Дата публикации: 19 сентября 2021
Автор(ы): Olga NAIDINA →
Публикатор: Научная библиотека Порталус
Рубрика: ЭКОЛОГИЯ →
Источник: (c) Science in Russia, №3, 2011, C.54-60 →
Номер публикации: №1632053135
Olga NAIDINA, (c)
by Olga NAIDINA, Cand. Sc. (Geol. & Mineral.), RAS Geology Institute, Moscow, Russia
How often did the global climate change in the geological past? The workings of these changes? The latest global warming? These and many other questions are answered by paleobotanists trying to see what the earth's natural conditions, its plant cover and climate were like after the Ice Age, that is over these last 11.7 thousand years.
GLOBAL WARMING: FIGMENT OR REALITY?
Representative of different sciences are closely involved with the global climate and its problems. Paleobotanists among them, too. They reconstruct its dynamics and the dynamics of vegetation in time, over thousands and thousands of years. It is highly important to understand the pattern of these processes: learning about the past, we may look into the future and try to model the probable scenario of natural conditions. Climatic changes look like a natural phenomenon--they have taken place in the geological history of the earth time and again. Like the global warming period in the Holocene* that occurred 9 to 6 thousand years ago. If we turn to more recent times, what seems to be an insignificant temperature rise of 0.6ºC in the Northern Hemisphere during the 20th century touched off changes in the distribution of precip-
* Holocene--the Recent geological epoch that set in after the lce Age about 11.7 thousand years ago and is still on. This is the last stretch of the Quaternary.--Ed.
itation and provoked more frequent floods, typhoons and hurricanes.
There are different viewpoint on this problem*: some see the causes of temperature shifts in the anthropogenic, man-related factor, while others insist such phenomena are of natural origin. According to Dr. Lev Karlin of the Russian State Institute of Hydrometeorol-ogy, the latest warming spell concurs with the 20th-century industrial revolution. Beginning in 1900, a solar activity rise led to a warming of the ocean and soil, and to a discharge of hothouse gases much more than did man and his activities.
Studies of ice samples extracted from deep boreholes in the Antarctic and Greenland confirm the conclusion made at the Chief Observatory in Pulkovo, St. Petersburg: the ongoing global warming is triggered by natural causes related to solar radiation intensity, and man's industrial activity has little, if any, effect. And yet, as Dr. Alexander Kislov, a climate scientist of Lomonosov Moscow State University, sees it, modeling data attest to the statistical vagaries of the present climate and invite an idea that the current rising temperature trend in the Northern Hemisphere might not be connected with natural processes after all.
The arctic regions feel the climate warming impact more than other areas. This is evident in the arctic shelf, the world's largest, taking up nearly a third of the Arctic Ocean. In the eastern part of the Russian Arctic* a great portion of the coast and isles is built of high-ice rock. Acted upon by summertime heat, the ice melts out, causing the shoreline to collapse; thus a vast amount of ground substance, including organic matter, methane and carbon dioxide, lands in the shelf zone. So, all of the coastal area is thought to be a generator of hothouse gases with the ensuing consequences.
The greater part of the arctic shelf zone formed over these last 11.7 thousand years is less than 100 m deep. This means that as late as the Holocene it was 200-300 m closer to the North Pole. Eurasia and America were one continent linked by the Bering bridge (a shallow-water strait 63 km wide today); arctic isles were part of the land, with mammoths and bisons out there just 5 to 3 thousand years ago.
Any next temperature rise and fall in the amount of precipitation would generate a set of ecological problems. Say, a 5ºC temperature rise in the Northern Hemisphere might kill Siberian woodlands that need lower temperatures for their sustainable growth. A per-
* See: Yu. Izrael, "Threat of Climatic Catastrophe?", Science in Russia. No. 4. 2003.--Ed.
* See: Yu. Leonov, "Important Phase of Polar Studies", Science in Russia, No. 1, 2010.--Ed.
mafrost melting in Yakutia and Chukotka would alter the coastal line and account for sea rises and for many other things.
A LAPTEV SEA PHENOMENON
The main formative factors responsible for the climate of northern Eurasia and Western Europe's weather depend on the activity of arctic ice. The Laptev Sea is the main ice "producer" in the Arctic Ocean. A great mass of pack ice formed there owing to abundant fluvial discharges has a significant effect on global climate changes. A transpolar ice drift westward starts out from the Laptev Sea; Fridtjof Nansen, a great Norwegian arctic explorer, naturalist, writer and statesman, was first to see that at first hand.* In 1898 Nansen was elected to the St. Petersburg Academy of Sciences as foreign member. His ship FRAM got frozen into polar floating ice near the Novosibirsk Isles and drifted on northwest as far as Franz Josef Land.
