Sediment microhabitat effects on carbon stable isotopic signatures of microcosm-cultured benthic foraminifera

Environmentally controlled sedimentary microcosms were used to experimentally determine temperature, salinity, and depth-horizon effects on delta(13)C(DIC) profiles of dissolved inorganic carbon (DIG) in sediment pore water (PW) and near-bottom water (BW). Simultaneously, benthic foraminiferal populations of Ammonia beccarii were cultured and assayed for calcite delta(13)C to establish how accurately foraminiferal shells recorded BW-PW delta(13)C(DIC). One treatment population was restricted to the 0-1-cm sediment depth; in another treatment, the population could freely ''roam'' up and down in the sediment microcosm. Pore-water DIC between the uppermost 0-0.5-cm sediment layer and the overlying BW (i.e. 3 cm above the sediment-water interface) showed steep gradients of core-top Delta delta(13)C(PW) averaging -3.6+/-1.6 parts per thousand at 25 degrees C and -2.6+/-0.5 parts per thousand at 20 degrees C. Correspondingly, shell calcite delta(13)C for A. beccarii collected from sediment microhabitats recorded a strong PW influence; delta(13)C shell values averaged 1.1+/-0.8 parts per thousand lighter than BW delta(13)C(DIC) but 1.6+/-0.6 parts per thousand enriched relative to average 0-5-mm PW delta(13)C(DIC). Foraminifera restricted to the uppermost 1-cm sediment depth throughout their lives exhibited shell calcite Delta delta(13)C values not significantly different from foraminifera allowed to free-roam through the sediment column.