Late Holocene foraminifera of Blake Ridge diapir: Assemblage variation and stable-isotope record in gas-hydrate bearing sediments

Abstract

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 CH 4 - or H 2 S-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 δ 18 O and δ 13 C 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 δ 13 C values of 0.1 to − 4.2 (‰VPDB), that are distinctly more 13 C-depleted relative to the δ 13 C of 0.4 to − 1.0 (‰VPDB) at the control (off seep) site. The species from a mussel-bed site exhibit more negative δ 13 C values than those from microbial mats, possibly reflecting different food sources and higher rate of anaerobic oxidation of methane. The positive δ 13 C values in the paired planktonic species suggest that authigenic carbonate precipitation did not overprint the observed 13 C depletions. Hence the probable cause of negative δ 13 C of benthic foraminifera is primary calcification from Dissolved Inorganic Carbon (DIC) containing mixed carbon fractions from (a) highly 13 C-depleted, microbially-oxidized methane and (b) a seawater source. • Benthic species from methane seeps record δ 13 C negative values reflecting seepages. • The diversity is reduced at seeps by the elimination of species intolerant to gases. • The agglutinated group is decimated at the seeps.