THE ROLE OF BASALT WEATHERING IN THE SR ISOTOPE BUDGET OF THE OCEANS

Abstract

Using a coupled fluid flow-mass transfer model to calculate the Sr flux resulting from the weathering of the young Columbia River basalts, it can be shown that the dissolution of the reactive phases in the extensive continental flood basalt could have caused the significant inflection in the marine Sr isotope record that occurred approximately 15 million years ago. The results of our model demonstrate the important and often overlooked role that the weathering of large igneous provinces can play in geochemical cycles. Furthermore, by approaching the question of continental Sr fluxes to the ocean using a mass transfer model, we are able to establish a quantitative relationship between the marine Sr record, chemical weathering rates and atmospheric CO2 concentrations. The CO2 drawdown rates that we calculate for the weathering of the Columbia River basalt suggest that, on a several million-year timescale, the formation of large igneous provinces represents a net sink for atmospheric CO2. The removal of CO2 via the rapid dissolution of the Columbia River basalts represents an alternative explanation for the glacial period believed to have followed its formation. We also consider the potential role that basalt weathering could play in the evolution of the 87Sr/87Sr ratio of the oceans since the early Cretaceous. Rapid Sr release during basalt dissolution combined with variations in the relative exposure of young vs. old lithologies could explain changes in the 87Sr/86Sr ratio of the oceans without calling on dramatic increases in the continental Sr flux over that time period. Our results demonstrate the importance of accounting for changes in the relative exposure of different lithologies when considering the nature of global geochemical cycles in the geologic past.