Pliocene and Pleistocene abundance of siliceous microfossil assemblages in ODP Leg 127 sites

A close examination of the siliceous microfossil assemblages from the sediments of ODP Leg 127, Japan Sea Sites 794, 795, and 797, reveals that upper Pliocene and Pleistocene assemblages have been subjected to more dissolution than have lower Pliocene assemblages. This conclusion is based on semiquantitative observations of samples processed for diatoms and radiolarians. Although preservation of opaline microfossils in some upper Pliocene and Pleistocene samples is better than others, in general, the poorly preserved state of these assemblages supports the notion that opal dissolution, in response to lowered productivity, is responsible for the paucity of siliceous microfossils in upper Pliocene and Pleistocene sediments. The lithological transition from diatomaceous oozes to silts and clays corresponds to a change between dominantly well preserved to more poorly preserved siliceous assemblages, and is termed the late Pliocene Japan Sea opal dissolution transition zone (ODTZ). The base of the ODTZ is defined as the uppermost occurrence of high abundances of moderately to well preserved valves of the diatom Coscinodiscus marginatus. The dissolution transition zone is characterized by partially dissolved refractory assemblages of radiolarians, the presence of C. marginatus girdles, C. marginatus fragments, siliceous sponge spicules, and a general decrease in weakly silicified, less solution resistant diatoms upward in the section. The top of the dissolution transition zone marks the level where whole C. marginatus valves and C. marginatus fragments are no longer present in significant numbers. Dissolution of the late Pliocene and Pleistocene opaline assemblages is attributed mainly to changes in paleoceanographic circulation patterns and decreased nutrient (dissolved silicon) contents of the water column, and possibly dissolution at the sediment/water interface, rather than to post-depositional dissolution or diagenesis. We suggest that the transition from silica-rich to silica-poor conditions in the Japan Sea was due to fluctuations of deep-water exchange with the Pacific through the Tsugaru Strait between 2.9 and 2.3 Ma.

Trace metal content in Pliocene to Pleistocene sediments of ODP Leg 127 holes

Quaternary sedimentation within the Japan Sea was controlled by the configuration of peripheral sills, seasonal and long-term climatic variability, and the resultant fluctuations in sea level (Tamaki, 1988). Prior to drilling in the area, piston cores recovered from its basins contained Pleistocene sediments having distinctive color and fabric variation. Sedimentological and geochemical studies conducted on those facies indicated that the variability in fabric was the result of fluctuating marine and/or terrigenous influx to the deep-water basins of the Japan Sea (see, for example, Chough, 1984; Matoba, 1984). The sequences recovered during Leg 127 at Sites 794, 795, and 797 contain long, virtually undisturbed sequences (92.3, 123, and 119.9 mbsf [Hole 797B], respectively) of upper Miocene, upper Pliocene, and Pleistocene/Holocene sediments. The majority of these sequences consists of dark-colored (dark brown, green, and black) silty-clays, many of which are enriched in biogenic components (majority silicious, some carbonate) and/or organic matter, some containing pyrite and/or ash. These facies alternate with light-colored silty-clays, some containing ash and some showing signs of bioturbation (for example, Tamaki, Pisciotto, Allan, et al., 1990, p. 425-433). The dark-to-light sequences are present throughout the section, although they are especially dominant throughout the Pleistocene (for a more detailed lithology of Quaternary sequences recovered at Sites 794, 795, and 797, see Follmi et al. 1992 and Tada et al., 1992). This data report provides trace metal information on Pliocene-Pleistocene-Holocene samples at Sites 794,795, and 797. These data can be used (1) to provide information related to the depositional environments of the Japan Sea during the Quaternary period, (2) to permit comparisons between the dark organic-rich sediments recovered from this semi-enclosed basin and those reported for other silled basins (for example, the Mediterranean and Black seas), and (3) to permit comparisons between these sediments and contemporary equivalents found, for instance, beneath areas of high biogenic productivity. By providing such data, one should be able (1) to determine more precisely the processes governing the deposition of sediments with various levels of organic matter within enclosed basins, (2) to compare individual basin-wide processes, (3) to look for and compare the signatures present as a result of climatic fluctuation, and (4) to attempt to identify the presence and/or absence of cyclicity within such sequences.

