Export of 13C-depleted dissolved inorganic carbon from a tidal forest bordering the Amazon estuary

Tidal wetlands play a significant role in the coastal carbon cycle and exchange material with the atmosphere and coastal ocean. Here, we report on changes in dissolved inorganic carbon speciation and isotopic composition throughout a 24 h cycle (2 tidal cycles) in Feb. 2007 in a channel connecting the Amazon estuary to the basin of a tidal forest. At this site, tropical forest soils are inundated at high tide by estuarine freshwater, and temporal concentration changes in the channel reflect exchanges between the forest and estuary. Our data show an export of dissolved inorganic carbon (DIC) in the form of excess CO2 and, to a much lesser extent, CH4. However, the tidal forest traps suspended sediments. Mixing plots of DIC versus conductivity showed that the DIC originated from the tidal forest soil, with a negligible contribution from the local watershed. Evolution of the isotopic signature of DIC reveals a 13C-depleted source (−56.9 ± 3.3‰), presumably originating from a dominant methanogenic pathway of carbon mineralization followed by almost complete CH4 oxidation in the organic clay-rich freshwater soil.