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This contribution summarizes the current state of understanding and recent advances made in the field of stable thallium (Tl) isotope geochemistry. High precision measurements of Tl isotope compositions were developed in the late 1990s with the advent of multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and subsequent studies revealed that Tl, despite the small relative mass difference of the two isotopes, exhibits substantial stable isotope fractionation, especially in the marine environment. The most fractionated reservoirs identified are ferromanganese sediments with ɛ 205 Tl ≈ +15 and low temperature altered oceanic crust with ɛ 205 Tl ≈ −20. The total isotopic variability of more than 35 ɛ 205 Tl-units hence exceeds the current analytical reproducibility of the measurement technique by more than a factor of 70. This isotopic variation can be explained by invoking a combination of conventional mass dependent equilibrium isotope effects and nuclear field shift isotope fractionation, but the specific mechanisms are still largely unaccounted for. Thallium isotopes have been applied to investigate paleoceanographic processes in the Cenozoic and there is evidence to suggest that Tl isotopes may be utilized as a monitor of the marine manganese oxide burial flux over million year time scales. In addition, Tl isotopes can be used to calculate the magnitude of hydrothermal fluid circulation through ocean crust. It has also been shown that the subduction of marine ferromanganese sediments can be detected with Tl isotopes in lavas erupted in subduction zone settings as well as in ocean island basalts. Meteorite samples display Tl isotope variations that exceed the terrestrial range with a total variability of about 50 ɛ 205 Tl. The large isotopic diversity, however, is generated by both stable Tl isotope fractionations, which reflect the highly volatile and labile cosmochemical nature of the element, and radiogenic decay of extinct 205 Pb to 205 Tl with a half-life of about 15 Ma. The difficulty of deconvolving these two sources of isotopic variability restricts the utility of both the 205 Pb– 205 Tl chronometer and the Tl stable isotope system to inform on early solar system processes.

The presence of large densities of the piston-pumping polychaete Lanice conchilega can have important consequences for the functioning of marine sediments. It is considered both an allogenic and an autogenic ecosystem engineer, affecting spatial and temporal biogeochemical gradients (oxygen concentrations, oxygen penetration depth and nutrient concentrations) and physical properties (grain size) of marine sediments, which could affect functional properties of sediment-inhabiting microbial communities. Here we investigated whether density-dependent effects of L. conchilega affected horizontal (m-scale) and vertical (cm-scale) patterns in the distribution, diversity and composition of the typical nosZ gene in the active denitrifying organisms. This gene plays a major role in N2O reduction in coastal ecosystems as the last step completing the denitrification pathway. We showed that both vertical and horizontal composition and richness of nosZ gene were indeed significantly affected when large densities of the bio-irrigator were present. This could be directly related to allogenic ecosystem engineering effects on the environment, reflected in increased oxygen penetration depth and oxygen concentrations in the upper cm of the sediment in high densities of L. conchilega. A higher diversity (Shannon diversity and inverse Simpson) of nosZ observed in patches with high L. conchilega densities (3,185–3,440 ind. m-2) at deeper sediment layers could suggest a downward transport of NO3− to deeper layers resulting from bio-irrigation as well. Hence, our results show the effect of L. conchilega bio-irrigation activity on denitrifying organisms in L. conchilega reefs.

WOS:000408783500011; International audience; Deep-sea fans are well known depot centers for organic carbon that should promote sulfate reduction. At the same time, the high rates of deposition of unconsolidated metal oxides from terrigenous origin may also promote metal-reducing microbial activity. To investigate the eventual coupling between the iron and sulfur cycles in these environments, shallow sediment cores ( \textless 50 cm) across various channels and levees in the Congo River deep-sea fan (similar to 5000 m) were profiled using a combination of geochemical methods. Interestingly, metal reduction dominated suboxic carbon remineralization processes in most of these sediments, while dissolved sulfide was absent. In some 'hotspot' patches, however, sulfate reduction produced large sulfide concentrations which supported chemosynthetic-based benthic megafauna. These environments were characterized by sharp geochemical boundaries compared to the iron-rich background environment, suggesting that FeS precipitation efficiently titrated iron and sulfide from the pore waters. A companion study demonstrated that methanogenesis was active in the deep sediment layers of these patchy ecosystems, suggesting that sulfate reduction was promoted by alternative anaerobic processes. These highly reduced habitats could be fueled by discrete, excess inputs of highly labile natural organic matter from Congo River turbidites or by exhumation of buried sulfide during channel flank erosion and slumping. Sulfidic conditions may be maintained by the mineralization of decomposition products from local benthic macrofauna or bacterial symbionts or by the production of more crystalline Fe(III) oxide phases that are less thermodynamically favorable than sulfate reduction in these bio-turbated sediments. Overall, the iron and sulfur biogeochemical cycling in this environment is unique and much more similar to a coastal ecosystem than a deep-sea environment.

