Insights from the first detailed record of Late Cretaceous seawater lithium isotopic composition

The late Cretaceous climate is represented by an 8-10 °C decline of global mean temperatures that terminated global warmth of mid-Cretaceous times. Causal mechanisms of the cooling are still not well constrained and discussed in the interplay of reduced volcanic greenhouse gas emission and intensified silicate weathering as a global carbon cycle feedback. The lithium isotopic composition (δ⁷Li) of marine carbonates is a proxy for the chemical weathering intensity of silicate rocks, and thus provides information about the role of silicate weathering as thermostat and sink for atmospheric CO₂.

Here, we present the first detailed chalk-derived Late Cretaceous δ⁷Li record (91-66 Ma) of the boreal white chalk in Northern Germany (Lägerdorf-Kronsmoor-Hemmoor) and from sections in southern England as archive for the seawater lithium isotopic composition. In the course of this study, we will also analyze the archives of skeletal calcite from brachiopods, belemnites and rudists, which should enable us to identify systematic offsets among different calcifiers related to vital effects by the direct comparison of fossilized shells and their surrounding sediments.

To handle the potential impact of clay contamination in bulk carbonates, we applied a pre-leaching and leaching procedure with 1 M ammonium acetate and 0.05 M nitric acid. The method was tested for a 1.85 Ma old sample of coccolith ooze from the Manihiki Plateau (equatorial West Pacific Ocean), which has consistent δ⁷Li values and shows a systematic negative 3-4 ‰ offset to modern seawater. In addition, the degree of potentially leached silicates is monitored by the analysis of E/Ca ratios, like Al/Ca.

In total, our late Cretaceous lithium isotope record shows a trend of rising δ⁷Li values between +16 and +25 ‰. Superimposed, the curve displays a rise in the Santonian, a local maximum in the early Campanian followed by a drop to a local minimum in the late Campanian. Subsequently, the δ⁷Li values rise again towards elevated values in the Maastrichtian. Overall, the shape of the δ⁷Li curve strongly resembles the evolution of deep-sea temperatures based on benthic oxygen isotopes suggesting a close link between climate and weathering. Thereby, more positive δ⁷Li values correspond to cooling periods and the late Campanian lowering of δ⁷Li values parallels the intermittent deep-sea warming. Such a pattern points towards a strong relationship between the congruency of silicate weathering and climate on a multi-million year time scale.