Humphreys, Matthew P., Daniels, Chris J., Wolf-Gladrow, Dieter A., Tyrrell, Toby and Achterberg, Eric P. (2018) On the influence of marine biogeochemical processes over CO2 exchange between the atmosphere and ocean. Marine Chemistry, 199 . pp. 1-11. DOI 10.1016/j.marchem.2017.12.006.

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Abstract

Highlights

• Biogeochemical processes create CO2 sources/sinks by altering seawater AT and CT
• Source/sink strength depends on local seawater ‘isocapnic quotient’ (Q)
• Q depends on seawater temperature and the state of the marine carbonate system
• Spatiotemporal variability in Q drives heterogeneous CO2 source/sink magnitude
• Future warming and CO2 emissions will modify Q and the size of CO2 sources/sinks

Abstract

The ocean holds a large reservoir of carbon dioxide (CO2), and mitigates climate change through uptake of anthropogenic CO2. Fluxes of CO2 between the atmosphere and surface ocean are regulated by a number of physical and biogeochemical processes, resulting in a spatiotemporally heterogeneous CO2 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 CO2 exchange. Biogeochemical processes exert an influence primarily through modifying seawater dissolved inorganic carbon (CT) and total alkalinity (AT), thus changing the seawater partial pressure of CO2 (psw). Here, we propose a novel conceptual framework through which the size of the CO2 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 (AT,CT) phase space for which there is no change in psw. 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 CO2 source or sink generated by processes that modify AT to vary spatiotemporally. Finally, we consider how the strength of each process as a CO2 source or sink may change in a warmer, higher-CO2 future ocean.

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