Continental or oceanic weathering processes – What controlled the Late Cretaceous lithium cycle?

The lithium isotopic composition (δ⁷Li) of marine carbonates is known as a proxy for the chemical weathering intensity of silicate rocks. To evaluate the role of weathering as a sink for atmospheric CO₂ during the Late Cretaceous, we generated a 20 Ma δ⁷Li record (86.3–66.0 Ma) using chalk from Northern Germany as an archive. The late Santonian to Maastrichtian record shows an overall increase of ~4.5‰ with superimposed increases in the late Santonian, early Campanian and across the Campanian–Maastrichtian transition.

The overall increase in δ⁷Li fits with the Late Cretaceous increase in seawater ⁸⁷Sr/⁸⁶Sr. Further, the δ⁷Li record strongly resembles both the evolution of deep-sea temperatures based on benthic oxygen isotopes and modeled changes in the rate of seafloor spreading. Thereby, increases of δ⁷Li in the early Campanian and across the Campanian–Maastrichtian transition correspond to major sea level falls, and the latter with a negative δ¹³C excursion.

These coherences allow three interpretative approaches: (1) Climate cooling controlled the weathering congruency and the magnitude of Li isotope fractionation by clay mineral formation on the continents. (2) Lowland exposure promoted erosion together with enhanced soil and clay formation after sea level falls. (3) A temperature-dependent fractionation during low-temperature basalt alteration affected the seawater δ⁷Li signal in a time, when more fresh oceanic basalts where available due to higher spreading rates. The best possible interpretation for the Late Cretaceous δ⁷Li record and the often-neglected aspects of basalt alteration and reverse weathering need to be further assessed and discussed.