Carbon geochemistry in serpentinites from and beneath the Lost City Hydrothermal Field

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and 17863_TC values ranging from -28.7? to +2.3?. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (d13C_TOC: -28.9? to -21.5?) and variations in d13CTC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C15 to C30. Longer-chain hydrocarbons (up to C40) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C16 to n-C20 alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important (as yet unidentified) reservoir for dissolved organic carbon (DOC) from seawater.

Results from ikaite investigations in the Kara Sea

The authigenic carbonate mineral ikaite is specific of low-temperature high latitude environments. The depletion of ikaite carbon in 13C isotopes in most cases implies a causal relation of ikaite generation with methane geochemistry. In this paper we present new data on ikaite minerals in Holocene sediments sampled along the Yenisei channel at the southern (74°N) and northern (77°N) ends. Stable carbon isotopes of the ikaite crystals were studied in conjunction with the hydrochemistry and isotope geochemistry of the sediments. Pore water and natural gas samples were separated from sediments to describe the methane carbon isotope distribution pattern throughout two sedimentary sequences embedding the ikaite crystals of different isotope composition (-24 per mil and -42 per mil). The biogenic nature of the methane is indicated by 51 C values being as low as -104.4 per mil. In the case of the moderately depleted sample (-24 per mil) from the southern location the small-scale ikaite formation fits best into the concept of a 'closed» sediment system, with a limited diagenetic carbon dioxide source being present. In the second case, formation of highly abundant and isotopically depleted ikaite crystals (-42 per mil) were caused by upwards flux of biogenic methane from below. Contribution of two main carbon sources to the ikaite crystals was estimated by using a isotope-mass balance equation. Organic-derived CO2 constitutes the principal source in both samples, amounting to 50 % of the total carbon of the strongly depleted ikaite crystals (-42 per mil) sampled at the northern end and 83 % for the moderately (-24 per mil) depleted crystals from the southern end. Methane-derived CO2 comes to 42 % for the isotopically light ikaite crystals and to 9% for the isotopically heavy crystals. The importance of sediment lithology and diffusive transport for ikaite formation is emphazied.