BENTHIC FORAMINIFERA FROM METHANE SEEPS IN MONTEREY BAY: ECOLOGY, STABLE ISOTOPIC COMPOSITION AND RELATION TO BIOGEOCHEMISTRY

Large shifts in stable isotopic compositions of fossil benthic foraminifera in the Quaternary and Late Paleocene have been linked to the release of large volumes of isotopically light methane, but this idea has not been universally accepted.

To gain insight into the use of benthic foraminifera as indicators of methane release, we examined relationships among the distribution of living (rose Bengal stained) benthic foraminifera, their stable isotopic signals, and the pore water biogeochemistry from two methane seeps in Monterey Bay. Several MBARI ROV Ventana push cores were collected from clam beds and bacterial mats at each seep site and from near-seep environments.

Results from this study suggest that there is an effect of methane-influenced pore waters on foraminiferal distributions and carbonate geochemistry. Despite the low oxygen availability and high concentration of sulfide present in subsurface microhabitats at seeps, infaunal densities of living foraminifera are higher than densities near the sediment-water interface. The presence of living foraminifera exposed to H₂S concentrations exceeding 16 mM indicates a high tolerance for sulfide in some species. Foraminiferal assemblages at clam bed environments have higher diversities and abundances than bacterial mat environments.

Carbon isotope values of living benthic foraminifera in Monterey Bay seeps are not much lower than those observed in non-seep environments. However, living foraminifera from seeps have much greater heterogeneity in d¹³C values than published d¹³C values of foraminifera living in non-seep habitats. The analysis of individual tests is critical to observe the variability and range of isotopic signals in seep foraminifera. Average isotopic signal from groups of foraminifera could dilute the methane seepage signal and mask isotopic variability.

Causes for the variability of d¹³C signatures within foraminiferal species associated with seeps are unknown, but we speculate that they likely reflect the variability of seep pore water geochemistry and possibly the influence of aggregations of organic material and bacteria. Additional work is needed to understand how seep foraminifera secrete carbonate with d¹³C values that are so far out of equilibrium with ambient pore water d¹³C values.