.

After operations were prematurely terminated at Site U1437 on 25 May, the science party decided to use the remaining time to core two or three additional holes and attempt to recover additional intervals of conspicuous black ash layers found 7 weeks earlier in Hole U1436A at ~50 mbsf that were severely disturbed by coring, in order to better constrain their thickness and thus the explosivity of the eruption that generated them. The short transit from Site U1437 to Site U1436 took from 0930 to 1700 h on 25 May, and contingency operations were conducted from 25 to 27 May.

Hole U1436B was positioned 20 m north of Hole U1436A and cored from 0 to 61.8 mbsf, obtaining 61.79 m (100% recovery). Hole U1436C is located 20 m south of Hole U1436A and was cored from 0 to 70.4 mbsf, obtaining 70.38 m (100% recovery). The final Hole U1436D was positioned 20 m east of Hole U1436A, and after washing down to 40 mbsf, coring from 40 to 62.0 mbsf obtained 22.07 m (100% recovery).

The ship remained on station at Site U1436 until 0900 h on 29 May before the short voyage to Yokohama, Japan, arriving on 30 May and ending Expedition 350.

Lithostratigraphy

Coring at Site U1436 recovered 71.64, 61.79, 70.38, and 22.07 m of sediment from Holes U1436A, U1436B, U1436C, and, U1436D, respectively. These sediments are described as a single lithostratigraphic unit (Unit I) consisting of intercalated (1) tuffaceous mud, (2) mafic ash and scoria lapilli ash (~80 intervals in Hole U1436A), and (3) evolved ash and pumice lapilli ash (~70 intervals in Hole U1436A), culminating in a total of ~40 m of tuffaceous mud and ~26 m of volcaniclastic sediment. The mafic intervals in Hole U1436A are 50% thicker than the evolved intervals, giving a mafic to evolved thickness ratio of 1.5:1. Ash and lapilli ash layers make excellent stratigraphic markers for hole-to-hole correlations. Tuffaceous mud intervals are massive, average 0.25 m in thickness, and are up to 4.20 m thick. They comprise >25% of vitric and crystal particles; foraminifers and bioturbation are present. The tuffaceous mud is light gray to dark gray-brown, commonly with a greenish hue. Rare glauconite (0.01–0.02 m thick layers) occurs at the top contacts of evolved ash intervals with tuffaceous mud.

The mafic ash and scoria lapilli ash intervals average 0.14 m in thickness and are up to 2.30 m thick (Core 350-U1436A-8H). Mafic ash and scoria lapilli layers are commonly severely disturbed because of their granular fabric. The evolved ash and pumice lapilli ash intervals are 0.09 m thick on average and up to 0.60 m thick. Pumice and scoria lapilli are ~30 mm (commonly <10 mm), and small lapilli are angular, whereas coarser lapilli are subrounded. Most mafic and evolved intervals are normally graded, with sharp bottom contacts and diffuse/gradational tops showing upcore increase in mud content. Most lapilli ash intervals are polymictic.

One very distinctive facies, the black glassy mafic ash, occurs in all holes between 49.1 and 53.6 mbsf (intervals 350-U1436A-8H-1, 0 cm, to 8H-2, 108 cm; 350-U1436B-10H-1, 0 cm, to 10H-2, 26 cm; 350-U1436C-11F-1, 106 cm, to 11F-2, 40 cm; and 350-U1436D-7F-2, 15–73 cm). It consists of massive, nongraded, nonstratified, very well sorted glass shards with minor feldspar and pyroxene crystals and foraminifers. Based on its undisturbed appearance in Holes U1436C and U1436D, the true thickness was determined to be 55 cm, with sharp upper and lower boundaries. The glass is brown to greenish brown with few microlites; glass particles are flat, blocky, curviplanar and/or plastically deformed, and contain vesicles. An apparent second black glassy mafic ash layer was observed at interval 350-U1436A-8H-3, 49–64 cm, but an equivalent was not found in the other three holes. Upon further examination of that interval, we concluded that it must have been formed by flow-in.

The tuffaceous mud is interpreted to record hemipelagic background sedimentation with substantial ash contribution from explosive eruptions or resedimentation from density currents, presumably predominantly 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 include far-field volcanism. The mode of transport and deposition of the mafic and evolved volcaniclastic layers include both vertical settling through the water column and/or water-supported density currents. The distinctive 0.55 m thick black glassy mafic ash facies is unusually homogeneous in componentry, grain size, and texture, suggesting an eruption-fed origin, and the angular and fluidal shapes of the ash particles suggest they are products of a subaqueous explosive eruption.

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 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. Pore water Na/Cl scatters around an average of 0.86 throughout the sampled interval, 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 caused by 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 ICP-AES and additional pXRF analysis. 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. 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 are low-K rocks and 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 (having <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, but these are basaltic andesite. One interval of evolved lapilli ash (at ~45 mbsf) has elevated K2O, indicating that it did not come from Aogashima or the East Aogashima Caldera ~45 km to the west, which have lower K2O. However, rhyolite-dominant submarine volcanoes with elevated K2O are common in the arc front, including three within 65 km of the drill site (Myojin Knoll, South Hachijo, and Myojinsho).

Physical properties

A significant shift in all physical properties is observed at ~50 mbsf: shear strength, NGR, and color reflectance L* parameter all decrease in the interval deeper than 50 mbsf, likely related to an increase in relative abundance of mafic ash layers in that interval. Physical properties data were examined for the specific intervals identified as mafic and evolved ash layers. Mafic ash layers have an average magnetic susceptibility value that is more than twice as high as the average value for evolved layers. The average NGR value for mafic layers is approximately half that for evolved layers. Reflectance L* values for mafic and evolved ash layers can be grouped into three luminance (lightness) ranges: 17–35 includes only mafic layers, 35–50 includes a mixture of mafic and evolved layers, and 50–78 includes only evolved layers. The 2.2 m thick mafic ash layer at ~50 mbsf yields a wide range of P-wave values, supporting the suspicion that the layer is affected by core disturbance (stretching). The general downhole shift in physical properties and ash types at ~50 mbsf also coincides with an inferred hiatus and significant decrease in recovery rate (see below).

