## 2 Materials and Methods  
### Sampling Locations  
Our studies were aimed at attempting to reconstruct the natural conditions that existed in the Pechora paleobasin (Figure 1) during the climatic optimums of the Pleistocene and Holocene. This may make it possible to analyze the dynamics of climate and landscape changes over time and predict changes in the natural environment in the future [50]. The names of sampling localities are given relative to nearby settlements. Fossil shells were taken at five localities (Table 1). Mikulin deposits are marked by bivalve mollusks _Artica islandica_ (Figure 2). Figure 1: Sites for sampling mollusk shells in the European North of Russia. 1: river Peza (vicinities of the village of Bychye), 2: river Sula (near the village of Kotkino), 3: Vastyansky Kon\({}^{\prime}\) (river Pechora), 4: Khongurei (river Pechora), 5: location 10 km north-west of the village Oksino, river Pechora. Shells from five sampling sites located in different parts of the Pechora paleobasin were used to analyze trace elements' composition. In July 2020, mollusk shells were sampled from coastal outcrops of the river Pechora (Vastyansky Kon'). On 3 July 2020, on the right bank of the river Pechora, mollusk shells were found in the southernmost part of the Vastyansky Kon'. Mollusk shells are confined to grey clays, which are covered with sand from above. On 4 July 2020, the location of mollusk shells was also found in the central part of the Vastyansky Kon' outcrop (Figures 2 and 3). The shells were confined to the lower part of the section (1 terrace) 1 m from the water's edge of the Pechora River in argillaceous deposits, with interlayers of sand and rounded pebbles. On 5 July 2020 in the vicinity of the village Khonguei, in an outcrop on the left bank of the river Pechora, mollusk shells were found (Figures Figures 4 and 6, Table 1). Mostly ferruginous sands are represented in the lower part with moraine. Some of the shells were found on a moraine beach. On 5 July 2020 10 km northwest of the village Oksino, for the first-time, mollusk shells were found in outcrops of the left bank of the Pechora River (Figure 6). The shells are confined to grey clays, which are overlain by sand and peat (Table 1). In June 2017 and July 2021, about 300 samples of mollusk shells were collected from coastal outcrops of the river Peza in the vicinity of Bychye (according to Table 1, Figures 5 and 6). In July 2016, 100 mollusk shells were sampled on the Sula River in the vicinity of Kotkino in MZT (according to Figures 5 and 6). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Locality** & **River Basin** & **Coordinates** & **Number of Samples** \\ \hline Kotkino village & Sula & \(\begin{array}{c}67^{\circ}0^{\prime}57.76^{\prime\prime}\ N\\ 50^{\circ}52^{\prime}37.02^{\prime\prime}\ E\end{array}\) & 100 \\ \hline
10 km north-west of the & & & \\ Oksino village & Pechora & \(\begin{array}{c}67^{\circ}39^{\prime}42.88^{\prime\prime}\ N\\ 52^{\circ}20^{\prime}19.25^{\prime\prime}\ E\end{array}\) & 36 \\ \hline Khonguei & Pechora & \(\begin{array}{c}67^{\circ}33^{\prime}4.21^{\prime\prime}\ N\\ 51^{\circ}56^{\prime}15.50^{\prime\prime}\ E\end{array}\) & 59 \\ \hline Vastyansky Kon’ & Pechora & \(\begin{array}{c}67^{\circ}15^{\prime}31.61^{\prime\prime}\ N\\ 52^{\circ}12^{\prime}0.07^{\prime\prime}\ E\end{array}\) & 200 \\ \hline Bychye village & Peza & \(\begin{array}{c}65^{\circ}47^{\prime}14.61^{\prime\prime}\ N\\ 44^{\circ}59^{\prime}55.33^{\prime\prime}\ E\end{array}\) & 300 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Locality and River basin of fossil shells of the European North. Figure 2: The subfossil shell of the marine mollusk Arctic islandica from the Sula outcrop. Figure 4: Correlation of Neopleistocene deposits in the north of the Bolshezemelskaya tundra reproduced with permission from [39].