Magnesium isotope ratios of pore-fluids and dolomites

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid d26Mg values increase with depth by as much as 2 per mil. Because carbonates preferentially incorporate 24Mg (low d26Mg), the increase in pore-fluid d26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid d26Mg values decrease with depth by up to 2 per mil. The decline in pore-fluid d26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion-advection-reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured d26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 per mil depleted in d26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in d26Mg by 0 per mil to +1.25 per mil relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.

Prokaryotic growth and presence of metabolic products and contamination tracers in rocks samples from ODP Leg 187 sites

The microbial population in samples of basalt drilled from the north of the Australian Antarctic Discordance (AAD) during Ocean Drilling Program Leg 187 were studied using deoxyribonucleic acid (DNA)-based methods and culturing techniques. The results showed the presence of a microbial population characteristic for the basalt environment. DNA sequence analysis revealed that microbes grouping within the Actinobacteria, green nonsulfur bacteria, the Cytophaga/Flavobacterium/Bacteroides (CFB) group, the Bacillus/Clostridium group, and the beta and gamma subclasses of the Proteobacteria were present in the basalt samples collected. The most dominant phylogenetic group, both in terms of the number of sequences retrieved and the intensities of the DNA bands obtained with the denaturing gradient gel electrophoresis analysis, was the gamma Proteobacteria. Enrichment cultures showed phylogenetic affiliation with the Actinobacteria, the CFB group, the Bacillus/Clostridium group, and the alpha, beta, gamma, and epsilon subclasses of the Proteobacteria. Comparison of native and enriched samples showed that few of the microbes found in native basalt samples grew in the enrichment cultures. Only seven clusters, two clusters within each of the CFB and Bacillus/Clostridium groups and five clusters within the gamma Proteobacteria, contained sequences from both native and enriched basalt samples with significant similarity. Results from cultivation experiments showed the presence of the physiological groups of iron reducers and methane producers. The presence of the iron/manganese-reducing bacterium Shewanella was confirmed with DNA analysis. The results indicate that iron reducers and lithotrophic methanogenic Archaea are indigenous to the ocean crust basalt and that the methanogenic Archaea may be important primary producers in this basaltic environment.

Boron concentrations and isotope ratios of authigenic carbonates from the Oregon margin

Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, Oregon margin. Carbonate precipitates are the most widespread authigenic phase in the shallow accretionary wedge and carry chemical information about long-term variations in pore fluid origin and flow paths in the Cascadia subduction zone. Drilling the first ridge (toe area including the frontal thrust) and the second ridge (or Hydrate Ridge) of the prism demonstrated different fluid regimes, with higher B contents in the authigenic precipitates at the toe. The delta11B of 18 authigenic precipitates analysed ranges from 13.9 per mil to as high as 39.8 per mil, extending the upper range of previously reported carbonate delta11B values considerably. When related to the delta11B ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with delta13C and delta18O, delta11B ratios of the carbonate concretions, nodules and crusts allow one to distinguish between precipitation influenced by (i) seawater, (ii) fluid reservoirs at different depth levels within the accretionary prism and (iii) cage water from dissociated gas hydrates, the latter possibly indicating a fluctuation of the bottom simulating reflector during most recent Earth's history. From this first systematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exceed those reported for other marine carbonates. Given the abundance of such precipitates at convergent margins, they represent a significant B sink in geochemical cycling. Isotopic compositions of the parent fluids to the carbonates mirror B chemistry of modern pore waters from convergent margins. The precipitates carry information of different subduction-related fluid processes over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow.