The Evolution of Glacial Conditions in the Southern Atlantic Ocean: A Depth Transect Approach
- Author(s): Foreman, Alan Dean
- Advisor(s): Charles, Christopher D
- et al.
Understanding the role of the deep ocean in governing glacial/interglacial cycles has been a central theme of paleoceanography since its inception. Historically attempts at resolving the changes that occurred in the deep ocean have focused on tying together sedimentary time series from disparate locations. However, the sparse geographic distribution and variable depositional conditions of the sedimentary cores that make up such compilations have been significant obstacles for the creation of a detailed picture of changes in circulation and carbon storage over ice-age cycles.
Vertical tracer profiles represent an alternative approach that circumvents a number of these problems. These profiles (‘depth transects’) compile proxy measurements from sediment cores collected at multiple depths in a single geographic area into a single ‘steady-state’ picture. By densely sampling the water column in a single location, vertical profiles act as ‘paleo-CTD’ casts that provide detailed observations of changes in water mass geometry using sediment cores that have experienced nearly identical depositional conditions.
In this thesis I present proxy observations from a depth transect in the Southeastern Atlantic Ocean for 6 different time points over the last glacial/interglacial cycle. Together these observations provide information about the timing and magnitude of changes in deep ocean temperature, salinity, and carbon storage as glacial conditions evolved over the last 140,000 years.
The efficacy of the vertical depth transect approach depends entirely on how accurately the cores that comprise the depth transect can be correlated and dated. In Chapter 1, I present X-Ray Fluorescence (XRF) observations of bulk sedimentary elemental abundances for the 20 sediment cores that comprise the core collection. In this chapter I demonstrate that these XRF-derived elemental traces have characteristic variability that can be used to correlate cores across the entire range of depths in the collection, and that they can be used to develop a precise stratigraphic framework.
With this knowledge in hand, Chapter 2 focuses on documenting the evolution of the deep ocean temperature structure over the last ice age cycle using paired measurements of benthic foraminiferal Mg/Ca and δ18O. From these measurements I infer that both a change in structure and a significant cooling occurred in the Southern Atlantic between 2-3 km across the Marine Isotope Stage (MIS) 5E/5D transition. At MIS 5A/4 a second cooling took place at mid-depths, and was likely accompanied by an increase in the production of cold, salty southern-sourced water. The deep South Atlantic appears to have mostly cooled to LGM temperatures by MIS 4, though the gradient between northern- and southern-sourced water continued to sharpen between MIS 4 and the LGM.
The changes in stratification implied by the conservative tracers presented in Chapter 2 should have had consequences for the cycling of carbon and nutrients in the deep ocean. Chapter 3 presents concurrent measurements of 7 different tracers (δ13C, U/Ca, Mn/Ca, Nd/Ca, B/Ca, Sr/Ca, and Na/Ca) that provide a way to infer the relative significance of remineralization, air-sea gas exchange, and other nonconservative properties over the last glacial/interglacial cycle. Taken together, the vertical profile measurements in Chapter 3 are consistent with the phenomenon of nutrient deepening in the South Atlantic during cold episodes of the full glacial cycle, and suggest that the increased production of cold, salty southern-sourced water trapped nutrients and carbon at depth in the Southern Atlantic glacial conditions strengthened.
Chapter 4 outlines a complimentary perspective by using newly developed records of the δ13C of atmospheric CO2 in tandem with two time series of deep ocean δ13C from this core collection to document the amount of excess carbon stored in the deep South Atlantic ocean during cold episodes. The magnitude and timing of the δ13C divergence record match other independent attempts to calculate vertical δ13C gradients in the Subantarctic South Atlantic and can be tied directly to the ice core atmospheric CO2 records. Each of these comparisons suggests that enhanced carbon storage at depth in the ocean was indeed responsible for much of the atmospheric δ13C CO2 record, and, by extension, the atmospheric concentration of CO2. Additionally, the similarity of the record of δ13C divergence and a time-series record of circumpolar deep water temperature suggests that carbon trapping in the deep South Atlantic ocean was associated with colder, saltier water, consistent with the mechanisms proposed in Chapters 2 and 3.