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.

Chemical and isotopic compositions of rocks and minerals from the Ashadze and Logachev hydrothermal fields, Mid-Atlantic Ridge

This study was aimed at reconstructing a sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites from internal oceanic complexes of the Ashadze and Logachev hydrothermal vent fields. Collections of plutonic rocks from Cruises 22 and 26 of R/V "Professor Logachev", Cruise 41 of R/V "Akademik Mstislav Keldysh", and from the Serpentine Russian-French expedition aboard R/V "Pourquoi pas?" were objects of this study. Data reported here suggest that the internal oceanic complexes of the Ashadze and Logachev fields formed via the same scenario in these two regions of the Mid-Atlantic Ridge. On the other hand, an analysis of petrological and geochemical characteristics of the rocks indicated that the internal oceanic complexes of the MAR axial zone between 12°58'N and 14°45'N show pronounced petrological and geochemical heterogeneity manifested in variations in degree of depletion of mantle residues and in Nd isotopic compositions of rocks from the gabbro-peridotite association. Trondhjemites from the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under influence of hydrothermal fluids. It was supposed that presence of trondhjemites in internal oceanic complexes of MAR can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone, in which petrologically and geochemically contrasting internal oceanic complexes are spatially superimposed, serves as an area for development of large hydrothermal clusters with considerable ore-forming potential.

Sulphate concentrations and sulphur isotopic composition in pore fluids from ODP Leg 168 sites

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO4**2- and d34S data from four sites drilled during Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48°N, where basement temperatures reach up to 62°C. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow, which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures. Pore fluid d34S data show the highest values (135 per mil) yet found in the marine environment. The bacterial sulfur isotopic fractionation factor, a, is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield erroneous values for a by as much as 15 per mil in diffusive and advective pore fluid regimes. Model results are consistent with a = 1.077+/-0.007 with no temperature effect over the range 1.8 to 62°C and no effect of sulfate reduction rate over the range 2 to 10 pmol/ccm/day. The reason for this large isotopic fractionation is unknown, but one difference with previous studies is the very low sulfate reduction rates recorded, about two orders of magnitude lower than literature values that are in the range of µmol/ccm/day to tens of nmol/ccm/day. In general, the greatest 34S depletions are associated with the lowest sulfate reduction rates and vice versa, and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates.

Hydrogen consumption by gills of Bathymodiolus spp. collected at the Comfortless cove and Lilliput hydrothermal vent fields in May 2006

The discovery of deep-sea hydrothermal vents in 1977 revolutionized our understanding of the energy sources that fuel primary productivity on Earth. Hydrothermal vent ecosystems are dominated by animals that live in symbiosis with chemosynthetic bacteria. So far, only two energy sources have been shown to power chemosynthetic symbioses: reduced sulphur compounds and methane. Using metagenome sequencing, single-gene fluorescence in situ hybridization, immunohistochemistry, shipboard incubations and in situ mass spectrometry, we show here that the symbionts of the hydrothermal vent mussel Bathymodiolus from the Mid-Atlantic Ridge use hydrogen to power primary production. In addition, we show that the symbionts of Bathymodiolus mussels from Pacific vents have hupL, the key gene for hydrogen oxidation. Furthermore, the symbionts of other vent animals such as the tubeworm Riftia pachyptila and the shrimp Rimicaris exoculata also have hupL. We propose that the ability to use hydrogen as an energy source is widespread in hydrothermal vent symbioses, particularly at sites where hydrogen is abundant.