Дата публикации: 07 октября 2021
Автор(ы): Mikhail KUZMIN, Galina KHURSEVICH →
Публикатор: Научная библиотека Порталус
Рубрика: ЭКОЛОГИЯ →
Источник: (c) Science in Russia, №3, 2012, C.56-64 →
Номер публикации: №1633596530
Mikhail KUZMIN, Galina KHURSEVICH, (c)
by Acad. Mikhail KUZMIN, Director of the Vinogradov Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences (Irkutsk); Galina KHURSEVICH, Dr. Sc. (Biol.), Maxim Tank Byelorussian State Pedagogical University (Minsk, Republic of Belarus)
Unicells include a stand-out group, the diatoms. They lead a solitary life or associate in colonies in the form of strands and chains, tubules and stars, rhipidia and clusters, teniae and films. Their single cell is enclosed in a silicon frustule. Its shape is most intricate and fanciful, and its structure is so nice that it looks like a creation of a master artist.
ROLE OF FOSSIL DIATOMS
Diatom images were first pictured 300 years ago when a light microscope was invented by Antony van Leeuwenhoek. The first detailed observations and study of diatom cell and frustule structure were conducted throughout the 19th century. A qualitatively new level of research began as of the mid-20th century owing to the use of transmission (fluorescent) and scanning (reflecting) electron microscopes which provided new valuable information on the frustule ultrathin structure in present-day and fossil diatoms.
The role of diatoms in solving biostratigraphic, paleolimnological and paleoclimatic problems is generally recognized. Silicon frustules and valves of diatoms can endure in rocks under favorable conditions for millions of years as carriers of information on the biological diversity of the past geological epochs, sedimentation time and conditions. Drilling of basin bed deposits is carried out for such purposes with a subsequent a layer-by-layer study of the species composition of petrified fossil diatoms bottom-upwards through the whole sedimentation section exposed by a core sample; thus the
change characteristics in their qualitative and quantitative composition are established. Proceeding from the information thus obtained, the levels of emergence or extinction of individual species/genera are identified as well as the levels of proliferation or drastic contraction in the number of any particular taxa weighed against the age model of the sedimentation section. Thereafter, the identified levels are compared with the known events characterizing climate changes on the planet as a whole or in a particular region. This allows to clear up the causes of extinction and also changes of individual taxa of diatoms in a particular ecosystem.
Concerning paleoclimatic "signals" related to "the messages from the past", it is well to bear in mind that data on changes in the diatom composition and in the number of valves cannot be transformed by any mathematical formulas to climate parameters, such as average annual temperatures, total precipitation and so on. However, it is quite real to conclude on a relative climatic variability in any particular epoch. Thus, we can learn in which period (glacial or interglacial) definite layers of bottom sediments of Lake Baikal were accumulated. Evaluation of the stability of the natural environment and climate conditions is another important aspect of paleoclimatic analysis. Besides, the boundaries of essential changes in the composition and number of algae are indicative of considerable changes of their habitat. Detailed records on diatoms help scientists define the general tendencies and particular paleoclimatic events as well as their age on the geological scale.
BAIKAL DRILLING PROJECT
The international Baikal Drilling Project (BDP) realized in 1991-2006 dealt with the causes of climate change in Central Asia. A look at its background history.
In 1989, on the initiative of the geologist and oceanologist Leo Sonenshein (RAS Corresponding Member), the program "Deep-water Ecology, Paleolimnology and Geodynamics of Lake Baikal" was drawn up; it got support from Acad. Valentin Koptyug, head of the RAS Siberian Branch, his deputy Acad. Nikolai Dobretsov and the RAS President Acad. Gury Marchuk. The program suggested a new stage of geological, geophysical and submarine studies of Lake Baikal.
The same year at the International Geological Congress in Washington, D.C., American specialists, above all professor of the University of South Carolina Douglas Williams, offered a joint US-Russia study of global changes in the natural environment and climate of Central Asia based on deep-water drilling at Lake Baikal. (The interest of foreign researchers in this region is not accidental. It was known then that obtaining continuous paleoclimatic records in the Northern Hemisphere was possible only at Lake Baikal.)
