Controls on the evolution of Cenozoic seawater chemistry

Abstract

A simple process-based model of the Li, Mg, Ca, Sr and alkalinity cycles in the ocean has been developed with the aim of better understanding the information contained in the evolution of Cenozoic seawater chemistry. In the model, changes in seawater chemistry are forced by changing climatic (temperature) and tectonic (ocean crust accretion rate and continental weatherability) conditions. These drive time-varying element and isotope fluxes from continental and seafloor weathering, mid-ocean ridge hydrothermal systems and sedimentation. Inverse modelling reveals that a range of model parameter values can simultaneously reproduce the key features of the Cenozoic history of seawater chemistry for all species considered. In addition to Cenozoic cooling, a decrease in the rate of ocean crust accretion and/or an increase in continental weatherability are required to fit the data. However, paleoseawater compositions do not allow the relative roles of these tectonic forcings to be determined. While other processes (e.g., changes in surface lithology) may have played a role in generating the observed variations in seawater chemical and isotopic composition, they are not required to explain the general tends in the data. The increase in Mg/Ca over the Cenozoic in acceptable models is largely driven by a decrease in the Ca/alkalinity of the net continental, diagenetic and hydrothermal fluxes, and not by an increase in the Ca/alkalinity of the carbonate mineral sink. Thus, the increase in seawater Mg/Ca over the Cenozoic is readily explained without any decrease in dolomite formation. Notably, all acceptable models have a stronger temperaturedependence of seafloor than continental weathering. This means that climatically driven changes in chemical fluxes in the model come dominantly from seafloor, not continental, weathering.