FORAMINIFERA FROM HYDRATE RIDGE, OREGON: CARBON ISOTOPIC VALUES AND IMPLICATIONS FOR PALEOMETHANE RELEASE

Methane has been implicated as a forcing mechanism for rapid climate change via massive releases from the hydrate reservoir. Because of the distinct carbon isotopic composition of methane, d13C excursions in marine sediments have been interpreted as methane releases throughout the geologic record, including the Quaternary. However, debate exists as to whether such late Quaternary d13C excursions actually represent methane release to the water column and atmosphere. Alternative hypotheses for these isotopic excursions include the oxidation of organic carbon in sediments and post-depositional diagenesis/authigenic carbonate alteration of foraminiferal shells. Resolution of this controversy is crucial for the interpretation of carbon isotopic excursions and understanding of massive methane release in the late Quaternary.

The presence of modern methane seeps at Hydrate Ridge, Oregon provide an opportunity to study the influence of methane seeps on the geochemistry of foraminifera from these environments. Samples taken from the R/V Atlantis using the submersible Alvin were preserved and stained (using Rose Bengal) to determine carbon isotopic signatures of foraminifera living in this methane-enriched environment. Three species of benthic foraminifera were analyzed in this study: U. peregrina, C. mckannai, and G. auriculata. Samples were picked for live (stained) and fossil specimens to assess the potential role of post-depositional authigenic carbonate on carbon isotopic values. All specimens appeared pristine under light microscopy and showed no indication of diagenesis or authigenic carbonate precipitation.

Carbon isotopic values ranged from 0‰ to –10‰ in surface sediments. No statistical difference exists between isotopic values of live vs. fossil specimens, ruling out a significant role for authigenic carbonate in the isotopic values. Isotopic values are more depleted at microbial mat sites than at clamfield and "control" sites, indicating that foraminifera can record both the presence and relative concentrations of methane in pore waters. These investigations clearly demonstrate that benthic foraminifera are capable of recording high concentrations of methane via their carbon isotopic composition.