Interactions between terrestrial dissolved organic matter and trace metals in the Amazon River plume

We present a study of the detailed molecular composition of dissolved organic matter (DOM) and metal-containing DOM along the Amazon River-to-ocean-continuum from the river mouth to the tropical North Atlantic Ocean during high discharge conditions in 2018. We covered the full salinity gradient along the river plume and sampled the mangrove belt south of the river mouth to serve as a reference ecosystem that is dominated by intertidal water exchange. We applied ultrahigh resolution mass spectrometry (FT-ICR-MS) and other geochemical proxies that identified a large influx of terrestrial DOM and macronutrients to the Atlantic Ocean. Aromatic compounds mainly derived from terrestrial land plant production, dissolved black carbon (DBC) and dissolved organic sulfur (DOS) were highly enriched in the river plume, but also in the coastal shelf with outwelling porewater from mangrove forests south of the Amazon River. DBC includes polycyclic condensed aromatics that are produced during incomplete combustion of organic matter on land, whereas DOS can be produced through abiotic sulfurization reactions of sulfide with DOM in anoxic systems. Both organic matter pools represent petagram inventories of recalcitrant DOM in the ocean with sources and residence times that need to be better constrained. Our data demonstrate that both, the Amazon River plume and Brazilian mangrove areas are important sources of these compounds to the Atlantic Ocean. Ongoing analyses of metal-containing DOM via FT-ICR-MS suggest that the organic iron- and copper-complexes were primarily aliphatic with a comparatively high abundance in the river mouth water but also in the offshore seawater. This may indicate that a part of the riverine metal-DOM complexes is stable enough to be transported to the coastal Atlantic Ocean.