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.

Geochemistry of Albian to Santonian black shale sequences on the Demerara Rise, South American margin

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian-Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, d13Corg values lie mostly between -30 per mil and -27 per mil, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C3 land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low d13Corg values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian-Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.

Mineral and organic geochemistry from Lake Anterne sediments (North French Alps)

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

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.

Organic geochemistry of Lower Cretaceous sediments from the northwetsern Pacific guyits

Organic geochemical data of Lower Cretaceous shallow water sediments from two sites (865 and 866) drilled during ODP leg 143 are presented. The organic matter is mainly terrestrial at the bottom of the sedimentary column at site 865, whereas algal and/or bacterial organic matter is dominant at site 866. This is the first evidence of shallow water deposition of organic matter during the Early Cretaceous in the Northwestern Pacific. The lower Aptian organic carbon-rich layers from the shallow water sediments of site 866 are geochemically similar to coeval mid-water sediments of site 463.

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.

Extractable hydrocarbon and carbon isotope geochemistry of Lower Cretaceous sediments of the Exmouth Plateau

The organic geochemical character of rocks selected from Aptian, Valanginian, and Berriasian clay stone and siltstone sequences encountered in Ocean Drilling Program (ODP) Holes 762C and 763C on the Exmouth Plateau was determined by means of a variety of analytical procedures. These sequences represent distal portions of the Mesozoic Barrow delta, in which petroleum source rocks and reservoirs exist on the Australian continent. The organic matter at the ODP sites is thermally immature type III material. Biomarker hydrocarbon compositions are dominated by long-chain, waxy n-alkanes and by C29 steranes, which reflect the land-plant origin of organic matter. Organic carbon d13C values ranged from -26 per mil to -28 per mil, consistent with a C3 land-plant source. Kerogen pyrolysate compositions and hopane isomerization ratios revealed progressively larger contributions of recycled organic matter as the depth of the deltaic sedimentary layers became greater.