Carbonate Compensation in the Glacial and Future Oceans – CARBCOMP

The response of the deep ocean carbonate system and CaCO3 dissolution to changes in the carbon cycle (‘carbonate compensation’) is a first order control on atmospheric CO2 on timescales of ~10^3 to 10^5 years. Carbonate compensation is thought to account for up to ~half of the glacial drawdown of CO2, and it will eventually help drawdown the CO2 humans are currently emitting to the atmosphere. However, large uncertainties currently exist in our understanding of the processes governing carbonate compensation. As such, both the role of carbonate compensation in driving glacial-interglacial CO2 variations, and the long-term effects of anthropogenic CO2 emissions on the Earth system remain poorly understood. In this proposal we will use glacial-interglacial CO2 as test case for both our conceptual understanding of carbonate compensation, and, along with new estimates of the acidification of the deep Atlantic over the industrial period, our ability to model the processes involved in carbonate compensation with an Earth System-Sediment model.
Using a highly innovative application of paired carbon and boron isotopes in benthic foraminifera, we will quantify changes in respired CO2 storage within the deep Pacific and Southern Oceans over the last glacial-interglacial cycle, and the response of the carbonate system and CaCO3 cycle to this addition/removal of respired CO2. Our results will provide the first quantitative estimates of the amount and timing of changes in the strength of the soft tissue pump, and the amount and timing of Alkalinity changes due to carbonate compensation – the two key processes invoked to lower glacial CO2. Crucially these data will enable us to constrain the timescale at which carbonate compensation can buffer the deep ocean carbonate system and drawdown CO2.
Anthropogenic CO2 has already begun to make its way into the deep Atlantic, acidifying the deep waters, and dissolving CaCO3. However, as the instrumental record of deep ocean carbonate chemistry is woefully short, there are large uncertainties in our understanding of exactly how much anthropogenic CO2 has been taken up by the deep Atlantic, and the response of the carbonate system and CaCO3 cycle to this CO2. In a highly novel application of benthic foraminiferal boron isotope pH proxy, we will apply state-of-the-art laser-ablation MC-ICP-MS methods to sediment cores in the North Atlantic with exceptional sedimentation rates, enabling us to generate the first record of deep Atlantic pH spanning the industrial period. Our deep Atlantic pH reconstruction will provide valuable new insights on how the largest cumulative sink of anthropogenic CO2 has evolved over the industrial era.
Using these new records of glacial carbonate compensation and industrial-era pH, we will optimise and validate the recently coupled iLOVECLIM-MEDUSA climate/carbon-cycle/sediment model, providing a state-of-the-art tool to model the processes and timescales required for carbonate compensation, and the feedbacks with the climate system. Using the model, we will conduct simulations to i) understand the roles of both the soft tissue pump and carbonate compensation in driving glacial-interglacial CO2, and, ii) predict the long-term effects of anthropogenic CO2 on Earth’s carbon cycle and climate. Given the improvements in our ability to model carbonate compensation that will arise from the optimisation/validation with the new glacial/industrial era datasets, these simulations will represent our best estimates of the long-term response of Earth’s carbon cycle and climate to a given anthropogenic CO2 emissions scenario. Our combined data-modelling approach will thus enable us to constrain the role of carbonate compensation in driving atmospheric CO2 and climate during both glacial times, and into the long-term future.