Impacts of coastal erosion

Permafrost soils store 1035 ± 150×10¹⁵ g of soil organic carbon within the upper three metres of soils [Hugelius et al., 2014], or 1672×10¹⁵ g when including deeper soil layers [Tarnocai et al., 2009]. This corresponds to approximately half of the carbon which is estimated to be stored in soils, worldwide [Tarnocai et al., 2009]. Several geomorphological processes disturb these carbon pools, which leads to the mobilisation of carbon, as well as other nutrients and sediments. Coastal erosion is such a process. It transfers sediments, carbon and nutrients from the terrestrial into the aquatic system [Dunton et al., 2006; Gustafsson et al., 2017].

Depending on their particle size, the sediments either stay in suspension, or settle on the sea bottom [Hjulström, 1932]. Thereby, strong sediment fluxes can change the water turbidity in the nearshore zone and the corresponding light availability, which is crucial for primary production [Neff and Asner, 2001; Dittmar and Kattner, 2003]. When carbon enters the nearshore zone, it is observed to take four major paths [Fritz et al., 2017]. Part of the carbon settles on the sea floor in the nearshore zone. Since the nearshore zone is a very dynamic area, carbon can be re-mobilized by wave action, bottom fast sea ice pick-up, or ice gauging [Aré et al., 2008; Vonk et al., 2012; Macdonald et al., 2015]. Further, carbon can be transported

beyond the nearshore zone and settle in deeper shelf areas or even off shelf [Vonk, 2014;

Gustafsson et al., 2017]. However, a considerable fraction of the carbon does not settle on the ground. Instead, it is mineralized by microbes, whereby a portion of the carbon is outgassed [Dunton et al., 2006; Battin et al., 2009; Gustafsson et al., 2017]. Further, the increased concentration of carbon and of other limited nutrients has the potential to enhance primary production in the ocean, upon which all organisms rely. Very high primary production can, in turn, lead to algae blooms and oxygen depletion, which can have massive effects on nearshore ecosystems [Fritz et al., 2017].

Rivers and coastal erosion are the two main sources of sediment and carbon fluxes into the Arctic Ocean [Rachold et al., 2004b]. However, the separate contributions of both delivery sources vary regionally. Along the Beaufort Sea, rivers, especially the Mackenzie River, are considered to deliver the bulk part of the carbon (0.89×10¹² g a^-1 by coastal erosion, 4.43×10¹² g a^-1 by rivers). Circum-Arctic estimations show that rivers supply approximately 30.04 to 34.04×10¹² g a^-1 of carbon per year [Rachold et al., 2004b; Holmes et al., 2012], whereas estimations on carbon fluxes derived from coastal erosion vary between 4.9 to 14×10¹² g a^-1 [Wegner et al., 2015 and references therein].

For the Yukon coast in particular, Harper and Penland [1982] published the first estimates of material fluxes, yielding 1.5×10⁶ m³ a^-1 of sediment. Using estimated cliff heights from video records and erosion rates from McDonald and Lewis [1973], this was a first rough estimation of how much material is annually released into the Beaufort Sea. A subsequent study conducted by Hill et al. [1991b] estimated a mean annual sediment flux of 7.12×10¹² g a^-1. By using an average total organic carbon concentration in coastal sediments of 5%, Macdonald et al. [1998] calculated a mean annual flux of total organic carbon of 0.06×10⁶ m³ a^-1 from the Yukon coast into the Beaufort Sea [Macdonald et al., 1998]. This value is used until present for the calculation of carbon fluxes to the Yukon coast [Rachold et al., 2000, 2004b].

A more accurate estimation of mobilized sediments and carbon is the initial step to assess the potential impacts these fluxes have on the aquatic system and which role they are playing in the general carbon and sediment cycle.

Figure 2.3: Modern sediment contribution (Tg a^-1) from coastal erosion into the Arctic Ocean. Figure from Wegner et al. [2015].

2.3.2 Retrogressive thaw slumps

Retrogressive thaw slumps are unique permafrost landscapes which are found along many Arctic coasts and rivers. They are back-wasting thermokarst features which can develop when a previously buried massive ice body gets exposed due to a disruption process, such as an active layer detachment or coastal erosion [Burn and Lewkowicz, 1990; Lantuit et al., 2012a;

Kokelj and Jorgenson, 2013]. When the massive ice body gets exposed to solar radiation, the ice melts or ablates and releases the previously frozen sediments and carbon therein. When the melting process of the massive ice body causes it to retreat faster than coastal processes are eroding the cliff, a retrogressive thaw slump occurs [Lewkowicz, 1987a]. A retrogressive thaw slump consists of a headwall, a slump floor and a slump lobe [Burn and Lewkowicz, 1990] (Figure 2.4). The headwall is comprised of the upper soil layer and the massive ice body. They are reported to recede up to 10 m a^-1 along the Yukon coast [Lantuit and Pollard, 2005]. The slump floor, which is fronting the headwall, contains part of the released material.

