Earth and Planetary Science Letters 292 (2010) 123–131

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Earth and Planetary Science Letters

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Strontium isotope ratios of the Eastern Paratethys during the Mio-Pliocenetransition; Implications for interbasinal connectivity

Iuliana Vasiliev a,⁎, Gert-Jan Reichart b, Gareth R. Davies c, Wout Krijgsman a, Marius Stoica d

a Palaeomagnetic Laboratory ‘Fort Hoofddijk’, Budapestlaan 17, 3584 CD, Utrecht, The Netherlandsb Department of Geochemistry, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD, Utrecht, The Netherlandsc Department of Petrology, Faculty of Earth and Life Sciences, Vrije University, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlandsd Department of Palaeontology, Faculty of Geology and Geophysics, University of Bucharest, Bălcescu Bd. 1, 010041, Romania

⁎ Corresponding author. Tel.: +31 30 253 1361; fax:E-mail addresses: vasiliev@geo.uu.nl (I. Vasiliev), rei

gareth.davies@falw.vu.nl (G.R. Davies), krijgsma@geo.uu.nmarius@geo.edu.ro (M. Stoica).

0012-821X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.epsl.2010.01.027

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 August 2009Received in revised form 14 January 2010Accepted 19 January 2010Available online 6 February 2010

Editor: M.L. Delaney

Keywords:Paratethys87Sr/86SrPlioceneDacian basinLago Mare

Paratethys represents the large basin that extended from central Europe to inner Asia, comprising the NorthAlpine foreland, Pannonian and Dacian basins, the Black Sea and Caspian Sea. Connectivity between thesesubbasins and the connectivity of Paratethys with the open ocean varied drastically because of pervasivetectono-climatic processes affecting the region. Here, we investigate the biogenically produced carbonates ofthe Dacian basin for strontium analyses to monitor changes in connectivity, water geochemistry andpalaeoenvironment during the Mio-Pliocene transition. Diagenetic evaluation showed that not allcontamination could be removed, but that the strontium content of our samples was not affected by post-depositional processes. 87Sr/86Sr ratios of ostracods and molluscs are in good agreement and show relativelyconstant values of 0.70865–0.70885. These are much lower than coeval Mio-Pliocene ocean water (0.7089–0.7090), which indicates that no long-standing connection existed to the Mediterranean. The newly obtainedstrontium ratios for Paratethys are best explained by a mixture of Danube, Dnieper and Don river waters,implying connectivity between Dacian basin and Black Sea during the latest Miocene–earliest Pliocene. Weobserved no evidence for connectivity to the Caspian Sea during this period. The 87Sr/86Sr ratios of the Dacianbasin are similar to the ones measured in the Mediterranean “Upper Evaporites/Lago Mare” facies. The majorfresh water deluge at the end of the Messinian salinity crisis could thus have been caused by drowning ofEastern Paratethys waters into the Mediterranean.

+31 30 253 1677.chart@geo.uu.nl (G.-J. Reichart),l (W. Krijgsman),

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

The strontium isotope ratio of ocean waters is the same at any onetime, regardless of where in the ocean it is measured, because thestrontium residence time is considerably longer than the mixing timeof ocean water. Oceanic 87Sr/86Sr ratios fluctuate through geologicaltime, showing a general increase from ∼50 Ma to present, and canthus be used as stratigraphic tool (McKenzie et al., 1988; Hodell et al.,1989a; Hodell et al., 1994; McArthur et al., 2001). Marine sedimentaryrecords can be dated by measuring strontium ratios (e.g. from well-preserved foraminifera), especially at time intervals where the 87Sr/86Sr curve shows steep changes (Hodell et al., 1991; Miller et al.,1991a,b; McArthur et al., 2001).

In (semi)isolated inland basins strontium isotope dating is farmore complicated because the 87Sr/86Sr ratio is controlled (in thesegeological settings) by mixing of ocean water and river water. The

87Sr/86Sr ratio of river water reflects the regional catchments geologyand may differ substantially between drainage systems (Major et al.,2006). Variations in input and mixing of different water sources willthus be reflected in the 87Sr/86Sr ratios, especially if these sources havemarkedly different isotope ratios. Consequently, the strontiumisotope ratio can be used to quantitatively study the influx of riverwater and to determine connectivity. River input, however, needs toexceed ∼50% of the total inflow and needs to have stronglycontrasting 87Sr/86Sr values and high strontium concentrations to beclearly identified (Flecker et al., 2002; Flecker and Ellam, 2006).

