High-resolution records of methane hydrate dissociation: ODP Site 893, Santa Barbara Basin

Abstract

Sediment records from ODP Site 893 (Santa Barbara Basin) have documented brief, negative carbon isotopic excursions in the late Quaternary in planktonic and benthic foraminifera. These excursions were interpreted as evidence of major releases of methane from the marine hydrate reservoir, affecting the entire water column and likely reaching the atmosphere. These carbon isotopic excursions are further recorded in the δ 13C values and concentration of lipid biomarkers of marine methanotrophs in the basin. Three previously described carbon isotopic excursions are re-examined during this study, at a resolution of every 3–7 years. The original isotopic excursions were sampled at a resolution of once every 50–70 years. Examination using light microscopy and environmental scanning electron microscopy (ESEM) indicates that specimens in this study are in excellent condition, with no apparent evidence of postdepositional diagenesis or alteration of the foraminiferal shells. Alternative hypotheses to account for these conspicuous carbon isotopic excursions include postdepositional carbonate diagenesis of the excursion intervals, and changes in surface ocean productivity and carbon export to the sediments. In this study, we further test the hypothesis that these excursions record the primary signal of methane in the water column by intensive analyses of the previously described carbon isotopic excursions. All four carbon isotopic excursions studied are associated with interstadial warming in the basin, time periods that are associated with shifts in the benthic foraminiferal assemblages toward low oxygen fauna. At high resolution, δ 13C excursions appear very brief (5–10 years duration), and within this time, planktonic and benthic species record a significant shift in δ 13C composition (planktonics to −3‰ and benthics to −6‰). Excursion events are synchronous in benthic and planktonic species within the resolution of these records. In one interval studied (∼37.5 ka), the bulk organic carbon δ 13C records a 1‰ negative shift, in concert with the carbon isotopic excursion in benthic and planktonic foraminifera, consistent with a carbon isotopic shift of photosynthetically derived marine organic carbon. We suggest that alternate explanations invoking changes in productivity and/or postdepositional alteration of these intervals cannot account for the entire magnitude of the carbon isotopic excursion nor the diversity of evidence presented here. Thus, methane hydrate destabilization remains a compelling hypothesis to explain the conspicuous δ 13C excursions observed in ODP Site 893 and similar late Quaternary excursions described elsewhere.