Paper No. 66-0
ISOTOPE RECORDS OF RECENT BENTHIC FORAMINIFERA FROM HYDRATE-BEARING SEDIMENTS: METHANE-HYDRATE DISSOCIATION EFFECTS
AHARON, Paul1, HACKWORTH, Matthew2, PLATON, Emil3, WHEELER, Christopher2, and SEN GUPTA, Barun2, (1) Geological Sciences, Univ of Alabama, Tuscaloosa, AL 35487, aharon@wgs.geo.ua.edu, (2) Geology and Geophysics, Louisiana State Univ, Baton Rouge, LA 70808, (3) Energy and Geoscience Institute, Univ of Utah, 423 Wakara Way, Salt Lake City, 84108

Anomalously negative d13C excursions in fossil benthic foraminifera tests coinciding with rapid climate changes were used as the smoking gun of hydrate-sourced methane released into the atmosphere. Although highly desirable, the effects of methane hydrate dissociation on stable isotopes of recent benthic foraminifera have not yet been tested. Here we present carbon and oxygen isotope records of benthic foraminifera species Bolivina ordinaria and B. albatrossi associated with methane hydrates which have important implications regarding the isotope effects of methane-hydrate dissociation.

A 24 cm-long core was acquired with a manned submersible in the Gulf of Mexico at 567 m depth where methane hydrates are at their threshold of stability. The Bolivina d13C record spanning 1800 years exhibits an upcore trend from -2.7 at the bottom and progressively shifting toward 7.9 at the top. d13C and d18O records are negatively correlated reaching peak values of 10.2 and 3.9 , respectively, at 15-19 cm core depth. Three lines of evidence attest to the complexity of the isotope records, caused by microbial processes fueled by methane-hydrate dissociation: (i) isotope records acquired from paired planktonic Globigerinoides and benthic Bolivina show similar trends; (ii) stable isotopes and distribution of % total carbonate trends are highly correlative, and (iii) carbonate overgrowths were documented on both the planktonic and benthic species. Disentanglement of the diagenetic overprint effect suggests that methane hydrate dissociation has contributed a maximum 13C depletion of 5.2 and 18O-enrichment of 1.7 relative to background values of 1.5 and 2.2 in recent Bolivina from isobathyal, hydrate-free, sites.

GSA Annual Meeting, November 5-8, 2001
General Information for this Meeting
Session No. 66
Foraminifera: Barometers of the Biotic and Abiotic World I
Hynes Convention Center: 312
8:00 AM-12:00 PM, Tuesday, November 6, 2001
 

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