Accumulation rates of bulk sediment and total organic carbon for Ontong Java Plateau sediments

Organic geochemical investigations were performed on sediments of Leg 130 to reconstruct the depositional environment of the Ontong Java Plateau. The Miocene to Quaternary sediments collected during the drilling campaign are characterized by extremely low organic carbon contents. As indicated by C/N ratios and Rock-Eval data, most of the organic matter is probably of marine origin. Based on mass-accumulation rates of organic carbon, the paleoproductivity for the Miocene-Pliocene and the late Pliocene-Pleistocene time intervals as well as the modern surface-water production were estimated. The productivity values of the surface sediments (25-59 gC/m2/yr) reflect the various influences of the equatorial upwelling cell on the different sites. The accumulation rates of organic carbon are generally low; however, they show a distinct increase at 8 Ma and a decrease at 2 Ma.

(Table 1) Carbohydrates, free sugars, and acid-extractable sugar at DSDP Leg 64 Holes

We analyzed 10 core samples of Pleistocene and Pliocene sediment for residual carbohydrates. All yielded positive results for total carbohydrates and acid-extractable glucose. We also detected galactose, mannose, arabinose, xylose, and traces of ribose and fucose in the Pleistocene samples. In the Pliocene samples we found only rare mannose. Only one Pleistocene sample yielded measurable cellulose and amylose.

Phosphorus components and barite in untreated and in carbonate-free sediments of ODP Hole 199-1221C

We determined changes in equatorial Pacific phosphorus (µmol P/g) and barite (BaSO4; wt%) concentrations at high resolution (2 cm) across the Paleocene/Eocene (P/E) boundary in sediments from Ocean Drilling Program (ODP) Leg 199 Site 1221 (153.40 to 154.80 meters below seafloor [mbsf]). Oxide-associated, authigenic, and organic P sequentially extracted from bulk sediment were used to distinguish reactive P from detrital P. We separated barite from bulk sediment and compared its morphology with that of modern unaltered biogenic barite to check for diagenesis. On a CaCO3-free basis, reactive P concentrations are relatively constant and high (323 µmol P/g or ~1 wt%). Barite concentrations range from 0.05 to 5.6 wt%, calculated on a CaCO3-free basis, and show significant variability over this time interval. Shipboard measurements of P and Ba in bulk sediments are systematically lower (by ~25%) than shore-based concentrations and likely indicate problems with shipboard standard calibrations. The presence of Mn oxides and the size, crystal morphology, and sulfur isotopes of barite imply deposition in sulfate-rich pore fluids. Relatively constant reactive P, organic C, and biogenic silica concentrations calculated on a CaCO3-free basis indicate generally little variation in organic C, reactive P, and biogenic opal burial across the P/E boundary, whereas variable barite concentrations indicate significant changes in export productivity. Low barite Ba/reactive P ratios before and immediately after the Benthic Extinction Event (BEE) may indicate efficient nutrient burial, and, if nutrient burial and organic C burial are linked, high relative organic C burial that could temporarily drawdown CO2 at this site. This interpretation requires postdepositional oxidation of organic C because organic C to reactive P ratios are low throughout the section. After the BEE, higher barite Ba/reactive P ratios combined with higher barite Ba concentrations may imply that higher export productivity was coupled with unchanged reactive P burial, indicating efficient nutrient and possibly also organic C recycling in the water column. If the nutrient recycling is decoupled from organic C, the high export production could be indicative of drawdown of CO2. However, the observation that organic C burial is not high where barite burial is high may imply that either C sequestration was restricted to the deep ocean and thus occurred only on timescales of the deep ocean mixing or that postdepositional oxidation (burn down) of organic matter affected the sediments. The decoupling of barite and opal may result from low opal preservation or production that is not diatom based.

Free and dissolved gases in the area of the Calypso hydrothermal vent, Bay of Plenty, New Zealand

Gas composition and hydrochemistry of bottom waters of the Bay of Plenty in the hydrothermally active zone of the Pacific island arc are investigated. Methane content in underwater vents is an order of magnitude greater than that in volcanic exhalations on the land. Salinity, pH, total content of CO2, its partial pressure, and silica content also differ. Correlations between gas parameters, hydrochemical parameters, and biological and microbiological parameters are identified.

