The most comprehensive history of fire activity in the Pur-Taz interfluve over the last 11,000 years (Western Siberia, Russia)

Abstract

Climate change in the Arctic may increase the incidence of tundra fires, which is expected to significantly transform tundra ecosystems. Therefore, it is promising to study the tundra paleofire data to assess whether the projected increase in fire numbers is unique. In this study, we used five types of ancient fire proxies to estimate the number of fire events during the Holocene in the southern tundra of Western Siberia. Our integrated analysis of soil and peat deposit proxies allowed us to distinguish, for the first time, three different fire activity periods for this region. In the first period from 11.2 to 7.5 cal kyr BP, high fire activity was recorded. In the second period from 7.5 to 3.5 cal kyr BP the number of fire events is minimal. In the third period from 3.5 cal kyr BP to the present day, the number of pyrogenic events is maximum, and the maximum occurs in the second and third millennia from our days. For the third period, a temporal relationship between fire frequency and archaeological cultures is noted. It was also established that in the second half of the Holocene in the southern tundra peat accumulation was not interrupted in fens permafrost polygonal peat plateaus. This allowed us to obtain a good time resolution of the peat charcoal record for the last 3.5 thousand years. Thus, this study demonstrates the occurrence of landscape fires in the southern tundra throughout the Holocene. And the predicted increase in the frequency of tundra fires has paleoecological analogues.

Acta Biologica Sibirica 10: 1755–1777 (2024)

doi: 10.5281/zenodo.14525710

Corresponding author: Nikita V. Shefer vchifz@mail.ru

Academic editor: R. Yakovlev | Received 1 December 2024 | Accepted 18 December 2024 | Published 21 December 2024

http://zoobank.org/E3FDC103-EA21-4955-A1F2-E174CC279D9A

Citation: Shefer NV, Loiko SV, Krickov IV, Lim AG (2024) The most comprehensive history of fire activity in the Pur-Taz interfluve over the last 11,000 years (Western Siberia, Russia). Acta Biologica Sibirica 10: 1755–1777. https://doi.org/10.5281/zenodo.14525710

Keywords

Paleofire, macro-charcoal, micro-charcoal, non-pollen palynomorphs (NPP), soil charcoal, Yamalo-Gydan ecoregion

Copyright Nikita V. Shefer et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

Landscape fires are an ancient and widespread exogenous ecological factor causing significant changes in biocenoses, altering the ratio of individual groups of living organisms (Kelly et al. 2020; Certini et al. 2021). The impact of fire has significantly transformed the soils of many of the Earth's natural zones (Li et al. 2021; Nelson et al. 2021). Fires are also an important geochemical factor that enhances the fluxes of elements from a catchment into rivers (Parham et al. 2013; Kuzmina et al. 2022; Kuzmina et al. 2024), and can reduce ecosystem productivity over long periods of time by reducing nitrogen stocks (Pellegrini et al. 2018). At the same time, the residual effect of fire on ecosystem processes persists for a long time not only due to interruption of successional development, destruction of litter, and leaching of chemicals elements (Cancelo-González et al. 2013), but also due to the penetration of charcoal on the soil surface (Luo et al. 2016), which has important ecological functions in boreal ecosystems (Hart and Luckai 2013). In subarctic ecosystems, fires can increase biodiversity for several decades (Heim et al. 2021).

Fire will become increasingly important in landscape dynamics due to current and near future climate warming (Kharuk et al. 2022; Mekonnen et al. 2022; Talucci et al. 2022). Fire dynamics in the boreal and subarctic regions are of particular concern, (Glückler et al. 2024; Shestakova et al., 2024), on the one hand because of high carbon stocks in vegetation and peat soil horizons, which is released into the atmosphere during fires (Nurrohman et al. 2024), and on the other hand because of the impact of fires on permafrost stability (Chen et al. 2021; Filimonenko et al. 2024). In order to make correct forecasts of the future impact of fires on ecosystems, it is necessary to refer to a distant retrospective, up to covering the entire Holocene and the previous warm and dry epochs. For these and other reasons, an increasing number of researchers are addressing fire dynamics in the Subarctic (Heim et al. 2022). However, there is still a long way to go before these issues are evenly studied in different regions of permafrost distribution (Heijmans et al. 2022). This way the Subarctic of North America and Europe is much better studied (Vachula et al. 2020; Scholten et al. 2024). The most studied in Russia in terms of reconstruction of paleofire dynamics are the central and southern parts of the East European Plain, as well as the south-east of Western Siberia (Pupysheva and Blyakharchuk 2023). The territory of Siberia is still being studied more by remote sensing, in the aspect of fire activity for the short-term retrospective.

