Calcification depths of planktic foraminifers constrained by geochemical
signatures from sediment-trap samples from the Bay of Bengal
Abstract
The Mg/Ca and oxygen isotope ratios (δ18O) of multiple species of
planktic foraminifers provide information on the hydrological conditions
between the surface and the thermocline. Knowledge of the apparent
calcification depth (ACD) of planktic foraminifers is key to
reconstructing paleoenvironments; however, ACDs exhibit seasonal
variations and differ over regional scales. We obtained the ACDs of
Globigerinoides ruber, Trilobatus sacculifer, and Neogloboquadrina
dutertrei in the Bay of Bengal using multiyear sediment-trap samples
collected at approximately 900 m depth. The sediment traps were moored
in the southwestern Bay of Bengal, with sampling intervals of 17-42
days. The temperature estimates obtained from the δ18O and Mg/Ca
patterns of G. ruber, T. sacculifer, and N. dutertrei indicate that G.
ruber reflects the temperature within the mixed layer, whereas N.
dutertrei precipitates its test in the upper thermocline and T.
sacculifer calcifies between these depths. The rapidly attenuating
photosynthetically active radiation constrains the living depths of
these symbiont-bearing species to within the upper 60 m of the euphotic
zone in the southwestern Bay of Bengal. Although G. ruber and N.
dutertrei calcify at different depths, as demonstrated by the different
δ18O values of the two species (Δ18Or-d), large Δ18Or-d values were not
obtained just in spring and summer when stratification is developed .
The flux-weighted δ18O value of a species corresponds to the mean annual
δ18O value of that species. Seasonal variations in species-specific test
fluxes can be averaged out because of recurring flux peaks during the
northeast and southwest monsoon seasons.