Modelling the dissolution of settling CaCO3 in the ocean

The production, transport and dissolution of carbonate shells plays an important rolein the global carbon cycle. For instance, these processes affect the ocean's CO3concentration which on glacial-interglacial timescales is involved in the regulationof atmospheric pCO2. The saturation state of the ocean with respect to calcium carbonatesuggests that any dissolution of calcium carbonate takes place not in the water columnbut at the seafloor. On the other hand, several authors suggest that a significant part of thedissolution takes place in the upper part of the water column. In the present paper, asimple model is presented to investigate under which circumstances dissolution of calciumcarbonate in the water column may be plausible. Our analysis also includes dissolutionfacilitated by CO2 release from organic carbon remineralization, whichmay generate a microenvironment that is undersaturated with respect toCaCO3. In marine snow aggregates, however, respiration driven dissolutionis shown to be insignificant due to the size and settling velocities of theaggregates. With respect to aragonite, dissolution in the water columnattributable to the bulk saturation state alone is significant and maywell contribute to the alkalinitymaximum in the undersaturated intermediate waters of the North Pacific Ocean.The suggested large loss of calcium carbonate dissolved in the water column(60-80% of CaCO3 production), however, cannot be explained by themechanisms investigated here.