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СПИСАНИЕ НА БЪЛГАРСКОТО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, год. 77, кн. 2–3, 2016, с. 23–31REVIEW OF THE BULGARIAN GEOLOGICAL SOCIETY, vol. 77, part 2–3, 2016, p. 23–31

The sedimentological study of core EUXRo01-1 (Northwestern Black Sea) – a proof for the Black Sea water level changes during the Early Holocene

Krasimira Slavova1, Ivan Genov1, Plamen Ivanov2, Nikolai Dobrev2 1 Institute of Oceanology, Bulgarian Academy of Sciences, 40 First May str., P.O. Box 152, 9000 Varna, Bulgaria; E-mails: slavova@io-bas.bg; idgenov@io-bas.bg 2 Geological Institute “Str. Dimitrov”, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 24, 1113 Sofia, Bulgaria; E-mails: plivanov62@yahoo.cа; nikolaidd5@yahoo.com

Седиментоложкото изследване на ядка EUXRo01-1 (cеверозападната част на Черно море) – доказателство за промяната на черноморското ниво през Ранния Холоцен

Красимира Славова1, Иван Генов1, Пламен Иванов2, Николай Добрев2

1 Институт по океанология, БАН, ул. „Първи май“ 40, ПК 152, 9000 Варна 2 Геологически институт „Стр. Димитров“, БАН, ул. „Акад. Г. Бончев“, бл. 24, 1113 София

Abstract. The present study is focused on the development of geological setting under the erosional basis of the Early Holocene regression in the upper Northwestern Black Sea continental slope. The new obtained data for the core EUXRo01-1 were correlated with proxy-records from other sedimentary cores from the Northwestern and Western Black Sea upper continental slope and they correspond to the researches of other authors. The deposits in core EUXRo01-1 have recorded Late Pleistocene–Holocene geological setting changes. The new obtained data show that the deposits during the Early Holocene Black Sea regression and subsequent transgression were ac-cumulated in rather variable lithodynamic conditions of the shelf, shelf edge and the continental slope. At some places of the continental slope, the processes of redeposition (e.g. gravity flows, turbiditic currents) were mainly controlled by water-level changes during the Early Holocene and they have played an important role in the forming of the sedimentary sequences.

Keywords: Black Sea, Early Holocene, sedimentary processes, sedimentary sequences, particle size analysis, mineral density.

Резюме. Настоящото изследване е фокусирано върху развитието на геоложката обстановка под ерозионния базис на ран-нохолоценската регресия в северозападния черноморски континентален склон. Новополучените данни от ядка EUXRo01-1 са корелирани с такива от други седиментни ядки от северозападния и западния черноморски горен континентален склон и те са в съгласие с изследванията на други автори. Отложенията в ядка EUXRo01-1 са записали промени в геоложката обстановка през късноплейстоценско–холоценско време. Новите данни показват, че отложенията на раннохолоценската черноморска регресия и последвалата я трансгресия са акумулирани в много променливи литодинамични условия на шелфа, ръба на шелфа и континен-талния склон. В някои части на континенталния склон процесите на преотлагане (напр. гравитачни потоци, турбидитни потоци) са били контролирани основно от промените на водното ниво по време на Ранния Холоцен и са играли важна роля при формира-нето на седиментните последователности.

Ключови думи: Черно море, Ранен Холоцен, седиментационни процеси, седиментни последователности, зърно-метричен анализ, минерална плътност.

Introduction

During the Late Pleistocene and Holocene the sedi-mentary sequences in the Black Sea were strongly af-fected by water level changes. When the Black Sea water level dropped below its outlet, it developed in-dependently from the global sea level. It is proved that variations in the sediment composition of the Black

Sea were strongly related to climate changes in the hinterlands. Within the scientific community there are different claims about the level of the Black Sea Lake during the Holocene regression – from 70 mbsl (m be-low sea level) to 105 mbsl (Ryan et al., 1997, 2003; Lericolais et al., 2009; Genov, 2015b, among many others). In this study it is assumed that the latest Black Sea regression has been with minimum water level up

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to ~90 mbsl, identified by seismic sections in the west-ern part of the Black Sea Basin (Genov, 2015b).

