The nitrogen isotope ratio of diatoms in high-latitude sediment cores has been used to probe mechanisms for glacial drawdown of atmospheric carbon dioxide. However, interpretations of this data rely on understanding the dynamics governing the nitrogen isotope composition of surface ocean nutrient reservoirs. In this research, I measured the nitrogen and oxygen isotope ratios of seawater nitrate and nitrate+nitrite, then calculated the isotopic ratio of nitrite. Based on these observations, I proposed that nitrogen isotopes are exchanged between seawater nitrate and nitrite across the nitrite oxidoreductase enzyme of marine organisms. A first-author manuscript based on this research was published in Global Biogeochemical Cycles in 2016 (pdf).

Ice age increases in the nitrogen isotope ratio of organic matter bound within diatom microfossils indicates higher fractional nitrate utilization in the glacial Southern Ocean surface than in the modern. In the Antarctic Zone of the Southern Ocean, associated declines in export production indicate a glacial reduction in the supply of nitrate to the photic zone. In this work, I built a seasonally resolved model of nutrient supply to the Southern Ocean surface calibrated against modern observations. By comparing model results with sediment core observations, I estimate that glacial gross nitrate supply was reduced by ~80% relative to modern conditions. A first-author manuscript based on this research was published in Paleoceanography and Paleoclimatology in 2018 (pdf).

Bubbles of ancient atmosphere stored in Antarctic ice are one of the only direct archives of ancient climate. However, standard ice coring methods have only recovered ice from the last ~800 thousand years, largely missing the mid-Pleistocene transition when the dominant frequency of climate oscillation shifted from 40 thousand year cycles to 100 thousand year cycles. In this project our team recovered the oldest ice ever found by drilling cores near topographic features, with ages derived from argon isotopes exceeding two million years. A manuscript based on this research was published in Nature in 2019 (pdf).