Paleomagnetism

Paleomagnetic analysis in Hole U1436A comprised archive-half demagnetization and remanence measurement at 10 mT steps up to 40 mT. Severe core disturbance resulted in complete destruction of the depositional remanence in many intervals, and discontinuous recovery 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 (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. We demagnetized these at 5 mT steps up to 20 mT. Less intense and magnetically softer overprinting of these discrete samples meant that the demagnetized inclination at 20 mT tightly clustered around the expected geocentric axial dipole (GAD) inclination of ±51°. For this reason, we halted demagnetization and measurement of most discrete samples at this level, continuing to 25 mT only in samples near and below the base of Chron C1n to improve the isolation of the reversed polarity remanence. Discrete samples in the discontinuous record below the hiatus allowed us to recognize two additional datums: the 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 the 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 SRM measurements indicate that Core 17X is all reversed polarity and probably still lies in the Matuyama interval (<1.778 Ma).

Biostratigraphy

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 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 are 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 (2.588 Ma) was placed within Cores 350-U1436A-18X and 20X based on the presence of the top of G. pseudomioceanica (2.39 Ma) in Sample 18X-CC and the top of G. decoraperta (2.75 Ma) in Sample 20X-CC. Benthic foraminifer assemblages (i.e., extinction of Stilostomelidae) corroborate this biochronology.

Calcareous nannofossils were also generally abundant and well preserved. The Late–Middle Pleistocene sequence is defined by the presence of Emiliania huxleyi in Sample 350-U1436A-3H-CC (16.9 mbsf) and Pseudoemiliania lacunosa in Sample 5H-6, 134–139 cm (34.6 mbsf). Between Samples 10F-CC (62.1 mbsf) and 13X-CC (65.05 mbsf), no additional Pleistocene bioevents were recorded. Sample 15X-CC (83.29 mbsf) contained Calcidiscus macintyrei, a marker for the early Pleistocene. The sequence below Sample 17X-CC (104.3 mbsf) contained few markers that characterize the Pleistocene/Pliocene boundary. Only rare specimens of diagnostic discoasterids were present, and some could be reworked. In this interval, Biozones CN12d, CN12c, and CN12b were tentatively identified on the basis of the top of Discoaster brouweri, top of Discoaster pentaradiatus, and top of Discoaster surculus, respectively.

Age model

Thirteen out of 16 biostratigraphic datums and one magnetostratigraphic datum were selected to construct an age-depth relationship for Site U1436. The model consists of two linear segments above (0–66 mbsf) and below (74–127 mbsf) the inferred hiatus, with LSRs of 71 and 45 m/My and MARs of 8.0 and 5.3 g/cm2/ky, respectively.

Site U1437

Background and objectives

Site U1437 is located in the Izu rear arc, ~330 km west of the axis of the Izu-Bonin Trench and ~90 km west of the arc-front volcanoes Myojinsho and Myojin Knoll at 2117 mbsl. Expedition 350 was the first expedition to be drilled in the Izu rear arc; all other ODP/Integrated Ocean Drilling Program sites have been in or near the Izu-Bonin arc front or fore arc, leading to an incomplete view of Izu arc magmatism as a whole. The main objective of Expedition 350 was to reveal the history of this “missing half” of the subduction factory.

Site U1437 (proposed Site IBM-3C) was chosen to provide a temporal record of rear-arc magma compositions, ideally from Eocene to Neogene time, allowing comparison with the previously drilled fore-arc magmatic record and determination of across-arc geochemical variation throughout the history of the arc system. A striking characteristic of volcanic arcs is the asymmetry in geochemical characteristics with distance from the trench, which was known prior to the advent of plate tectonics. The Izu arc shows this asymmetry, and its rear-arc magmas are much more similar to continental crust than the Izu arc-front magmas. The Izu rear arc is therefore important for understanding how intracrustal differentiation produces crust that is similar to the “average continent.”

The Izu arc front is a ~900 km long volcanic chain whose Quaternary geology can be characterized by (1) basalt-dominated volcanoes spaced at ~100 km intervals, some of which form islands, and (2) submarine rhyolite-dominated calderas. All Neogene volcanic rocks behind the Izu arc front are referred to as rear-arc volcanic rocks and include three types: (1) the east northeast–trending basaltic to dacitic rear-arc seamount chains (~17–3 Ma); (2) a broad extensional zone with small bimodal volcanoes referred to the back-arc knolls, which overlaps with the eastern part of the rear-arc seamount chains (<3 Ma); and (3) a narrow active rift with bimodal volcanic rocks that lies immediately behind the arc front (<1.5 Ma). The chemistry of the arc-front magmas should be distinguishable from that of the rear-arc seamount chains because the rear-arc seamount chains are enriched in alkalis and high–field strength elements and other incompatible elements and have less enriched Sr, Nd, Hf, and Pb isotopes, compared to the volcanic front. However, the <3 Ma bimodal volcanism is transitional between the two in chemistry as well as space; therefore, we can distinguish rear-arc and arc-front magmas for all rocks older than 3 Ma but not for rocks younger than 3 Ma. Because the objective was to study the temporal evolution of rear-arc magmatism, Site U1437 was chosen in a location that should be topographically shielded from arc front–derived sediment gravity flows (although arc front–derived ash fall may be present). Another objective at Site U1437 was to determine if arc geochemical asymmetry was present early in the history of the arc (in the Paleogene) or if it is strictly a Neogene feature.

Site U1437 lies in a volcano-bounded basin that formed between the Manji and Enpo rear-arc seamount chains, which are two of several Izu rear-arc seamount chains that are up to ~50 km long and strike N60°E. Three main hypotheses have been proposed for the origin of the seamount chains:

  1. They are related to compression caused by collision between the southwest Japan and Izu arcs, associated with opening of the Japan Sea.
  2. They formed along Shikoku Basin transform faults.
  3. They overlie diapirs in the mantle wedge, such as the hot fingers proposed for northeast Japan, which predicts that the rear-arc seamount chains young from west to east.

Site U1437 was chosen to learn about the temporal evolution of the Manji and Enpo seamount chains.

Science results

Operational overview

After the 7.5 h, 77 nmi transit from Site U1436 to U1437, operations at this site were conducted from 10 April 2014 to 24 May (44 days).