It is proved, that the Black Sea could not have been in a continuous outflow mode throughout the last glacial period, because the Black Sea Lake lev-el clearly experienced periods of draw down below the Bosphorus sill depth (Ryan et al., 1997, 2003; Lericolais et al., 2009; Genov, 2015b; Major et al., 2002; Major, 2002, among many others). Chaotically accumulated sediments on the outer shelf or on the upper slope during the Early Holocene regression of the basin were in disequilibrium because they were exposed to various hydrological factors. The subse-quent abrupt transgression changed the hydrological situation (wave and littoral currents regime, their ar-eas of activity, rim currents location, etc.) which led to the enhancement of the imbalance of unconsolidated or poor-consolidated sediments. Processes of redepo-sition, mainly controlled by sea-level changes, have played an important role in the geological setting dur-ing the Holocene in Black Sea.

The studied gravity core ЕUXRo01-1 was lo-cated in the NW Black Sea continental slope. The NW Black Sea is mainly characterized by a very wide shelf area and receives the largest tributaries of the Pontic basin: the Danube, Dniester, Dnieper and Southern Bug rivers. Presently the Danube in-fluence is determinant for the sedimentation on the NW Black Sea shelf area (Panin, Jipa, 2002). Today the surface water circulation in the Back Sea con-sists of two large cyclonic (counterclockwise) cen-tral gyres that define the eastern and western basins. The gyres are bounded by the wind-driven rim cur-rent that flows along the edge of the continental shelf and above the continental slope around the whole ba-sin (Oguz, 2009). Data from autonomous profiling floats has shown currents typically have a velocity of 15 cm/s at 200 m depth along the rim current jet around the basin (Korotaev et al., 2006).

The aim of the present study is to complement the reconstruction of geological setting under the erosion basis of the Еarly Holocene regression in the upper NW Black Sea continental slope by involving the re-ceived new data. For this end, the following tasks were solved: 1) interpretation of the new obtained data from the core EUXRo01-1; 2) correlation between them and similar data from other cores from the NW and W Black Sea slope. For tracking of the sedimentary processes were used: а) proxy-records of the cores BLKS 9809 and BLKS 9810 from the upper continen-tal slope in the Western Black Sea (Major et al., 2002); b) lithology of the core BLKS 9808 (Major, 2002); c) data of cores published by various other authors (e.g., Bahr et al., 2006; Giunta et al., 2007; Genov et al., 2014); 3) study of the sedimentary characteristics of the core EUXRo01-1 and the associated with them geological processes as transgression, regression, pro-cesses of redeposition of sediments (e.g., gravity flows, turbiditic currents, etc.) in the NW Black Sea area.

Material and methods

The studied gravity core ЕUXRo01-1 was collected by a GeoEcomar team during a scientific cruise on board R/V Mare Nigrum from the Romanian locations of the MARINEGEOHAZARD project observatories. The core was recovered from a water depth of 228 m in front of the Danube Delta (Fig. 1). The investigated length of the core is 400 cm. Sedimentological descrip-tion and sampling of the core ЕUXRo01-1 for car-bonate, grain size and mineral density analyses were performed in the Institute of Oceanology, Bulgarian Academy of Sciences. The samples were analyzed in the Geological Institute, Bulgarian Academy of Sciences.

For determination of carbonate contents of the sampling intervals of the core a classic approach of decarbonatization was used. Dry sample with an exact weight (10 g) is treated with 5% HCl. After finishing of the reaction the solid remainder was desiccated and measured. Based on its weight the carbonate contents were calculated. The particle size distribution is one of the most important physical characteristics of sedi-ments. The percentage of sand, silt and clay in the in-organic fraction of sediments was measured using the Stoke’s method based on the rate of sedimentation of particles suspended in water. In this article, the parti-cle size analyses were done according to the following Bulgarian standard: BDS 2762-83 for particle-size dis-tribution of soils by hydrometer analysis. The mineral density analyses were done according to the following Bulgarian standards: BDS 646-81 for mineral density (ρs), using 100 ml volumetric flask. A stratigraphic ap-proach to the study of core ЕUXRo01-1 sediments in-cludes the application of the stratigraphy on the basis of investigations of Ross and Degens (1974).

