Determination of the lithium isotopic composition of planktic foraminifera and its application as a paleo-seawater proxy

To investigate the potential use of Li isotopes in foraminifera as a paleo-lithium seawater proxy, the Li isotopic compositions of planktic foraminifera (Orbulina universa, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia truncatulinoides) were determined by thermal ionization mass spectrometry (TIMS), using phosphate as an ion source, and multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Rigorous cleaning procedures were employed to purify foraminiferal tests of detrital, metal oxide, and organic phases in single species analyses. Data generated using both methods reveal that δ⁷Li in planktic foraminifera resemble seawater such that the isotopic fractionation from the precipitation of foraminiferal calcite appears to be minimal. Down-core measurements indicate that the δ⁷Li of O. universa has remained constant over the last 44 ka in the North Atlantic, suggesting little change of Li isotopic composition of ocean water during the last glaciation, consistent with the long mean residence time of Li. Results suggest that planktic foraminifera may provide a reliable record of the lithium isotopic composition of ocean water in the past. The first measurements of seawater δ⁷Li from the Indian Ocean average 33.0 ± 1.2‰ (2σ) throughout the water column. This value is within error of values previously reported for the Atlantic and Pacific Oceans as is expected based on the long mean residence time of Li in the ocean. There is no evidence for a significant decrease in Li/Cl ratios, as reported by some workers, with Indian Ocean waters averaging 9.10 ± 0.11 μg/g.