Isotopes Trace Biogeochemistry and Sources of Cu and Zn in an intertidal soil

River floodplain soils are sinks and potential sources for toxic trace metals like Cu and Zn. We hypothesize that stable Cu and Zn isotope ratios reflect both the mobilization and the sources of metals. We determined the soil properties, the concentrations and partitioning of Cu and Zn, and variations in delta Cu-65 and delta Zn-66 values in a core obtained from an Aquic Udifluvent developed on a freshwater intertidal mudflat of the River Elbe, Germany. The core was sampled at 2 cm intervals to a depth of 34 cm, which corresponds to approximately 9 yr of sedimentation. Elevated concentrations of Cu (up to 320 mu g g(-1)) and Zn (up to 2080 mu g g(-1)) indicated anthropogenic pollution. At the time of sampling the redox conditions changed from oxic (Eh 200 to 400 mV, above 22 cm deep) to strongly anoxic conditions (-100 to -200 mV, below 22 cm deep). The delta Cu-65 values varied systematically with depth (from -0.02 to 0.16 parts per thousand) and were correlated with the Fe, C, and N concentrations. Although pre-depositional variations cannot be ruled out, the systematic variation with depth suggests post-sedimentation fractionation of delta Cu-65 in response to seasonally variable organic matter deposition and redox conditions. In contrast, the delta Zn-66(IRMM) values were uniform (from -0.07 to 0.01 parts per thousand) throughout the core, indicating that the Zn isotopes did not significantly fractionate after deposition and that the Zn sources were homogeneous throughout the sedimentation.