Comparisons of Distributions and Isotopic Geochemistry of Benthic Foraminifera from Seep and Non-seep Environments, Offshore of Costa Rica

Abstract

Vertical distribution patterns and stable isotopic geochemistry of benthic foraminifera labeled with CellTracker Green and stained with Rose Bengal were compared at sites of active methane seepage and adjacent non-seep habitats off the Pacific coast of Costa Rica. Sediment cores of bacterial mats from Costa Rica revealed vertical distribution patterns more similar to those seen previously in clam beds, suggesting increased levels of bioturbation compared to nonseep sites. Similar taxa were found at both seep and non-seep sites including: Chilostomella oolina, Uvigerina peregrina and hispida, Cibicides mckannai, and Cassidulina braziliensis. Within active methane seep habitats, elevated substrate such as carbonate rocks, and vestimentiferan tubeworms were examined for living foraminifera. Vestimentiferan tubeworms had highly variable numbers of attached epibenthic foraminifera, dominated by Cibicides wuellerstorfi and Carpenteria monticularis. Stable carbon isotopic comparisons between epibenthic foraminiferal species of Cibicides wuellerstorfi and the vestimentiferan tubeworms on which they reside revealed 10‰ to 30‰ differences between the foraminiferal carbonate and substrate, suggesting that the geochemical signatures of elevated epibenthics were not significantly influenced by the geochemical signature of the substrate on which they reside. This study finds no apparent methane influence on the foraminiferal calcite of elevated epibenthic foraminifera from the three active seep sites studied (Mound 11, Mound 12, and Jaco Scar). This may be because the elevated epibenthics were not exposed to seep-influenced fluids by inhabiting raised substrates. This study also provides a quantitative analysis of coiling directions in elevated epibenthic species at seeps, which has never previously been reported. Statistical analysis revealed that there were no significant differences in isotopic composition between sinstral (left)and dextral (right) coiling Cibicides wuellerstorfi. The results of this study suggest that coiling direction of elevated epibenthic oraminifera, such as Cibicides wuellerstorfi and Carpenteriamonticularis, is a result of biologic factors rather than environmental influences.