Eastward Routing of Glacial Lake Agassiz Runoff caused the Younger Dryas Cold Event

• Main Author: Leydet, David J.
• Other Authors: Carlson, Anders E., Clark, Peter U., Nolin, Anne W., Rosenberger, Randall, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University. Graduate School
• Format: Master Thesis
• Language: English; unknown
• Published: Oregon State University
• Subjects: Meltwater > Agassiz; Lake; Global cooling; Climatic changes; Runoff > Agassiz; Paleoclimatology; Ocean-atmosphere interaction; Ice sheets > Agassiz; Agassiz Lake; Arctic; Arctic Ocean; Glacial Lake; Mackenzie River; Climate change; Ice Sheet; Mackenzie river; North Atlantic
• Online Access: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/b2773z14t

The purpose of this thesis is to analyze an abrupt case of climate change in the past as a means to understand the mechanisms that force climate change. By looking to past analogs of climate change, we hopefully will gain an understanding of these events, which could be used to further our understanding of future climate change. In this light, I analyze the case of the Younger Dryas (YD), an abrupt cooling event that occurred from ~12.9 ka to ~11.7 ka. We investigate several hypotheses regarding the cause of the YD and attempt to determine the forcing mechanism for this abrupt cooling event. I use ¹⁰Be surface exposure dating as our method for dating retreat of the Laurentide Ice Sheet from the eastern outlets of glacial Lake Agassiz, a large pro-glacial lake that formed during the last deglaciation whose drainage into the North Atlantic is hypothesized to have caused the YD via a slowing of ocean overturning circulation. I find that the eastern outlets of glacial Lake Agassiz begin to deglaciate at 14.0 ± 0.3 ka with ice retreating from the key Lake Kaministikwia outlet at 13.0 ± 0.3 ka, concurrent with the onset of the YD. I also date retreat from the Steep Rock moraine at 13.8 ± 0.2 ka and retreat from the Marks moraine by 11.0 ± 0.4 ka. I use our chronology along with other terrestrial and marine proxies to reconstruct the meltwater routing history of Lake Agassiz. Specifically, the Gulf of St. Lawrence isotopic record indicates meltwater routing through Eastern Outlets, peaking at ~12.6 ka. Subsequently, the isotopic record of the Arctic Ocean near the mouth of the Mackenzie River indicates meltwater routing beginning at ~12.4 ka and peaking at 12.2 ka. I argue that the timing of these meltwater pathways support the hypotheses that the YD was caused by freshwater forcing, weakening the Atlantic Meriodional Overturning Circulation, and thus cooling the climate of the Northern Hemisphere. The results demonstrate the importance of meltwater routing on the climate system and will be important in understanding ...