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Si f eology.
Journal of Geologyt Geography and Geoecology
Journal home page: geology-dnu-dp.ua
ISSN 2617-2909 (print) ISSN 2617-2119 (online)
Journ.Geol. Geograph.
Geology, 28(2), 301-312.
doi: 10.15421/111930
D.V. Hlavatskyi Journ. Geol. Geograph. Geoecology, 28(2), 301-312.
Refined magnetostratigraphic position of the Shyrokyne unit in loess sequences from Central Ukraine
D.V. Hlavatskyi
Institute of Geophysics of National Academy of Sciences of Ukraine, Kyiv, Ukraine. e-mail: [email protected]
Received: 31.01.2019 Abstract. The youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary
Received in revised form: 10 04 2019
Acce ted 16 04 2019 (MBB), which occurred 780 kyr ago, is a "golden spike" in the age calibration of sediment
V ••• sequences. The use of palaeomagnetic method as a stratigraphic tool in the study of loess
sequences from Ukraine originated some 50 years ago. One major problem in using the available data is the contradictory position of the MBB in different stratigraphic units, which affected historic evolution of the chronostratigraphic models of the Quaternary in Ukraine. The most important units in this regard are the Shyrokyne and Martonosha units, in which the MBB had been defined most often. This paper provides the careful analysis of the previous magnetostratigraphic data and new preliminary results from key loess-palaeosol sections in Central Ukraine. Shyrokyne palaeosol complex in four loess-palaeosol sections located in the Middle Dnieper and Podolia regions has been palaeomagnetically studied. It is shown that the transition zone of the Matuyama-Brunhes palaeomagnetic reversal is most likely located at the base of the soil complex. In the Vyazivok section the MBB has been found in the lowermost part of Shyrokyne palaeosol shr Preliminary palaeomagnetic studies of the Stari Kaydaky section reveal that the MBB cannot be defined at least above shx subunit. Medzhybizh and Holovchyntsi sections were deposited after the Matuyama-Brunhes reversal; however, the palaeomagnetic informativeness of the part of studied strata is doubtful. Magnetostratigraphic position of the Shyrokyne unit below the MBB in some previous studies is explained by methodological reasons and inconsistent chronostratigraphic models. The paper substantiates that normal magnetic polarity zone in the Pryazovya loess and upper part of the Shyrokyne soil is not associated with the influence of secondary processes on the palaeomagnetic record.
Key words: magnetostratigraphy, Matuyama-Brunhes boundary, palaeomagnetic method, loess-palaeosol sequence, Pleistocene, marine isotope stage 19.
Уточнене магштостратиграфiчне положення широкинського горизонту у лесових серiях Центрально! УкраУни
Д.В. Главацький
1нститут геофiзики Нацюналъног академи наук Украгни, Кигв, Украгна, e-mail: [email protected]
Анотащя. Наймолодша геомагштна iнверсiя полярносл, границя Матуяма-Брюнес (МБ), що вщбулася 780 тис. р. тому, е «золотим цвяхом» у вшовому калiбруваннi осадових серш. Використання палеомагнггного методу як стратиграфiчного шструменту при дослщженш лесових серш Украши бере початок близько 50 роюв тому. Одшею з головних проблем використання наявних даних е суперечливе положення границ МБ у рiзних стратиграфiчних горизонтах, що позначилося на юторичному розвитку хронотратиграфiчних моделей четвертинного перюду в Укршш. Найбшьш важливими шдроздшами у цьому контекст е широкинський та мартоноський горизонти, в яких границя МБ визначалася найчастше. У цш статт проведено ретельний аналiз попередшх магнiтостратиграфiчних даних та нових попередшх результат ключових лесово-грунтових розрiзiв у Центральны Укршш. Широкинський грунтовий комплекс дослщжено палеомагнггним методом у чотирьох лесово-грунтових розрiзах, що знаходяться у Середньому Придншров'1 та на Подшьськш височиш. Показано, що перехщна зона палеомагштно! шверси Матуяма-Брюнес найiмовiрнiше розташована у пiдошвi грунтового комплексу. У розрiзi В'язiвок гра-ницю МБ було знайдено у нижнш частиш широкинського грунту shr Попередш палеомагштш дослщження розрiзу Старi Кай-даки свщчать про те, що границя МБ не може бути визначена принаймш вище тдгоризонту shr Вщклади розрiзiв Меджибiж i Головчинщ сформувалися тсля шверси Матуяма-Брюнес; однак, палеомагштна шформативнють частини дослщжених шарiв сумшвна. Магнiтостратиграфiчне положення широкинського горизонту нижче гранищ МБ у деяких попередшх робо-
тах пояснюеться методолопчними причинами та непослiдовними хроностратиграфiчними моделями. У статп обгрунтовано, що зона прямоï полярностi у приазовському леи та широкинському грунтi не пов'язана i3 впливом вторинних процес1в на палеомагштний запис.