Problems bearing on arctic climate changes and their effect on Europe's weather are the target of national and international research programs. One, "Ecoclimatic System of the Laptev Sea" launched fifteen years ago, is a joint effort of Russian and German scientists.
As found in the course of cooperative interdisciplinary sea and ground studies, the Laptev Sea is a unique natural complex that has no peer elsewhere in the world. New valuable data have been obtained with the use of up-to-date methods, such as biochemical, paleontolog-
* See: V. Markin, S. Khorkina, "Cooperation in the Arctic", Science in Russia, No. 6, 2005.--Ed.
ical and palynological*, supplemented by isotope, radiocarbon dating and space observation techniques.
We might as well note here that bottom sediment studies are important for learning a succession of events and conditions responsible for the arctic paleoclimate: the run-off of big rivers like the Lena, Khatanga, Anabar, Olenek and the Yana impacted glacial, sedimental, biological and other processes. For this reason further
* Palynology (palynologia,--Gr. palýno--broadcast, asperse)--a branch of botany studying plant pollen and spores, their morphology, growth and dissemination.--Ed.
explorations--taking in northern Yakutia, too--may help elucidate certain singularities of the present natural environment and climate of these vast expanses.
Yakutia, for one, is known for its extracontinental climate: the temperature differential between winter and summer seasons may be as high as 100 degrees on the centigrade scale. At Oimyakon and Verkhoyansk, the "pole of cold" in the Northern Hemisphere, the mercury may drop to minus 70 Celsius. But it may be very hot in summer. Such temperature differences are a unique phenomenon. The permafrost layer in some of Yakutia's regions may be as deep as 1,000 meters. The land next to the Laptev Sea is a hillocky tundra plain crisscrossed by sedge and cotton-grass bogs.*
The Lena is the biggest river flowing into the Laptev Sea. Its delta is most impressive. Its mouth is an endless succession of swamps in the shape of regular polygons. The southeast of the estuary has moss- and lichen-overgrown cliffs here and there, some up to 250 meters high. The delta proper is rich in fertile, generous soil on which many plants grow. Each year they are succeeded by other plants of the same kind due to the sagging of suspended material brought in by freshets. The plant remains and deposits of floated timber on the bottom are a rich source of organic carbon.
Continuous weather observations and all-round study of paleoclimates, including those of the Holocene**, are
* See: G. Rusanova, "Bolshezemelskaya Tundra (Cold Desert): Flashback", Science in Russia, No. 1, 2007.--Ed.
** See: A. Herman, "'Extinct' Climate of the Arctic". Science in Russia, No. l, 2007.--Ed.
needed for an accurate warming prediction. As said above, the Holocene set in 11.7 thousand years ago, and it concurs with the Interglacial period still on, and global warming.
There exist several methods for learning the succession of events of the distant past. One major method is that of reconstruction, palynological reconstruction in particular, involved with microscopic plant remains, such as pollen grains and spores of higher plants. Growing in huge amounts (in hundreds of thousands and millions) in generative plant organs, such grains are broadcast and disseminated, sometimes over very large areas. Getting onto surface soil or into sediments, a considerable part of pollen and spores is buried and becomes fossile. The matter contained within is destroyed, and what is left over are just husks of pollen and spore grains. Be that as it may, the reconstruction method is highly informative with respect to the flora and dominant types of vegetation, their change in course of time. It allows to rehabilitate the landscape and climatic features of the past.
It is climate that evolves as the main formative factor for land plants that respond to all temperature and humidity variations. Changes in natural conditions affect the taxonomy of phytocenoses and cause significant migrations of vegetation zones. The surviving material bits of evidence of these processes are good as indicators of such changes of interest to us. The very position of the northern boundary of the woodland zone carries all-important paleoclimatic information: it coincides with the mean month-of-July position of the polar atmospheric front on the border between the air of polar (temperate) and tropical latitudes. Warming periods that caused woodlands to move farther north had a mighty
effect on the radiation balance.* During the Holocene the variations of this border in Siberia reached hundreds of kilometers. Accordingly, the area of perpetually frozen ground (permafrost) changed as well, and so did the amount of atmospheric precipitation and fluvial flow conditions. Again and again: palynological evidence registers also transitions of plant zones in time.
Pollen showers over sea waters show that their minute biological particles carry cumulative data on the plant
* Radiation balance-the difference between incoming and outgoing solar radiation on terrestrial surface; it depends on the sun's position (height) in the sky at a particular time of day and year, latitude and other factors like surface albedo (reflectance), atmospheric transparence and so forth. Measured in W/m2.--Ed.
kingdom in adjacent lands and, consequently, on the climate of large areas. In the Arctic's eastern sector the joining of two elements, "land and the Laptev Sea shelf, is realized through the above-mentioned big rivers. Therefore their subwater run-off has a great part to play in the formation of pollen marine spectra. The great Lena and Yana rivers carry off a huge amount of dissolved and suspended particles (plant pollen as well) building up on the sea shelf. As demonstrated by our initial studies, pollen spectra in shelf sediment samples reveal reliable information on changes in the land plant cover off the Laptev Sea under the effect of climatic factors.
