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doi:10.2204/iodp.proc.346.108.2015 LithostratigraphyDrilling at Site U1427 penetrated to a maximum subbottom depth of 548.6 m in Hole U1427A, recovering a total of 542.6 m of sediment for a recovery rate of 99% (see “Operations”). The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides (143 from Hole U1427A and 101 from Hole U1427B) (see Site U1427 smear slides in “Core descriptions”) and bulk mineralogic analysis by X-ray diffraction (XRD) (57 samples; Table T2). These objective criteria were used to describe the sediment succession and to define facies and facies associations. The sedimentary sequence recovered at Site U1427 extends from the early Pleistocene to Holocene and is dominated by clayey silt and nannofossil- or biosiliceous-rich clayey silt. Volcaniclastic material represents a minor component throughout the succession, except in tephra (i.e., volcanic ash) layers. Shell fragments of shallow-water origin are observed throughout the recovered section. The character of the sedimentary physical properties, including natural gamma radiation (NGR), magnetic susceptibility, color reflectance parameters, and density, also records the distribution of the various sediment components and lithologies (see “Physical properties”). The major characteristics of the sedimentary sequence at Site U1427, together with some of these additional properties, are summarized in Figures F2, F3, and F4. A hole-to-hole correlation based on the distribution of lithologic units is shown in Figure F5. The lithology at Site U1427 consists of one unit based on the investigation of sediment composition and sedimentary structures. The name “Unit A” is used for the complete sequence at Site U1427 in order to distinguish it from “Unit I,” which was previously used at other sites occupied during Expedition 346. Unit I, as defined by Tada (1994), is recognized in the marginal basin by centimeter- to decimeter-scale alternation of dark and light sediment layers. Because the sediment succession at Site U1427 does not show this characteristic, the name “Unit I” is not used for this site. Unit A
Lithology and structuresUnit A consists of clayey silt, silty clay, nannofossil-rich clayey silt, biosiliceous-rich clayey silt, and nannofossil ooze (Figs. F2, F3, F4). The sediment is generally heavily bioturbated with a fairly homogeneous, structureless appearance (Fig. F6). The major lithologies are interbedded with several centimeter- to decimeter-thick tephra layers. Hole U1427A contains a total of 33 tephra beds >0.5 cm thick (Fig. F7; Table T3). Unit A is divided into Subunits A1 and A2 based on the difference in minor lithology. Bulk mineralogyThe results of XRD analyses are listed in Table T2. In general, Pleistocene sediment at Site U1427 is composed mainly of quartz, calcite, plagioclase, K-feldspar, and clay minerals (including smectite, illite, and kaolinite and/or chlorite), as well as minor amounts of halite, pyrite, and biogenic opal-A. Hornblende and pyroxene, which were not seen at the previous sites, are also found at this site and show relatively high values in the upper ~200 m CSF-A. Figure F8 shows the downcore variations in peak intensity of the identified minerals at Site U1427. Quartz and plagioclase contents show a long-term increasing trend toward the top of the hole. In contrast, clay minerals (smectite, illite, and kaolinite and/or chlorite) and opal-A show a long-term decreasing trend toward the top of the hole. Plagioclase and K-feldspar show high values in some intervals. TephraNumerous visible tephra beds were observed in cores from Hole U1427A. Because most of the tephras occurred as small patches or thin discontinuous layers reflecting strong bioturbation throughout the cores and contained high amounts of quartz with volcanic glass shards, it is hard to distinguish between original and reworked beds. Therefore, tephra beds that occurred as small patches or bioturbated thin layers were not counted when documenting tephra abundance except for beds composed of nearly pure volcanogenic materials such as volcanic glass shards and phenocrysts. Tephra beds with a thickness >0.5 cm and thin tephra layers (<0.5 cm) that are probably correlatable between holes are listed in Table T3. In this table, tephra layers from the two holes that appear on the same line are thought to be correlative. Thick tephra layers (thickness >10 cm) occur at intervals 346-U1427A-2H-7, 21–31 cm (10.68–10.78 m CSF-A); 5H-7, 55 cm, to 5H-CC, 3 cm (39.25–39.35 m CSF-A); 11H-3, 74–94.5 cm (91.02–91.225 m CSF-A); 25H-6, 14 cm, to 25H-7, 51 cm (226.89–228.71 m CSF-A); and 27H-1, 25.5 cm, to 27H-2, 29 cm (233.655–234.02 m CSF-A). The total thickness of tephra beds shows some peaks at horizons of the thick beds, especially in Cores 23H to 27H, with a small peak in Cores 45H to 49H. However, no clear peak is found in the number of thick tephras with thicknesses >0.5 cm in each core (Fig. F7). Several examples of tephra layers recovered at Site U1427 are shown in Figure F9. The thickest tephra layer is >180 cm thick and straddles Sections 346-U1427A-25H-6 to 25H-7. This tephra layer is composed of two parts. The upper 6 