Major nannofossils, carbon and oxygen isotopes and carbon composition of ODP leg 194 site survey gravity cores

Biostratigraphic, sedimentologic, and geochemical analyses of hemipelagic periplatform sediments from shallow gravity cores taken during the Ocean Drilling Program Leg 194 site survey reveal that, despite the strong currents and almost infilled intraplatform bathymetric depressions, recent sedimentation at the location of the Leg 194 drill sites recorded glacial-interglacial cycles. Sediment analyses included determination of sediment type, carbonate content, bulk stable oxygen isotope composition, and calcareous nannofossil zones. Glacial periods, identified by elevated bulk d18O, are characterized by darker sediment color, coarser grain size, and lower carbonate content, whereas interglacial periods yield lighter-colored, finer, and carbonate-rich sediments. These data from the shallowmost few meters of Marion Plateau sediments complement the subsurface information of Leg 194 holes, in which the top few meters have not been analyzed in such a high-resolution fashion. In addition, these gravity cores are more likely to have recovered the sediments closest to the sediment/water interface as compared to the hydraulic piston cores collected during Leg 194.

Lithology and organic geochemistry at DSDP Hole 77-535

Analyses of extractable organic matter from selected core samples obtained at DSDP Site 535 in the eastern Gulf of Mexico show that the asphalt (or tar) and adjacent oil stains in Lower Cretaceous fractured limestones have a common origin and are not derived from the surrounding organic-matter-rich limestones. Organic matter indigenous to those surrounding limestones was shown to be thermally immature and incapable of yielding the hydrocarbon mixture discovered. In contrast, the oil-stained and asphaltic material appears to be a post-migration alteration product of a mature oil that has migrated from source rocks deeper in the section, or from stratigraphically equivalent but compositionally different source-facies down-dip from the drill site. Further, hydrocarbons of the altered petroleum residues were shown to be similar to Sunniland-type oils found in Lower Cretaceous rocks of South Florida. The results suggest that shallowwater, platform-type source-rock facies similar to those that generated Sunniland-type oils, or deeper-water facies having comparable oil-generating material, are present in this deep-water (> 3000 m) environment. These findings have important implications for the petroleum potential in the eastern Gulf of Mexico and for certain types of deep-sea sediments.

C1-C8 hydrocarbons in DSDP Leg 64 Holes sediments

Sediments from the Baja California Continental Margin Transect - Sites 474 and 476 - showed small amounts of C2-C8 hydrocarbons and functionalized compounds (alkenes) typical of organic-rich, Recent, cold (<30°C) marine sediments. In contrast, some samples from Sites 477, 478, 479, and Hole 481A in the Guaymas Basin, an active spreading center, showed the characteristics of thermally generated hydrocarbons. These include an increase (sometimes exponential) in amount and diversity of C2-C8 hydrocarbons and a decrease in alkenes in more thermally mature sediments. The results indicate that the injection of basaltic sills has minimal effect on C2-C8 hydrocarbon generation except in the immediate vicinity of the sill. The absence of light hydrocarbons close to the hottest sills suggests that the compounds distill away as they are formed in these areas of very active hydrothermal circulation. A sample of young sediment exposed to very high temperatures (>300°C) from deeper thermal sources at the hottest site, 477, showed a very limited hydrocarbon distribution, including primarily ethane, benzene, and toluene, together with smaller amounts of propane and butane.

Manganese contents and accumulation rates in reduced bottom sediments of the Sea of Okhotsk

Manganese contents in reduced sediments and accumulation rates were investigated. Their values in sediments of most of cores are background (0.03-0.07 %).Anomalous concentrations (up to 2.5 %) and accumulation rates (up to 60 mg/cm**2/ka) occur near the known region of hydrothermal barite mineralization in the Derugin Basin. High accumulation rates of Mn (>10 mg/cm**2/ka) also occur in Holocene sediments to south-east from the Derugin Basin. It can be assumed that high Mn contents and accumulation rates occur there due to transportation of Mn-rich water from the Derugin Basin in the near-bottom layer under the lower border of the Sea of Okhotsk Intermediate Water. Intensive Mn accumulation is also typical for the South Okhotsk Basin near the Bussol Strait. Mn accumulation rates of glacial sediments of the second oxygen isotope stage are less significant, which is presumed to be caused by paleoceanological reasons.

Stable carbon isotopes, Methane Index, and distributions of different GDGTs

Gas hydrates represent one of the largest pools of readily exchangeable carbon on Earth's surface. Releases of the greenhouse gas methane from hydrates are proposed to be responsible for climate change at numerous events in geological history. Many of these inferred events, however, were based on carbonate carbon isotopes which are susceptible to diagenetic alterations. Here we propose a molecular fossil proxy, i.e., the "Methane Index (MI)", to detect and document the destabilization and dissociation of marine gas hydrates. MI consists of the relative distribution of glycerol dibiphytanyl glycerol tetraethers (GDGTs), the core membrane lipids of archaea. The rational behind MI is that in hydrate-impacted environments, the pool of archaeal tetraether lipids is dominated by GDGT-1, -2 and -3 due to the large contribution of signals from the methanotrophic archaeal community. Our study in the Gulf of Mexico cold-seep sediments demonstrates a correlation between MI and the compound-specific carbon isotope of GDGTs, which is strong evidence supporting the MI-methane consumption relationship. Preliminary applications of MI in a number of hydrate-impacted and/or methane-rich environments show diagnostic MI values, corroborating the idea that MI may serve as a robust indicator for hydrate dissociation that is useful for studies of global carbon cycling and paleoclimate change.

