The Holocene events in the Caspian Sea

Introduction. of different origin. Since the beginning of instrumental observations (۱۸۳۰) of the sea level,the amplitude equal to ۴ m, from −۲۵.۳m in the ۱۸۸۰'s and −۲۹m in ۱۹۷۷. In ۱۹۷۸ the sea level began to riseand reached −۲۶.۵m in ۱۹۹۴, thus in ۱۶ years the level increased by more than ۲ m. In ۱۹۹۵ a phase of sealevelstabilization and decrease began. Paleogeographic data give evidence to considerable leveloscillations, whose amplitude was more than ۱۵۰m during the Quaternary. The task of our work is topresent the main transgressive and regressive events in the Caspian Sea during the Holocene and to findout a dependence of fluctuations of the sea level with the climate changes. The paper is based on thestudying of seismic-acoustic profiles, as well as on the drilling material from different parts of the shelf.The obtained core was studied with lithological, faunistic, and radiocarbon methods. The Holocenesequence occurs on top of the sedimentary thickness in the Caspian basin. Mangyshlakian sediments arepreserved in valleys formed by river incision during periods of low lake stands; and in closed depressionsof different size, representing relict lows in the relief of draining past deltas. Deltaic deposits of the sameage occur along the southern edge of the shelf zone, while a vast alluvial fan formed at the edge of thenorthwestern flank of the Middle Caspian depression in depth interval ۴۵–۶۰m below the present level. Asfollows from the radiocarbon dates, the paleodepression infilling with loose deposits occurred within theinterval ۹۸۶۰–۶۳۵۰ ۱۴С yr BP. The New Caspian sediments overlie unconformably the Mangyshlakiansediments. There are ۵ seismoacoustic complexes (nk۱-nk۵) distinguishable in the New Caspian series.Complexes nk۱ and nk۳ are distinguished for mainly stratified structure as suggested by extensivesubhorizontal reflecting surfaces; locally the bedding is complicated with an irregular or cross-laminatedsmall-scale texture typical for infilling facies. The complexes are composed of sandy-clayey sediments withmollusk shells. Complexes nk۲ and nk۴ are essentially facies of channel (or lake basins) infilling. The largestof the erosional paleolandforms cut through the entire series of sediments to a depth of ۱۴ m. The sedimentscontain abundant remains of aquatic plants, shells of freshwater mollusks. Thin uppermost complex nk۵consisting of sand and shell material overlies unconformably nk۴ and nk۳ deposits. The radiocarbon datespermit to ascertain chronological boundaries of the recognized subdivisions of the New Caspian horizonthe lower New Caspian layer (nk۱) was deposited in the time interval from ۸۳۰۰ to ۵۲۰۰ ۱۴С yr BP. Theerosional landforms were being filled with deposits (nk۲) between ۴۹۰۰ and ۳۵۰۰ ۱۴С yr BP. The middleNew Caspian layer (nk۳) is dated to ۳۴۰۰-۳۲۰۰ ۱۴С yr BP, and deposits filling the erosional landforms ofthe next stage (nk۴) – to ۲۹۰۰-۲۲۰۰ ۱۴С yr BP. The ۱۴С dates obtained for the uppermost layer (nk۵) fallwithin the last ۲۰۰۰ years. The analysis of the data on the Holocene sediments of the Northern Caspian Searevealed palaeogeographic events of different scale recorded in the sequence. The most significant of themare the Mangyshlak regression and the multiple-stage New Caspian transgression, both resulting fromclimatic changes varying in scale and direction. The large-scale Mangyshlak regression having left marksof the sea level at depths about −۹۰m occurred in the Boreal period (according to the Blytt-Sernanderscheme) of the early Holocene. The New Caspian transgression developed in several stages. The first stageof transgression developed under conditions of a warm and wet climate of the Holocene optimum. Thesecond stage could be a response to the Subboreal cooling and increase in moisture supply on the EastEuropean Plain. The third transgressive stage occurred after ۲۳۰۰ yr BP. The two important climatic phasesof the late Holocene (the Medieval Climatic Anomaly and the Little Ice Age) can be interpreted from thesedimentary sequence as belonging to that stage. However, there is a hiatus in the series of dates obtainedfor the transgressive sequence, which corresponds to the warm and dry medieval interval and suggests alowered level of the Caspian Sea at that time. The second group of the dates fits the time interval of theLittle Ice Age and gives grounds to assume that the Caspian Sea level rose during that episode of a cooland wet climate. The first regressive phase corresponded to the Subboreal thermal maximum of theHolocene marked by a low moisture supply over the European part of Russia. The second phase isconsidered to be the Caspian Sea response to the warming and rainfall decrease in the Volga drainagebasin. Smaller-scale fluctuations of the sea level resulting from regional climatic changes account forvarious coastal landforms and characteristics of the deltaic deposits. The work was supported by the RFBRProject ۲۰-۳۹-۷۰۰۲۰. The authors are thankful to colleagues from geochronological laboratories of theMoscow and St. Petersburg state universities, from St. Petersburg Pedagogical University, of the Instituteof Geography RAS (Moscow) for performing radiocarbon dating of core samples.

کلیدواژه ها:

Northern Caspian Sea ، Holocene sediments ، stratigraphy ، Mangyshlakian regression ، New Caspian transgression ، climate change