| Summary: | This study uses the coiling direction of the planktic foraminifer Globorotalia truncatulinoides as a proxy for reconstructing past changes in upper ocean hydrography in the northwestern subtropical Atlantic Ocean during the mid-Pleistocene climate transition (~0.4 – 1.2 Ma). The mid-Pleistocene transition is a time interval of interest because it encompasses the evolution of glacial-interglacial cycles from less severe, 41 kyr-paced glaciations to more severe, 100 kyr-paced glaciations, without any observable changes in corresponding orbital forcing. I hypothesize that the establishment and persistence of the more extreme glaciations characteristic of the current 100 kyr world was associated with changes in the hydrography of the subtropical North Atlantic and the related poleward heat transport, evident in the downcore coiling ratio of G. truncatulinoides. The Gulf Stream and surrounding North Atlantic current system transport a substantial amount of heat from the equator to the poles, and therefore have a significant climatic impact on large temporal and spatial scales. Here I test whether or not changes in subtropical gyre dynamics, particularly the relative strength and position of the Gulf Stream and subtropical gyre, played a role in contributing to the onset of the 100 kyr cycle. To do this, I will construct a record of downcore variations in the coiling direction of G. truncatulinoides spanning the mid-Pleistocene transition. Studies have shown that there are spatial and temporal differences in the coiling direction of G. truncatulinoides through time as a result of changes in oceanic conditions; for example, sediments dominated by left coiling specimens characterize interglacial periods and deeper permanent thermoclines, while sediments dominated by right coiling specimens characterize glacial periods and shallower permanent thermoclines. Deeper permanent thermoclines are characteristic of interglacial periods due to enhanced flow of the subtropical gyre and Gulf Stream, and therefore greater poleward ...
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