Biogeochemical investigation of cold seep sediments in the Eastern Mediterranean Sea

Cold seep ecosystems are highly productive, fragmented ecosystems of the deep-sea floor. They form worldwide where methane reaches the surface seafloor, and are characterized by rich chemosynthetic communities fueled by the microbial utilization of hydrocarbons. Here we investigated with in situ (benthic chamber, microprofiler) and ex situ (pore water constituents, turnover rates of sulfate and methane, prokaryote abundance) techniques reduced sites from three different seep ecosystems in the Eastern Mediterranean deep-sea. At all three cold seep systems, the Amon Mud Volcano, Amsterdam Mud Volcano and the Nile Deep Sea Fan Pockmark area, we observed and sampled patches of highly reduced, methane-seeping sulfidic sediments which were separated by tens to hundreds of (kilo)meters with non-reduced oxygenated seafloor areas. All investigated seep sites were characterized by gassy, sulfidic sediments of blackish color, of which some were overgrown with thiotrophic bacterial mats. Fluxes of methane and oxygen, as well as sulfate reduction rates varied between the different sites.

Data on multidisciplinary research in the Tajura Rift (Gulf of Aden) during Cruise 7 of R/V Akademik Mstislav Keldysh in winter 1983-1984

In the collective monograph results of geological and geophysical studies in the Tadjura Rift carried out by conventional outboard instruments and from deep/sea manned submersibles "Pisces" in winter 1983-1984 are reported. Main features of rift tectonics, geology, petrology, and geochemistry of basalts from the rift are under consideration. An emphasis is made on lithology, stratigraphy, and geochemistry of bottom sediments. Roles of terrigenous, edafogenic, biogenic, and hydrothermal components in formation of bottom sediments from the rift zone are shown.

Bromine and iodine concentrations in sediments and pore fluids of ODP Leg 112

At the Peruvian convergent margin, two distinct pore fluid regimes are recognized from differences in their Cl- concentrations. The slope pore fluids are characterized by low Cl- concentrations, but elevated Br- and I- concentrations due to biogenic production. The shelf pore fluids exhibit elevated Cl- and Br- concentrations due to diffusive mixing with an evaporitic brine. In the slope pore fluids, the Br-, I-, and NH4+ concentrations are elevated following bacterial decomposition of organic matter, but the I- concentrations are in excess of those expected based on mass balance calculations using NH4+ and Br- concentrations. The slope sediment organic matter, which is enriched in iodine from oxidationreduction processes at the oxygenated sediment-water interface, is responsible for this enrichment. The increases in dissolved I- and the I- enrichments relative to NH4+ and Br- correlate well with sedimentation rates because of differential trapping following regeneration. The pore-fluid I-/Br- ratios suggest that membrane ion fiitration is not a major cause of the decreases in Cl- concentrations. Other possible sources for low Cl- water, including meteoric water, clathrate dissociation, and/or mineral dehydration reactions, imply that the diluting component of the slope low-Cl- fluids has flowed at least 1 km through the sediment. The low bottom-water oxygenation in the shelf is responsible for the low (if any) enrichment of iodine in the shelf sediments. Fluctuations in bottom-water oxygen concentrations in the past, however, may be responsible for the observed variations in the sediment I/Br ratios. Comparison of Na+/Cl- and Br-/Cl- molar ratios in the pore fluids shows that the shelf high-Cl- fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution. This dense brine, located below the sediment sections drilled, appears to have flowed a distance >500 km through the sediment.

Interstitial water chemistry at DSDP Leg 74 Holes

Interstitial water studies from sites drilled during a transect of the Walvis Ridge indicate that concentration increases in calcium and decreases in magnesium toward and into the basement. These trends can be understood principally in terms of reactions taking place in Layer 2 of the oceanic crust. At Site 525, however, some removal of magnesium occurs within the sediment column. Concentration maxima of dissolved strontium clearly indicate that carbonate recrystallization occurs throughout the carbonate sediments, and studies of the Sr/Ca ratio in carbonates indicate that in chalks and limestones recrystallization is essentially complete. Predictions of dissolved strontium maxima generally fail; this can be understood as removal of strontium in basal sediments and/or basalts.

