Reconstruction of the silicate weathering feedback in the mid-Cretaceous high-CO2 world

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The interaction and secular relationship between tectonic and climatic processes are key issues for our understanding of global carbon cycle variability. The mid-Cretaceous period (130-90 Ma) witnessed elevated rates of continental rifting and oceanic crust production, resulting in increased atmospheric CO2 and pronounced greenhouse climate conditions. Atmospheric CO2 reduction, in turn, is achieved either by organic matter burial, by changes in the intensity of silicate weathering or a combination of both. The lithium isotopic composition of marine carbonates is a proxy for the intensity of the silicate weathering feedback and records secular changes in seawater δ7Li. The seawater δ7Li reflects the balance of sources and sinks controlled by variations in the weathering of continental and oceanic silicates due to the isotopic fractionation during the formation of clays. With this project we target the generation of a new mid-Cretaceous seawater lithium isotope curve for the interval between 130-90 Ma ago to assess the silicate weathering feedback, analogue to the already existing record for the Cenozoic. We will use the archives of bulk carbonate and pristine preserved skeletal calcites (shells of brachiopods, belemnites and bivalves) for which a well-dated collection of specimens exists. The new record includes the two oceanic anoxic events OAE1a and OAE2 as well as intermittent cold phases in the late Aptian/early Albian and Cenomanian and will serve a reference for weathering during extreme greenhouse climate conditions. The main expected deliveries of this project will be 1) an improved knowledge about the secular evolution of mid-Cretaceous seawater δ7Li values, 2) information about mid-Cretaceous background δ7Li values to assess its long-term variability relative to OAEs, the seawater strontium isotope record, and the amount of basalt weathering, 3) new information about the congruency (intensity) of weathering relative to intermittent cold phases, and 4) estimates about potential uncertainties in δ7Li relative to different degrees of skeletal calcite preservation and taxon-specific vital effects of lithium isotope fractionation among different calcifiers.