Nestled alongside the large Darkhad Depression (Fig 1) to the west of Khuvsgul Lake along the Siberian border, the Ulaan Taiga Special Protected Area is an expansive area of tundra and larch forest, home to the reindeer-herding Tsaatan people(ethnic Tuvan or Dukha).These people settled in the region when Tuva was annexed into the Soviet Union during World War II—although nearby areas may have been used by the Dukha for centuries [18]. Today, the Tsaatan are estimated to comprise a group of around 200 people, or about 30 families [19]. Most of these families occupy the Baruun Taiga area in the west, with a smaller number residing in the Zuun Taiga region along the eastern side of the Depression, outside the park’s protected area (Fig 1).The region is home to distinct wild reindeer populations. The animals exhibit markedly different behaviors, but occasionally crossbreed with domesticated stock (as do domestic bulls occasionally brought across the Russian border from Tuva).

The study area, known as Mengebulag, a large tundra valley and popular summer pasture for Tsaatan reindeer herders located above 2000 masl within Baruun Taiga in the Sayan Mountains along the western lip of the Darkhad Depression. On a preliminary visit to the region in 2017, we identified ice patches between 1850–2580 masl in the vicinity of 51° N. Verbal discussions with residents indicated that during the summers of 2016–2018, previously permanent patches in Mengebulag melted for the very first time in collective memory (Fig 2). Recognizing the significance of this region to the understanding of reindeer pastoralism in Mongolia, and that organic cultural and biological materials contained within ice patches can decompose rapidly after their exposure[16], we selected this region for archaeological survey in the summer of 2018 (Fig 3).

Results from these preliminary ethnographic and archaeological investigations of persistent snow and ice patches in the high mountain zones of northern Mongolia indicate that these locations play a crucial role in domestic reindeer pastoralism, which is directly threatened by summer melting resulting from climate change. Ethnographic informants describe reindeer using munkh mus for drinking water, regulating body temperatures, and for relieving predation by insects. In some cases, recent melting has resulted in the complete loss of these historically permanent landscape features.

The absence of ice patches curtails the animals’ ability to thermoregulate, and their inability to escape insects may also increase exposure to parasites and vector-borne disease (Laaksonen et al. 2010). Such behaviors are also noted among the semi-domesticated reindeer of the Saami people in northern Scandinavia and wild caribou of North America [14,22–26]. In many of these cases, herds are adapting to the disappearance of ice patches by moving to higher elevations or more northern latitudes [23,27]. However, this adaptation is not available to the Tsaatan reindeer herders, as they already occupy the highest available zones within Mongolia, and are hemmed in to the north and west by the international border with Russia.

Access to fresh water for humans reindeer is also an important function provided by ice patches that is threatened by summer warming. Several informants described proximity to suitable pasture and camp locations as the most important factor in determining patch use for pastoral purposes, and indicated that those closest to camp were subjected to the most severe recent melting (e.g. Fig 2). Pastorally-utilized ice patches in the study area the lowest elevation among identified patches. While there is still enough drinking water for humans and stock today, informants complained of drying pastures. If the melting of ancient ice, and the trend toward increasingly drier conditions continues, it may have a disastrous impact on reindeer, as they are especially sensitive to water availability [9,28].

The use of new pastures in the Darkhad region is also prohibited by regulations governing the area’s “strictly protected” conservation zones. Despite receiving financial state support for increasing their herds, with these restrictions in place, current pastures suffer increasing degradation due to larger herd sizes. In 2018 conversations, informants mentioned that lichen and plants eaten by reindeer typically need three years to fully recover—meaning that restricted mobility has dire consequences for pastured degradation. Under these paired socioeconomic and ecological circumstances, ice patches appear even more important for the Tsaatan and their reindeer. Other patch qualities, beyond those explicitly mentioned by informants (such as having a gentle, accessible slope or limited snow/ice depth) may also be important, as well as influenced by the melting process. As continued warming is anticipated in the future, we expect that ongoing melting of ice patches will cause new sources of stress and disease for livestock (such as harassment from oestrid flies, heat stress, and mosquito-borne parasites), reducing the viability of summer pastures threatening the sustainability of reindeer pastoralism.

Our archaeological survey provides support for the idea that ice patches contain key insights into the role of mountain zones in the poorly understood history of domestic reindeer in Mongolia. Our investigation identified one patch that showed extensive use by modern reindeer, as evidenced by a large amount of previously frozen dung, hair, and other organic material. Future investigation of this patch and others yielding plant and animal material during peak summer melt, paired with radiocarbon dating and more nuanced paleoecological analysis, may provide important new data necessary to reconstruct the antiquity of domestic reindeer use in northern Mongolia.

Moreover, the discovery of at least two artifacts shows that taiga environments were exploited by Tsaatan people for acquiring wild resources–possibly fish–after their arrival to the region in the mid-20th century. The radiocarbon dates suggest the most likely period for the deposition of the artifacts was the early 1960’s during the Soviet-era collectivization period, or in the late 1970’s. Ethnographers have documented the former interval as a particularly harsh transition for the Dukha people, and a period during which traditional hunting practices were largely banned, and reindeer herding was not supported under state-run livestock policies [18]. During this time most young people in the region moved to the county seat of Tsagaan Nuur to work on the newly-established collective fishery [18].

Ice patch melting in the region is part of a larger global phenomenon [29]. Our results indicate that northern Mongolia may have experienced historically unprecedented ice patch melting over recent decades. The extreme melting observed during the 2018 season, and the degraded condition of the exposed artifacts that were recovered at Khets Davaa, indicates that summer warming is an urgent threat to the material culture preserved in the ice patches in Darkhad and other subarctic regions of northeast and Central Asia.

While the emergence of pastoral economies in Central Asia was one of the most important economic transitions in human history, many of the dynamics underlying this transition are poorly understood. This is particularly true of the archaeological record of high-mountain zones. Strong circumstantial evidence suggests that these areas may have been crucial to the early development of herding lifeways; however, due to their environmental and depositional characteristics, these areas retain little direct evidence of how they were used in the deep past. In Mongolia, mountain zones host unique species of domestic animals (reindeer and yak), that provide the economic foundation of some modern lifeways, but evidence of the process by which they were domesticated is almost absent from the archaeological record.

Our investigations in northern Mongolia indicate that persistent snow and ice patches–long known to be important loci for reindeer hunting–play a central role in modern reindeer pastoralism. The degradation and loss of these features in summer pastures threaten the viability of this herding lifestyle. Archaeological surveys and ethnographic research of ice patches in one important tundra zone in Mongolia’s Darkhad Basin provide the first direct material evidence of exploitation of this region in the mid-twentieth century, and suggest that contemporary melting exceeds levels seen in at least half a century. Culturally modified wood recovered from a melting ice patch suggest use of the area for summer pastures and travel corridors in modern times (as well as for wild resource gathering such as fishing) in the mid-20th Century, after documented migration to the region from southern Russia. While future research may yield important cultural and paleoenvironmental material relevant to understanding the deeper chronology of reindeer domestication and pastoral use of high altitudes, ice patches and the material they contain are urgently threatened by warming temperature and climate change.