Faith Duffy, S.M., 2022
Olivier Marchal, Advisor
This project investigates the relationship between paleoceanographic radiocarbon records and the ventilation history of deep oceanic basins during the last 40 kyrs. Deep ocean ventilation changes, especially in Atlantic meridional overturning circulation (AMOC), are often invoked to explain the deglacial rise in atmospheric carbon dioxide (CO2) concentration. Much of our current understanding of deep ocean ventilation during the deglaciation comes from radiocarbon concentration records of benthic foraminifera and deep-sea corals (paleo-D14C data). We combine a global compilation of paleo-D14C data (40 – 0 ka) with a 16-box ocean model to address two key questions: (1) To what extent can the paleo-D14C data be explained by atmospheric D14C variations when flow rates are fixed to modern ocean estimates? and (2) To what extent can the paleo-D14C data be explained by atmospheric D14C variations when flow rates are allowed to vary? To address these, the box model is fitted to the paleo-D14C data using sequential methods of optimal estimation theory (linear Kalman filter, Extended Kalman Filter, Rauch-Tung-Striebel (RTS) smoother, linearized RTS smoother).
We find that 62–76% (depending on the assumptions about air-sea 14CO2 exchange) of the paleo-D14C data can be explained by the modern flow rates as represented in the box model, if the model is forced with the IntCal20 reconstruction of atmospheric D14C. When flow rates in the model are allowed to vary, 74-89% of the data can be explained, with the range reflecting assumptions about the error in air-sea 14CO2 exchange and the random walk used to model flow rate evolution. It is concluded that changes in ventilation may have occurred from 20-10 ka, thereby contributing to the deglacial CO2 rise, but the spatial pattern of ventilation changes may have been complex, with a strengthening of the downwelling branch of AMOC and weakening of its deep southward branch.