Geochemistry, mineralogy, and geochemical mass balance on major elements in two peat bog profiles (Jura Mountains, Switzerland)

The mineralogical and chemical composition of peats from two Sphagnum bogs in the Franches Montagnes region (Jura Mountains, Switzerland) were compared. The peats in the top 102 cm at Etang de la Gruère (EGr) represent 2110 years of peat formation compared with 1730 years for the first 84 cm at Tourbière de Genevez (TGe). Scandium was used as a conservative tracer to distinguish between the primary sources of major elements to the bogs: atmospheric deposition of soil-derived aerosols (EGr and TGe); physical incorporation of elements along with mineral matter from the underlying sediments (TGe); and adsorption or complexation by the peat following diffusion from groundwater (TGe). These results support previous findings which showed that the EGr core (102 cm) is exclusively ombrogenic compared with TGe where the peats below approx. 30 cm become predominantly minerogenic. dern rates of Sc accumulation (past 100 years) are similar in the two cores: 39 μg m−2 a−1 at EGr versus 52 μg m−2 a−1 at TGe. However, comparison with deeper sections of the EGr core reveals that the present-day rates of atmospheric Sc deposition are nearly 3 times greater than the long-term average rate, reflecting the higher concentrations of soil-derived atmospheric aerosols today. The rates of Sc accumulation in the TGe core are slightly higher than at EGr; this reflects the incorporation of mineral matter from the underlying sediments In the deeper, minerogenic peats at TGe, the long term rates of Fe and Ca accumulation are 5 and 7 times higher, respectively, than in the EGr core. These fluxes greatly exceed the difference in Sc flux, and cannot be explained by the differences in amount of mineral matter in the peats. These enrichments require an independent explanation, and are most likely the result of adsorption and/or complexation of cationic Ca and Fe species diffusing from deeper layers in the profile. neralogical composition of the ash is mainly quartz (60–90%), with feldspar (5–15%) and muscovite (5–15%), although various other minerals (5–10%) are commonly present. Biogenic Si represents another important fraction of ash and is abundant (30–70%) at discrete depths. The vertical profiles showed no significant changes in mineralogy with depth: assuming a constant composition of source materials, the lack of progressive mineralogical change suggests that the fine-grained silicates supplied by soil dust have not been measurably weathered during the past two millennia. A number of peaks in Fe/Sc are found in both cores above peaks in ash content and density. These appear to correspond with temporal fluctuations in wet/dry conditions: accumulation of Fe-oxides during dry periods and leaching during wetter conditions.