Five holes were completed at Site U1437. Hole U1437A (10 April) was a jet-in test with an APC/XCB bottom-hole assembly (BHA). A seafloor camera survey was first conducted to confirm that no subsea cables were present at the site.

Hole U1437B (11–14 April), located 10 m north of Hole U1437A, was cored with the APC (0–89.2 mbsf), the HLAPC (89.2–145.7 mbsf), and the XCB (145.7–439.1 mbsf), with a total recovery of 243 m (55%).

The plan for Hole U1437C (14–15 April), located 20 m south of Hole U1437B, was to drill without coring to 425 mbsf, which is several meters above the total depth cored in Hole U1437B, and then start RCB coring. At 309.7 mbsf, the bit got stuck and had to be dropped at the bottom of the hole to free the pipe, ending operations in this hole.

Hole U1437D (15–26 April) was positioned 10 m west of Hole U1437A in a second attempt to drill and core a deep RCB hole. Drilling without coring extended from the seafloor to 427.2 mbsf. RCB coring from 427.2 to 980.4 m recovered 434.56 m (79% recovery). At this time, the bit had been rotating for 51.5 h and required changing, so we decided to stop coring, deploy a free-fall funnel, and collect wireline log data.

Three logging strings were successfully deployed in Hole U1437D: the triple combo with the magnetic susceptibility sonde (MSS) (92 to ~960 mbsf), the Formation MicroScanner (FMS)-sonic (92–950 mbsf), and the Vertical Seismic Imager to acquire a vertical seismic profile (VSP) (14 stations; 175–875 mbsf). The hole was in excellent condition, with a diameter barely exceeding the bit size for most of the hole. The data recorded were therefore of high quality.

RCB coring in Hole U1437D resumed with a new bit, and the hole was deepened from 980.4 to 1104.6 mbsf, recovering 69.29 m (56%). Drilling problems forced us to pull out of the hole at this point. The total cored interval in Hole U1437D was 677.4 m, with 503.8 m recovered (74%).

We decided to drill and case a new hole to the total depth of Hole U1437D and then core and log it as deep as time permitted. This decision was prompted by (1) the increasing risk with penetration depth of not being able to clean the hole and (2) the fact that the ship was carrying ~1100 m of 11¾ inch casing, just enough to cover the interval cored to date, which made this the optimal time to install the casing.

The ship moved ~20 m northeast to begin operations in Hole U1437E (26 April–24 May). The casing deployed in this hole consists of a 20.7 m long 20 inch casing connected to the reentry cone, a 264 m long 16 inch casing string hung from the casing hanger in the reentry cone, and a R/V JOIDES Resolution record-breaking 1085.6 m long 10¾ inch casing string, also hung from the reentry cone. It took ~12 days to complete the total casing installation.

RCB coring for the duration of three 50 h bit life cycles extended Hole U1437E from 1104.0 to 1806.5 mbsf and recovered 387.45 m of core (55% recovery). When preparing for the fourth bit deployment, the fiber-optic cable for the camera needed to reenter Hole U1437E had broken. This prematurely ended the expedition’s operations in Hole U1437E. Hole U1437E is currently in excellent condition for logging and/or further coring operations, preferably attempted soon, before the hole deteriorates.

At 0930 h on 24 May, the ship was under way for contingency operations at Site U1436.

Lithostratigraphy

Cores from three consecutively cored holes at Site U1437 recovered a coherent stratigraphy from 0 to 1806.50 mbsf: Cores 350-U1437B-1H through 55X (0–439.10 mbsf), 350-U1437D-2R through 73R (427.20–1104.60 mbsf), and 350-U1437E-4R through 79R (1104.00–1806.5 mbsf). Overlap between the bottom of one hole and the top of the next is minimal. The majority of the recovered core is sedimentary, dominated by tuffaceous mud and mudstone with intercalated volcaniclastic layers in the uppermost 1300 m (Units I–V), with a gradual change to dominantly volcaniclastic layers in the lowermost 500 m (Units VI and VII). Overall, 62% of the cored depth interval was recovered and described. Tuffaceous mud and mudstone comprise 58.5% of the described rocks, ash and tuff comprise 21.0%, and lapilli-sized volcaniclastics comprise 20.3% (with <0.5% volcaniclastics coarser than lapilli).

The seven lithostratigraphic units (I–VII) are distinguished from each other based on the proportion and characteristics of volcaniclastic intervals relative to tuffaceous mud and mudstone. Their upper and lower boundaries are defined by the appearance or reappearance of distinct marker layers as described in the following paragraphs.

The tuffaceous mud and mudstone in all units contain clay minerals, foraminifers, igneous crystals, and glass shards in varying proportions. Bioturbation is intense. Ash and tuff intervals often are normally graded, consist of glass shards and crystals, and commonly have a crystal-rich layer at the base of the interval. Layer bases usually are sharp, whereas upper contacts often grade into increasingly bioturbated tuffaceous mud and mudstone such that there is a continuum between the lithofacies. Matrix- and clast-supported polymictic lapilli tuff intervals become very thick and highly abundant at certain depths and are typically the basis of unit boundaries. The clasts in these coarse-grained intervals become more lithic rich toward the base of the recovered sequence. Only one igneous unit was defined (igneous Unit 1) based on the presence of a 1.2 m thick rhyolite with chilled margins and peperitic contacts with surrounding volcaniclastic material.

Compaction of sediment at Site U1437 increases linearly from ~0% shallower than ~410 mbsf to ~36% at the base of Hole U1437D. Although the transition from unconsolidated to lithified rocks occurred progressively, sediments were considered lithified from 427 mbsf (top of Hole U1437D) downhole. Alteration becomes more pervasive downhole in Holes U1437B and U1437D; initially it is predominantly glauconitic-smectitic and eventually becomes more chloritic. Fresh glass is observed from Unit I to the top of Unit III. Within Unit III, glass is progressively altered to cryptocrystalline aggregates of clay minerals, probably dominated by smectite. From Units IV to VII, glass is totally altered with the exception of the uppermost vitric-rich part of Unit VII, where colorless glass is preserved. Crystals are not affected by alteration until Unit VII, where orthopyroxene is replaced by cryptocrystalline brown aggregates of clay minerals and plagioclase is partly altered to clay minerals and epidote. The downhole increase in alteration intensity and the transition from smectite to chlorite and prehnite indicates alteration is partly related to burial. Iron sulfides occur as aggregates throughout all holes, especially as replacements of worm burrows, and according to rock magnetic properties, greigite is progressively replaced by pyrite downhole in Hole U1437D. Glauconite layers and reduced iron sulfides associated with burrows are a result of biogenic alteration under reducing conditions. Some volcanic clasts in Units IV–VII display higher temperature alteration assemblages comprising prehnite, biotite, and epidote, related to high-temperature (>300°C) alteration before final deposition.

