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doi:10.2204/iodp.proc.346.107.2015 LithostratigraphyDrilling at Site U1426 penetrated to a maximum subbottom depth of 396.7 m CSF-A in Hole U1426A, recovering a total of 418.78 m of sediment for a recovery rate of 106%. The detailed visual assessment of sediment composition, color, sedimentary structures, bioturbation intensity, and drilling disturbance was supplemented by petrographic analysis of smear slides (175 from Hole U1426A, 7 from Hole U1426B, 87 from Hole U1426C, and 29 from Hole U1426D) and bulk mineralogic analysis by X-ray diffraction (XRD) (43 samples; Table T2). Results from these analytical methods provide the basis for identification of sedimentary facies and facies associations and for the division of the stratigraphy into major lithologic units (Figs. F2, F3, F4, F5). The sedimentary succession recovered at Site U1426 consists of a wide variety of lithologies: clay and silty clay with varying contents of biosilica, foraminifers, and calcareous nannofossils, as well as diatom, biosiliceous, and nannofossil ooze. Major lithologies are interbedded with tephra (volcanic ash) layers. Carbonate-rich sections are restricted to the upper 185 m CSF-A and to the interval between 296 and 341 m CSF-A of the succession. Intervals of laminated sediment are observed to 125 m CSF-A. At 54.8 and 297.6 m CSF-A in Hole U1426A, two diagenetic carbonate layers, each several centimeters thick, were recovered. The stratigraphy is divided into two lithologic units (I and II) based on sedimentary structures and sediment composition. Unit I is further divided into two subunits (IA and IB). These lithologic units are not equivalent to the lithologic division of nearby Site 798 (Shipboard Scientific Party, 1990). Instead the division of Site U1426 follows in principle the classification of Tada (1994) and is consistent with the division of Units I and II at Sites U1424 and U1425. In the case of Site U1426, the boundaries mark distinct changes in the frequency of change of major lithologies within the unit, rather than changes of major lithology from one unit to the next. As at Sites IODP U1422–U1425, Unit I shows alternations of clay and diatom-rich clay to diatom ooze. Subunit IA shows characteristic, albeit somewhat less pronounced than at Site U1425, centimeter- to decimeter-scale alternations of diatom-rich clay and clay with a distinct dark–light color banding. In contrast, Subunit IB is characterized by intermediate-frequency alternations (3–5 m scale) of clay with diatom and nannofossil oozes, with subtle color changes between major lithologies. Unit II is characterized by thick intervals of diatom ooze and clay and shows a lower frequency of variability in general. Physical properties measurements, including natural gamma radiation (NGR), magnetic susceptibility, color reflectance, and dry bulk density profiles, reflect the various lithologies and unit boundaries (see “Physical properties”). Figures F2, F3, F4, and F5 summarize the major characteristics and physical property profiles of the sedimentary sequence. Hole-to-hole correlation based on the distribution of lithologic units is shown in Figure F6. Site U1426 appears to provide a complete pelagic to hemipelagic sedimentary sequence without major disturbances. At nearby Site 798, slumped sediment was described in interval 128-798C-7H-4, 86–120 cm (Shipboard Scientific Party, 1990). In Hole U1426A, the same interval was recovered in Sections 346-U1426A-7H-5 and 7H-6 (Fig. F7), and the description of Leg 128 was revisited. Clear indications for slumping processes are not present in the succession of Site U1426. However, several coarse-grained layers (including tephra layers and foraminifer-sand layers; Fig. F8) in Core 7H might indicate resedimentation and grain-size sorting by bottom currents. In interval 346-U1426D-9H-5, 138–147 cm, a layer with normal grading and foraminifer sand at its base is observed (Fig. F9). Unit I
Unit I consists of a wide range of major lithologies that alternate on different scales (from centimeter to meter). Dominant major lithologies are clay, silty clay, nannofossil-rich clay, and diatom ooze. The alternations show a distinct change in frequency. Based on this frequency change, Unit I is divided into Subunits IA and IB (Tada, 1994). Subunit IA