Pore water chemistry of sediment core GeoB15101-7 and water chemistry of CTD casts GeoB15101-1 and GeoB15101-6

Submarine brine lakes feature sharp and persistent concentration gradients between seawater and brine, though these should be smoothed out by free diffusion in open ocean settings. The anoxic Urania basin of the Eastern Mediterranean contains an ultra sulfidic, hypersaline brine of Messinian origin above a thick layer of suspended sediments. With a dual modeling approach we reconstruct its contemporary stratification by geochemical solute transport fundamentals, and show that thermal convection is required to maintain mixing in the brine and mud layer. The origin of the Urania basin stratification was dated to 1650 years before present, which may be linked to a major earthquake in the region. The persistence of the chemoclines may be key to the development of diverse and specialized microbial communities. Ongoing thermal convection in the fluid mud layer may have important, yet unresolved consequences for sedimentological and geochemical processes, also in similar environments.

Water chemistry, and abundance and carbon biomass of under-ice fauna during POLARSTERN cruise ARK-XIX/1 to the Storfjord area in 2003

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7-10/10) prevailed and ice thickness ranged over <0.1-1.6 m covered by <0.1-0.6 m of snow. Air temperatures ranged between -1.8 and -27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0-5 m depth) were not stratified (T=-1.9 to -2.0°C and S=34.2-34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 µg chlorophyll a/l), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2-241.3 and 5.3-16.4 µg/l, respectively, the C/N ratio over 11.2-15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0-1.8 ind/m**2 for Apherusa glacialis, 0-0.7 ind/m**2 for Onisimus spp., and 0-0.8 ind/m**2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181-2,487 ind/m**3 (biomass: 70-2,439 µg C/m**3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34-1,485 ind/m**3), contributing 19-65% to total abundances, followed by copepod nauplii (85-548 ind/m**3) and the cyclopoid copepod Oithona similis (44-262 ind/m**3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.

Geochemistry, microbiology and phospholipid fatty acid content of ODP Site 169-1036 sediments

Phospholipid fatty acids were measured in samples of 60°-130°C sediment taken from three holes at Site 1036 (Ocean Drilling Program Leg 169) to determine microbial community structure and possible community replacement at high temperatures. Five of six samples had similar concentrations of phospholipid fatty acids (2-6 pmol/g dry weight of sediment), and biomass estimates from these measurements compare favorably with direct microscopic counts, lending support to previous microscopic measures of deep sedimentary biomass. Very long-chain phospholipid fatty acids (21 to 30 carbons) were detected in the sediment and were up to half the total phospholipid fatty acid measured; they appear to increase in abundance with temperature, but their significance is not known. Community composition from lipid analysis showed that samples contained standard eubacterial membrane lipids but no detectable archaeal lipids, though archaea would be expected to dominate the samples at high temperatures. Cluster analysis of Middle Valley phospholipid fatty acid compositions shows that lipids in Middle Valley sediment samples are similar to each other at all temperatures, with the exception of very long-chain fatty acids. The data neither support nor deny a shift to a high-temperature microbial community in hot cores, so at the present time we cannot draw conclusions about whether the microbes observed in these hot sediments are active.

Ion concentrations and extracelluar pH of Calanus glacialis in Billefjorden, Svalbard

Arctic shelf zooplankton communities are dominated by the copepod Calanus glacialis. This species feeds in surface waters during spring and summer and accumulates large amounts of lipids. Autumn and winter are spent in dormancy in deeper waters. Lipids are believed to play a major role in regulating buoyancy, however, they cannot explain fine-tuning of the depth distribution. To investigate whether ion exchange processes and acid-base regulation support ontogenetic migration as suggested for Antarctic copepods, we sampled C. glacialis in monthly intervals for 1 yr in a high-Arctic fjord and determined cation concentrations and the extracellular pH (pHe) in its hemolymph. During the winter/spring transition, prior to the upward migration of the copepods, Li+ ions were exchanged with cations (Na+, Mg2+, and Ca2+) leading to Li+ concentrations of 197 mmol/L. This likely decreased the density and promoted upward migration in C. glacialis. Our data thus suggest that Li+ has a biological function in this species. Ion and pHe regulation in the hemolymph were not directly correlated, but the pHe revealed a seasonal pattern and was low (5.5) in winter and high (7.9) in summer. Low pHe during overwintering might be related to metabolic depression and thus, support diapause.

Composition of sediments, hydrothermal manifestations, interstitial waters, and aeolian material from the TAG Hydrothermal Field and Central Atlantic

The book is devoted to regularities of spatial distribution, mineralogy and geochemistry of hydrothermal and hydrothermal-sedimentary manifestations of the Mid-Atlantic Ridge rift zone.