Ключовi слова: магнiтостртиграфiя, границя Матуяма-Брюнес, палеомагштний метод, лесово-Грунтова серiя, плейстоцен, морська iзотопна стадiя 19.
Introduction. Loess sequences, alternating loess and palaeosol horizons, are unique continental formations of the Quaternary. They contain one of the most complete records of global changes in the climate of this geological period, in particular, glaciations and interglaciations of the last million years, and are significantly widespread, mostly at intermediate latitudes of the Northern Hemisphere (Evans and Heller, 2003; Buggle, 2011). For this reason, loess sediments are carefully studied by various methods of environmental research, including the palaeomagnetic method.
The Matuyama-Brunhes polarity reversal, which occurred 780 kyr ago (Tauxe et al., 1996) is the key benchmark of correlation of loess-palaeosol sequences of the Quaternary, defining the Lower-Middle Pleistocene boundary. In deep-sea sediments, this reversal is fixed in the interglacial marine isotope stage (MIS) 19 (Tauxe et al., 1996).
Loess-palaeosol exposures along Dnieper, Sula and Southern Bug rivers in Central Ukraine reveal a complexsuccessionofQuaternarypalaeoenvironments over the past 0.7-1.2 million years (Veklich, 1982; Bolikhovskaya and Molodkov, 2006; Matviishyna et al., 2010; Buggle, 2011). For magnetologists the most important unit in the loess sequences is the Shyrokyne horizon (hereinafter the stratigraphic terminology is used in accord with the Stratigraphic Framework of the Pleistocene of Ukraine (Veklich et al., 1993)) - palaeosol complex consisting of three subunits: brown clayey soils of earlier optimum sh1 and later optimum sh3, and uncommon loess-like loam subunit sh2 (Gozhik and Gerasimenko, 2011; Sirenko, 2017). Previously the Matuyama-Brunhes transition zone was defined by colleagues from the Institute of Geophysics (Tretyak et al., 1987; Tretyak et al., 1989; Tretyak and Vigilyanskaya, 1994; Vigilyanskaya, 2000, 2001a, 2001b, 2002) in wide range of sections most often in the lowermost sh1 subunit. According to their generalized Pleistocene Magnetostratigraphic Scale of Ukraine (Tretyak and Vigilyanskaya, 1994), which is based on the palaeomagnetic studies of almost 60 sections in Ukraine, Moldavia and Pryazovya, the Matuyama-Brunhes boundary (MBB) is located in the Shyrokyne horizon (according to stratigraphic nomenclature of (Veklich et al., 1984)), the age of which was estimated at 730 kyr. Thus, the Shyrokyne unit should be evidently correlated with MIS 19 (Bogucki et al., 2012).
However, in the stratigraphic models, which are proposed for the Ukrainian Quaternary, the chronological placement of the Shyrokyne unit is different. It is generally argued that this pedocomplex and the respective interglaciation occurred during 0.85-1.2 Ma (Matviishyna et al., 2010) and corresponds to MIS 25 (Veklich, 1995, cited in Bolikhovskaya and Molodkov, 2006) or 21-33 (Lindner et al., 2006). Therefore, the younger Martonosha palaeosol unit is placed either in MIS 19-23 (Veklich, 1995, cited in Bolikhovskaya and Molodkov, 2006) or 17-19 (Lindner et al., 2006). Theoretical correlation with marine oxygen isotope stages of stratigraphic schemes of Ukraine, resulting from the different opinions on the chronological placement of the Shyrokyne and Martonosha units, is given in (Bakhmutov et al., 2017).
