Organic geochemistry of Upper Albian sediments from the Kirchrode I borehole

In the framework of a multidisciplinary research program, an organic geochemical study was carried out on a drill core which comprises a 245 m thick sequence of light-colored, Upper Albian marlstones that were deposited in the central part of the Lower Saxony basin (northern Germany). For part of the Upper Albian sequence, high-resolution measurements of carbonate contents reveal cycles which can be related to earth orbital forcing. Based on these data, sediment accumulation rates were calculated to be in the order of 15 g/m**2/yr. These high accumulation rates contrast with very low organic carbon contents and an extremely poor preservation of the autochthonous organic matter. Most of the sedimentary organic matter is of terrigenous origin and mainly derived from the erosion of older sedimentary rocks. Organic petrography reveals only a very small fraction of marine organic particles. Carbon/sulphur ratios, pristane/phytane ratios as well as the predominance of resedimented organic particles over autochthonous organic particles suggest that aerobic degradation processes rather than anaerobic processes (sulphate reduction) were responsible for the degradation of the organic matter. Furthermore, the scarcity of terrigenous organic particles (vitrinite) indicates that there was little vegetation on nearby land areas. To explain these analytical results, a depositional model was developed which could explain the scarcity of organic matter in the Upper Albian sediments. This model is based on downwelling of oxygen-rich, saline waters of Tethyan origin, which reduces the nutrient content of surface waters and thus primary bioproductivity while degradation of primary organic matter in the water column is enhanced at the same time. These conditions contrast to those which existed in Barremian and early Aptian times in this basin, when limited water exchange with adjacent oceans caused oxygen deficiency and the deposition of numerous organic carbon-rich black shales. The thick, organic matter-poor Upper Albian sequence of northern Germany also contrasts with comparatively thin, time-equivalent, deep-sea black shales from Italy. This discrepancy indicates that local and regional oceanographic factors (at least in this case) have a greater influence on organic matter deposition than global events.

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.

Pleistocene and upper Pliocene sapropels in the Tyrrhenian Sea, Mediterranean Sea

A most significant finding of the ODP Leg 107 drilling campaign was the recovery of at least 56 distinct sapropel intervals in upper Pliocene to Pleistocene sediments of six sites drilled in the Tyrrhenian Sea. Except for 3 repots of disturbed organic-rich sediments - recovered in Core 201 of the Swedish Deep-Sea Expedition, in Core 2R-1,107 cm of Site 373 (Leg 13 DSDP) and at Site 373, Core 1-2,O-5 cm of DSDP Leg 42A - sapropels had previously only been described from the eastern Mediterranean and the Black Sea. Scientific deep-sea drilling in the Tyrrhenian Sea during DSDP Legs 13 and 42A apparently missed most of these deposits due to spot coring and rotary drilling techniques; high sedimentation rates may have precluded recovery by conventional gravity coring devices. The recovery of multiple layers of sapropels and sapropelic sediments in the Tyrrhenian Sea demonstrates that oceanographic conditions conducive to sapropel formation were not confined to the Black Sea and eastern Mediterranean, but occurred sporadically and possibly simultaneously in the entire Mediterranean during the Pliocene and Pleistocene. In the light of this finding, previous models of sapropel genesis may need reconsideration. In this paper, we present some initial data on the Tyrrhenian sapropels and suggest some implications of their massive occurrence in the western Mediterranean realm. We end by outlining possible causes for deposition of sapropels in an attempt to revive the interest in sapropels and their paleoceanographic significance.

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.

Sea ice diatoms from Late Quaternary sediments in the Scotia Sea

Sea-ice growth and decay in Antarctica is one of the biggest seasonal changes on Earth, expanding ice cover from 4x10**6 km**2 to a maximum of 19x10**6 km**2 during the austral winter. Analyses of six marine sediment cores from the Scotia Sea, SW Atlantic, yield records of sea-ice migration across the basin since the Lateglacial. The cores span nearly ten degrees of latitude from the modern seasonal sea-ice zone to the modern Polar Front. Surface sediments in the cores comprise predominantly diatomaceous oozes and muddy diatom oozes that reflect Holocene conditions. The cores exhibit similar down-core stratigraphies with decreasing diatom concentrations and increasing magnetic susceptibility from modern through to the Last Glacial Maximum (LGM). Sediments in all cores contain sea-ice diatoms that preserve a signal of changing sea-ice cover and permit reconstruction of past sea-ice dynamics. The sea-ice records presented here are the first to document the position of both the summer and winter sea-ice cover at the Last Glacial Maximum (LGM) in the Scotia Sea. Comparison of the LGM and Holocene sea-ice conditions shows that the average winter sea-ice extent was at least 5° further north at the LGM. Average summer sea-ice extent was south of the most southerly core site at the LGM, and suggests that sea-ice expanded from approximately 61°S to 52°S each season. Our data also suggest that the average summer sea-ice position at the LGM was not the maximum extent of summer sea-ice during the last glacial. Instead, the sediments contain evidence of a pre-LGM maximum extent of summer sea-ice between ab. 30 ka and 22 ka that extended to ab. 59°S, close to the modern average winter sea-ice limit. Based on our reconstruction we propose that the timing of the maximum extent of summer sea-ice and subsequent retreat by 22 ka, could be insolation controlled and that the strong links between sea-ice and bottom water formation provide a potential mechanism by which Southern Hemisphere regional sea-ice dynamics at the LGM could have a global impact and promote deglaciation.