One of the Siberian regions sensitive to landscape fire activity is Western Siberia due to the high rate in permafrost peatlands (Vasil’chuk and Vasil’chuk 2016). With regard to the study of paleofire activity in Western Siberia, its boreal part has been mainly investigated. In pioneering reseearch, charcoal layers in peat deposits were counted (Turunen et al. 2001) and periods of high fire activity were dated. In the last decade, work has been carried out to study the proxies of landscape fires in soils, peat and lake deposits of boreal landscapes of Western Siberia (Lamentowicz et al. 2015; Amon et al. 2020; Feurdean et al. 2022; Loiko et al. 2022; Startsev et al. 2022; Blyakharchuk et al. 2024; Pupysheva and Blyakharchuk 2024; etc.). There is currently little information on the paleofire dynamics of subarctic permafrost landscapes in Western Siberia (Shefer et al. 2023), although in a number of interfluves, almost all landscapes have been affected by fires, and in relatively recent times (Moskovchenko et al. 2020; Sizov et al. 2021). All this emphasizes the importance of studying paleofire activity in the permafrost zone in the Holocene and identifying the causes of its fluctuations.

In this paper, we have studied the paleofire history in one of the typical low-tundra regions of Western Siberia using several proxies. The northern part of the Pur-Taz interfluve of the Yamal-Gydan ecoregion was chosen as such a region. The choice of this area is related not only to its good transport accessibility, but also to its location near the large Taz River, along which the presence of large settlements (Tazovsky factoria, Mangazeya town) has been known for several centuries even according to written sources.

This study combines paleofire data from both peat deposits of frozen polygonal peat plateaus and mineral soils located near peat plateaus. This is the first time that these different types of charcoal proxies have been compared for the Yamalo-Gydan ecoregion. Plant macrofossils and pollen in the peat that have had very limited peat growth in the last four thousand years have been previously investigated in this region (Pastukhov et al. 2021). In the present study, not only such a peat deposit was used, but also peat deposits that have had a marked increase in the last four thousand years, allowing a higher resolution for the second half of the Holocene for the distribution of charcoal in peat.

Materials and methods

Study area

The tundra ecosystems of the Pur-Taz interfluve are located in the continuous permafrost region, in the southern tundra subzone of Western Siberia (Zarov et al. 2021). The studied key area (Fig. 1) is located in the northeast of the Pur-Taz interfluve, within the third lacustrine-alluvial plain. Ridges, thermokarst-erosional hollows and basins of drained thermokarst lakes are common within this terrace (Loiko et al. 2020; Opokina et al. 2024). The studied part of the interfluve has a large area of permafrost polygonal peat plateaus, and the soils are mostly of loamy texture (Zarov et al. 2021). Active peat accumulation in this area began in the early Holocene (Tikhonravova et al. 2023) and is thought to have slowed significantly in the late Holocene, and permafrost polygonal peat plateaus are defined as relict (Fotiev 2017). Their microtopography is represented by alternating elevations (polygons), elongated interpolygonal depressions (cracks), and fens through which supraper- mafrost water flows into streams (Loiko et al. 2019).

According to observations from the nearest meteorological station in the village of Tazovsky (67°28′ N; 78°44′ E) for 1991-2021, the average temperature of the coldest month (January) is -25.4 °C, the warmest month (July) is +14.4 °C, and the average annual temperature is -6.8 °C. The average annual precipitation reaches 515 mm (Climate data for cities around the world, https://ru.climate-data.org/).

Figure 1.Maps and locations of the study sites.

The plant communities of the upland tundra are represented by moss-sedge-lichen tundra in the upper parts of the slopes and willow-dwarf birch-moss communities at the foot of the slopes. On the convex upper parts of the slopes, alder bushes with tundra meadows in the centre are found in places (Loiko et al. 2022). Single larch trees are rare. Peatland vegetation is widespread in extensive thermokarst depressions, ancient drained basins of thermokarst lakes, thermokarst-erosional hollows. Shrub-lichen and sedge-sphagnum communities occur on polygons of permafrost peat plateaus. In fens and cracks are cotton grass-sedge-sphagnum. Tall thickets of alder and willow grow in the floodplains along the rivers, with occasional larch, birch (Loiko et al. 2023). Highly productive grass communities grow under the cover of trees and shrubs.

