Vertical distributions and stable isotopic compositions of live (stained) benthic foraminifera from the North Carolina and California continental margins

The vertical distributions of live (Rose Bengal stained) benthic foraminifera were determined in Soutar box cores from six sites on the North Carolina continental margin (337–1477 m) and three sites on the California continental margin (786–3705 m). Stained specimens of the most abundant taxa were analyzed for their carbon and oxygen isotopic compositions. Bottom water δ13C values and pore water δ13C profiles were determined at seven of the sites to aid in interpretation of the live benthic foraminiferal δ13C data. undance profiles of most benthic foraminiferal species show consistent patterns within the sediments at each site. These patterns enable us to characterize taxa as epifaunal and shallow, intermediate, and deep infaunal. The vertical range of the living assemblage is small (2–4 cm) in several of the cores, presumably as a consequence of the relatively high organic carbon fluxes and correspondingly small oxygen penetration depths in these continental margin environments. At each site, species with deeper within-sediment microhabitats have lower average δ13C values than do shallow-dwelling species. The δ13C offset from bottom water for each species is larger at high bottom water oxygen sites (North Carolina) than at low bottom water oxygen sites (California). Both of these observations are consistent with a pore water influence on benthic foraminiferal δ13C. The Atlantic-Pacific differences rule out a constant species-specific fractionation as the explanation for the δ13C values of these foraminifera. Despite substantial downcore changes in pore water δ13C, foraminiferal δ13C values for most species change very little over the entire depth range of stained individuals within each core. This lack of vertical δ13C variation may imply that calcification takes place in a relatively small sub-zone of the microhabitat range suggested by the distribution data.