Research
Below is brief description of various research themes going in on the group. They include experiments, construction of records from the modern and Earth's past, and data compilations.
Carbonate clumped isotopes
We apply carbonate clumped-isotope measurements to a variety of problems ranging from paleoclimate applications to understanding the processes that set and modify the isotopic compositions over geologic time using both experiments and environmental samples. A few example recent projects are given below.
Hackberries as a paleoclimate proxy
Former postdoctoral scholar Rebekah Stein (now an assistant prof. at Quinnipiac) calibrated carbonate-clumped isotope measurements of hackberry seeds (carbonate seeds in fruit). The above figure (from Stein et al., in press; P&P) shows that for samples that formed in equilibrium, reconstructed temperatures are reasonable growing-season temperatures.
Deep-sea diagenesis
Andrew Turner (a current PhD student) is working to study how burial of carbonate in deep-sea sediments modifies the clumped-isotope composition of carbonates with a focus on conversion of sediments to limestone via pressure solution processes. We are studying this to better understand how lithification may affect the final stable isotope compositions of carbonate rocks preserved in the rock record. The unpublished figure below shows how at depths of ~800 meters below sea floor, clumped isotope temperatures increase rapidly, which we interpret to be due to the conversion of chalk to limestone via pressure solution increasing measured clumped-isotope temperatures.
Origin of modern carbonate mud
Ziman Wu (a current PhD student) is collaborating with Adam Maloof's group at Princeton to study how carbonate mud and ooids from the Bahamas form and then are diagenetically modified to form rocks. In the below figure from Geyman et al. (2022; PNAS), the carbonate clumped-isotope temperatures (Δ47 temperatures) were used to show that carbonate mud forms as water upwells onto the Bahamas platform vs. forming on during whitings on the platform itself.
Isotopic composition of methane
We are engaged in a variety of studies to better constrain the processes that control the carbon, hydrogen, and clumped isotope composition of methane using experiments, models, and environmental samples for both microbial and thermogenic gases. Some example projects are shown below.
Experimental determination of the equilibrium hydrogen isotopic composition of methane
Andrew Turner (a current PhD student) made the first ever experimental determinations of the equilibrium isotopic composition between methane and water at microbially relevant temperatures (<120°C). He found that the environment in which the microbial methane was formed determined its hydrogen and carbon isotopic composition. Figure from Turner et al. (2021; GCA)
Origin of methane from larger hydrocarbons
Former postdoc Daniel Eldridge (now at Los Alamos National Laboratory) studied what processes controls the isotopic composition of methane generated via the breakdown of larger hydrocarbons. This is important to understand in order to fingerprint sources thermogenic methane in the environment and atmosphere. There are very few controlled experiments to study this. As shown in the figure above (from Eldridge et al., in review) the generation of methane from ethane is controlled by both reversible and irreversible chemical reactions that set the ultimate isotopic composition of methane.
Clumped isotopic composition of wood
We have developed the first techniques to measure the clumped isotopic composition of methoxyl groups (CH3-O-R) from wood and are currently studying it as a proxy for past photorespiration. We are quite excited about where this project could lead.
Former postdoc Max Lloyd (now an assistant professor at Penn State) developed this technique while at Berkeley. The figure to the left from Lloyd et al. (2021; GCA) shows our initial survey of Δ13CH2D and Δ12CHD2 values derived from methoxyl and methyl groups from wood, methanol, chloro- and fluoromethane, methyl iodide, and syringaldehyde. We observe wood samples are systematically different from other methyl groups in Δ13CH3D values and are currently exploring the controls on this as a proxy for past wood photorespiration.
History of ocean-mantle interactions as seen by island arcs and altered oceanic crust
We have been using the chemical and isotopic composition of ancient island arcs and altered oceanic crust to reconstruct how the Earth's surface and mantle have co-evolved. Most prior work has involved data compilations, but new work involves field work and new measurements. This work is an active collaboration with Don DePaolo at UC Berkeley and Claire Bucholz at Caltech.
Figure from Stolper and Keller (2018; Nature) showing that the amount of oxidized iron (Fe3+) vs. total ion (ΣFe) in submarine basalts changes with time with a significant increase ~540-400 million years. We interpreted this to represent the time when the deep ocean became oxygenated and hydrothermal circulation could oxidize submarine basalts.
In a follow on study to that given to the left, we (Stolper and Bucholz, 2019; PNAS) studied the Fe3+/ΣFe ratio of island arcs through time and found they also increase ~540-400 million years. We interpreted this change to be due to the subduction of oxidized material to the mantle that began at this time. This idea is summarized in the figure below.
Conceptual figure from Stolper and Bucholz (2019; PNAS) summarizing our working model for how the oxygenation of the deep ocean altered the oxidation state of oceanic crust, the sub-arc mantle, and island-arc rocks.