Results and interpretation

Lithological composition with stratigraphic subdivi-sions and the results from carbonate, particle size dis-tribution and mineral density are graphically plotted on Figures 2 and 3.

The three units typical for deep-water sediments from the Black Sea Basin according to Ross and Degens (1974) were described in the core EUXRo01-1: lacustrine Unit 3 – lutite, deposited under the lake to brackish conditions, Unit 2 – organic-rich microlami-nated sediments, deposited under marine environment, and Unit 1 – marine carbonate-rich microlaminated sediments consisting mainly of the coccolithophore Emiliania huxleyi Lohmann (Fig. 2). In this study is accepted the age of the boundary between units 3/2 to be about 7.16 14C ka BP and the boundary between units 2/1 to be about 3.33 14С ka BP as reported by Jones and Gagnon (1994).

In the core EUXRo01-1 from 400 to 260 cm the sediments are typical lacustrine – gray clays with

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Fig. 1. Location of gravity core EUXRo01-1 and seismic line XXIV

Фиг. 1. Местоположение на ядка EUXRo01-1 и на сеизмичен профил XXIV

rare whole disarticulated small Dreissena whole and fragment shells (Fig. 2). The presence of a major-ity of fragments and disarticulated Dreissena shells and rarely articulated and small Dreissena shells in a clay matrix with a specific structure (sometimes soft,

sometimes hard, but predominantly soft) shows that during the deposition of those sediments there were conditions for a relative proximity of the core to the shore, i.e. regression of the basin. From the beginning of this interval to its end, a slight trend of increase in

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Fig. 2. Lithological description and stratigraphic subdivisions of core EUXRo01-1 with curves of particle size distribution and carbonate contents (%)

Фиг. 2. Литоложко описание и стратиграфски единици в ядка EUXRo01-1 с криви на гранулометричното разпре-деление и карбонатното съдържание (%)

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Fig. 3. Mineral density distribution (g/cm3) in cores EUXRo01-1 and EUXRo03-3

Фиг. 3. Разпределение на минералната плътност (g/cm3) в ядка EUXRo01-1 и ядка EUXRo03-3

coarse and medium coarse fractions is observed. The values of fine components are slightly lower (Fig. 2). These changes in the particle size of the core are in-dicative of the regression of the basin, too. Following the comparative lithological method and the location of sediments in the considered interval, under sapropel (Unit 2), their deposition refers to the Early Holocene regression, as proven for core EUXRo03-3 (Genov et al., 2014).

The Early Holocene regression of the basin is also confirmed by seismostratigraphic investigations (Genov, 2015b) (Fig. 4).

An evidence for low sea level during the Early Holocene in core GeoB 7610-1 (465 mbsl) gives the

trend towards δ18O isotopic values lighter than those in the deeper cores during that period too (Bahr et al., 2006; Fig. 4). Probable reason for these lighter δ18O isotopic values is that during a period of low sea level a progradation of the Danube delta to the shelf edge, shifting core GeoB 7610-1 gradually to a more proxi-mal position to the freshwater source than the deeper cores (Bahr et al., 2006).

Within the range from 260 to 252 cm there is no change in the lithology of core EUXRo01-1. The change occurs at the 252nd cm of the core. The change in grain size is also at the 252nd cm but it is plotted in Fig. 2 at 250th cm because the sample for grain size analysis was taken from the 250th cm and it does not

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coincide with the lithological sharp contact at 252nd cm (Fig. 2).

The lithological description of the interval 252–225 cm and the significant changes in particle size distribution in the same interval are typical for tur-bidite deposits, i.e. gradual transition from olive gray fine sand to silty clay (enriched with organic mat-ter) (Fig. 2). Such turbidites, characterized by fine sand to silt turbidite base and normally graded, are often described as dominant facies in the deep cores (Lericolais et al., 2013).

