Etang De La Gruere Research Articles

Interpreting the ash trend within ombrotrophic bog profiles: atmospheric dust depositions vs. mineralization processes. The Etang de la Gruère case study

Introduction Ash peaks along ombrotrophic bog profiles may arise from several different processes. In a recent paper, Leifeld and co-authors (Plant Soil 341:349–361, 2011) argued that ash peaks along the Etang de la Gruere (EGr) peat bog profiles are signs of previous periods of higher peat decomposition rather that an indication of periods of elevated dust inputs.

Response to Interpreting the ash trend within ombrotrophic bog profiles: atmospheric dust depositions vs. mineralization processes. The Etang de la Gruère case study

In ‘Interpreting the ash trend within ombrotrophic bog profiles: atmospheric dust depositions vs. mineralization processes. The Etang de la Gruere case study’, Zaccone et al. (2011a) question our interpretation of enhanced ash contents in depth profiles of ombrotrophic bogs. We see them as an indication for C-loss through peat decay (Leifeld et al. 2011). We highly appreciate the work by Claudio Zaccone, William Shotyk and co-authors on peatlands, including their discussion of methods and their limitations. The approach described in Leifeld et al. (2011) has the potential to overcome some limitations of other methods to estimate C-loss from disturbed peatlands but needs to be carefully evaluated. The criticism of Zaccone et al. (2011a) relates to one (EGr) of the four different bogs discussed in our study. EGr is a rather small bog (5 ha). Therefore, the concerns raised in this single case may not be generalized. In our study, EGr served as a reference site and its ash peak has been attributed mainly to human disturbance but less to changing patterns in atmospheric deposition or drainage. Zaccone et al. (2011a) challenge this assumption and raise three points of concern, (1) sampling methodology, (2) normalization of ash peaks to depth, and (3) discussion and interpretation of ash peaks. We reply to each of them.

Distribution patterns of selected PAHs in bulk peat and corresponding humic acids from a Swiss ombrotrophic bog profile

An ombrotrophic peat core was collected in 2005 from Etang de la Gruere, Jura Mountains, Switzerland. The concentrations of nine among the U.S. Environmental Protection Agency priority polycyclic aromatic hydrocarbons (PAHs) (i.e., acenaphthene, phenanthrene, fluorene, pyrene, fluoranthene, benzo[jbk]fluoranthene, benzo[a]pyrene, benzo[ghi]perylene, and indeno[1,2,3-cd]pyrene) were determined in both bulk peat and corresponding humic acids (HA) samples by gas chromatography equipped with a mass spectrometry detector (GC-MS). The maximum PAHs concentrations in peat (around 1,250 μg Σ PAHs kg−1 dry matter) were found at 28–30 cm of depth, which correspond to ca. 1920–1930, when coal inputs to Switzerland reached their maximum level. Amongst the nine PAHs analyzed in the peat samples, pyrene (Pyr) was the predominant species, accounting for ca. 20–100% of the total PAHs throughout the profile. In the HA fraction, that represents 24.7% (average value) of the bulk peat, only phenanthrene (Phe), and sporadically Pyr and fluoranthene (Fth), were detected. In particular, HA showed Phe concentrations that were ten–150 times higher than corresponding bulk peat samples, thus suggesting its preservation against biodegradation due to the incorporation into HA molecules.

Lead in Three Peat Bog Profiles, Jura Mountains, Switzerland: Enrichment Factors, Isotopic Composition, and Chronology of Atmospheric Deposition

