New Scientific Drilling in the Bering Sea--Results Imply Contribution of Thermogenic Methane to Beringian Margin BSR and Miocene Termination of Subduction Beneath Bowers Ridge

METHANE BENEATH THE BERINGIAN MARGIN: Based on an acoustically prominent BSR (~450 m), it has long been surmised that large volumes of methane gas and methane hydrate occur within the Cenozoic deposits that thickly (5-10 km) drape the Beringian continental margin. In 2009, IODP Exp. 323 drilled three sites along the margin to recover the late Cenozoic paleoceanographic history stored there. On deck, warming sediment cores vigorously released interstitial methane gas. Disassociation of pore-space methane hydrate was inferred, but hydrate was not visually seen. P/T conditions at the BSR (24 C at 360 bars) matched those of the expected phase transition from methane gas below to methane hydrate above. Reflection records below bright sectors of the BSR display gas-blanking effects traceable to subsurface depths of several km, at which the thermal gradient of 50-55 deg C/km implies thermogenic methane would be generated. Since at least the early Miocene, surface water overlying the Beringian margin has been a biologically productive garden spot. Large quantities of organic matter can be expected to have accumulated in the underlying slope deposits. Deep, glacial-age canyon cutting, normal faulting, and diagenetic fracturing of siliceous shale (opal-A to opal-CT) provide venting paths for deep-generated methane. It is posited that ascending thermogenic methane importantly nourishes the sub-margin hydrate BSR. END OF SUBDUCTION AND ARC VOLCANISM AT BOWERS RIDGE: Submarine Bowers Ridge, which is not volcanically active, projects oddly northward and curvingly westward into the Bering Sea Basin from the mid point of the Aleutian Arc. The ridge rises as high as 3500 m to flatten near 600 m at wave-planed platforms cut across basement rock. The ridge's magnetic, velocity, and gravity characteristics are typical of an arc massif. A sediment-filled, trench-shaped trough along the base of the ridge's northern flank implies a former subduction zone underthrust the ridge to the south. At Exp 323 Site U1342, 42 m of basement core were recovered from the southern side of Ulm Plateau, a subsided, wave-cut platform. The upper part of the ridge is here constructed of well-lithified, commonly reddish, arc-generated volcaniclastic sediment and lesser andesitic lava (Kawabata et al., 2011: http://publications.iodp.org/proceedings/323/323toc.htm). It seems likely the recovered volcanic material is from a subaerial stratavolcano--Whiskey Mt--that formerly rose above Ulm Plateau. Wave-base destruction of Whiskey Mt occurred about 14 Ma, possibly recording the termination of arc volcanism and subduction below the curving Bowers spur of the Aleutian Arc complex (Site U1342; http://publications.iodp.org/proceedings/323/323toc.htm). Ar-Ar dating of recovered material is underway.