The Ancient Peat Plateaus' Vulnerability in Western Siberia

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1		Alexander Pastukhov	+ 2257 word(s)	2257	2021-12-22 05:21:06	\|
2	format correct	Nora Tang	+ 25 word(s)	2282	2022-01-19 05:15:43	\|

This entry is adapted from the peer-reviewed paper 10.3390/plants10122813

Based on the data of the plant macrofossil and palynological composition of the peat deposits, the evolution and current state of polygonal peatlands were analyzed at the southern limit of continuous permafrost in the Pur-Taz interfluve. Paleoreconstruction shows that peat accumulation began in the Early Holocene, about 9814 cal. year BP, in the Late Pre-Boreal (PB-2), at a rate of 1 to 1.5 mm year−1. Intensive peat accumulation continued in the Boreal and early Atlantic. The geocryological complex of polygonal peatlands has remained a stable bog system despite the predicted warming and increasing humidity. However, a rather rapid upper permafrost degradation and irreversible changes in the bog systems of polygonal peatlands occur with anthropogenic disturbances, in particular, a change in the natural hydrological regime under construction of linear objects.

cryolithozone degradation arctic peatlands permafrost soil–geocryological complex palynological spectra

1. Introduction

Peatlands, occupying only 3% of the land surface, contain about 15–30% of global soil organic carbon reserves ^[1], thereby playing a significant role in the regulation of general planetary processes, such as biogeochemical and biogeophysical cycles, greenhouse gases, and activity and species diversity of vegetation and soil biota. In Western Siberia, peatlands cover 592,440 km², exceeding 50% of all regional area in the taiga zone, and have the peat thickness up to 8–10 m ^[2]. The Western Siberian Plain stands out among other boreal plains by phenomenal bogging, which has both global and regional significance. The polygonal peatlands and peat plateaus are the most extensive bog types in the study area ^[3]. However, in the tundra, polygonal peatlands with a thick peat deposit (from 2 to 5 m and more) are not uncommon ^[3]^[4]^[5].

Current climate warming and wetting in Siberia led to a lake area increase by 0.89% detected for the 1999–2013 period and an increase by 4.15% for the 1999–2018 period. In Eastern Siberia, a lake area expansion trend was detected in high yedoma fraction areas, indicating ongoing Yedoma degradation by lake thermokarst ^[6]. In western Siberia, climate warming, increased precipitation, and permafrost thaw are also accompanied by an increase in the frequency of full or partial drainage of thermokarst lakes. After lake drainage, highly productive plant communities on nutrient-rich sediments may develop, thus increasing the influencing greening trends of Arctic tundra ^[7]. Thermokarst features, such as thaw ponds of thermokarst lakes, are hotspots for methane emissions in warming lowland tundra ^[8].

In contrast to the northeastern European tundra, where both the climatic optima of the Atlantic and Subboreal of the Holocene, and, at present, permafrost has been thawing due to warming ^[9], in the tundra zone of Western Siberia, even the maximum warming occurred within the range of negative mean annual soil temperatures and, thus, did not lead to the permafrost retreat ^[5]^[10]. However, very-high-resolution (VHR) images showed various types of disturbances over permafrost areas’ extensive networks of hydrocarbon exploration and infrastructure occurring in the Yamal Peninsula in the last several decades, stimulating the initiation of new thermokarst features ^[11]^[12]. The significant warming and seasonal variations of the hydrologic cycle, in particular, increased snow water equivalent acting in favor of deepening of the active layer; thus, an increasing intensification of the processes of thawing of underground ice wedges and destruction of soil–geocryological complexes of polygonal peatlands ^[13]. Peatlands are the most stable from the standpoint of the temperature state of permafrost ^[14], but the high amount of ice wedges in soil–geocryological complexes makes them very vulnerable to anthropogenic impacts ^[15]. Our earlier studies in the European Northeast with the use of high-frequency, ground-penetrating radar showed that the construction and operation of a road embankment with a hard cement-concrete coating crossing peat plateaus in the southern permafrost was limited, leading to the permafrost table retreating down to 8 m, and the warming effect of the road construction affected the field 50 m wide ^[16].

Nowadays, there is increasing attention to the study of peat genesis and properties, since peat supports and affects bog ecosystems, which have a unique communities’ structure and a high biodiversity level ^[4]^[10]^[17]^[18]. However, publications on the features of the permafrost peatlands’ evolution are extremely insufficient ^[7]^[19]^[20]^[21]^[22] and spore-pollen spectra studies are limited ^[4]^[23]^[24]. Significant reserves of soil organic carbon are conserved as a peat. Therefore, the peat pool plays an important role in the biogeochemical carbon cycle and climate change processes ^[25]. Peat monoliths are archives of information on paleoenvironmental conditions ^[26].

