Alexandra Auderset
Alexandra Auderset is a paleoceanographer and currently an Anniversary Fellow at the University of Southampton. She earned her PhD in 2020 through a joint program between ETH Zurich and the Max Planck Institute for Chemistry, followed by a Swiss National Science Foundation-funded postdoctoral fellowship at Princeton University. Her research focuses on reconstructing past climates, particularly Cenozoic warm periods and the Miocene. Using lipid biomarkers and fossil-bound nitrogen isotopes, she investigates biogeochemical cycling at multiple scales—ranging from ocean oxygen minimum zones to nitrogen recycling in planktic foraminifera. She is actively involved in PAGES working groups PO2 and MioOcean (part of PlioMioVAR), which explore past ocean oxygenation and Miocene temperature dynamics. Looking ahead, her research aims to refine our understanding on how nutrient dynamics influence ocean (de)oxygenation and their broader connections to the global carbon cycle and climate.
Nutrient (re)cycling in the past: What nitrogen isotopes can tell us about foraminifera-symbiont interactions and ocean oxygenation
The growing use of fossil-bound nitrogen isotopes in paleoceanography reflects both its versatility and complexity. This emerging proxy captures a wide spectrum of oceanographic, biogeochemical and ecological signals, from large-scale ocean nutrient cycling to small-scale processes in microorganism metabolisms. In this talk, I present two applications of foraminifera-bound nitrogen isotopes (FB-δ15N), spanning different time periods and ocean basins.
First, I assess FB-δ15N as a tool for identifying symbiotic relationships in planktic foraminifera. Using a new global compilation of core-top and Holocene data, along with glacial/interglacial records from the Atlantic, we observe consistent δ15N differences between symbiont-bearing and symbiont-barren species. Symbiont-bearing species appear to retain low-δ15N foraminiferal ammonium waste, reducing their trophic enrichment signature. Intriguingly, these differences may even help distinguish between different types of symbionts, offering a new way to study symbiosis. Building on this finding, I use FB-δ15N to reconstruct species-specific symbiosis in extinct foraminifera, a key information in the application of many geochemical proxies.
The second case study explores the biogeochemical evolution of marine oxygen-deficient zones (ODZs) using FB-δ15N. Drawing on records from across all ocean basins, we focus on distinct warm intervals of the Cenozoic. Contrary to future projections, we find evidence that the eastern equatorial Pacific ODZ contracted during Cenozoic climate optima, including the PETM, whilst the Arabian Sea and Atlantic display a more complex, regionally variable response. Finally, I will present new preliminary FB-δ15N and lipid biomarker records to better understand mechanisms and identify drivers of the global biogeochemical cycles throughout Earth’s history to present day.