Fieldwork

Fieldwork was carried out during the summers of 2015–2021, during which time two cores were collected from the central parts of adjacent polygonal bog polygons and a series of soil charcoal. The peat core CKT-15-3D (400 cm) was collected in 2015 by geological core drilling using the UKB-12/25-01 rotary drilling rig (Russia, Yekaterinburg, 'Machine-Building factory V. V. Vorovsky' LLC) with a compartment length of one metre and a diameter of 59 mm. After drilling, the column was cut into 2.5 cm thick samples which were packed in hermetic bags and frozen. Peat core CKT-21 (48 cm) was sampled in 2021 as a monolith (Fig. 1). The column was later cut into 1 cm thick samples which were packed into hermetic bags and frozen. To describe the morphology of mineral and peat soils, the International Soil Classification System WRB (IUSS... 2022) was used.

Charcoal was collected from the soil pits by extracting it from the front wall, which had been cleared with a shovel. Charcoal larger than 2 mm in size, easily visible to the naked eye, was collected. All charcoal that was found in the range from the mineral surface of the soil to the soil-forming deposit or permafrost was collected. No charcoal was collected from under the plant litter, i.e. from the mineral surface of the soil. All the soils from which the charcoal were collected were mineral and were located either on tundra upland plains or in the basins of drained thermokarst lakes. Charcoal in tundra soils are small in size, as they are usually formed from dwarf shrubs. They are scattered, as they are immersed in the soil during cryoturbation. They are often sealed in dense loam and are destroyed during extraction. Therefore, all more or less large undestroyed charcoal was selected. If charcoal formed a cluster in the form of a morphon (patch), then the largest charcoal from this patch was selected. In general, the methodology for extracting soil charcoal and its subsequent preparation for radiocarbon dating was completely analogous to previous works (Bobrovsky et al. 2019; Loiko et al. 2022).

Radiocarbon dating and age-depth modelling

Radiocarbon analysis of peat using liquid scintillation spectrometry was carried out at the Kiev Radiocarbon Laboratory, Ukraine (7 samples, index Ki) and the Institute of Monitoring of Climatic and Ecological Systems, SB RAS, Russia (3 samples, index IMCES). Radiocarbon analysis using accelerator mass spectrometry was carried out at the Poznań Radiocarbon Laboratory, Poland (10 samples, index Poz), the National Taiwan University AMS Lab Taiwan (2 samples, index NTUAMS). Preparing for radiocarbon dating of plant remains samples with IGANAMS index (1 sample) was carried out at the Laboratory of Radiocarbon Dating and Electron Microscopy of the Institute of Geography of the Russian Academy of Sciences (Moscow) and radiocarbon analysis using accelerator mass spectrometry was carried out at the Isotope Research Centre of the University of Georgia (USA). Calibration of the radiocarbon age in the calendar was carried out at the OxCal 4.4. program using IntCal20 calibration curve (Reimer et al. 2020). Errors are expressed at the two sigma level of confidence, and dates are quoted in calendric years BP (Tables 1, 2).

An age-depth models was established using “rbacon” package in R (Blaauw and Christen 2011) and IntCal20 calibration curve. Dating obtained for samples at depths of 275–277.5 cm and 320–322.5 cm is unreliable, in the model they are marked as "outlier". Age-depth model for the sediment core (CKT-15-3D) suggest that the sediment covers 11.2 cal kyr BP and lack of sedimentation between 4.4 and 0.8 cal kyr BP (Fig. 3). The upper part of the sediment (0–20 cm) started to accumulate recently.

Laboratory analyses

We divide a charcoal particles into micro-charcoal (DM > 10 µm, where DM is the maximum dimension of the particle) and macro-charcoal (DM > 125 µm) particles.

