Absolute abundances of benthic foraminifers and stable isotopes from the Alvin dives 3709-3712

The presence of gas hydrates on the Blake Ridge diapir, northeastern Atlantic Ocean, offers an opportunity to study the impact of methane seepage on the ecology and geochemistry of benthic foraminifera in the late Holocene. Three push cores, covering a time span of ~ 1000 yrs, were retrieved from three distinct microhabitats at the top of the diapir at a water depth of ~ 2150 m: (i) sediments away from seepage (control core), (ii) sediments overlain by clusters of methanotrophic and thiotrophic bivalves, and (iii) chemoautotrophic microbial mats. The foraminiferal assemblages at the two seep sites are marked by a reduction in benthic foraminiferal species diversity, coupled with a near-absence of agglutinated species. However, an opportunistic population rise in CH4- or H2S-tolerant calcareous species (e.g., Globocassidulina subglobosa and Cassidulina laevigata) that utilize the abundant trophic resources at the seeps has led to an increase in the overall assemblage density there. The delta18O and delta13C values of three species of benthic foraminifera - Gyroidinoides laevigatus, Globocassidulina subglobosa, and Uvigerina peregrina - and the planktonic species Globorotalia menardii were acquired from all three cores. The benthic species from methane seeps yield delta13C values of 0.1 to - 4.2 (per mil VPDB), that are distinctly more 13C-depleted relative to the delta13C of 0.4 to - 1.0 (per mil VPDB) at the control (off seep) site. The species from a mussel-bed site exhibit more negative delta13C values than those from microbial mats, possibly reflecting different food sources and higher rate of anaerobic oxidation of methane. The positive delta13C values in the paired planktonic species suggest that authigenic carbonate precipitation did not overprint the observed 13C depletions. Hence the probable cause of negative delta13C of benthic foraminifera is primary calcification from Dissolved Inorganic Carbon (DIC) containing mixed carbon fractions from (a) highly 13C-depleted, microbially-oxidized methane and (b) a seawater source.