One metre cores were taken from three peat bogs in the Jura Mountains of Switzerland: Etang de la Gruere (EGr), La Tourbiere des Genevez (TGe), and Praz Rodet (PRd). Dried peat samples were analyzed for lead (Pb) using the EMMA XRF and scandium (Sc) using INAA. Enrichment factors (EF) were calculated by normalizing to the background Pb/Sc ratio at EGr. Age dates were obtained using 210Pb (CRS Model) and confirmed using pollen chronostratigraphic markers in replicate cores. The isotopic composition of Pb in selected peat samples from EGr and TGe was determined using TIMS. Two pronounced peaks in Pb EF occur in the uppermost sections of all three bogs: the upper one corresponds to the late 1960's to late 1970's and the lower one to approximately 1900 to 1920. At EGr, sample 2f5 (11 cm, 86 µg/g Pb, EF = 91, and dated at A.D. 1967 ± 2) has an isotopic composition similar to that of leaded gasoline used in Berne in the 1970's. For comparison, the older peak at EGr (sample 2fl 1 at 29 cm, 84 µg/g Pb, EF = 79 and dated at A.D. 1905 ± 6) is significantly different. In contrast to these two samples, the isotopic composition of sample 2fl 5 (41 cm, 30 µg/g Pb, EF = 13 and pre-dating ca. A.D. 1800) approaches the present day "average terrestrial lead" and is likely to be predominately lithogenic. Therefore, the isotopic data show that the Pb introduced to the bog from leaded gasoline has not penetrated to this depth in the profile. A simple mass balance using the measured Pb concentrations and the isotopic compositions shows that vertical migration of gasoline Pb cannot explain the deeper, older peak in Pb EF. A more likely explanation is that the older peak reflects the rates of atmospheric Pb deposition during the first two decades of the 20th century. Taken together, the Pb concentrations, age dates, and isotopic data suggest that these peat profiles have preserved the record of changing rates of atmospheric Pb deposition. In addition, the results indicate that the isotopic composition of Pb deposited on the surface of the bogs gradually shifted away from lithogenic ratios as long ago as the middle of the 19th century, a change which clearly pre-dates the introduction of leaded gasoline.

Lead-210 Age Dating of Three Peat Cores in the Jura Mountains, Switzerland

Peat cores collected from three sites in the Jura region of Switzerland, La Tourbiere des Genevez, Etang de la Gruere and Praz Rodet, were analysed for the fallout radionuclides 210Pb, 137Cs, 134Cs and 241Am, and the minerogenic radionuclide 226Ra. Unsupported 210Pb inventories of the cores were all in the range 3900-4784 Bq m-2 and are consistent with their having largely retained the atmospheric flux (ca. 130 Bq m-2y-1). In contrast, comparisons with earlier (1986) cores from these sites suggest that there have been significant losses of fallout 137Cs. Samples from all three sites had low but significant levels of supported 210Pb (226Ra) activity, the origin of which is presumably wind blown soil dust. 210Pb dates calculated using the CRS model were independently validated by 241Am and pollen stratigraphy. The core from La Tourbiere des Genevez had a fairly conventional 210Pb activity versus depth profile indicating more or less constant net accumulation during the past 100 years. At the other two sites however the 210Pb profiles contained significant deviations from simple exponential decline that may record episodes in the bog during which there were major variations in the net dry mass accumulation rate. These episodes are dated 1930-50 (Etang de la Gruere) and 1960-80 (Praz Rodet). Although the depths at which total 210Pb reaches equilibrium with the supporting 226Ra range widely, from ca.37 cm in Etang de la Gruere to ca.60 cm in Praz Rodet, mean net dry mass accumulation rates for the past 100 years are remarkably similar, the values for all three sites are in the range 0.023-0.027 g cm-2y-1.

Atmospheric Lead Deposition from 12,400 to ca. 2,000 yrs BP in a Peat Bog Profile, Jura Mountains, Switzerland

The bog at Etang de la Gruere (Jura Mountains, Switzerland) consists of 420 cm of Sphagnum-dominated bog peat overlying 230 cm of Carex-dominated fen peat. One hundred cm below the bog surface, there is a pronounced peak in lead (Pb) concentration (approx. 10 µg/g) which has been dated at 2110 ± 30 BP and can be attributed to Roman Pb mining and smelting.