The above frameworks are based in turn on initial contradictory magnetostratigraphic data (Tretyak and Volok, 1976; Tretyak, 1980, 1983; Tretyak et al., 1980, 1986; Veklich, 1982), in which the MBB had been defined in the Berezan, Shyrokyne, Martonosha, Sula and Lubny units. Eventually, the normal polarity zone within Przyazovya loess and the upper part of the Shyrokyne soil units had been interpreted by geologists as Jaramillo event (0.98 Ma) and the MBB had been placed at the top of the Martonosha soil unit, which became the basis for following palaeogeographic reconstructions (Veklich, 1987; Lindner et al., 2004; Lindner et al., 2006; Matviishyna et al., 2010; Gozhik and Gerasimenko, 2011 and others). However when considering them, surprising unanimity in position of the MBB in several key Pleistocene sections is troubling. For example in (Lindner et al., 2004, fig.
2) the MBB in the Vyazivok section had been defined based on only one sample with anomalous polarity at the base of Sula loess (Veklich, 1982, fig. 26), and in the Roxolany section (Lindner et al., 2004, fig.
3) - one sample of reversed polarity in Martonosha soil (Tretyak, 1980, fig. 1; Tretyak, 1983, fig. 6.5). It should be recognized that most of previous results (Tretyak and Volok, 1976; Tretyak, 1980, 1983; Tretyak et al., 1980, 1986; Veklich, 1982; Veklich, 1987), carried out without effective minimization the acquisition of present-day viscous magnetization, and defined abundant magnetic events therein, cannot be taken as reliable as any stratigraphic correlations based in them (Bakhmutov and Hlavatskyi, 2016).
However, later in several sections of Southern Ukraine and Ukrainian Shield the MBB was found exactly at Martonosha horizon level (Bakhmutov et al., 2005; Sirenko et al., 2008; Slivinskaya et al., 2012), which supports the existing charts. In addition, in the Roxolany section, one of the key loess sequences of the Black Sea area, the MBB was defined recently at the contact of two soils (Bakhmutov and Hlavatskyi, 2014a, b), which were stratified by (Gozhik et al., 2007; Bogucki et al., 2013) as the Lubny and Martonosha units.
In order to resolve the problem of inconsistency between different stratigraphic positions of the MBB in loess cover of Ukraine, the most complete sections of the Quaternary should be investigated. In this study initial results, focusing on determination the MBB in key loess-palaeosol sequences of the Middle Dnieper area at Vyazivok and Stari Kaydaky are given. Additionally, new results of palaeomagnetic study of the Podolian Upland sequences - the Lower Palaeolithic sites - at Medzhybizh and Holovchyntsi are presented.
Geological setting. The Vyazivok section is located in the village of Vyazivok (49°33' N, 32°98' E) about 8 km south of Lubny of Poltava oblast on the western bank of the Sula River, a tributary of the Dnieper (Fig. 1). It represents one of the most complete Quaternary records in Ukraine and the longest section studied within Dnieper Lowland. It is characterized by 59-meter sequence of several well
developed palaeosols which alternate with thick loess units. The section was studied earlier by (Veklitch et al., 1967; Matviishyna et al., 2001; Rousseau et al., 2001). The detailed description of the section which the author was guided in rock sampling is presented in (Matviishyna et al., 2001). The Shyrokyne unit in the Vyazivok section is represented by two subunits. The lower subunit sh1, which is 1.15 m thick, consists of two substages: greyish-brown clayey soil sh1b1 and reddish-brown sandy-clayey soil sh1b2. The upper soil subunit sh3, which is 2.35 m thick, is chocolate-brown soil, composed of clay, more compact, prismatic, with abundant carbonate concretions at the base of soil.