Cold-water coral ecosystems are considered hot-spots of biodiversity and biomass production and may be a regionally important contributor to carbonate production. The impact of these ecosystems on biogeochemical processes and carbonate preservation in associated sediments were studied at Røst Reef and Traenadjupet Reef, two modern (post-glacial) cold-water coral reefs on the Mid-Norwegian shelf. Sulfate and iron reduction as well as carbonate dissolution and precipitation were investigated by combining pore-water geochemical profiles, steady state modeling, as well as solid phase analyses and sulfate reduction rate measurements on gravity cores of up to 3.25 m length. Low extents of sulfate depletion and dissolved inorganic carbon (DIC) production, combined with sulfate reduction rates not exceeding 3 nmol S cm−3 d−1, suggested that overall anaerobic carbon mineralization in the sediments was low. These data showed that the coral fragment-bearing siliciclastic sediments were effectively decoupled from the productive pelagic ecosystem by the complex reef surface framework. Organic matter being mineralized by sulfate reduction was calculated to consist of 57% carbon bound in CH2O groups and 43% carbon in -CH2- groups. Methane concentrations were below 1 μM, and failed to support the hypothesis of a linkage between the distribution of cold-water coral reefs and the presence of hydrocarbon seepage. Reductive iron oxide dissolution linked to microbial sulfate reduction buffered the pore-water carbonate system and inhibited acid-driven coral skeleton dissolution. A large pool of reactive iron was available leading to the formation of iron sulfide minerals. Constant pore-water Ca2+, Mg2+ and Sr2+ concentrations in most cores and decreasing Ca2+ and Sr2+ concentrations with depth in core 23–18 GC indicated diagenetic carbonate precipitation. This was consistent with the excellent preservation of buried coral fragments.

Burial of organic matter (OM) plays an important role in marine sediments, linking the short-term, biological carbon cycle with the long-term, geological subsurface cycle. It is well established that low-oxygen conditions promote organic carbon burial in marine sediments. However, the mechanism remains enigmatic. Here we report biochemical quality, microbial degradability, OM preservation and accumulation along an oxygen gradient in the Indian Ocean. Our results show that more OM, and of biochemically higher quality, accumulates under low oxygen conditions. Nevertheless, microbial degradability does not correlate with the biochemical quality of OM. This decoupling of OM biochemical quality and microbial degradability, or bioavailability, violates the ruling paradigm that higher quality implies higher microbial processing. The inhibition of bacterial OM remineralisation may play an important role in the burial of organic matter in marine sediments and formation of oil source rocks.

The first comprehensive dataset (492 samples) of dissolved Mn in the Southern Ocean shows extremely low values of 0.04 up to 0.64 nM in the surface waters and a subsurface maximum with an average concentration of 0.31 nM (n=20; S.D.=0.08 nM). The low Mn in surface waters correlates well with the nutrients PO4 and NO3 and moderately well with Si(OH)(4) and fluorescence. Furthermore, elevated concentrations of Mn in the surface layer coincide with elevated Fe and light transmission and decreased export (Th-234/U-238 deficiency) and fluorescence. It appears that Mn is a factor of importance in partly explaining the HNLC conditions in the Southern Ocean, in conjunction with significant controls by the combination of Fe limitation and light limitation. No input of Mn from the continental margins was observed. This is ascribed to the protruding continental ice sheet that covers the shelf and shuts down the usual biological production, microbial breakdown and sedimentary geochemical cycling. The low concentrations of Mn in the deep ocean basins (0.07-0.23 nM) were quite uniform, but some elevations were observed. The highest deep concentrations of Mn were observed at the Bouvet Triple Junction region and coincided with high concentrations of Fe and are deemed to be from hydrothermal input. The deep basins on both sides of the ridge were affected by this input. In the deep Weddell Basin the input of Weddell Sea Bottom Water appears to be the source of the slightly elevated concentrations of Mn in this water layer. (C) 2010 Elsevier Ltd. All rights reserved.

