Multi-proxy Reconstruction of Seawater Chemistry Across K-Pg Boundary: Tracking Weathering Feedbacks in Response to Extreme Carbon Cycle Perturbation

Abstract:

On geologic time scales concentrations of atmospheric CO₂, a greenhouse gas and critical mediator of Earth’s surface temperature and climate, is thought to be controlled by a balance between CO₂ input from mantle degassing through volcanism and metamorphism and consumption via temperature-sensitive chemical weathering of tectonically uplifted continental rocks. This interplay between global climate and tectonic uplift also controls the delivery of cations to the oceans. Hence, past changes in seawater chemistry provide a powerful archive of the interplay and feedback between climate and tectonics. Mass Extinction Events, like that at K-Pg boundary, are characterized by rapid, global Carbon Cycle Perturbations either from increased mantle degassing or by incineration of the continents due to extra-terrestrial impact. It is hypothesized that enhanced chemical weathering of continental silicate rocks consumes this excess CO₂ and restores steady-state.

Lithium, B, and Mg are conservative ions in seawater that are isotopically homogeneous with a residence time much longer than the oceanic mixing time. As a result, δ⁷Li_SW, δ¹¹B_SW, and δ²⁶Mg_SW, recorded by marine calcites reflect a global picture and secular variations in isotopic composition of these elements within periods shorter than their residence time must thus reflect imbalances between the sources and sinks of these elements to and from the ocean. Cenozoic δ⁷Li_SW shows an abrupt 5‰ drop across the K-Pg boundary, simultaneous with the seawater Ir and Os isotope spikes. This rapid decrease in δ⁷Li_SW is due to a large instantaneous delivery of isotopically light Li to the oceans and cannot be produced by an impactor nor by Deccan trap volcanism, suggesting large-scale continental denudation. We will create high-resolution δ⁷Li_SW, δ¹¹B_SW, and δ²⁶Mg_SW records across K-Pg boundary using planktonic and benthic foraminifera from multiple ODP/DSDP sites to quantify the amount of C excursion and the response of continental weathering feedbacks to regain climatic steady states. This multi-proxy approach will help quantify the extent of CCP; time scale and magnitude of continental chemical weathering response; and the contribution of both carbonate rock and silicate rock weathering in CO₂ drawdown across carbon cycle excursion episodes.