Lithologies and structuresSubunit IA consists of clay, silty clay, and nannofossil-rich clay with interbedded foraminifer- and diatom-rich layers (Figs. F2, F3, F4, F5). The major lithologies are interbedded with several centimeter-thick tephra layers (vitric and scoriaceous). Subunit IA in Hole U1426A contains a total of 38 tephra beds >0.5 cm thick (Fig. F10; Table T3). The most characteristic sedimentary structure of Subunit IA is the decimeter-scale color banding with alternations of dark brownish, organic-rich intervals interspersed with lighter colored, bioturbated organic-poor intervals (Fig. F11). These alternations are reflected in the L*, a*, and b* records (see “Physical properties”). In detail, the light intervals are mainly composed of light greenish gray and pale yellowish gray diatom-rich clay. These light intervals are moderately to heavily bioturbated. The organic-rich dark intervals are composed of clay and in general have a sharp base. The characteristics of these layers have been described in detail for Sites U1424 and U1425. Compared to these other sites, the dark layers at Site U1426 are more enriched in foraminifers and nannofossils, which is consistent with its shallower water depth. The transition from dark to light is generally gradational but also very often disturbed by bioturbational mixing. Bioturbation is more intense in the lower part of Subunit IA (deeper than 50 m CSF-A), where color contrast between the bands is also reduced. In this portion of the sequence, bioturbation often impacts heavily on the preservation of the thin, darker banding and its contact with the light colored bands (Fig. F12). Many dark to light transitions show evidence of Chondrites burrows. In general, the upper part of Subunit IA is dominated by brownish colors, whereas the lower part of the unit is dominated by light greenish colors, indicating a decrease of organic carbon content toward the base of the unit. Some intervals of Subunit IA are laminated. These intervals are typically dark gray in color and consist of biosiliceous silty clay with increased occurrence of pyrite (e.g., Section 346-U1426B-3H-1). This type of lamination occurs only in the uppermost 45 m of Subunit IA. In contrast to the dark gray laminated sections, a second type of laminated sediment is composed of nannofossil ooze and has a pale yellowish gray color and no particularly high pyrite content. This especially noticeable pale yellow nannofossil ooze occurs only at ~42 m CSF-A. The Subunit IA/IB boundary is well defined by the occurrence of a third type of laminated sediment. This interval consists of diatom ooze and occurs in Section 346-U1426A-14H-7. CompositionThe principal components of lithologies in Subunit IA are of terrigenous and biogenic origin (see Site U1426 smear slides in “Core descriptions”). Terrigenous components are dominated by clay and fine silt size fractions. Volcanic glass is always present in the sediment as a minor dispersed composition (usually 5%–10% by visual estimate); however, volcanic glass and pumice account for nearly 100% of the tephra layers. Nannofossils dominate the biogenic fraction, with minor components of foraminifers and biogenic silica (mostly diatoms and sponge spicules). Preservation of nannofossils and foraminifers is very good throughout. The persistently high nannofossil content (Fig. F13) throughout Subunit IA is the most prominent feature at Site U1426, which is distinctly different from Sites U1422–U1425. Pyrite can be found in almost all sediment with concentrations from 1% to 20% and is especially rich in darker colored sediment. It usually occurs as framboids and/or in irregular masses and can even be found with well-developed crystal shapes (polyhedrons). Although dolomite is difficult to identify in smear slides, XRD analysis suggests that it is present in almost all intervals, although its concentration is very low (<1%–3%) throughout. However, it is enriched (20%–30%) in some layers (e.g., Section 346-U1426A-3H-2A) (Fig. F14). Subunit IB