The Stari Kaydaky (or Stari Kodaky) section (48°22' N, 35°07' E) is located south of Dnipro at the Dnieper River. The sequence comprises eight major loess-palaeosol couples. It was stratified by (Veklich and Sirenko, 1972) and was declared as reference Pleistocene section of Ukraine. The uppermost part of the section (from the Lubny unit to the Holocene unit) had been studied in particular by rock magnetic methods (Buggle et al., 2008, 2009, Buggle, 2011) and it was correlated with loess sequences of Danube basin. The Shyrokyne unit in studied exposure at Stari Kaydaky is not less than 8 m thick, which is almost twice as much its thickness in the stratotype sequence at Shyrokyne (4.5 m) (Veklich and Sirenko, 1972). It includes three subunits: subunit sh1, which is at the minimum 3.9 m thick, subunit sh2 (0.55 m thick) and the upper soil sh3 (3.6 m thick). The lower subunit sh1
Fig. 1. Location of studied sections on the map of Ukraine.
consists of the solid chocolate-brown soil, composed of clay, compacted, prismatic, with rare carbonate concretions at the base of soil, and the upper dark humus subhorizon. The middle subunit sh2 includes thin reddish-brown loam and dark humus layer. The upper subunit sh3 is brown clayey soil.
The Medzhybizh-A section (49°25' N, 27°23' E) is located in the village of Medzhybizh about 33 km east of Khmelnytskyi on the nothern bank of the River Southern Bug. Loess sequence covers the time interval from the Shyrokyne unit to the Holocene, and lies on the marine sediments of the Sarmatian time (Stepanchuk et al., 2014). It is famous archaeological site of the Lower Palaeolithic. Archaeological artifacts were found in the palaeosols of the Zavadivka, Lubny, Martonosha and Shyrokyne stratigraphic units (Stepanchuk et al., 2014, 2016). The Shyrokyne horizon is very thin (a few tens of centimeters) and is composed of sandy clay.
The Holovchyntsi-1 section (49°25'E, 27°29'N) is located 1.3 km north of the village of Holovchynt-si and 7.5 km east of Medzhybizh-A section on the northern bank of the River Southern Bug. The section is the southern part of the current Holovchynskyi granite quarry. It was investigated by archaeologists from the Lower Palaeolithic expedition of the Institute of Archeology of the National Academy of Sciences of Ukraine in cooperation with geologists (Yu.M. Veklich, S.P. Karmazinenko, A.V. Nadvirnyak) for the first time in 2015. Initially, studied soil horizons, which are 3 m thick, were attributed by geologists to the Zavadivka or Lubny unit. Conclusions based on topography and stratigraphy of the location, as well as artifact typologies, clearly indicated the discovery of a new Lower Palaeolithic location in the region (Vetrov, 2016). Research was continued in 2016-2017 under the guidance of V.N. Stepanchuk. According to new stratigraphic interpretation of Zh.M. Matviyishina and S.P. Karmazinenko the palaeosols were already stratified as the Shyrokyne horizon and it was possible to establish a preliminary conclusion about the oldest artifacts found in Ukraine - 0.9-1.2 Ma. The interpretation of stratigraphic subdivision by Zh.M. Matviyishina and S.P. Karmazinenko has been used in this paper.
Sampling and methods. Samples for palaeomagnetic studies in the Vyazivok section have been collected in 2014-2015. 173 oriented rectangular blocks were taken from the whole sequence, which were cut into 692 oriented cubes with an edge of 2.0 cm. Samples from Shyrokyne horizon have been processed in 2014 from 2 exposures (Zupynka 1 and Zupynka 2) which represent the same units of the Lower Pleistocene.
In 2015 a total of 81 blocks were taken from the
Medzybizh-A section and in 2017 - 17 blocks from the Holovchyntsi-1 section. Considering the loose state of the oriented rectangular blocks selected in the Medzhybizh-A section, it was not possible to make stable cube samples for magnetometric measurements. It was possible to make only a few oriented samples mostly from the lowermost part of the section.
