HR: 09:30h
AN: OS21A-07    [Abstracts]
TI: Accumulation and Migration of Gas Hydrates in Sediments of the Peru Margin Revealed by Rock Magnetism
AU: * Musgrave, R J
EM: palmbob@dodo.com.au
AF: PALM Laboratory, School of Environmental and Life Sciences, University of Newcastle, University Drive, Callaghan, NSW 2308, Australia
AU: Vega, M E
EM: palmlab@dodo.com.au
AF: PALM Laboratory, School of Environmental and Life Sciences, University of Newcastle, University Drive, Callaghan, NSW 2308, Australia
AB: Gas hydrate disseminated in marine sediments encourages microbial activity, including bacterial metabolism that results in production of magnetic iron sulphides. Previous studies of sediment cores from continental margins where gas hydrate occurs have shown that the rock magnetic parameter D_JH, an index based on hysteresis properties representing position on a Day plot, is useful as a proxy for the modern and glacial distribution of gas hydrate. Ocean Drilling Program sites 688 (Leg 112) and 1230 (Leg 201) provided samples through hydrate-bearing accretionary margin sediments on the Peru margin. Perhaps surprisingly, results from samples from Site 688, which have been curated for 13 years, are broadly similar to those from Site 1230, sampled immediately after core recovery, indicating that protracted storage does not entirely destroy the rock magnetic signal. Profiles of D_JH at both sites suggest a preferential concentration of gas hydrate within the porous upper lithological unit, with the highest values of D_JH occurring near the base of the unit, where hydrates would be expected to accumulate, and in other intervals where the high concentrations of hydrate have been observed or inferred. Viable bacterial counts parallel the trend in D_JH, suggesting that the principal control on bacterial numbers in this profile is hydrate concentration. Site 688, which penetrated below the modern base of gas hydrate stability (BGHS), displays a feature on the D_JH profile similar to that interpreted at the Cascadia margin as the marker of the BGHS at the last glacial maximum. This feature lies about 65 m below the bottom-simulating reflector that marks the modern BGHS. Thermal modelling based on this determination of the glacial BGHS gives a bottom-water temperature at the Peru margin during the last glacial maximum of - 1.5°C, and serves to explain the anomalously low temperature predicted for the modern BGHS at the Peru margin on the basis of geothermal gradient measurements in the upper 50 m of the sediment column.
DE: 0416 Biogeophysics
DE: 0714 Clathrate
DE: 1505 Biogenic magnetic minerals
DE: 3004 Gas and hydrate systems
DE: 4926 Glacial
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting