Fractionation of del30Si and del7Li during supply limited chemical weathering: Towards unified models of stable isotopic responses to weathering

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The conversion of the rock that makes up the bulk of the solid Earth into the soil that blankets the planet's surface takes place through a process called chemical weathering. This process is responsible for many characteristics of the surface environment, impacting not only rivers and soils but also the atmosphere and oceans. Weathering is particularly important in setting the pace of global chemical cycles, including the cycle of carbon, because the solid Earth actually holds vastly more carbon, in the form of carbonate like limestone, than the atmosphere, oceans, and biosphere together. Because the pace of chemical weathering depends on climate, which in turn depends on atmospheric carbon concentrations, weathering is a key piece in the how the Earth system operates, and how it may be respond to human disturbance. It is often difficult to accurately measure weathering rates on today's Earth, and it is even more challenging to try to assess how these have changed over geologic time - making this a central question for today's scientists. One way of trying to get at past changes is to look at records in sediments, for example in lakes or the oceans. Looking at these records requires reliable and well-understood proxies, in other words, measurable characteristics that provide information about weathering processes in the past. This is not a new science; it has been a core part of geological research for decades, and the main outcome of previous work has been to demonstrate that it is difficult to interpret any one single proxy on its own. This realisation has stimulated efforts to develop multiple new proxies of chemical weathering, efforts that have been facilitated by technological advancements allowing improved measurement of the isotopic ratios of key elements involved in weathering reactions. Two elements of particular interest are silicon (Si) and lithium (Li). These are relevant because they are released when minerals in fresh rocks dissolve, but also partially retained in clays that are created during soil formation. During both processes, different isotopes of Si and Li are preferentially released into the waters that drain into streams and rivers. As a result, the isotopic composition of these rivers, and of sediments that may eventually form from them, potentially provides information about the relative rates of chemical reaction and physical removal of material (because this removal inhibits soil formation). This balance is referred to as the 'weathering intensity.' Several years of work, in the labs run by the partners in this study as well as other institutions internationally, have helped to begin quantitatively understanding how the Si and Li isotopic systems behave, and particularly how they respond to weathering intensity in natural settings. This provides a tantalizing glimpse of their potential. However there is a major gap in the present picture, because their behaviour at very high weathering intensities, when soils are thick in tropical environments, is not understood. The research that is proposed in this study will try to fill this gap by measuring the Si and Li isotopic composition of streams draining tropical catchments in Cameroon and Costa Rica. Working at two different sites will make it possible to compare effects on different types of rock. In addition to measuring these isotopic compositions, the proposed work will involve measurements and lab experiments designed to understand the mechanism generating these compositions, in other words, what chemical processes are responsible and how these differ from processes identified in other settings. Finally, the results of the project will be used to make a better quantitative model that may allow application of Si and Li isotopes as proxies for chemical weathering intensity.