Unit I extends from 0 to 682.12 mbsf and encompasses all of Hole U1437B to interval 350-U1437D-28R-2, 112 cm, in Hole U1437D. Unit I is dominated by tuffaceous mud and mudstone (88% of total described material) alternating with thin evolved, mafic, or bimodal ash/tuff intervals and minor lapilli ash/lapilli tuff intervals. Average bed thicknesses are 0.36 m for mud and mudstone, 0.08 m for ash and tuff, and 0.06 m for lapilli ash and lapilli tuff. The tuffaceous mud and mudstone from this unit are interpreted to result from background sedimentation of hemipelagic clay and carbonate mixed with substantial volumes (>25%) of dispersed volcanic material. This background sedimentation is punctuated by episodic deposition of ash layers, derived by eruption-fed or resedimented seafloor-hugging density currents, or suspension settling of subaerially distributed ash through the water column. The Unit I/II boundary is marked by the first appearance of monomictic lapilli tuff, which is the characteristic lithofacies within Unit II; this change is reflected in the physical properties (e.g., increase in magnetic susceptibility, see below).

Unit II extends from 682.12 to 726.50 mbsf in Hole U1437D (interval 350-U1437D-28R-3, 0 cm, to 32R-CC, 7 cm). It is characterized by abundant intervals of evolved thick- to very thick-bedded evolved tuff (40% of total described rocks) and monomictic pumice lapilli tuff and lapillistone (37%) intercalated with lesser tuffaceous mudstone (23%). The top of Unit II is defined by the first lapilli tuff in a 44.38 m thick sequence dominated by intervals of lapilli-sized tephra. Average bed thicknesses are 0.16 m for evolved tuff, 0.20 m for lapilli tuff and lapillistone, and 0.19 m for tuffaceous mudstone. Unit II volcaniclastics are interpreted to have been deposited by seafloor-hugging density currents that transported lapilli, pumice grains, and crystal fragments. The thickest and graded intervals are inferred to be eruption fed. Minor (22%) intercalated tuffaceous mudstone indicates periods of volcanic quiescence during deposition of Unit II. The Unit II/III boundary is marked by the first interval of tuffaceous mudstone below the last occurrence of intercalated lapilli tuff and lapillistone.

Unit III extends from 726.50 to 1017.88 mbsf in Hole U1437D (interval 350-U1437D-32R-CC, 7 cm, to 64R-1A, 8 cm). It is dominated by tuffaceous mudstone (63% of the described rocks) with intercalated intervals of evolved tuff and minor evolved lapilli tuff and lapillistone. Average bed thicknesses are 0.23 m for the tuffaceous mudstone, 0.11 m for the tuff, and 0.15 m for the lapilli tuff. Near the top of Unit III (interval 34R-3, 78 cm, to 34R-4, 118 cm) lies a single distinctive 1.91 m thick tuffaceous breccia with mudstone intraclasts (up to ~20 cm in size) interpreted as a submarine debris flow deposit. The overall abundance of mudstone and lack of coarse-grained volcaniclastic material in Unit III suggests an origin similar to that described for Unit I. Unit III shows an increase in fine-grained tuff (relative to tuffaceous mudstone) in its basal ~80 m, possibly produced by phreatomagmatism. The Unit III/IV boundary is defined by an increase in grain size and abundance of pumiceous lapilli tuff; tuffaceous mudstone decreases below this boundary.

Unit IV extends from 1017.88 to 1120.11 mbsf, from the bottom of Hole U1437D and into Hole U1437E (interval 350-U1437D-64R-1A, 8 cm, to 350-U1437E-6R-3, 122 cm). The unit is characterized by coarse sand-sized tuff (53% of described rocks), polymictic lapilli tuff, and minor polymictic lapillistone (25%). Tuffaceous mudstone is only a minor part of the unit (22% of the described rocks), becoming more frequent toward the base. The top of Unit IV is defined by the uppermost interval of a succession of massive intervals of clast-supported polymictic lapilli tuff intervals. Two tuff lithofacies are found in Unit IV: a light green tuff showing planar and convolute stratifications and a massive and coarser dark gray to black tuff. Average bed thicknesses are 0.24 m for tuff, 0.43 m for lapilli tuff, and 0.16 m for tuffaceous mudstone. Volcaniclastic intervals in Unit IV are interpreted to have been deposited by voluminous high-concentration density currents derived from mass wasting of islands or seamounts or by pyroclastic eruptions that remobilized large volumes of lithic clasts. The base of Unit IV is defined by the last interval of polymictic lapilli tuff >0.50 m thick for the next 146.66 m (well into Unit V).

Unit V (1120.11–1320.00 mbsf; interval 350-U1437E-6R-3, 122 cm, to 27R-CC, 15 cm) is similar to Units I and III in that it is dominated by tuffaceous mudstone intercalated with tuff. However, a distinctive characteristic of Unit V is the presence of multiple intervals of monomictic, reversely graded fiamme-rich lapilli tuff with mudstone. The top of Unit V is defined by the interval immediately beneath the final >70 cm thick polymictic lapilli tuff of Unit IV. Heavily bioturbated silt-sized tuffaceous mudstone comprises 69% of the described rocks in the unit, evolved tuff comprises 15%, and lapilli tuff and lapillistone constitute 16%. Average bed thicknesses are 0.27 m for tuffaceous mudstone, 0.06 m for evolved tuff, and 0.18 m for lapilli tuff. The distinctive monomictic, reversely graded lapilli tuff lithofacies is thin to medium bedded and consists of a sharp basal contact filled with tuff that grades upward into a matrix-supported, reversely coarse-tail graded, fiamme-rich lapilli tuff with tuffaceous mudstone. Pumice lapilli tuff and lapillistone occur as ~5–10 cm thick intervals in the middle of the unit; they are similar to the dominant lithofacies in Unit IV. Unit V is interpreted to have an origin similar to Units I and III, with the addition of monomictic lapilli tuff with a likely origin as eruption fed seafloor-hugging density currents described for Unit II. The base of Unit V is defined as the last interval above the first thick interval of polymictic lapilli tuff of Unit VI.

