An assessment of foraminiferal species distribution and stable-isotope anomalies at a methane-hydrate mound in the Gulf of Mexico

摘要

Using a manned submersible, a 25-cm-long sediment core was acquired at upper bathyal depth in the Gulf of Mexico from a mound underlain by methane hydrates at the threshold of stability. The main objective of the core investigation was to elucidate little-understood effects of gas-hydrate dissociation on benthic foraminiferal ecology, stratigraphy, and stable isotopes. Our results show that intense methane seepage has decimated, but not obliterated, the benthic foraminiferal community that includes many facultative anaerobes, dominated by two species of Bolivina. The remnant community has continued to exist at this site for over 2000 years. A shift from a much higher foraminiferal density to persistent low values is observed in the lowermost part of the core. A major oil spill in the Gulf of Mexico in 2010 caused similar devastation to the foraminiferal community, and drastically reduced the density. Planktonic (Globigerinoides ruber) and benthic foraminiferal tests (Bolivina ordinaria and Bolivina albatrossi) yield δ~(13)C and δ~(18)O profiles that are inversely correlated and exhibit anomalously negative carbon and positive oxygen isotope values up to maxima of -10.2‰ and 3.9‰ (VPDB), respectively. The isotope anomalies are attributed to anaerobic oxidation of ~(13)C-depleted biogenic methane and intake of ~(18)O-rich fluids released during hydrate dissociation. Two sequential sulfate-methane transition zones (SMTZs), with sedimentation rates of 8.2 and 27.5 cm Ka~(-1), are coeval with two distinct intervals in the G. ruber isotope profiles. Three lines of evidence attest to the complexity of reconstructing methane flow from foraminiferal records. (i) Both planktonic and benthic foraminiferal tests serve as templates for secondary carbonate overgrowths. (ii) Decoupling of the overgrowths from the primary biogenic calcite tests is intractable, and therefore stable isotopes cannot be used as a tool to confirm whether Bolivina may live and thrive in anoxic sedime