Abstract. So far, the exploration of possible mechanisms for glacial atmospheric CO2 draw-down and marine carbon sequestration has focussed almost exclusively on dynamic or kinetic processes (i.e. variable mixing-, equilibration- or export rates). Here an attempt is made to underline instead the possible importance of changes in the standing volumes of intra-oceanic carbon reservoirs (i.e. different water-masses) in setting the total marine carbon inventory. By way of illustration, a simple mechanism is proposed for enhancing the carbon storage capacity of the deep sea, which operates via an increase in the volume of relatively carbon-enriched AABW-like deep-water filling the ocean basins. Given the hypsometry of the ocean floor and an active biological pump, the water-mass that fills more than the bottom 3 km of the ocean will essentially determine the carbon content of the marine reservoir. A set of simple box-model experiments confirm the expectation that a deep sea dominated by AABW-like deep-water holds more CO2, prior to any additional changes in ocean overturning rate, biological export or ocean-atmosphere exchange. The magnitude of this "standing volume effect" might be as large as the contributions that have been attributed to carbonate compensation, the thermodynamic solubility pump or the biological pump for example. If incorporated into the list of factors that have contributed to marine carbon sequestration during past glaciations, this standing volume mechanism may help to reduce the amount of glacial – interglacial CO2 change that remains to be explained by other mechanisms that are difficult to assess in the geological archive, such as reduced mass transport or mixing rates in particular. This in turn could help narrow the search for forcing conditions capable of pushing the global carbon cycle between glacial and interglacial modes.
Benthic grazing and carbon sequestration by deep-water glass sponge reefs