Roland Polygon

The active layer core from the elevated centre of Roland Polygon (YC12-RP-Mc) showed a hiatus but no age inversions (Table 5.1). The upper three dated samples at 12 cm and 14 cm depth originated within the last 300 years. At 21 cm depth the calibrated median age was 603 cal. yrs BP.

Figure 5.5. Stratigraphic diagrams showing sediment parameters and established zonation (a) in the centre core and (b) in the rim core of Roland Polygon. Parameters used in the CONISS analysis are shown in black, while additional parameters not used in this analysis are shown in grey. The age ranges shown are calibrated 1 sigma ranges based on AMS radiocarbon dates (Table radiocarbon dates). The presence of aquatic organisms in the macrofossil record is indicated by hatching.

A hiatus of nearly 6500 cal. years lay between this sample and the next dated sample at 26 cm depth, which had a median age of 7058 cal. yrs BP. On the basis of CONISS analysis of TOC, TOC/TN and δ¹³C we established two stratigraphic zones in the core (Figure 5.5a). The

boundary between RP_c1 and RP_c2 reflects the hiatus at 21 cm depth and 603 cal. yrs BP. The upper zone RPc2 was divided into two subzones RPc2A and RPc2B. RPc1 (22-27 cm depth) was characterized by low TOC values between 8.2 wt.% and 12.1 wt.% (mean: 9.8 wt.% ± 1.3) and low TOC/TN ratios between 13.0 and 21.9 (mean: 17.8 ± 2.9). Relatively high δ¹³C values from -27.2‰ to -28.9‰ (mean: -27.8‰ ± 0.6) were measured in this zone. The sediment texture in RPc1 was clayey silt (Figure 5.6).

Table 5.4. Summary of identified vascular plant macrofossils from the centre and rim cores of Roland Polygon. The overall composition of the sieving residue is described by giving the amount of plant material after sieving through 1 mm mesh size and the respective estimated amounts of Bryophyte, Cyperaceae and wood remains in each sample. Plant macrofossil that have been picked and further identified are ordered by hydrological requirements from taxa found under mesic conditions typical for ice-wedge polygon rims to taxa found in wet conditions typical for ice-wedge polygon centres. Finally, aquatic plant remains typical for subarctic ponds and lakes are listed.

Depth (cm) median age (cal yrs BP) amount plant material in sample (ml) amount Bryophytes in sample (ml) amount Cyperaceae in sample (ml) amount wood in sample (ml) Betula glandulosatwig Betula glandulosaleaf (fragment) Betula glandulosafruit Betula glandulosacatkin scale Empetrum nigrum seed Ledum decumbensleaf Vaccinium vitis-idaealeaf Eriophorum vaginatumseed dwarf shrub twig fragment dwarf shrub leaf fragment cf.Luzulaseed Carexsp. seed Hippuris vulgarisseed Menyanthes trifoliata seed Potentilla palustrisseed Potamogetonsp. seed Charophyta oogonia Daphniasp. ephippiae

Zone

Plant macrofossils were abundant and relatively diverse in this zone, with remains of mesic terrestrial dwarf shrubs (Betula glandulosa, Empetrum nigrum, Ledum decumbens) alongside seeds of wet terrestrial Carex sp. and emergent aquatic Hippuris vulgaris, Menyanthes trifoliata, and Potentilla palustris (Table 5.4). Remains of submerged aquatics (Potamogeton sp., Charophyta oogonia, Daphnia ephippiae) were frequent in this zone.

Figure 5.6. Ternary diagram illustrating grain size composition in samples from all six cores.

Symbols are colour-coded to individual ice-wedge polygons, with lighter colours representing rim or margin cores, and darker colours representing centre cores. Narrow crosses represent the uppermost zone in the corresponding core, while bold crosses represent the lower zone in each core. A general trend towards more fine-grained material downcore is visible in all except the Roland Polygon margin core.

The grain size analyses classified inorganic particles from RP_c2 as sandy silt (Figure 5.6).

Mesic terrestrial plant macrofossils (Betula glandulosa, Ledum decumbens, Eriophorum vaginatum) became particularly abundant in RP_c2, while Carex sp. seed occurrence declined gradually and aquatic remains disappeared entirely.

The active layer core from the margin of the high-centred Roland Polygon (YC12-RP-Mr) showed a similar hiatus and had a median basal age of 7085 cal. yrs BP that was nearly identical to the one in the centre core YC12-RP-Mc from the same polygon (Table 5.1). The age to depth relationship was also remarkably similar to the one found in the rim core of Komakuk Polygon. The upper part of the core showed modern ages or ages of up to 300 cal.

yrs BP at 9, 12, and 14 cm depth, an age of 5035 cal. yrs BP at 17 cm depth, 6691 cal. yrs BP at 19 cm, and 7085 cal. yrs BP at 27 cm. The core showed no age inversion. A sedimentary facies break was present at 10-11 cm core depth. Two stratigraphic zones RPr1 and RPr2 were delineated based on CONISS ordination of parameters characterizing organic matter (TOC, TOC/TN, and δ¹³C), the lower zone was subdivided into RP_r1A and RP_r1B (Figure 5.5b).

RPr1 (12-27 cm depth) had high TOC contents from 31.5 wt.% to 42.5 wt.% (mean: 37.5 wt.% ± 4.6), TOC/TN ratios were between 21.3 and 32.1 (mean: 26.0 ± 3.4). A slight decrease was observed in δ13C, which ranged from -27.3‰ to -29.0‰ (mean: -28.2‰ ± 0.6).

TOC decreased from subzone RPr1A (18-27 cm depth, mean: 40.9 wt.% ± 1.9) to RPr1B (12- 17 cm depth, mean: 32.7 wt.% ± 0.8), while TOC/TN ratios stayed similar (means: 24.5 ± 2.3 vs. 28.2 ± 3.6), and δ¹³C decreased within subzone RPr1A (mean: -27.8‰ ± 0.4) and stabilized in subzone RP_r1B (mean: -28.7‰ ± 0.2). The grain size composition changed from silty sand in RP_r1A to sandy silt in RP_r1B (Figure 5.6). Plant macrofossils were abundant in zone RPr1, and were dominated by terrestrial taxa (Betula glandulosa, Empetrum nigrum, Ledum decumbens, Vaccinium vitis-idaea, Eriophorum vaginatum, Carex sp.) (Table 5.4).

Remains of Betula glandulosa were rare in RPr1A and became abundant in RPr1B, while seeds of the wet terrestrial Carex sp. were abundant in RP_r1A, and decreased strongly towards RP_r1B. The only aquatic indicators were Daphnia ephippiae found in RP_r1A. The trend towards more mesic taxa was mirrored by the occurrence of Eriophorum vaginatum seeds, which were missing from the lower part of RPr1A, and increased towards the upper part of RPr1B.

In RP_r2 (0-11 cm depth), TOC contents were between 37.7 wt.% and 43.2 wt.% (mean: 41.2 wt.% ± 1.5), and TOC/TN ratios increased strongly within this zone, ranging from 27.9 to 81.9 (mean: 48.7 ± 13.8), while δ¹³C fluctuated between -27.6‰ and -28.8‰ (mean: -28.2‰

± 0.4). There was no information on grain size composition for RP_r2, as the peat contained very little inorganic material. Plant macrofossils were dominated by abundant remains of the mesic terrestrial taxa Betula glandulosa, Ledum decumbens, and Eriophorum vaginatum (Table 5.4).

5.6 Discussion