| Along the ocean margins of the world, hydrocarbons are typically generated from organic matter within thick sediment accumulations and released into the oceans and atmosphere because of faulting, over-pressuring, and changes in depositional rates, sea level, climate, or tectonic regime. Ephemeral hydrocarbon migration in marine sediments leaves robust geological signatures including carbonates resulting from microbial and inorganic processes. The goal of this study was to utilize the calcium carbonate tests (shells) of foraminifera to develop a proxy for identifying and characterizing methane seepage in ancient sediments. Specific objectives were to: (i) assess the effects of methane seepage on foraminiferal assemblages: (ii) integrate carbon and oxygen isotopic data from foraminiferal carbonate and associated authigenic carbonate: and (ii) identify and characterize diagenetic alterations of carbonates resulting from methane seepage. Samples from fossil methane seeps of Eocene through Pliocene age were obtained from the Cascadia Margin of northwest Oregon, western Washington, U.S.A. and southern Vancouver Island, Canada. Samples from a modern analogue were recovered from the Hikurangi Margin off the east coast of New Zealand. Foraminiferal assemblages from both fossil and Recent seeps did not definitively reflect the presence of methane-influenced water. No endemic seep species were found. and assemblage density and diversity were not significantly different in seep and non-seep samples. Stable isotopes. on the other hand, reflected the influence of methane-derived dissolved inorganic carbon in their depleted and variable delta13C values. In Recent samples, however, these values are generally in disequilibrium with surrounding pore- and seawater. Combining isotope analyses with carbonate elemental ratios (Mg/Ca) revealed that the greatest value in using foraminifera for evaluation of methane seepage lies in recognizing diagenetic alteration and exploiting the record. Near seeps, foraminifera become nucleation centers for diagenetic mineralization. These alterations mimic authigenic carbonates and carry complex geochemical information about prevailing conditions and processes. Because foraminifera are small and nearly ubiquitous in marine sediments, they are present where other carbonates are absent. Thus, they facilitate fine scale measurements and reconstruction of modern and fossil methane seep environments. |
