Organic geochemistry of Cretaceous black shales

Organic-carbon-rich 'black shales' and adjacent organic-carbon-poor rocks from three different Cretaceous settings encountered during ODP Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich marlstone turbidites, large amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian/Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deep-water setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deep-water anoxia may have been involved.

Element and isotope compostion of basalts and sediments from the Japan Sea

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Japan Sea, a Miocene-age back-arc basin in the western Pacific Ocean, recovered incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a significant sedimentary component in their mantle source. Isotopically, the volcanic rocks cover a wide range of compositions (e.g., 87Sr/86Sr = 0.70369 - 0.70503, 206Pb/204Pb = 17.65 - 18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched component with the composition of EM II. At Site 797, the combined isotope and trace element systematics support a model of two component mixing between depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been determined by analyzing samples of differing lithology from DSDP Sites 579 and 581 in the western Pacific, east of the Japan arc. The sediments have large depletions in the high field strength elements and are relatively enriched in the large-ion-lithophile elements, including Pb. These characteristics are mirrored, with reduced amplitudes, in Japan Sea enriched tholeiites and northeast Japan arc lavas, which strengthens the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than the enriched component inferred fiom mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/REE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-arc basalt source. Samples from other potential sources of an enriched, EM II-like component beneath Japan, such as the subcontinental lithosphere or crust, have isotopic compositions which overlap those of the Japan Sea tholeiites and are not "enriched" enough to be the EM II end-member.

Organic chemistry of sediments from the Tyrrhenian Sea

Thirty sediment samples from Tortonian to Pleistocene age of five ODP locations (Holes 650A, 651A, and 652A, and Sites 654 and 655) in the Marsili Basin, Vavilov Basin, and Sardinia Margin (Tyrrhenian Sea) were studied by organic geochemical methods including total organic carbon determination, Rock-Eval pyrolysis, bitumen extraction, pyrolysis-gas chromatography, and organic petrography. Six organic facies, including open ocean anoxia with variable terrestrial input, oxic open ocean, oxic tidal flat, mildly oxic lagoon, and anoxic lacustrine algal-bacterial mat environments, have been recognized in these sediments. The sediments below 500 m in Sardinia Margin are mature for significant hydrocarbon generation. Possible mature source-rock (Type I and IIB/III kerogen) and migrated bitumen occur in the deeper part of the section in Vavilov Basin and Sardinia Margin sediments. Sporadic sapropel formation observed in the studied Pliocene-Pleistocene sediment section is probably controlled by organic productivity due to nutrient supply by the rivers and terrestrial input associated with open ocean anoxia or anoxia caused by the material balance between rate of organic matter supplied by turbidites and organic matter consumption. Pliocene and Pleistocene sapropels are mostly immature and lie within Type II-III (precisely as IIA-IIB and IIB source rocks) kerogen maturation path.

Geochemistry of Peru margin sediments

This data report tabulates results of chemical analyses of sediments from four sites (680, 682, 685, and 688) drilled during Leg 112 offshore Peru. These sediments were recovered from the forearc basins underlying the Peru upwelling area. They are equivalent in facies and age to the Pisco and Monterey formations, both of which are of considerable economic and geological interest as hydrocarbon source rocks deposited under conditions of coastal upwelling. Sediments recovered from the shelf (Site 680) and slope (Sites 682, 685, and 688) during Leg 112 are unconsolidated and are thermally immature. A lack of consolidation and thermal catagenesis makes these deposits ideal targets for chemical investigation into effects of early diagenesis in organic-carbon-rich siliceous muds.

Chemistry of organic matter of glacial and Cretaceous sediments from the Prydz Bay

Organic matter in Miocene glacial sediments in Hole 739C on the Antarctic Shelf represents erosional recycled continental material. Various indications of maturity in bulk organic matter, kerogens, and extracts imply that an exposed section of mature organic carbon-rich material was present during the Miocene. Based on biomarker, n-alkane, and kerogen analysis, a massive diamictite of early Eocene/Oligocene age at Hole 739C contains immature organic matter. Visual and pyrolysis analyses of the kerogens suggest a predominance of terrestrial organic matter in all samples from Hole 739C. A reversal of thermal maturities, i.e., more-mature overlying less-mature sections, may be related to redeposition generated from glacial erosion. Siliciclastic fluviatile sediments of Lower Cretaceous age from Hole 741A were analyzed. The organic matter from this hole contains immature aliphatic and aromatic biomarkers as well as a suite of odd carbon number-dominated nalkanes. Visual examination and pyrolysis analysis of the kerogen suggests that predominantly immature terrestrial organic matter is present at Hole 741A. The similarities between Hole 739C Unit V and Hole 741A suggest that the source of the organic matter in the glacial sediments in Unit V at Hole 739C could be Cretaceous in age and similar to sediments sampled at Hole 741A in Prydz Bay.

