Nitrogen Isotopes unveiling Earth's mysteries – From changing sea level and ocean oxygenation to foraminifera ecology

Alexandra AUDERSET^1
1 School of Ocean and Earth Science, University of Southampton, Southampton, United Kingdom; a.auderset@soton.ac.uk

Foraminifera-bound nitrogen isotopes (FB-δ¹⁵N) is an emerging geochemical proxy capable of unveiling Earth’s mysteries through its diverse applications in reconstructing both environmental and ecological changes in the geological past. In this keynote presentation, I will showcase three studies using FB-δ¹⁵N to explore different aspects of the evolving Earth system.

In the first case study, a link is identified between the FB-δ¹⁵N and variations in sea level over time. FB-δ¹⁵N is used to reconstruct changes in Nitrogen fixation across glacial/interglacial cycles. The data shows that Nitrogen fixation, a process that supplies the ocean with essential bioavailable nitrogen, is coupled to sedimentary denitrification. In turn, sedimentary denitrification depends on sea level with reduced denitrification rates during glacials when sea level is low and the shelf areas are exposed to air.¹

Moving further back in time, the second case study uses FB-δ¹⁵N to reconstruct the bacterial process of water column denitrification, which is sensitive to ocean oxygen concentration. We explore ocean oxygenation during the Cenozoic era and reveal the surprising behaviour of oxygen-deficient zones getting smaller during climate optima and hyperthermals in the Miocene and Eocene. This stands in contrast to the deoxygenation trend predicted for global warming of the 21^st century.^2,3

Finally, I show how FB-δ¹⁵N can serve as a window into ecological niches and feeding strategies of planktic foraminifera in the modern and past ocean. Evidence from South Atlantic sediments indicates a pronounced δ¹⁵N difference between shallow and deep dwelling planktic foraminifera, as well as between dinoflagellate-bearing and asymbiotic species.⁴

In conclusion, these case studies underscore the transformative potential of FB-δ¹⁵N as a versatile tool for probing Earth’s evolutionary history. As this proxy is still in its early days, the applications will continue to expand, e.g. for more deep-time questions like the emergence and evolution of symbiosis. But this is for another time.

References:
¹ Auderset, A., et al. (in review). "Sea level modulation of Atlantic nitrogen fixation over glacial cycles." Paleoceanography and Paleoclimatology.
² Auderset, A., et al. (2022). "Enhanced ocean oxygenation during Cenozoic warm periods." Nature 609(7925): 77-82.
³ Moretti, S., et al. (2024). "Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum." Science 383(6684): 727-731.
⁴ Auderset, A., et al. (in prep.). "Foraminifer-bound nitrogen isotopes as a proxy of photosymbiont activity: Evidence from South Atlantic sediments." Climate of the Past.