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Magnesium/calcium paleothermometry is an established tool for reconstructing past surface and deep-sea temperatures. However, our understanding of nonthermal environmental controls on the uptake of Mg into the calcitic lattice of foraminiferal tests remains limited. Here we present a combined analysis of multiple trace element/calcium ratios and stable isotope (delta O-18 and delta C-13) geochemistry on the subpolar planktonic foraminifera Neogloboquadrina incompta to assess the validity of Mg/Ca as a proxy for surface ocean temperature. We identify small size-specific offsets in Mg/Ca and delta O-18(c) values for N. incompta that are consistent with depth habitat migration patterns throughout the life cycle of this species. Additionally, an assessment of nonthermal controls on Mg/Ca values reveals that (1) the presence of volcanic ash, (2) the addition of high-Mg abiotic overgrowths, and (3) ambient seawater carbonate chemistry can have a significant impact on the Mg/Ca-to-temperature relationship. For carbonate-ion concentrations of values > 200 mu mol kg(-1), we find that temperature exerts the dominant control on Mg/Ca values, while at values < 200 mu mol kg(-1) the carbonate-ion concentration of seawater increases the uptake of Mg, thereby resulting in higher-than-expected Mg/Ca values at low temperatures. We propose two independent correction schemes to remove the effects of volcanic ash and carbonate-ion concentration on Mg/Ca values in N. incompta within the calibration data set. Applying the corrections improves the fidelity of past ocean temperature reconstructions.

Eastern North Atlantic Central Water formation and circulation are linked to the strength of the Atlantic Meridional Overturning Circulation (AMOC) and therefore an important component of our climate system. Today, the North Atlantic Oscillation is one of the controlling modes in climate variability of atmosphere-ocean linkages over the subpolar and subtropical basins and influences central water formation and circulation. However, the impact of longer-term forcings such as total solar irradiance, sudden or gradual changes in the freshwater budget of the North Atlantic, and the nature of climate signal propagation from high to low latitudes within central water circulation remains poorly understood. This thesis presents a newly developed Mg/Ca temperature calibration of the calcitic benthic foraminifera Hyalinea balthica, which allows the reconstruction of decadal to centennial scale climate variability in Eastern North Atlantic Central Water during the Holocene.

Core top samples from Indonesian and northeast Atlantic depth transects were used to calibrate Mg/Ca and δ18O in tests of the calcitic benthic foraminifer Hyalinea balthica to bottom water temperature between 4°C and 13°C. This shallow infaunal species is primarily abundant in neritic to upper bathyal sediments (

Modern warming of the Arctic Region has been linked to extreme weather events in the Northern Hemisphere, including severe winters, increased precipitation, summer heatwaves, and increased storminess. Further, sea-ice decline and enhanced background melting of the Greenland Ice-Sheet may affect the Atlantic Meridional Overturning Circulation (AMOC) by altering surface water buoyancy at deep-water formation sites. However, our observation and historical data series are too short to ascertain the impact Arctic warming might have on the climate system, resulting in a gap in our knowledge and uncertainty on how the climate system may respond to future warming. Contradictory conclusions between observations and model studies add to these uncertainties. For example, many models predict a reduction in deep-water formation during times of enhanced freshwater input, while palaeo evidence suggests vigorous deep-water formation despite freshening, highlighting our incomplete understanding of the mechanisms driving climate change. Past climate archives provide us with a tool to assess the ocean-atmosphere climate system during times of enhanced high-latitude warming. However, as the climate system responds not only to direct forcing but also to the forcing history (i.e., boundary conditions), it is crucial we examine the climatic response to enhanced high-latitude warming under various boundary conditions. This research focuses on three new palaeoceanographic investigations during periods of Arctic warming. Two focus on interglacial boundary conditions that were warmer than present, and the third investigates a deglacial period when the Arctic was warming but the boundary conditions were in transition from glacial to interglacial states. This research thus provides an opportunity to assess how different boundary conditions modulated the climate response of the North Atlantic Region. The first investigation focuses on the transition from the warmer than present Holocene Thermal Maximum (HTM) (~4 – 7 kiloannum (ka)) to the cooler Late Holocene. The HTM was characterised by significantly lower sea-ice extent at high northern latitudes and warm sea surface temperature and surface air temperature anomalies in the Barents Sea and subpolar North Atlantic Region. This was followed by a transition to high-latitude cooling and sea-ice growth during the Late Holocene. Using geochemical proxies to reconstruct past movements of the Irish Shelf Front, this study reveals how past changes in atmospheric circulation resulted in enhanced storm magnitude and frequency for the UK and Ireland during warmer than present climates of the HTM. The second study focuses on an abrupt climate event during Marine Isotope Stage (MIS) 11 (~424 – 403 ka) when prolonged warming of the Arctic and continuous background melting of the Greenland Ice-Sheet led to a cold, and relatively fresh surface ocean in the Nordic Seas. Analysing both surface ocean hydrography and deep-water flow strength, preserved in the same sample from a sediment core collected in the eastern North Atlantic, allows us to assess the relative timing of the onset, duration, and recovery of a surface and deep-water climate event. Interestingly, despite the addition of freshwater, Nordic Seas Deep-Water formation remained strong during early MIS 11, supporting the hypothesis that deep-water formation may not be as susceptible to future Greenland Ice-Sheet melting as previously thought. However, our analysis reveals deep-water flow strength weakened when the freshwater lens over the Nordic Seas drained into the subpolar North Atlantic, also causing an abrupt surface cooling event. Finally, the third study focuses on the Glacial-Interglacial transition, Termination 5 – TV (~430 – 424 ka), when the demise of the largest continental ice-sheets of the late Quaternary occurred under relatively weak orbital forcing. Here, the paired surface and deep-water proxies enables an assessment of the relative timing of surface water properties and deep circulation changes (i.e., lead/lags) at our core site in the eastern North Atlantic. Specifically, our analysis reveals that the primary onset of the deglaciation occurred in the Nordic Seas rather than at low-latitudes, since the reinvigoration of overflows in the Nordic Seas at the end of MIS 12 precedes the recovery of the surface ocean by several centuries. Both the investigations on MIS 11 and TV suggest that fluctuations in Nordic Seas Deep-Water formation are precursors to abrupt climate change in the eastern North Atlantic. Further, both studies identify the density gradient between the Nordic Seas and the subpolar North Atlantic as crucial in maintaining overflows, during both warm and cold climates. Thus, Nordic Seas Deep-Water may not be as susceptible to freshwater forcing as previously hypothesised. Moreover, enhanced advection of cold, and relatively fresh Polar Waters to the subpolar North Atlantic can rapidly initiate an abrupt cold event, within centuries, during both glacial and interglacial conditions. This research improves our understanding of how the climate system responds to enhanced high-latitude warming under different boundary conditions. Further, it highlights that boundary conditions are fundamental to how the climate responds. This is important going forward and must be considered when climate models are being developed. 2024-01-30