Rare earth element data for sediments from ODP Leg 127

The relative effects of paleoceanographic and paleogeographic variations, sediment lithology, and diagenetic processes on the recorded rare earth element (REE) chemistry of Japan Sea sediments are evaluated by investigating REE total abundances and relative fractionations in 59 samples from Ocean Drilling Program Leg 127. REE total abundances (Sum REE) in the Japan Sea are strongly dependent upon the paleoceanographic position of a given site with respect to terrigenous and biogenic sources. REE concentrations at Site 794 (Yamato Basin) overall correspond well to aluminosilicate chemical indices and are strongly diluted by SiO2 within the late Miocene-Pliocene diatomaceous sequence. Eu/Eu* values at Site 794 reach a maximum through the diatomaceous interval as well, most likely suggesting an association of Eu/Eu* with the siliceous component, or reflecting slight incorporation of a detrital feldspar phase. Sum REE at Site 795 (Japan Basin) also is affiliated strongly with aluminosilicate phases, yet is diluted only slightly by siliceous input. At Site 797 (Yamato Basin), REE is not as clearly associated with the aluminosilicate fraction, is correlated moderately to siliceous input, and may be sporadically influenced by detrital heavy minerals originating from the nearby rifted continental fragment composing the Yamato Rise. The biogenic influence is largest at Site 794, moderately developed at Site 797, and of only minor importance at Site 795, reflecting basinal contrasts in productivity such that the Yamato Basin records greater biogenic input than the Japan Basin, while the most productive waters overlie the easternmost sequence of Site 794. Ce/Ce* profiles at all three sites increase monotonically with depth, and record progressive diagenetic LREE fractionation. The observed Ce/Ce* record does not respond to changes in oxygenation state of the overlying water, and Ce/Ce* correlates slightly better with depth than with age. The downhole increase in Ce/Ce* at Site 794 and Site 797 is a passive response to diagenetic transfer of LREE (except Ce) from sediment to interstitial water. At Site 795, the overall lack of correlation between Ce/Ce* and La_n/Yb_n suggests that other processes are occurring which mask the diagenetic behavior of all LREEs. First-order calculations of the Ce budget in Japan Sea waters and sediment indicate that ~20% of the excess Ce adsorbed by settling particles is recycled within the water column, and that an additional ~38% is recycled at or near the seafloor (data from Masuzawa and Koyama, 1989). Thus, because the remaining excess Ce is only ~10% of the total Ce, there is not a large source of Ce to the deeply buried sediment, further suggesting that the downhole increase in Ce/Ce* is a passive response to diagenetic behavior of the other LREEs. The REE chemistry of Japan Sea sediment therefore predicts successive downhole addition of LREEs to deeply-buried interstitial waters.

Calcareous nannofossil biostratigraphy of ODP Leg 127 sites

Calcareous nannofossils were studied by light microscopy in Neogene sedimentary rocks recovered at four sites of the Ocean Drilling Program Leg 127 in the Japan Sea. Nannofossils occur sporadically at all sites, and allow recognition of seven zones and two subzones; four zones in the Holocene to the uppermost Pliocene, and three zones and two subzones in the middle to lower Miocene. Forty-eight nannofossil species are recognized in 95 of the 808 irregularly-spaced samples taken from all the sites. The nannofossil assemblages in the Miocene are more diverse than those in the Holocene to Pliocene sedimentary interval. The greater diversity and the presence of warm-water taxa, such as Sphenolithus and discoasters in the upper lower Miocene to lower middle Miocene, suggest a relatively warm and stable surface-water condition, attributed to an increased supply of warm water from the subtropical western Pacific Ocean. Site 797 in the southern part of the Yamato Basin contains the most complete and the oldest nannofossil record so far reported from the Japan Sea. The lowermost nannofossil zone at this site, the Helicosphaera ampliaperta Zone (15.7-18.4 Ma) gives a minimum age for the Yamato Basin. This age range predates rotation of southwest Japan, an event previously believed to be caused by the opening of the Japan Sea.

Mineral composition and geochemistry of sediments from ODP Leg 127 sites

The lithostratigraphy of Neogene hemipelagic sediments recovered from the Japan Sea during Leg 127 was revised to improve intersite consistency and to remove confusion stemming from diagenetic modification of the lithology through the opal-A to opal-CT transformation. Special emphasis was put on the presence and nature of dark-light cycles in revising the lithostratigraphy. Mineral composition analysis was conducted for samples from Sites 794, 795, and 797. In addition, major element chemical composition analysis was conducted for these same sample sets from Site 794. The result of mineral composition analysis suggests that the detrital component, which consists of such minerals as quartz, plagioclase, illite, and kaolinite plus chlorite, is diluted to various degrees by biogenic silica (opal-A) and its diagenetic equivalents (opal-CT and quartz). Smectite, on the other hand, may be a diagenetic or hydrothermal alteration product of volcanic material, although more study is necessary to confirm its origin. As a whole, vertical variation in the sediment composition is consistent with the revised lithostratigraphy and helps to characterize the redefined lithologic units quantitatively.