Biomarker records of DSDP Hole 24-231 and terrestrial sediments

ntegrated terrestrial and marine records of northeast African vegetation are needed to provide long high resolution records of environmental variability with established links to specific terrestrial environments. In this study, we compare records of terrestrial vegetation preserved in marine sediments in the Gulf of Aden [Deep Sea Drilling Project (DSDP) Site 231] and an outcrop of lacustrine sediments in the Turkana Basin, Kenya, part of the East African Rift System. We analyzed higher plant biomarkers in sediments from both deposits of known equivalent age, corresponding to a ca. 50-100 ka humid interval prior to the b-Tulu Bor eruption ca. 3.40 Ma, when the Lokochot Lake occupied part of the Turkana Basin. Molecular abundance distributions indicate that long chain n-alkanoic acids in marine sediments are the most reliable proxy for terrestrial vegetation (Carbon Preference Index, CPI = 4.5), with more cautious interpretation needed for n-alkanes and lacustrine archives. Marine sediments record carbon isotopic variability in terrestrial biomarkers of 2-3 per mil, roughly equivalent to 20% variability in the C3/C4 vegetation contribution. The proportion of C4 vegetation apparently increased at times of low terrigenous dust input. Terrestrial sediments reveal much larger (2-10 per mil) shifts in n-alkanoic acid delta13C values. However, molecular abundance and isotopic composition suggest that microbial sources may also contribute fatty acids, contaminating the lacustrine sedimentary record of terrestrial vegetation.

Organic geochemistry of DSDP Site 467

Results of the analyses of twenty-three samples from the Middle Miocene to Lower Pliocene strata from DSDP Site 467, offshore California, are presented. The analyses were performed with the aim of determining the origin of the organic matter, the stratigraphic section's hydrocarbon generation potential and extent of organic diagenesis. Organic carbon contents are an order of magnitude greater than those typically found in deep sea sediments, suggesting an anoxic depositional environment and elevated levels of primary productivity. Hydrocarbon generation potentials are above average for most samples. The results of elemental analyses indicate that the kerogens are primarily composed of type II organic matter and are thermally immature. Analysis of the bitumen fractions confirms that the samples are immature. In cores from 541 to 614 meters, the gas chromatograms of the C15+ non-aromatic hydrocarbon fractions are dominated by a single peak which was identified as 17*(H), 18*(H), 21beta(H)-28, 30-bisnorhopane. This interval is the same area in which the highest degrees of anoxia are observed as reflected by the lowest pristane/phytane ratios. This correlation may have some implications with regard to the origin of the bisnorhopane and its possible use as an indicator of anoxic depositional conditions within thermally immature sediments.

Hydrocarbons in hydrothermal sulfides from the Rainbow Hydrothermal Field

The Rainbow Hydrothermal Field (36°N, Mid-Atlantic Ridge) is one of three presently known fields related to serpentinization of ultramafic rocks accompanied by formation of hydrogen- and methane rich solutions. Gas chromatographic and molecular gas chromatographic - mass spectrometric investigations of sulfide ores and sediments from this field confirmed predominantly biological nature of bitumoids related to high-temperature transformation of biomass of the hydrothermal biological community. At the same time ores of the Rainbow field contain significant amounts of compounds that are not directly related to biogenic synthesis. This fact suggests possibility of abiogenic synthesis of methane and even complex hydrocarbons during serpentinization of ultramafic rocks.

Data on multidisciplinary research in the Tajura Rift (Gulf of Aden) during Cruise 7 of R/V Akademik Mstislav Keldysh in winter 1983-1984

In the collective monograph results of geological and geophysical studies in the Tadjura Rift carried out by conventional outboard instruments and from deep/sea manned submersibles "Pisces" in winter 1983-1984 are reported. Main features of rift tectonics, geology, petrology, and geochemistry of basalts from the rift are under consideration. An emphasis is made on lithology, stratigraphy, and geochemistry of bottom sediments. Roles of terrigenous, edafogenic, biogenic, and hydrothermal components in formation of bottom sediments from the rift zone are shown.

Organic chemistry of sediments of the Peru margin

Organic-carbon-rich anoxic sediments from the continental shelf (Site 680) and the lower continental slope (Site 688) off Peru were studied to determine factors controlling the accumulation of reduced sulfur. High concentrations of organic matter in diatomaceous muds, its thermal immaturity, and the presence of abundant hydrogen-containing organic compounds lead to the conclusion that organic matter is not limiting for reduced sulfur formation. Rather, high degrees of iron pyritization at Site 680 limit pyrite accumulation in sediments from this shelf site. The low degree of iron pyritization and nearly complete reduction of dissolved sulfate at Site 688 suggest that a lack of interstitial sulfate is limiting pyrite formation there. Although factors that limit the formation of sedimentary iron sulfide are different at each site, the resulting average reduced-sulfur concentrations are remarkably similar (0.85 wt.% at Site 680 and 0.86 wt.% at Site 688). Carbon to sulfur (C/S) ratios are higher in samples containing in excess of 3 wt.% organic carbon than the average of 2.8 in normal marine sediments and have been primarily influenced by variations in organic matter concentrations.