Organic carbon chemistry of late Tertiary to Quaternary sediments ODP Leg 107

We analyzed samples from ODP Holes 652A and 654A (Leg 107, Tyrrhenian Sea) for the amount, type, and thermal maturity of organic matter. The sediments encompass clastic and biogenic lithologies, which were deposited on the passive margin east of Sardinia since the late Miocene to the Pleistocene. Marine, hypersaline/evaporitic, lacustrine/riverine, and finally hemipelagic marine conditions with occasional anoxic(?) interludes gave rise to very diverse sedimentary facies. The majority of samples is lean in organic matter (<0.2% TOC). Notable exceptions are Tortonian sediments (TOC average 0.3%), Messinian oil shales from Core 107-652A-64R (up to 11% TOC), Messinian lacustrine/fluvial sediments from Hole 652A (TOC average 0.42%,), and Pleistocene sapropel samples (>2% TOC). The Messinian oil shale in Hole 652A appears to be the only mature hydrocarbon source rock. In general, Pliocene sediments are the leanest and least mature samples. Pleistocene and Pliocene samples derive organic matter from a marine source. In spite of obvious facies differences in the Messinian between the two sites, pyrolysis results are not conclusive in separating hypersaline facies of Site 654 from the fresh water facies of Site 652, because both appear to have received terrestrial organic tissue as the main component of TOC. It is apparent from the distribution of maximum pyrolysis temperatures that heat flow must have been considerably higher at Site 652 on the lower margin in the Messinian. Molecular maturity indices in lipid extracts substantiate the finding that the organic matter in Tortonian and Messinian samples from Hole 654A is immature, while thermal maturation is more advanced in coeval samples from Hole 652A. Analyses of lipid biomarkers showed that original odd-even predominance was preserved in alkanes and alkylcyclohexanes from Messinian samples in Hole 654A, while thermal maturation had removed any odd-even predominance in Hole 652A. Isomerization data of hopanes and steranes support these differences in thermal history for the two sites. Hopanoid distribution further suggests that petroleum impregnation from a deeper, more mature source resulted in the co-occurrence of immature and mature groups of pentacyclic biomarkers. Even though the presence of 4-methylsteranes may imply that dinoflagellates were a major source for organic matter in the oil shale interval of Hole 652, we did not find intact dinoflagellates or related nonskeletal algae during microscopic investigation of the organic matter in the fine laminations. Morphologically, the laminations resemble bacterial mats.

Geochemical characterizations of the Cretaceous formations at DSDP Holes 93-603B and 11-105

Geochemical characterizations of the Cretaceous formations at Site 603 are quite comparable with those at Site 105. In the Blake-Bahama and the Hatteras formations, the petroleum potential is medium (<5 kg HC/t of rock) to very low (<0.5 kg HC/t of rock), and the organic matter is mainly of type III origin, that is, terrestrial. At the top of the Hatteras Formation, there is a condensed series, which chiefly contains organic matter of type II origin, with up to 20 wt.% total organic carbon content in Core 603B-34 and 25 wt.% in Core 105-9. This accumulation corresponds to the Cenomanian/Turonian boundary event. An examination of dinoflagellates in the kerogen concentration assigns dates to the samples studied by organic geochemistry. The Cenomanian and Turonian age of the organic-matter-rich black claystones indicates a low rate of sedimentation, about 1 m/Ma. Furthermore, the occurrence of type II organic matter indicates an anoxic environment with insufficient oxygen renewal to oxidize the sinking hemipelagic organic matter. This organic enrichment is not related to local phenomena but to sedimentation over an extended area, because deposits are well known in various areas with different paleodepths in the North Atlantic.