Isotope Systematics of Sulfate-oxygen and Sulfate-sulfur in Six European Peatlands

Abstract

Oxygen (O) and sulfur (S) isotope systematics in bog water sulfates were determined for six Sphagnum dominated wetlands located in the British Isles and the Czech Republic, Central Europe. Comparison of a polluted and unpolluted site showed that 4 times higher atmospheric S inputs led to 3 times higher bog water sulfate concentrations and substrate S concentrations, 3 times increased ranges of substrate S concentrations, and 3 times increased ranges of δ34S values. Sites with elevated atmospheric S inputs exhibited greater geochemical variability in wetland S species. Sulfate O–S isotope composition of bog pore water at a depth of 40 cm below surface differed from that of surface bog water, indicating that dissimilatory bacterial sulfate reduction, a process known to discriminate against the heavier isotopes 18O and 34S, occurred in surface peat layers. While bacterial sulfate reduction remained to be one of the main isotope-selective processes for sulfate in peat, it could not fully explain the O–S isotope systematics of peat waters. The ‘residual’ sulfate was not simultaneously enriched in the heavier isotopes 18O and 34S. Mixing of residual sulfate following bacterial sulfate reduction with the product of S2− reoxidation, cleavage of esters, and isotope exchange reactions may have contributed to the decoupling of the δ34Sso4 and δ18Sso4 values. Large within-site differences in δ18Sso4 and δ34Sso4 (up to 13 and 15‰, respectively) indicated little communication between the 0 and 40 cm peat depth at some sites. Extremely high δ18Sso4 and δ34Sso4 values found in several peat bog water samples from Connemara (Ireland), Thorne Moors (England) and Ocean (Czech Republic) were not seen in streams draining the wetlands. Direct runoff of atmogenic sulfate constituted a significant portion of the bog outflow. At the wetland scale, zones of dissimilatory bacterial sulfate reduction form pockets whose lateral hydrological fluxes are small.