The influence of riverine inputs on biogeochemical cycling and organic matter recycling in sediments on the continental shelf off the Rhone River mouth (NW Mediterranean Sea) was investigated by measuring sediment oxygen uptake rates using a combination of in situ and laboratory techniques. Four stations were investigated during two cruises in June 2001 and June 2002, with depths ranging from 9 to 192 m and over a distance to the Rhone River mouth ranging from 4 to 36 km. Diffusive oxygen uptake (DOU) rates were determined using an in situ sediment microprofiler and total oxygen uptake (TOU) rates were measured using sediment core incubations. There was good agreement between these two techniques which indicates that the non-diffusive fraction of the oxygen flux was minimal at the investigated stations. DOU rates ranged from 3.7 +/- 0.4 mmol O-2 m(-2) d(-1) at the continental shelf break to 19.3 +/- 0.5 mmol O-2 m(-2) d(-1) in front of the Rhone River mouth. Sediment oxygen uptake rates mostly decreased with increasing depth and with distance from the Rhone mouth. The highest oxygen uptake rate was observed at 63 m on the Rhone prodelta, corresponding to intense remineralization of organic matter. This oxygen uptake rate was much larger than expected for the increasing bathymetry, which indicates that biogeochemical cycles and benthic deposition are largely influenced by the Rhone River inputs. This functioning was also supported by the detailed spatial distribution of total organic carbon (TOC), total nitrogen (TN) and C/N atomic ratio in surficial sediments. Sediments of the Rhone prodelta are enriched in organic carbon (2-2.2%) relative to the continental shelf sediments (< 1%) and showed C/N ratios exceeding Redfield stoichiometry for fresh marine organic matter. A positive exponential correlation was found between DOU and TOC contents (r(2) = 0.98, n = 4). South-westward of the Rhone River mouth, sediments contained highly degraded organic matter of both terrestrial and marine origin, due to direct inputs from the Rhone River, sedimentation of marine organic matter and organic material redeposition after resuspension events. (c) 2008 Elsevier Ltd. All rights reserved.

A novel sulfate-reducing bacterium, strain HB1(T), was isolated from an upflow anaerobic sludge blanket (UASB) reactor treating paper-mill wastewater operated at 37 degrees C. Cells of strain HB1(T) were oval to rod-shaped, 1-1.3 microm wide and 2.6-3.5 microm long and Gram-negative. The optimum temperature for growth was 28-30 degrees C. In the presence of sulfate, the isolate was able to grow on H(2)/acetate, formate, ethanol, propionate, fumarate, succinate, butyrate, crotonate, catechol, benzoate, 4-hydroxybenzoate, palmitate and stearate. The isolate only grew on H(2) when acetate was added as a carbon source; when grown on formate, acetate was not required. Growth was also possible on pyruvate and crotonate without an electron acceptor. The isolate showed very poor growth on acetate. Thiosulfate and sulfate were used as electron acceptors. Phylogenetic analysis of 16S rRNA gene sequences revealed that strain HB1(T) represents a novel lineage within the Deltaproteobacteria; sequence similarities between strain HB1(T) and members of other related genera were less than 91%. Strain HB1(T) was also distinguished from members of related genera based on differences in several phenotypic characteristics. It is a member of the family Desulfobacteraceae. The major cellular fatty acids of strain HB1(T) were C(16:0), iso-C(15:0), anteiso-C(15:0) and C(14:0). beta-Hydroxy fatty acids were also present in the range of C(14:0) to C(18:0), of which C(16:0) was the most abundant. The G+C content of the DNA was 55.1 mol%. Based on physiological, biochemical and chemotaxonomic traits together with results of comparative 16S rRNA gene sequence analysis, strain HB1(T) is considered to represent a novel species in a new genus, for which the name Desulfatirhabdium butyrativorans gen. nov., sp. nov. is proposed. The type strain of Desulfatirhabdium butyrativorans is HB1(T) (=DSM 18734(T) =JCM 14470(T)).