HR: 0830h
AN: T61A-16
TI: Evidence for a Link Between Bottom Simulating Reflections and Elevated Fluid Flow, Lima Basin, Peru Margin
AU: * Pecher, I A
EM: ingo@ig.utexas.edu
AF: Inst. Geophysics, U. Texas, 4412 Spicewood Springs Rd., Austin, TX United States
AU: Kukowski, N
AF: GFZ Potsdam, Telegrafenberg C22 6NB, Potsdam, Germany
AU: Huebscher, C
AF: Inst. Geophysik, U. Hamburg, Bundesstr. 55, Hamburg, Germany
AU: Greinert, J
AF: Geomar, Wischhofstr. 1-3, Kiel, Germany
AU: Bialas, J
AF: Geomar, Wischhofstr. 1-3, Kiel, Germany

AB: Bottom Simulating Reflections (BSRs) are commonly assumed to be caused by free gas at the base of gas hydrate stability (BGHS) in marine sediments. Many regional and global estimates of gas hydrate amounts are based on BSR occurrences. However, gas hydrates have been encountered at locations without BSRs. We therefore need to investigate BSR formation for tighter constraints on gas hydrate quantities. Two possible formation mechanisms are a generation of free gas from gas hydrate dissociation at the BGHS and a supply of gas from below. BSRs are ubiquitous on the Peru Margin's lower slope. A regional lack of BSRs in the upper-slope Lima Basin, where conditions are very favorable for gas hydrate formation, was attributed to rapid tectonic subsidence. Subsidence leads to a pressure increase which results in a downward movement of the BGHS with respect to the sediment column and absorption of gas at the BGHS into the gas hydrate stability zone (GHSZ). This downward movement of the BGHS was predicted to be stronger than the signal from sedimentation, which causes an upward movement of isotherms and hence, of the BGHS. We have detected localized BSRs beneath two morphologically elevated areas in Lima Basin. We studied these areas during a recent campaign of the {\it R/V Sonne}. High-resolution seismic data suggest that these areas do not experience sedimentation. The BGHS therefore is predicted to move downward with respect to the sediments and we may rule out dissociation of gas hydrates as the gas source for these BSRs. The subsurface structure appears to focus fluid flow and hence, methane flux, into the GHSZ. With a TV-sled, we observed slabs and boulders on the seafloor which we interpret as authigenic carbonates. A sediment sample recovered from the seafloor was cemented by carbonates. These observations are consistent with high rates of methane flux. Our results therefore suggest that the BSRs in these areas are maintained solely by gas that migrates from below into the GHSZ. Sufficient gas must be supplied to offset its absorption into the GHSZ caused by tectonic subsidence. We know from studies during Ocean Drilling Program Leg 112 that the gas for gas hydrate formation on the lower slope is biogenic methane. We speculate that the gas for the formation of Lima Basin BSRs may also be of biogenic origin. This would suggest high bacteriological activity deep in the Lima Basin sediments emphasizing the significance of the deep biosphere for gas hydrate systems.

DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
DE: 3099 General or miscellaneous
SC: T
MN: Fall Meeting 2000