cm consists of fine-grained white volcanic glass, and the lower part consists of well-sorted coarse pumice, biotite, and quartz. The lower part of this thick tephra was not recovered in Hole U1427B, and white-colored fine-grained volcanic glass sits directly above clayey silt with an erosional contact. In Hole U1427C, the same tephra consists of 6 cm of fine-grained white glass above 10 cm of a coarse pumiceous layer with a contact that shows no evidence of erosion. Based on these observations, it is concluded that the coarse basal part of this tephra is possibly flow-in in Hole U1427A that is missing in Hole U1427B. Some tephra beds contained “pumice-type” volcanic glass shards and orthopyroxene, as well as hornblende as phenocrysts. Biotite is also found in some tephras. These characteristics are similar to those of the Daisen and Sanbe Volcanoes located near the site. Therefore, a stronger contribution from local volcanoes to this site relative to the other offshore sites may be expected. Subunit A1
Lithologies and structuresSubunit A1 consists of clayey silt, silty clay, silty sand, nannofossil-rich clayey silt, biosiliceous-rich clayey silt, and nannofossil ooze (Fig. F10). The sediment is characterized by meter- to several tens of meters–scale alternations of biogenic component–rich clayey silt and biogenic component–poor clayey silt (Fig. F6). The biogenic component is mainly composed of nannofossils and secondarily of diatoms and sponge spicules. The biogenic component–rich clayey silt is generally olive-gray or dark greenish gray in color, and the biogenic component–poor clayey silt typically is more grayish green. Although the contrast in color between these lithologies is low and the transition from one to the next is gradual, color variations over depth scales of tens of meters can be observed. These alternations of lithologies are reflected in records of NGR, magnetic susceptibility, and color reflectance. The clayey silt intervals present relatively high NGR and magnetic susceptibility and lower b* values. In contrast, the biogenic component–rich clayey silt intervals are marked by relatively lower NGR and magnetic susceptibility and higher b* values. As minor lithologies, very dark gray and/or very dark greenish gray clayey silt with heavy bioturbation are observed (Fig. F11). These dark colored intervals occur only in the upper ~120 m CSF-A with ~50–400 cm thickness. They are divided into two types (I and II) based on appearance. Type I dark intervals (e.g., intervals 346-U1427A-5H-1, 20–90 cm [30.5–31.2 m CSF-A]; 7H-1, 0 cm, to 7H-2, 130 cm [49.3–51.26 m CSF-A]; and 9H-1, 0 cm, to 9H-4, 45 cm [68.3–71.94 m CSF-A]) are composed mainly of siliciclastic-rich sediment (i.e., silty clay) and are black to dark greenish gray. Type II dark intervals (e.g., intervals 12H-3, 0 cm, to 12H-5, 20 cm [99.54–102.74 m CSF-A]; and 13H-2, 80 cm, to 13H-4, 10 cm [108.61–110.87 m CSF-A]) are composed of nannofossil-rich clayey silt and nannofossil ooze and are very dark greenish gray to grayish brown. CompositionThe major lithologies of Subunit A1 are dominated by fine- to medium-grained terrigenous materials and biogenic carbonate and biosiliceous components. The relative abundances of terrigenous and biogenic components change alternately (see Site U1427 smear slides in “Core descriptions”). Volcanic glass and pyrite are observed throughout the sediment, and scattered shell fragments and wood fragments can be found in some intervals. Carbonate concretions were observed at intervals 346-U1427A-6H-5, 120–140 cm (45.87–46.07 m CSF-A), and 346-U1427B-6H-2, 90–100 cm (45.20–45.30 m CSF-A), and probably represent the same diagenetic horizon. The dark color intervals contain organic matter and pyrite, although the pyrite contents of these layers are not significantly higher than other intervals. Subunit A2
Lithologies and structuresSubunit A2 is distinguished from Subunit A1 by the lack of dark color intervals and the occurrence of laminated sediment. Subunit A2 consists of clayey silt, silty clay, silt, nannofossil-rich clayey silt, biosiliceous-rich clayey silt, and nannofossil ooze (Fig. F10). The major lithology of Subunit A2 is similar to that of Subunit A1 and is characterized by meter- to several tens of meters–scale alternations of biogenic component–rich clayey silt and biogenic component–poor clayey silt. The biogenic component is mainly composed of nannofossils; however, diatoms are the major component deeper than 449.6 m CSF-A. As a minor lithology, laminated sediment is observed at three intervals in Hole U1427A: Sections 346-U1427A-17H-6 to 18H-7 (149.8–163.1 m CSF-A), 23H-6 to 24H-6 (208.8–219.8 m CSF-A), and 45H-1 to 49H-CC (318.00–342.19 m CSF-A) and three intervals in Hole U1427B: Sections 346-U1427B-19H-1 to 20H-4 (149.3–163.8 m CSF-A), 25H-1 to 27H-1 (207.5–224.8 m CSF-A), and 47H-1 to 51H-CC (320.7–343.0 m CSF-A). These intervals can be correlated between the two holes. Laminations are generally millimeter-thick and greenish gray in color (Fig. F12) and are composed of silty clay with minor amounts of diatoms. CompositionThe major lithologies of Subunit A2 are dominated by fine- to medium-grained terrigenous materials