Evidence against clathrate‐derived methane release to Santa Barbara Basin surface waters?

Abstract

The Clathrate Gun Hypothesis suggests that methane from the marine sedimentary gas hydrate reservoir was episodically released into the atmosphere where it generated rapid atmospheric warming that drove late Quaternary abrupt millennial‐scale climate change. Brief, large negative δ13C excursions recorded by planktonic (up to −3‰) and benthic (up to −5‰) foraminifera from Santa Barbara Basin sediments have been interpreted as evidence of clathrate‐derived methane release to surface waters during or immediately before marine isotope stage (MIS) 3 interstadials. Molecular biomarkers from the δ13C excursion horizons have been interpreted to support this scenario. However, recent work at modern methane seeps show that fossil specimens can be contaminated with authigenic overgrowths or calcite replacement that may be difficult to detect through microscopy. The possibility that planktonic specimens are contaminated, and thus their δ13C values not a primary signal, warrants further inquiry. We generated a high‐resolution continuous total organic carbon (TOC) δ13C record from fortuitously obtained samples across the younger foraminiferal δ13C excursion during Interstadial 8. The record exhibits variations similar in magnitude and timing to those that occurred in the basin over the last 600 years. Furthermore, previously published TOC δ13C values of discreet samples from this and the three other MIS 3 foraminiferal δ13C excursions are also similar to recent values. Although there are factors that could obscure a surface water methane signal in the TOC δ13C record, these results question the interpretation that methane was released to surface waters. An alternative hypothesis to explain the available data is that the foraminiferal δ13C excursions are an artifact of diagenetic processes at or below the sediment‐water interface. This interpretation may be compatible with the molecular biomarker data. Further work is necessary to differentiate between the two hypotheses.