Composition of sedimentary rocks from the Mountainous Crimea and the Northwestern Caucasus

Detailed data obtained on chemistry of sedimentary rocks from the Mountainous Crimea and the Northwestern Caucasus that were dated at the Cenomanian/Turonian boundary and formed during Oceanic Anoxic Event 2 make it possible to calculate dissolved oxygen concentration in bottom waters of the sedimentation basin. Enrichment factors of trace elements in black shales are revised and an explanation is suggested for genesis of the rocks with regard for unusual climatic changes.

Concentration and isotope composition of sulphur in sediments and pore water at DSDP Leg 70 Holes

Data obtained while investigating the mounds area near the Galapagos Spreading Center demonstrate the direct influence of solutions derived from the interaction of seawater and young oceanic crust on the sedimentary cover. Investigation of metalliferous sediments from the mid-oceanic ridges, the Galapagos mounds, and the FAMOUS-area zone formations have shown that this influence and the resulting products are dependent on composition, temperature, and conditions of solution input. The study of sulfur in upwardly migrating solutions and the interaction of these solutions with sediments is of great interest. Investigations of different types of hydrothermally derived formations (Edmond, et al., 1979; Spiess et al., 1980; Styrt et al., 1981; Rosanova 1976; Grinenko et al., 1978) have shown the significant role of sulfur-bearing minerals in deposits formed from hightemperature solutions. In contrast, the addition of hydrothermal sulfur is negligible in those metalliferous sediments that precipitated as a result of the interaction between the solutions and open seawater (Bonatti et al., 1972, 1976; Gordeev et al., 1979; Migdisov, Bogdanov, et al., 1979). For example, sulfides are absent in clearly oxidized metalliferous sediments from the East Pacific Rise (EPR). Barite sulfur from these sediments is identical with seawater sulfate sulfur in isotope composition (Grinenko et al., 1978). Gurvich and Bogdanov (1977) have suggested that barium from EPR metalliferous sediments results completely from biological activity and from the components of ocean waters. Edmond et al. (1979) report that low-temperature springs from the Galapagos Rift axis contain two types of solutions: those with and those without H2S.

Stable isotope and Mg/Ca record of Globigerinoides ruger morphotypes

Tests of the planktonic foraminifer Globigerinoides ruber (white; d'Orbigny) have become a standard tool for reconstructing past oceanic environments. Paleoceanographers often utilize the Mg/Ca ratios of the foraminiferal tests for reconstructing low-latitude ocean glacial-interglacial changes in sea surface temperatures (SST). We report herein a comparison of Mg/Ca measurements on sample pairs (n = 20) of two G. ruber (white) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) from surface and downcore samples of the western Pacific and Indian Oceans. G. ruber s.s. refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide, high arched aperture, whereas G. ruber s.l. refers to a more compact test with a diminutive final chamber and small aperture. The G. ruber s.s. specimens generally show significantly higher Mg/Ca ratios compared to G. ruber s.l. Our results from the Mg/Ca ratio analysis suggest that G. ruber s.l. specimens precipitated their shells in slightly colder surface waters than G. ruber s.s. specimens. This conclusion is supported by the differences in delta18O and delta13C values between the two morphotypes. Although it is still unclear if these two morphotypes represent phenotypic variants or sibling species, our findings seem to support the hypothesis of depth and/or seasonal allopatry within a single morphospecies.

Organic geochemistry of sediment cores from the Arbian Sea

Two deep-sea sediment cores from the northeastern and the southeastern Arabian Sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles. Core 136KL was recovered from the high-productivity area off Pakistan within the modern oxygen-minimum zone (OMZ). By contrast, modern primary productivity at the site of MD900963 close to Maldives is moderate and bottom waters are today well oxygenated. For both cores, we reconstructed the changes in palaeoproductivity using a set of biomarkers (alkenones, dinosterol and brassicasterol); the main result is that primary productivity is enhanced during glacial stages and lowered during interstadials. The proxies associated with productivity show a 23 kyr cyclicity corresponding to the precession-related insolation cycle. Palaeoredox conditions were studied in both cores using a new organic geochemical parameter (C35/C31-n-alkane ratio) developed by analysing surface sediments from a transect across the OMZ off Pakistan. The value of this ratio in core 136KL shows many variations during the last 65 kyr, indicating that the OMZ was not stable during this time: it disappeared completely during Heinrich- and the Younger Dryas events, pointing to a connection between global oceanic circulation and the stability of the OMZ. The C35/C31 ratio determined in sediments of core MD900963 shows that bottom waters remained rather well oxygenated over the last 330 kyr, which is confirmed by comparison with authigenic metal concentrations in the same sediments. A zonally averaged, circulation-biogeochemical ocean model was used to explore how the intermediate Indian Ocean responds to a freshwater flux anomaly at the surface of the North Atlantic. As suggested by the geochemical time series, both the abundance of Southern Ocean Water and the oxygen concentration are significantly increased in response to this freshwater perturbation.

Dissolved oxygen, nitrate, nitrite, and ammonum in waters of the Arabian Sea

Distribution of ammonium, nitrite and nitrate nitrogen is examined in a section along 65-67°E between 18°S and 23°N during the transition period from winter to summer monsoons. It is shown that, under conditions of very large oxygen deficit in the 200-400 m layer, denitrification process results in formation of the second deep-sea maximum of nitrites and the intermediate minimum of nitrate nitrogen.

Geochemistry of oceanic crust of the Cocos Plate

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ?100 km beyond the morphological hotspot track.

Concentrations of labile dissolved forms of Cu, Zn, Pb, and Cu in waters of the Kara Sea and Ob and Yenisey estuaries

Concentrations of labile dissolved forms of Cu, Zn, Pb, and Cu in waters of the Kara Sea and Ob and Yenisey estuaries measured on board during Cruise 49 of R/V Dmitry Mendeleev.