1МГУ им. М.В. Ломоносова, Москва, Россия; firstname.lastname@example.org
2НИЛ геохронологии четвертичного периода, ТТУ, Таллин, Эстония
Представлен обзор результатов, полученных по представительной коллекции образцов из 7-метровой толщи отложений юго-восточного побережья Финского залива посредством датирования палинологической записи палеоклиматических сигналов методом оптически инфракрасно-стимулированной люминесценции (ИК-ОСЛ). Совместное использование этих методов позволило впервые выполнить хронологически обеспеченную климато-фитоценотическую реконструкцию палеоклиматических событий и последовательность палинологических зон примерно от 100 тыс. л. н. до 50 тыс. л. н. и проследить эволюцию природных обстановок в этом интервале. Репрезентативные спектры пыльцы убедительно свидетельствуют о теплом интервале межледникового ранга во второй половине морской изотопной стадии (МИС) 5, отвечающей стандартным микулинским палинозонам М6, М7, М8, а также о периоде значительного похолодания МИС 4, характеризующегося развитием в районе наших исследований растительности перигляциального типа, что противоречит точке зрения о развитии в это время обширного калининского оледенения, продвигавшегося, как полагают, на многие сотни километров южнее Финского залива. Результаты, полученные в ходе настоящего исследования по интервалу МИС 5, подтверждаются также данными наших предыдущих работ по установлению хронологии палеоклиматических событий по коллекциям образцов малакофауны, отобранным из климатически чувствительных регионов палеошельфа Евразийской Арктики. Эти результаты показали, что подавляющее большинство ЭПР датировок по раковинам моллюсков, отобранным из морских трансгрессивных отложений МИС 5 арктического палеошельфа (около 82%), приходится на интервал 110–70 тыс. л. н. (МИС 5d–5a), что предполагает существование наиболее благоприятных климатических условий именно во второй половине МИС 5.
Ключевые слова: МИС 5, МИС 4, ИК-ОСЛ хронология, палинология, динамика эволюции природной среды
New palaeoenvironmental and chronological data, which may be integrated with the results from the neighbouring and more remote regions, were collected from a highly unique outcrop discovered in the south-eastern coast of the Gulf of Finland.
The greater part of the coastline here is bordered by the steep northern margin of the limestone plateau, widely known as the North Estonian Clint. The clint makes up approximately a quarter of the up to 1200 km long Baltic Clint, which extends from the western coast of the Swedish island of Öland to the southern shore of Lake Ladoga (Fig. lA).
The outcrop studied is situated in a clint depression ‒‒ the Voka clint bay (Fig. lB). The Quaternary cover in the area adjacent to the clint bay does not usually exceed 0.5 m, and only occasionally is 2–3 m thick. In contrast, the Voka clint bay has revealed a well-exposed continuous stacked sequence of sandy to clayey subaqueous deposits. Immediately on the shore, the tops of the outcrops are at about 22 m a.s.l. The sandy to clayey sedimentary sequence in the profile V1-05 discussed is conventionally subdivided into two lithostratigraphic units with a few centimetres thick sandy-gravelly layer between them at a depth of 5.6 m (Fig. 2). The upper Unit in the section is overlain by ca. 1-m-thick brownish-grey to light-brown soil resting upon ca 0.7-m-thick layer of the light-brown silt-like deposits. The lower one is represented by fine- to medium-grained sands with clayey interlayers.
The main purpose of the study of this section is to identify the Late Pleistocene palaeoenvironmental evolution and palaeoclimatic dynamics in the study area during one of the most controversial periods in the Late Pleistocene history, which is the MIS 5. Our approach to achieving the goal is based on the investigation of the feldspar-based infrared optically stimulated luminescence (IR-OSL) dated records of pollen-induced palaeoclimate and palaeoenvironmental proxies.
To achieve the intended goal, 23 samples for IR-OSL chronological and 33 samples for palynological analyses were taken from the V1-05 section (59°24.86' N, 27°35.94' Е) at a depth interval between 12.8 m and 5.5 m.
According to the age-depth model the age of the deposits directly overlying the above-mentioned marker layer is 57.4 ka at a depth of 5.6 m, under the layer at a depth of 5.7 m — 58.1 ka, that implies the formation of this marker layer as a result of erosion or a brief hiatus in sedimentation, which most likely could be associated with the climate change in the Late Pleistocene towards more favourable climate conditions after MIS 4/MIS 3 transition.
