Pedogenetic response to climatic fluctuations within the last glacialinterglacial cycle in the Lower Volga basin

سال انتشار: 1400
نوع سند: مقاله کنفرانسی
زبان: انگلیسی
مشاهده: 136

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IQA04_040

تاریخ نمایه سازی: 27 آبان 1400

چکیده مقاله:

Soils form under the direct influence of climatic parameters, and they retain environmental information intheir features (soil memory: Targulian and Goryachkin, ۲۰۰۸). Thus, buried soils provide an excellentopportunity to reconstruct paleoenvironments preceding burial. The area of the lower Volga basinexperienced considerable changes due to fluctuations in Caspian Sea level together with other responsesto glacial-interglacial cycling in the Quaternary. Numerous horizons of buried soils have been recorded insedimentary sequences, and they have been used for stratigraphic correlations and paleogeomorphicreconstructions in the area (Konstantinov et al., ۲۰۱۶). But the study of paleosols as a paleoenvironmentalproxy has not been performed until now. The study of the ۲۰ m exposure on the terrace in the middlestream of the Akhtuba River provides a unique opportunity to reveal the complex interplay among severalfactors: Caspian Sea level cycling, related fluctuations in fluvial activity, continental (aeolian)sedimentation, the local environment of the Akhtuba River valley, and the pedogenetic response toenvironmental and geomorphic changes. The site is situated on the terrace of the Akhtuba River with anelevation of ۲۱ m asl and ۲۰ km from Volzhskiy city. The exposure was thoroughly examined in the field.The stratigraphy includes a designation of loess layers separated by marine clays and fluvial deposits. Sixpedogenetic levels were examined morphologically according to the FAO Guidelines for Soil Description(IUSS, ۲۰۰۶: ۱۰۹) and classified according to the WRB (۲۰۱۴). Soil samples for hierarchical morphological(mesomorphology, micromorphology, SEM) and analytical studies were taken from all horizons. OSLdating (described in the paper of Yanina et al., this volume) allows us to bracket the pedogenetic events tovarious stages within the last interglacial-glacial cycle.The ۲۰ m high exposure is capped by a ۱۲۰ cm upper(MIS۲) loess layer with modern surface soil (Kastanozem) typical for dry steppe areas. The loess isunderlain by marine clays ۱۲۰-۵۲۰ cm. OSL dates for the middle (۱۳,۰۰۰±۵۰۰) and lower parts (۱۵,۰۰۰±۱۰۰۰yrs) characterizes marine clays of the Early Khvalynian transgression of the Caspian Sea (Arslanov et al.,۲۰۱۳). The clays consist of fine plates with a typical chocolate hue. The clay strata are slightly touched bypedogenesis down to ۱۱۰±۲۰ cm (۲۳۰±۲۰ cm from the day surface); vertical cracks and sub-angularstructural units are covered by weakly developed clay cutans with overlying carbonate coatings. Marineclays are underlain by a soil-sedimentary sequence formed during the Atel-Akhtuba regression of theCaspian Sea (MIS ۳) with three pedogenetic levels developed in loess and correspond-ing to MIS ۳interstadial paleosols. The humus horizon of the upper well-developed soil (۵۲۰-۶۰۰ cm) is truncated tothe AhB horizon. The remaining profile could be preliminarily attributed to a Relictigleyic LuvicChernozem with Bt horizons showing well-developed clay-humus cutans covered by carbonate films. Theprofile is underlain by fluvial sand (۶۰۰-۷۷۰ cm), showing a gradual transition to silty loess loamdownward. Sand merges laterally into the ۱۰-۲۰ cm loess layer with large gypsum crystals, confirming anarid environment at the time of loess sedimentation. The second pedogenetic level (۷۷۰-۸۰۰ cm) ispresented by loess strata with weakly developed soil without clear horizonation. Pedogenetic featuresinclude platy and angular structural units, clay cutans, rhizoliths, manganese mottling, carbonateconcretions, and gypsum crystals. Dark spots indicate the remains of displaced humus horizons. The uppersurface of the pedogenetic level is complicated by a regular network of sand cracks, indicating severeseasonal frost and possible permafrost. The third pedogenetic level is concise with shallow loess patches(۸۸۰-۹۱۰ cm), merging laterally and covered by a fluvial sand interlayer (۸۰۰-۸۸۰ cm). Pedogenetic featuresare recorded in clay cutans and biopores. Remains of a carbonate horizon are visible in the upper ۷ cm ofloess. The third pedogenetic level is disturbed by cryogenesis. The lower part of the loess is intermixedwith sandy lenses. The study of the MIS ۳ soil-sedimentary complex confirms that the soils formed in acold, arid environment during short periods of mesomorphic pedogenesis coinciding with loesssedimentation and interrupted by an increase in fluvial activity. A cold environment and possiblepermafrost are indicated by frost wedges and involutions disturbing the soil horizons. Gley mottlingcontradicting the aridity features could be the result of waterlogging owing to long-seasonal overfloodingof the terrace surface. The lower part of the Atel-Akhtuba strata (۹۱۰-۱۵۳۰ cm) is represented by a carbonateloess without noticeable pedogenetic transformation. The Mesin pedocomplex (MIS ۵a-e, ۱۵۳۰-۱۶۸۰ cm) isattributed to