Total organic carbon content and redox-sensitiv trace metal concentrations of selected late Cenoman claystones from DSDP Hole 93-603B

The Cenomanian/Turonian (C/T) intervals at DSDP Sites 105 and 603B from the northern part of the proto-North Atlantic show high amplitude, short-term cyclic variations in total organic carbon (TOC) content. The more pronounced changes in TOC are also reflected by changes in lithology from green claystones (TOC<1%) to black claystones (TOC>1%). Although their depositional history was different, the individual TOC cycles at Sites 105 and 603B can be correlated using stable carbon isotope stratigraphy. Sedimentation rates obtained from the isotope stratigraphy and spectral analyses indicate that these cycles were predominately precession controlled. The coinciding variations in HI, OI, delta13Corg and the abundance of marine relative to terrestrial biomarkers, as well as the low abundance of lignin pyrolysis products generated from the kerogen of the black claystones, indicate that these cyclic variations reflect changes in the contribution of marine organic matter (OM). The cooccurrence of lamination, enrichment of redox-sensitive trace metals and presence of molecular fossils of pigments from green sulfur bacteria indicate that the northern proto-North Atlantic Ocean water column was periodically euxinic from the bottom to at least the base of the photic zone (<150 m) during the deposition of the black claystones. In contrast, the green claystones are bioturbated, are enriched in Mn, do not show enrichments in redox-sensitive trace metals and show biomarker distributions indicative of long oxygen exposure times, indicating more oxic water conditions. At the same time, there is evidence (e.g., abundance of biogenic silica and significant 13C-enrichment for OC of phytoplanktic origin) for enhanced primary productivity during the deposition of the black claystones. We propose that increased primary productivity periodically overwhelmed the oxic OM remineralisation potential of the bottom waters resulting in the deposition of OM-rich black claystones. Because the amount of oxygen used for OM remineralisation exceeded the amount supplied by diffusion and deep-water circulation, the northern proto-North Atlantic became euxinic during these periods. Both Sites 105 and 603B show trends of continually increasing TOC contents and HI values of the black claystones up section, which most likely resulted from both enhanced preservation due to increased anoxia and increased production of marine OM during oceanic anoxic event 2 (OAE2).

Alkenon concentrations of Creatcesous sediments of DSDP Hole 76-534A (Table 1)

We here report the discovery of unusual distributions of long-chain alkenones (C37-C42) in two Cretaceous black shales from the Blake-Bahama Basin, western North Atlantic. These sediments are Cenomanian (c. 95 Ma) and mid-Albian (c. 105 Ma) in age, thus significantly extending the geological range of these compounds. The precise source of these lipids is, as yet, unknown, although they may derive from an ancient ancestor of Emiliania huxleyi.

Thallium concentrations and isotope composition of MORB glasses and altered oceani crust from DSDP/ODP Hole 504B and DSDP Hole 52-417D

Hydrothermal fluids expelled from the seafloor at high and low temperatures play pivotal roles in controlling seawater chemistry. However, the magnitude of the high temperature water flux of mid-ocean ridge axes remains widely disputed and the volume of low temperature vent fluids at ridge flanks is virtually unconstrained. Here, we determine both high and low temperature hydrothermal fluid fluxes using the chemical and isotopic mass balance of the element thallium (Tl) in the ocean crust. Thallium is a unique tracer of ocean floor hydrothermal exchange because of its contrasting behavior during seafloor alteration at low and high temperatures and the distinctive isotopic signatures of fresh and altered MORB and seawater. The calculated high temperature hydrothermal water flux is (0.17-2.93)*10**13 kg/yr with a best estimate of 0.72*10**13 kg/yr. This result suggests that only about 5 to 80% of the heat available at mid-ocean ridge axes from the crystallization and cooling of the freshly formed ocean crust, is released by high temperature black smoker fluids.The residual thermal energy ismost likely lost via conduction and/or through the circulation of intermediate temperature hydrothermal fluids that do not alter the chemical budgets of Tl in the ocean crust. The Tl-based calculations indicate that the low temperature hydrothermal water flux at ridge flanks is (0.2-5.4)*10**17 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1 to 3.6 °C relative to ambient seawater. If these low temperatures are correct then both Sr and Mg are expected to be relatively unreactive in ridge-flank hydrothermal systems and this may explain why the extent of basalt alteration that is observed for altered ocean crust appears insufficient to balance the oceanic budgets of 87Sr/86Sr and Mg.

(Table 1) Organic carbon, hydrogen and oxygen index and hydrocarbons at DSDP Hole 93-603B

C2-C8 hydrocarbon concentrations (about 35 compounds identified, including saturated, aromatic, and olefinic compounds) from 27 shipboard-sealed, deep-frozen core samples of DSDP Hole 603B off the east coast of North America were determined by a gas-stripping/thermovaporization method. Total yields representing the hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces of the sediments vary from 22 to 2400 ng/g of dryweight sediment. Highest yields are measured in the two black shale samples of Core 603B-34 (hydrogen index of 360 and 320 mg/g Corg, respectively). In organic-carbon-normalized units these samples have hydrocarbon contents of 12,700 and 21,500 ng/g Corg, respectively, indicating the immaturity of their kerogens. Unusually high organic-carbonnormalized yields are associated with samples that are extremely lean in organic carbon. It is most likely that they are enriched by small amounts of migrated light hydrocarbons. This applies even to those samples with high organic-carbon contents (1.3-2.2%) of Sections 603B-28-4, 603B-29-1, 603B-49-2, and 603B-49-3, because they have an extremely low hydrocarbon potential (hydrogen index between 40 and 60 mg/g Corg). Nearly all samples were found to be contaminated by varying amounts of acetone that is used routinely in large quantities on board ship during core-cutting procedures. Therefore, 48 samples from the original set of 75 collected had to be excluded from the present study.

(Table 1) Low molecular weight hydrocarbon concentrations and organic carbon contents at DSDP Hole 71-511

C2-C8 hydrocarbons (36 compounds identified) from 56 shipboard sealed, deep-frozen core samples of DSDP Leg 71, Site 511, Falkland Plateau, South Atlantic, were analyzed by a combined hydrogen stripping-thermovaporization method. Concentrations, which represent hydrocarbons dissolved in the pore water and adsorbed to the mineral surfaces of the sediment, vary from 24 ng/g of dry weight sediment in Lithologic Unit 4 to 17,400 ng/g in Lithologic Unit 6 ("black shale" unit). Likewise, the organic carbon normalized C2-C8 hydrocarbon concentrations range from 104 to 3.5 x 105 ng/g Corg. The latter value is more than one order of magnitude lower than expected for petroleum source beds in the main phase of oil generation. The low maturity at 600 meters depth is further supported by light hydrocarbon concentration ratios. The change of the kerogen type from Lithologic Unit 5 (Type III) to 6 (Type II) is evidenced by changes in the C6 and C7 hydrocarbon composition. Redistribution phenomena are observed close to the Tertiary-Cretaceous unconformity and at the contact between the "black shale" unit and the overlying Cretaceous chalks and claystones. Otherwise, the low molecular weight hydrocarbons in Hole 511 are formed in situ and remain at their place of formation. The core samples turned out to be contaminated by large quantities of acetone, which is routinely used as a solvent during sampling procedures onboard Glomar Challenger.

Molybdenum and sulphur isotope ratios of early Pleistocene sapropels of ODP Hole 160-969D

Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes. In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition. Molybdenum isotopes of these 8 sapropels encompass a range of values (d98Mo = +0.2 to +1.7), but are all 98Mo-depleted relative to seawater (d98Mo = +2.3 per mil), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest d98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative d98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments.