Mid- to Late Holocene landscape and vegetation development of the Yukon Coastal Plain

6.1.1 Long-term trends

The most important climatic change on the Yukon coast during the last 6000 years was an increasing maritime influence on summer climate caused by sea level rise and shoreline transgression (Burn 1997, Fritz et al. 2012a). This led to a cooling of summer temperatures.

Winter temperatures remained largely unaffected because the frozen sea acts as a land surface. Yet, sea ice cover is already decreasing and is projected to decrease further, both in spatial extent and seasonal duration (e.g. Holland et al. 2006, Comiso et al. 2008, Stroeve et al. 2012). A reduction in sea ice extent will further alter climatic seasonality, and could lead to increased annual precipitation by extending the length of cool and moist summers (e.g.

Higgins & Cassano 2009). Impacts of sea ice decline on coastal areas also include increased permafrost thaw (Lawrence et al. 2008), a lengthening of the growing season and shrub

expansion (Bhatt et al. 2010, Post et al. 2013), an increase in tundra fires (Hu et al. 2010), or changes in carbon flux (Parmentier et al. 2013).

Thaw lake initiation and decline

The landscape on the Yukon Coastal Plain was strongly affected by the thawing of ice-rich permafrost over time. Thaw lake initiation was most widespread in the early Holocene, when increased thermokarst created a well-documented thaw unconformity during the Holocene thermal maximum (ca. 11500-9000 cal yrs BP) (Rampton 1982, Burn 1997, Fritz et al.

2012b). Thaw subsidence of ice-rich sediments initiated numerous large water-filled depressions, the so-called thermokarst lakes or thaw lakes. The modern topography of the coastal plain shows partially or entirely drained thaw lakes, in which ice-wedge polygons abound. These thaw lakes were already declining in spatial extent in the Mid-Holocene, as evident from our study of sediment cores from ice-wedge polygons, which were situated in marginal reaches of drained thaw lake basins (Chapter 5). The studied sites were only shallowly submerged or already part of a peaty ice-wedge polygon by 6000 cal. yrs BP. At the study site on Herschel Island, the thaw lake that preceded the modern ice-wedge polygon drained at approx. 4000 cal. yrs BP due to coastal erosion (Chapter 4). Climatic cooling after the Holocene thermal maximum likely decreased thermokarst activity, leading to thaw lake decline as more thaw lakes were drained than initiated.

Ice-wedge polygon initiation and stages

This thesis documented ice-wedge polygon initiation and maturation through different stages during the Mid- to Late Holocene. All studied ice-wedge polygons formed in drained thaw lake basins. The results of this thesis show that the initiation of ice-wedge polygon and peat development started before 7000 cal. yrs BP at the sites where an intermediate- and a high- centred polygon are found today. The modern low-centred polygons, however, were initiated during the Late Holocene (Chapters 4, 5). The Mid-Holocene decline in thaw lake extent facilitated ice-wedge polygon initiation by providing newly exposed waterlogged sediments.

This agrees with findings from Alaska (e.g. Jones et al. 2011). The inception of ice-wedge polygons was followed by periods of stable peat accumulation in individual low-centred ice- wedge polygons on the Yukon Coastal Plain and Herschel Island, which lasted for millenia (Chapters 4, 5). This thesis showed that shallow lake environments developed into wet low- centred polygons and into mesic low-centred polygons at all investigated sites. Two of the sites dried and degraded further and became intermediate-centred or high-centred,

respectively (Chapter 5). This supports the theory of successional stages of ice-wedge polygon development, as found in other studies (Ovenden 1982, Vardy et al. 1997, De Klerk et al.

2011). Yet, no thaw lake cycles with renewed thaw lake initiation were found, as found by (Billings & Peterson 1980, Jorgenson & Shur 2007, Lenz et al. 2016b) for other locations in the North American Arctic. These cycles may simply not have been captured in the studied records, as they take many millennia to unfold. On the other hand, drastic changes in geomorphology caused by increased thaw and thaw subsidence, coastal erosion and thermal erosion in the context of recent warming are likely to disrupt these cyclicities. Some studies even found recurrent phases of wet and dry conditions in peat-forming ice-wedge polygons (Ellis et al. 2008, Teltewskoi et al. 2016), underlining that multiple pathways are possible.

Vegetation development

During the Holocene Thermal Maximum, the treeline extended further north into the Tuktoyaktuk Coastlands, persisting well into the Mid-Holocene (Ritchie et al. 1983). No such treeline advance was recorded on the Yukon Coastal Plain, where tundra vegetation prevailed throughout the entire Holocene (Fritz et al. 2012a), which could be supported by this thesis.