Lithology and structuresSubunit IB consists of intermediate-frequency alternations (3–5 m scale) of brown and olive-gray clay, brownish and olive-green and greenish gray biosiliceous-rich nannofossil ooze, biosiliceous-rich clay, and diatom ooze (Figs. F2, F4, F15). Subunit IB is distinguished from Subunit IA on the basis of sediment color and the frequency of color and clay content changes. Subunit IB sediment is moderately bioturbated, which leads to an overall decrease in the preservation of original sedimentary structures. Tephra layers are a minor but common component of Subunit IB, and several tephra layers are >5 cm thick. They vary in color and type (vitric and scoria). Subunit IB lacks the characteristic centimeter- to decimeter-scale color banding of Subunit IA but instead shows clear color and lithologic cycles at a 3–5 m scale. These cycles mainly reflect alternations in clay content. Intervals with low clay content are diatom or nannofossil ooze. CompositionThe major lithologies in Subunit IB are dominated by fine-grained terrigenous and biosiliceous materials, the latter mostly diatoms. Calcareous nannoplankton and foraminifers are common in the upper part of Subunit IB. The lower part of the unit (172–283 m CSF-A in Hole U1426A) lacks biogenic carbonate. Biosilica rapidly becomes more abundant (>50%) from Section 346-U1426A-24H-5A downhole. Diatoms and siliceous sponge spicules are dominant in the biosilica fraction, whereas radiolarians and silicoflagellates are found only in rare or trace amounts (1%–5%). The above-mentioned siliceous fossil assemblages are observed in both the brownish and greenish color sediment in the “diatom ooze” category. Pyrite and volcaniclastic materials represent a minor component of sediment throughout Subunit IB. Unit II
Lithology and structuresUnit II shows lower frequency alternations of uniform dark green to gray clay with gray biosiliceous-rich clay and gray diatom ooze (Fig. F16). Sediment is generally slightly to heavily bioturbated throughout and has a fairly homogeneous, structureless appearance. Some intervals have high abundances of biogenic carbonate. Sediment deeper than 380 m CSF-A shows a distinct decrease in biogenic silica. Deeper than this depth, the major lithology is moderately indurated clay but with no clear evidence of opal-CT. CompositionThe major lithologies in Unit II are dominated by fine-grained terrigenous materials and biosiliceous components that are mostly diatoms. However, with the transition from Subunit IB to Unit II, the abundance of nannofossils rapidly increases again, ranging from common, abundant, and even dominant between Sections 346-U1426A-37H-2A and 48H-1A (296–341 m CSF-A). This phenomenon is not observed in Unit II of Sites U1422–U1425. The biosiliceous components rapidly decrease from >50% to <10% at around Section 54H-4A (Fig. F17). The preservation of diatoms decreases dramatically at this depth. These features may be related to diatom dissolution during diagenesis. Bulk mineralogyThe results of XRD analysis of samples from Site U1426 are listed in Table T2. In general, the Pleistocene–Pliocene sediment at Site U1426 is composed mainly of quartz, calcite, plagioclase, and clay minerals (including smectite, illite, and kaolinite and/or chlorite), as well as biogenic opal-A and minor amounts of halite and pyrite. Calcite is a major mineral component in the upper part of the sedimentary succession but disappears deeper than ~200 m CSF-A. Figure F18 shows the downcore variations in peak intensity of the identified minerals at Site U1426. In general, the contents of quartz, plagioclase, smectite, illite, kaolinite and/or chlorite, K-feldspar, calcite, and opal-A show high variability throughout the sedimentary succession and especially between 50 and 300 m CSF-A. This large variability was not observed in Units I and II at Sites U1422–U1425. As expected, Site U1426 samples with a high opal-A or calcite content show a consistently lower abundance of silicate minerals. In summary, the average bulk mineralogy does not significantly change across unit boundaries. Subunit IB appears to show a larger amplitude of variability than Subunit IA and Unit