G02_04

Climate induced thermocline aging and ventilation south of the Azores front over the last 32,000 years

Beisel E¹,  Frank N¹, Lausecker M¹, Friedrich R², Therre S¹, Schröder-Ritzrau A¹, Butzin M³

¹Institute of Environmental Physics, Heidelberg University, Heidelberg, Germany, ²Curt-Engelhorn-Center Archaeometry, Mannheim, Germany, ³MARUM-Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany

The radiocarbon analysis of uranium-thorium-dated cold-water corals (CWC) provides an excellent opportunity for qualitative reconstruction of past ocean circulation and water mass aging. While mid-depth water mass aging has been studied in the Atlantic Ocean, the evolution of the thermocline, tightly coupled to the atmosphere, remains largely unknown. Here we present a high-resolution dataset of combined ¹⁴C and U/Th data obtained from thermocline-dwelling CWCs at various sites in the Atlantic Ocean, directly compared to simulation results of the ¹⁴C-equipped Large Scale Geostrophic ocean general circulation model for the last 32 ka. CWCs off Angola provide the link between previous records from the equatorial Atlantic and Southern Ocean at greater depths, opening the possibility of a unified southern ¹⁴C signal in the Last Glacial Maximum (LGM). In contrast to the South Atlantic and to modeling results, North Atlantic CWCs show strong variations of a well-ventilated water mass near the Azores Front. Our results confirm previous observations of enhanced ventilation during the Bølling-Allerød interstadial (B/A), both shallower and deeper water layers exhibit the same radiocarbon signal. We conclude that the North and South Atlantic must be considered as separately acting reservoirs during the LGM, subsequent deglaciation and B/A. Respired carbon is stored in the dynamic mid-depth to deep Atlantic south of the Azores Front, while the subpolar North Atlantic waters remain a persistent well-ventilated ocean. Consequently, CWC-¹⁴C records from the South Atlantic and Southern Ocean provide the opportunity to determine a high-precision calibration curve for the radiocarbon content of the thermocline.