The technical support of works was undertaken by the Nedra R&P enterprise of the USSR Ministry of Geology. In 1992, Japanese scientists of the University of Kyoto, Japan (Professor Shoji Horio) and of the Baikal Association (Dr. Takaeshi Kawai) joined the project. At that time the Russian Federation Ministry of Science and Technologies also supported this work.
In 1992 and 1993, members of the South Branch of the Shirshov Institute of Oceanology (RAS) and participants of the expedition jointly with the United States Geological Survey carried out multichannel seismic investigations and assessed the structure and thickness of the Baikal sedimentary formation. In the course of the implementation of the program, the bottom of the lake was studied by the manned Pisces submersible vehicles. Besides, the drilling facilities were installed on a barge, which, together with a tug boat, was to be frozen into ice in the performance of operations.
Prior to the start of the drilling the scientists determined more exactly the thickness (up to 8 km) of the lake sedimentary mantle and established the rhythmicity of the sedimentary framework determined by alternation of beds enriched with diatom valves formed in warm interglacial periods, and also beds formed by terrigenous (clastic) clays deposited in cold glacial periods.
The first version of the drilling facilities was mounted on a 400t barge. The installed equipment allowed to drill 200-300 m wells under the 300-400 m water layer, and that was done during the 1993 and 1996 field expeditions.
In 1997, the Baikal-2000 drilling facilities were mounted already on a 1,000 t barge. During the expeditions, which took place in the same and in two subsequent years, it became possible to drill wells up to 1 km deep with the water depth up to 900 m. Core receivers up to 6 m long (but not 2 m long as it was before) were used, which resulted in a marked gain in the speed of drilling. In the end, the core recovery made up 95-98 percent, which was assessed as a breakthrough by specialists of the international programs of oceanic and continental research drilling.
On the whole Lake Baikal consists of three deep depressions separated by underwater rises. For example, the Northern Depression is separated from the Central Depression by the Akademichesky ridge, and the Central Depression is separated from the Southern Depression
Bathymetrical chart of Lake Baikal. Dots denote the sites of wells.
by the Selenga-Buguldeika Divide. The very first drilling was performed in 1993 at this divide approximately 6 km southeast of the Buguldeika river estuary at 52°31'05" N and 106°09'11" E at a depth of 354 m.
Another attempt was realized in 1996, this time on the Akademichesky ridge rise at 53°41'48" N and 108°21'06" E in the water thickness of 321 m. Two wells were drilled at a short distance, namely, BDP-96-1, 300 m deep (core sampling was performed only to the 200 m depth mark) and BDP-96-2, 100 m deep.
In 1998 drilling on the Akademichesky ridge was carried at 53°44'48" N and 108°24'34" E located 5 km north of the above two points of BDP-96 drilling. The depth of the BDP-98 well was 670 m under a water column of 333 m. A complete core recovery was performed down to the mark of 600 m, and the core recovery amounted to 90 percent.
The next year drilling was conducted at 52°05'23" N and at 105°50'24" E, where the water depth was 205.56 m and that of the well 350.5 m. This place fits the so-called Posolsky Bank, where deposits come largely from the suspended material brought in by the Selenga. Here high-resolution paleoclimatic data were obtained, which reflected the ecology of the Selenga river basin in the very distant past and provided ample information on the history of the Baikal Depression.
By and large, in 10 years of the Baikal Drilling Project five drilling parties were undertaken that sank seven wells 40 m to 630 m long and recovered bottom sediments more than 1,500 m long and above 8 mln years of age. The resolution (chronological accuracy) of the obtained records is high and makes up from 20 to 200 years in one centimeter of the sample core. The total amount of the evidence thus obtained was so great that it took more than 10 years to sort it out and process: this work is still on.