Samples from Shyrokyne palaeosol in the Stari Kaydaky section have been collected over two field seasons during 2017-2018. It was difficult to reach the primary exposure because of a thick deluvium layer. 26 oriented rectangular blocks were taken. For now 65 oriented cubes from the uppermost part were cut from 11 rectangular blocks, taken in 2017. Sampling density is about every 10-20 cm.
Palaeomagnetic measurements were carried out in the laboratory of the Institute of Geophysics of the National Academy of Sciences of Ukraine (Kyiv). Partly the measurements were performed in the laboratories of Palaeomagnetism Department of the Institute of Geophysics of the Polish Academy of Sciences (Warsaw) and The Ivar Gi^ver Geomagnetic Laboratory hosted by the Centre for Earth Evolution and Dynamics at the University of Oslo (Oslo).
The natural remanent magnetisation (NRM) was measured using JR-6, JR-6A magnetometers and SQIUD 2G Enterprises magnetometer accompanied by an alternating field demagnetizer. All samples were subjected to stepwise thermal demagnetization (THD) up to 240-350° C and alternating field demagnetization (AFD) in fields of up to 100 mT. All measurements have been performed inside the magnetically shielded rooms to minimize the acquisition of present-day viscous magnetization.
Directions of characteristic remanent magnetization (ChRM) component has been calculated by multicomponent analysis of the demagnetization path (Kirschvink, 1980), using Remasoft 3.0 software (Chadima and Hrouda, 2006). Polarity zones were built in MPS program (Man, 2008). Brief overview of previous results and new preliminary data. Vyazivok section. Previously the Vyazivok profile had been palaeomagnetically studied in the 1970's (Veklich, 1982). In one sample at the lowermost level of the Sula loess (the samples below were not selected) anomalous polarity had been established. It was announced that the MBB lies at the top of the Martonosha palaeosol, and it had long been thought that the Martonosha unit belongs to Matuyama chron. This conclusion was the basis for futher correlation charts of the Quaternary in Ukraine and neighboring territories (Lindner et al., 2004).
The whole sequence had been studied by (Vigilyanskaya, 2001b; composite palaeomagnetic
section is in fig. 9 in (Matviishyna et al., 2001)). 233 samples were thermally demagnetized up to 250° C and 132 samples were demagnetized by alternating field up to 50 mT. The MBB was clearly defined in Shyrokyne palaeosol.
Preliminary results of palaeomagnetic and rock magnetic study of the Vyazivok section were published by author of this study with co-authors in (Hlavatskyi et al., 2016b). Most ofthe samples from the Shyrokyne unit and surrounding Pryazovya and Illichivsk loess units containe a high-coercivity component, removed totally only by 80-100 mT field demagnetization. In some cases it could not be destroyed even by 100 mT alternating field or 300°C temperature. According to thermomagnetic analysis the main carriers of NRM in studied rocks are magnetite and hematite. Paramagnetic minerals also play a significant role in the magnetic properties of this loess-palaeosol sequence (Hlavatskyi et al., 2016b).
Samples demagnetized by alternating field and temperature have similar positive inclination and northerly declination values for the depth interval 51.5-56.2 m. Only in two specimens above 56.2 m (from upper part of the Shyrokyne unit), which were thermally demagnetized, was a full reversal of directions observed. Other samples keep steady inclinations in average at 66°. However, data from samples below suggest a major polarity change at a depth of 56.2 m which is interpreted as evidence of the Matuyama-Brunhes polarity boundary. Samples below this level have mean ChRM inclination direction -52° and declination direction 181°. The Jaramillo subchron cannot be identified in the Vyazivok section.
Thus, the MBB was found at a depth of approximately 56.2 m in the lowermost Shyrokyne soil sh1. After removal of viscous component, loess and soil samples display the presence of a stable hard component carried mainly by hematite.
