Carbon and oxygen isotopic disequilibrium during calcification of Globigerina bulloides in the Southern ocean

Abstract:

Oxygen and carbon isotopes in planktonic foraminifera Globigerina bulloides recovered from the water column of 0–1000 m depth across the meridional transect i.e. 10°N to 53°S of Indian ocean were compared with the available data from the core-top samples across the same transect. We also recorded in situ temperatures of the water column based on probe (CTD) profiles. The δ¹⁸O and δ¹³C values measured in the core top samples matches with the tow results. The equilibrium δ¹⁸O of calcite calculated from known temperature and δ¹⁸O of water column allowed us to compare the observed δ¹⁸O of formaminieral shell with the expected equilibrium values. Our comparison of carbonate composition in the samples between 10°N till 40°S showed excellent match with the expected equilibrium δ¹⁸O values established from the water collected at depth range of ~75-200m, however beyond 40°S the disequilibrium was pronounced with heavier δ¹⁸O (enriched by ~1.5‰) recorded in the carbonate as compared with the expected equilibrium δ¹⁸O values established from water. This observation was further verified with δ¹³C measurement of shell carbonates comparing with the equilibrium δ¹³C of calcite calculated with known temperature and δ¹³C of dissolved inorganic carbon in the water column. The δ¹³C of the shell carbonate was found heavier as compared to the expected equilibrium δ¹³C. Both δ¹⁸O and δ¹³C showed simultaneous enrichment signature in the region beyond 40°S suggesting role of processes such as leaching along with dissolution of shell carbonate in a relatively acidic condition.