The Pleistocene Subaerial Loess-Soil Sequence of West Siberia: A Climate Change Archive and a Standard for Regional and Global Correlations

Zykin, V.S.¹, Zykina, V.S.¹, Chirkin, K.A.¹, Smolyaninova, L.G.¹

¹Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia,

Subaerial and limnic sedimentary sections in continental Asia are incompletely deciphered unique natural archives that store a record of Cenozoic climate history of the region. Minute investigation of these objects can provide clues to the evolution of global and regional environment and climate changes, and establish natural systems probable reactions on global changes, as well as to the moisture change trend of internal Asia areas controlled by warming progress.

The continental subaerial loess-soil sequence of West Siberia is one of the most completed stratigraphy scale, with its units fitting the cold and warm stages of the marine oxygen isotope scale (δ18O) and other records of global climate change. Correspondence of fossil pedocomplex constitution and warm stage of marine isotope oxygen scale and other global records is the base of their correlation. This consistency with the global records makes the West Siberian loess sequence the only standard scale for intra-regional correlations. The structure loess-soil sequence demonstrates different intensity of air circulation in Pleistocene warm and cold periods. Pleistocene warm and wet periods corresponded to soil formation epochs in conditions of weak air circulation. All pedocomplexes have thickest lowermost soils corresponding to longest and most intense soil formation in warm conditions, which is evidence of a greater duration and warmer and wetter environments of the initial substages of the warm climate stages. The soil formation stages in the sequence differ in the structure of pedocomplexes, in automorphic soil genetic types, and in geographic patterns, which allowed to distinguish diagnostic or morphotypic features for each warm stage. The constitution of fossil pedocomlexes in the West Siberian loess-soil sequence reproduce the structure of global odd warm stages consisting of closely spaced warm events interfered with brief cold spells. For the prognosis of climate changes and environment of the near future and definition of the modern warm epoch duration the problem of installation of duration and periodicity of Pleistocene warm epoch has great value. Compared with the modern (Holocene) soil formed in similar geomorphic conditions, interglacial fossil soils have thicker profiles in the early and middle Pleistocene section but are less mature and thinner in the late Pleistocene. Therefore, large interstadials, including the last one, were much longer than the Holocene, which appears to be the initial stage of warming.

Cold times were associated with climate drying and more intense air transport of dust which maintained loess deposition. During these periods were cold deserts, deflation surfaces and closed basins. Judging by the distribution of subaerial, mostly eolian, deposits and related landforms in the West Siberian Plain during the last Glacial (16-20 kyr BP), the area was then a vast cold desert with respective typical deposition environments. The spatial pattern of loess and fossil soil profiles indicates more intense deflation in the Omsk Irtysh area than in the east of the Plain.

A special focus in the studies was made on atmospheric circulation during glacials. Some researchers assume significant weakening of effect the west-to-east motions of the atmosphere in mid-latitude as a result of general North Atlantic air circulation change [e.g.; Dodonov, 2002]. The dominant wind directions in the past could be inferred from the landform pattern of deflation and deposition related to cold periods. This pattern is best preserved in the southern West Siberian Plain as hilly topography formed in Last Glacial time. The ridges in the area are of subaerial origin indicated by boundaries between layers in their sections which are free from traces of water erosion, water transport and gleying of sediments but bear signature of wind erosion, have thin soil profiles, and small desiccation cracks. The ridge history consisted of alternating events of eolian deposition, deflation, and brief formation of soil which became desiccated and development carbonate and gypsum deposition. The position of deflation basins (lake basins) west of transversal ridges shows strong erosion and deposition control from westerly cyclonic wind transport in middle latitudes. In eastern Asia, the more intense westerly air transport during glacials coincided with intensification of winter monsoons.

The Siberian loess-soil sequence stores record of longand short-period changes of paleoclimates. Spectral analysis of frequency-dependent magnetic susceptibility time series Siberian loess-soil sequence revealed a periodicity corresponding to the orbital cycles of eccentricity (100- kyr cycles), obliquity (40-kyr cycles), and precession (23- kyr cycles) [Kravchinsky et al., 2008]. Thus, the climate history of northern Central Asia located in the very continental interior is consistent with global change even though the Asian studied regions are thousands of kilometers far from the Atlantic Ocean, China loess plateau, and Lake Baikal which are the areas of best resolved paleoclimatic records. The FD highresolution profiles of the loess–paleosol sections can be used as one more quantitative proxy to study the periodicity of climatic variability encoded in the geologic record.

The West Siberian loess-soil sequence was correlated with its counterparts in other loess provinces of the world (Central Asia, North China, Eastern Europe, including Ukraine and Russian Plain), which have been exhaustively studied in terms of stratigraphy and paleogeography. The dry and wet climate stages of the West Siberian sequence showed synchronicity with those in the sections from zones of both westerly and monsoon air circulation. This synchronicity confirms the global scale and a single driving mechanism of the climate evolution. The records from all loess provinces within the Brunhes chron contain at least nine pedocomplexes separated by loess layers, with coeval pedocomplexes having the same structure. Correlation was based on the age ties of the last interglacial pedocomplex (Kazantsevo, Mikulinian, Eemian stages, etc.), Brunhes-Matuyama paleomagnetic reversal (780 kyr BP), and 14C and TL data.

Short-period climate variations were investigated in sedimentary sections of closed deflation lake depressions and deposits, which formed Late Glacial and Late Holocene eolian landforms. Late Holocene dunes in the fore-Altai plain are composed of eolian sands with buried soils and are overlain by an immature modern soil. They are localized along the eastern sides of lake basins and reach 10 m high. Radiocarbon ages and correlation with tree-ring data indicate that the subaerial deposition represents a 200-300 year quasi-periodicity of dry and cold (to 1°C, Little Ice age) cycles alternating with periods of wet climate at air temperatures about those at present. The dry and cold cycles were associated with eolian processes, fall of lake levels, deflation, and accumulation of dunes, while the relatively wet and warm stages in the fore-Altai plain were the times of soil formation.

The study was supported by grant 07-05-01109 from the Russian Foundation for Basic Research.

Corresponding author: Zykin, V.S. | zykin@uiggm.nsc.ru