Sediment geochemistry of ODP Leg 127 sites

The relative effects of paleoceanographic and paleogeographic variations, sediment lithology, and diagenetic processes on the final preserved chemistry of Japan Sea sediments are evaluated by investigating the rare earth element (REE), major element, and trace element concentrations in 59 squeeze-cake whole-round and 27 physical-property sample residues from Sites 794, 795, and 797, cored during ODP Leg 127. The most important variation in sedimentary chemical composition is the increase in SiO2 concentration through the Pliocene diatomaceous sequences, which dilutes most other major and trace element components by various degrees. This biogenic input is largest at Site 794 (Yamato Basin), moderately developed at Site 797 (Yamato Basin), and of only minor importance at Site 795 (Japan Basin), potentially reflecting basinal contrasts in productivity with the Yamato Basin recording greater biogenic input than the Japan Basin and with the easternmost sequence of Site 794 lying beneath the most productive waters. There are few systematic changes in solid-phase chemistry resulting from the opal-A/opal-CT or opal-CT/quartz silica phase transformations. Most major and trace element concentrations are controlled by the aluminosilicate fraction of the sediment, although the effects of diagenetic silica phases and manganese carbonates are of localized importance. 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 overall correspond well to aluminosilicate chemical indices and are strongly diluted by SiO2 within the upper 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. SumREE at Site 795 also is affiliated strongly with aluminosilicate phases and yet is diluted only slightly by siliceous input. At Site 797, SumREE 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. Ce/Ce* profiles at all three sites increase essentially monotonically with depth and record progressive diagenetic LREE fractionation. The observed Ce/Ce* increases are not responding to changes in the paleoceanographic oxygenation state of the overlying water, as there is no independent evidence to suggest the proper oceanographic conditions. Ce/Ce* correlates slightly better with depth than with age at the two Yamato Basin sites. The downhole increase in Ce/Ce* at Sites 794 and 797 is a passive response to the diagenetic transfer of LREE (except Ce) from sediment to interstitial water. At Site 795, the overall lack of correlation between Ce/Ce* and Lan/Ybn suggests that other processes 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. 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.

(Table 1) Isotopic and trace-element composition of ODP Leg 127/128 basalts

The western Pacific includes many volcanic island arc and backarc complexes, yet multi-isotopic studies of them are rare. Basement rocks of the Sea of Japan backarc basin were encountered at Sites 794,795, and 797, and consisted of basaltic sills and lava flows. These rocks exhibit a broad range in isotopic composition, broader than that seen in any other western Pacific arc or backarc system: 87Sr/86Sr = 0.70369 to 0.70499, 143Nd/144Nd = 0.51267 to 0.51317, 206Pb/204Pb = 17.64 to 18.36. The samples form highly correlated arrays between very depleted mid-ocean ridge basalt (MORB) and the Pacific pelagic sediment fields on Pb-Pb plots. Similarly, on plots of Sr-Pb and Nd-Pb, the Sea of Japan samples lie on mixing curves between depleted mantle and enriched mantle ("EM II"), which is interpreted to be of average crustal or pelagic sediment composition. The source of these backarc rocks appears to be a MORB-like mantle source, contaminated by pelagic sediments. Unlike the Mariana and Izu arc/backarc systems, Japanese arc and backarc rocks are indistinguishable from each other in a Sr-Nd isotope plot, and have similar trends in Pb-Pb plots. Thus, sediment contamination of the mantle wedge appears to control the isotopic compositions of both the arc and backarc magmas. Two-component mixing calculations suggest that the percentage of sediments in the magma source varies from 0.5% to 2.5%.

Description and chemical composition of manganese nodules collected in the Pacific Ocean

Chemical and X-ray analyses were performed on the fifteen manganese nodules collected from the Pacific Ocean floor. The results were discussed compared with the previous data on the manganese nodules. Minerals were found to be todorokite, delta-MnO2 and other silicates, montmorillonite, illite, phillipsite and alpha-Si02. Average composition shows that copper is concentrated on the deep sea nodules more than the shallow ones, and that the todorokite rich nodules contain more copper and nickel than the delta-MnO2 rich ones. The analyses of fresh water iron-manganese precipitates by bacterial activity suggest that biological process is one of the important factors on the genesis of the sedimentary iron-manganese deposits, in¬cluding the manganese nodule.

Benthic foraminifers from the Japan Sea

Five holes were drilled at two sites in the Sea of Japan during Ocean Drilling Program (ODP) Leg 128. Site 798 is located on Oki Ridge at a depth of about 900 m. Sediment age at Site 798 ranges from Pliocene to Holocene. Site 799 is located in the Kita-Yamato Trough at depth of 2000 m and below the present calcite compensation depth (CCD); the sediment ranges from Miocene to Holocene in age. Samples from all holes contain benthic foraminifers. Faunal evidence of downslope displacement is frequent in Holes 799A and 799B. The vertical frequency distribution of some dominant species shows that significant faunal changes occur in Holes 798A-C on Oki Ridge. Based on the faunal change and the thickness of sediments, it appears that the Oki Ridge was uplifted more than 1,000 m during last 4 m.y. Benthic foraminifers also demonstrate that the water depth of Site 799 rapidly changed from upper bathyal to lower bathyal during middle Miocene time. The appearance of benthic foraminifer species common to anaerobic environments suggests that the dysaerobic to anaerobic bottom conditions existed during the evolution of the Sea of Japan. Faunal distributions also suggest that the 'Tertiary-type' species recognized in the Neogene strata of the Japan Sea coastal regions disappeared sequentially from the Sea of Japan during Pliocene to late Pleistocene.