Isotopic evidence for the incorporation of methane-derived carbon into foraminifera from modern methane seeps, Hydrate Ridge, Northeast Pacific

摘要

The presence of modern methane seeps at Hydrate Ridge, offshore Oregon, provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera. A series of cores were collected from the southern summit of Hydrate Ridge in 2002. Samples were preserved and stained to determine the delta(13)C composition of three species of live (stained) and dead benthic foraminifera: Uvigerina peregrina, Cibicidoides mckannai, and Globobulimina auriculata. Specimens were examined under light and Scanning Electron Microscopy (SEM) and exhibit no evidence of diagenesis or authigenic carbonate precipitation. Individual living foraminifera from seep sites recorded delta(13)C values from -0.4parts per thousand to -21.2parts per thousand, indicating the isotopic influence of high methane concentrations. Average delta(13)C values (calculated from single specimens) range from -1.28 to -5.64parts per thousand at seep sites, and -0.81 to -0.85parts per thousand at a control (off seep) site.Two distinct seep environments, distinguished by the presence of microbial mats or clam fields, were studied to determine environmental influences on delta(13)C values. Individual foraminifera from microbial mat sites exhibited more depleted delta(13)C values than those from clam field sites. We interpret these differences as an effect of food source and/or symbiotic microbes on foraminiferal carbon isotopic values, acting to magnify the negative delta(13)C values recorded via the DIC pool. No statistical difference was found between delta(13)C values of live vs. dead specimens. This suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions. However, a few dead specimens with extremely negative delta(13)C composition (<-12parts per thousand) do indicate potential evidence for an authigenic influence on the recorded delta(13)C composition. Copyright (C) 2004 Elsevier Ltd.