Lithium isotope stratigraphy and Ordovician weathering

Abstract

Understanding the role of silicate weathering in controlling the long-term carbon cycle is essential in exploring multi-million-year scale climate variabilities and links to tectonics and changes in biodiversity. The lithium isotopic composition of carbonate rocks (δ⁷Li) is a non-traditional proxy that offers a more comprehensive understanding of silicate weathering dynamics in the geologic past in combination with traditional weathering proxies such as strontium isotopes (⁸⁷Sr/⁸⁶Sr). However, questions related to the effects of diagenesis and the relationship between changes in δ⁷Li and CO₂ consumption during silicate rock weathering hamper the use of this novel proxy to its full potential. Here we present measurements of bulk carbonate δ⁷Li that span the Ordovician Period from six sections across the Laurentian paleocontinent. Using a rigorous textural and geochemical screening, we produce the first high-resolution seawater δ⁷Li (δ⁷Li_sw) curve that spans the Tremadocian to Sandbian of the Ordovician Period (∼487 – 453 Ma). The δ⁷Li_sw curve produced in this study exhibits a ∼4‰ negative shift during the Early to Middle Ordovician. Using comparisons to existing weathering proxies and a numerical modeling approach, we suggest a progressive change toward a low-intensity, congruent weathering regime was a likely driver for the observed trend. This finding highlights potential links between island arc migration toward the tropics, global increase in exposed landmass, possible terrestrialization by non-vascular land plants and changes in silicate weathering.