Methane-derived authigenic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin: Discussion and reply

Abstract

Methane-derived authigenic carbonates in terrigenous sediments described from the Pacific Ocean along the Oregon/Washington margin (Ritger and others, 1987) have their counterparts in the outer continentalshelf setting of the western Atlantic Ocean. Lithified sandstones, containing a modern microfauna and resulting from submarine cementation, underlie extensive stretches of the shelf between New Jersey and Florida (Allen and others, 1969; Friedman, 1975; Friedman and others, 1971; Friedman and Sanders, 1978, esp. p. 157-158). In a dredged shelly molluscan sandstone, the radiocarbon age of the molluscan shells is 4,390 ± 120 yr and that of cement is 15,600 ± 250 yr. At first sight, the radiocarbon ages give what appears to be an impossible result: cementation of young shells by old cement. To explain these ages, one may invoke the idea that the actual precipitation of the cement occurred less than 4,390 yr ago but that the carbon in the aragonite did not come directly out of sea water (in which case, the carbon would be new, that is, 4,390 yr old) and rather was derived from a source of older carbon. Analyses of stable-carbon isotopes show that the cement is enriched in the light stable isotope 12C; its S13C value is -44.8. This contrasts with a S13C value of -8.4 for the molluscan shells of the rock. The strongly negative 613C value of the cement suggests that carbon of the aragonite cement came from methane (CH4) (Allen and others, 1969; Friedman and others, 1971; Friedman, 1975); no other material is known in which the lighter carbon isotope has been that much enriched. The stable isotopic compositions of cement and skeletal carbonates overlap with those of the Oregon/Washington margin (Fig. 7, p. 151 of Ritger and others, 1987).