Dr Sarah Fawcett will present the Science Faculty seminar with a talk entitled, "Using the nitrogen isotopes to understand the past and present polar ocean: a plan for research capacity development and student training".  

Abstract: Nitrogen is one of two major nutrients required universally by life on Earth such that studying the nitrogen cycle is fundamental to understanding how our planet works. In the ocean, nitrogen exists in many forms (organic and inorganic, particulate and dissolved, reduced and oxidised) and limits the growth of phytoplankton (the microscopic plants inhabiting the sunlit upper layer) throughout tropical, subtropical, and temperate waters. While not the primary limiting nutrient in the Southern Ocean, nitrogen nonetheless exerts a dominant control on Antarctic productivity and atmospheric CO₂ drawdown. Observations show that nitrate, a nutrient mixed up from deep waters by the wind, is seldom fully consumed in Southern Ocean surface waters due to a combination of iron and light limation of phytoplankton. However, if phytoplankton were to more completely consume this nitrate, they would remove more atmospheric CO₂, thus altering global climate. Nitrogen has two stable isotopes, the natural abundance distributions of which provide a sensitive and integrated view of biogeochemical and physical processes that are highly variable in time and space. The “denitrifier-isotope ratio mass spectrometry  method” can be used to measure the isotopes of almost all nitrogen species. Since its development in 2001, it has become the global standard for marine nitrogen cycle research, facilitating the analysis of samples >100 times smaller than conventional techniques allow and permitting the simultaneous analysis of the oxygen isotopes of oxidised nitrogen species. However, this method has yet to be implemented in any African laboratory. Here, I will describe the effort to develop it at UCT and discuss its utility for Antarctic science. For example, (1) the nitrogen and oxygen isotopes of nitrate can be used to quantify seasonal nitrate drawdown in the surface ocean, which provides a measure of atmospheric CO₂ removal, as well as to disentangle overlapping nitrogen cycle processes that can complicate such estimates of CO₂ drawdown; (2) the nutritional preference and ecological role of important phytoplankton taxa can be deduced from the nitrogen isotopes of organic biomass; and (3) the hypothesis that more complete Southern Ocean nitrate consumption drove the ice-age decreases in atmospheric CO₂ can be tested using the nitrogen isotopes of sedimentary microfossils.

Biography: Dr. Sarah Fawcett is a marine biogeochemist interested in the complex relationships between biogeochemical fluxes (particularly nitrogen fluxes) and primary productivity in the ocean, with implications for past and future climate, ecosystem structure and function, ocean fertility, and global biogeochemical cycles. She is primarily a sea-going and lab-based scientist, who uses measurements of the stable isotopes of the various nitrogen species as a principal analytical tool. She enjoys combining these measurements with other (at-times unconventional) techniques to yield high resolution insights into past and present marine ecosystems. Sarah holds a Bachelor’s degree in Earth and Planetary Sciences from Harvard University and a Ph.D. in Geoscience from Princeton University. She has worked in the California Upwelling System and the subtropical and subpolar North Atlantic Ocean, and her current focus is on the vast Southern Ocean, the southern Benguela Upwelling System, and the greater Agulhas Current System. In 2017, Sarah was awarded an NRF P rating and was selected as one of the Mail and Guardian’s Top 200 Young South Africans (Science and Technology). She also received the SANCOR Young Researcher Award and the Claude Leon Merit Award for Early Career Researchers.