UC Davis

The Arctic Ocean is currently experiencing increases in temperature twice as fast as lower latitudes. Changes in air and surface water temperatures as well as fluctuations in rates and volume of deep-water formation in the North Atlantic will have substantial impacts on global climate. Therefore, understanding how the structure of the Arctic Ocean has changed in response to variations in past climate will help us to better predict what changes we can expect as the Earth warms over the next century. To do this, we rely on one of the fundamental tools used to reconstruct past ocean conditions- the geochemistry of foraminiferal calcite. Neogloboquadrina pachyderma is the species of planktonic foraminifera that dominates high latitude assemblages and is therefore the principal species utilized to study the paleoceanography of the polar regions. The calcification of N. pachyderma is unusual in that it grows its primary (lamellar) calcite in the surface waters and subsequently adds a thick CaCO3 crust at depths below the mixed layer. Therefore, geochemical analyses on whole shells produce results that are difficult to interpret as they are derived from mixed lamellar and crust calcite contributions that are unequal in mass. Analytical tools capable of high spatial resolution such as electron microprobe, laser ablation ICPMS (LA-ICPMS) and secondary ion mass spectrometry (SIMS) can measure the geochemistry of discrete regions of N. pachyderma shells, therefore allowing us to analyze ontogenetic and crust calcite separately. For the first portion of my dissertation, I analyze the in situ Mg/Ca and 18O in discrete calcite zones using LA-ICPMS, EPMA and SIMS within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. I compare shell geochemistry to the measured temperature, salinity and 18O sw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. The results include a new relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. Further, this data revealed that the 18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma 18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions. Next, I evaluate intrashell trace element (TE)/Ca data from N.pachyderma shells that were grown and subsequently produced a crust in culture at various temperatures. These shells reveal marked shifts in the Mg/Ca, Ba/Ca, Zn/Ca, Al/Ca, and Mn/Ca between the lamellar and crust calcites grown in culture with no associated changes in either the environmental conditions or trace element concentrations of the seawater. The TE/Ca data were compared to the hydrologic conditions (i.e. temperature, salinity, pH, [TE]), to better understand the relationships between the crust and lamellar geochemistry and the environment in which the shells calcified. My results demonstrate that the Mg/Ca in the LC agrees with the calibration relationship previously described in Livsey et al. (2020), while the study reveals that Mg/Ca in the crust calcite is not primarily controlled by temperature. Though these results cannot definitively resolve what is controlling the transition in geochemistry between the LC and crust, they do reveal that neither changes in the environmental conditions, [Te]sw, nor a change in the calcification rate are the dominant factors. Finally, I quantified patterns of freshwater outflow from the Arctic associated with the draining of glacial Lake Agassiz. This meltwater event at ~11.5 ka likely triggered a slowdown of global ocean currents and initiated the northern hemisphere cold interval known as the Younger Dryas. I combine LA-ICPMS trace element measurements with δ18Ocalcite analyses on N. pachyderma shells to reconstruct the δ18O of Lake Agassiz-derived Mackenzie River meltwater. Remarkably, these results indicate that the geochemistry of the meltwater entering the Arctic Ocean varies throughout the Younger Dryas event, revealing the inaccuracy of current model simulations. Further, I illustrate that the meltwater pulse was not continuous throughout the interval, with at least one cessation of drainage that is linked to a shift in the chemistry of the meltwater. Overall, the time-resolved isotopic evolution of meltwater in the Arctic Ocean through the Younger Dryas reveals the unexpected complexities of meltwater pulses, which have large implications for our understanding of volumes and rates of freshwater discharge into the North Atlantic in the past and provides insights on the source of the meltwater for the Younger Dryas event.