Accurate and Precise Determination of Li Isotopes in Foraminiferal Carbonate

The rate of continental weathering plays a central role in regulating atmospheric carbon dioxide concentration, and hence global climate, on both long and short timescales. However, despite this recognition there are only a few methods available that allow a reconstruction of the amount of weathering in the past. The flux of Li and the Li isotopic composition of rivers draining continental rocks have been shown to be largely independent of rock composition and are instead primarily controlled by weathering intensity. The Li isotopic composition of seawater is therefore potentially a sensitive measure of global weathering rates. Provided an archive of oceanic Li isotopic composition can be found, this proxy has enormous potential for studies of weathering rate in the past. In this contribution we examine how well planktic foraminfera record the Li isotopic composition of seawater. We have analysed a number of sizes of several common species of Holocene planktic foraminfera from sites in the Atlantic and Pacific oceans using multicollector inductively coupled mass spectrometry. High machine sensitivity ensures that as little as ~1 mg of foraminiferal carbonate is required for each analysis (~1 ng of Li), and rigorous cleaning and screening ensures clay contamination is not significant. In contrast to previous studies of this kind, we find that well preserved G. sacculifer and G. ruber accurately record modern seawater (within our analytical precision of ~0.5 - 2 sd). In sites where preservation is poor however, we find that the measured isotope ratio in G. sacculifer is heavier than seawater by up to a permil. Also, when clay removal has been inefficient we find Li isotope ratios significantly lighter than seawater. We have also extended our record down core and document lighter Li isotope values at 20 Ma. At face value this reflects a record of changing weathering inputs to the ocean but a better understanding of variations in core top Li isotope ratios needs to be obtained before more detailed interpretion can be undertaken.