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, understanding the possible future state of the ocean remains critical. About half of all the anthropogenic carbon is sequestered in the ocean, with 40% in the Southern Ocean. This capacity largely depends on the degree to which phytoplankton use the available nutrients. The Southern Ocean hosts a richness of endemic species that have evolved unique physiological adaptations to a rapidly changing environment. Multiple lines of evidence, ranging from historical measurements to numerical modelling, suggest that multi-faceted shifts are already occurring in the Southern Ocean, threatening both ecosystems and the services they provide. Available data is largely limited to the past few decades of observations, hindering our ability to disentangle the relative effects of natural and anthropogenic variability. My previous results provided vital information to fill this knowledge gap. Nitrogen (N) isotope composition of proteinaceous organics trapped within the skeletal matrix of diatoms frustules (DB-δ15N) for the West Antarctic region and the carbonate of deep-sea corals (CB-δ15N) in the Ross Sea, respectively, suggest unprecedented changes in nutrient consumption and food web structure over the last century. On the same materials, together with a collaborator involved in this project, I discovered dwarfing of Antarctic benthic foraminifera in the last 150 years. This observation underpins metabolic stress likely induced by the combined effect of warming and deoxygenation in the West Antarctic region. Three important questions that arise from these findings are: 1) what is the true magnitude and spatial significance of these perturbations all around antarctica? 2) what mechanisms are driving them? and 3) what is the overall impact on the rich benthic ecosystems? In this project, I propose to address these points with the study of five Antarctic high-resolution sedimentary archives covering the last millennium across a wide range of longitudes and coastal environments. First, I will build a circumpolar dataset of DB-δ15N on these well-dated paleoenvironmental archives. In parallel and on splits of the same materials, the network of collaborators involved in this project will provide support to create a solid chronology and independent geochemical proxies to further constrain the interpretation of nitrogen isotopes based on Th-normalization mass fluxes and authigenic uranium and foraminifera size records. This project will provide a present-assessment and a historical context for several ongoing circumantarctic changes in the Southern Ocean biogeochemistry and ecosystem stressors in the Anthropocene.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Alfredo Martínez García