Micro-charcoal analysis

Samples for analysis (~1 cm³) were taken throughout the thickness of peat sediments every 1 cm for column CKT-21 and every 2.5–22.5 cm for column CLT-15-3D. Samples were treated according to palynological methodology (Erdtman 1943), using acetolysis. One tablet of Lycopodium clavatum spores was initially added to each weighted sample as a reference for concentration calculation (Stockmarr 1971). For the CKT-15-3D column material, we used tablets with batch numbers 1031 and 3862 (containing 20848 ± 1546 and 9666 ± 212 spores, respectively). For the CKT-21 column material, we used only tablets with batch number 3862. CKT- 15-3D column material from depths of 320 cm and below contained silicates and was treated with HF for 24 hours.

Micro-charcoal and non-pollen palynomorphs (NPP) were counted using a Zeiss Axiolab A1 microscope at 400× and 1000× magnification. Identification of NPPs was carried out using database NPP-ID (Shumilovskikh et al. 2022a; Shumilovskikh et all. 2022b).

The following formula was used to calculate the micro-charcoal concentration (M) per cm³:

M = C * Lyc^t / Lyc1 (1),

where:

C – the identified number of micro-charcoal;

Lyc¹ – the identified number of Lycopodium spores;

Lyc^t – the number of Lycopodium spores in tablet.

Macro-charcoal analysis

Samples for analysis (~2 cm³) are taken throughout the thickness of peat sediments every 1 cm. Standard sample processing technique is used for macro-charcoal analysis (Whitlock and Larsen 2001; Mooney and Tinner 2011) with using a 10% solution of KOH and 6% solution of H₂O₂. Macro-charcoal was counted with a “Zeiss Axio Zoom.V16” microscope at 45× magnification using a Bogorov’s camera to count the total number of macro-charcoal. The statistical processing of the obtained data on the concentration of macro-charcoal in sediments is carried out in the "CharAnalysis" adapted for the "R" program (Higuera 2009). The number of macro-charcoal particles, their depth, and the upper and lower limits of their age (by age-depth model), served as the initial data for analysis.

Statistical interpretation of the data was carried out in several steps similar to the articles by Kupriyanov and Novenko (Kupriyanov and Novenko 2019):

1. Based on the obtained concentration values, the rate of macrocharcoal accumulation was calculated. Concentration values, sampling depth, and age were interpolated to bring them to a single time resolution of 70 years, the median time resolution of the selected samples calculated on the basis of the Age-depth model;

2. Background values of charcoal accumulation rate were calculated to smooth short-period fluctuations and subsequent separation of local and regional signals. We chose the Locally Estimated Scatterplot Smoothing (LOESS) regression method with a smoothing interval of 400 years, which best described the charcoal particle accumulation curve to identify the background values;

3. To identify local pyrogenic events, a threshold value was calculated assuming that the ‘noises’ are distributed according to the Gaussian model of impurity dispersion within a user-defined time window. The Signal-to-Noise Index (SNI) was applied for each time window to assess statistical validity. To unify the statistical functions, we used a window resolution of 3300 years, the range of SNI values was from 2 to 5, which satisfies the statistical requirements of the analysis. We set the 95th percentile of the noise distribution as the threshold. If the determined values of coal accumulation rate were higher than this threshold, it was classified as a reliable local fire event.

Results

Dating for soil charcoal

The results of soil charcoal dating are summarised in Table 1. Figure 2 shows examples of localisation of dated charcoal in relation to patches composing the morphological pattern of the soil profile. This figure shows that depth in mineral soils does not affect the age of the charcoal as it usually does in peat soils. Charcoal more than 8 cal kyr BP old can be as little as 10 cm below the mineral soil surface, while charcoal about 2.5 cal kyr BP can be at 60–70 cm deep.

Figure 3 shows a ranked series of charcoal dates. The dates are grouped into three areas, two of which are confined to the Early Holocene and two to the Late Holocene. The largest number of charcoal dates are from the last 2.5 thousand years. Between 8 and 3 thousand years is a large time interval without dates. The charcoal with the oldest date to be 10366±133 cal yr BP and sampled from a depth of 80 cm. In this case it is confined to an ancient humus structure of dark grey colour, which arose on the site of a melted ice wedge during the Holocene climate warming, which occurred not earlier than the considered date.

Figure 2.Examples of soils studied and locations of dated charcoalA – Gleyic Cryosol at the foot of a slope; B – Gleyic Cryosol at the summit of a slope; C – Cryic Epifibric Endohemic Histosol in a depression on a polygon of the peat plateau; D – Hemic Cryic Histosol on a polygon of the peat plateau.

