Seasonal variations in the Amazon plume-related atmospheric carbon sink

The Amazon River plume is a highly seasonal feature that can reach more than 3000 km across the tropical Atlantic Ocean, and cover similar to 2 million km(2). Ship observations show that its seasonal presence significantly reduces sea surface salinity and inorganic carbon. In the western tropical North Atlantic during April-May 2003, plume-influenced stations exhibited surface DIC concentrations lowered by as much as 563 mmol C kg(-1) (similar to 28%) and pCO(2) as low as 201 mu atm. We combine our data with other data sets to understand the annual uptake and seasonal variability of the plume-related CO2 sink. Using flux estimates from all seasons with monthly plume areas determined by satellite, we calculate the annual carbon uptake by the outer plume alone ( 28 < S < 35) to be 15 +/- 6 TgC yr(-1). Diazotroph-supported net community production enhanced the air-sea CO2 disequilibrium by 100x and reversed the typical CO2 outgassing from the tropical North Atlantic. The carbon sink in the Amazon plume depends on climate-sensitive conditions that control river hydrology, CO2 solubility, and gas exchange.