Stari Kaydaky section. For the first time the Stari Kaydaky section was studied by A.N. Tretyak and L.I. Vigilyanskaya and the MBB was determined in the Shyrokyne palaeosol (V.G. Bakhmutov and N.P. Gerasimenko, personal communication, 2018). But any publications of A.N. Tretyak and L.I. Vigilyanskaya concerning palaeomagnetic research of the Stari Kaydaky profile cannot be found in the Institute of Geophysics library filing cabinet, as there are no appropriate references in their works. Only composite palaeomagnetic section is available. Probably, this section was studied before 1994, because Stari Kaydaky outcrop is marked as studied section on the map, attached to Pleistocene Magnetostratigraphic scale of Ukraine (Tretyak and Vigilyanskaya, 1994).
Preliminary investigations by V.G. Bakhmutov in 2006 (personal communication, 2018), in which high precision measurement equipment (including 2G Enterprises magnetometers) had been used, could not detect the geomagnetic polarity change of the MBB. The studied part of the profile included interval from the top of the Shyrokyne unit (uppermost part with a thickness of 1.5 m) to the Holocene soil. In all layers, including top of sh3 subunit as the overlying Pryazovya loess and Martonosha palaeosol units, exceptionally normal polarity was observed.
Works on demagnetization of samples of the lowermost layers are currently in progress. For now, a pilot collection of 24 specimens from the entire thickness of the sh3 and sh2 subunits, and from upper humus subhorizon of sh1 subunit have been treated by temperature up to 300°C. Since most of the specimens were fragile, they cannot be heated to temperatures above 300°C and few steps of demagnetization at temperatures 210, 240, 270 and 300°C have been carried out. After stepwise THD objectively one stable component is observed on the orthogonal vector diagrams (Fig. 2a,b), which is not totally removed by temperature of 300°C. Probably the secondary component overprint was demagnetized at temperatures lower than 210°C, but incomplete demagnetization also is possible. In most samples the component, which was accepted as characteristic, decays linearly towards the origin, with only normal directions indicating the formation of studied subunits during the Brunhes chron.
Average value of bulk magnetic susceptibility is 310*10-6 unit SI, which is the same as in the Shyrokyne unit of the Vyazivok section (318*10-6 unit SI). These values indicate typical for loess-palaeosol series of Ukraine concentration of magnetic minerals.
Full details of the further palaeomagnetic data from the Stari Kaydaky section require a separate paper and will be presented in future submissions.
Medzhybizh and Holovchyntsi sections. Complex rock magnetic and palaeomagnetic studies of the Medzhybizh-A and Holovchyntsi-1 loess-palaeosol sections for the first time were performed by (Bakhmutov et al., 2018) in order to determine the suitability of these objects for palaeomagnetic study and the establishment of magnetostratigraphic markers.
The deposits of the Medzhybizh-A and Hol-ovchyntsi-1 sections are characterized by low values of magnetic susceptibility, which indicates an insignificant content of magnetic minerals, especially in the Medzhybizh-A section (in average 82*10-9 m3/ kg, in contrast to 178*10-9 m3/kg in the Holovchyntsi section, Fig. 3). Investigated sections by rock mag-
Fig. 2. Examples of stepwise thermal (a, b) demagnetization of soil samples from the Shyrokyne unit in the Stari Kaydaky section, and alternating field and following thermal (c, d) demagnetization of soil samples from the Shyrokyne unit in the Holovchyntsi-1 section.
1 - stereographic projections of demagnetization directions, 2 - orthogonal demagnetization projections, 3 - intensity decay curves of NRM.
netic characteristics are closest to the sections of the Volynian Upland, and refer to the intermediate "Chinese" type of formation of magnetic properties with an admixture of the "Alaskan" mechanism (Hlavatskyi et al., 2016a; Bakhmutov et al, 2017).
Most of the samples from both sections have visually disturbed texture, which is confirmed by anomalous data on the anisotropy of the magnetic susceptibility. In interpretation of (Bakhmutov et al., 2018), it reflects the orientation of ferrimagnetic grains due to currents, slope processes, winds etc. Samples, in which the primary magnetic texture is not broken, were selected for further laboratory palaeomagnetic studies.
