Anthropogenically enhanced fluxes of water and carbon from the Mississippi River

TLDR: An unprecedented high-temporal-resolution, 100-year data set from the Mississippi River is introduced and it is shown that the large increase in bicarbonate flux that has occurred over the past 50 years is clearly anthropogenically driven.

Abstract: The flow of dissolved inorganic carbon from rivers to the oceans is an important net flux connecting the terrestrial and marine carbon reservoirs. Now a remarkable 100-year record of bicarbonate determinations, made at water treatment plants in the towns of Carrollton and Algiers, has been used as a basis for a study of Mississippi River water and carbon fluxes. Previous work revealed a significant increase the amount of dissolved inorganic carbon, mostly bicarbonate, exported by the Mississippi to the ocean over the past 50 years, but the cause for the increase remained uncertain. The Carrollton/Algiers data, together with sub-watershed and precipitation data, point to a mainly anthropogenic origin — increased bicarbonate discharge from agricultural watersheds that was not balanced by a rise in precipitation. A high temporal resolution, 100-year data set from the Mississippi River is coupled with sub-watershed and precipitation data to reveal that a ∼40 percent increase in flux of bicarbonate that has occurred over the last 50 years is clearly anthropogenically driven. This is caused by an increase in discharge from agricultural watersheds not balanced by a rise in precipitation. It is suggested that land use change and management are arguably more important than changes in climate and carbon dioxide fertilization. The water and dissolved inorganic carbon exported by rivers are important net fluxes that connect terrestrial and oceanic water and carbon reservoirs1. For most rivers, the majority of dissolved inorganic carbon is in the form of bicarbonate. The riverine bicarbonate flux originates mainly from the dissolution of rock minerals by soil water carbon dioxide, a process called chemical weathering, which controls the buffering capacity and mineral content of receiving streams and rivers2. Here we introduce an unprecedented high-temporal-resolution, 100-year data set from the Mississippi River and couple it with sub-watershed and precipitation data to reveal that the large increase in bicarbonate flux that has occurred over the past 50 years (ref. 3) is clearly anthropogenically driven. We show that the increase in bicarbonate and water fluxes is caused mainly by an increase in discharge from agricultural watersheds that has not been balanced by a rise in precipitation, which is also relevant to nutrient and pesticide fluxes to the Gulf of Mexico. These findings demonstrate that alterations in chemical weathering are relevant to improving contemporary biogeochemical budgets. Furthermore, land use change and management were arguably more important than changes in climate and plant CO2 fertilization to increases in riverine water and carbon export from this large region over the past 50 years.