Temporal evolution of the deep-sea carbonate system and implications for the role of the oceans in glacial-interglacial changes in atmospheric CO2

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Large areas of the floor of the oceans are draped with sediment chiefly composed of biogenic calcium carbonate, the remains of calcareous organisms (foraminifera, coccolithophores, pteropods) whose shells are composed of the CaCO3 minerals calcite or aragonite. The CaCO3 contents of marine sediments in many oceanic regions have varied with climate over glacial-interglacial cycles: lower contents of CaCO3 coinciding with the build-up of continental ice sheets. The existence of 'CaCO3 sediments' in the deep ocean has been crucial for moderating the limits of variation in atmospheric CO2. This is because there is an inverse relationship between the concentration of carbonate ion in the deep ocean and the concentration of atmospheric CO2. Carbonate ion concentration, [CO32-], is a major factor controlling the solubility of CaCO3. Because of this inverse relationship, palaeoceanographers have strove for many years to find a proxy for deep sea [CO32-]. The records of deep-ocean CaCO3 content provide important evidence of how ocean chemistry changed with climate but the evidence is indirect because the CaCO3 records represent a response to changes in the carbonate chemistry of ocean waters or of pore waters. Other methods in use are also indirect and rely on the dissolution of the shells of the calcareous organisms. We have developed a new method to estimate deep sea [CO32-] in past oceans using the incorporation of boron (B) in benthic (deep sea) foraminiferal calcite. Benthic B/Ca allows us to define [CO32-] of ocean waters and thus the depth of the water column 'saturation horizon' above which water is oversaturated, and below which is undersaturated, with respect to CaCO3 solubility. We aim to generate records of deep-ocean [CO32-] in critical regions of the oceans that should add significantly to understanding the role of the oceans in atmospheric CO2 cycles.