Influences of thermal and fluid characteristics of methane and hydrothermal seeps on the stable oxygen isotopes of living benthic foraminifera

The serendipitous discovery of methane seep fluids with elevated temperatures (3.6-5.2 °C), described as a “hydrothermal seepage” by Levin et al. (2012), has led to questions about the prevalence of this previously overlooked habitat. Stable oxygen isotopic analysis of Cibicidoides wuellerstorfi (Schwager, 1866) attached to tubeworms near the hydrothermal seep described by Levin et al. (2012) reflect δ18O values consistent with elevated seepage temperatures. Temperature reconstructions, based on C. wuellerstorfi δ18O, from additional seeps in the Pacific (Mounds 11 & 12 offshore Costa Rica, and Hydrate Ridge) produced variability that is likely a result of influences of seep source, temperature and salinity. Larger ranges in δ18O (σ > 0.10) paired with traditional isotopic seep indicators, (e.g., δ13C; σ > 0.15), have the potential to geochemically discern whether foraminifera were calcifying in areas of active methane seepage or nearby surrounding off-seep environments. Elevated temperatures derived from foraminiferal reconstructions at Jaco Scar indicate that it is possible to use foraminiferal δ18O to identify the presence and extent of these recently discovered environments. Seep fluids with elevated temperatures may be common seafloor features, and benthic foraminiferal δ18O may be an effective means to detect and determine the spatial and temporal extent and possible origin of hydrothermal seepage.