Assessment of Chalk as an Archive for the Lithium Isotope Composition of Seawater

The understanding of silicate weathering and its role as a sink for atmospheric CO2 is important to get a better insight into how the Earth shifts from warm to cool climates. The lithium isotope composition (delta 7Li) of marine carbonates can be used as a proxy to track the past chemical weathering of silicates. A high-resolution delta 7Li record would be helpful to evaluate the role of silicate weathering during the late Cretaceous climate cooling. Here, we assess chalk as a potential archive for reconstructing Late Cretaceous seawater Li isotope composition by comparing Maastrichtian chalk from Northern Germany (Hemmoor, Kronsmoor) to a Quaternary coccolith ooze from the Manihiki Plateau (Pacific Ocean) as a lithological analog to modern conditions. We observe a negative offset of 3.9 +/- 0.6 parts per thousand for the coccolith ooze relative to the modern seawater Li isotope composition (+31.1 +/- 0.3 parts per thousand; 2SE; n = 54), a value that falls in the range of published offsets for modern core-top samples and for brachiopod calcite. Further, the negative offset between the Li isotope compositions of Manihiki coccolith ooze and modern planktonic foraminifera is 2.3 +/- 0.6 parts per thousand. Although chalk represents a diagenetically altered modification of pelagic nannofossil ooze, manifested by changes in the composition of trace elements, we observe a consistent offset of Li isotope data between Maastrichtian chalk and Maastrichtian planktonic foraminiferal data (-1.4 +/- 0. 5 parts per thousand) that lies within the uncertainty of modern values. We therefore suggest that chalk can be used as a reliable archive for delta 7Li reconstructions. Chalk is a reliable archive for the Li isotope composition of seawater Coccolith ooze has a negative offset of 3.9 +/- 0.6 parts per thousand from modern seawater for Li isotope ratios The estimated mean value for the late Maastrichtian seawater Li isotope composition is +27.5 +/- 1.0 parts per thousand