Holocene environmental conditions in the Western part of Eastern Sayan low-mountain relief: based on comprehensive study of the Mina mire deposits

Field: Earth and Planetary Sciences (Physical Sciences)

Mires in the foothill areas have high palaeoecological information content. Pollen and spores, which record composition and abundance changes of the main forest-forming species’ pollen in combination with pollen of shrubs and grasses, make it possible to trace altitudinal shifts in vegetation belts caused by relative warming or cooling [Blyakharchuk, 2011; Borisova, Panin, 2019; Blyakharchuk Kurina, 2021; Bezrukova et al., 2022]. The feature of peat strata to retain various organogenic and mineral fractions that fall on their surface as a result of deluvial and river runoff [Volkova, 2005; Chernova, 2005] makes it possible to identify periods of increased erosion, including those of a pyrogenic factor. To date, within the Altai-Sayan region, the features of palaeoecological conditions in the western part of the Eastern Sayan have been less studied. To understand the main trends in the development of mountain taiga landscapes in specific physical-geographical, climatic and forest growth conditions, a comprehensive study of peat deposits seems extremely important. The study site is located on the north-western macroslope of the Eastern Sayan in the floodplain of the Mina River (right bank of the Yenisei River). The river valley lies between the slopes of the Kuturchinsky and Koysky Belogorye, north of the Manskoye Belogorye ridge (the western end of the main watershed ridge of the Eastern Sayan), and belongs to the northern part of the Mansko-Kansky low-mountain region. The mires are confined to the widest sections of the Mana and Mina rivers valleys. At the river mouth of the Mina the terrace part is swampy; in the high-mountain belt, small areas of mires are confined mainly to the shores of overgrown lakes. The studied mire area is located on the right bank of the Mina River in the middle reaches above the mouth of the left-bank tributary of the Kuturchin River. The modern mire vegetation cover is represented by a mixed sparse forb-sphagnum-green moss forested mire. Using botanical analysis of peat, three columns were studied: 1) in a terrace depression at a point with coordinates 54.92° N, 94.28° E and an absolute mark of 560 m, where the thickness of the deposits was 2.40 m, of which: peat - 2.05 m, peaty loam - 0.35 m; 2) at a distance of 450 m from the slope depression, the total thickness is 1.95 m, of which 1.25 m is peat, 0.7 m is loam; 3) at a distance of 750 m from the slope, where peat is 0.8 m, below there is gravel. Samples of the thickest column were studied using a combination of methods: pollen [Grichuk, Zaklinskaya, 1948], botanical analysis [Kulikova, 1974], macrocharcoal analysis [Clark, 1988], determination of peat ash content was carried out according to [GOST 11306-2013, 2019]. AMS dating was performed in Poznań Radiocarbon Laboratory, Poland. Peaty loam (depth interval 2.40–2.05 m, 7900–5700 cal. yr BP) includes remains of the bark of Picea obovata and Pinus sibirica, as well as tissues of green and sphagnum moss. The peat core has a two-layer structure; in the interval of 2.05–1.35 m, the deposit is formed by lowland woody-sphagnum peat, with ash content values varying from 15 to 30%, except for the depth interval of 1.87–1.81 m (4500–4100 cal. yr BP), where the maximum value of 53% is observed. A sample from this stratigraphic layer was separated in an aqueous medium with subsequent examination of the fine and medium-dispersed phase using a TESCAN VEGA 3 SBH scanning electron microscope with an OxfordX-Act energy-dispersive microanalysis system. The content of Si (6%) and Al (2.2%) indicates a high proportion of terrigenous admixture in the formation of the stratigraphic layer, likely associated with post-pyrogenic erosion in the study area. The upper part of the core (1.35-0.07 m, approximately from 1100 to 60 cal. yr BP) is formed by sphagnum peat. Starting from 7970±23 cal.yr BP on the Kuturchinsky and Koysky Belogorye slopes fir-spruce- siberian pine forests grew. In the Mina River valley with a wide floodplain conditions developed for the pinching off of an oxbow lake with its gradual silting and overgrowing. The time interval of 7200–5700 cal. yr BP was characterized by high fire activity and the beginning of peat accumulation in the Mina River floodplain (around 5700 cal. yr BP), which may reflect the response of landscapes to the Holocene Thermal Maximum. The period 5300–4100 cal. yr BP is characterized by consistently high humidity, with slopes covered by fir-spruce-cedar forests and a forb-fern ground cover. The time interval 4500-4100 cal. yr BP is characterized by the