Unit VI extends from 1320.00 to 1459.80 mbsf (interval 350-U1437E-28R-1, 0 cm, to 42R-3, 60 cm). As with Units II and IV, this unit is characterized by a higher abundance of tuff (32% of described rocks) and lapilli tuff (57%) and lesser tuffaceous mudstone (11%) than in the unit above. The top of Unit VI is marked by the first appearance of multiple intervals of thickly to very thickly bedded (up to 2.8 m) matrix-supported polymictic lapilli tuff and continues downward through intervals that are dominated by tuff and lapilli tuff rather than tuffaceous mudstone. Unit VI is intruded by the 1.2 m thick rhyolite-dacite sheet of igneous Unit 1 (see below). Lapilli tuff intervals include monomictic and polymictic varieties and matrix- and clast-supported varieties. Average bed thicknesses are 0.25 m for tuffaceous mudstone, 0.24 m for tuff, and 0.41 m for lapilli tuff and lapillistone. Igneous vitric and lithic clasts >2 cm become more common toward the base of the unit. Unit VI is interpreted similarly to Unit IV. The base of Unit VI is marked by the lapilli tuff above the first normally graded dense glass–rich interval of Unit VII.

Unit VII begins at 1459.80 mbsf and extends to the bottom of Hole U1437E at 1806.50 mbsf (interval 350-U1437E-42R-3, 60 cm, to 79R-03, 83 cm). The bulk (89%) of Unit VII consists of thin to extremely thick intervals of graded or nongraded lapilli tuff, lapillistone, tuff breccia, and breccia (0.63 m average bed thickness) and contains angular lithic andesite clasts ranging from pebble to cobble in size. The remaining 11% is tuff (0.21 m average bed thickness). There is only one thin-bedded (0.11 m) tuffaceous mudstone in the entire 340 m thick unit. Unit VII comprises two main lithofacies. A distinctive black evolved lapilli tuff and lapillistone occurs mostly in the upper part of the unit and has a matrix of slightly to moderately altered glass plus isolated plagioclase and pyroxene crystals. Polymictic evolved lapilli tuff, lapillistone, tuff-breccia, and volcanic consolidated breccia exclusively occur in the lower part of the unit. It is more altered (green-colored, altered glass) and coarser than the upper lithofacies. Clasts with unbroken quench margins suggest proximity to the source. Evidence of in situ emplacement is restricted to a few levels, where jigsaw-fit textures (hyaloclastite) and/or baked and peperitic margins are observed.

Igneous Unit 1 occurs at 1388.86 to 1390.07 mbsf (interval 350-U1437E-35R-1, 76 cm, to 35R-2, 55 cm). It is a 1.21 m thick moderately phyric quartz-hornblende-feldspar rhyolite-dacite interpreted as an intrusive sheet with chilled margins and basal peperite. It occurs within a continuous interval of clast-supported polymictic pumice lapilli tuff that is baked at both contacts with igneous Unit 1. The unit has a porphyritic texture with sieve-textured subhedral plagioclase (up to 4 mm, ~7%), euhedral hornblende (up to 0.5 mm, ~3%), anhedral to subhedral quartz (up to 8 mm, ~1%) with fresh glassy melt inclusions, minor opaque minerals, and rare zircon (20 µm in size). The groundmass varies from cryptocrystalline near the upper and lower contacts to fine-grained in the center of the unit. Flow banding is observed across the entire unit in various orientations.

Geochemistry

Headspace. 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. At shallow depths, methane abundances gradually increase with depth with the highest abundances at ~750–1459 mbsf. The zone of methanogenesis is unusually deep because of a release of sulfate below the sulfate reduction zone that may be buffering the methanogenesis by anaerobic methanogens. Deeper than ~1459 mbsf, methane concentrations decrease again. Minor amounts of ethane occur at 1105 mbsf. In samples with detectable methane (C1) and ethane (C2), C1/C2 values are <100, which is consistent with mature organic matter producing thermogenic aliphatic hydrocarbons. C1/C2 never reached critical thresholds to halt drilling operations according to the shipboard safety program.

Pore water. Chemical analyses for interstitial water samples from Site U1437 (0–684 mbsf; n = 67) display systematic downhole trends. Depth variations in pore fluid chemistry suggest at least three major processes controlling the changes in major and trace element distribution in a fluid that started out as sediment-trapped seawater. These processes may be interdependent and operate simultaneously but can be ranked according to increasing impact with depth: (1) biologic activity (0–100 mbsf), which is primarily controlled by sulfate reduction of organic material; (2) lateral fluid transport (deeper than 100 mbsf) interpreted from increasing alkalinity and sulfate back toward near-seawater values away from the zone of microbial sulfate reduction, with spikes in Li indicating that this process is likely fracture or formation controlled; and (3) diagenesis (0–684 mbsf) evident through progressive downhole alteration of volcanic glass and dissolution of siliceous diatoms producing gradual decreases in Mg and increases in Si, respectively. Deeper than ~450–500 mbsf, increases in Ca and Na coupled with a sharp decrease in Mg may indicate progressive clay formation by alteration of volcanic ash.

Mud and mudstone bulk geochemistry. A total of 218 sediment samples were collected at Site U1437 and analyzed for concentrations of CaCO3, TC, TOC, and TN. TOC/TNat ratios were calculated to determine the source of the sedimentary organic matter. TC and CaCO3 contents are highly variable and show generally the same trend over the cored sequence, suggesting that most of the TC consists of inorganic carbon. Highly variable sedimentary CaCO3 abundances over short vertical distances likely result from varying inputs of volcanic ash or terrigenous clay, which dilute biogenic carbonate. TOC and TN contents are generally low, but TOC and TN are comparatively elevated in the uppermost ~230 m of the cored sequence. Deeper, TN contents decrease continuously, probably as a result of nitrogen loss during diagenesis. TOC/TNat ratios vary from 2.33 to 213 with an average of 18.6, suggesting a mixed input of both marine-derived and terrestrial-derived organic matter.

