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

Abstract

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. Benthic foraminiferal B/Ca, an indicator of past seawater carbonate ion saturation is the main paleoceanographic proxy that was used in this study. 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 con- centrations 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 im- portant precursor to deglacial CO2 release. The abrupt and stepwise deglacial increase in atmospheric CO2 is mirrored in distinct CO32- maxima at 835 m water depth, while carbonate ion histories of deeper waters between 1390 and 2498 m are negatively offset from this trend. Opposing deep and intermediate water carbonate ion histories throughout the deglacial period do not follow model predictions that are based on the calcite compensation- and rain-ratio-theories, however similar LGM and Early Holocene carbonate ion concentrations in all cores can be explained in line with calcite compensation feedbacks. A close correlation of carbonate ion concentrations at 1390 and 2498 m water depth throughout the deglaciation, is interpreted in line with previous arguments for a shallower convection of Antarctic intermediate waters at this time. Glacial carbon sequestration in the highly stratified Southern Ocean likely occured within Pacific Deep Water and a gradual improvement of interior mixing during the LGM allowed a stepwise transfer of old carbon towards shallower depth. Then starting at the glacial termination, enhanced upwelling of deep waters close to the Polar Front released CO2 and this signal of carbon loss was subsequently transmitted within a shallow layer of Antarctic sourced intermediate waters that subduct from the zone of air-sea equilibration and are exported throughout the world ocean.