Lithium isotopes in foraminifera shells as a novel proxy for the ocean dissolved inorganic carbon (DIC)

Past ocean pH and pCO(2) are critical parameters for establishing relationships between Earth's climate and the carbon cycle. Previous pCO(2) estimates are associated with large uncertainties and are debated. In this study, laboratory cultures of the foraminiferan genus Amphistegina were performed in order to examine the possible factors that control the Li isotope composition (delta Li-7) of their shells. delta Li-7 is insensitive to temperature and pH variations but correlates positively with the Dissolved Inorganic Carbon (DIC) of seawater. Li/Ca ratio in the shells shows negative correlation with delta Li-7, consistent with published data for planktonic foraminifera from core tops and from short periods during the Cenozoic. We propose that the sensitivity of delta Li-7 and Li/Ca ratio to DIC is a biological phenomenon and is related to biomineralization mechanisms in foraminifera. We used the published foraminiferal delta Li-7 records, and our experimental results, to determine the paleo-ocean DIC and pH for the last glacial-interglacial cycle. The results are consistent with published estimates of pH and pCO(2) based on boron isotopes and ice cores. We suggest Li and its isotopes may serve as a new complementary proxy for the paleo-ocean carbonate chemistry. (C) 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.