Carbon chemistry of cretaceous marine organic matter

Geochemical studies of Cretaceous strata rich in organic carbon (OC) from Deep Sea Drilling Project (DSDP) sites and several land sections reveal several consistent relationships among amount of OC, hydrocarbon generating potential of kerogen (measured by pyrolysis as the hydrogen index, HI), and the isotopic composition of the OC. First, there is a positive correlation between HI and OC in strata that contain more than about 1% OC. Second, percent OC and HI often are negatively correlated with carbon isotopic composition (delta13C) of kerogen. The relationship between HI and OC indicates that as the amount of organic matter increases, this organic matter tends to be more lipid rich reflecting the marine source of the organic matter. Cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter. Average delta13C values for organic matter from most Cretaceous sites are between -26 and -28 per mil, and values heavier than about -25 per mil occur at very few sites. Most of the delta13C values of Miocene to Holocene OC-rich strata and modern marine plankton are between -16 to -23 per mil. Values of delta13C of modern terrestrial organic matter are mostly between -23 and -33 per mil. The depletion of terrestial OC in 13C relative to marine planktonic OC is the basis for numerous statements in the literature that isotopically light Cretaceous organic matter is of terrestrial origin, even though other organic geochemical and(or) optical indicators show that the organic matter is mainly of marine origin. A difference of about 5 per mil in delta13C between modern and Cretaceous OC-rich marine strata suggests either that Cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis, or that OC derived from Cretaceous marine plankton was isotopically lighter by about 5 per mil relative to modern plankton OC. Diagenesis does not produce a significant shift in delta13C in Miocene to Holocene sediments, and therefore probably did not produce the isotopically light Cretaceous OC. This means that Cretaceous marine plankton must have had delta13C values that were about 5 per mil lighter than modern marine plankton, and at least several per mil lighter than Cretaceous terrestrial vegetation. The reason for these lighter values, however, is not obvious. It has been proposed that concentrations of CO2 were higher during the middle Cretaceous, and this more available CO2 may be responsible for the lighter delta13C values of Cretaceous marine organic matter.

Processing results from drill hole VNIIO-1988-Sht-01 on the shelf of the Barents Sea (Report 6404, Murmansk)

Interareal correlation has been carried out; composition of the deposits has been determined; sections recovered by marine drilling have been compared; reconstructed paleogeographic conditions confirm previous views on Jurassic and Cretaceous sedimentation in the area: 1. Determinate changes of continental and shallow marine mainly sandy Middle Jurassic deposits by sandy-clayey marine ones to the north and west occur. This indicates similar direction of clastic material migration and converse direction of Jurassic marine transgressions. 2. Increase of sand contents in the deposits also to the east and to the southeast indicates an important source of clastic material. It can result from incipience and development of the epiplatform orogen of Novaya Zemlya - Pai-Khoi in the Late Triassic - Early Jurassic. 3. Compositional and facial changes as well as changes in thicknesses of some Early Cretaceous lithologic-stratigraphic complexes indicate fast change of terrigenous material transport from the north to the south - south-east in the Late Valanginian - Hauterivian. Besides within the South Barents Sea region up to the Shtokman area there occurs weak variability in lithologic parameters of Neocomian avandeltaic deposits and turbidites composed of clays, claystones, and clayey siltstones. Correlation of drilling sections from the Shtokman area and from the South Basin of the Barents Sea together with paleotectonic analysis result to the conclusion about significant structure-forming movements in the Late Jurassic - Early Neocomian. During this time there occurred maximal growth of the Shtokman structure and likely of many other structures belonging to the South Basin of the Barents Sea.

(Table 1) CPI17-23, hopanoic acid, and n-C35 + lycopane/n-C31 ratios as well as 17beta(H), 21beta(H) homohopane, long-chain alkanes, and alkenones concentrations in IODP Exp302

During Integrated Ocean Drilling Program Expedition 302 (Arctic Coring Expedition (ACEX)) a more than 200 m thick sequence of Paleogene organic carbon (OC)-rich (black shale type) sediments was drilled. Here we present new biomarker data determined in ACEX sediment samples to decipher processes controlling OC accumulation and their paleoenvironmental significance during periods of Paleogene global warmth and proposed increased freshwater discharge in the early Cenozoic. Specific source-related biomarkers including n-alkanes, fatty acids, isoprenoids, carotenoids, hopanes/hopenes, hopanoic acids, aromatic terpenoids, and long-chain alkenones show a high variability of components, derived from marine and terrestrial origin. The distribution of hopanoic acid isomers is dominated by compounds with the biological 17beta(H), 21beta(H) configuration indicating a low level of maturity. On the basis of the biomarker data the terrestrial OC supply was significantly enriched during the late Paleocene and part of the earliest Eocene, whereas increased aquatic contributions and euxinic conditions of variable intensity were determined for the Paleocene-Eocene thermal maximum and Eocene thermal maximum 2 events as well as the middle Eocene time interval. Furthermore, samples from the middle Eocene are characterized by the occurrence of long-chain alkenones, high proportions of lycopane, and high ratios (>0.6) of (n-C35 + lycopane)/n-C31. The occurrence of C37-alkenenones, which were first determined toward the end of the Azolla freshwater event, indicates that the OC becomes more marine in origin during the middle Eocene. Preliminary UK'37- based sea surface temperature (SST) values display a longterm temperature decrease of about 15C during the time interval 49-44.5 Ma (25° to 10°C), coinciding with the global benthic d18O cooling trend after the early Eocene climatic optimum. At about 46 Ma, parallel with onset of ice-rafted debris, SST (interpreted as summer temperatures) decreased to values <15°C. For the late early Miocene a SST of 11°-15°C was determined. Most of the middle Eocene ACEX sediments are characterized by a smooth short-chain n-alkane distribution, which may point to natural oil-type hydrocarbons from leakage of petroleum reservoirs or erosion of related source rocks and redeposition.