INTRODUCTION

Fires in natural ecosystems are a serious danger for both the integrity and populations of these territories. Current human activity has resulted in a significant increase in the amount of fires in many regions around the world [1, 2]. The amount and intensity of fires also depend on climatic factors, thus requiring an adequate forecast of fire conditions for all of Earth’s regions under current climatic changes. The adequate forecast, in turn, requires information on past fire dynamics. The comparison of present fire conditions with those in the preindustrial period allows estimation of the contribution of the anthropogenic factor to this process [2]. Lacustrine sediments can be among the best archives of information on fires, because their quantitative indicators (charcoal particles) occur in the atmosphere as a result of plant combustion and precipitates on the lake bottom, where they are preserved for thousands of years [2, 3]. It was shown that charcoal particles of <100–125 µm in size characterize the regional fire background, whereas larger particles (>100 µm) characterize the fire conditions in the vicinity of a certain lake [4].

Fires on steppe and forest–steppe territories of South Siberia are a regular event. They are often caused by deliberated or occasional arson with dry grass in spring and autumn. In particular, the activity of fires on the territory of the Minusinsk Basin is 1–10 fires/1000 km2 day–1 (NASA Observatory, data from 2015). The fires regularly damage homes and outbuildings sometimes leading to human casualties [5]. The steppe band of southern Siberia is also affected by significant oscillations of humidity and has undergone long dry and wet periods over the Holocene [6], which is probably reflected in oscillations of the natural fire background. In particular, the Minusinsk Basin is of special interest from the viewpoint of the paleoclimate. This is a plain territory surrounded by mountains, which prevent the penetration of wet air masses resulting in the formation of softer semiarid microclimate in comparison with the surrounding territory. This territory contains a unique amount of archeological sites, which were left by several successive and partly crossing cultures of the Stone, Bronze, and Iron ages. It is suggested that exactly this favorable climate is a reason for the relatively high level of evolution of ancient people; thus a link between human activity and climatic oscillations (e.g., the change from husbandry to agriculture) may be especially evident [7]. The reconstruction of the dynamics of fires on lacustrine sediments can help in revealing the principles of climatic changes and anthropogenic impacts. The vast territory of Siberia is insufficiently studied from the angle of paleofires. The available reconstructions on wooden rings [8], peat columns [9], and lacustrine sediments [3] have been conducted for forest ecosystems of higher latitudes in contrast to the few works on steppe and forest–steppe regions of southern Siberia [3]. No mention of Siberia exists in recent reviews on paleofires [1, 2]; thus new information on their dynamics in this previously unstudied region is of fundamental interest.

In this work, we analyzed for the first time the content of charcoal particles >100 µm in size in sediments of two lakes located in southern Siberia within the forest–steppe zone of the North Minusinsk Basin (Republic of Khakasiya and Krasnoyarsk krai). We revealed that the amount of particles has strongly increased over the past 100 years, which is most likely caused by enhanced human activity including fuel combustion in adjacent settlements. Our data confirmed a trend of an unprecedented increase in the amount of charcoal particles typical of many world regions during the industrial period, which is also typical of this previously unstudied steppe region of southern Siberia.

OBJECTS OF STUDY

Shira Lake (54°30′ N, 90°11′ E) is located in the Shira region of the Republic of Khakasiya, 15 km from an eponymous regional settlement. The lake has an oval shape, a size of 5.3 × 9.3 km, an area of 35.9 km2, and a maximum depth of 24.5 m (2021). The lake is balneological with the famous year-round resort Ozero Shira on the southwestern shore and the settlement of Zhemchuzhnyi, which is surrounded by summer houses and recreation centers.

Uchum Lake (55°05′ N, 89°43′ E) is located 70 km northwest of Lake Shira, on the territory of the Uzhur region of Krasnoyarsk krai, 30 km south of the town of Uzhur. The lake has an irregular oval shape, a size of 1.5 × 4 km, an area of ~4 km2, and a maximum depth of 9.2 m (2022). The mud and water of the lake also exhibit balneological properties, and the southwestern coast of the lake includes the Ozero Uchum resort and an eponymous settlement.

