Reconstruction of Seawater Carbonate chemistry during the last Glacial-Interglacial transition from Boron isotopic ratios and concentrations in foraminifera

The symptoms of carbon cycle perturbations (CCP), either natural or induced by human activities are global warming, ocean acidification (OA) and dysoxia. These turn out to be a deadly mix as natural CCP have been identified as the main cause of at least 4 of the 5 mass extinctions in Earth history (Honisch et al. 2009; Bijma et al. 2013a). Anthropogenic activities are releasing CO2 ten times faster than at any time in the last 65 million years, and possibly the last 300 Myr, making the management of the anthropogenic carbon perturbation one of societies major challenges. To accurately project the consequences of anthropogenic CCP, it is vital to first understand the fluctuations and variability of the natural sinks and sources of the Earths carbon cycle. This requires accurate reconstruction of the oceanic carbonate chemistry because changes in the carbon storage in the deep ocean are the key to explain the glacial/interglacial atmospheric CO2 variations observed in ice core records (Köhler et al. 2005; Yu et al. 2010). Herein, we propose to quantify the processes that led to the ca. 100ppmv increase in atmospheric pCO2 over the glacial/interglacial transition. Processes in the Southern Ocean, where most of the deep water is ventilated, are suspected to play a central role in this regard. It is believed that the sluggish glacial ocean could store more carbon, that the biological pump was more efficient (through iron fertilization and ballasting) and that increased stratification of the water column reduced carbon leakage from the glacial Southern Ocean back to the atmosphere (Keeling and Visbeck 2001). During the deglaciation, this deep ocean carbon capacitor becomes reconnected with the atmosphere and leads to rapid CO2 outgassing because of incomplete nutrient utilization, which is characteristic for the Southern Ocean during warm periods (so called High Nutrient Low Chlorophyll (HNLC) region). To date, all of this remains hypothetical, albeit supported by circumstantial evidence (Martinez-Boti et al. 2015; Ronge et al. 2015), but not proven by direct reconstructions of the glacial/interglacial carbonate chemistry evolution. The overarching goal of our proposal is to analyse two independent carbonate chemistry proxies on benthic and planktonic foraminiferal tests from sediment cores recording the last G/IG transition in order to quantify natural CO2 outgassing and contribute to a better understanding of natural carbon storage and release. In order to improve the reconstructions, we will produce the first d11B pH and B/Ca bicarbonate ion calibration on deep sea benthic foraminifera under in situ pressure. We will also optimize the methods and the analytical tools towards smaller sample size, allowing for single shell analysis.