3.4. Cluster analysis
To explore the patterns in elemental and ionic constituent concentrations, together with DOC, we performed cluster analysis. Grouping variables (Figure 6a), at the 33% maximum distance level, yielded 5 clusters: C1 (Zn & Cu), C2 (Mo, Li, Cd & Ba), C3 (Si, Ni, Mn & Fe), C4 (Be, Co, As, Al and K+), and C5 (V, DOC, Se, Sb, Sr and all other ions). At the 67% level, C1 and C2 form one cluster together, and another is formed by C3 and C4, joined. Grouping samples (Figure 6b), on the other hand, resulted in three distinct clusters at the 33% maximum distance level: P1 & P2; L14 & L15; and all other samples. Notably, permafrost ice P3 was classified as relatively similar to the samples from the bottom of Lake 2 and one Kolyma river sample (K31). The variables distinguishing Lake 2 and the P3 ice from other samples were the concentrations of DOC, Al, Co, As, Mn, Ni, Fe, Zn, while the variables setting the permafrost creeks P1 and P2 apart were the concentrations of Si, Sr, Sb, V and the ions Na+, Mg2+, F-, and SO42- (cf. Figures S3 & S4, Suppl. Mat. 1 ).
An additional cluster analysis (Figure 7), performed for river waters alone (Kolyma, Omolon and Anyui), yielded three clusters at 67% maximum distance level. Cluster c1 contained rock-derived elements, organics and some more typically anthropogenic contaminants (Si, Mn, Fe, Zn, Sb, Cu, V, Co, As, Ni and DOC); c2 grouped Li, Mo, Cd, Ba, Be, Al and Sr; and c3 joined Sr, Se, and all inorganic ions. Around 33% maximum distance, the c1 cluster divided into c1a (Si, Mn & Fe) and c1b (Zn, Sb, Cu, V, Co, As, Ni and DOC); cluster c2 consisted of c2a (Li, Mo & Cd) and c2b (Ba, Be & Al); and cluster c3 was composed also of two sub-clusters: c3a (Sr, Se, K+, Mg2+, Ca2+ & SO42-) and the saline c3b (Cl-, Na+ and F-).