The lithium isotopic composition of Late Cretaceous seawater: Implications for a correlation between weathering, carbon cycle and climate

The cause and feedback mechanisms of Late Cretaceous climate cooling are not well understood to date. To evaluate the role of silicate weathering as a thermostat and sink for atmospheric CO2, we generated a lithium isotopic record (δ7Li) of Late Cretaceous seawater using chalk as an archive.

The Late Cretaceous δ7Li record (91–66 Ma) shows an overall increase by 3.5‰. Superimposed, five positive δ7Li events occur during this period, which mostly correlate with δ13C anomalies in the late Santonian, latest Campanian to earliest Maastrichtian and mid-Maastrichtian. In contrast, the δ7Li event in the mid-Campanian shows no correlation with a δ13C anomaly. The overall increase in δ7Li correlates positively with the increase in seawater 87Sr/86Sr. The general shape of the δ7Li curve resembles the evolution of deep-sea temperatures based on benthic foraminiferal δ18O suggesting a link among the weathering flux, lowered atmospheric CO2 concentrations and climate cooling. The apparent covariation between δ7Li and δ18O indicates a strong connection between the congruency of silicate weathering and climate change on a multi-million-year time scale. In contrast, the five positive δ7Li excursions correspond to periods of decreased rates of change in seawater 87Sr/86Sr, typical for a decelerated continental weathering flux.

Here, we suggest that enhanced clay mineral formation in lowlands, increasingly exposed during sea level falls, promoted more incongruent weathering and strong lithium isotope fractionation. The relation between the positive δ7Li and δ13C events appears to be more complex and requires further investigation through box models.