Major et al. (2002) reported high-resolution record of clay mineralogy in cores BLKS 9810 (378 mbsl) and BLKS 9809 (240 mbsl) from the upper continental slope of Romania in the Western Black Sea. The grain size data from core BLKS 9810 show major peak in the coarse fraction within the dark green mud (T) at the top of Unit 3 (Major et al., 2002 – Fig. 4). The layer “Т” indicates a dark green mud below the base of Unit 2 and it is limited in time between 8.4 and 7.16 14С ka BP. For Unit 3 in core BLKS 9810 is reported that it сontains rare mollusks, usually individual valves of small speciments of the Dreissena rostriformis and Turricaspia caspia, as well as in the core EUXRo01-1 (Fig. 2). For the shallower core BLKS 9809 Major et al. (2002) reported that the uppermost part of Unit 3 down to the base of light grey muds is missing. The dating which is reported for Dreissena sp. under sap-ropel Unit 2 is 12.31 14С ka BP (Major, 2002). The lack of the uppermost part of Unit 3, core BLKS 9809, which is 18 m deeper than core EUXRo01-1 is prob-ably caused by an subaqueous erosion or landslide of sediments just below the sapropel base. In core BLKS 9808 (186 mbsl) there is also a lack of sediments (Unconformity I), referring to the upper part of Unit 3 (Major, 2002 – Figs. 3–9). Giunta et al. (2007) re-ported for silty-sand intercalations into Unit 3 of core B2 KS38 (355 mbsl), immediately below Unit 2.

The foregoing confirms that the described litho-logical characteristics of core EUXRo01-1 are not an isolated case for the western upper slope of the Black Sea. Redeposion of sediments, lack of sediments and silty-sand intercalations just below the sapropel base in the above mentioned cores according to reference sources and core EUXRo01-1, are their common litho-logical characteristics. But they are probably caused by different mechanisms – subaqueous erosion, grav-ity flows, turbiditic currents, etc.

The lithology of the interval 252–225 cm and Unit 2 (225–151 cm) of core EUXRo01-1 as well as the results of particle size distribution analyses prove un-doubtedly an overall turbidite structure. The tendency expressed in reduction of the coarser fractions and in-crease of the finer fractions from the base to the top of the interval 252–151 cm as well as detritus inter-calations, approx. 1 mm from freshwater species at 214 and 216 cm and Dreissena shells fragments up to 151st cm (Fig. 2), are indicative of sapropel redeposi-tion in its major part. Therefore the abrupt decrease of mineral density in the interval 252–220 cm is sig-nificant and display fast deposition (i.e. weak sorting out and compaction). Similar decrease of the mineral density was measured in the base of sapropel deposits of the core EUXRo03-3 (Fig. 3).

The foregoing corresponds to the recent studies which have shown that the rising of the water level due to the sudden invasion of the Mediterranean waters into the Black Sea was fast enough to cause erosion of the lowstand shoreline and upper slope (Lericolais

et al., 2010) by acting on the deposits accumulated by the preceding the Early Holocene regression that are in disequilibrium (Fig. 4).

Most probably turbidites in the interval from 252 to 151 cm were caused from the gravity flows (turbid-ity currents) mainly controlled by rising of the sea-lev-el. Chaotically deposition of the sediments during the

Fig. 4. Seismic stratigraphic interpretation of the seismoacoustic profile XXIV (emended after Genov, 2015a): a, genetic units: FR (forced regressive), LNR (lowstand normal regressive), T (transgressive); b, sequence stratigraphic surfaces: SU (subaerial unconformity − dotted red line), RSME (regressive surface of marine erosion − pink line), MRS (maximum regres-sive surface − yellow line), MFS (maximum flooding surface − dotted blue line), TRS (transgressive ravinement surface − dotted green line) (Catuneanu et al., 2009); c, disturbance: gravity fault or slip fault − red line