The IR-OSL ages obtained on the VI-05 section below the marker layer fall according to the age-depth models within the period between ca. 92 ka to ca. 58 ka that indicates at least two rare circumstances, namely, the existence here a continuous sedimentary record spanning the most contradictory phase of the palaeoenvironmental development during the latter half of MIS 5, and, apparently, the unique record of sedimentation dynamics under the huge ice sheet of the Kalinin/Middle Weichselian (MIS 4) glacier, whose boundary southward of the Gulf of Finland stretched by about 800 km according to the classical scheme of the boundaries of the Pleistocene ice sheets in the East European Plain [see, e.g., Fig. 1 in Molodkov and Bolikhovskaya, 2010].
In the study section, palynological data (34 spore-pollen spectra) characterise in detail a 7-meter stratum formed in a fairly extensive freshwater reservoir. The climatic-phytocenotic reconstructions and the chronology of paleoclimatic events in the latter half of the MIS 5 and of the whole MIS 4 are illustrated by the time scale and spore-pollen diagram in Fig. 2.
According to the entire set of palynological data obtained, the deposits dated from ca. 92 ka to ca. 70 ka were definitely formed in the warm climate of interglacial rank. The composition of the spore-pollen spectra indicates that the sediments of this interval correspond to the standard palynozones M6, M7 and M8 [after Grichuk, 1961] or i‒g [after Jessen and Milthers, 1928] of the last interglacial.
Period of ca. 92 ka to 81.8 ka (see Fig. 2) corresponds to palynozone (PAZ) M6 (hornbeam with the participation of oak, linden, ash, elm, Scots and Siberian pine, birch and spruce), which is subdivided into 5 sub-palynozones (SPAZ), reflecting successive changes in the development of the dominant forest formations. The following phases in the development of vegetation in this interval have been reconstructed.
SPAZ M6a –– coniferous-broadleaf (pine-hornbeam-spruce, pine-hornbeam with oak and elm, pine-oak-hornbeam) forests;
SPAZ M6b –– mixed forests, consisting of coniferous (spruce-Siberian pine-Scots pine, spruce-Siberian pine) and broadleaf hornbeam with an admixture of oak, linden, ash, elm communities;
SPAZ M6c –– spruce-Siberian pine-Scots pine forests with an admixture of hornbeam;
SPAZ M6d –– birch-pine forests with hornbeam and oak;
SPAZ M6e –– pine-birch with an admixture of broad-leaved forests.
The sequence of forest successions, the maximum of broadleaf pollen (sum of broad-leaved trees ‒‒ 28% and 29%, Corylus avellana ‒‒ up to 20%), and the presence of broadleaf pollen in the autochthonous complex, which is among the representative taxa of the Mikulino dendroflora ‒‒ hornbeam (Carpinus betulus), linden (Tilia cordata, T. tomentosa, T. cf. dasystyla), oak (Quercus robur, Q. petraea), elm (Ulmus laevis), hazel (Corylus avellana), alder (Alnus glutinosa, A. incana), etc. indicate warm climatic conditions of interglacial rank occurred in the interval between ca. 92 ka and 82 ka. In the interval under consideration, the sub-palynozone M6c, which reflects a significant reduction (up to 6%) in the proportion of pollen of broad-leaved trees, is likely due to endothermal cooling (cold spell) during the warm period.
Sediments accumulated during the interval between 81.8 ka and 70.4 ka, are characterised by two palynozones ‒‒ M7 and M8. Palynozone M7 reflects the phase of the predominance of Siberian pine-spruce and pine-spruce forests. The spore-pollen spectra of PAZ M8 illustrate the vegetation cover of the final phase of the warm period of interglacial rank. At the final stage of this warm period, the investigated area was dominated by birch-Scots pine-Siberian pine forests and sparse woodlands with Siberian pine (Pinus sibirica) as the dominant and meadow-bog communities.
As a whole concerning the studied MIS 5 interval, it is noteworthy that the palynological records obtained on the Late Pleistocene sequence in the VI-05 profile do not contradict the well-known spore-pollen data of the warm Mikulino deposits of interglacial rank in the north-western sector and the centre of the East European Plain, i.e. unambiguously indicate the same warm climatic conditions on the south-eastern coast of the Gulf of Finland during the latter part of MIS 5.
