Doctor of Philosophy (PhD)
Geology and Geophysics
The trace element content of calcareous foraminifera provides a powerful tool to the study of glacial-interglacial changes in the physical and chemical properties of the ocean. Foraminifera incorporate barium in direct proportion to its concentration in seawater. Using barium as a nutrient proxy, Ba/Ca in benthic Planulina wuellerstorfi is used to reconstruct changes in thermocline ventilation and mid-depth circulation in the North Atlantic during the last glacial and deglacial time. Rivers are concentrated in barium compared to surface seawater. Therefore, barium in planktonic Neogloboquadrina pachyderma is used to identify deglacial meltwater in the Arctic Ocean. Foraminiferal Li/Ca was analyzed to elucidate factors influencing incorporation behavior, including interspecies differences, temperature, pressure, dissolution, and shell mass. To investigate the use of lithium isotopes as a proxy for paleo-seawater chemistry, d6Li was determined in planktonic Orbulina universa. During the last glacial maximum, nutrients in the thermocline and the intermediate water of the North Atlantic was lower than today due to increased ventilation and the presence of nutrient-depleted Glacial North Atlantic Intermediate Water (GNAIW). During deglacial time, GNAIW was replaced by southern component water, resulting in an enrichment of nutrients in the mid-depth Atlantic water. Increased Ba/Ca in the surface Arctic Ocean indicates an increase in meltwater discharge between 12.4 and 11.3 14C ka BP. This may have triggered an increase in the export of freshwater to the North Atlantic, contributing to a shutdown in GNAIW production, and leading to the onset of the Younger Dryas. A second meltwater event at 9.4 14C ka BP may be the result of glacial Lake Agassiz draining through the Clearwater spillway to the Mackenzie River. Foraminiferal Li/Ca shows systematic glacial-interglacial variation coincident with d18O. The incorporation behavior of lithium in foraminifera does not appear to be dominated by changes in temperature, pressure, dissolution, or shell mass, but is potentially controlled by changes in growth conditions, including calcification rate. Preliminary work indicates that d6Li remained constant throughout the Holocene and the last glacial maximum at 30.5 ± 1.1‰. Further developmental studies are necessary to fully engage lithium isotopes as a tracer of seawater composition.
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Hall, Jenney M., "Barium and lithium in foraminifera: glacial-interglacial changes in the North Atlantic" (2002). LSU Doctoral Dissertations. 3135.