Carbon stable isotopic composition of benthic foraminifera from Pliocene cold methane seeps, Cascadia accretionary margin

Abstract

Cold methane seep carbonates preserve the distinctive carbon stable isotope signatures of their ambient porewaters, thus facilitating the identification of methane sources and fluid migration pathways. This study employs an isotopic indicator from microfossils in a diffuse fossil seep for the recognition and characterization of methane seepage in the rock record. Benthic foraminifera from the Pliocene Quinault Formation, western Washington State, are plentiful and provide a temporal link between modern and more ancient seeps. Stable carbon isotope analyses of individual foraminiferal tests revealed a clear distinction between seep and non-seep foraminifera. Infaunal species Globobulimina pacifica and Nonion basispinatum collected from seep samples exhibited δ 13C values that were highly variable and often strongly depleted: values ranged from − 33.1‰ to − 55.3‰ PDB in G. pacifica and from + 2.1‰ to − 42.9‰ PDB in N. basispinatum. The epifaunal species Cibicidoides mckannai, though less depleted, also showed considerable variability with δ 13C values ranging from + 1.7‰ to − 3.1‰ PDB. In contrast, non-seep specimens were much more enriched and less variable: non-seep specimens of N. basispinatum displayed δ 13C values clustering between − 0.5‰ and − 1.5‰ PDB, while non-seep specimens of C. mckannai yielded a range of + 1.1‰ to + 1.8‰ PDB. SEM analyses and oxygen isotope values of foraminiferal tests indicated minimal diagenetic alteration. δ 13C values from authigenic blebs (small nodules with wispy, indistinct boundaries) ranged from + 8.7‰ to − 38.4‰ PDB. By combining these inorganic carbonate data with the foraminiferal values, the association of seep-specific bivalves, and the present-day leakage of thermogenic methane from the nearby Garfield gas mound, we conclude that these foraminifera precipitated their tests in the presence of methane-derived carbon. Thus they record pore-water 13C depletion and syn-depositional fluid-flow pathways near the sediment/water interface in a seep system active 3.4 mya and continuing onshore and offshore today. The range and degree of 13C depletion in the Pliocene microfossils is due to the heterogeneity of seepage caused by different methane sources (i.e. thermogenic and biogenic), the tortuosity of fluid migration pathways reaching the seafloor sediments, and mixing of seep fluids with dissolved inorganic carbon (DIC) and marine organic carbon.