Geochemistry of molybdenum in the Chao Phraya River estuary, Thailand: Role of suboxic diagenesis and porewater transport

The concentration of dissolved molybdenum (Mo) was measured in the Chao Phraya River estuary, Thailand to examine the sources and geochemical behavior of Mo. Water samples in the salinity range of 0.02-24.5 collected during periods of contrasting river discharge have Mo concentrations of 3.2-117 nM. In the estuary, Mo and salinity show strong linear correlations suggestive of conservative mixing. However, at salinity>5, Mo concentrations are much higher than that can be achieved from mixing of river water and average seawater. Desorption from river-borne and estuarine sedimentary particles contribute Mo that is significantly less than the observed “excess” Mo in the Chao Phraya estuary. Groundwater discharge is unlikely to be an important source of Mo in the estuary. Reductive mobilization of Mo in the sediments of the Gulf of Thailand, its transport to the overlying water and finally to the estuary is likely to be the dominant mechanism that is responsible for the observed “excess” Mo. This inference is drawn from higher salinity and “excess” Mo in deepwater of the Gulf of Thailand, strong correlation between dissolved Mo and 228Ra, and the observation of suboxic diagenesis in the Gulf sediments. Contribution of Mo to the estuary via porewater transport is likely to be enhanced by higher water temperature, tidal currents, sediment re-working and bottom water upwelling which are all documented in the northern Gulf of Thailand. Porewater flux of Mo, estimated using 228Ra flux, is much higher than the average riverine Mo flux. Temporal changes in redox condition and circulation pattern in the Gulf seem to be responsible for the observed seasonal variation of Mo abundances in the estuary.