Earth History

Reconstructing past Earth system change from the stratigraphic record

Past changes in global biogeochemical cycling frequently are reconstructed using
geochemical proxies preserved in ancient sedimentary rocks. Reconstructions of
pre-Cenozoic Earth system change rely primarily on sediments deposited in shallow-water environments, which have complex depositional histories, are prone to diagenetic alteration, and may be influenced by both local and global processes. These complexities can lead to inaccurate and biased reconstructions of past Earth system change.

To tackle these complexities, I developed a new statistical model (preprint) for reconstructing regional and global proxy change over time from the stratigraphic record. By combining stratigraphic proxy data and age constraints from multiple locations, the model infers changes in proxy values over time directly from the data. Bayesian statistics is used to incorporate a wide range of geological information, including superposition relationships, both geochronological and correlative age constraints (e.g., biostratigraphic and lithostratigraphic information), and paleoenvironmental context. In addition to testing hypotheses about past Earth system change, the model provides a framework for stratigraphic correlation and age model construction. The model code is available as an open-source and fully-documented Python package (StratMC).

Spatial geochemical variability

The carbon isotopic composition of ancient carbonate rocks (δ¹³C_carb) is used to reconstruct past perturbations to Earth's surface carbon cycle. However, local carbon cycling, sediment transport processes, and diagenesis partially decouple the chemistry of shallow-water carbonate sediments from the open ocean. As a result,  δ¹³C_carb varies both within and between different depositional environments. To learn about past global carbon cycling from the rock record, we must understand how this local variability is translated to and ultimately preserved in the stratigraphic record. 

Currently, I am studying Paleozoic shallow-water carbonates in the western United States and Canadian Rockies to understand how spatial patterns in carbonate geochemistry can be used to fingerprint different primary and post-depositional processes in the rock record. Ultimately, this work will help us to better understand how information about both global and local seawater chemistry is encoded in shallow-water carbonate sediments. This project combines field observations, drone photogrammetry, computational modeling, and stable isotope geochemistry. 

Pleistocene carbonate geochemistry

Shallow-water carbonate sediments from the most recent Pleistocene glacial cycles provide an opportunity to evaluate the sensitivity of δ¹³C_carb to geologically rapid changes in climate and environment. However, the chemistry of pre-Holocene shallow-water carbonates was partially overprinted by meteoric diagenesis during the last glaciation. To study older time periods, we instead must rely on sediment that was produced in shallow waters and then transported to adjacent slope and basin environments, which are protected from meteoric alteration during sea level fall.

I studied short piston cores from deep waters around the Bahamas carbonate platforms to quantify differences in shallow-water carbonate production and geochemistry between the current Holocene interglacial, the last glacial period, and the last interglacial period. I found that despite differences in regional climate, surface conditions, and sediment production, the δ¹³C of aragonite mud remained constant between the Holocene and the last interglacial period. In addition, mineralogical and stable isotope data suggest that aragonite mud production persists in slope environments during glacial sea level lowstands. This work was published in Earth and Planetary Science Letters.