Post depositional alteration of foraminiferal shells in cold seep settings: New insights from flow-through time-resolved analyses of biogenic and inorganic seep carbonates

摘要

Transient hydrocarbon migration within a sediment package leaves behind robust geological signatures in the biogenic and authigenic carbonate record. Here we apply Flow-Through Time Resolved Analyses (FT-TRA) to unravel the compositional changes in foraminifera from coastal fossil methane seeps exposed from Oregon to Vancouver Island: The Eocene-Oligocene Keasey Formation, the Oligocene-Miocene Pysht and Sooke Formations, and the Pliocene Quinault Formation. Our data show that secondary mineralization can be traced with the use of Mg/Ca ratios, which in altered foraminifera are significantly higher than the biogenic ratio (< 3 compared to values as high as 69 mol/mol). Analogous to the record in authigenic carbonate, secondary mineralization contains valuable information about seep characteristics and their geologic history. Data from the Quinault Formation reflect the influence of anaerobic oxidation of biogenic methane in both bleb (δ~(13)C: δ29.8‰ to δ14.0‰) and foraminiferal (δ~(13)C: δ43.0‰ to 2.0‰) carbonate. Oxygen isotopes from blebs and foraminifera indicate precipitation at bottom water temperatures in an environment comparable to conditions observed in modern seeps on the Oregon slope and elsewhere. The carbonates in these seeps are enriched in barium and strontium over biogenic values, and such elevated values may be used a diagnostic tool to identify methane-related carbonates. In contrast, in the Pysht and Sooke formations, carbonate precipitation (including secondary mineralization of foraminifera), was fueled by a thermogenic carbon source (δ~(13)C: δ14 to 3.4‰). These carbonates reflect a more complex paragenetic history and suggest alteration driven by post-depositional warm and/or meteoric fluids. The high manganese (up to 12mmol/mol in foraminifera and 60.1mmol/mol in a carbonate nodule) and low oxygen isotope values (δ~(18)O as low as δ7.7‰ in foraminifera and δ11.6‰ in a carbonate pavement) observed for the Pysht and Sooke samples are consistent with carbonates recovered from fault zones on the Oregon slope, and suggest the intriguing possibility that these carbonates may reflect manganese-dependent anaerobic methane oxidation.