Significant modern carbon present in microbial methane at gas seeps and gas charged sediments along the northern U.S. Atlantic margin

Previous studies of methane from cold seeps in gas hydrate provinces typically report the radiocarbon (14C) age of methane as completely or nearly fossil (i.e., 14C-free), meaning the organic matter and other carbon contributions used to synthesize the methane became isolated from the atmosphere at least 50,000 years ago. The implication of that observation is that methane seepage is thought to inject ancient carbon into the ocean. Recent studies have identified hundreds of methane seeps on the passive northern U.S. Atlantic margin (USAM) at water depths from < 180 m to greater than 1,000 m. Methane sampled from sediment pore fluids at four USAM seep fields, including in Hudson Canyon (540-640 m) and at the deep water (~1,100 m) Chincoteague seep field, was uniformly microbial in origin and contained substantial 14C, equating to a radiocarbon age of 10,350 to 16,520 years before present (yBP) at the slope and seep sites and 2,505 yBP in Hudson Canyon. Curiously, the 14C ages of sediment organic matter (OM) at the slope sites and deep-water seep were older (19,510 to 31,060 yBP) than the associated methane and correspond with the last glacial maximum period (marine isotope stage 2). By contrast, for Hudson Canyon, the 14C age of methane (2,505 yBP) was older than associated sediment OM (1,565 yBP). To account for these age discrepancies, we developed a 14C mass balance model that balances inputs from a deep-fossil source, buried seawater, and associated OM. The OM input includes an early diagenetic fractionation effect to account for preferential degradation of more modern and hence presumably more labile OM. We suggest that researchers should not assume that methane released from cold seeps is radiocarbon-dead (i.e., fossil). Further, the recognition of modern methane inputs into the deep ocean suggests that, in some locations, a fraction of the carbon released from methane seepage is recycled from recently deposited OM.