The material is transported from the mud pool downslope towards the ocean, and creates a

slump lobe. The removal of the sediments at the base of the retrogressive thaw slump by waves maintains a steep shore gradient, which is important for the material transport. If material does not get transported shoreward any longer, it will accumulate in front of the headwall, causing its insolation [Lantuit and Pollard, 2005]. In this case, the development of the retrogressive thaw slump decelerates or even stops. A re-exposure of the ice-body by thermokarst processes or coastal erosion leads to the re-activation of parts of the initial retrogressive thaw slump. Many retrogressive thaw slumps are observed to have such a polycyclic nature [Wolfe et al., 2001; Lantuit and Pollard, 2005, 2008; Lantuit et al., 2012a].

Along the Canadian Beaufort Coast, a phase of enhanced retrogressive thaw slump initiation and re-activation is observed within the last 20 years [Lantz and Kokelj, 2008; Lantuit et al., 2012a; Segal et al., 2016].

Depending on their size, retrogressive thaw slumps release considerable amounts of sediments, carbon and nutrients to the nearshore [Lantuit and Pollard, 2005; Obu et al., 2017;

Tanski et al., 2017]. Hence, it is important to consider these landforms for estimations of material fluxes into the sea. The main challenge relates to the association between retrogressive thaw slump occurrence and terrain characteristics, such as substrate, relief or ground ice contents. A comprehensive statistical and empirical assessment of the relation between slumps and terrain is needed to understand the main drivers of slump initiation.

Figure 2.4: Morphology of a retrogressive thaw slump, modified after Lantuit and Pollard [2005].

2.3.3 Socio-economic impacts

Over the last decades much work has focused on the biogeochemical processes induced by arctic coastal erosion (Chpt. 2.3.1). However, it is the pictures of collapsing houses and eroding roads which are illustrating the importance of studying arctic coastal erosion processes best. Climatic warming in the Arctic is both a threat and an opportunity. It is a

threat for coastal infrastructure, such as the former DEW line stations along the US-American and Canadian Arctic coasts [Jones et al., 2008]. It is also threatening industry infrastructure such as the Varandei oil terminal at the Pechora Sea coast [Ogorodov et al., 2016]. It directly threatens settlements, such as Tuktoyaktuk at the Northwest Territories Beaufort coast [Johnson et al., 2003; Forbes et al., 2013] as well as cultural and archaeological sites [Jones et al., 2008; Kroon et al., 2010; Radosavljevic et al., 2015; O’Rourke, 2017]. On the other hand, it is an opportunity for the shipping industry to establish new Arctic-Pacific shipping routes [Smith and Stephenson, 2013; Stephenson and Smith, 2015], for the oil and gas industry to explore new offshore plays [Zöckler et al., 2011], and for the tourism sector to open new destinations [Olsen et al., 2011].

There are no permanent settlements along the Yukon coast at present times. However, the coastal area is still crucial for the Inuvialuit to pursue their traditional lifestyle, as well as for other indigenous and non-indigenous peoples for subsistence harvesting, recreation and transportation [Alunik et al., 2003; Hacquebord, 2011; Zöckler et al., 2011]. The Yukon coast has a long settlement history [Friesen, 2015; Arnold, 2016; Jensen, 2016]. The region is preserving valuable information about human history in the North in form of numerous cultural and archaeological heritage sites. These are containing remains from the Thule Inuit in the Washout Site at Herschel Island [Friesen and Arnold, 2008], are documenting the life of the Inuvialuit [Adams, 2004; Lyons, 2004], and are reporting on the arrival and activities of the Europeans [Bockstoce, 1986; Saxberg, 1993]. The recognition of the cultural and natural richness of the Ivvavik National Park and Herschel Island made this region a candidate for UNESCO World Heritage Site status [UNESCO, 2004].

In the late 1990s, a comprehensive study of all cultural and archaeological heritage sites within the Ivvavik National Park (Figure 2.5) was carried out by Thomson et al. [1998], who systematically inventoried each site and the corresponding artifacts. This study was accompanied by shoreline change studies in 1996 and 1997 along five main heritage sites [Solomon, 1996; Forbes, 1997]. Further, a detailed investigation of potential erosion and flood hazards of a former whaling settlement at Simpson Point, Herschel Island was carried out, and showed that sea-level rise, together with an intensification in storms, is substantially increasing the flood risk potential [Radosavljevic et al., 2015]. Despite this increasing hazard potential and its impact on cultural heritage sites along the Yukon coast, no comprehensive study of the future risk of coastal processes for cultural heritage exists. The potential impact of erosion on important infrastructure such as the DEW line airstrips, and on highly

frequented travel routes, are also unknown. Thus, there is a drastic need to assess the effects coastal processes have on the human component in the Arctic system.