## 2 Materials and Methods  
### Study Area and Sample Collection  
The selection of the ISNO-1 peat core (64\({}^{\circ}\)18\({}^{\prime}\)55.3\({}^{\prime\prime}\) N 40\({}^{\circ}\)41\({}^{\prime}\)15.6\({}^{\prime\prime}\) E) was carried out on 12 August 2020 within the large Ilassky bog massif, which is a typical representative of the raised bogs of the northern taiga (Belomorsky type), occupying more than 90% of the total area of the bogs in the Subarctic region of European Russia [39].  
The Ilassky bog massif is located ~20 km south of the city of Arkhangelsk. The layout of the sampling site and photos of the bog surface are shown in Figure 1 (Appendix B).  
The area belongs to the White Sea basin. Geomorphologically, the territory is represented by a water-glacier accumulative relief type with lacustrine-glacial plains. The relief of the territory was formed during several epochs of the Quaternary glaciation, the last of which ended about 11.7 thousand years ago. Peat formation began immediately after the disappearance of ice. The glacial deposits underlying peat bogs have a variegated lithological composition--sands, loans, pebbles, and sandy loans.  
The climate is transitional from temperate maritime to subarctic. The average annual rainfall is 600-700 mm per year. Winter is long and cold, the number of days with snow cover is from 180 to 190 days, and the snow depth reaches 60 cm. Summer is cool and short with a frost-free period of 90 to 100 days [40].  
The vegetation cover of the sampling area of the ISNO-1 peat profile makes it possible to identify this bog facies as a carpet facies without a pronounced microrelief. There is no tree layer in the study area. The herbaceous-shrub layer is composed mainly of vaginal cottongrass (_Eriophorum vagnitatum_), marsh scheuchzeria (_Scheuchzeria palustris_), and common boletus (_Andromeda polifolia_). The moss-lichen layer is composed of several species of sphagnum mosses (_Sphagnum majus, Sphagnum cuspidatum, Sphagnum balticum, Sphagnum medium_) (Appendix B, Figures A1 and A2).  
Figure 1: Location of the studied Ilasskoe peatland where the peat profile ISNO-1 was collected (Arkhangelsk Region). The red cross, arrow and rectangle in the figure indicate the sampling location.  
The peat deposit at the sampling site of the ISNO-1 column has a thickness of 2.2 m. The deposit is homogeneous in botanical composition--it is composed of sphagnum moss with a small admixture of cottongrass throughout the peat profile with an increase in its share in the bottom layer. The extent of degradation estimated in the field varies between 5-20% with increasing depth. The underlying rocks are moraine of light mechanical composition (sand, sandy loam). The groundwater level during the entire research season was at the level of 0-5 cm. Before taking a peat core in the field, measurements of the Eh of the peat deposit were carried out with depth gradation (Appendix B, Figure A3).  
A peat core with undisturbed layering up to a depth of 49 cm was extracted using a PVC pipe. After delivery to the laboratory, the peat column was divided into layers of 2 cm, except for the uppermost layer of 0-3 cm, and prepared (drying, grinding) for further analytical procedures. A brief description of methods for determining the activity of radionuclides and physicochemical parameters is presented below.