Sensitivity of atmospheric CO₂ to regional variability in particulate organic matter remineralization depths

The concentration of CO₂ in the atmosphere is sensitive to changes in the depth at which sinking particulate organic matter is remineralised: often described as a change in the exponent "b" of the Martin curve. Sediment trap observations from deep and intermediate depths suggest there is a spatially heterogeneous pattern of b, particularly varying with latitude, but disagree over the exact spatial patterns. Here we use a biogeochemical model of the phosphorus cycle coupled with a steady-state representation of ocean circulation to explore the sensitivity of preformed phosphate and atmospheric CO₂ to spatial variability in remineralisation depths. A Latin hypercube sampling method is used to simultaneously vary the Martin curve indepedently within 15 different regions, as a basis for a regression-based analysis used to derive a quantitative measure of sensitivity. Approximately 30% of the sensitivity of atmospheric CO₂ to changes in remineralisation depths is driven by changes in the Subantarctic region (36°S to 60°S), simliar in magnitude to the Pacific basin despite the much smaller area and lower productivity. Overall, the absolute magnitude of sensitivity is controlled by export production but the relative spatial patterns in sensitivity are predominantly constrained by ocean circulation pathways. The high sensitivity in the Subantarctic regions is driven by a combination of high export production and the high connectivity of these regions to regions important for the export of preformed nutrients such as the Southern Ocean and North Atlantic. Overall, regionally varying remineralisation depths contribute to variability in CO₂ of between ±5–15 ppm relative to a global mean change in remineralisation depth. Future changes in the environmental and ecological drivers of remineralisation, such as temperature and ocean acidification, are expected to be most significant in the high latitudes where CO₂ sensitivity to remineralisation is also highest. The importance of ocean circulation pathways to the high sensitivity in Subantarctic regions also has significance for past climates given the importance of circulation changes in the Southern Ocean.