CELL STRUCTURE IMPACTS FORAMINIFERAL TEST &delta13C SIGNATURE: COMPARISON BETWEEN HYDROCARBON SEEP AND NON-SEEP SPECIMENS

The dogma that the δ¹³C values of benthic foraminiferal carbonate correlate predictably with ambient δ¹³C_DIC values has recently been drawn into question because calcite of foraminiferal tests (shells) in cold methane-seep habitats are as much as 40‰ out of equilibrium with the ambient δ¹³C_DIC of bulk pore water. Disequilibrium between ambient pore water and co-occurring foraminiferal calcite has important consequences for our understanding of past seep activities. The causes of such disequilibrium are likely biological and/or environmental, and must be understood before the isotopic composition of foraminiferal calcite can be used as a reliable proxy for any paleoenvironmental record. Our novel method, which provides both the stable carbon isotope values and cellular ultrastructure of individual foraminifera, allows insights into the biological controls on an important paleoceanographic proxy. Specimens of six common calcareous species from both seep and non-seep samples were analyzed for their stable carbon isotopes and then examined for (1) food vacuole contents such as inorganic detritus and bacterial, algal, and metazoan remains, (2) heterotrophic symbiont presence/absence, and (3) intact organelles (e.g., nuclei, mitochondria). The mean δ¹³C value of non-seep conspecifics was less depleted than that of conspecifics collected from under seep clams. The range in δ¹³C in non-seep rose-Bengal stained specimens was 4.7 ‰ (-1.5 to 3.2‰) while the range in δ¹³C of seep specimens was more than doubled (10.6‰; -8.5‰ to 2.1‰). Cellular variations exist among and between species: diet and symbiont presence is inconsistent in both settings. Further, rose Bengal stained foraminifera often lack identifiable organelles but house abundant bacteria, suggesting that bacterial attack can cause foraminiferal mortality. A synthesis of isotope and cellular ultrastructural results will be presented. Supported by NSF projects OCE-0551001, OCE-0550396, & OCE-0550401.