Scientific output

• Dissolved organic carbon content is higher in deeper soil layers.
• The deep dissolved organic matter is older and more humified than at the surface.
• The vertical gradients studied are amplified during neap tides.
• Cr and Mn follow the dissolved organic carbon vertical gradient.
• Ni and Mo follow an opposite vertical gradient to dissolved organic carbon.

Estuaries exchange greenhouse gases with the atmosphere and can either potentially warm or cool the climate. Most studies focus on CO2, but other gases such as CH4 and N2O can also be important. In several tropical deltas, mixing between river water and sea water can lower CO2 levels, leading these systems to act as CO2 sinks even without biological uptake. Here, we measured air‐water CH4 and N2O fluxes in a tropical estuarine delta in Brazil and combined these data with CO2 flux measurements collected at the same time and locations.