The regional vascular plant species diversity likely did not change throughout the Mid- to Late Holocene, yet the decline of submerged surfaces in the context of the lake decline addressed in section 6.1.1.1 has altered local vegetation composition and cover. A Late Holocene decrease in aquatic and semiaquatic plant taxa in favour of terrestrial wetland and mesic taxa was found in the studied ice-wedge polygons (Chapters 4, 5). Palaeoecological studies of ice-wedge polygon development in northwest Canada have found a similar unidirectional vegetation succession (Ovenden 1982, Vardy et al. 1997), while other studies from the Canadian High Arctic and the Siberian Arctic have found repeated switches between taxa typical for wet conditions and taxa typical for dry conditions (Ellis & Rochefort 2006, Teltewskoi et al. 2016).

6.1.2 Short-term trends

Thaw lake deepening

Chapter 3 presents impacts of short-term climatic fluctuations of the last three centuries on an extant thaw lake. This lake deepened around AD 1910, after the regional Little Ice Age (~AD 1600-1850) (D'Arrigo et al. 2006, McKay & Kaufman 2014) ended and twentieth century warming started. Warming-induced deepening of thaw lakes is generally attributed to

increased thaw of ice-rich permafrost under the lakes (Hopkins 1949, Rampton 1982, Lenz et al. 2013). This process may unfold within a few years to decades (Chapter 3).

Rapid ice-wedge initiation and degradation

Ice-wedge initiation and degradation can happen equally rapidly. Chapter 4 showed that revegetation of the exposed thaw lake floor after drainage was accompanied by initiation of ice-wedge cracking, probably within the first winters after drainage. This process has been observed on recently drained lakes on the Beaufort Sea coastal plains (Hopkins 1949, Mackay 1974b, 1999). Ice-wedge degradation can act on decadal timescales (Mackay 1974b, Jorgenson et al. 2006, Fortier et al. 2007). This is supported by findings presented in Chapters 2 and 5.

All four wedge polygons studied in this thesis experienced drying (Chapter 5) and ice-wedge degradation (Chapter 2) during the last century, likely within a few decades.

Vegetation dynamics

Short-term vegetation dynamics are often linked to geomorphic disturbances providing bare ground for seedling establishment or to rapid and strong climatic change such as the modern warming trend. Chapter 4 reconstructed the rapid revegetation of a thaw lake floor within about 100 years after drainage (ca. 4000 cal. yrs BP), with pioneer taxa typical for disturbed sites in the Arctic being succeeded by wetland vegetation typical for low-centred ice-wedge polygons (see also Chapter 2). Such local change as a response to geomorphic disturbances is very common in the Western Canadian Arctic (Ovenden 1986, Cray & Pollard 2015).

Chapter 3 showed that the regional tundra vegetation remained fairly stable over the past 300 years, but that short-term local change influenced its composition in the lake catchment. Local lake-margin vegetation declined within about 10-20 years at the beginning of the twentieth century, while the regional vegetation signal remained largely stable. A similar pattern has been found by Niemeyer et al. (2015) for the same time period on the Taymyr Peninsula in the Siberian Arctic. As their study site was closer to tree-line than the sites from this thesis, a slight increase in larch (Larix sp.) pollen was found in that study, yet the overall vegetation composition remained stable. Similarly, a slight recent increase of Alnus pollen was found in the study presented in Chapter 3, which could indicate an approaching Alnus shrubline. Shrub increase during recent decades has been reported from Herschel Island (Myers-Smith et al.

2011a) and from other sites in the Alaskan (Tape et al. 2006), Canadian (Ropars & Boudreau 2012), northeast European (Forbes et al. 2010) and Siberian Arctic (Frost & Epstein 2014). It is also indicated on the Yukon Coastal Plain (Chapters 2, 3, 5) (Fraser et al. 2014), yet pollen

analysis did not capture it well in the study presented in Chapter 3, possibly because of low pollen production of some of the taxa involved and a time lag between climate forcing and vegetation response. The investigation of modern vascular plant taxa composition and cover in the four studied polygons showed that dwarf and low shrubs (predominantly Betula glandulosa, various Ericales, and Salix spp.) were present in all of them on slightly elevated sites with some drainage (Chapter 2). This shows that these taxa are intrinsically associated with local geomorphology and that their expansion under the observed and projected higher temperatures and longer growing seasons in the Low Arctic (Høye et al. 2007, Post et al.

2009) is likely if geomorphic conditions sustain or expand existing microhabitats.