Unravelling the land source: an investigation of the processes contributing to the oceanic input of DIC and alkalinity

In models of the marine carbon system, it is important to correctly represent riverine and aerial inputs of dissolved inorganic carbon (DIC) and alkalinity. We have examined the different processes contributing to this exchange. In terms of DIC, we have divided the fluxes into their internal component, constituting the carbon ultimately derived from the atmosphere, and their external component originating from rocks. We find that the only process contributing to external DIC input is carbonate and fossil carbon weathering and that erosion of organic matter ultimately constitutes a DIC sink. A number of both riverine and aerial inputs affect the alkalinity. Beside carbonate and silicate weathering, we examine processes of pyrite weathering, aerial input of sulphuric acid, and riverine and aerial inputs of various nitrogen species. Using the observation that, in the ocean, the nitrate concentration follows that of phosphate, we assume a steady state in nitrate. This leads to the surprising result that the only processes affecting the alkalinity is riverine input of nitrate, constituting an alkalinity source and input of ammonia, constituting an alkalinity sink. Furthermore, we compare the flux sizes. As expected, carbonate and silicate weathering has the largest effect on alkalinity, though we note that burial of pyrite might be of importance during periods of large-scale anoxia.