On the influence of marine biogeochemical processes over CO2 exchange between the atmosphere and ocean

The ocean holds a large reservoir of carbon dioxide (CO 2 ), and mitigates climate change through uptake of anthropogenic CO 2 . Fluxes of CO 2 between the atmosphere and surface ocean are regulated by a number of physical and biogeochemical processes, resulting in a spatiotemporally heterogeneous CO 2 distribution. Determining the influence of each individual process is useful for interpreting marine carbonate system observations, and is also necessary to investigate how changes in these drivers could affect air-sea CO 2 exchange. Biogeochemical processes exert an influence primarily through modifying seawater dissolved inorganic carbon ( C T ) and total alkalinity ( A T ), thus changing the seawater partial pressure of CO 2 ( p sw ). Here, we propose a novel conceptual framework through which the size of the CO 2 source or sink generated by any biogeochemical process, denoted Φ, can be evaluated. This is based on the ‘isocapnic quotient’ ( Q ), which defines the trajectory through ( A T , C T ) phase space for which there is no change in p sw . We discuss the limitations and uncertainties inherent in this technique, which are negligible for most practical purposes, and its links with existing, related approaches. We investigate the effect on Φ of spatiotemporal heterogeneity in Q in the present day surface ocean for several key biogeochemical processes. This leads the magnitude of the CO 2 source or sink generated by processes that modify A T to vary spatiotemporally. Finally, we consider how the strength of each process as a CO 2 source or sink may change in a warmer, higher-CO 2 future ocean.