Bulk mud compositions by ICP-AES (n = 11) and pXRF (n = 38) indicate downhole increases in clay and ash content (represented by TiO2) relative to CaCO3. The increasing prevalence of clay and ash correlates approximately with the transition from lithostratigraphic Units III to IV (~1019 mbsf) and continues to the bottom of Hole U1437E, where mud intervals become rare. Zr/Y in mud decreases with depth from ~4 (6 mbsf) to ~2 (1424 mbsf), paralleling a decrease in Rb from 103 to 11 ppm. These depth trends suggest that shallow mud intervals are more strongly influenced by terrigenous clay or distal ash (e.g., from the Ryukyu arc where Zr/Y and Rb are high relative to the proximal Izu arc-front and rear-arc volcanoes) than deeper intervals.

Volcanic geochemistry. In an effort to constrain the geochemistry of various volcanic deposits observed at Site U1437 (tuff, lapilli, and lava), both a pXRF (n = 227) and ICP-AES (n = 50) were utilized. ICP-AES analyses were completed for samples of ash and tuff (n = 26), lapilli tuff and lapillistone (n = 14), igneous clasts (n = 10), and a rhyolite intrusion (n = 1). The discrete ash layers in lithostratigraphic Unit I (0–682 mbsf) plot within the low-K field relative to SiO2 and have low Zr/Y. Their low K2O and Zr/Y values indicate that the ash is likely derived from the Izu volcanic front. A few deposits, commonly coarser grained with cross bedding and hornblende, have elevated K2O and Zr/Y. The samples younger than 3 Ma (0–440 mbsf) that contain elevated K2O and Zr/Y relative to SiO2 are more ambiguous in origin. Pervasive alteration at depths >720 mbsf compromises most major and trace elements. Major element compositions of samples >720 mbsf trend toward compositions equivalent to hydrothermally altered Manji Seamount rocks. Fluid-immobile Zr and Zr/Y are elevated for coarse-grained volcaniclastic rocks between ~684 and 1120 mbsf (top of Unit II to Unit IV), indicating the effects of higher degrees of differentiation, higher abundances of incompatible trace elements in the source, and possibly hornblende fractionation. All three characteristics are consistent with a proximal rear-arc origin. The presence of low Zr/Y samples within the same depth interval, however, indicates continued deposition of tephra and volcaniclastic sediment from sources similar to modern Izu arc-front volcanoes. Variability in Zr/Y and K2O values in Unit V (1120–1312 mbsf) indicates a mixture of lithologies similar to dredged rear-arc and Quaternary volcanic front lava from a comparable stratigraphic interval. Deeper than 1320 mbsf to the total depth of 1807 mbsf (comprising Units VI and VII), most ICP-AES samples (typically representing large single clasts) are dominantly intermediate in composition. The samples have Zr abundances as low as 34 ppm and Zr/Y < 3, which are characteristic for modern basalt-dominant island arc volcanoes of the Izu arc front. One exception is a conspicuous quartz-, hornblende-, and zircon-bearing rhyolite (igneous Unit 1) in Sections 350-U1437E-35R-1 and 35R-2 (1388.9–1391.1 mbsf), which has the lowest CaO analyzed for any rock collected at Site U1437 but comparatively high Zr/Y (5.9). Overall, the presence of Manji seamount chain-like rocks identified by high Zr/Y is well established for Units II–IV, whereas such rocks are more infrequently encountered in Units I, V, VI, and VII. Preliminary geochemical modeling using major and trace element compositions for mud and volcaniclastic sediment or rock, weighed by their abundance throughout the section drilled at Site U1437, indicates that the Izu rear arc is significantly depleted in incompatible trace elements (e.g., K2O and Rb) relative to upper continental crust but less so than the arc-front magmas.

Physical properties

Physical properties measurements were performed to obtain information on the density, porosity, NGR, shear strength, thermal conductivity, MS, P-wave velocity, and reflectance of the recovered sequence. After letting the core reach thermal equilibrium with the ambient temperature at ~20°C, gamma ray attenuation density, MS, and P-wave velocity were measured using a Whole-Round Multisensor Logger and NGR was measured on the whole-round Natural Gamma Radiation Logger. Thermal conductivity was measured in soft sediments and rocks with the needle and puck probes, respectively. Discrete measurements of P-wave velocity, moisture and density, and shear strength were performed on working section halves. Finally, color reflectance and MS were measured on the archive section halves using a spectrophotometer and the point magnetic susceptibility sensor on the Section Half Multisensor Logger.

Thirteen physical properties (PP) units were differentiated based on distinct offsets in otherwise continuous profiles that define boundaries between intervals and interval characteristics such as magnitude, rate of change, and relative scatter of measurements.

PP Unit 1 (0–430 mbsf) is characterized by a downhole increase in bulk density and P-wave velocity and a corresponding downhole decrease in porosity.

The top of PP Unit 2 (430–550 mbsf) is marked by an initial increase in porosity followed by a downhole continuation of the decrease observed through PP Unit 1, as well as a decrease in the reflectance L* and a* values.

The upper part of PP Unit 3 (550–682 mbsf) is characterized by a sharp increase in porosity and corresponding decrease in bulk density. This initial offset is followed by more gradual downhole trends, similar to those seen in PP Unit 2. The reflectance ratios of a*/b* data show a decrease in the scatter, indicating that color is less variable through PP Unit 3 than in PP Units 1 and 2.

The upper boundary of PP Unit 4 (682–728 mbsf) corresponds to the lithostratigraphic Unit I/II boundary and is marked by an increase in the scatter of density and porosity, an increase in P-wave velocity and MS values, and a decrease in the NGR and reflectance b* values.

The top of PP Unit 5 (728–794 mbsf) corresponds to the upper boundary of lithostratigraphic Unit III and is defined by an increase in NGR values and a decrease in the scatter of P-wave velocity. This is followed downhole by the continuation of the increasing P-wave trend observed in PP Unit 3.

PP Unit 6 (794–846 mbsf) is defined by an increased scatter in P-wave velocity and reflectance a*/b* values.

The upper boundary of PP Unit 7 (846–1018 mbsf) is marked by an initial decrease in bulk density and a corresponding increase in porosity, followed downhole by trends similar to those in PP Unit 6. The top of this unit is also characterized by an abrupt decrease in MS values.