It should be noted that access to the drilling site was not a pleasure trip. The floating platform was brought to this site at the very beginning of a freezing period, and therefore it could find itself in hummocking areas. In this connection, we take the liberty to digress a little from our narration and cite a description of one of the authors of this article, Mikhail Kuzmin of a situation, when the drilling facilities get into a dangerous zone like that. "Suddenly some roar was heard. Puzzled, I ran to the command bridge. Complete silence in the deck house, nobody spoke, but the roar was growing. It was clear that the ice drift started with the formation of ice hummocks. The scene was striking! At the starboard side there appeared cracks in the ice filled with water, though there was solid ice there several minutes ago... Blocks of ice coming ever closer and piling up. Separate blocks of compact ice of up to one meter and more high were thrown on the sheet ice like small cubes and formed hummocking zones. Big cracks gave rise to transversal and slanting cracks. Ice blocks were piled up just in these cracks. It seemed that the whole ice around us was moving. Massive ice blocks, 5-8 m x 4-5 m large and 20-30 cm thick, plunged easily under or piled up on the icefield blocking their way. A grand and formidable scene... Such outburst of nature continued only about 45 minutes. Everybody on board, the crew and the expedition members, stood by the boards on, the bow or on the command bridge and watched silently that scene. We were on a small ship among a living and moving ice, which broke and piled up, forming hummocking fields. Such ice knows no bounds, it can crush everything. One feels insignificant and
unprotected in ice captivity, helpless in the face of nature and realizes that it is not to be trifled with."
But back to the point. The results of studies of fossil diatoms in the deep-water cores of Lake Baikal obtained in the past 15 years are of global significance. As mentioned earlier, Lake Baikal is located at high latitudes where variations of solar radiation are manifested especially clear, and seasonal climatic variations are well expressed. The continuity and depth of drill cross sections (accumulation of a 600 m sedimentary mass took place during the last 8 mln years) were likewise important. We should also point to a detailed testing interval (every 2 cm for the drill cores of BDP-96-1 and BDP-96-2, and every 5-10 cm and seldom 15 cm in the core of BDP-98). And one more thing. It became possible to achieve a high core recovery (90-95 percent) and an exact age control of the extracted deposits. The obtained material was studied using a package of methods (diatom analysis, distribution of biogenic silica in cross sections, palynological data and also indices of the mineralogical and chemical composition of sediments, etc.), which helped interpret the global and regional climatic changes in the region. The obtained paleoclimatic records can be considered to be model ones for Asia and, on the whole, for the continents of the Northern Hemisphere.
In the course of work sedimentary columns were opened up, containing a unique record of diatom evolution, and thereupon the age span of extinct endemics and relicts was established; also, changes in the morphological characters in time were traced, and the supposed phylogenetic links of separate genera and species were identified.
As a whole, the Upper Cenozoic* sediments of Baikal revealed a rich composition of plankton diatoms of the Coscinodiscophyceae class represented by 16 genera covering 81 species and 22 intraspecies taxa. The following is new to science: one endemic family (Thalassiobeckiaceae), three Baikal endemic genera (Mesodictyopsis, Stephanopsis, Thalassiobeckia), 42 species and 6 intraspecies taxa belonging to the extinct ones. Traced were the emergence, efflorescence and extinction of diatom plankton species due to multiple changes both in the environment and in paleoclimate.
EVOLUTION OF DIATOMS IN THE LATE MIOCENE
The Upper (Late) Miocene** deposits were opened by the BDP-98 as deep as 600-286 m. Their age is 8.00 to
* Cenozoic era is the youngest in the geological history of the Earth, it covers also the recent epoch. It started 66 mln years ago and is divided into the Palaeogene, Neogene and Quarternary (Anthropogene) periods.--Ed.
** Miocene is the lower subdivision of the Neogene, which set in ~23.5 mln years ago and continued approximately for 20 mln years.--Ed.
5.32 mln years. Eight biostratigraphic zones were identified on the basis of changes in the species composition of diatoms and their number. While the lower part of this well log (600-492.32 m) is characterized by a relatively low content of diatom valves and prevalence of shallow-water bottom-living forms (Staurosira, Fragilariforma, etc.) on some of its levels, the overlying deposits (492.32-286 m) reveal a substantially higher number of plankton diatoms in the form of extinct representatives of the Concentrodiscus, Actinocyclus and Mesodictyon genera and also a considerable content of the Aulacoseira ancient species as well as the intraspecific variety and variability of the dying out Cyclotella iris species.
