Source or Sink: What is the fate of Carbon in Upper and Intermediate Waters in the South Atlantic?

• Applicant: Dr. Karl Meier
• Subject Area: Palaeontology ; Oceanography
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541094686

Rising atmospheric CO2 concentrations, driven by natural and anthropogenic factors, have profound implications for Earth's climate. The Ocean is recognized as a carbon sink; however, uncertainties persist regarding the fate of carbon in key oceanic features during periods of climate change. This research proposal aims to investigate dynamics of carbon fluxes in the South Atlantic from a paleoclimatic perspective. The proposed project is based on a multiproxy approach, focusing on Antarctic Intermediate Water (AAIW) and the South Atlantic Subtropical Gyre (SASG) to unravel their roles in the global carbon cycle during times of rapid climate change.The proposed proxy records will be obtained from marine sediment core M125-25-4, situated on the SE Brazilian margin (~960 m water depth), bathed in the center of AAIW, and within the surface Brazil Current, the western branch of the SASG. The paleoclimatic proxies include planktic and benthic foraminiferal Mg/Ca (calcification temperatures), stable isotopes δ18O (stratigraphy and salinity combined with Mg/Ca), δ13C (paleo-productivity), and δ11B for seawater pH/CO2 concentrations and Cd/Ca of benthic foraminifera to constrain AAIW dynamics.AAIW is a pivotal water mass within the Atlantic Meridional Overturning Circulation (AMOC) return flow. Today, the formation process of AAIW results in being a sink for atmospheric CO2. Yet, as increased (anthropogenic) carbon concentrations in AAIW proceed northward and feed equatorial upwelling regions, a higher oceanic-to-atmospheric pCO2 gradient may be expected, implying more efficient oceanic CO2 outgassing, thus turning AAIW into a potential CO2 source for Earth’s atmosphere. This emphasizes the ambiguous role of AAIW, either evolving into a potential source or sink during climate transitions.As the SASG and AAIW are interconnected via the downwelling limb of the shallow overturning in the Southern Ocean, where subtropical waters are entrained into the northward flowing AAIW, the question arises of how preformed SASG waters impact the carbon budget of AAIW. A second major attempt within this project is to reconstruct carbon fluxes between the atmosphere and the SASG with respect to physical and biological variability and unravel the impact of preformed carbon budget of the SASG on AAIW throughout past rapid climate change.To understand these systems, this project attempts to investigate their dynamics and impacts on Earth’s climate during rapid climate shifts of the geological past, which are comparable to recent rates of climate change and in atmospheric CO2 changes. This comprises the last deglacial period from the Last Glacial Maximum towards the Holocene, and the last glacial period. Both intervals are marked by phases of rapid climate change (Dansgaard/Oeschger-cycles and Heinrich Events), which are associated with distinct AMOC perturbations and pronounced by atmospheric CO2 fluctuations.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Eleni Anagnostou, Ph.D.