The subsequent interval from ca. 70 ka to ca. 58 ka is characterized by significantly more severe conditions and corresponds to the time when it was assumed that the study area was hidden under the Kalinin/Middle Weichselian (MIS 4) ice sheet. However, as follows from the paleoenvironmental record of the section, the process of sedimentation and the development of organic life has been continued, although in much more severe conditions than in the previous period (MIS 5 d-a) of its heyday. As evidenced by the continuous record, periglacial-tundra and forest-tundra landscapes predominated in this period with a predominance of shrub communities of dwarf birch and alder (Betula nana, Alnaster fruticosus), meadow-bog formations and areas of sparse woodlands of Scots pine, Siberian pine and spruce. The obtained palynological records (PAZ B1-B4) indicate the evolution of phytocenoses of the periglacial type under influence of corresponding climatic conditions in the studied area during the MIS 4 period.
Thus, our studies at the Voka site indicate that, contrary to expectations, neither glaciogenic deposits were found in the MIS 4 interval, which would serve as evidence of the post-MIS 5 glacial passage through the Gulf of Finland, nor the interruption of the sedimentation. The development of vegetation in the study area also did not stop during this period.
Moreover, there has not been identified the expected onset of harsh climatic conditions due to, as believed, the development of the huge Barents-Kara ice sheet in northern Eurasia [see, e.g., Svendsen et al., 2004] in the latter half of MIS 5. On the contrary, our study unambiguously indicates that, contrary to expectations, the sediments of the Voka section studied in the interval from ca. 94 ka to ca. 70 ka, i.e. during the latter half of MIS 5, which are likely should be located in the zone of expected periglacial processes, in fact are characterized by warm climatic conditions that fully correspond to interglacial rank.
The findings obtained during the present study are also convincingly confirmed by those of our previous long-term investigations of warm climate-related shell-bearing deposits of various origins from Northern Eurasia including climatically highly sensitive Eurasian Arctic palaeo-shelf area. These results indicated that the overwhelming majority of the warm climate-related dates (ca. 82%) is also concentrated in the latter half of MIS 5 in the time range between 110 ka and 70 ka [Molodkov, 2020].
Acknowledgements. This research was supported through grants nos. 5440, 6112 and 8425 from the Estonian Science Foundation. The palynological studies of N.S. Bolikhovskaya is a part of GM program “Paleoclimates, the natural environment evolution and long-term prediction of the environmental change”.
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PALAEOENVIRONMENTAL CHANGES BETWEEN CA. 100 KA AND CA. 50 KA AS EVIDENCED BY PALYNO- AND CHRONOSTRATIGRAPHICAL DATA FROM THE SOUTH-EASTERN COAST OF THE GULF OF FINLAND
1Bolikhovskaya N.S., 2Molodkov A.N.
1Lomonosov Moscow State University, Moscow, Russia
2Research Laboratory for Quaternary Geochronology, Tallinn, Estonia
An overview of the results obtained by collaborative use of feldspar-based infrared optically stimulated luminescence (IR-OSL) dated records of pollen-induced palaeoclimate and palaeoenvironmental proxies is presented. Palynological analysis and IR-OSL dates for the first time produced a comprehensive detailed and dated continuous sequence of palynological zones ranging from ca. 100 ka to ca. 50 ka and made it possible to trace climate and palaeoenvironmental evolution in this interval. Representative pollen spectra provided convincing evidence of a warm interval of interglacial rank at least during the latter half of MIS 5 and the period of significant cooling during MIS 4. The findings obtained during the present study are also convincingly confirmed by those of our previous long-term investigations of warm climate-related shell-bearing deposits of various origins from Northern Eurasia including climatically highly sensitive Eurasian Arctic palaeo-shelf area. These results indicated that the overwhelming majority of the warm climate-related dates (ca. 82%) is also concentrated in the latter half of MIS 5 in the time range between 110 ka and 70 ka.
Keywords: MIS 5, MIS 4, IR-OSL dating, chronology, palynology, age and palaeoenvironmental conditions
Ссылка на статью:
Болиховская Н.С., Молодьков А.Н. Эволюция природных обстановок в интервале 100–50 тыс. л. н. по данным палино- и хроностратиграфических исследований отложений юго-восточного побережья Финского залива // Рельеф и четвертичные образования Арктики, Субарктики и Северо-Запада России. 2021. Выпуск 8. C. 26-30. doi: 10.24412/2687-1092-2021-8-26-30