Our research in the area of paleoclimate focuses on two key aspects for understanding the evolution of Earth's surface and climate over time: (1) the role of chemical weathering processes in regulating the geologic carbon cycle over million-year time scales, and (2) the history of rainfall and soil water infiltration over thousand-year or glacial-interglacial time scales.
Over geologic timescales, mineral-fluid or “chemical weathering” reactions transfer carbon, sulfur, phosphorous, and major rock-forming elements to the oceans. The rate of transfer of these elements between different reservoirs establishes geochemical cycles that control the composition of the sediments, the continents, the atmosphere, and the oceans. For example, chemical weathering transfers calcium and bicarbonate from continents to the oceans where it precipitates irreversibly as calcium carbonate, effectively moderating atmospheric CO2 over million year time scales. However, there are few reliable measures of past weathering rates. Most current models of elemental cycles therefore rely on empirical relationships between weathering rates and factors such as erosion rates, river discharge, temperature, rock type, ecosystem type, and the age of the weathering minerals. Only if we can determine the fundamental mechanisms that link weathering rates to these variables, can we build models that accurately recreate Earth’s past environments and accurately forecast the impact of future anthropogenic perturbations.
Our current research in this area seeks to extend information from detailed studies of modern systems to consider the global carbon cycle. Using reactive transport theory and model simulations, we have developed a simple but mechanistic approach to explain the chemical response of large river systems to rainfall, topography, and erosion rate.
Mineral-fluid reactions in terrestrial environments leave behind isotopic signatures that are stored in soil minerals such as calcite and opaline silica. These isotopic signatures can provide clues to past conditions at Earth’s surface, similar to the climatic information provided by ice cores and marine sediments. However, compared to our knowledge of paleoceanography from marine sediments, our knowledge of terrestrial paleoclimate is less extensive. High spatial resolution isotopic analyses of soil materials, such as calcite, clays, and opal present many opportunities for reconstructing spatially variable processes such as atmospheric circulation and rainfall patterns.We are currently working to determine if uranium isotopes provide a quantitative measure of past rainfall in soils and speleothems (i.e. secondary mineral deposits formed in limestone caves). This approach would be useful as there are very few ways to reconstruct past rainfall.
A pre-requisite for any paleoclimate study is precise knowledge of when the rock or sediment formed. Using two analytical facilities in the School of Earth, Energy & Environmental Sciences, we have developed a range of techniques for dating minerals such as carbonates, and other common materials, such as opal. We have developed MC-ICPMS methods that yield precise age information on bulk samples using both the uranium-thorium and uranium-lead age dating approaches. For complex samples that cannot be analyzed as bulk samples, we have developed methods using the SHRIMP-RG. This approach allows us to date the materials at a spatial resolution of approximately 100 micrometers, also using either uranium-thorium or uranium-lead age dating approaches.