Tempo and longevity of methane efflux along the US Atlantic Margin

Abstract:

The newly-discovered US Atlantic margin (USAM) methane seep province presents an unprecedented opportunity to investigate the timing and evolution of methane emissions along a passive continental margin, across a range of water depths (~300-2000meters), and at seeps linked to myriad processes (dissociation of upper slope gas hydrates, flow through fractured Eocene rock, and salt diapirism). The USAM seep province stretches nearly 1300 km from Nygren Canyon near Georges Bank in the north to the well-studied Blake Ridge Diapir seeps offshore South Carolina. Here we use methane derived authigenic carbonate (MDAC) samples retrieved by DSV Alvin on the July 2015 SeepC expedition led by C. Van Dover, supplemented by carbonates obtained on earlier expeditions, to date methane efflux at selected USAM seep sites using U-Th geochronology. MDAC U-Th analysis, carried out in conjunction with petrographic and tracer isotope analyses (e.g., δ¹³C), and a robust assessment of detrital Th corrections, will provide absolute dates for MDAC formation and inferentially methane efflux. Multiple dates, with associated information on petrographic context, can be obtained from each sample, and multiple samples were collected from seeps situated both on the upper continental slope, and in deepwater settings. The resulting dataset will constrain the tempo of methane efflux at each site, and the distribution of ages obtained at each seep may be used to distinguish between short-lived, or prolonged and/or episodic records of methane emission. Prior U-Th geochronologic analyses on two archive samples from Baltimore Canyon and Norfolk Canyon yielded ages corresponding to the Last Glacial Maximum and the end of the Holocene Thermal Maximum, respectively.