The top of PP Unit 8 (1018–1140 mbsf) corresponds to the top of lithostratigraphic Unit IV and is defined by an increase in the MS values and abrupt decreases in P-wave velocity and thermal conductivity.

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 decreased scatter in density, porosity, P-wave velocity, and NGR measurements, as well as a downhole decrease in thermal conductivity.

The upper boundary of PP Unit 10 (1315–1460 mbsf) coincides with the top of lithostratigraphic Unit VI and is marked by an abrupt decrease in the NGR values relative to PP Unit 9, as well as a slight downhole increase in thermal conductivity values.

The top of PP Unit 11 (1460–1580 mbsf) corresponds to the top of lithostratigraphic Unit VII. PP Unit 11 is characterized by an abrupt decrease in MS and decreased scatter in density, porosity, and P-wave velocity values. NGR data 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 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 to the maximum drilling depth.

The upper boundaries of PP Units 2, 3, 6, 7, 9, 12, and 13 do not correspond to lithostratigraphic boundaries. Therefore, we suggest that changes in the physical properties at the mentioned boundaries may reflect postdepositional processes like diagenetic and hydrothermal dissolution and recrystallization, which may affect primary color, porosity, density, and consequently, P-wave velocity of the recovered sediments and rocks.

Magnetostratigraphy

Magnetostratigraphy at Site U1437 was determined from 30 mT demagnetization and measurement of natural remanent magnetization of archive section halves using the SRM, supported by polarity identified by measurement of discrete samples after alternating field or thermal demagnetization. A total of 29 magnetostratigraphic datums, marking the tops and bases of normal polarity chrons and subchrons, were identified. Every chron and subchron in the sequence down to Subchron C3An.2n (6.436 Ma; 1056.65 mbsf) was recognized. Where biostratigraphic datums were available, they agreed very well and consistently with the magnetostratigraphic interpretation, but magnetostratigraphy became the main control on the depositional age model deeper than 550 mbsf.

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. We suggest that a normal fault at or near the base of Hole U1437D has caused a loss of section between the two holes. Pattern matching of the polarity record in Hole U1437E indicated that the reversed polarity interval began immediately above normal Subchron C4n.1n, and the magnetostratigraphy could then be followed down as far as the top of Chron C4An (8.771 Ma) at 1302 mbsf. Magnetostratigraphy in Units VI and VII was impossible to recognize, with the exception of reversed polarity seen in Sample 350-U1437E-35R-1, 125 cm, from igneous Unit 1, which was the first indication that coring had proceeded below the base of normal Chron C5n.2n (9.984–11.056 Ma) spanning the upper part of the lowest nannofossil age range.

Extrapolating the magnetostratigraphy from the last datum at 8.771 Ma (1302 mbsf) near the base of lithostratigraphic Unit V, to Sample 350-U1437E-36R-CC (1402 mbsf) substantially underestimates the age of the nannofossil datum of 10.97–11.85 Ma at this depth. The most likely explanation is a hiatus at the lithostratigraphic Unit V/VI boundary.

Although magnetostratigraphy was surprisingly successful at Site U1437, this is not to say that the polarity record was easily obtained; magnetic sulfides in Holes U1437B and U1437D and a dominantly multidomain magnetite mineralogy in Hole U1437E required a variety of strategies for recognizing polarity, including the use of liquid nitrogen cooling in field-free space to reduce the effect of high-stability overprints in discrete samples.

The paleomagnetic team also provided direct support to the lithostratigraphers. The sample in igneous Unit 1 demagnetized along a simple single-component path after removal of the drilling overprint, and this had an appropriate inclination; this provided part of the evidence that this feature was indeed emplaced in situ. Demagnetization analyses of samples of selected clasts in Unit VII were also conducted in order to determine if they were emplaced hot or cold. These experiments were only partly successful due to multidomain overprinting, but clasts hosting Samples 350-U1437E-66R-5, 106–108 cm, and 66R-6, 16–18 cm, display characteristic reversed polarity that might be consistent with hot emplacement.

Rock magnetism

Rock magnetic studies at Site U1437 spanned the gamut of techniques available onboard, including SIRM acquisition, backfield demagnetization of SIRM by a 300 mT field to yield the S–0.3T ratio, stepwise acquisition of pARM, thermal demagnetization analysis, and AMS. Both pARM coercivity spectra and thermal demagnetization behavior confirmed the presence of magnetic iron sulfide (presumed to be greigite in the absence of shipboard X-ray diffraction data during Expedition 350), at least within the interval cored in Holes U1437B and U1437D. This sulfide is present in addition to magnetite, which is the dominant magnetic mineralogy.

Sooty black sulfides typically including greigite were visibly identified in association with worm burrows, bioturbation, and glaucony/glauconite. They are likely to host concentrated magnetic sulfides and were avoided in sampling discrete paleomagnetic samples. This practice, and the weaker intensity of the drill string overprinting field near the center of the core, made the record of polarity in discrete samples often more reliable than the SRM record.

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, interstitial water geochemistry, and organic 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. After initial sulfate reduction, sulfate recovers and reaches a plateau concentration from ~275 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. Transport of sulfate in pore fluid appears to have been responsible for a history of renewed magnetic authigenesis. The step reduction in AMS foliation at ~400 mbsf can also be explained by renewed growth of magnetic sulfides, which would have a compaction history reset at zero at this subbottom depth. The deep methanogenesis zone also matches the rock magnetic record, corresponding to the highest S–0.3T values encountered at this site to date. Such high S–0.3T, indicating a very low coercivity magnetic assemblage, suggests not only complete conversion of greigite to pyrite but probably also dissolution of fine-grained magnetite; both may reflect further reduction of the sediments related to deep and ongoing microbial activity stimulated by influx of sulfate-rich pore fluids.

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.

Biostratigraphy

The biochronology for Site U1437 was established based on planktonic foraminifers and calcareous nannofossils. Core catcher samples and several extra samples from within the core from Holes U1437B, U1437D, and U1437E were analyzed for planktonic foraminifer and calcareous nannofossil content. Deeper than 1403 mbsf, no age-diagnostic microfossils were found. Both fossil groups show that the upper 1403 m part of the succession spans from the lower Pleistocene to the upper Miocene (maximum age detectable ~12 Ma). The timing of bioevents agrees well with magnetostratigraphic data. Deeper than 1403 mbsf, the bioevents were difficult to establish because of poor preservation and low microfossil abundance. The decrease in preservation/abundance corresponds to a lithologic change from a succession dominated by mud and mudstone to one dominated by volcanic material.