The diatomic analysis and the lithological characteristics of the interval in question (tendency towards a considerable decrease in the content of gravel, sand interlayers, plant deposits and parts of wood towards the upper boundary of the 600-286 m depth of deposits) indicate that these deposits were formed initially in the coastal zone of the ancient basin under the conditions of a large inflow of terrigenous material of the ancient paleo-Barguzin river and their gradual substitution by fine silts due to the extension and further deepening of the basin in the region of the underwater Akademichesky ridge just where the well was sunk. The extinct Mesodictyopsis genus represented by 7 endemic species evolved actively at the time interval of 6.6 to 4.8 mln years.
EVOLUTION OF DIATOMS IN PLIOCENE
The first data testifying to an obvious relationship between changes in the species composition and the content of diatom valves, and the changes in paleoclimate of East Siberia in the last 2.5 mln years were obtained and published already in 1998. The more detailed studies on diatoms in a 200 m core sample (well BDP-96) were presented later. The age of this stratum base makes up approximately 5.4 mln years. A complete Pliocene* section was made only by the well BDP-98. Besides, its deposits were divided into 13 biostratigraphic zones reflecting the main biological and paleoclimate events which had taken place in the ancient basin and the Baikal region. The extinction of the Mesodictyopsis species and the consequent proliferation and extinction of a set of the Cyclotella iris forms correspond probably to a long cooling wave at the beginning of the Pliocene (5.3-4.7 mln years).
The emergence, proliferation and extinction of the Tertiariopsis species in combination with the Aulacoseira mass ancient species took place under the conditions of
* Pliocene is an epoch of the Neogene period. It began about 5.3 mln years ago and came to a close about 1.8 mln years ago.--Ed.
a climatic optimum (4.7-4.44 mln years). Domination of the Stephanopsis new genus together with the Aulacoseira ancient species reflects a long postoptimal span of favorable conditions in the paleo-basin (4.44-3.5 mln years). The monodominant development of a new and later extinct Stephanopsis costatus endemic and the temporary disappearance of the Aulacoseira ancient species from the lake plankton correspond to the onset of a significant cooling at the time interval of 3.5-2.85 mln years. Besides, a sudden proliferation of the small short-living Tertiarius and Stephanodiscus species followed by a short outburst of the new and subsequently extinct Thalassiobeckia genus testifies to a short-time improvement of the paleoecological conditions in the ancient basin against the background of the progressive climate cooling of the climate in the second half of Middle Pliocene (2.85-2.64 mln years).
The complete absence of representatives of three extinct genera Stephanopsis, Tertiarius and Thalassiobeckia and the emergence of the new and later extinct Cyclotella species in the Baikal plankton reflect dramatic changes in the environment of the paleo-basin during the most substantial cooling of the local climate (2.64-2.46 mln years) caused, most probably, by the development of the earliest mountain glaciations in this region about 2.5 mln years ago. Finally, the proliferation of the two subsequently extinct Cyclotella distincta and Cyclotella tempereiformica endemics, whose quantitative content in the plankton of the ancient lake changed repeatedly, characterizes a response of its ecosystem to periodic variations of the climate at the time interval of 2.46-1.81 mln years.
It should be noted that in the Early and Middle Pliocene (5.32-2.5 mln years) four diatom genera emerged. They grew actively and died out (initially Tertiariopsis, then Stephanopsis, later Tertiarius and Thalassiobeckia), which attests to the high evolutionary rates of freshwater plankton diatoms.
DIATOMS IN PLEISTOCENE AND HOLOCENE
Detailed studies of the Pleistocene* deposits stripped most fully in the BDP-96-2 well log at a depth of 72.8-0.11 m made it possible to divide them into 43 diatoma-ceous zones reflecting the glacial-interglacial rhythmicity of the climate. Such contrast conditions of the natural environment caused frequent and major changes in the lake ecosystem.
