Cadmium isotope fractionation in an intertidal soil induced by tidal pumping

Estuarine floodplain soils are both sinks and potential sources of toxic trace metals such as Cd. Mobilization of Cd has been identified through spatiotemporal monitoring in intertidal sediments during the last decades, but no information was yet available as to what extent these biogeochemical dynamics change the Cd isotopic composition. Cores of an Aquic Udifluvent soil from an intertidal mudflat of the Elbe River, Germany, were sub-sampled at 2 cm intervals to a depth of 40 cm corresponding to a sedimentation of about 10 years. Strong redox zonation was found in the cores, with an upper suboxic zone and a lower anoxic zone of significantly different Cd isotopy. Cadmium partitioning and pore water data determined in 1985 and 1988 using a sequential extraction procedure were reassessed to explain the variations in the δ114Cd values determined in this study. The redox conditions in the mudflat soil were found to change from suboxic (Eh 300 mV) to anoxic (Eh –100 mV) at 20 cm. This was caused by semi-diurnal flooding with oxygenated river water (tidal pumping). The δ114Cd values varied systematically with depth and were correlated with the redox profile. The intense tidal water flow caused lighter Cd isotopes to desorb more readily from the parent soil in the upper suboxic zone and to subsequently redepositing in the deeper anoxic (sulfidic) zone, where the lighter Cd mobilized from Fe/Mn oxyhydroxides was found to be scavenged in sulfidic form. Tidal advection of dissolved lighter Cd down to the anoxic zone and sulfidic reprecipitation led to a decrease in δ114Cd value of up to -0.2‰ with signals of ±0.1‰ even on a seasonal timescale. It became clear that apportioning sources using Cd isotopes would be difficult given the sensitivity of the isotope record to such early-diagenetic isotope fractionation reactions.