"Chemical weathering exerts a key control on global CO2 cycles and ocean chemistry. Magnesium is a particularly important aspect of seawater chemistry, given that it controls the nature of carbonate precipitation is important tool in the reconstruction of past ocean temperatures. Stable isotopic tracers are increasingly being used to identify sources of Mg and other constituents in rivers and oceans and in marine sediments and sedimentary rocks to deduce paleoclimates, ancient ocean chemistry and circulation, and other aspects of Earth’s history. However these interpretations are entirely dependent on the processes that may fractionate the isotopes during mineral weathering and (bio)geochemical cycling.This proposal seeks to develop a novel biogeochemical multi-tracer using Mg and Li isotopes and apply it to test and interrogate the hypothesis that the isotopic composition of Mg in rivers is influenced by chemical weathering reactions in catchment soils. In particular, it seeks to understand processes in soils, the ultimate reactor in which the riverine flux to the oceans is determined. As an intensive case study, the isotopic multi-tracer technique will be applied to a large set of natural samples previously obtained from the Luquillo Mountains in Puerto Rico. This dataset will be used to demonstrate the potential of the multi-tracer technique and to move forward the models of biogeochemical mineral nutrient cycling that I am developing for the Luquillo Critical Zone Observatory (LCZO). The Mg-Li multi-tracer will also be applied on a global scale, by analyzing a large suite of soil samples from a variety of climates, latitudes, and rock types, which are currently archived by Professor Vance in Bristol."