Radon‐traced pore‐water as a potential source of CO2 and CH4 to receding black and clear water environments in the Amazon Basin

Abstract Groundwater is a primary source of dissolved CO2 and CH4 in Amazonian headwaters, yet in higher order rivers, a groundwater/pore‐water source is difficult to constrain due to the high spatial and temporal heterogeneity of pore‐water exchange. Here, we report coupled, high resolution measurements of pCO2, CH4, and 222Rn (a natural pore‐water and groundwater tracer) during receding waters in the three major water types of the Central Amazon Basin: black (Negro River); clear (Tapajós River); white (Madeira River). Considerable spatial heterogeneity was observed in pCO2, CH4, and 222Rn concentrations ranging from 460 μatm to 8030 μatm, 7 nM to 281 nM, and 713 dpm m−3 to 8516 dpm m−3, respectively. The significant correlations between pCO2 and CH4 to 222Rn in the black and clear waters suggests that pore‐water further enhanced CO2 supersaturation by 18–47% and is a driver of CH4 dynamics in these waters.