The North Atlantic Ocean: A force for change at Earth’s Greenhouse-Icehouse Transition

Earth’s landmark greenhouse-icehouse transition at the Eocene Oligocene Transition (EOT) is arguably one of the most important climatic events in geological history. The dominant global geochemical signature of this dramatic shift in Earth’s climate (approximately 33.7 million years ago), a 1.2 - 1.5%o increase in the stable oxygen isotope ratio (18O) of benthic foraminifera, represents a significant period of global cooling that occurred contemporaneously with the initiation of widespread Antarctic glaciation. The EOT represents the first establishment of a widespread, continental-scale, somewhat stable ice sheet on Antarctica and yet significant debate remains as to the forcing mechanism responsible for this substantial perturbation marking the end of the Eocene greenhouse world. Initiation of the step-wise shift in 18O occurred during an interval of low eccentricity and low-amplitude change in obliquity; an orbital configuration considered the ultimate trigger for glaciation at the EOT and the pacemaker for subsequent ice sheet growth. However, some other forcing mechanism(s) must have been important in conditioning the climate leading up to the EOT as these orbital configurations are found at other points in geological history where such large-scale changes in climate are absent. 
\nThe overall aim of this thesis is to explore the evolution of temperature, ice volume and ocean circulation, the interactions between these key climatic parameters and the relative importance of different forcing and feedback mechanisms across the EOT. This thesis will place particular emphasis on paleoceanographic changes that occurred in the Atlantic Ocean region across this interval and their subsequent impacts. To this end, multiple records of foraminiferal stable isotopes and trace elements, and fish teeth neodymium isotopes are presented from sediment cores largely within the Atlantic Ocean or subject to its influence at the EOT. 
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\nFindings of this thesis suggest a fascinating link between deep water formation in the North Atlantic Ocean and Antarctic ice sheet dynamics at the EOT. A new highresolution Nd record derived from fossil fish teeth at ODP Site 1263 (Walvis Ridge) in the South Atlantic provides evidence for a significant strengthening of Northern Component Water (NCW), a paleo pre-cursor to North Atlantic Deep Water (NADW), at 34.7 Ma one million years prior to the onset of the EOT. The increasing influence of northern-sourced deep waters, and associated cross-hemisphere heat piracy, provided optimal conditions for landmark climate change prior to the occurrence of the orbital parameters considered to be the ultimate trigger of the EOT. From this point on, NCW played a significant role in the distribution of heat and salinity within the Atlantic Ocean basin and is likely partly responsible, along with enhanced production of Antarctic Bottom Water (AABW) due to Antarctic glaciation, for the interhemispheric temperature asymmetry documented within this thesis. Benthic foraminiferal Mg/Ca-derived bottom water temperatures from IODP Site U1406 (Newfoundland Sediment Drifts) in the North Atlantic show a much lesser degree of cooling in the high northern latitudes when compared to their southern counterparts; in line with previous evidence from records of sea surface temperature (SST). 
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\nBenthic foraminiferal trace element data from IODP Site U1406, along with ODP Site 925 (Ceara Rise) in the Equatorial Atlantic, demonstrate a regional, transient excursion in primary productivity that begins coeval with Antarctic glaciation at the EOT. Arctic Ocean continental shelves served as a nutrient-rich source fuelling the 
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\nIII Cardiff University Amy Sparkes 
\nproductivity pulse in the Atlantic at the EOT when glaciation-induced sea level fall exposed organic-rich shelf sediment nutrients to weathering. This new source of bioavailable nutrients was efficiently distributed through the North Atlantic region by surface water overflows between the Greenland and Norwegian Seas and the North Atlantic causing increased primary productivity and associated export production leading to a steepening of the overall decline in pCO2 and hence acting as a positive feedback on Antarctic Ice Sheet (AIS) growth. 
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\nOverall this thesis makes significant strides in clarifying the timeline of key climatic events in the Atlantic Ocean in the lead up to and across the EOT and highlights the importance of the influence of the Arctic Ocean and North Atlantic in modulating climate and ocean chemistry at the EOT.

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