The Role of River Fluxes and Ocean Skin Temperature

An interhemispheric CO transport in the deep ocean which might reconcile Tans et al. with the ocean models would seem to be ruled out by the lack of sufficiently large pCO deficits in the North Atlantic. If the story ended here we would clearly be at an impasse. However, Sarmiento and Sundquist (1992) reminded us that the oceanic CO sink determined by pCO actually underestimates the oceanic sink because the ocean naturally outgasses CO to the atmosphere to balance terrestrial sinks.

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. If rock weathering on land is in balance with the burial of CaCO on the sea floor and decarbonation reactions during sediment metamorphism, then an amount of CO is outgassed from the ocean which balances the CO uptake on land (Sarmiento and Sundquist, 1992). 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. Sarmiento and Sundquist estimate that as much as half of the 1.0-1.5 GtC/yr discrepancy between the oceanic uptake in Tans et al. (1990) and in ocean models can be explained by aspects of the natural carbon cycle involving rivers.

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. This means that the oxidation of riverine organic matter should be especially large in the North Atlantic and Arctic on a per unit area basis. The polar seas of the North Atlantic are also dominated by organisms which produce much greater sinking fluxes of CaCO than organisms in polar seas elsewhere (Honjo, 1990). Both of these effects tend to focus the CO outgassing due to river fluxes in the North Atlantic and to mask the flux of anthropogenic CO that one infers from pCO.

Sarmiento and Sundquist (1992) and Robertson and Watson (1992) raise the issue of ocean ``skin'' temperature. The temperature of ocean water within 1 mm of the ocean-atmosphere interface is known to be about 0.3C cooler than water in the bulk mixed layer due to thermal radiation and evaporation. This cooling while small has a rather large effect on the pCO that governs ocean-atmosphere CO exchange. A skin effect of 0.3C lowers the pCO at the interface by 4 ppm. Since the average ocean-wide pCO needed to move 2 GtC/yr into the ocean is only about 8 ppm, it is easy to see how important the skin effect can be. Robertson and Watson estimate that the skin effect may account for an additional 0.7 GtC/yr of ocean uptake.

River fluxes and the skin temperature effect are large enough to bring the global system close to balance and still allow substantial CO uptake by the ocean (Sarmiento and Sundquist, 1992). However, it remains to be seen whether these effects can produce a big enough effect in the North Atlantic to satisfy the atmospheric transport models.