2.Vulnerability of the Ancient Peat Plateaus in Western Siberia

The active process of peat accumulation started in the Preboreal stage. Bogs were formed as a result of the end of the Sartan glaciation and the beginning of climate warming, but their development proceeded in local sites in well-pronounced microdepressions ^[10]. As with the oldest and largest peat deposits in the Eastern European forest-tundra and tundra ^[21]^[27], the studied peatland TZ began to develop as a result of the paludification of mixed birch and coniferous woodlands earlier than 9814 cal. yr. BP at a rate of 1 mm per year during the Preboreal (PB) (10,300–9000 cal. yr. BP) (Figure 1). No other ancient dates were obtained for a thick peatland (peat deposit exceeds 5 m) developed on the high sea terraces of the Gydan Peninsula, 9940 cal. yr. BP at a rate of 0.7 mm/year ^[4]. The severe continental climatic conditions of the Late Dryas changed to a milder and humid climate ^[28], and the bog system development began with the overgrowing and peat formation of lakes of various genesis, which were mostly thermokarst with submerged taliks. During the second half of the Preboreal (PB, zone I, 9580–9350 cal. yr. BP), birch-spruce woodlands predominated in the study area. Groups of shrub and dwarf birches played a great role, while pine was an admixture. Bog–tundra plant communities are widespread.

Figure 1. Plant macrofossil composition of TZf. +—0.5–2% of the sum of all species encountered.

At the beginning of the Boreal (BO-1, zones II, IV), the climate became warmer and more humid. There was a gradual increase in the role of spruce and the participation of Betula sect. Albae had more than halved. The presence of dwarf and shrub birches was greatly reduced. In open sites, bog–herbaceous communities were widespread, with a significant increase of sphagnum mosses. The rate of peat accumulation reached 1.5 mm/yr., up to the end of the Boreal climatic optimum. However, after 8898 cal. yr. BP (BO-2, zone III), there was a reduction in forest vegetation and an increase in groups of dwarf and shrub birches, alder, and willow. At the same time, the rate of peat accumulation was reduced by more than two times, to 0.7 mm/year.

During the entire Atlantic (AT, zone V) (8000–4800 cal. yr. BP), spruce–birch forests predominated in the study area. The forest area periodically increased and then decreased depending on climate fluctuations. In open sites, fen–herbaceous plant communities were developed. If, at the beginning of the Atlantic period, the peat accumulation rate reached 1 mm/yr., then, from 7749 cal. yr. BP, it was about 0.26 mm/yr., and from 6801 to 5657 cal. yr. BP it was only 0.13 mm/year, after which it almost stopped. In the Subboreal (SB, zone VI) (4800–2500 cal. yr. BP), there was a decrease in taiga and an increase in areas occupied by dwarf thickets of Betula sect. Fruticosae and B. nana. Herbaceous associations of sedges and mesophilic grasses were developed in fens.

In the Subatlantic (SA, zone VII) (2500 cal. yr. BP—present), spruce–birch woodlands are widespread, which alternates with extensive fens. The data of the plant macrofossil composition indicate the transition of the eutrophic stage of the bog transformation into the oligotrophic stage, accompanied by a change in the species’ composition of paleocommunities (the replacement of fen grass–moss peats with dwarf shrub–hypnum peats). The possible mechanisms for that could not be explained by climatic changes but was a result of restorative succession after vegetation cover disturbances under local factors (fires, since carbonaceous residues are recorded, and/or changes in hydrological conditions due to permafrost heaving) in the surrounding area.

Thus, paleoreconstruction shows that intensive peat accumulation began as early as the Preboreal, when the temperature and moisture content were even lower than contemporary ones, and the slowing to almost complete cessation of peat accumulation occurred already in the middle of the Atlantic period, when the maximum climatic optimum still went on. For such arctic peatlands, a fairly clear temporal limitation of the period of their most active vertical growth was noted, from 9000 to 6000 years ago, with a rate reaching 1.5–4.4 mm/year. These environmental conditions (due to a unique combination of temperature and moisture) had been no longer repeated either in the second half of the Atlantic period or later ^[5]. The irregularity of the peat accumulation rates is explained both by active soil heave ^[29] and following peat cryoturbations, as well as by the predominance of texture-forming, congelation, and wedge-shaped ice in the peat soil, when the mass fraction of biogenic material (peat) is commonly less than 10% of the total mass ^[5]. In the interpolygonal depression (site TZf, Figure 2), an inversion of two radiocarbon dates was observed: The median age of peat at a depth of 70–75 cm is 7885 cal. yr. BP, and at a depth of 90–95 cm—520 cal. yr. BP. Obviously, this inversion is explained by the result of the peat turbation due to the formation and growth of an underground ice wedge. Thus, in the studied site, a space of 60–65 cm was filled with a horizontal ice vein, expanded to 20 cm on the left wall, and turned into a large block of ice wedges, over 3 m.

