Organic Matter of Buried Soils as a Paleoclimatic Archive of Holocene.// Advances in Natural organic Matter and Humic Substances Research 2008-2010, XVтезисы доклада
Дата последнего поиска статьи во внешних источниках: 29 мая 2015 г.
Аннотация:Global warming and impending sea-level rise have attracted increasing interest in regional climate changes and in the methods of their investigation. The soil cover of the extensive territories of Eurasian is represented by polygenetic Holocene soils along with soils buried by late Pleistocene and Holocene loess. Specific features of these soils are due to the action of elementary modern and relic soil processes, whose age and intensity varied under influence of glacier movements and climatic fluctuations. These properties could help to elucidate conditions of soil formation during the Holocene and, as well as, soil organic matter can be used as indicator of paleoclimatic changes. Soil organic matter is the more informative paleoarchive of Holocene climatic events, since it may be dated by the radiocarbon, and isotopic curves may be obtained for soil humic substances.
Based on the radiocarbon chronology and soil organic matter properties the following major climatic events can be reconstructed: a glacial conditions during the late Pleistocene, a dry and cold climate during the early Holocene, warm and dry conditions of soil formation in the middle Holocene, and a moderate humid climate of the late Holocene.
For example, the cold and dry climatic period with mean annual temperature about 5-7 0C above 0, with little alpine desert vegetation of C-3 type and slightly weathered interstadial soils can be reconstructed based on the δ13C data at 15000 years BP. At the 8-6 kyr BP, we reconstructed the warm and dry period with mean annual temperature about + 15 degree above 0, when peats spread out in the piedmonts and lowlands due to glacial melting. Meadows with C-3 type of plants developed, based on δ13C values ranging from –19 to –29 ‰.
Low δ13C values in humus may indicate a climate regime with a contrasting dry and wet season. Based on the low aromatisity of humic asids in this horizon – about 20 %, low values of the humification index and the coefficient of extinction, proportions of lignin phenols we can assume, that it was Cambisoils under dry juniperus forest in Tian-Shan mountains.
In contrast, in humus horizon with radiocarbon data 10 000 y BP on the altitude about 1000 m in Caucasus we detected mean values of the humification index, the spectral curve slope with the color coefficient Q>4. The results obtained provide some evidence for the existence of Leptosoil Lithic and Leptosols Humic under the wet alpine meadows and warm and wetter climate of early Holocene.
The Holocene optimum events with a warm and dry climate, air temperatures higher than at the present, a C-3 type of vegetation on the Chernozem soils can be reconstructed at 6-4 kyr BP in Tian-Shan, Caucasus and Russian plane. A thick black humus horizon of a relic soils provides evidence for a warmer and drier period than at present. IR specters of it is rich by amids and Hydrogen. Very high values of the humification index, high values of the coefficient of extinction, and the spectral curve slope with the color coefficient 3<Q<4 are intrinsic to the organic matter of buried horizons. The latter being indicative of a more complex structure of humic acids rich in benzoid compounds typical for mountain Chernozem profiles. The middle Holocene landscapes of Russian plane were presented by steppe.
Favorable and relatively stable climatic conditions of Middle Holocene gradually changed to the Late Holocene. Decreasing of δ 13C values in humus, the traces of trees phenolic compaunds in soil humus, optical properties of humus acids detect more wet conditions than in Middle Holocene. The profiles of Chernozem Mollic can only be subdivided into upper and lower parts, which are assessed as recent and early Holocene, respectively. The criteria to subdivide the profile are the values of the color coefficient (Q > 4 and 3< Q < 4), the coefficient of extinction (0.08 and 0.19), the degree of humification (3.40 and 9.86). This subdivision is in agreement with the climate of the modern meadow-steppe and the effect of the early and middle Holocene xerothermic period on the formation of ancient Chernozems.
Thus, Late Holocene climatic changes toward cooling and wet caused the formation of a new soil type on the profile of mountain Chernozem. A horizons forming under modern bioclimatic conditions in the subalpine zone of mountains studied differ appreciably from the underling horizons with respect to the condensation of aromatic nucleus which are closer to typical alpine soils.
As δ 13C and characteristics of organic matter shown, the Arhys interval of middle centures was characterized by warmer and dryer climate, than at present. Deformatic fluctuations dominate in IR spectra of the mordern suface horizons in comprise with the high energetic valent fluctuations in the Middle Holocene horizons. The band of aromatic rings C=C bonds is more intensity in spectra of humic acids of Arhus interval soils.
Little ice age advance in mountaines was established at about 200-300 years ago and can be recognized by low values of humic acids optical density, increasing of Pg –fraction content in soil organic matter and magnetic susceptibility. It was a cool and wetter period. Soils in initial stages of formation characterized by the high humus content (up to 15 %), humate-fulvate type of it, low value of the extinction coefficient (0.06), the considerable green humic acids content, all of which indicate a humid and cold current climate.
Conclusions. That is way Holocene climatic changes in the central Asia seem to coincide with those of Eastern Europe, northwestern Caucasus, Russian plane and correspond to global climate changes. Organic matter of buried soils can be used successfully for paleoenvironmental and paleoclimatic reconstructions.