Figure 3.A – Gleyic Cryosol at the foot of a slope; B – Gleyic Cryosol at the summit of a slope; C – Cryic Epifibric Endohemic Histosol in a depression on a polygon of the peat plateau; D – Hemic Cryic Histosol on a polygon of the peat plateau.

Age-depth models of peat deposits of polygonal permafrost peat plateaus

The peat deposit of the studied peat plateau is about 340 cm deep, with lake sediments below it, which were drilled to a depth of 400 cm. The highest intensity of peat accumulation was during the period from 8 to 5.5 cal kyr BP. Thereafter, the intensity of peat accumulation generally decreased and became highly variable depending on the microtopography. Thus, on polygons with convex center it almost ceased and for about 5 thousand years it was no more than 25 cm (Fig. 4). At the same time, polygons with concave center accumulated 48 cm of peat over a period of 3.5 thousand years.

Peat accumulation rates in profile CKT-21 slowed throughout the studied period, especially from 2.3 cal kyr BP (Fig. 4A). These differences in peat accumulation rates are also reflected in the morphology of the peat soils. Thus, in the CKT-21 profile, the upper 20 cm of peat has a predominantly sedge and sphagnum composition of plant macrofossils (Fig. 2C, D). This horizon is diagnosed as Oi. At the same time, in the upper 20 cm of profile CKT-15-3D the peat consists of macrofossils of shrubs, lichens and green mosses. This horizon is diagnosed as Oe. Thickness of the active layer also differs, in profile CKT-21 it is 10–15 cm more.

Micro-charcoal and NPP

The full CKT-15-3D column contain 400 cm and consist of lake sediment (400–340 cm) and peat sediment (340–0 cm). The concentration of microcharcoal at lake sediment part (11.2–9.7 cal kyr BP) is quite high (56K–230K particles). The peat sediments of core CKT-15-3D can be divided into 2 periods based on the number of micro-charcoal. The period 11.2–8 cal kyr BP, for which a high concentration of micro-charcoal (1.3K–806K particles) and the period 8–4.4 cal kyr BP, for which a low concentration of micro-charcoal (1.3K–19K particles) is noted. The spores of Gelasinospora are presence at: 11.2, 10.7, 10.6, 9, 8.2, 8.1, 7.7, 5.4 cal kyr BP. The spores of Glomus are presence at: 11.2, 8.9, 8.3, 8.1, 8, 7.9, 6.7, 6.4, 4.9 cal kyr BP (Fig. 5A).

The 48 cm column CKT-21 is divided into four periods. Period 3.4–3 cal kyr BP, a minimum concentration of charcoal (8K–22K particles) is noted. In the period 3–2.3 cal kyr BP, the concentration of charcoal increases, ranging from 25K–88K particles. The maximum charcoal concentration is during the period 2.3–1.1 cal kyr BP (20K–106K charcoal particles). Around 1.1 cal kyr BP, the concentration of micro-charcoal decreases and during the period 1.1–0 cal kyr BP makes 2K–31K particles. The spores of Gelasinospora is presence at: 2.9, 2.8, 2.6, 2.5, 2.4, 2.3, 2.2, 1.8, 1.7, 1.5, 1.4, 1.2, 1.1, 1 cal kyr BP, ~251 cal yr BP and ~24 cal yr BP. The spores of Glomus are presence at: 3.4, 3.3, 2.6, 2.4, 2.3, 2.2, 1.8, 1.5, 1.2, 1, 0.8 and 0.5 cal kyr BP (Fig. 5B).

Figure 4.Bacon age-depth models for (A) column CKT-21 and (B) CKT-15-3D. Calibrated 14C AMS dates are shown in transparent blue, and the age-depth model is shown in grey (darker greys indicate more likely calendar ages; grey dashed lines show 95% confidence intervals).

Macro-charcoal

Results of macro-charcoal accumulation rate calculations obtained using the CharAnalysis software show the dynamics of local fires, over ~3.5 cal kyr BP (Fig. 6).