Figure 2. Spore-pollen diagram of TZf. •—0.5–2% of the sum of all species encountered.

In contrast to the tundra and forest–tundra of the European Northeast, where peat accumulated in non-permafrost bogs covered with woody plants, sedges, and mosses and permafrost appeared only at the beginning of the Subboreal (4600–4300 cal. yr. BP), after which, during warming, the degradation of permafrost repeatedly occurred, in the West Siberian tundra, even the maximum warming of the Atlantic optimum climate occurred within negative temperatures and did not lead to a significant degradation of the permafrost table. Model calculations show that by 2050 yr. in Western Siberia, due to global warming, the air temperature may rise by 0.9–1.5 °C and the humidity will increase by 12–39% ^[30]. Similar climate changes occurred during the Atlantic (about 8000 cal. yr. BP), and in such a climate the peatlands absorbed significant amounts of atmospheric carbon, with peat accumulation rates reaching 0.5 mm/yr. ^[31]. Therefore, under natural conditions, it can be assumed that the soil–geocryological complexes of studied and other Arctic peatlands will remain a stable carbon sink. It is believed that the West Siberian peatlands in the 21st century will not only remain a carbon sink, but will also increase its absorption ^[32]; however, the methane emission will obviously also increase ^[33]. Taking into account the higher Global Warming Potential of CH₄ compared to CO₂ (28–34 times) ^[34], increased methane emissions can significantly affect the overall impact of thawing permafrost, the degradation of arctic peatlands, and climate change.

Under the present-day climate warming, the vegetation cover of polygonal peatlands and peat plateaus protects the permafrost from thaw. When the surface of polygons and peat plateaus dries up, moss ground layer is replaced by lichens and peat circles are formed. However, dry peat provides thermal insulation, preventing further permafrost thawing. Such peatlands might collapse due to wind abrasion and thermal erosion, but thermokarst processes are uncommon there. The thawing of peat plateaus and polygonal peatlands occurs in case they are destroyed or under hindered surface runoff, when lake and fen formation occur. The main indicator of bog stability is the average long-term ground water level, and for permafrost peatlands, the thickness of the active layer is also equal to the averaged long-term depth of seasonal thawing of peat deposits ^[35]. Since the water levels in the bogs are close to the soil surface, any anthropogenic linear objects crossing them would change the bog runoff. Therefore, the construction of the road favored the flooding of the studied peatland, which led to a change in the hydrothermal regime and vegetation cover and an increase in the temperature of the upper permafrost as well as soil-geocryological complex degradation. Thus, technogenic impacts exceeded the potential stability of the studied peatland as a bog system, and, therefore, irreversible destruction processes may occur.

The analysis by ground-penetrating radar studies showed that the zone of the warming effect of the road as a result of changes in the hydrological regime of the investigated peatland already exceeds 50 m. The most significant retreat of the permafrost table (up to 2–3 m) was observed directly at the bottom of the slope of the road embankment, composed of loose sandy and sandy-loamy soils. The main reasons for the subsidence of the permafrost table are the transformation of the natural conditions serving the polygonal peatland ecosystem functioning. In roadside depressions, favorable conditions are created for waterlogging, the growth of tall shrubs, and snow accumulation as well ^[16]. The combination of these factors activates the thermokarst processes ^[36], resulting in a significant permafrost table retreat in the sites under road impact.

In natural regional ecosystems, the permafrost subsidence is less pronounced, in comparison with the zone of discontinuous permafrost in the European North. This is due to the greater stability of low-temperature, continuous permafrost in the North of Western Siberia. According to the classification scheme ^[37], the studied permafrost is classified as climate-conditioned, ecosystem-protected. In the case of ecosystem disturbances, permafrost in soil–geocryological complexes partially thaws, but repeated permafrost aggradation is also possible under favorable environmental conditions.

In this regard, it is necessary to pay attention to the protection and preservation of permafrost peatlands and to apply the concept of ecosystem services for the peatland use in the construction of infrastructure facilities. When designing and constructing the linear objects, it is required to arrange sufficient culverts to maintain the natural level regime of bog waters. Measures to prevent or reduce negative consequences on linear structures are the drawing up of grid lines of bog water runoff, determination of locations, and calculation of the size of culverts ^[35].

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1. Introduction

2.Vulnerability of the Ancient Peat Plateaus in Western Siberia

References