PHYTOLITES AS POSSIBLE BIOMARKERS IN RECONSTRUCTION OF VEGETATION COVER AND PALEOCLIMATE

PHYTOLITES AS POSSIBLE BIOMARKERS IN RECONSTRUCTION OF VEGETATION COVER AND PALEOCLIMATE

Natalya Leusova

Candidate of biological sciences, scientific secretary, Institute of Geology & Nature Management FEB RAS,

Russia, Blagoveshchensk

At present, along with spores and pollen, phytoliths are widely used for stratification and dating of vegetation of past eras. However, data on phytoliths of coal deposits, according to the information available to the authors, are not presented in the literature. The use of an integrated approach, including the determination of morphostructural features, chemical composition, and biomarkers, makes it possible to fairly reasonably determine the main types of coal-forming plants that produce phytoliths, reconstruct regional aspects of environmental change, and identify the manifestations of global and regional paleoecological events that took place during the Cenozoic. In addition, detailing the species composition of the flora and the situation of its burial in the future may become the basis for substantiating the localization of precious and rare metals in coals.

The sampling of coals and host rocks of the Sergeevskoe lignite deposit was carried out in four separate sections from the top of the seam to its soil at the points of artificial and natural outcrop of coal seams to the surface. The study of the fine structure of coals was carried out by optical (Axio Scope A1) Zeiss) and electron microscopy (JEOL JSM-6390LV with an analytical microanalysis system Oxford INCA Energy 350Wave (England). Dominant morphotypes are shown in Figure 1. The amount of data obtained on the microstructures of phytoliths makes it possible to identify their characteristic groups. We have identified 7 categories of morphotypes according to the classification of Fredland and Tieszen (1), which are produced by the subfamilies Panicoideae, Pooideae, Cloridoideae. In the ensemble of phytoliths, there are also morphotypes that are rather difficult to describe, and they are probably represented by dicotyledonous plants. The 4 main morphotypes of phytoliths of monocotyledonous plants that we have identified: the panicoid type, serrated rods (spiny cells), are characteristic of the subfamilies Panicoideae (Millet) and Arundinoideae (Reed). The panicoid type of phytoliths is produced by C4 plants (photosynthetic pathway) that grow in warm and humid climates.

Figure 1. Morphotypes of phytoliths from brown coals of the Sergeevskoe deposit (mark 5 µm)

The subfamily Pooideae (Bluegrass) is represented by such dominant morphotypes as crenate cells. Phytoliths of such type as saddle-shaped forms are characteristic of the subfamily Chloridoideae. According to the works of Twiss (2) and a number of other researchers (1), the complex of plant phytoliths reflects the paleoclimatic changes in the area in terms of the distribution ratio of C3 and C4 plant types (3). These authors introduced calculations of the temperature coefficient and humidity index:

Temperature Coefficient (TC) = Pooid/(Pooid+Panicoid+Cloridoid)×100                                                              (1)

Large values indicate a cold climate at high latitudes or altitude (where C-3 plants predominate), low values suggest a warm climate at low latitudes or lowlands. Similarly to this coefficient, the humidity index is also introduced as the ratio of chloridoid to the sum of chloridoid and panicoid phytoliths:

Moisture Index (HI) = Cloridoid/(Cloridoid +Panicoid)×100                                                                    (2)

Values close to 100 suggest an arid climate, while low values indicate a very humid climate. We obtained the following ratios of the distribution of types of coal seam phytoliths: the panikod type of phytoliths showed a high relative abundance (≈70%), while bluegrass and chloris phytoliths were ≈20% and 10%, respectively. Based on these data, the calculated values of the temperature coefficient and the humidity index are derived, indicating in favor of a very warm and humid climate:

ТC= 10/(10+70+20)×100= 10; MI=10/(10+70)×100= 12,5                                                                    (3)

The morphology of the phytolith types themselves - a large number of elongated forms - is evidence of the abundance of plants in humid habitats. A stable position among plant communities was occupied by angiosperms (monocots), the presence of which in these deposits is also evidence of the youth of the coal seam, since the isolation of cereals, with the formation of current taxa, occurred in the Paleogene or Neogene, while tropical plants that evolved parallel to their adaptation are considered ancestors to habitat conditions (4). According to a number of authors (Sorokin A.P., 1972(5)), the composition of the Miocene flora was represented exclusively by coniferous - broad-leaved forests (dominance of tropical and subtropical flora), coniferous, mainly pine, dominated in the watersheds. The abundance and composition of cereals confirms that the formation of the coal seam took place in lowland bogs under conditions of a humid and warm (humid) climate, since the accumulation of silicon by cells in an arid climate, as a rule, does not occur. The data obtained by us are in addition to those previously available and indicate the role of cereal plants in the process of coal formation, which is presented for the first time.

Thus, the main types of plants - coal-formers - producers of phytoliths have been identified: plants of the subfamilies Panicoideae (Millet) and Arundinoideae (Reed); the regional aspects of environmental changes are reconstructed, - it is concluded that the formation of the coal seam in the Miocene period - early or middle Miocene - since the climate became much colder in the later period, took place in a humid warm (humid) climate. Phytoliths can be considered as paleogeographic markers, which makes it possible to use them to understand the trends in the development and genesis of the protolith in a given period.

In addition, information about the presence of phytoliths in modern Far Eastern plants and paleosols established during archaeological research, which indicates their wide distribution in the past and the possibility of long-term preservation in a variety of landscape conditions.

Exploring the problem of stability, L.E. Novorossova (1951(6) noted that in podzolic soils, under acidic conditions, the dissolution of phytoliths and the removal of silicon cannot be significant and, over time, phytolith silica accumulates in the soil profile in significant amounts. CM. Kutuzova (1970) (7) when studying the impact of microorganisms - acid-forming and alkali-forming on biogenic silica, the resistance of phytoliths in an acidic environment (PH<7) was registered. At an alkaline pH value (pH> 7), their dissolution and silicon mobilization occur. The dissolution process is enhanced with an increase in pH, and the duration of the alkaline conditions of the environment also plays an important role. In addition, geodynamic factors influence the distribution and composition of siliceous formations, including plate movements that determine paleogeographic settings and the supply of nutrients. Possibly, favorable conditions exist in uplift zones for both high production and preservation of biogenic opal in sediments. Oxidizing environmental conditions also contribute to the high preservation of phytoliths. All this can serve as indirect evidence of the uplift of the basin in the acidic oxidative nature of the environment, which determines the high preservation of biogenic silica from the Sergeevskoe lignite deposit, the formation of which is associated with one of the stages of coal accumulation within the Zeya-Bureya basin, determined by the interval of 23-11.6 million years (Sorokin A.P. (5)). Thus, the presence of phytoliths of angiosperms in the coals of the Sergeevskoe deposit indicates a wide age range of its existence (from the Miocene to the Holocene) with a high degree of preservation of its forms.

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