INCREASE IN THE EXPORT OF ALKALINITY FROM NORTH AMERICA’S LARGEST RIVER: CLIMATE AND LAND-USE CONTROLS ON ALKALINITY EXPORT FROM THE MISSISSIPPI RIVER

Chemical weathering and the subsequent export of carbonate alkalinity (HCO3- +CO3-2 ) from soils to rivers is a significant fate of terrestrially-sequestered atmospheric CO2 . We show here that during the past half-century the export of this alkalinity has increased dramatically (59%) from North America’s largest river, the Mississippi. This increased export is in part the result of increased flow resulting from higher rainfall in the Mississippi basin. Global change models predict a continuation in the alterations to the global water balance over the following decades with a general increase in precipitation. Thus it appears that the rate of alkalinity export and terrestrial CO2 sequestration through weathering could continue to increase in future years.

Sub-catchment data of the Mississippi suggest that the increase in the export of alkalinity is also strongly linked to land cover. Croplands, which make up >30% of the Mississippi watershed, export 4 to 9 times more alkalinity per unit area than forests. The direct mechanism is unclear, but sub-catchment observations indicate that cropland systems respond differently to rainfall events than forested systems. These observations have important implications for potential management of C sequestration in the United States and suggest that reconverting abandoned cropland fields to forest will decrease terrestrial carbon sequestration through weathering at a rate comparable to above ground forest biomass accumulation.