II. However, the sampling resolution for the XRD analyses is not sufficient to resolve potential differences in the frequency of alternations in mineralogy within the units. TephraNumerous (>200) visible tephra beds were observed in cores from Hole U1426A. Tephra beds with a thickness >0.5 cm are listed in Table T3. Several thick tephra layers (>20 cm) occur in the lower part of Unit I (intervals 346-U1426A-11H-3, 63–86.5 cm [91.11–91.345 m CSF-A], and 18H-6, 13–48 cm [162.13–162.48 m CSF-A]) and in Unit II (intervals 26H-4, 1.5–23 cm [230.645–230.86 m CSF-A], and 44H-2, 5–47 cm [322.18–322.60 m CSF-A]), whereas most of the beds have a thickness <1 cm. The total thickness of tephra beds and the number of discrete tephra layers with a thickness >0.5 cm in each core reach a maximum in Cores 7H to 9H. The number of tephra layers decreases deeper than Core 10H near the Brunhes/Matuyama paleomagnetic boundary. Some peaks in tephra thickness below this horizon indicate that rare but thick tephra deposition occurred during the period. Several tephras were correlated to well-dated on-land and/or marine tephras such as U-Oki (~10.7 ka: intervals 346-U1426A-1H-2, 20–25 cm [1.70–1.75 m CSF-A], and 346-U1426D-1H-2, 19–20.5 cm [1.69–1.705 m CSF-A]), AT (~29.3 ka: intervals 346-U1426A-2H-1, 60–72 cm [3.10–3.22 m CSF-A], and 346-U1426D-1H-3, 64–75.5 cm [3.64–3.755 m CSF-A]), U-Ym (~35 ka: interval 346-U1426A-2H-2, 29–32 cm [4.29–4.32 m CSF-A]), and Aso-4 (~88 ka: interval 346-U1426D-2H-4, 66.5–81 cm [9.565–9.71 m CSF-A]) tephras based on their characteristics and stratigraphic occurrence. Tephras at intervals 346-U1426A-5H-3, 125–134 cm (34.75–34.84 m CSF-A), 346-U1426D-4H-3, 0–2.5 cm (26.29–26.315 m CSF-A), and 346-U1426D-6H-3, 17–20.5 cm (45.28–45.315 m CSF-A), are probably correlative to Ata-Th (240 ka), Aso-1 (250–270 ka), and Kb-Ks (520–530 ka) tephras, respectively, based on volcanic glass shard characteristics and heavy mineral compositions. Tephra beds at intervals 346-U1426A-11H-3, 63–86.5 cm (91.11–91.345 m CSF-A), and 18H-3, 1–7 cm (157.51–157.57 m CSF-A), are potentially correlated to the Ss-Pnk tephra (~1.6 Ma; Tamura et al., 2008; Satoguchi and Nagahashi, 2012) and the Eb-Fkd tephra (~1.7 Ma; Tamura et al., 2008; Satoguchi and Nagahashi, 2012), respectively, from the glass morphology. These initial correlations will be tested by shore-based petrographic and geochemical analyses. DiscussionThe sedimentary succession at Site U1426 documents the paleoceanographic history of the Yamato Basin from the Pliocene to the Holocene. Like the other sites drilled during Expedition 346, the lithostratigraphic characteristics appear to vary in a manner that is highly sensitive to global climate variability. The relatively rapid sedimentation at Site U1426 will allow for paleoclimate and paleoceanographic reconstructions at high resolution. For division of the lithostratigraphic units, particular attention was paid to the frequency of lithologic changes. For nearby Site 798, a unit boundary was placed at ~220 meters below seafloor (mbsf), marking the downhole transition from the upper carbonate-rich part to the lower carbonate-poor sediment. Although such a division seems sensible from a lithologic point of view, it would be inconsistent with the lithostratigraphic framework for this marginal basin previously introduced by Tada (1994). Tada’s (1994) classification of the marginal basin’s lithologic units is based on sediment records from deeper water depths (>2500 m), well below the CCD. Site U1426 is substantially shallower (941 m) and therefore shifts in the depth of the CCD will have a greater impact on the local lithology, namely on carbonate preservation and content. Oceanographic as well as potential tectonic processes, such as uplift of the Oki Ridge, lifted the location of Site U1426 above the CCD during the early Pleistocene (~220 m CSF-A). However, this boundary is not correlatable across the basin because it strongly depends on local conditions, in particular the water depth of Site U1426 relative to other sites. Although Subunits IA and IB and Unit II at Site U1426 show some different lithologic characteristics compared to Site U1425 in