Hydrological study of Lyngmossen bog, Sweden: Isotopic tracers (³H, δ²H and δ¹⁸O) imply three waters with different mobilities

The ³H concentration and stable isotope ratio of hydrogen and oxygen, δ²H and δ¹⁸O, of waters extracted from a Sphagnum-dominant raised bog in Lyngmossen, Sweden, were measured in order to understand where the precipitation is retained and how mobile it is. Three types of waters, which were defined by extractability, were collected from the peat. Two waters were extracted by compressing samples with different pressures (SQW1 and SQW2). The other water was obtained by distilling the compressed samples (DW). ³H was detected in all types of water from depths of 0–50 cm: the concentrations in SQW1, SQW2 and DW ranged 1.17–3.07 Bq/L, 0.98–2.03 Bq/L, and 1.02–1.54 Bq/L, respectively. The maximum ³H concentrations of SQW1, SQW2 and DW were all detected at a depth of around 15 cm, whose ¹⁴C age covers the year of the atomic bomb experiments. The ³H results of SQW1/2 indicate that SQW consists of at least two waters of different mobility, water flowing rapidly downward and immobile water. Sphagnum hyaline cells may be responsible for the immobile water. The δ¹⁸O and δ²H relationship exhibited independent trends between SQW and DW. The distinct difference observed between the two waters at the surface (0–5 cm) indicates that the two waters may be supplied by precipitation at different times of the year, or alternatively that DW comprises plant water taken in from hyaline cells. The δ¹⁸O and δ²H values of both SQW and DW in the shallow layer increased with increasing depth, and in the layer around 30 cm depth, those of SQW showed a distinct decrease with depth. Isotope fractionation caused by evaporation and/or plant utilization of water at the surface layer are considered to be the main causes of such isotopic variation at the surface. Evaporation is likely to take place in much drier conditions for DW than for SQW, probably through stems by capillary action. In SQW freezing may be a possible cause for the decrease of δ¹⁸O and δ²H around 30 cm depth. DW is isotopically very well separated from two SQW1/2. Integrating all isotopic information, we conclude the presence of three different waters: least mobile water at shallow depth perhaps in hyaline cells, which can be extracted by squeezing peat with low pressure; most mobile water in a deeper layer than 30 cm, extracted also by squeezing peat; mobile but least extractable water, which is likely water inside plant tissues.