Ba, Al and SO4 concentrations in sediments and pore fluid of ODP Leg 127/128 samples

The barium distribution in sediments and pore fluids from five sites drilled in the Japan Sea have been used to illustrate the geochemical behavior of this element as it pertains paleoproductivity reconstructions, diagenetic remobilization, and barite precipitation in authigenic fronts. Sites where sulfate is depleted in the pore fluids also show high concentrations of dissolved barium, reflecting dissolution of biogenic barite. The high rate of sedimentation at Sites 798 and 799 results in a rapid sulfate depletion, which in turn leads to barite dissolution and reprecipitation in diagenetic fronts. The dissolved barium distribution at these sites has been used to quantify the rate of barite dissolution; we estimate a first-order rate constant for barite dissolution to be 2*10**-6/s at Site 799 and 2*10**-7/s at Site 798. Authigenic barite has been documented in sediments from Site 799 at 323 meters below seafloor by scanning electron microscopy and X-ray fluorescence analysis. These results indicate barite precipitation in a diagenetic front near the zone of sulfate depletion by upward migration of dissolved barium and downward diffusion of sulfate. Barite precipitation has also been inferred at Sites 796 and 798 based on sedimentary and dissolved barium distributions. Sulfate is not depleted in the pore fluids of Site 794. The lack of diagenetic remobilization of biogenic barium at this site preserves the high barium signal associated with the high-productivity sequences deposited during the late Miocene to Pliocene. Significantly, the organic carbon distribution does not indicate high accumulation rates during the periods of high opal and barium deposition. Instead, higher organic carbon accumulations are recorded in the Quaternary and middle Miocene sequences; intervals that are also characterized by deposition of siliciclastic turbidites. The presence of a terrestrial component in the organic carbon record renders barium a more useful indicator than organic carbon for paleoproductivity reconstructions in this marginal sea.

Stable and radiogenic isotope ratios in pore waters from ODP Leg 127 sites

Interstitial waters from four sites of the Japan Sea (794 to 797) have been analyzed for stable isotopes (delta D, delta11B, delta18O, and delta34S) and 87Sr/86Sr, besides major and minor ions. The isotopic composition is dominated by organic matter degradation, alteration of ash layers and volcaniclastic sands, silica transformation (opal A/CT), and basement alteration. Organic matter degradation and corresponding sulfate reduction leads to 32S depletion and is dependent upon sedimentation rate. The remaining sulfate reservoir is characterized by very "heavy" delta34S ratios, up to +93 ? (rel. CDT = Canyon Diabolo Troilite). "Barite fronts," which may develop in such sediments, should also be characterized by very "heavy" sulfur isotopes. The alteration of volcaniclastic material in the Quaternary sections influences the delta18O (-1.5 ? shift) and delta11B (desorption and later adsorption of "labile"11B). A pronounced positive delta11B anomaly at Site 795 represents the depth range of preferential 10B uptake by alteration products of the ash layers. At Site 796 delta D, delta11B, and 87Sr/86Sr are severely affected by alteration processes of volcaniclastic sands. The opal A/CT transformation may influence the oxygen isotopes and serves as a potential source for B, which is liberated at this interval at Site 795. This positive B anomaly is not reflected in the delta11B profile. Basement alteration processes dominate the sedimentary sequence below the opal A/CT transition, which serves as a chemical and physical boundary. The decreases in delta D and delta18O are probably related to a "paleo ocean water reservoir" situated in the permeable Layer II of the oceanic crust, as is indicated by the positive correlation between these two parameters. Besides Mg, alkalies and delta18O basement rocks also serve as a sink for 11 B (Site 795) and are the source for the Ca and Sr increases, as is documented by the less radiogenic 87Sr/86Sr ratio. 87Sr/86Sr ratios for the lowermost pore waters from Site 795 (0.70529) are comparable to those from volcaniclastic rocks from the "Green Tuff' region (0.704 to 0.706) and oil field brines from the Niigata Oil Field.