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 delta(13)C values and pore water delta(13)C profiles were determined at seven of the sites to aid in interpretation of the live benthic foraminiferal delta(13)C data. The abundance profiles of most benthic foraminiferal species show consistent patterns within the sediments al 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 delta(13)C values than do shallow-dwelling species. The delta(13)C 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 delta(13)C. Th, Atlantic-Pacific differences rule out a constant species-specific fractionation as the explanation for the delta(13)C values of these foraminifera. Despite substantial downcore changes in pore water delta(13)C, foraminiferal delta(13)C values for most species change very little over the entire depth range of stained individuals within each core. This lack of vertical delta(13)C variation may imply that calcification takes place in a relatively small sub-zone of the microhabitat range suggested by the distribution data. (C) 1997 Published by Elsevier Science Ltd.