Abstract:
The late Quaternary carbon isotope record from ODP Site 893, Santa Barbara Basin, shows brief, negative delta (super 13) C spikes in both benthic and planktonic foraminifera between 60 and 25 ka. A sudden massive input of methane into all levels of the water column could explain the spikes. High-resolution seismic reflection profiles exhibit Bottom-Simulating Reflections (BSRs) at unusually shallow sediment depths (2 to 25 m) that extend over large areas of the western basin, but are most evident in a large submarine slide area. We propose that upwards-migrating methane might diffuse through relatively permeable late Quaternary siltstones and sandstones and pool beneath impermeable layers, including shallow methane hydrate. Pore fluid pressure reduces the effective normal stress across potential slide planes, and earthquakes trigger slides, releasing both the methane in the hydrate and the free gas beneath. The area of methane stability has expanded in the basin following the last glacial maximum (after 10 ka) as sea level rose. Therefore, the BSR is shallow because there has been insufficient time for accumulation of the inferred clathrate. Coring this highly-accessible BSR to confirm methane hydrate would also produce estimates of methane flux since initiation of hydrate stability. A field of large seep mounds exists at the western sill of the basin whose activity may be related to clathrate instability. Work by Sorlien and others (this meeting) has documented a down to the north tectonic tilting and subsidence of the slope and island shelf south of the basin. We predict that central and western Santa Barbara basin has only been deep enough for clathrate stability during the late Quaternary (perhaps since 0.5 Ma), and that associated large submarine slides have become more common.