If the story ended here we would clearly be at an impasse. CO is taken up on land by chemical reactions when carbonate and silicate rocks are weathered. Cations released by the weathering reactions and bicarbonate ions from atmospheric CO flow into the ocean via rivers. Riverine bicarbonate is released back to the atmosphere as CO when calcium is taken up by marine organisms to form CaCO . This outgassing reduces the ocean-wide pCO and reduces the amount of anthropogenic CO uptake that one would infer from pCO measurements.
Rivers also carry particulate and dissolved organic carbon from the land to the sea.
Some of this organic matter is buried in sediments, but much of it is remineralized in the ocean. The remineralized carbon must also be outgassed from the ocean and cycled back through the atmosphere in order to maintain a steady cycle. Thus river fluxes of carbon, both as bicarbonate and as organic matter, tend to mask the flux of anthropogenic CO going into the ocean. River fluxes are also important because of the location within the ocean where CaCO is formed and where riverine organic matter is oxidized. The North Atlantic and Arctic Oceans receive the drainage from a disproportionate share of the world's land area.