Publication Date:
2022-05-26
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2017
Description:
Significant changes occurred during the last deglaciation (roughly 10-20 thousand
years (ka) before present) throughout the climate system. The ocean is a large
reservoir of carbon and heat, however, its role during the deglaciation is still not
well understood. In this thesis, I rely on radiocarbon measurements on fossil
biogenic carbonates sampled from the seafloor to constrain deglacial ocean
ventilation rates, using new data, an extensive data compilation, and inverse
modeling. First, based on a sediment core that is absolutely dated from wooden
remains, I argue that the deglacial 14C reservoir age of the upper East Equatorial
Pacific 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 CO2
released directly from the ocean (e.g., in the Southern Ocean) to the atmosphere
rather than first mixed through the upper ocean. Then using a high-deposition-rate
sediment core located close to deep water formation regions in the western North
Atlantic, I show that compared to today, the mid-depth water production in the
North Atlantic was probably stronger during the Younger Dryas cold episode, and
weaker during other intervals of the late deglaciation. However, the change was not
as large as suggested by previous studies. Finally, I compile published and
unpublished deep ocean 14C data, and find that the 14C activity of the deep ocean
mirrors that of the atmosphere during the past 25 ka. A box model of modern ocean
circulation is fit to the compiled data using an inverse method. I find that the
residuals of the fit can generally be explained by the data uncertainties, implying
that the compiled data jointly do not provide strong evidence for basin-scale
ventilation changes. Overall, this thesis suggests that, although deep ocean
ventilation may have varied at some locations during the last deglaciation, the
occurrence of basin-scale ventilation changes are much more difficult to be put on a
firm footing. An imbalance between cosmogenic production and radioactive decay
appears as the most natural explanation for the deglacial 14C activity decline
observed in both the atmosphere and the deep ocean.
Description:
This research was supported by NSF-OCE grants 0822854 to Lloyd Keigwin,
1031224 to Lloyd Keigwin, 1204045 to Lloyd Keigwin, 1301907 to Olivier Marchal,
Geoffrey Gebbie and Lloyd Keigwin, and 1405160 to Lloyd Keigwin, WHOI Academic
Programs Endowed Funds, the Ocean Ventures Fund from WHOI, and a graduate
student internship from NOSAMS.
Keywords:
Climatology
;
Climatic changes
;
Glaciers
;
Carbon dioxide
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Permalink
