Last Glacial to Holocene changes of deep and intermediate water carbonate ion concentrations in the Southern Ocean: constraints from foraminiferal Boron/Calcium ratios

Carbon cycling between the ocean and the atmosphere is an integral part of glacial to interglacial climate change, the nature of trigger versus feedback mechanisms is however a subject of ongoing debate. Atmospheric CO2 communicates with the marine carbonate ion reservoir via air–sea equilibration, and glacial to interglacial CO2 fluctuations are thus expected to be mirrored in seawater chemistry. In this thesis, the first records of intermediate and deep water carbonate ion concentrations in the South Pacific were generated in order to study carbon cycle dynamics throughout the past 30,000 years, a time that is characterized by rapid environmental changes as the global climate transitioned out of the last ice-age into the present warm interval, the Holocene. Benthic foraminiferal B/Ca, an indicator of past seawater carbonate ion saturation is the main paleoceanographic proxy that was used in this study. Analyses were primarily carried out with Laser Ablation–ICP–MS and it was shown that this method is well suited for trace element measurements on samples with low abundances of foraminiferal shells. This represents an important step towards extending paleoceanographic proxy coverage to regions that are characterized by low carbonate contents in sediments, such as the Southern Ocean. To enable an application of the B/Ca proxy in the Southern Ocean, an extension of the existing global core-top calibration was necessary. The South Pacific core-top study that was conducted as part of this thesis, included the species C. wuellerstorfi, C. cf. wuellerstorfi and C. mundulus. B/Ca in C. cf. wuellerstorfi, a C. wuellerstorfi morphotype that is abundant in this region, shows the highest sensitivity to carbonate saturation changes reported to date and is thus very suitable for B/Ca proxy studies. Trace element profiles through C. wuellerstorfi and C. mundulus shells revealed an intra-shell B/Ca variability of ±36 % around the mean value, and roughly inverse patterns of B/Ca and Mg/Ca, suggestive of vital effects. The core-top study furthermore indicated that foraminiferal B/Ca supplemented by δ13C can function as an indicator of water mass age and ventilation status and thus possibly be used as a non-conservative water mass tracer for the past. Down-core proxy studies carried out within the scope of this thesis were used to address currently unresolved questions about the origin, mechanism and pathway of deglacial CO2 release and to test the validity of model outputs aimed at explaining glacial to interglacial CO2 fluctuations. For this purpose, B/Ca reconstructed carbonate ion concentrations were complemented by stable oxygen and carbon as well as radiogenic carbon isotope data. Carbonate ion histories revealed a stepwise redistribution of old carbon from deep to intermediate depths at the end of the Last Glacial, which represents an important precursor to deglacial CO2 release. 14C data of intermediate, deep and abyssal waters furthermore indicated that the oldest glacial waters in the Southern Ocean occu-