In order to determine characteristic component of NRM a pilot collection of the samples from both sections has been demagnetized by stepwise AFD up to 100 mT (using cryogenic SQUID magnetometer 2G Enterprises with an in-line alternating field demagnet-izer), and THD up to 240-350°C. Most of the samples show ChRM directions of normal polarity (Fig. 2c, d). Below the soil horizon sh3 in the Holovchyntsi-1
section the NRM and ChRM values decrease in average to 1x10-3 mA/m and 0.5*10-3 mA/m, respectively (Fig. 3), and the samples show a large scatter of the ChRM directions with predominantly anomalous polarity.
Taking into account the secondary changes in the deposits, the minimum values of the remanent magnetization in the soil, the small thickness of the section as a whole, it is not possible to reliably determine the primary component of the magnetization in the deposits within the lowermost layers. Statistical characteristics of NRM and ChRM after demagnetization by magnetic field and temperature are shown in fig. 3 in (Bakhmutov et al., 2018). The average values for the directions of the NRM and ChRM components within sh3 subunit agree within 95% and 99% confidence limits and indicate a direction close to the current geomagnetic field in the area (Fig. 3).
The results of palaeomagnetic studies turned out to be uninformative for the most layers of the Medzhybizh-A section and for the lowermost part of the Holovchyntsi-1 section (shx subunit) because of
Fig. 3. Results of palaeomagnetic study of the Holovchyntsi-1 section: lithostratigraphy (Matviishyna and Karmazinenko, 2017), magnetic susceptibility, normal remanent magnetization, ChRM directions, discriminant function and palaeomagnetic chart.
the almost complete absence of ferrimagnetic material and the disturbance of the sedimentary magnetic texture of the rocks. The entire loess-soil stratum of the Medzhybizh section according to preliminary data can be attributed to the Brunhes chron, i.e. the age of the rocks in the section does not exceed 780 kyr. In the Holovchyntsi-1 section the Matuyama-Brunhes boundary also was not clearly identified. The polarity is reliably allocated only in the upper soil sh3. Discussion. Preliminary palaeomagnetic data obtained in key Pleistocene sections of the Middle Dnieper region in contrast to most of current palaeogeographic reconstructions (Veklich, 1995 cited in Bolikhovskaya and Molodkov, 2006; Matviishyna et al., 2010; Gozhik and Gerasimenko, 2011) and in agreement with the earlier conclusions in (Tretyak and Vigilyanskaya, 1994; Vigilyanskaya, 2001a, b) and the latter correlation model (Bogucki et al., 2012) manifest the position of the MBB in the lowermost part of the Shyrokyne pedocomplex. In the Vyazivok section the MBB was found in the Shyrokyne palaeosol subunit sh1, which is in full agreement with previous
data, obtained by Vigilyanskaya (2001b). Preliminary investigations, obtained by V.G. Bakhmutov in 2006 and ongoing study of Stari Kaydaky profile cannot indicate evidence of the MBB boundary at least above sh1 subunit (Fig. 4).
The Medzhybizh-A section and the lowermost part of the Holovchyntsi-1 section from Podolia region are characterized by low concentration of fer-rimagnetic material, the destruction of the primary sedimentary magnetic texture, which makes them unsuitable for qualitative magnetostratigraphic studies. Additionally, the Medzhybizh-A section has very small thickness of stratigraphic units (Fig. 4). A zone of normal polarity, probably the Brunhes chron, has been reliably determined in the uppermost part of the Holovchyntsi-1 section (sh3 subunit). The palaeomag-netic veracity of the remaining investigated layers is questioned. Unfortunately, reliable data on the MBB and, correspondingly, data on the age of any layers with artifacts in the Medzhybizh-A and the Hol-ovchyntsi-1 sections more than 780 kyr have not been obtained from the results of palaeomagnetic studies.
Fig. 4. Comparison chart of Shyrokyne palaeosol complex in the Vyazivok, Stari Kaydaky, Holovchyntsi and Medzhybizh sections.
