Benthic foraminifera associated with cold methane seeps on the northern California margin: Ecology and stable isotopic composition

Abstract

Release of methane from large marine reservoirs has been linked to climate change, as a causal mechanism and a consequence of temperature changes, during the Quaternary and the Paleocene. These inferred linkages are based primarily on variations in benthic foraminiferal stable isotope signatures. Few modern analog data exist, however, to assess the influence of methane flux on the geochemistry or faunal characteristics of benthic foraminiferal assemblages. Here we present analyses of the ecology and stable isotopic compositions of living (Rose Bengal stained) and dead (fossil) foraminifera (>150 μm ) from cold methane seeps on the slope off of the Eel River, northern California (500–525 m), and discuss potential applications for reconstructions of methane release in the past and present. Calcareous foraminiferal assemblages associated with Calyptogena clam bed seeps were comprised of species that are also found in organic-rich environments. Cosmopolitan, paleoceanographically important taxa were abundant; these included Uvigerina, Bolivina, Chilostomella, Globobulimina, and Nonionella. We speculate that seep foraminifera are attracted to the availability of food at cold seeps, and require no adaptations beyond those needed for life in organic-rich, reducing environments. Oxygen isotopic values of the tests of living foraminiferal assemblages from seeps had a high range (up to 0.69‰) as did carbon isotopic values (up to 1.02‰). Many living foraminiferal isotope values were within the range exhibited by the same or similar species in non-seep environments. Carbon isotopic values of fossil foraminifera found deeper in the sediments (18–20 cm), however, were 4.10‰ (U. peregrina) and 3.60‰ (5. subargentea) more negative than living δ 13C values. These results suggest that δ 13C values of foraminiferal tests reflect methane seepage and species-specific differences in isotopic composition, and can indicate temporal variations in seep activity. A better understanding of foraminiferal ecology and stable isotopic composition will enhance paleo-seep recognition, and improve interpretations of climatic and paleoceanographic change.