Over geologic timescales, the chemical weathering of continental silicate rocks regulates atmospheric CO2 levels and thereby global climate. Quantifying this mechanism and its controls is crucial for understanding how Earth became and remains habitable. Calcium (Ca) is coupled to the long-term carbon cycle because as rocks weather, Ca is released in dissolved form, transported by rivers to the oceans, and sequestered during carbonate precipitation from seawater. The Ca isotope composition (delta 44Ca) of marine carbonate thus reflects the history of continental weathering and related changes in atmospheric CO2 concentrations. However, development of the marine Ca isotope record has outpaced understanding of processes eliciting Ca isotope variation during mineral weathering and subsequent transport of aqueous Ca. By analyzing the Ca isotope composition of rivers draining the New Zealand Southern Alps, the goal of this project is to help resolve questions concerning if, how, and the extent to which tectonic and climatic processes influence the Ca isotope budget of seawater. The mountain range has a uniform lithology but large gradients in tectonics and climate. The western side experiences a wet climate, rapid uplift, and high rates of physical erosion, whereas the eastern side experiences an arid climate, low uplift, and low rates of physical erosion. The PI has worked previously in the New Zealand Southern Alps to quantify mineral weathering reactions, atmospheric CO2 consumption rates, and riverine Sr isotope fluxes. Using this framework, the PI will test specific hypotheses concerning controls on the Ca isotope geochemistry of rivers. Sample collection will include waters, suspended sediments, rocks, and vegetation. The PI will measure delta 44Ca values by thermal ionization mass spectrometry and synthesize data using mass-balance modeling, carbonate equilibria calculations, hydrograph separations, and elemental and isotope mixing equations. The intellectual merit of the project is to improve knowledge about the Ca isotope geochemistry of weathering and thereby aid reconstructions of past atmospheric CO2 levels using the marine Ca isotope record. Broader impacts include the teaching and training of a post-doctoral research fellow.
