Geochemistry of the peat bog at Etang de la Gruère, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 14C yr BP

Abstract

A 650 cm core from a Swiss bog represents 12,370 14C years of peat accumulation and provides the first complete record of atmospheric Pb deposition for the entire Holocene. Excess, non-atmospheric Sr in the peat was calculated by normalizing Sr/Sc to crustal abundance; this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral-water interactions contribute metals to the peat. While sediment dissolution contributes significantly to the Sr inventory of the minerogenic section of the peat profile, there is no measurable effect of this process on the Pb burden. Isotopic analyses (204Pb, 206Pb, 207Pb, 208Pb) show that effectively all of the Pb in the peat profile was supplied exclusively from the atmosphere.To separate natural and anthropogenic Pb, Sc was selected over Ti, Y, Zr, Hf and REE as a conservative, reference element which is supplied by soil dust aerosols derived from rock weathering. Enrichment factors (EF) were calculated using the Pb/Sc ratio in the peat samples, normalized to the “natural, background” Pb/Sc which is found in peats dating from 8030 to 5320 14C yr BP. The results show that anthropogenic source have dominated the supply of atmospheric Pb to the peat core continuously since 3000 14C yr BP. The aerosols supplied to the bog can be divided into 3 classes: a) Pre-Anthropogenic (older than 3000 14C yr BP with Pb EF < 2 and 206Pb/207Pb > 1.194); b) Pre-Industrial (dating from 3000–240 14C yr BP, with Pb EF ≥ 2 but <20 and 206Pb/207Pb 1.193–1.179); c) Industrial (more recent than 240 14C yr BP, with Pb EF > 20 and 206Pb/207Pb < 1.179).Elevated soil dust fluxes are observed at 5320, 8230 and 10,590 14C yr BP; the latter corresponds to the Younger Dryas. Aluminum, Zr, Hf, and REE/Sc ratios also are elevated at the same depths, suggesting differences in particle size, wind strength, or source regions. Pre-Anthropogenic aerosols deposited since 8230 14C yr BP reveal Pb/Sc which is significantly higher, and 206Pb/207Pb which is less radiogenic, than during the early Holocene. While the trend toward increasing Pb/Sc could be due to chemical weathering and soil development, this could not explain the shift in Pb isotopic composition. The changes which took place at 8230 14C BP, therefore, may be related to a large scale climatic reorganization which, at present, is poorly understood.