Trace metals in Holocene coastal peats and their relation to pyrite formation (NW Germany)

Abstract

Three drill cores from the marshlands of NW Germany, which cover the entire Holocene, were analyzed at high-resolution for bulk composition, Al, Fe, selected trace metals, and stable sulfur isotopes. The drill cores contain two lithological types of peat: (i) basal peats overlying Pleistocene sands and (ii) intercalated peats situated between clastic sediments of predominantly marine origin. The peat layers are characterized by distinct enrichments in pyrite due to microbial sulfate reduction under almost open system conditions with respect to seawater sulfate as shown by sulfur isotope partitioning. The main Fe source seems to be the freshwater environment. The determination of dissolved and particulate Fe of channels and small rivers close to the study area revealed a 50-fold higher Fe content of the freshwater environment when compared with North Sea water. Pyrite enrichments are explained by two scenarios: (i) pyrite formation coincides with fen reed peat growth (basal and intercalated) under the influence of a brackish water zone (salinity app. 5–15) and (ii) pyrite was formed after peat growth in the lowest limnic basal peat intervals. Maximum pyrite accumulation (TS 28%) occurs in latter peats that contain thin clastic layers as a result of tidal channel activities after peat formation. The occurrence of clastic layers may have favoured the inflow of saline groundwater. The peat layers are also characterized by enrichments in redox-sensitive trace metals (As, Mo, Re, U) and Cd, whereas Co, Cr, Cu, Mn, Ni, Pb, Tl, and Zn reflect the geogenic background. Leaching experiments have shown that As, Co, Cu, Mo, Re, and Tl are predominantly fixed as sulfides and/or incorporated into pyrite. The remaining trace metals show no distinct trends, only Cr reveals a strong relation to the lithogenic detritus. Seawater is the dominating source for As, Cd, Mo, Re, and U. The remaining trace elements seem to have a freshwater source similar to Fe. In contrast to the distribution of pyrite, highest amounts of redox-sensitive trace metals are seen in fen reed peats (basal and intercalated) that were formed under a direct influence of seawater and brackish water, respectively. Therefore, we suggest that saline groundwater entering the basal peats was probably depleted in redox-sensitive trace metals, e.g. owing to microbially induced reduction of trace metals and subsequent precipitation as sulfides or fixation by organic matter.