Over the past several million years, Earth's climate has alternated between warm "interglacials" like today, with land-based ice sheets limited to Antarctica and Greenland, and "glacials," when ice sheets expanded to cover North America as far south as Long Island, NY. Although changes in Earth's orbit are primarily believed to cause these climate changes, both ocean circulation and carbon dioxide represent critical amplifiers of climate change. Around one million years ago, a time dubbed the "Mid-Pleistocene Transition," glacial intervals became much harsher than previously. Recent research has shown that ocean circulation drastically shifted in this interval (Figure from Pena and Goldstein, 2014). I seek to address how this change in ocean circulation affected carbon.
Direct measurements of ocean chemistry only exist for the past few decades. Understanding how ocean chemistry changes in the past, particularly over large climate changes, requires "proxies". Using a proxy to infer past changes in ocean chemistry has two fundamental requirements: 1. The proxy can be reliably and reproducibly measured in the laboratory, and 2. The underlying mechanisms by which the proxy represents the true chemical parameter of interest are known.

The boron isotope-pH proxy is a powerful approach used to interpret how the ocean's carbon content changed in the past. I seek to improve the rigor of our analytical measurements by performing the first direct comparison of two major techniques for boron isotope measurement in a single laboratory (just published!). Also, in collaboration with colleagues at the University of Hawaii-Manoa, University of California-Santa Cruz, and Yale University, I am analyzing a suite of carbonates grown in the laboratory under varying seawater chemistry. Our results will provide better constraints on the inner workings of the boron isotope-pH proxy.
Deep-sea coral skeletons tell us about how the concentration of radiocarbon changes in the ocean abyss. Three corals live-collected in the early 2000s show a pronounced increase in radiocarbon content of the North Atlantic around 1980 (see image). This increase records the input of bomb radiocarbon from nuclear weapons testing to the deep ocean, and give us important information on both the rate and magnitude of human perturbations to the deep sea. In collaboration with colleagues at Princeton University, I am using these rate and magnitude data to test ocean circulation models, and hopefully better constrain how much human-produced carbon is stored in the ocean.
Formation of dense waters in the North Atlantic today fills up nearly half of the deep ocean, serving as an important conduit for heat and carbon transfer. Given the immense climate changes that occurred around the North Atlantic over the last 3 million years, it is likely that North Atlantic ocean circulation and dense water formation was affected. But how this happened, and how this either forced or amplified past climate changes, remains unknown. As part of my postdoctoral studies at Princeton University and Max-Planck Institute for Chemistry, I will measure the nitrogen isotopic composition of organic material bound in foraminifera to test whether the North Atlantic Ocean stratified during this critical climate transition.