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Paleoclimate, Weathering & Paleohydrology

Our research in this area focuses on both the long-term geologic carbon cycle and the relationship between climate and weathering, and the short-term shifts in the water balance over the western U.S. over glacial-interglacial climate states.
M.S. student Claire Kouba and her field of lysimeters, gas wells and nascent sprouts of poison oak at the Feather River, CA field site where an interdisciplinary group of researchers are examining the coupling between erosion, chemical weathering and hydrology.
M.S. student Abe Torchinsky tends to a tensiometer at the Feather River, CA field site.
M.S. students Valerie Rosen and Claire Kouba ponder weathering processes from a steep slope in the Feather RIver, CA.
Professor Kate Maher supervises the water sampling from a bed of poison oak, Feather River, CA.
Our team of research collaborators, including the poison oak - proof tyvek suit, congregate at the end of a successful field trip.
The goal of our paleohydrology research is to develop to use lakes and soils to reconstruct changes in precipitation over thousand-year timescales. This information then allows us to assess climate model predictions and better understand large-scale controls on rainfall.
This map shows the precipitation anomaly (%) predicted at the Last Glacial Maximum by a climate model (red= drier than modern, blue=rainier). Using isotope measurements of lakes and soils we are able to ground truth model predictions like this.

>>See our research in the news: "Stanford scientists solve mystery of ancient American lakes" in the Stanford Daily (original paper in GSA Bulletin)

>> Read about the long-term moderation of Earth's climate by chemical weathering processes in: "How Earth Can Cool Without Plunging Into a Deep Freeze" (original paper in Science)

Research overview: 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.

Chemical Weathering: 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. 

Paleohydrology: 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. 

Geochronology: 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.