and biogenic carbonate and biosiliceous components similar to Subunit A1. Calcareous nannofossils are the most dominant of the biogenic components. However, calcareous microfossils, including nannofossils and planktonic and benthic foraminifers, are rarely observed deeper than ~430 m CSF-A. Dispersed volcanic glass and pyrite are observed throughout the sediment. Shell fragments can also be frequently found, especially from ~370 to 450 m CSF-A. Summary and discussionThe sedimentary succession at Site U1427 records the paleoceanographic history of the marginal sea from the early Pleistocene to Holocene. The lithostratigraphic characteristics appear to respond sensitively to regional and global climate change. The lithology of Site U1427 consists of one unit (A), which is characterized by alternation of clayey silt and nannofossil-rich clayey silt over depth scales of meters to tens of meters. Unit A is further divided into two subunits (A1 and A2) based on differences in minor lithology. Sediment appearance at Site U1427 is much different from what is observed at other sites drilled during Expedition 346. Unit I sediment at sites deeper than 900 m are characterized by centimeter- to decimeter-scale alternations of dark and light colored layers. Those dark–light alternations are correlatable in deeper parts of the basin (Tada et al., 1992). Ikehara et al. (1994) showed that the most recent distinct dark layer, deposited during the Last Glacial Maximum (LGM), is observed in all sediment cores except for those shallower than 500 m water depth. The lack of dark–light alternations at Site U1427, which is related to deepwater oxygenation, can be attributed to the shallow setting of this site (~330 m). The characteristic tens of meters–scale lithologic changes at Site U1427 are reflected in the physical properties. Figure F13 shows variations in NGR, color reflectance b*, and maximum grain size, which is determined by smear slide observation. Silt/clay-rich sediment has high NGR and low b* values, and nannofossil/biosiliceous-rich sediment has low NGR and high b* values. These data show cyclic changes, in particular in the upper ~350 m CSF-A. Considering the shallow setting of this site, sea level change must have had a strong influence on the sedimentary environment. With an average sedimentation rate of ~40 cm/k.y. (see “Stratigraphic correlation and sedimentation rates”), tens of meters–scale changes are probably linked to orbital-scale sea level change. The maximum grain size variation in Hole U1427A ranges from 50 to 200 µm (Fig. F13). Relatively fine grain sizes (<100 µm) are observed at 155, 175, 217–230, 327–352, and 480 m CSF-A. These intervals correspond to the biogenic component–poor silty clay, except for 175 m CSF-A, which is biosiliceous-rich calcareous ooze, and some of them correspond to laminated intervals. The relationship between the maximum grain size and NGR changes with depth. They are well correlated in Subunit A1 (the upper ~130 m CSF-A). Namely, coarser grains are observed in siliciclastic-rich intervals (i.e., high NGR with low b* value). In contrast, the relationship is the opposite in Subunit A2, (~130–480 m CSF-A). In particular, relatively long intervals of siliciclastic-rich sediment (~140–170, 200–250, and 320–350 m CSF-A) consist of fine grain size material. The timing of this change in the relationship between maximum grain size and NGR is consistent with the subunit boundary, and it may be related to the occurrence of dark color intervals and laminated intervals. The dark color intervals are observed only in Subunit A1, which are the intervals where NGR is positively correlated with maximum grain size. The Type II dark intervals (99.54–102.74 and 108.61–110.87 m CSF-A), which are in nannofossil-rich sediment, are probably related to periods of high productivity in the Late Pleistocene. The Type I dark intervals seem to be transitional from nannofossil-rich to siliciclastic-rich sediment and may be related to perturbations in surface ocean environments during global sea level change. In contrast to dark color intervals, the laminated intervals are observed only in Subunit A2, which are the intervals where NGR is negatively correlated with maximum grain size. Three laminated intervals are composed of siliciclastic-rich and fine-grained sediment (Fig. F13). Considering that the pyrite contents, which are based on both smear slide observation and XRD analyses, do not show high values in laminated intervals, these laminations may not be indicative of anoxic environments. Overall, changes in sedimentation observed at Site U1427 since the early Pleistocene most likely reflect some combination of climate oscillations and eustatic sea level changes. Site U1427 also records the volcanic history of Japan, in particular local volcanoes in western Japan and East Asia. The changes in the relative amount of biogenic components and siliciclastics are plausibly related to the orbital-scale global sea level changes. The high sedimentation rates and good preservation of calcareous fossils at Site U1427 will allow for a suite of high-resolution paleoceanographic studies. |
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