Due to the significant icing of the Laptev Sea, the local land areas are remarkable for treeless landscapes. Yet trees, shrubs and grasses are pollen-prolific and, in cor-
responding bottom sediment spectra, such pollen residues are proof positive with respect to Yakutia's flora and thus can well be used in paleoclimatic reconstructions.
We have examined several core samples recovered between 1993 and 2000 from Holocene deposits by joint Russian-German Transdrift parties within the framework of the Ecoclimatic System of the Laptev Sea project. Proceeding from numerous radiocarbon datings of spore samples, their calendar age was determined by Dr. Henning Bauch, IFM-GEOMAR, Kiel, Germany, using proximate mass spectrometry.
Sediments of the early and late Holocene are best represented in the recovered samples. Core samples from deposits built up 11.3 to 5.3 thousand years ago show an alternating pattern of the basic types of vegetation in land bordering on the sea, with predominant tundra herbage, and dwarf shrubs, or else trees and bushes. Most abundant was the pollen of the dwarf arctic birch Betula sect, nanae, Duschekia fruticosa as well as grains of Pinus and the prostrate pine Pinus pumila. The pollen of sedges Cyperaceae and Poaceae grasses were most common in the group of grasses and dwarf shrubs. Likewise common were grains of the Ericaceae heather, Caryophyllaceae cloves, Asterceae composite plants and other species of the tundra herbage. Now and then came solitary grains of the valerian Valeriana, willow-herb Epilobium and sorrel Rumex. Sporophytes are represented by Sphagnum and Bryales moss species. Spores of the cold-demanding Selaginella rupestra are quite common there.
Judging by the makeup of spore-pollen spectra, the plant kingdom in the early Holocene was of the arctic
tundra type. The climate was close to what we are having now. Alternating pollen maxima of conifers and dwarf birches were registered higher up. Tundra dwarf shrub landscapes change to those of the forest-tundra with pine and Pinus pumila here and there. The forest-tundra came closer to the sea coast, and the climate must have been warmer than it is today. By our data the previous climate warming spell must have occurred 9.3 to nearly 8 thousand years ago.
Palynological assays of another core sample (with sediments accumulating over the last 9.7 thousand years) show that the growing concentration and amount of pollen grains 6 to 7 thousand years ago concurs with an increase in organic carbon content. This increase, coming with the higher air temperature, coincides with the contraction of ice in the Arctic Ocean. In the high-latitude Arctic the air temperature was 2 to 4ºC higher than nowadays. It is quite probable that arborescent pollen was carried on by wind from small sylvan patches. Such islets of trees were azonal ecogeosystems, i.e. not characteristic of the territory, like it is with the world's northernmost woodland in the isolated terrain locality of Ary-Mas in the lower reaches of the Khatanga, or the groves on the islands of the river Undiulug, a right tributary of the Lena. It has been established that taiga forest vegetation could be carried into the tundra plains through gas-emitting spaces of tectonic discussion that gave rise to river valleys.
Overall, the thus reconstructed vegetation exhibits a mosaic distribution pattern within one and the same tundra zone. The probable cause of the emergence of such azonal patches as Ary-Mas resides in a peculiar geological medium of mineral alimentation and in geot-hermal anomalies of the terrestrial surface. Zonal temperatures are a major parameter for the appearance of plant communities. The gas exchange between them and ambient surface air results in an increase of a sum total of active temperatures, i.e. in a longer growing season. It is common knowledge that processes implicated in nitrogen and carbon dioxide assimilation and binding are quite sensitive even to minor temperature changes. A temperature increase or decrease, no matter how small (by fractions of a degree on the centigrade scale, let alone 1-2ºC) alters the plant kingdom composition.
The greater export of conifer pollen (mostly from Pinus and the prostrate Pinus pumila) began 9 thousand years ago possibly as a consequence of woodland encroachment toward the sea coast in the wake of a warming spell ending 3.8 thousand years ago. Comparative studies of regional ground data show the higher transfer of tree pollen to the sea shelf concurs with the movement of the forestland boundary northward.
So, the beginning of the Holocene climatic optimum (the warmest time span) for the arctic eastern regions dates to 8.9 thousand years, and its end, 5.5 thousand years from now. The amount of atmospheric precipitation was above the present-day level, and air temperature was about 3ºC higher than today.
The author of the present article would like to thank Drs. Heidi Kassens and Henning Bauch (IFM-GEOMAR, Kiel, Germany) for the materials they have kindly provided to us as well as for radiocarbon datings and support.
Опубликовано на Порталусе 19 сентября 2021 года
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