HR: 1340h
AN: B43B-0144    [Abstracts]
TI: Carbon Isotopic Signatures in Living Benthic Foraminifera from Methane-Soaked Gas-Hydrate-Bearing Sediments in Guaymas Basin, Gulf of California
AU: * herguera, j
EM: herguera@cicese.mx
AF: CICESE, Km107 Carretera TransPeninsular, Ensenada, BC 22850 Mexico
AU: Perez, E
EM: geperez@isugw.indstate.edu
AF: Los Angeles Museum of Natural History, Downtown LA, Los Angeles, CA 93039 United States
AU: Paull, C
EM: paull@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Ussler, B
EM: methane@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Peltzer, E
EM: etp3@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Brewer, P
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039-9644 United States
AU: Cortina, A
AF: CICESE, Km107 Carretera TransPeninsular, Ensenada, BC 22850 Mexico
AB: There is an ongoing discussion on the role of methane outgassing from costal basins to explain the rapid warmings in the Northern Hemisphere during the last Glacial cycle. The recurring anomalous depletions in the benthic foraminifera carbon isotopic composition, observed in a few high resolution cores in carbon rich coastal basins, has been interpreted as an indication of the assimilation of methane derived carbon by these organisms. The magnitude and timing of these depletions, coincident with rapid warmings in the Northern hemisphere, have been interpreted in terms of large methane releases from these carbon loaden sediments into the atmosphere. However, it is still poorly known whether benthic foraminifera do in fact assimilate methane derived carbon and if at all the observed carbon isotopic depletions are related to periods of methane release. Here we report on some recent results from an extensive ROV-based exploration along the NE transform margin of the Guaymas Basin in the Gulf of California with the objective of characterizing the patterns of carbon isotopic assimilation into the benthic foraminiferal tests along known gradients of present methane venting. Cores were retrieved from beds of living calyptogenid clams, tubeworms, and white bacterial mats, commonly found on exposed strata on the flanks of the basin; from a methane venting site visible by a continuous emanation of gas bubbles from the seafloor in 1,582 m of water; and from background sites at the same depth in Guaymas Basin. Rose bengal-stained specimens of Uvigerina peregrina (Up), Planulina wllerstorfi (Pw), Globobulimina pacifica (Gp), and Bulimina mexicana (Bm) were hand-picked and their carbon and oxygen isotopic compositions were determined in 117 samples. The mean carbon isotopic values (relative to PDB) in any group of these benthic foraminifera are not distinctly lighter than expected for these sites (-1.14% Up, -0.28% Pw, -1.71% Gp, and -0.5% Bm). Their range of values are small (1.26% Up, 1.14% Pw, 2.17% Gp, and 0.81% Bm) and can be fully explained in terms of the pore water carbon isotopic composition, that results from the oxidation of organic carbon in the upper few cm of the sediment column further mirrored in the strong sulphate reduction gradients with depth. DIC isotopic composition measured in several pore water sample range from -2.9% to -35.85%. The most depleted values indicate that while some methane derived carbon has entered the authigenic DIC pool within the seafloor sediments, there is no record of the assimilation of this carbon on the calcitic shells of living benthic foraminifera. These results question the extent to which strong carbon isotopic depletion signals in the geological record are features recorded by living foraminifera or the result of a posterior diagenetical imprint.
DE: 4806 Carbon cycling
DE: 4835 Inorganic marine chemistry
DE: 4804 Benthic processes/benthos
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting