Lithium isotope compositions of various biogenic carbonates throughout the mid-Cretaceous – Challenges of reconstructing δ7LiSW in deep time

The evolution of the seawater Lithium isotope composition (δ⁷Li_SW) depends on the prevalent weathering regime. To reconstruct δ⁷Li_SW, and thus, weathering congruency, two main archives are widely used: calcareous fossils and bulk carbonate sediments. Either of these archives has its strong and weak points. While carbonate sediments are prone to diagenesis and clay contamination, biogenic skeletons may exhibit strong vital effects on δ⁷Li_SW. Here, we present δ⁷Li_SW data for the mid-Cretaceous derived from various biogenic carbonates such as bivalves (mainly rudists), cephalopods, and brachiopods from well-known sample sites across Europe.

Additionally, we assessed the extent of taxon-specific vital effects on δ⁷Li for well-preserved Albian molluscs from Madagascar (Mahajanga Basin, age 110.5 +/-0.5Ma) and compared them with data for modern taxonomic groups.

Our most complete data set derives from rudist shells (mainly requieniid rudists) from the mid-Barremian to early Aptian sample locations of Sausset (Urgonian Limestone Formation, France), Ericeira (Crismina Formation, Portugal), Kanfanar (Kanfanar unit, Croatia), and Miravete (Villarova de los Pinares Formation, Spain). We compare their δ⁷Li compositions and trends therein with that of cotemporaneous carbonate-rich sediments. We provide preliminary δ⁷Li_SW for several time intervals which range between 22-25‰ on average with values exceeding 30‰ in the late Barremian.

Relative to published bulk carbonate data, we observe a systematic offset for rudist-shell δ⁷Li values of 3-8‰ towards heavier values. We assign this offset to vital effects similar to those known for modern calcitic molluscs.