## 1 Introduction  
The climate warming in high latitudes leads to the change in frequency of the seasonal thawing-freezing regime [1, 2] which will be mostly pronounced during autumn and spring, i.e., the so called \"shoulder\" seasons or transition periods [3]. It is known that spring and autumn are extremely important for the export of dissolved and particulate load by rivers and element cycling in the lakes of Arctic wetlands [4; 5; 6]. During these transition periods, the majority of organic carbon and inorganic solutes originate from leaching of supra-permafrost soil and vegetation [7; 8]. In addition to the effect on the carbon and major elements cycling, these processes largely control the release of many trace elements from soils to the aquatic systems [5; 9; 10]. However, in contrast to relatively good empirical and modeling understanding of the physical aspects of soil freeze/thaw processes [11; 1], the behavior of aqueous solutes (\(<\)0.45 \(\upmu\)m) upon freezing and thawing of organic matter-rich surface and soil waters of permafrost peatlands remains poorly understood. It is known that both the peat porewaters [12; 13] and surface waters [5; 7] of these regions are strongly enriched in dissolved organic carbon (DOC) and some trace metals. These solutes are often present in the form of organic and organo-mineral colloids [10; 14; 15], which are likely to coagulate upon ice formation, for example, during the full freezing of surface depressions and shallow thermokarst lakes and ponds [4]. In contrast to the sizable number of works devoted to characterizing the impact of freezing and thawing on mineral soil properties [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34], the freezing/thawing impact on the chemical composition of porewater and ice has not been well described, and most often, the researchers have to rely on modeling predictions [35]. Although laboratory experiments allow for the identification of governing factors during freezing/thawing effects [36; 37; 38; 39; 40; 41; 42; 43], their applications to naturally relevant aquatic settings of high latitude permafrost peatlands remain very restricted. Previous works in permafrost peatlands reported translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, DOC, major and trace element concentrations) after the thawing and bidirectional freezing of peat cores [44; 45]. Another recent study examined the impact of freezing/thawing cycles (FTC) on the surface waters from Northeast European permafrost peatland [46]. The latter authors reported only minor (\(<\)5-15%) changes of DOC and labile ion concentrations, whereas several trace elements (Fe, Al, P, Mn, As, and REE) exhibited a sizable decrease in their concentrations after FTC. These authors concluded an overwhelming impact of colloidal status of DOC and trace metal on this coagulation/dissolution processes during freezing and thawing. However, the quantitative assessments of these effects are still missing. In the present study, we aimed at (1) elaborating an optimal experimental procedure allowing for a discrete sampling of fluid during complete freezing progress; (2) quantifying the partitioning coefficients of elements between water and ice, and (3) testing the relationship between different elements, depending on their speciation and affinity to dissolved organic matter. We hypothesize the existence of several group of contrasting solutes--(1) those incorporating into forming ice; (2) those remaining in the fluid phase via concentrating due to freezing front migration but not subjected to coagulation, and (3) less soluble complex organo-mineral amorphous compounds or certain minerals, subjected to reversible or irreversible coagulation. Testing these mechanistic hypotheses constituted the first objective of this study.