Calcium isotope ratios in calcitic tests of benthic foraminifers

摘要

We analysed Ca isotope ratios of the infaunal foraminiferal species Cassidulina laevigata, Elphidium spp., Gyroidinoides spp., and Uvigerina peregrina as well as the epifaunal Cibicidoides wuellerstorfi over a depth-temperature gradient. The calcium isotope ratios of the benthic foraminifers' tests show a distinct anomaly around 4 degrees C. This anomaly is observed in both epi- and infaunal species, but there are also characteristic differences between both. The delta(44/40) Ca characteristics of benthic foraminifers may be explained by the interference of temperature and carbonate chemistry of the fluid on Ca isotope fractionation. We suggest that Ca isotope composition of benthic foraminifer tests is dominantly controlled by temperature at temperatures higher than 5 degrees C, while in the temperature range below 5 degrees C, calcite saturation (Omega(Cc)) becomes an increasingly important controlling factor, due to the non linear shape of the Omega(Cc)-delta(44/40) Ca relation of actively transported calcium. The potential applicability of Ca isotope ratios in benthic foraminifers for reconstruction of either temperatures or delta Ca-44/40(sw) is limited by the complex fractionation pattern below 5 degrees C. However, due to the minor temperature dependence in the temperature range above 5 degrees C, benthic foraminifers might provide a possible alternative archive for delta Ca-44(sw) avoiding some uncertainties associated with planktic records (relocation, habitat/calcification depth, short time climate variability) as long as a defined bottom water temperature above 5 degrees C can be secured.