the Late Khazarian transgressive epoch and includes three distinct soils formed in loess. Theage of the upper soil (MIS ۵a-d) is characterized by OSL as ۶۸,۰۰۰ ± yrs. The Gleyic Phaeozem has anaccretionary humus horizon (about ۱ m) owing to synsedimentary pedogenesis at the onset of loesssedimentation. The soil surface is disturbed by a network of frost wedges ۴۰-۵۰ cm apart with a broad partgoing down to ۴۰ cm and narrow fissures much lower than the soil profile. The wedges start in theoverlying Atel-Akhtuba loess layer, indicating the beginning of the last glacial cycle (MIS ۴). Krotovinas,mostly expressed at ۲۰-۳۰ cm from the buried surface, show a steppe environment, and gleyic featuresindicate seasonal overflooding. Lower horizons (۱۶۵۰-۱۶۷۰ cm) are developed in loess and interlayeredwith fine sandy layers owing to contrasting sedimentation. The soil profile gradually merges into themiddle soil of the Mesin pedocomplex (۱۶۷۰-۱۷۹۰ cm). The upper ۵ cm of the humus horizon of the middlesoil (Gleyic Chernozem) are intermixed with the Bg horizon of the upper soil (welded paleosol). Carbonatesin the upper layer are most probably diagenetic; in the Bg horizon, the complex assemblage of carbonateneoformations is a result of steppe pedogenesis. Gleyic features are due to seasonal overflooding. Narrowendings of frost fissures penetrate from the Atel-Akhtuba loess down to the middle part of the profile. Thelower soil of the Mezin pedocomplex (MIS ۵e) is separated from the middle soil by a transportic layer (۱۷۹۰-۱۸۰۰ cm) and represented by a Mollic Calcic Gleysol (۱۷۹۰-۱۸۶۰ cm) formed in loess sedimentsaccumulated during the penultimate glaciation (MIS ۶). The presence of the Mezin pedocomplex formedin loess confirms that the area was beyond the Late Khazarian transgression of the Caspian Sea.Nevertheless, the high stand of the Akhtuba River caused long-term seasonal overflooding resulting ingleyic features. Three soils of the Mezin pedocomplex have common features: well-developed humushorizons and a complex assemblage of carbonate neoformations formed in a steppe environment; gleyicfeatures owing to long-term seasonal overflooding; welded and/or synlithogenic profiles owing tocontrasting sedimentation on the river terrace. The lower part of the sequence is represented byinterlayering of loess and clay bands without clear pedogenetic influence that indicates quicksedimentation with contrasting changes in the water level of the Akhtuba River. However, carbonatepseudomicellia indicate at least primitive pedogenesis in clays on the river terrace or dried firth surface.The soil sedimentary sequence is formed in loess layers intermixed with marine and fluvial sediments. Theupper loess (MIS ۲) is underlain by marine chocolate clays. Such a unique feature allows setting a reliablecorrelation for Caspian marine and lower Volga loess stratigraphic patterns. Loess sedimentation on theriver terrace frequently changed to fluvial sedimentation according to fluctuations of the Caspian Sea level.Various loess units are divided by stream sands and terrace clays. Six pedogenetic levels are linked toperiods of loess sedimentation between MIS ۵ and MIS ۱. The longer interval of geomorphic stability afterloess sedimentation results in better developed loess profiles. All soils were formed in an arid environmentthat is indicated by a complex assemblage of carbonate neoformations and krotovinas. Nevertheless, allsoils except the modern one show hydromorphic features (gley mottling, iron and manganese concretions).Such a mutually exclusive combination of soil features indicates polygenesis caused by river influence:long-term seasonal overflooding. There are three levels of interstadial paleosols (MIS ۳). They formed in acold and arid environment. This is especially obvious in the upper pedogenetic level that was influencedby deep seasonal freezing during the LGM. Interglacial soils (MIS ۵a-d and especially MIS ۵e) are muchbetter developed. In contrast to the last interglacial soils of the center of the East European Plain (Mikulino)and Western Europe (Eemian), they are represented by steppe soils with gleyic features—GleyicChernozems and Mollic Calcic Gleysols. Soils are partly synlithogenic, their upper horizons formedsimultaneously with loess sedimentation. The Mezin pedocomplex is represented by welded profiles. Theupper horizons of some profiles are truncated. Interlayering and a gradual transition between loess andsand layers, contrasting pedogenetic features (humus accumulation and clay cutans, a complex assemblageof carbonate neoformations, gypsum crystals and gleyic features) indicate contrasting sedimentation andpedogenetic environments. Pedogenetic horizons serve as good stratigraphic markers that will helpcorrelate late Pleistocene soil-sedimentary sequences of the whole Caspian-Azov-Black Sea region of theEast European Plain and link it with global stratigraphic schemes. Detailed analytical and further fieldstudies are required to reveal further pedogenetic responses to environmental changes in the area.

نویسندگان

A Makeev

Soil Science Faculty, Moscow State University, ۱/۱۲ Leninskye Gory, Moscow, Russia ۱۱۹۹۹۱