Temporal record of lithium in seawater: A tracer for silicate weathering?

This paper presents multi-species records of the Li/Ca ratio and Li isotopic composition (δ7Li) of planktonic foraminifera from the Pacific and Atlantic oceans for the past 18 Ma. The Li/Ca record is corrected for interspecific offsets determined from recent (Holocene) foraminifera; interspecific offsets in δ7Li are not significant. Despite different diagenetic histories, the records produced from both oceans are remarkably consistent. Corrected planktonic foraminiferal Li/Ca ratios range from 6.3 to 10.9 μmol/mol, while planktonic foraminiferal δ7Li ranges from 25 to 31‰. Both records are interpreted in terms of long-term changes in seawater Li/Ca and δ7Li, enabling issues related to higher-resolution variability in Li/Ca and δ7Li to be ignored. By assuming that the hydrothermal flux of Li into the oceans, and the flux of Li removed from the oceans during low-temperature uptake by marine basalts and sediments, have not changed significantly since 18 Ma, and using published records for the seawater calcium concentration, the seawater Li/Ca and δ7Li records can be used to estimate global average river δ7Li and Li fluxes. Our records indicate that the river flux of dissolved Li decreased between 16 and ∼8 Ma while the δ7Li value of the river input increased. These data imply that both silicate weathering rates and weathering intensity decreased over this interval which may have been responsible for putative increases in levels of atmospheric CO2. In contrast, the riverine flux of Li has increased since ∼8 Ma while its δ7Li value has increased. This implies that the silicate weathering rate has increased, while weathering intensity has decreased, since that time.