Both lakes are located on the territory of the North Minusinsk Basin. The climate of this area is strongly continental: the average July and January temperatures are about +18°С and –19°С, respectively. The potential evaporation from the territory (600 mm year–1) exceeds the average annual amount of precipitation (300 mm year–1) [10]. The semiarid climate facilitates the formation of the steppe landscape; the ice cover of the lakes in winter is about 1 m thick. In summer, the lakes become a popular recreation place with numerous tent and wild camps.

MATERIALS AND METHODS

Drill cores of sediments were collected on March 4, 2021, from Shira Lake (coordinates of the sampling place 54°30.387′ N, 090°11.652′ E) and on March 16, 2022, from Uchum Lake (coordinates of the sampling place 55°05.670′ N, 089°43.406′ E). The length of the core was 110 and 70 cm, respectively. The core was sampled in a central deep part of the lakes from the ice cover through a hole. The core was sampled using a UWITEC hammer-gravitation corer (Austria), transported vertically to the laboratory, cut along the vertical axis, photographed with a scale bar, and divided into transverse sections with a step of 1 cm. The charcoal particles in samples were calculated following the methods described in [4, 11]. A known volume of wet sediments was emplaced into a deflocculated solution (6% Na hexametaphosphate) for three hours and was further sieved under wet conditions through a fabric with a cell size of 100 µm (a mesh gauze). The residual was exposed for one hour in 6% Na hexametaphosphate for whitening and was again sieved through the same fabric. The residual was placed into a Bogorov chamber, and charcoal particles were calculated under a stereomicroscope in reflected light at a magnification of 25. The comparative objects included fragmented wooden and activated coal. The charcoal particles were recognized by the presence of a metallic luster, acute edges, and brittleness. The quantitative analysis was conducted in the CharAnalysis program (http://CharAnalysis.googlepages.com) [12] including the interpolation of the amount of charcoal particles in samples through even time periods and smoothing for the identification of the background, which reflects the averaged intensity of combustion in the region, as well as the processes of redeposition and gain of material by washout from the drainage territory [12].

The age of sediments of Shira Lake based on the measurement of activity of the 137Cs, 210Pb, and 14С isotopes in combination with calculation of the annual layers (varves) was previously estimated for another core sampled at the same point [13]. A visible light layer at a depth of ~13 cm was used as a reference for the correlation of different cores of Shira Lake. The age of sediments of Uchum Lake was previously estimated by 137Cs and 210Pb activity for the upper part of sediments [10]. In this work, we corrected the age estimation taking into account the decrease in the thickness of varves with depth. The amount of varves was calculated in several core areas by two people, and the average values of the thickness of annual layers were accepted for the age estimation. In areas with poorly visible varves, the thickness of the annual layer was accepted as the average value between adjacent areas from both sides with visual varves.

RESULTS AND DISCUSSION

The analysis of the charcoal particles allowed the identification of three typical groups of different sizes, which correspond to those described in the literature [11, 14]. The first group of particles has an elongated morphology, and these likely relics of grassy plants, fine roots, conifers, leaf veins, etc. [11, 14], were conditionally called “grass.” The second group has a morphology of thin flat scales and is interpreted as relics of leaves and local bark and wood [11, 14]. The third group included volumetric particles of various irregular morphologies, which are probably relics from the combustion of wood, coal, and other fuel solid materials [11, 14]. These morphological types corresponded to those previously found in sedimentation traps of Shira Lake [5].

The charcoal particles of all three types in lacustrine sediments were found at all depths, however, with a heterogeneous distribution along the depth (Figs. 1, 2). The most striking heterogeneities were revealed in the layers that correspond to the past century: a strong increase in the accumulation rate of particles of types 2 and 3 (Figs. 1, 2). The amount of particles of type 1 at present demonstrates no significant increase (Figs. 1, 2).

Fig. 1.
figure 1

Accumulation rate of charcoal particles in sediments of Shira Lake.