the Kita-Yamato Trough or Site U1424 in the southeastern Japan Basin, certain features are persistent throughout. Therefore, the same lithostratigraphic concept has been applied to Site U1426 to emphasize correlatable features in the lithology that are not only basin wide (Japan Basin as well as Yamato Basin) but also across a wide range of water depths. The most striking basin-wide feature is variations in the frequency of change of the major lithology. This frequency decreases stepwise downhole. At Site U1426, the major steps occur at ~125 and 285 m CSF-A. At the same time, the amplitude of the lithologic changes decreases as well. These features reflect the changes in global climate variability across the Pliocene–Pleistocene. The characteristic centimeter- to decimeter-scale color banding of Subunit IA is a recognizable lithostratigraphic feature that occurs basin wide during the Quaternary (Tada, 2004). Tada (1994) suggested that the deposition of organic-poor gray clay in the basin occurred under oxic conditions, whereas organic-rich dark brown layers were deposited during suboxic conditions, reflecting changes in ventilation of the marginal sea. Deposition of gray colored, laminated, and pyrite-rich intervals occurred potentially under euxinic conditions during the glacial maxima. The alternations between dark and light layers can be linked to global millennial-scale climate variability (Tada, 2004). At Site U1426, dark–light cycles of Subunit IA are well developed in the upper 45 m. In contrast, the lower part of Subunit IA is strongly affected by bioturbation and the color cycles become less evident. At ~42.5 m CSF-A, a remarkable interval of partially laminated nannofossil ooze occurs that can be linked to a carbonate-rich layer in nearby Core MD01-2407 (Kido et al., 2007). This layer coincides with marine isotope Stage (MIS) 13. Because this interval of nannofossil ooze looks different from any other interval within Site U1426, one can conclude that oceanographic conditions in the marginal basin during MIS 13 were distinctly different. Unusual climatic and oceanographic conditions of unknown origin during MIS 13 have also been reported based on Chinese Loess Plateau records (e.g., Guo et al., 2000). MIS 13 marks the onset of large-amplitude glacial–interglacial cycles after the mid-Pleistocene transition. Therefore, the intensified dark–light color banding in the upper 45 m appears to be linked to the recent period of more extreme glaciations. Gray pyrite-rich laminated intervals are also restricted to the upper 45 m CSF-A, indicating the extreme sea level lowstands of the last 450,000 y have a particularly strong impact on the oxygenation of the marginal basin. In contrast to Subunit IA, Subunit IB lacks centimeter- to decimeter-scale color banding. This is similar to what is observed at Sites U1422–U1425. Instead, Subunit IB is characterized by distinct 3–5 m alternations in the abundance of siliciclastic components, mainly the clay fraction. These alternations are particularly well depicted in the NGR profile (see “Physical properties”), with high NGR values indicating higher clay content in the sediment. With a sedimentation rate of ~10 cm/k.y. (see “Stratigraphic correlation and sedimentation rates”), these cycles seem likely to be linked to orbital cyclicity (41 k.y., obliquity), which dominated global climate and sea level fluctuations during the early Pleistocene. Orbital forcing has also been suggested as a possible explanation for the lithostratigraphic cycles at Site 798 (deMenocal et al., 1992). Pliocene sedimentation at Site U1426 corresponds to the deposition of Unit II. Unit II is mainly composed of moderately to heavily bioturbated clay and diatom ooze with intervals of abundant nannofossils. Well-ventilated deep waters with high oxygen levels are indicated by the intensity of bioturbation, whereas the low frequency of lithologic changes reflects lower amplitude and lower frequency changes in Pliocene global climate. The decreased abundance and preservation of diatoms deeper than 380 m CSF-A represents the diagenetic dissolution of biogenic silica. |
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