A MOLECULAR-ISOTOPIC RECORD OF METHANE HYDRATE DISSOCIATION DURING THE LATE QUATERNARY

Recently, Kennett et al. reported compelling evidence from carbon isotopic compositions of benthic and planktic foraminifera buried in sediments from ODP Site 893, Santa Barbara Basin, for repeated dissociation events of methane hydrate and subsequent release of methane into the basin (Science 288, 128-133, 2000). More recently, the hypothesis by Kennett and co-workers was supported by the discovery of a biosynthetic product of methane-consuming microorganisms. This compound, the hopanoid diplopterol, has exhibited isotopic compositions of around -60‰ and occurred only in samples with unusually low carbon isotopic compositions of benthic foraminifera (Hinrichs, G-Cubed, #2000GC000118, 2001). Accordingly, aerobic methanotrophic bacteria oxidized methane that may have formed plumes in bottom waters of the basin.

We have extended this work to a stratigraphic investigation of occurrence, distribution, and isotopic composition of biomarker products from methanotrophic microorganisms in relation to the foraminiferal isotopic record. Similarly, molecular products from marine photosynthetic primary producers were examined for their relationship to steep negative isotopic excursions observed in planktonic foraminifera during some interstadials. We will discuss the molecular-isotopic data in relation to the timing and magnitude of methane release and to the associated ecological and environmental response.