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Fig. 2.
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Accumulation rate of charcoal particles in sediments of Uchum Lake.

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The presence of charcoal particles >100 µm in size is interpreted as the relics of plants, which were burnt closely to the lake, within a radius of ~10 km [4, 11]. Human activity is the most likely reason for the increasing accumulation rate of charcoal particles of types 2 and 3 in the lakes studied during the present-day period. Along the shores of the lakes, these particles are sourced from pipes of boiler plants and private houses, as well as from fires made by people in summer. The previously analyzed seasonal dynamics of the amount of charcoal particles in sediments of Shira Lake using sedimentation traps for several years convincingly showed that this amount increases each year during the cold season [5]. In all seasons the traps contained the dominant volumetric particles, whereas grass and leaves were almost absent or their amount was insufficient for identification of the seasonal dynamics [5]. These data combined with our data from cores indicate a direct anthropogenic impact in the form of combustion of firewood and coal for heating and other economic needs, which evidently increased from the beginning of the 20th century to the present. Natural fires caused by human activity can also contribute to this increase. Particles of type 1 are indicators of natural fires; however, their amount was constant during the present-day period (Figs. 1, 2). It is likely that the intensity of steppe and forest fires in ambient ecosystems increase a little in comparison with the past.

An increase in the amount of charcoal particles in sediments in recent decades has been registered in many world regions including Siberia [6, 9], even on the poorly inhabited Putorana Plateau [3]. At the same time, the absence of a similar increase in the sediments of the three lakes of another poorly populated region of Siberia (Central Tunguska Plateau, Krasnoyarsk krai, Evenkia) [15] indicates a heterogenous anthropogenic load on different territories, therefore, generating interest in its estimation for all of Siberia.

Note that, despite similar dynamics, the absolute values of particles of types 2 and 3 in Uchum Lake are higher by an order of magnitude in comparison with Shira Lake (Figs. 1, 2). It is probably explained by the fact that Uchum Lake is protected from the dominant westerly and southwesterly winter winds by high hills. This results in enhanced precipitation of particles on the lake surface from the settlement located on the southwestern coast rather than from their dispersion over large distances. In contrast, Shira Lake is open to all winds; thus, the particles propagated over greater distances resulting in a decreased amount on the lake surface.

CONCLUSIONS

For the first time for this region, we found an increase in the deposition of charcoal particles in lacustrine sediments for about the past 100 years, which is unprecedented for the entire Late Holocene. This is evidence of an anthropogenic impact on the dynamics of charcoal particles in lacustrine sediments of this region.

This information can be used for interpretation of charcoal profiles in cores of older lacustrine sediments and reconstruction of the paleoclimate of this region. Drainless lakes in arid and semiarid regions react sensitively to changes in climate moisture by the change in the volume and therefore salinity, which is anticorrelated with the volume. The changes in salinity are reflected in the geochemical and biochemical composition of lacustrine sediments, as well as the composition of pollen and spores buried in them. The sediments of Shira and Uchum lakes are currently being studied to reconstruct the climate moisture from geochemical and biochemical paleoindicators, as well as spore–pollen spectra. The distribution of charcoal particles will be compared with the profiles of pollen and plant spores, elemental and mineral composition, and lipids and pigments of photosynthesized microorganisms, thus increasing the reliability of reconstructions of climate humidity.

The comparison of the composition of charcoals in the cores with that from sedimentation traps gives grounds for differentiation of various types of particles by their informative value. We showed that the “grass” in the present-day period mostly reflects a natural fire background, whereas volumetric particles are associated with an anthropogenic contribution. In older sediments, in core areas, which exhibit no correlations between different types of particles, therefore, the “grass” will mostly reflect climatic changes and will be more informative for the climate reconstructions. At the same time, the volumetric particles in these areas could reflect the dynamics of the population and could depend weakly on the climate. The local peaks of the volumetric particles thus could indicate episodes of the mass presence of ancient people in the vicinities of the lakes studied.