HR: 14:15h
AN: U33A-04 INVITED     [Abstracts]
TI: New Perspectives on Methane in Quaternary Climate Change: Modern Methane Seeps, Intermediate Water Temperature History and Thermogenic Seepage
AU: * Hill, T M
EM: tmhill@ucdavis.edu
AF: Department of Geology, University of California, Davis, CA 95616 United States
AU: Kennett, J P
EM: kennett@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Behl, R J
EM: behl@csulb.edu
AF: Department of Geological Sciences, California State University, Long Beach, CA 90840 United States
AB: Several lines of evidence from late Quaternary sediment records implicate methane as a forcing mechanism for rapid climate change via releases from the hydrate reservoir. One such line of evidence, negative δ13C excursions in marine sediments, is interpreted to reflect the transport of methane to the water column and atmosphere. However, it is controversial whether such late Quaternary δ13C excursions actually represent methane release, or instead local changes in sediment-porewater geochemistry. Clarification of this issue is crucial for the interpretation and understanding of the nature of past rapid climate change. We present the results of several studies in Santa Barbara Basin, California and Hydrate Ridge, Oregon that focus on understanding methane release in the modern environment and geologic record. Modern methane seeps at Hydrate Ridge, Oregon provided an opportunity to investigate the influence of methane on the δ13C composition of foraminifera from these environments. Three species of benthic foraminifera were picked for live (stained) and fossil specimens to assess the potential role of post-depositional authigenic carbonate δ13C values. Individual living foraminifera from seep sites recorded δ13C values to negative 21.2‰, indicating the isotopic influence of methane. No statistical difference existed between isotopic values of live vs. fossil specimens, ruling out a significant role for authigenic carbonate. These investigations demonstrate that living foraminifera are capable of recording high concentrations of methane via their carbon isotopic composition. Previous high-resolution studies of Santa Barbara Basin (ODP Site 893) indicated that δ13C excursions associated with interstadial warming were brief (5-10 years duration) and synchronous within benthic and planktonic species. Newly collected cores provided material to investigate a continuous deglacial record of intermediate waters, which reside at the critical depth range for the destabilization of methane hydrate. Intermediate water temperatures appear to warm synchronously with surface waters (2-3°C), indicating early warming on the California margin approximately 2 kyr prior to Termination IA. Negative δ13C values of planktonic foraminifera indicate the influence of an isotopically-depleted carbon source in surface waters at this time. In an exciting and new discovery, the amount of tar preserved in basin sediments was quantitatively analyzed for the past 32 kyr. These records, from two sites in the basin, indicate that tar seepage from thermogenic hydrocarbon reservoirs increased during Termination IA and IB deglacial warming. These records indicate that during time periods of abrupt warming, both the hydrate and thermogenic methane reservoirs may increase outputs of methane to the water column and atmosphere. These and other new records of methane hydrate destabilization indicate that geologic evidence is growing in support of late Quaternary instability of the gas hydrate reservoir and associated episodic transfer of methane into the ocean/atmosphere system.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 3030 Micropaleontology
DE: 4267 Paleoceanography
DE: 4806 Carbon cycling
DE: 4870 Stable isotopes
SC: Union [U]
MN: 2005 Joint Assembly