passage of strong fires and increased surface erosion, which contributed to a high input of mineral particles to the surface of the mire, which together may reflect the manifestation of pyrogenic erosion. Starting from 4100 cal. yr BP, a significant reduction in the amount of dark coniferous species pollen is noted (up to 40–44% in total): Pinus sibirica – 25–27%, Picea – 5–8%, with a slight increase in the content of Abies pollen (up to 7–9%) and Betula sect. Nanae (up to 18–23%). The total content of grass pollen increases to 20%, representatives of the following taxons are noted: Rosaceae, Caryophyllacea, Poaceae, Artemisia, Thalictrum. In the mire the spruce-sphagnum community is replaced by green moss-sphagnum yernik. This period is marked by the maximum extremum in the content of macrocharcols indicating the close localization of the fire to the study point. The totality of the identified paleosignals may indicate a decrease in overall humidity in the period 4100-3300 cal. yr BP and high fire activity. The increase in fire activity during this period is in good agreement with the Subboreal Thermal Maximum of the Holocene (from 4200 to 3200 cal. yr BP), which was identified by N.A. Khotinsky [Khotinsky, 1982] for Northern Eurasia, or on a global scale with “event 4.2” (4.2–3.8 thousand cal. yr BP) [Mayewski et al., 2004; Wang et al., 2010]. The cooling period around 2600 cal. yr BP, known for the temperate latitudes of the Northern Hemisphere [Shnitnikov, 1957], was also noted in the highlands of the Eastern Sayan [Bezrukova et al., 2004], in Altai [Galakhov et al., 2012], and in the Baikal region [Vorobyeva, 2010]. In the Mina core, this period was manifested by an increase in the amount of Betula sect. Nanae (up to 25% in the pollen sum), a high content of Equisetum (16% of the proportion of spores), and a complete absence of traces of macrocharcoal. Beginning at 2400 cal. yr BP, the pollen content of dark coniferous species. In the range of 2400-2000 cal. yr BP the content of spruce is noted – up to 13.6%, which may indicate a wide distribution of Picea and a consistently high soil moisture content. At the same time, sphagnum moss dominates in the ground cover of the mire. Sphagnum angustifolium dominates, the tree layer is absent, the mire passes from the eutrophic-mesotrophic stage of development to the mesotrophic-oligotrophic one, which coincides with a significant decrease in the amount of solar insolation [Berger, Loutre, 1991], for 55° N approximately to 480-490 Wm-2. Fires occurred 1550, 1100 and 900 cal.yr BP which in the observations may reflect a change in humidification conditions towards lower humidity. In general, the interval 1600-1100 cal.yr BP correlates well with the Cooling of the Dark Ages (410-775 AD). The interval 1100-900 cal. yr BP is characterized by a peak value of dark coniferous species (68-73%– the maximum extremum for the entire reconstruction period), the participation of Pinus sibirica – 50-52%, Picea – up to 11%, Abies – up to 12%. This period is consistent with the Medieval Warm Period, which covered significant areas of the Northern Hemisphere from approximately 830 to 1100 AD [PAGES 2k Consortium, 2013; Moberg et al., 2005]. The most dramatic changes in vegetation composition occurred during the period 750–650 cal. yr BP: pollen concentration was extremely low, the contribution of conifers to the pollen sum was minimal, and the majority consisted of Betula sect. Nanae grains (over 65%), Ericaceae pollen, and Sphagnum spores. In the interval of 600-500 cal. yr BP stable humid conditions are recorded, fir-spruce- siberianpine forests are developed. Later, around 500–450 cal. yr BP, a high proportion of Siberian pine in the forest composition is noted (41% of the pollen sum), with a decrease in the proportion of other dark coniferous species (up to 4–7%), a reduction in spore content to 20%, and a maximum of Ericaceae and Artemisia in the grass and shrub group, which may reflect increased continentality. Further, at 450–400 cal. yr BP, while Siberian pine remained dominant, relatively low pollen productivity was noted. It is known that 1600-1826 AD became the coldest period of the Little Ice Age. Later, consistently humid and cool conditions were observed in the study area, with fir-spruce-Siberian pine forests continuing to develop on the slopes. At the final stage, an increase in the pollen content of Pinus sylvestris (up to 13%) and a decrease in the proportion of Pinus sibirica (up to 27%) were recorded. The content of macrocharcoal in peat has remained consistently high over the past 1000 years, reflecting the increasing intensity of fires characteristic of the entire Northern Hemisphere [Goldammer et al., 2013; Valendik et al., 2014; Ponomarev, Haruk V.I., 2016].