Climate vs. geomorphic disturbances as drivers of change in ice-wedge polygons

The prerequisites for ice-wedge polygon development (waterlogged ground, permafrost, extreme ground-penetrating cold during winter) are determined by climate and geomorphology. Ice-wedge polygon initiation and conversion of low-centred into high- centred polygons is therefore strongly related to the dynamics of and the interplay between both.

Investigations into radiocarbon dates have revealed broad climate-induced simultaneous patterns of peatland initiation (MacDonald et al. 2006, Jones & Yu 2010). Strong seasonality and high summer temperatures have been suggested as drivers of intensive peatland formation during the Holocene Thermal Maximum in Alaska (Jones & Yu 2010). Our study of Mid- to

Late Holocene ice-wedge polygon development found spatially heterogeneous peat formation in polygons around 7000 cal. yrs BP (after the regional Holocene Thermal Maximum), under conditions much wetter than today (Figure 5.7). We found no climate-induced peat initiation in following millennia, when regional climatic patterns were largely stable. In the last millenium, however, re-initiation of ice-wedge polygon development and peat growth in Komakuk Polygon and Roland Polygon during the regional Little Ice Age (ca. AD 1600-1850 (D'Arrigo et al. 2006, Bird et al. 2009, McKay & Kaufman 2014)), suggested a climatic link.

Topographic evidence suggests geomorphic causes for ice-wedge polygon initiation on the Yukon Coastal Plain, where most polygon fields, including the ones we studied, are situated in drained thaw lake basins. Additionally, the initiation of Ptarmigan Polygon was likely linked to sea level rise. When Workboat Passage was flooded 1600-600 cal. years ago (Forbes 1980, Hill et al. 1985, Burn 2013), the relative topography in the area was flattened, with very low coastal bluffs (1-2m). This increased water retention on land, facilitating ice-wedge polygon development and peat growth.

The conversion of low-centred polygons to high-centred polygons is thought to be linked to internal self-organisation (Zoltai & Pollett 1983, Mackay 2000) or improved drainage (e.g.

Hussey & Michelson 1966). Shifts from aquatic to high-moisture wetland vegetation and finally to mesic wetland vegetation were evident in our cores (Figure 5.7, Tables 5.2, 5.3, 5.4). The conversion of low-centred polygons to better drained forms likely happened during twentieth century in all polygons (Figures 5.3, 5.4, 5.5, 5.7, Tables 5.2, 5.3, 5.4). Komakuk Polygon switched from a low-centred polygon with dwarf shrub growth on the rims to an intermediate-centred polygon where dwarf shrubs had also established in the polygon centre.

Ptarmigan Polygon was the most stable, yet the polygon rim changed from Cyperaceae- dominated to dwarf-shrub-dominated, indicating drying (Table 5.3). Roland Polygon showed a complete development from low-centred to high centred. All three polygons have been reported to show signs of recent ice-wedge degradation (Wolter et al. 2016).

The conversion of one polygon type to another may result from internal self-organization through two main processes: lateral movement of material adjacent to ice wedges may widen ice-wedge troughs and displace material towards the polygon centre, where a mound establishes (Mackay 2000). Vegetation growth in polygon centres exceeding the upwards growth of the surrounding ice wedges, may also result in a well-drained mound of peat surrounded by water-filled trenches (Zoltai & Pollett 1983, Ellis & Rochefort 2004). Both processes act on time-scales of centuries to millennia, contrasting with the rapid conversions we found.

Improved drainage may result from a change in topographic gradient and thus in surface flow patterns, or from ice wedge degradation promoting drainage of polygon centres into the surrounding ice wedge troughs. The modern position of Komakuk Polygon and Roland Polygon on elevated surfaces above lakeshore bluffs of several meters height (Figure 5.2a,c) indicate that drainage outweighs water input to these polygons, facilitating conversion to high-centred polygons. The climate-induced process of ice-wedge degradation is also evident in the polygons and may be rapid: Ice-wedge degradation and establishment of drainage channels within a few decades have been reported from the Arctic Coastal Plain of Alaska (Jorgenson et al. 2006, Liljedahl et al. 2016), the Eastern Canadian Arctic (Fortier et al. 2007) and Siberia (Czudek & Demek 1970).

In the two studied ice-wedge polygons that experienced conversion from low-centred to intermediate-centred (Komakuk Polygon) or high-centred (Roland Polygon), both rim cores and one centre core showed a hiatus of at least 5000 cal. years caused by erosion of sedimentary material (Figure 5.7), indicating significant disturbance. Several processes might have caused material loss: lateral material displacement caused by ice wedge growth (Mackay 2000), increased runoff (Liljedahl et al. 2016) facilitating thermal erosion, erosion as a result of ice-wedge degradation (Fortier et al. 2007), or peat decomposition as a result of better aeration, higher temperatures and increased microbial activity (Zoltai & Pollett 1983). No disturbances in peat accumulation were indicated in low-centred Ptarmigan Polygon (this study), nor in a low-centred ice-wedge polygon studied on Herschel Island (Fritz et al. 2016), which showed undisturbed peat formation for 3000 cal. years. The question whether disturbance triggered later drainage of the polygon centres and finally led to relief inversion cannot be answered at this stage, but will be worth investigating. To our knowledge, no similar erosion event in an ice-wedge polygon has been reported elsewhere in the Arctic.

The changes we observed (peatland initiation, change from low-centred to high-centred) were mostly caused by geomorphological change such as sea-level rise, tapping and draining of adjacent lakes, or changes in drainage pathways across the landscape. In permafrost-affected landscapes, climatic change may trigger widespread geomorphological change, especially where unconsolidated ice-rich sediments dominate. Such climate-induced geomorphological change may have locally variable impacts, but its frequency is likely to increase under climatic change. Regionally synchronized ice-wedge polygon development requires a higher amplitude and seasonality of temperature and precipitation change than evident for the Mid- to Late Holocene. Our findings indicate that modern warming, however, may have triggered

regionalized conversion from low-centred polygons to high-centred polygons. This process may rapidly initiate irreversible self-enhancing erosion of ice-wedge polygons.