Previous research has shown that the atmospheric pCO₂ sensitivity to changes in low-latitude sea-surface chemistry (“low-latitude sensitivity”) depends on both the volume of the ocean ventilated from low latitudes and on the degree of air-sea disequilibrium at high latitudes. However, it is not clear which effect is more important. In this paper we present a diagnostic framework for quantifying the relative importance of low-latitude ventilation versus high-latitude air-sea disequilibrium in determining the low-latitude sensitivity of ocean carbon cycle models. The diagnostic uses a Green function that partitions the ocean's carbon inventory on the basis of whether the carbon last interacted with the atmosphere in the low latitudes or in the high latitudes. The diagnostic is applied to a simple 3-box model, a box model with a ventilated thermocline, and a suite of OGCM runs meant to capture a range of possible ocean circulations for present and last-glacial-maximum conditions. The diagnostic shows unambiguously that the OGCM has a greater low-latitude sensitivity than the box models because of the greater amount of water ventilated from low latitudes in the OGCM. However, when applied to the suite of OGCM runs, the diagnostic also reveals that the effect of high-latitude air-sea disequilibrium can sometimes dominate the effect of low-latitude ventilation and is highly sensitive to the state of the ocean circulation. In particular, the magnitude of the high-latitude disequilibrium effect correlates strongly with the strength of the Atlantic meridional overturning circulation and the volume of water ventilated from northern high latitudes.

A major impediment to understanding long-term changes in the marine nitrogen (N) cycle is the persistent uncertainty about the rates, distribution, and sensitivity of its largest fluxes in the modern ocean. We use a global ocean circulation model to obtain the first 3-D estimate of marine denitrification rates that is maximally consistent with available observations of nitrate deficits and the nitrogen isotopic ratio of oceanic nitrate. We find a global rate of marine denitrification in suboxic waters and sediments of 120–240 Tg N yr⁻¹, which is lower than many other recent estimates. The difference stems from the ability to represent the 3-D spatial structure of suboxic zones, where denitrification rates of 50–77 Tg N yr⁻¹ result in up to 50% depletion of nitrate. This depletion reduces the effect of local isotopic enrichment on the rest of the ocean, allowing the N isotope ratio of oceanic nitrate to be achieved with a sedimentary denitrification rate about 1.3–2.3 times that of suboxic zones. This balance of N losses between sediments and suboxic zones is shown to obey a simple relationship between isotope fractionation and the degree of nitrate consumption in the core of the suboxic zones. The global denitrification rates derived here suggest that the marine nitrogen budget is likely close to balanced.