Фиг. 4. Сеизмостратиграфска интерпретация на сеизмоакустичен профил XXIV (Genov, 2015a, с изменения): а − генетични единици: FR (регресивна), LNR (преходна от регресия към трансгресия), T (трансгресивна); b − страти-графски повърхности: SU (субаерално несъгласие − прекъсната червена линия), RSME (регресивна повърхност на морска ерозия − розова линия), MRS (максимална регресивна повърхност − жълта линия), MFS (максимална трансгресивна по-върхност − прекъсната синя линия), TRS (ерозионна повърхност, образувана при трансгресия − прекъсната зелена линия) (Catuneanu et al., 2009); c − нарушение: гравитационен разсед или разсед на хлъзгане − червена линия

→

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Early Holocene regression has generated instability of the sediments on shelf edge/upper slope in rather vari-able lithodynamic conditions. Under the subsequent rapid transgression the hydrological conditions were changed. The waves, coastal and rim currents changed the primary sediments position against the old coast-lines after the rapid rise in the level of the basin due to the invasion of the saltier Mediterranean waters.

As an important lithological feature in the base of Unit 2 – sapropel are described the white carbonate laminae at 225 cm and 217 cm. The presence of rice-like aragonite at the base of Unit 2 in core EUXRo03-3, as a marker for redeposition of sediments, has been previously proved (Genov et al., 2014). Hence, the tur-bidite structure of Unit 2 in core EUXRo01-1 and the interval below it confirm the redeposition of aragonite from the shelf at the base of Unit 2.

The carbonate values in the core EUXRo01-1 were measured within the interval from 160 cm to the top of the core and correspond to the lithological description of the core (Fig. 2). The lower values at 160 cm (at the end of Unit 2), compared with the values obtained for Unit 1, are a typical sign of sapropel sediments (Georgiev, 1984). The increase of the carbonate val-ues at 150th cm, although the boundary between Unit 2 and Unit 1 is described at 147 cm, is due to coc-colithic intercalation in the sapropel within the range of 151 to 147 cm (Fig. 2). The indisputable increase in carbonate values from 150th cm to the top of the core is typical of Unit 1 (Georgiev, 1984) because it is composed almost entirely of the remains of Emiliania huxleyi Lohmann.

The boundary between Unit 2 and Unit 1 at 147 cm in core EUXRo01-1 was described lithologically. In the description of Unit 1 (147–0 cm) of core EUXRo01-1 the intercalation of molluscs shells, dis-articulated Modiolus phaseolinus Philippi and darker intercalation enriched with organic matter, interbed-ded between coccolithic ooze layers, show probably local periodical redepositions as a result of the dynam-

ics of rim currents in the Black Sea, i.e. it is a reflec-tion of the erosion-accumulative activity of currents near the edge of the shelf. This is expressed clearly by the variations in the particle size distribution of the core for this interval (Fig. 2). The data show that the deposits were accumulated in rather variable lithody-namic conditions.

Conclusions

At some places of the continental slope the rising of the water level, as a result of the sudden incursion of the Mediterranean salt waters into the Black Sea, has caused erosion of the lowstand shoreline and the upper slope unconsolidated or poor-consolidated sediments accumulated in the preceding regression. This in turn has led to turbiditic currents or slippages toward the greater depths of the basin resulting in redeposition of sediments. Data obtained from the carbonate, particle size and mineral density analyses of core EUXRo01-1 and the sedimentary sequences at the end of Unit 3 and Unit 2 in the same core prove that statement and are in accordance with the proxy-records and lithology of the other cores used in this study.

Sediment formation at Unit 1 in core EUXRo01-1 shows probable local periodic redepositions resulting from the dynamics of rim currents in the NW Black Sea, i.e. it is a reflection of the erosion-accumulative activity of currents near the edge of the shelf that have occurred on the recent geological timescales.

Acknowledgements: The present article has been written as a part of MIS-ETC 641 – MARINEGEO- HAZARD Project through financial support by CBC Romania-Bulgaria Programme 2007–2013. We are grateful to Stamenka Milosheva from Geological Institute “Str. Dimitrov”, Bulgarian Academy of Sci-ences for the performed precise carbonate analyses.

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(Постъпила на 06.10.2016 г., приета за печат на 27.02.2017 г.)Отговорни редактори Б. Вълчев и Елена Колева-Рекалова