Seasonal variations in the Amazon plume‐related atmospheric carbon sink

Abstract

The Amazon River plume is a highly seasonal feature that can reach more than 3000 km across the tropical Atlantic Ocean, and cover ∼2 million km2. 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 μmol C kg−1 (∼28%) and pCO2 as low as 201 μatm. 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.