* Pleistocene began approximately 1.8 mln years ago and ended about 10,000 years ago. Characterized by a general cooling and periodic vast continental glaciations in the middle latitudes.--Ed.
For example, at the beginning of Early Pleistocene (1.81-1.51 mln years) the small population and sporadic development of actually one dominant and later extinct Cyclotella comtaeformica species in the ancient basin are indicative of a substantial cooling. During the next period (1.51-1.25 mln years) the development of this species came to an end with the emergence of the Aulacoseira and Stephanodiscus species in small numbers showing yet another change in the lacustrine environment.
A comparative study of the composition of diatom communities typical of the subsequent warm and cold intervals the Pleistocene allows to identify four periods in their evolution. Each had a certain level of stability determined by changes in solar radiation. The boundaries of the periods concur with sharp changes in the climatic and paleoecological conditions of Baikal and, consequently, with substantial structural changes in diatom communities. Their onset agrees with certain sea oxygen-isotope stages (due to the differences between oxygen isotopes, they show warm and glacial periods).
The period of 1.25-0.612 mln years ago is notable for intensive species formation and extinction within the Stephanodiscus genus. The age span of its individual representatives does not exceed 13-15 thous. years, which speaks for their dependence on a rapid change in the natural environment. Compared with Stephanodiscus taxa, the extinct Cyclotella praeminuta species emerged and developed in the ancient basin for a longer period (about 170,000 years).
The time span of 595-365 thous. years, when the amplitudes of Earth insolation changes were insignificant, is characterized by the abundance of diatoms in Baikal and the stable, actually continuous growth of the plankton community represented mainly by the extinct Stephanodiscus distinctus et var. excentricoides endemic. Besides, this time interval saw the first appearance and a maximum in the population of a new species, Cyclotella minuta.
The period covering 342-74 thous. years is remarkable for the emergence, proliferation and extinction of species in the Stephanodiscus grandis group and also for the first appearance and a population maximum of the new Baikal taxa-Cyclotella baicalensis, Cyclotella ornata and Aulacoseira baicalensis, as caused by greater changes of insolation amplitudes.
At the end of the Late Pleistocene and Holocene* (58 thous. years ago to the present time) the modern phytoplankton flora of the lake was formed. Since then Aulaco-
* Holocene is a post-glaciation epoch embracing an unfinished span of the Quarternary (Anthropogene) period of geological history. It began about 11,000 years ago.--Ed.
seira baicalensis and species of the Cyclotella minuta group have been dominant as part of the diatomaceous flora; they are still there.
In the first half of the Holocene an extinct species closely related to Stephanodiscus flabellatus developed in the lake. Besides, the qualitative diversity of the diatom community increased owing to the Aulacoseira skvortzowii, Cyclostephanos dubius, Stephanodiscus inconspicuous species notable for high population numbers.
Thus, whereas the evolution of plankton diatoms of the Coscinodiscophyceae class in the lake during the Late Miocene and Pliocene was characterized by the emergence and extinction of both species and genera, it took place in the Pleistocene mainly on the species level.
Baikal is a unique object for studying processes implicated in endemic species formation. The boundaries of the extinction of the majority of species in the lake agree with the boundaries of the onset of cold (glacial) periods and characterize significant ecological changes. Extinction of the dominant diatom species during glaciation periods vacated niches domesticated quickly by new taxa. Frequent climatic changes (and, consequently, also of the paleoecological conditions of the environment) in the Pleistocene contributed to a rapid evolution of plankton diatoms in the lake (above all, the Stephanodiscus species). A close relationship between the climate and species formation demonstrates control of the former over the evolution of diatoms and their diversity.
In its details, its exact age model and duration (8 mln years), the diatom record of ancient Baikal deposits has no parallel in the world. It may well be used as a standard for interpretation of other continental records, including the "chronicles" of lakes, in for drilling projects. Besides, the data obtained by the field expeditions provide a basis for climate comparisons in oceans and on the continent.
Опубликовано на Порталусе 07 октября 2021 года
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