Anaerobic oxidation of methane and sulphate reduction rates, hydrocarbons, sulphate and sulphide concentrations of sediment cores from the Larsen B embayment at the eastern site of the Antarctic Peninsula

First videographic indication of an Antarctic cold seep ecosystem was recently obtained from the collapsed Larsen B ice shelf, western Weddell Sea (Domack et al., 2005). Within the framework of the R/V Polarstern expedition ANTXXIII-8, we revisited this area for geochemical, microbiological and further videographical examinations. During two dives with ROV Cherokee (MARUM, Bremen), several bivalve shell agglomerations of the seep-associated, chemosynthetic clam Calyptogena sp. were found in the trough of the Crane and Evans glacier. The absence of living clam specimens indicates that the flux of sulphide and hence the seepage activity is diminished at present. This impression was further substantiated by our geochemical observations. Concentrations of thermogenic methane were moderately elevated with 2 µM in surface sediments of a clam patch, increasing up to 9 µM at a sediment depth of about 1 m in the bottom sections of the sediment cores. This correlated with a moderate decrease in sulphate from about 28 mM at the surface down to 23.4 mM, an increase in sulphide to up to 1.43 mM and elevated rates of the anaerobic oxidation of methane (AOM) of up to 600 pmol cm**-3 d**-1 at about 1 m below the seafloor. Molecular analyses indicate that methanotrophic archaea related to ANME-3 are the most likely candidates mediating AOM in sediments of the Larsen B seep.

Gas hydrate structures and C1-C5 hydrocarbons at individual sites in the Gulf of Mexico

Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities. The crystallographic structures were established by X-ray diffraction, using both conventional X-ray sources and high-energy synchrotron radiation. The microstructures were examined by cryo-stage Field-Emission Scanning Electron Microscopy (FE-SEM). Good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses. Gas hydrate structure II samples from the Green Canyon in the northern GOM had methane concentrations of 70-80% and up to 30% of C2-C5 of measured hydrocarbons. Hydrocarbons in the crystallographic structure I hydrate from the Chapopote asphalt volcano in the southern GOM was comprised of more than 98% methane. Fairly different microstructures were identified for those different hydrates: Pores measuring 200-400 nm in diameter were present in structure I gas hydrate samples; no such pores but dense crystal surfaces instead were discovered in structure II gas hydrate. The stability of the hydrate samples is discussed regarding gas composition, crystallographic structure and microstructure. Electron microscopic observations showed evidence of gas hydrate and liquid oil co-occurrence on a micrometer scale. That demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix.

Sedimentary climate indicators of sediment core MD08-3167 and GeoB1711-4, southwestern Africa

The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax dD and View the MathML source taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax dD and higher View the MathML source (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

Geochemistry and K-Ar dates of the Mazagan granodiorite at V30-225, VA79-96KD and DSDP Hole 79-544A, off Moroccan coast

Gneissic granodiorite was recovered by drilling at the base of the Mazagan escarpment, 100 km west of the Casablanca, Morocco, at 4000 m water depth. Coarse, predeformative muscovite yielded dates of -515 Ma, fine-grained muscovite of -455 Ma, biotite -360 and 335 Ma, and feldspar -315 Ma. These dates are tentatively correlated with the microscopic results. We assume a minimum age of middle Cambrian for the granodiorite, an Ordovician deformation and mylonitization, and a Late Carboniferous overprint under upper greenschist facies conditions.

Geochemistry of basalts from ODP Hole 109-648B

Legs 106-109 achieved the first basaltic bare-rock drill hole, on a small volcano (Serocki volcano) located on the flanks of the rift valley in the MAR about 70 km south of the Kane fracture zone. Because of severe technical difficulties only 50.5 m of basalt below seafloor was recovered. Geochemical analysis shows that the recovered basalts display typical N-MORB characteristics as expected in this segment of the Mid-Atlantic ridge. The lava flows display rather equivalent geochemical characteristics all over the drilled section and show the imprint of a previous magmatic differentiation suffered by the magmas before their emission, indicative of a fractional crystallization of plagioclase-bearing cumulates. The incompatible and alkali element content of these 648B magmas is very low, a feature which resembles those of other N-MORB. The geochemical characteristics of these basalts look closely similar to those of basalts from the same flow line emitted respectively 10 m.y. (Hole 395, Legs 45-46), and 110 m.y. (Hole 417A, Legs 51-53) ago, supporting the persistence in this ridge segment of a mantle source with depleted characteristics over the last 110 m.y., but with some variations in the degree of depletion of the source along this period. Although these rocks appear fresh, the imprint of an incipient low temperature alteration can be noticed in a few samples, as evidenced by slight increases of alkali, U elements, and 87Sr/86Sr isotopic compositions.

Age of bottom sediments and minerals from the Atlantic and Pacific oceans

Application of nuclear geochronology methods in study of recent sedimentation processes, in paleoceanology, tectonics, geomorphology, and other problems associated with accumulation of sedimentary material in oceans and seas are under consideration in the book. A comparative analysis of dating results obtained by biostratigraphy, paleomagnetic and nuclear geochronology methods is given.

Chemical composition and K-Ar age of basalts from the floor of the Indian Ocean

Petrographic description as well as data on chemical composition and K-Ar age of basalts from the floor of the Indian Ocean are reported in the paper.

Concentrations of particulate organic carbon in the Kara Sea and in the Obskaya Guba (Ob River estuary) in September 1993

Contents and distribution of particulate lipids were studied by thin-layer chromatography technique with flame ionization detection (Iatroscan TH-10) along the transect from the Ob River towards the Kara Sea. Lipid contents range from 18.4 to 266 µg/l with, average 84.97 µg/l, which comprises from 4.06 to 58.32 % of total particulate organic matter. Principal constituents of particulate lipids are hydrocarbons (32.14 % of total lipids on the average), polar compounds (29.85 %), wax and sterol esters (13.04 %), and mono- and diglycerides (12.52 %). Secondary components are presented by fatty acid esters (5.14 %), free fatty acids (4.56 %), triglycerides (2.32 %), and sterols (1.04 %). Specific composition of particulate lipids along the Ob River - Kara Sea transect is formed under strong impact of river run-off. Particulate lipid composition reflects differences between processes of organic matter transformation in estuarine and marine parts of the transect, as well as peculiarities of species composition of Arctic living organisms.