Foraminifers. A total of 146 core catcher samples were examined for planktonic and benthic foraminifer content. In the upper ~543 m of the succession (Samples 350-U1437B-1H-CC through 350-U1437D-13R-CC; Holocene to lower Pliocene) planktonic foraminifers are generally abundant, diverse, and show moderate to good preservation. Recognizing foraminifer datums deeper than ~543 mbsf (from Sample 350-U1437D-14R-CC downhole) became increasingly challenging because of a combination of low foraminifer abundance, lack of age-diagnostic species in most of the assemblages, poor preservation, and/or induration of sediments. Induration posed great difficulties in extracting the foraminifers. In interval 350-U1437D-40R-CC through 63R-CC (805–1009 mbsf, lithostratigraphic Unit III), many samples are barren of foraminifers, or where foraminifers are present, they show strong evidence of both plastic deformation and recrystallization, presumably due to compaction. Less deformed foraminifers were recovered from Samples 350-U1437D-64R-CC through 72R-CC (1021–1088 mbsf), but most are present as internal molds.

In terms of datums, the Pliocene/Pleistocene boundary (2.588 Ma) is placed somewhere between 372 and 416 mbsf based on the recognition of the top of G. decoraperta (top of 2.75 Ma ± 0.03 Ma) in Sample 350-U1437B-53X-CC (416 mbsf), and the top of G. pseudomiocenica (top of 2.39 Ma) in Sample 48X-CC (372 mbsf). Many typical age-diagnostic fauna (e.g., Globigerinoides fistulosus and Globoturborotalita nepenthes) are very rare or absent in this succession. The datums in the lower part of the succession (deeper than ~570 mbsf) are tentatively assigned to include the top and bottom of Globorotalia margaritae (3.85 ± 0.03 Ma and 6.08 ± 0.03 Ma, respectively), bottom of Globorotalia crassaformis sensu lato (4.31 ± 0.04 Ma), and top of Sphaeroidinellopsis kochi (4.53 ± 0.17 Ma). An additional bioevent, the extinction of the benthic foraminifer genus Stilostomella in the Middle Pleistocene (Sample 11F-CC; 94 mbsf), is not used to establish the biochronology but corroborates it.

Calcareous nannofossils. Calcareous nannofossils were abundant and well preserved throughout Holes U1437B and U1437D down to Sample 350-U1437D-26R-CC (669 mbsf). From 27R-CC downhole (677–1806 mbsf), moderate to poor preservation is recorded and several samples are barren in nannofossils. The Middle–Upper Pleistocene sequence is defined by the bottom of E. huxleyi in Sample 350-U1437B-3H-CC (19.51 mbsf) and the top of P. lacunosa in Sample 6H-5, 75–76 cm (51.48 mbsf). The top common occurrence (Tc) and bottom common occurrence (Bc) of Reticulofenestra asanoi (Samples 15F-3, 80–81 cm [110 mbsf], and 16F-CC [113 mbsf], respectively) and bottom of Gephyrocapsa omega (113 mbsf) define the bottom of Chron Cn14a. The bottoms of Chrons CN13a, CN12d, and CN12c are defined by the top of D. brouweri (248 mbsf), top of D. pentaradiatus (337 mbsf), and top of D. surculus (367 mbsf), respectively. In Hole U1437D, the succession spans the upper Miocene to Pliocene. The bottom of Chron CN12b is defined by the top of Discoaster tamalis (384 mbsf). The top of Reticulofenestra pseudoumbilicus in Sample 350-U1437D-19R-3, 32 cm (595 mbsf), defines the bottom of Chron CN12a. The last reliable datum indicator is the top of Triquetrorhabdulus rugosus (Sample 48R-CC; 871 mbsf). Since the preservation of nannofossils quickly deteriorates below Sample 27R-CC (677 mbsf), it was not possible to recognize other bioevents in the middle Pliocene to upper Miocene part of the succession. The preservation in Hole U1437E is very poor, and from Sample 350-U1437E-27R-CC (1312 mbsf) downhole the majority of the samples are barren in nannofossils. A broad age range is provided for Sample 36R-CC (1403 mbsf) by the presence of the species Coccolithus miopelagicus (top = 10.97 Ma) and the absence of Coccolithus floridanus (top = 11.85 Ma), which confirms a sequence falling within the biozone Chron CN5b. Biozones from Chrons CN10a to CN6 are not identifiable because preservation issues affect the presence of markers (e.g., the different species of Discoaster, Catinaster, and Minylitha convallis).

Age model

At Site U1437 it was possible to identify a Pleistocene to upper Miocene succession. 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 in the interval 1303–1806 mbsf. Also, no biostratigraphic datums are recognized deeper than 867 mbsf; thus, the age model for the interval 867–1303 mbsf was constructed only using magnetic reversal datums. Deeper than 1303 mbsf, the only age constraint from microfossils is given by a calcareous nannofossil assemblage, which suggests an age range between 10.97 and 11.85 Ma at 1403 mbsf. This age range is consistent with the magnetostratigraphic 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 LSR between intervals 868–1056 and 1122–1302 mbsf corresponds to the change from Hole U1437D to Hole 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 LSR within lithostratigraphic Unit V is extrapolated to Unit VI, ages are not consistent with the age constraint given by the nannofossil assemblage in Sample 350-U1437E-36R-CC (10.97–11.85 Ma at 1403 mbsf); a hiatus is the most likely explanation for this.

The total MAR, calculated from dry bulk density, ranges between 12 and 35 g/cm2/ky, with the highest values in the intervals 2–2.5 and 4.2–4.7 Ma. High MAR (29 g/cm2/ky) is also recorded in the lower part of Unit III (860 mbsf) to the upper part of Unit IV.

The carbonate accumulation rates are low over the entire succession ranging between 3 and 8.3 g/cm2/ky, with the highest values in the intervals 4.2–4.7 and 5.2–6.4 Ma.

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