Planktic foraminiferal dissolution in the twilight zone

摘要

Calcite dissolution of planktic foraminifers that settle from the surface ocean to depths has been assessed using the weight/size relationship of tests and correlated to carbonate ion concentration ([CO_3~(2-)]) of ambient sea water. Globigerina bulloides and Globigerinita glutinata were sampled from the North Atlantic and the Arabian Sea during different seasons (spring and fall and the monsoons, respectively). Test weight has been determined for single tests from narrow size classes (250 ± 8 μm and 300 ± 7 μm minimum test diameter). Initial test weight of both species in surface waters differs between regions and seasons. Weight loss of settling tests in the twilight zone between 100 and 1000 m water depths averaged 19%. Below the twilight zone (1000-2500 m) no weight loss occurs on average, and tests may even gain weight. Remineralization of settling planktic foraminiferal tests is most pronounced at maximum ΔCO_3~(2-). Accordingly, strong dissolution occurs in the twilight zone, at the depth of maximum decrease in ΔCO_3~(2-). It is assumed that dissolution of planktic foraminiferal calcite is caused by CO_3~(2-) (under-) saturation inside the test, buffered by diffusive exchange of CO_3~(2-) ions with the ambient sea water through the pores and aperture. Consequently, reconstruction of small-scale differences in seawater [CO_3~(2-)] from test weight alone is not feasible and systematic correlation between test weight and ΔCO_3~(2-) of the ambient sea water is statistically not significant. At constant [CO_3~(2-)], remineralization cannot be deduced from test weight, neither at high nor low CO_3~(2-) saturation. In total, our data suggest that the global vertical planktic foraminiferal CaCO_3 flux is about 19% lower than calculated by Schiebel [2002. Planktic foraminiferal sedimentation and the marine calcite budget. Global Biogeochemical Cycles, 16 (4), 1065].