Abstracts Category : Other

Reconstructing deglacial ocean ventilation using radiocarbon : data and inverse modeling

by Ning Zhao

Significant changes occurred during the last deglaciation (roughly 10-20 thousandyears (ka) before present) throughout the climate system. The ocean is a largereservoir of carbon and heat, however, its role during the deglaciation is still notwell understood. In this thesis, I rely on radiocarbon measurements on fossilbiogenic carbonates sampled from the seafloor to constrain deglacial oceanventilation rates, using new data, an extensive data compilation, and inversemodeling. First, based on a sediment core that is absolutely dated from woodenremains, I argue that the deglacial 14C reservoir age of the upper East EquatorialPacific was not very different from today. Combined with stable carbon isotope data,the results suggest that the deglacial atmospheric CO2 rise was probably due to CO2released directly from the ocean (e.g., in the Southern Ocean) to the atmosphererather than first mixed through the upper ocean. Then using a high-deposition-ratesediment core located close to deep water formation regions in the western NorthAtlantic, I show that compared to today, the mid-depth water production in theNorth Atlantic was probably stronger during the Younger Dryas cold episode, andweaker during other intervals of the late deglaciation. However, the change was notas large as suggested by previous studies. Finally, I compile published andunpublished deep ocean 14C data, and find that the 14C activity of the deep oceanmirrors that of the atmosphere during the past 25 ka. A box model of modern oceancirculation is fit to the compiled data using an inverse method. I find that theresiduals of the fit can generally be explained by the data uncertainties, implyingthat the compiled data jointly do not provide strong evidence for basin-scaleventilation changes. Overall, this thesis suggests that, although deep oceanventilation may have varied at some locations during the last deglaciation, theoccurrence of basin-scale ventilation changes are much more difficult to be put on afirm footing. An imbalance between cosmogenic production and radioactive decayappears as the most natural explanation for the deglacial 14C activity declineobserved in both the atmosphere and the deep ocean.