Sensitivity of the marine carbon cycle to mechanistic uncertainties in the biological pump

The exchange of carbon dioxide (CO2) between the ocean and the atmosphere is strongly mediated by the fate of organic carbon formed by phytoplankton. Organic carbon is predominantly exported as sinking particles that are either degraded in the water column or buried at the seafloor, which in part set the efficiency of the biological pump. Modern field and laboratory observations have identified a wide range of mechanisms that potentially control organic carbon fluxes and which could be influenced by various environmental conditions, including (but not limited to!) temperature. For example, colder temperatures during glacial periods are hypothesized to have reduced the export of organic carbon but also slowed its degradation in the water column. Key questions remain such as: Is pCO2 more sensitive to one process over another? Are there feedbacks between these processes? Are they the same on different timescales? These questions have significance for understanding how CO2 responds to changes in the biological pump and for interpreting productivity proxies and sedimentary carbonate records such as the CCD. Here we present results using the Earth system model cGENIE to quantify the sensitivity of pCO2 and the marine carbon cycle to uncertainties in the mechanisms that are driving particulate organic matter fluxes. We identify three key processes (export, remineralisation, and flux to sediments) and vary them systematically. Our approach explores the different hypothetical modes of operation of the biological pump over different timescales. We show that the sensitivity of pCO2 to individual processes and their interactions is dependent on the timescale considered due to the response of deep-sea CaCO3 sediments. We also highlight that some modes of operation are contradicted by the deep-sea CaCO3 record presenting an opportunity to help constrain these processes for modern and future modelling.