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

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences


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.


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Description The main objective, of identifying the controls on isotopic fractionation in these environments, was met through a detailed study of weathering at La Selva Biological Preserve in Costa Rica, as originally proposed. Problems with availability of samples from the Congo basin (another site where we originally proposed to work), and the publication of results from this region by another group, meant that we focused attention in greater detail on the La Selva site. During a ten-day field trip in May 2010, we collected 21 surface water samples, 5 groundwaters, 7 soil pore waters, and 1 rainwater sample, as well as several river sediment (bedload and suspended load) samples, 4 rock samples, and soil samples from two 4m-deep soil pits. These samples were prepared for chemical analysis (including crushing of rock samples and separation of clay fractions from soils) and analyzed for major and trace element compositions, and for _30Si and _7Li. Additional work beyond that originally proposed included _26Mg analyses, and we have archived sample aliquots, including samples of colloids from river and stream waters, that are available for other future geochemical work. A major advantage of the La Selva study site is the direct juxtaposition of depleted-soil weathering (reflected in waters with local sources) and fresh-bedrock weathering (reflected in waters with deep groundwater sources, which emerge within the Preserve at discrete springs). This made it possible to identify the specific isotopic signature of supply limited soil weathering, which was found to have much lighter _30Si and _7Li composition than bedrock weathering, consistent with the limited prior data from tropical rivers. This suggests that the use of these isotope systems as proxies for weathering in the past must carefully consider the possibility that extremely high weathering intensities (as reflected in tropical, supply-limited environments) may deliver dissolved material to the oceans with relatively light dissolved isotopic values. This result is critical for the geochemical and paleoclimate communities as they move forward in thinking about applying these proxies. Beyond this broad characterization, the more detailed analysis of the isotopic composition of the weathering system at La Selva yielded a number of valuable insights. The Si and Li isotope systematics of the bulk soils, pore waters, and fine clay separates are all consistent with a soil weathering system which is evolving through progressive re-weathering of clay minerals. One interesting possibility that this suggests is that the isotopic composition of clay minerals may hold tantalizing information about the soil weathering environment where these clays form. This could provide an exciting avenue for further geochemical research. Another important observation is that fractionation during clay formation, rather than vegetation uptake, appears to be controlling the isotopic systematics of Si at La Selva. Though there is fractionation associated with vegetation uptake, this appears to have limited influence on soil and pore water compositions, a surprising result given the dense biomass. Further collaborative work with US partners to complete Ge/Si analyses promises additional insight into this problem. In constrast to Si, the _26Mg isotope composition appears significantly influenced by biological uptake, with a strong fractionation varying with depth in soils (i.e. highly fractionated Mg at depth in the soils, reflecting the sequestration of Mg in the biomass and in the organic soil horizons above).
Exploitation Route Controls on the chemistry of natural waters The work at La Selva yields valuable new data on weathering and erosion rates in a basaltic environment, which fills an important piece in the broader puzzle of understanding global controls on weathering fluxes to the oceans.
Sectors Environment

Description Summary of impact The main users of the research are other academics, and perhaps conservation bodies. Ultimately, the results from this research may serve to inform policymakers and regulators
First Year Of Impact 2010
Sector Environment
Description Goldschmidt Conference, Prague 2011 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact questions and discussions after talk

Talk led to future collaborations between principal research fellow working on project and new colleagues
Year(s) Of Engagement Activity 2011
Description Goldschmidt Conference, Yokohama, Japan 2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact presentation at Goldschmidt 2016
Year(s) Of Engagement Activity 2016