These results further supports that the Shyrokyne palaeosol unit should be comparable to the marine oxygen isotope stage 19 (as well as in (Bogucki et al., 2012)) rather than 25 (Veklich, 1995, cited in Bolik-hovskaya and Molodkov, 2006) or 21-33 (Lindner et al., 2006).
Inconsistencies in the stratigraphic position of the MBB in the Shyrokyne and Martonosha units relative to the national stratigraphic scheme (Veklich et al., 1993) in different studies could be caused by following factors: 1) ambiguous physical definition of the palaeoclimatic boundaries; 2) the relatively low sampling resolution of some previous studies; 3) methodological difficulties in determination of the ChRM component; 4) inconsistent or incorrect stratigraphic subdivisions of Pleistocene loess-palaeosol series in some studies. They are not evidently associated with the influence of secondary magnetization processes on the palaeomagnetic record because of a huge distance between the Shyrokyne and Martonosha units in the Middle Dnieper sequences (3-13 m). Even for thick Chinese loess sequences lock-in effect exceed not more 2-3 m (Tauxe et al., 1996; Zhou and Shackleton, 1999; Spassov et al., 2003; Liu et al., 2008). Since the MBB was defined in loess series of Ukraine much more often in palaeosol, it should be stratigraphically the same horizon.
Furthermore, the determination of the position of the MBB in loess sections should take into account the fact that loess sediments are affected by soil formation processes less than compared to soils (Bolshakov, 2008). Therefore, the overprinting effect of chemical magnetization in loess is less significant. Thus, loess can serve as a barrier against palaeomag-netic lock-in depth due to secondary chemical processes. In other words, if the Matuyama-Brunhes reversal is synchronous with the formation of a part of soil horizon, the palaeomagnetic record of the reversal in general cannot be displaced appreciably below the boundary between the soil and underlying loess (Bolshakov, 2008). The Pryazovya loess unit below the Martonosha unit in the Vyazivok and Stari Kay-daky sections has completely normal polarity, which excludes secondary processes overprint on the palae-omagnetic record in above layers. Notably, in earlier works (Veklich, 1982; Veklich, 1987), in which the MBB had been placed in the Martonosha unit, the entire underlying Pryazovya loess has normal polarity and it was mistakenly attributed to Jaramillo excursion.
Magnetic and palaeomagnetic characteristics of the key sections of the Ukrainian loess sequences composed of various types of loess-palaeosol horizons should be carefully studied and an integrated
palaeogeographic analysis of the sediments should be conducted in order to compare climatic conditions of the formation of loess and soil units located around the MBB in different sequences. Clarifying the question of stratigraphic position of the MBB in key loess series of Ukraine allows recognizing the most objective chronostratigraphic marker and would help to correlate them with other loess sequences across Eurasia. Conclussions.
Using multiple tools of measurements in different palaeomagnetic laboratories, the MBB was identified in the Vyazivok section at the Shyrokyne lowermost palaeosol sh1 subunit.
In the Stari Kaydaky section the MBB cannot be defined at least above Shyrokyne sh1 subunit. Further study is currently in progress to identify polarity reversal in underlying layers.
Among studied sections in Podolian Upland only the uppermost part of the Holovchyntsi section (subhorizon sh3) has been turned out to be acceptable for palaeomagnetic studies, in which the Brunhes chron of normal polarity has been reliably determined. The palaeomagnetic informativeness of the rest of the studied strata is questionable.
In the present Ukrainian stratigraphic system the chronological setting of the Shyrokyne and Martonosha units is regarded in different ways. It appears most reasonable (at least in studied sequences) to correlate the Shyrokyne complex with MIS 19, and the Mar-tonosha unit with MIS 17. This correlation supports the stratigraphic model of (Bogucki et al., 2012). Acknowledgements. This research was supported by grant 0117U003498 from National Academy of Sciences of Ukraine. The author wishes to thank D.Sc. N.P. Gerasimenko, D.Sc. Zh.M. Matviishyna, D.Sc. V.M. Stepanchuk and PhD S.P. Karmazinenko for their assistance during field works.
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