Riverine-driven interhemispheric transport of carbon

Controversy surrounds the role of the ocean in interhemispheric transport o f carbon. On one hand, observations in the atmosphere and in the ocean both seem to imply that the preindustrial ocean transported up to 1 Pg C yr(-1) from the Northern to the Southern Hemisphere. On the other hand, three dim ensional (3-D) ocean models suggest that global interhemispheric transport of carbon is near zero. However, in this debate, there has been a general n eglect of the river carbon loop. The river carbon loop includes (1) uptake of atmospheric carbon due to inorganic and organic erosion on land, (2) tra nsport of carbon by rivers, (3) subsequent transport of riverine carbon by the ocean, and (4) loss of riverine carbon back to the atmosphere by air-se a gas exchange. Although carbon fluxes from rivers are small compared to na tural fluxes, they have the potential to contribute substantially to the ne t air-sea fluxes of CO2. For insight into this dilemma, we coupled carbon f luxes from a global model of continental erosion to a 3-D global carbon-cyc le model of the ocean. With rivers, total southward interhemispheric transp ort by the ocean increases from 0.1 to 0.35 +/-0.08 Pg Cyr(-1), in agreemen t with oceanographic observations. Resulting air-sea fluxes of riverine car bon and uptake of CO2 by land erosion were installed as boundary conditions in a 3-D atmospheric model. The assymetry in these fluxes drives a preindu strial atmospheric gradient of CO2 at the surface of -0.6 +/-0.1 mu atm for the North Pole minus the South Pole and longitudinal variations that excee d 0.5 mu atm. Conversely, the gradient for Mauna Loa minus South Pole is on ly -0.2 +/-0.1 mu atm, much less than the -0.8 mu atm gradient extrapolated linearly from historical atmospheric CO2 measurements from the same two si tes. The difference may be explained by the role of the terrestrial biosphe re. Regardless, the river loop produces large gradients both meridionally a nd zonally. Accounting for the river carbon loop changes current estimates of the regional distribution of sources and sinks of CO2, particularly conc erning partitioning between natural and anthropogenic processes.