One of the key questions in paleoclimate research is to improve our conceptual and quantitative understanding of changes in atmospheric CO₂ contents over past glacial/interglacial cycles. Based on Antarctic ice core records, it is well known that glacial atmospheric CO₂ contents were 80-100 ppmv lower compared to the pre-industrial values of the Holocene and other interglacials. Coupled ocean atmosphere-biosphere box models suggest that more than 30 ppmv of the glacial decrease may be due to decreases in ocean alkalinity and carbon inventories. To further constrain this important role of the oceanic carbonate system in past and potentially future climate evolution, reliable proxy records for variations in deep ocean carbonate ion concentrations are urgently needed.

Up to now, different approaches have been used to reconstruct this important proxy, primarily including carbonate dissolution-based studies (e.g., carbonate contents, foraminifera dissolution indices, etc.) that revealed however partly conflicting results. A recently published new approach (Yu & Elderfield, 2007 EPSL) is based on B/Ca ratios in benthic foraminifera and has been successfully applied on sediment core-tops and down core records in the North Atlantic. Here, we plan to apply this promising new tool on sediment cores from the Southern Ocean to reconstruct the carbonate ion signature of the major deep and intermediate water masses between ~600 and ~4600 m water depth (suitable sediment cores with already existing well-constrained age models are available at AWI). In this respect the Southern Ocean plays a key role as a global distributing centre for the import and export of water masses from and into the different ocean basins. Our studies are expected to provide new insights into the question whether the Southern Ocean acted more as a source or a sink for atmospheric CO₂.