ISOTOPE RECORDS OF RECENT BENTHIC FORAMINIFERA FROM HYDRATE-BEARING SEDIMENTS: METHANE-HYDRATE DISSOCIATION EFFECTS

Anomalously negative d¹³C excursions in fossil benthic foraminifera tests coinciding with rapid climate changes were used as the smoking gun of hydrate-sourced methane released into the atmosphere. Although highly desirable, the effects of methane hydrate dissociation on stable isotopes of recent benthic foraminifera have not yet been tested. Here we present carbon and oxygen isotope records of benthic foraminifera species Bolivina ordinaria and B. albatrossi associated with methane hydrates which have important implications regarding the isotope effects of methane-hydrate dissociation.

A 24 cm-long core was acquired with a manned submersible in the Gulf of Mexico at 567 m depth where methane hydrates are at their threshold of stability. The Bolivina d¹³C record spanning 1800 years exhibits an upcore trend from -2.7 ‰ at the bottom and progressively shifting toward –7.9 ‰ at the top. d¹³C and d¹⁸O records are negatively correlated reaching peak values of –10.2 ‰ and 3.9 ‰, respectively, at 15-19 cm core depth. Three lines of evidence attest to the complexity of the isotope records, caused by microbial processes fueled by methane-hydrate dissociation: (i) isotope records acquired from paired planktonic Globigerinoides and benthic Bolivina show similar trends; (ii) stable isotopes and distribution of % total carbonate trends are highly correlative, and (iii) carbonate overgrowths were documented on both the planktonic and benthic species. Disentanglement of the diagenetic overprint effect suggests that methane hydrate dissociation has contributed a maximum ¹³C depletion of –5.2 ‰ and ¹⁸O-enrichment of 1.7 ‰ relative to background values of –1.5 ‰ and 2.2 ‰ in recent Bolivina from isobathyal, hydrate-free, sites.