Microhabitat effects on Cd/Ca and δ13C of benthic foraminifera

Cd/Ca and δ13C were measured on bottom and pore water samples, and samples comprising dead individuals of six species of benthic foraminifera, including Cibicidoides wuellerstorfi, Uvigerina peregrina and Melonis barleeanum, from throughout the sediment mixed layer at three well-characterised sites in the Northeastern Atlantic. ‘Living’ (i.e., Rose Bengal stained) U. peregrina and M. barleeanum from one of the three sites were also analysed. Co-existing living and dead foraminifera of the same species from the same site have similar Cd/Ca and δ13C, and show no significant down core variability. Therefore, comparison of δ13C in foraminifera with bottom water and pore waters was used to estimate average calcification depths within the sediment for each species and thereby determine DCd based on the Cd concentrations at these depths. Pore waters are 2–4 times more enriched in Cd than bottom waters; consequently, DCd values are different from estimates based on bottom water Cd. Results give DCd of ∼1 for all the infaunal species, with no significant water depth dependence. DCd for C. wuellerstorfi based on bottom water Cd are 3.2±1.1 at 3600 m water depth and 3.9±1.3 at 1900 m water depth, being consistent with DCd estimated from culture experiments. The results suggest that the depth dependence of DCd based on bottom water Cd may be partly explained by a pore water influence on the test chemistry for infaunal species.