Evaluating the biogeochemical cycling of nutrients in the Southern Ocean over the past centuries

• Applicant: Dr. Simone Moretti
• Subject Area: Oceanography
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 529282003

As anthropogenic carbon continues to accumulate in the ocean-land-atmosphere system and global average temperature rises, understanding the possible future state of the ocean remains critical. About half of all the anthropogenic carbon is sequestered in the ocean, with 40% of this sink in the Southern Ocean. Multiple lines of evidence, ranging from historical measurements to numerical modelling suggest that large and multi-faceted changes are already occurring in the Southern Ocean circulation, ecosystems and biogeochemical cycles. However, the specific response of the different biogeochemical zonations of this important ocean might be very different. Available data is largely limited to the past few decades of observations, hindering our ability to disentangle the relative effects of natural variability from global warming effects. Preliminary data indicates that the nitrogen (N) isotope composition of proteins trapped with the skeletal carbonate of deep-sea corals (CB-delta15N) in the Southern Ocean holds promise to reconstruct Southern Ocean’s nutrient repletion and dynamics beyond the temporal extent of instrumental record. In this proposal, we combine a large collection of Antarctic deep-sea corals covering a vast array of circumantarctic environments, four high-resolution well-dated sediment cores and three sediment trap records covering a time-span of the last 30 years. First, we will quantify how CB-delta15N in modern corals track the delta15N of seawater nitrate and sinking particulate organic matter, and thus the degree of nitrate consumption in Southern Ocean surface waters. We additionally aim to evaluate how possible changes in food-web structure affect CB-deltas15N in modern corals by comparing their isotopic signature to that of sediment trap sinking diatom and selected zooplankton species. Secondly, we aim to create an historical circumpolar compilation of CB-delta15N and diatom-bound delta15N (DB-delta15N) measurements, covering several centuries of Southern Ocean history in different Antarctic sectors and environments. The proposed analyses aim to provide both a present-assessment and an historical context for the several ongoing changes in Southern Ocean biogeochemistry in the Anthropocene.

• DFG Programme: Research Grants