Maps and acoustic profiles of free gas distribution, Western Baltic Sea and Kattgat-Skagerrak area

A shallow gas depth-contour map covering the Skagerrak-western Baltic Sea region has been constructed using a relatively dense grid of existing shallow seismic lines. The digital map is stored as an ESRI shape file in order to facilitate comparison with other data from the region. Free gas usually occurs in mud and sandy mud but is observed only when sediment thickness exceeds a certain threshold value, depending on the water depth of the area in question. Gassy sediments exist at all water depths from approx. 20 m in the coastal waters of the Kattegat to 360 m in the Skagerrak. In spite of the large difference in water depths, the depth of free gas below seabed varies only little within the region, indicating a relatively fast movement of methane in the gas phase towards the seabed compared to the rate of diffusion of dissolved methane. Seeps of old microbial methane occur in the northern Kattegat where a relatively thin cover of sandy sediments exists over shallow, glacially deformed Pleistocene marine sediments. Previous estimates of total methane escape from the area may be correct but the extrapolation of local methane seepage rate data to much larger areas on the continental shelf is probably not justified. Preliminary data on porewater chemistry were compared with the free gas depth contours in the Aarhus Bay area, which occasionally suffers from oxygen deficiency, in order to examine if acoustic gas mapping may be used for monitoring the condition of the bay.

Total organic carbon content, methane isotopic composition and hydrocarbon characteristics of ODP Leg 180 sites

The organic geochemistry of Sites 1108 and 1109 of the Woodlark Basin, offshore Papua New Guinea, was studied to determine whether thermally mature hydrocarbons were present in the penetrated section and, if present, whether they are genetically related to the penetrated "coaly" interval. Both the organic carbon and pyrolysis data indicate that there is no significant hydrocarbon source-rock potential at Site 1108. The hydrocarbons encountered during drilling appear to be indigenous and not migrated products or contaminants. In contrast, the coaly interval at Site 1109 contains zones with significant hydrocarbon-generation potential. Several independent lines of evidence indicate that the coaly sequence encountered at Site 1109 is thermally immature. The Site 1108 methane stable-carbon isotope composition does not display a clear trend with depth as would be expected if it was solely reflecting a maturation profile. The measured isotopic composition of methane has most probably been altered by fractionation during sample handling and storage. This fractionation would result in isotopically heavier values than would be obtained on free gas. The organic geochemical data gathered indicate that Site 1108 can be safely revisited and that the organic-rich sediments encountered at Site 1109 were not the source of the gas encountered at Site 1108.

(Table 1) Redox potential and chemical element contents in CaCO3 and amorphous silica free matter in surface layer bottom sediments of the Pacific Ocean from Hawaiian Islands to Mexico coast

Distribution of Fe, Mn, P, Ti, Cu, Ni, Co, V, Cr, W, Mo, and As in the surface sediment layer on the section from the Hawaiian Islands to the coast of Mexico (Mexico section) is studied. Contents of all studied elements increase from biogenic-terrigenous sediments off the coast of Mexico to pelagic red clays of the Northeast Basin, and more sharply for mobile elements - Mn, Mo, Cu, Ni, Co, and As. In near Hawaii sediments rich in coarsely fragmented volcanic-terrigenous and pyroclastic material of basaltic composition with high contents of Ti, Fe, V, Cr, W, and P, contents of these elements increase sharply, and contents of Mn, Mo, Ni, Co, and Cu for the same reason decrease sharply in comparison with red clay. Abnormally high contents of Mn, Mo, Cu, Ni, Co, and As in the upper layer of hemipelagic and transition sediments of the Mexico section result from diagenetic redistribution and their accumulation on the surface. Processes of diagenetic redistribution in hemipelagic and transition sediment mass of the Mexico section are more rapid than in similar sediments of the Japan section due lower sedimentation rates and higher initial concentrations of Mn. Basic similarity of element distribution regularities in sediments of Japan and Mexico sections is shown.