SEDIMENT MICROHABITAT EFFECTS ON CARBON STABLE ISOTOPIC SIGNATURES OFMICROCOSM-CULTURED BENTHIC FORAMINIFERA

Environmentally controlled sedimentary microcosms were used to experim entally determine temperature, salinity, and depth-horizon effects on delta(13)C(DIC) profiles of dissolved inorganic carbon (DIG) in sedime nt pore water (PW) and near-bottom water (BW). Simultaneously, benthic foraminiferal populations of Ammonia beccarii were cultured and assay ed for calcite delta(13)C to establish how accurately foraminiferal sh ells recorded BW-PW delta(13)C(DIC). One treatment population was rest ricted to the 0-1-cm sediment depth; in another treatment, the populat ion could freely ''roam'' up and down in the sediment microcosm. Pore- water DIC between the uppermost 0-0.5-cm sediment layer and the overly ing BW (i.e. 3 cm above the sediment-water interface) showed steep gra dients 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 degr ees C. Correspondingly, shell calcite delta(13)C for A. beccarii colle cted 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 u ppermost 1-cm sediment depth throughout their lives exhibited shell ca lcite Delta delta(13)C values not significantly different from foramin ifera allowed to free-roam through the sediment column.