Influence of the Amazon River on the composition of particulate organic carbon in the western tropical Atlantic Ocean

Abstract

The Amazon River exports ∼13.7 Tg yr⁻¹ of particulate organic carbon (POC) to the ocean, a portion of which is transported vertically through the water column. Here, we investigated in detail the composition of POC using a multi-tracer approach, including POC concentration, stable carbon isotopes (δ¹³C) and a wide range of molecular biomarkers to assess the contribution of POC export from the Amazon River to the western tropical Atlantic Ocean (WTAO). For this purpose, suspended and sediment trap POC were collected at multiple depths along the Amazon River plume under high and low river discharge conditions. Concentrations of POC detected in the plume were ∼2 to 3-fold higher than those below the plume. Overall, δ¹³C signatures of suspended and sediment trap POC samples were enriched, averaging −19.9 ‰ and –22.8 ‰, respectively. At the surface, POC composition was mainly characterized by low molecular weight n-alkanoic acids (<C₂₀), mono- and disaccharides (e.g., glucose, scyllo-inositol and melibiose) and triglyceride derivatives (e.g., glycerol and 2-O-glycerol-α-D-galactopyranoside), indicating fresh inputs of organic matter (OM) presumably derived from algal blooms. These biomarkers were observed in lower concentrations in sediment traps. Trap samples were marked by zooplankton- (e.g., cholesterol, occelasterol) and bacteria-derived (e.g., C₁₅ and C₁₇ branched n-alkanoic acids) biomarkers, indicating metabolic alteration of OM at those depths. Solvent-extractable terrestrial biomarkers (e.g., dehydroabietic acid and levoglucosan) were only found in minor concentrations in most samples, which contrasts with the dissolved fraction which has been previously shown to have a primary signature of terrigenous inputs. Our results clarify the Amazon River plume’s impact on the biological pump of the WTAO, consistent with a river plume fueling primary production, and with increased zooplankton and bacteria contributions to POC composition at depth and in the POC that is vertically exported.