Assessing Changes in Neogene Continental Silicate Weathering Rates Using Lithium (Li) Isotopic Compositions of Foraminifers.

The global climate of the Neogene period is characterised by a dramatic transition from hothouse to icehouse climate modes. Most hypotheses as to the cause of this long-term cooling invoke changes in carbon partitioning between the carbon reservoirs of the atmosphere, biosphere, oceans and mantle. Continental silicate weathering has long been proposed as an important process to change this partitioning. Despite this recognition the interplay between climate, tectonics and weathering is still poorly constrained. Therefore in this study we investigate this mechanism further by development of the promising Li proxy. We measure the Li isotopic and elemental composition of Neogene foraminifers with a view to documenting perturbations in the Li geochemical cycle, which likely reflect variations in continental silicate weathering. Before a reliable temporal record can be obtained, it is important to assess environmental controls on Li isotopic fractionation during Li incorporation into foraminiferal tests. Our high-precision multi-collector plasma mass spectrometry measurements (± 0.3‰ 2std) demonstrate a resolvable δ7Li variation within Holocene species whilst analyses of Pleistocene Globigerinoides ruber (white) are consistently lighter (~ -0.7‰) at two sites during the Last Glacial Maximum. All these species grew in seawater with the same ambient δ7Li and suggest environmental parameters are important in Li incorporation during biomineralisation. This variability of δ7Li in all planktic and benthic foraminifers appears to be linearly correlated with the temperature at which they grew. The response of foraminiferal Li/Ca to ambient conditions is more complex showing both temperature and carbonate ion dependence. Despite these complexities it is possible to recreate the Li isotope composition of the oceans using a multi- proxy approach. Our temperature corrected record shows that Miocene seawater had δ7Li ± 3‰ lower than present day. This suggests weathering rates were lower in the early Miocene and that they increased during the late Miocene to the current (relatively) high level over the last 5Ma. In contrast to previous tracers and treatments of Li archives this demonstrates that Li weathering flux varies in concert with climate variation, yet not with any significant documented variation in pCO2.