Lead 210 and silicate profiles from sediments of the equatorial Pacific and South Atlantic

Particle mixing rates have been determined for 5 South Atlantic/Antarctic and 3 equatorial Pacific deep-sea cores using excess 210Pb and 32Si measurements. Radionuclide profiles from these siliceous, calcareous, and clay-rich sediments have been evaluated using a steady state vertical advection diffusion model. In Antarctic siliceous sediments210Pb mixing coefficients (0.04-0.16 cm**2/y) are in reasonable agreement with the 32Si mixing coefficient (0.2 or 0.4 cm**2/y, depending on 32Si half-life). In an equatorial Pacific sediment core, however, the 210Pb mixing coefficient (0.22 cm**2/y) is 3-7 times greater than the 32Si mixing coefficient (0.03 or 0.07 cm**2/y). The difference in 210Pb and 32Si mixing rates in the Pacific sediments results from: (1) non-steady state mixing and differences in characteristic time and depth scales of the two radionuclides, (2) preferential mixing of fine-grained clay particles containing most of the 210Pb activity relative to coarser particles (large radiolaria) containing the 32Si activity, or (3) the supply of 222Rn from the bottom of manganese nodules which increases the measured excess 210Pb activity (relative to 226Ra) at depth and artificially increases the 210Pb mixing coefficient. Based on 32Si data and pore water silica profiles, dissolution of biogenic silica in the sediment column appears to have a minor effect on the 32Si profile in the mixed layer. Deep-sea particle mixing rates reported in this study and the literature do not correlate with sediment type, sediment accumulation rate, or surface productivity. Based on differences in mixing rate among three Antarctic cores collected within 50 km of each other, local variability in the intensity of deep-sea mixing appears to be as important as regional differences in sediment properties.

Stable carbon and oxygen isotope ratios of Planulina wuellerstorfi from sediments of the Sierra Leone Rise

Causes of change in deep water delta13C can be either global or local in extent. Global causes include (1) climatically-induced changes in the amount of terrestrial biomass which alter the average carbon isotopic composition of the oceanic reservoir (Shackleton, 1977), and (2) erosion and deposition of organic-rich, continental shelf sediments during sea level fluctuations which change the mean oceanic carbon: phosphorus ratio (Broecker, 1982 doi:10.1016/0079-6611(82)90007-6). Regional gradients of delta13C are created by remineralization of organic detritus within the deep ocean itself thus reflecting the distribution of water masses and modern thermohaline flow. Changes in a single geological record of benthic foraminiferal delta13C can result from any combination of these global and abyssal circulation effects. By sampling a large number of cores collected over a wide bathymetric range yet confined to a small geographical region we have minimized the ambiguity. We can assume that each delta13C record was equally affected by global causes of delta13C variation. The differences seen between the delta13C records must, therefore, reflect changes in the distribution of delta13C in the deep ocean. We interpret these differences in distribution in terms of changes in the ocean's abyssal circulation. Benthic foraminiferal carbon isotopic evidence from a suite of Sierra Leone Rise cores indicates that the deeper parts of the eastern Atlantic basins underwent a reduction in [O2] during the maximum of the last glaciation. Reduced advection of O2-rich deep water through low-latitude fracture zones, associated with increased delivery of organic matter to the deep ocean, lowered the delta13C of deep water SumCO2 at all depths below the sill separating the eastern and western Atlantic basins (Metcalf et al., 1964 doi:10.1016/0011-7471(64)91078-2). This decreased advection into the eastern Atlantic Ocean coincides with the overall decrease in deep water production in the North Atlantic during the last glacial maximum (Curry and Lohmann, 1982 doi:10.1016/0033-5894(82)90071-0; Boyle and Keigwin, 1982 doi:10.1126/science.218.4574.784; Schnitker, 1979 doi:10.1016/0377-8398(79)90020-3; Streeter and Shackleton, 1979 doi:10.1126/science.203.4376.168).

Chemical and isotope composition of carbonate nodules and host bottom sediments from Core DM9-666, Gulf of California

Results of mineralogical and isotopic analyzes of sulfur and carbon in carbonate nodules and host bottom sediments and results of 14C measurement in carbonate nodules are reported. It is proved that the carbonate nodules formed 11-22 thousand years ago in anaerobic diagenesis of bottom sediments rich in organic matter. Isotopic light metabolic carbon dioxide was a source of carbonate for nodules. It formed during microbial degradation of organic matter of bottom sediments.