The role of basalt weathering in the Sr isotope budget of the oceans

Using a coupled fluid flow-mass transfer model to calculate the Sr flux res ulting from the weathering of the young Columbia River basalts, it can be s hown that the dissolution of the reactive phases in the extensive continent al flood basalt could have caused the significant inflection in the marine Sr isotope record that occurred approximately 15 million years ago. The res ults of our model demonstrate the important and often overlooked role that the weathering of large igneous provinces can play in geochemical cycles. F urthermore, by approaching the question of continental Sr fluxes to the oce an using a mass transfer model, we are able to establish a quantitative rel ationship between the marine Sr record, chemical weathering rates and atmos pheric 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 t he Columbia River basalts represents an alternative explanation for the gla cial period believed to have followed its formation. We also consider the p otential role that basalt weathering could play in the evolution of the Sr- 87/Sr-87 ratio of the oceans since the early Cretaceous. Rapid Sr release d uring basalt dissolution combined with variations in the relative exposure of young vs. old lithologies could explain changes in the Sr-87/Sr-86 ratio of the oceans without calling on dramatic increases in the continental Sr flux over that time period. Our results demonstrate the importance of accou nting for changes in the relative exposure of different lithologies when co nsidering the nature of global geochemical cycles in the geologic past. (C) 1999 Elsevier Science B.V. All rights reserved.