Background char accumulation rates ranged from 0.1 to 4.2 char particles per cm²/yr (c. p. per cm²/yr). The three events with the highest charcoal accumulation rates were observed between ~2.2 cal kyr BP and 3–2.3 cal kyr BP (4.2 c. p. per cm²/ yr), 2.6 cal kyr BP (3.1 c. p. per cm²/yr), and 2.9 cal kyr BP (3.8 c. p. per cm²/yr). The minimum background accumulation rate was observed between ~0 and 0.8 cal kyr BP (0.1–0.6 c. p. per cm²/yr). CharAnalysis interpretation of fire events recorded 13 local fire episodes (Fig. 5): 0.6, 1.1, 1.2, 1.3, 1.5, 1.6, 2, 2.3, 2., 2.6, 2.7, 2.9 and 3.2 cal kyr BP.

Figure 5.Charcoal and non-pollen palynomorphs for columns CKT-15-3D(A) and CKT- 21(B).

Discussion

Our complex study of five terrestrial paleofire proxies for the landscapes of the Pur-Taz interfluve is the first such work for the Yamal-Gydan ecoregion. Charcoal in peat appears not only due to fires in the peatlands itself, but also due to wind-borne input from the surrounding upland landscapes. The study of charcoal in the soils (Figs 2, 7) of the latter allowed the two archives to be compared. Such research has never been carried out before, not only in the region under study, but also in the whole of Western Siberia. In this connection, some features of the location of charcoal in soils should be considered.

We identified four main types of charcoal transport mechanisms in mineral soils. (1) Transport by cryoturbation (Fig. 2B). This process is more characteristic of the thixotropically patterned summit slope of an upland tundra. (2) Charcoal deposition in a microtopographic depression following fire, activating erosion processes (Fig. 7A). (3) Charcoal deposition as a result of melting of ice veins in the first half of the Holocene during climate warming (Fig. 7B). (4) Transfer of charcoal to thermokarst lakes by coastal abrasion, its deposition on the bottom with subsequent accumulation of peat after lake drainage (Fig. 7C).

Figure 6.Local fire frequency for column CKT-21.

Figure 7.Examples of soils studied and locations of dated charcoalA – Gleyic Cryosol; B – Gleyic Cryosol (Ochric); C – Histic Gleysol (Gelic).

Analyses of charcoal dates show that relatively young charcoal (less than 2,000 years old) can be found at rather deeper depths (more than 50 cm). This is associated with active cryoturbation in the areas of thixotropically patterned ground. This weakly depth-dependent distribution of radiocarbon dates is similar to the distribution of the age of organic matter in the profile of cryogenic soils, for which relatively young radiocarbon dates have also been recorded at deep depths (Palmtag et al. 2015). Cryoturbations lead to inversion of radiocarbon dates in peat deposits as well, which is well known (Pastukhov et al. 2021; Tikhonravova et al. 2023).

However, there are also cases of ancient dates located near the surface (see Fig. 2A). Such soils are found at the foot of slopes, where the soil is more stable due to the organogenic horizon.

The presence of charcoal of about 8 thousand cal. yr BP in the surface horizons of Gleyic Cryosol indicates that permafrost soils protect charcoal from decomposition. Earlier it was stated that charcoal in soil can be completely decomposed in 4–5 thousand years (Kuzyakov et al. 2014). Obviously, under conditions of permafrost and gleyisation, charcoal decomposes much more slowly. Cryosols can be considered a good archive of paleofires. These soils provide not only high spatial resolution, but also high temporal resolution.

The Gelasinospora and Glomus fungi used in this work have not been previously used for the tundra of Western Siberia. Therefore, when interpreting the fact of their presence in a peat deposit, it is necessary to take into account some features of the tundra environment. For example, the fungus Gelasinospora is considered to be carbonicolous, which means that it grows on both burnt soil and burnt wood (Lageard and Ryan 2013; Stivrins et al. 2019), and the fungus Glomus grows on open eroded soil (Van Geel et al. 2011). In the boreal region, a significant part of the communities are woodlands, whereas in the tundra, the woody biomass that is able to burn is much lower. Therefore, in the tundra, the number of Gelasinosporas pores will be strongly dependent on the proportion of shrubs and dwarf shrubs and will be significantly lower compared to similar spectra in boreal regions. And Glomus spores can probably form in the tundra not only after post-pyrogenic erosion, but also without fires at all, during cold periods, when solifluction and patterned ground formation processes are activated. It is therefore possible that Glomus spores are not always associated with other proxies.

