Research Interests

The 13C-18O carbonate paleothermometer: The hottest thing off our presses is a carbonate paleothermometer based on the ordering of 13C and 18O into bonds with each other in the carbonate mineral lattice. This differs in important ways from Urey's classic carbonate-water oxygen isotope thermometer (or similar ones based on other sets of phases. Most significantly, the 13C-18O thermometer is based on a homogeneous equilibrium (an exchange reaction involving only components of a single phase). Therefore, it rigorously constrains temperature without needing to know the oxygen isotope composition of water, or anything else besides carbonate). This property of the 13C-18O thermometer promises to resolve many long-standing problems in paleoclimate research and paleothermometry. Our first application focused on determining the growth temperatures of soil carbonates from the Altiplano, which can be compared to the 'surface lapse rate' to constrain its uplift history. Ongoing applications include the thermal histories of aqueously altered meteorites, marine paleoclimate studies in 'deep' geologic history (the mesozoic, paleozoic and late pre-cambrian), and the body temperatures of the dinosaurs (roar!). These applications are anticipated to be the tip of a large iceberg. This is an ideal program for an ambitious new student or postdoc to join...

Multiply-substituted isotopologues of CO2 in air: The same set of instruments used to study 13C-18O ordering in carbonates can be used to measure the abundances of the rare carbon dioxide isotopologue, 13C18O16O, in air and other natural gases. This molecule has several interesting and exotic properties, and as a result it can provide unique constraints on the origins of CO2 and the budget of atmospheric CO2. For example, the concentration of this isotopologue is sensitive to whether or not CO2 comes from high temperature sources (car exhaust or forest fires) vs. low-temperature sources (e.g., respiration), independent of the ∂13C of that CO2. We are applying this measurement to understand the atmospheric budget of CO2 and the mechanisms of its production and consumption in model systems. In the near future, we will begin similar studies of multiply-substituted isotopologues of other atmospheric gases.

Earth's atmospheric H2 budget: Molecular hydrogen (H2) is the tenth most abundant molecule in earth's atmosphere, and the second most abundant reduced gas, after methane. It participates in or indirectly influences photochemical cycles of OH, methane and ozone, and has a poorly understood role in the microbiology of soils. The budget and environmental chemistry of H2 is not well known, and this ignorance poses a significant problem for predicting the possible consequences of anthropogenic emissions of H2 that might be associated with the transition to a 'hydrogen economy'. We have been examining these problems by modeling the effects of loading the atmosphere with H2, characterizing the stable isotopic (D/H) variability of atmospheric H2, and determining the isotopic characteristics of H2 of different origins. This work is rooted in a new analytical capability we developed for precise analysis of the ^D of H2 in small (liter and smaller) samples of air. Our most recent work on this problem focuses on H2 uptake by soils.

Hydrogen budget of the earth's interior: A large fraction, perhaps the majority, of earth's hydrogen atoms are contained in nominally anhydrous mantle minerals (e.g., pyroxene and olivine). This hydrogen exerts strong influences on the mantle's rheology and melting properties, but little is known about its distribution and budget. We are studying the mantle H cycle through hydrogen isotope analyses of nominally anhydrous minerals in mantle xenoliths and of glasses and minerals in mantle-derived lavas. This work has revealed that most nominally anhydrous minerals a strongly D-depleted relative to most of earth's H, including that extracted from the mantle dissolved in silicate melts. We believe this property can be understood most of the H contained in the earth's nominally anhydrous mantle minerals is the residue of near-complete dehydration and partial melting of subducted slabs. We are in the process of constructing a more general model for the mantle H cycle that integrates this exotic property of nominally anhydrous minerals with D/H ratios of other mantle materials.

Origin and evolution of igneous rocks: Basaltic lavas are the most widespread (if indirect) samples of the chemistry of the Earth's mantle. We have a range of projects involving uses of the isotopic composition of oxygen as a monitor of the presence and abundance of crustal materials (both subducted and from the existing lithosphere) as components to the sources of basaltic lavas. Oxygen isotopes offer unusual constraints on these issues because oxygen is the only element that is present in nearly equal abundances in most natural solids, melts, and fluids, and the isotopic composition of oxygen is sensitive to fractionations that occur at or near the Earth's surface; therefore discriminating between materials that have resided in the crust and those that were produced by differentiation within the mantle. Recent developments in this program include study of global geochemical variations of mid-ocean ridge basalts, an examination of the impact crustal contamination has on high-magnesium Icelandic basalts, detailed study of the geochemistry of Central American arc lavas, and study of samples recently recovered by drilling Mauna Kea and Koolau volcanoes in the Hawaiian islands.

Aqueous alteration of meteorites: Aqueously altered meteorites are rare but provide some of our only windows onto the origins and geochemical cycles of water on bodies other than the earth. We have previously studied the H and O isotope geochemistry of the aqueously altered Martian meteorites and carbonaceous chondrites. A project currently underway applies the 13C-18O carbonate paleothermometer to carbonate minerals in these rocks. These measurements will determine the temperatures of aqueous alteration and the oxygen isotope compositions of waters that infiltrated the SNC and carbonaceous chondrite meteorites - two key pieces to the puzzle of aqueous chemistry in the crust of Mars and the interiors of primitive solar system bodies.

Stable isotope geochemistry of 'cold' environments: The stable isotopes of light elements are useful for study of the atmospheres and other near-surface volatiles on the earth and other planets. In many environments of interest (the earth's upper atmosphere, the atmospheres of Mars and the satellites of the outer planets, and comets), fractionations of stable isotopes take place at temperatures far below that at the earth's surface. Stable isotope fractionations under such conditions are largely unknown. We have conducted experiments constraining equilibrium and kinetic isotopic fractionations in such systems. These data will eventually be needed to interpret measurements of the isotopic composition of the Martian atmosphere. They also reveal fascinating and counter-intuitive fractionation behaviors (e.g., 'reversed' vapor pressure isotope effects) and are intrinsically interesting as insights on the physical chemistry of isotopically substituted molecules. In the future, this work will focus on vapor-pressure isotope effects for multiply-substituted isotopologues.