Aragonite cements around methane seeps: Comment

Abstract

Lithified rocks underlie extensive stretches of the Atlantic continental shelfoff the eastern United States. A sample of shelly sandstone resembling beachrock and cemented by aragonite wasrecovered from thisshelfat a depth of79 moff the New Jersey coast (lat, 38OZ2'N, long. 73°45'W). The radiocarbonageofthemolluscan shells is439O±120yearsand that of the cement is 15,6OO±250 years. At first sight, the radiocarbon agesgivewhatappears tobe an impossible result: cementation of "young" shellsby "old" cement Assuming that the radiocarbon measurements are correct, they require someexplanation. Onepossible waytoexplain theminvokes the idea that the actual precipitation of the cementoccurred less than4390 years ago, but that thecarbon in the aragonite didnotcomedirectly outofseawater (inwhich casethecarbon would be "new", that is, 4390 years old), but was rather derived from a sourceof older carbon. Analyses of stablecarbon isotopes showthat the cement is enriched in the light stableisotope(12C); its513C valueis -44.8. Thiscontrasts with a 013C value of -8.4for themolluscan shells of therock. The strongly negative 013C value of the cement suggests that carbon of thearagonite cementcamefrom methane (CH.J; no other material is known in which the lighter carbon isotope has been that much enriched. Although such strongly negative 513C values are diagnostic of methane, diagenetic processes ultimately will restore a normal isotopic composition (Friedman, 1975). Photographs of thesecements are given in papers by Allen et al. (1969) andFriedman et al. (1971).