To discuss the fire history in the region, we collected all the proxy data used in the study. Microcharcoal, which records more regional fires. Macrocharcoal, soil charcoal, and fungal spores, which record local fires. Given the different resolution of the CKT-15-3D and CKT-21 cores, we visualized the results with a time resolution of 500 years for a more relevant presentation of the results in Fig. 8.

Figure 8.Indicators of fire events for the last 11.2 cal kyr BP.

We identified three time periods that reflect millennial changes in fire activity. In the first period from 11.2 to 7.5 cal kyr BP, high fire activity was recorded. In the second period from 7.5 to 3.5 cal kyr BP the number of fire events is minimal. In the third period from 3.5 cal kyr BP to the present day, the number of pyrogenic events is maximum, and the maximum occurs in the second and third millennia from our days. Fire events of the first period are recorded in all proxy types. High fire activity at the beginning of the Holocene has previously been reported for the study area (Peteet et al. 1998), and our data confirm this. The most intense fires occurred immediately after the 8.2 cal kyr BP event. This event has been associated with climate cooling (Zhao et al. 2022), but it is currently difficult to assess the mechanism of fire occurrence after this cooling.

In the second period, only one fire was recorded, which occurred 5.4 cal kyr BP. Also, the appearance of Glomus spores was noted for this period, in the absence of other proxies, which may be associated with deflationary processes. A decrease in the frequency of fires during this period is consistent with data for the middle taiga of Western Siberia (Loiko et al. 2022, Pupysheva and Blyakharchuk 2024). In contrast, for north Yenisei Siberia has a significant frequency of fire events during the periods 6.3-5.8 cal kyr BP (Novenko et al. 2023). There are other regions of the Arctic that experienced a significant number of fire events during this period. In the southeast of the Kola Peninsula, for example, at least 12 fire events were recorded between 7000 and 5000 cal BP (Mergelov et al. 2024).

The third period is characterised by the highest fire frequency. In published studies for the studied region, this period is the one with the least data, as peat deposits with extremely slow (or interrupted) peat accumulation were analysed during this period (Peteet et al. 1998; Tikhonravova et al. 2023; Pastukhov et al. 2021). The peat deposit with active peat accumulation in the second half of the Holocene selected in our work, as well as the use of soil charcoal dating, made it possible to record high fire activity during this period. Similar results have been obtained for the middle taiga (Loiko et al. 2022; Pupysheva and Blyakharchuk 2024) and for Putorana Plateau (Novenko et al. 2022). Since this period was not the driest, we can assume the influence of ancient anthropogenic activities. There are archaeological sites a few kilometers from the field site. We know of settlements that existed in the second half of the fourth millennium, the middle of the second millennium and the beginning of the first millennium. For the last five hundred years there have been Russian settlements in the lower reaches of the Taz River (Stepanova and Syuzyumov 2019; Tkachev and Tkachev 2019; Tkachev 2018). A similar relationship between fire frequency and features of archaeological cultures has been noted previously for the southwest of Western Siberia (Ryabogina et al. 2024).

Conclusion

The study is the first to compare different landscape paleofire proxies with very different recording mechanisms. This allowed information on fire activity to be obtained for almost the entire Holocene. The work demonstrates the prospects of such an approach, due to which the records of peat columns of large peat depressions and soils of upland plains complement each other.

For paleoecological studies in the Yamalo-Gydan ecoregion, the analysis of peat deposits of concave polygons of permafrost peat plateaus turned out to be very promising. In these polygons, peat accumulation has been uninterrupted for the last four millennia.

The mechanisms of charcoal transfer to soils are considered. Charcoal transfer in mineral soils depends strongly on the microtopographic position of the soil profile. Under the influence of suprapermafrost water with the formation of a peat horizon, the intensity of cryoturbation decreases. Ancient charcoal may occur near the mineral surface. In soils with the highest cryoturbation activity, young charcoal often occurs at depth.

Three periods with different frequencies of fire events were identified. According to the generalised data, the number of fire events was estimated for intervals of 0.5 millennia. The frequency of pyrogenic events in these periods was determined by both climatic events and ancient human influence.

Acknowledgement

The study was performed within the framework of a state assignment of the Ministry of Science and Higher Education of the Russian Federation (No. FSWM-2024- 0006). The authors would like to thank M.A. Pupysheva for consultation with CharAnalysis.

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