The Black Sea, or its geological precursor Paratethys (Fig. 1),represents an inland basin that is very suitable to study regionalhydrological patterns and interbasinal water exchange by determin-ing the strontium isotope values. Connectivity to the Mediterraneanwould result in 87Sr/86Sr ratios that are extremely close to oceanicvalues (0.709155), while an exclusively fresh water supply would bereflected in values typical for the rivers feeding the Black Sea basin(0.708792; Palmer and Edmond, 1989). Additionally, ocean water has∼30 times more Sr dissolved than the rivers entering the Black Sea(Table 1). During the last glacial, when the Black Sea became a lake bycomplete isolation from the open ocean, 87Sr/86Sr values (Table 1;

mailto:vasiliev@geo.uu.nlmailto:reichart@geo.uu.nlmailto:gareth.davies@falw.vu.nlmailto:krijgsma@geo.uu.nlmailto:marius@geo.edu.rohttp://dx.doi.org/10.1016/j.epsl.2010.01.027http://www.sciencedirect.com/science/journal/0012821X

Fig. 1. Schematic palaeogeographic map of Paratethys region, comprising Lake Pannon, the Dacian basin (DB), Black Sea and Caspian Sea. The star indicates the position of theRîmnicu Sărat section in the Dacian basin as a Paratethys sub-basin. DS indicates the location of the Dobrogea sill and MS the position of the Marmara Sea.

124 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

Major et al., 2006) were indeed close to a weighted average of themajor rivers entering the basin (Major et al., 2006). It has also beensuggested that the Paratethys was temporarily isolated from the openocean during the Mio-Pliocene transition (Hsü and Giovanoli, 1979;Popov et al., 2006), especially when the Mediterranean water leveldropped because of the Messinian salinity crisis (MSC). There is littleinformation so far on strontium isotope changes in the Paratethysduring this period.

In this paper, 87Sr/86Sr analyses are applied to assess the changes inbasin water geochemistry and palaeoenvironment of the Dacian basin(Romania) during the Mio-Pliocene transition. We selected themagnetostratigraphically well-dated successions of the FocşaniDepression (Vasiliev et al., 2004; Vasiliev et al., 2007), and focussedon the interval between 6.3 and 4.1 Ma (Fig. 2), when Paratethyswater level may have dropped because of the MSC. During this period,the Dacian basin formed, together with the Black Sea and Caspian Sea,the Eastern Paratethys (Fig. 1). The three subbasins were separated byshallow sills at Dobrogea and the Caucasus, and minor changes intectonic uplift, sea level or hydrological budgets could have seriouslyinfluenced interbasinal connectivity. We selected both ostracod

Table 187Sr/86Sr ratios of various rivers and open marine domains around the Paratethys. The Ave

Water body Remarks Sr(ppm)

Present-day ocean water 7.62Global river waterAOW during Lower EvaporitesAOW during Upper evaporitesMessinian sea waterMarmara Sea (modern)Aegean Sea (modern)Black Sea (modern)Black Sea (Last Glacial Maximum)Danube 53% of freshwater runoff to Black Sea 0.24Dnieper 14% of freshwater runoff to Black Sea 0.22Don ∼16% of freshwater runoff to Black Sea 0.22Sakarya ∼4% of freshwater runoff to Black SeaRiver average Danube, Dnieper, Don and Sakarya 0.24Caspian Sea (modern) 0.48Volga 82% of freshwater runoff to Caspian Sea 9.92

valves and mollusc shells for strontium analyses, because of theiromnipresence throughout the succession and excellent state ofpreservation. Prior to Sr analyses we determined trace and minorelements to assess the preservation state of the biogenic carbonates.The results will be used to analyse the hydrological balance of theDacian basin, and to determine the interbasinal connectivity withinthe Eastern Paratethys and the Mediterranean. The data will furtherserve as crucial constraints for ongoing strontium models ofMessinian evaporites (Flecker et al., 2002) and may help to decipherthe alleged Paratethys influx into the Mediterranean in the final (LagoMare) stage of the MSC (Hsü et al., 1973; Orszag-Sperber, 2006).

2. Geological setting

The Paratethys has been a semi-enclosed basin since the beginningof the Oligocene, when it extended from central Europe to inner Asia(Rögl, 1996; Ramstein et al., 1997; Popov et al., 2006). Alpinecontinental collision during the Neogene caused restricted circulationand progressive rearrangement of individual subbasins. Paratethysgradually transformed into a restricted marine and finally into a giant

rage Ocean Waters (AOW) is also reported.

87Sr/86Sr Reference

0.709155 Henderson et al. (1994)0.712 Palmer and Edmond (1989)0.708999 Howarth and McArthur (1997)0.709012 Howarth and McArthur (1997)0.708983–0.709028 Howarth and McArthur (1997)0.70915 Major et al. (2006)0.709157 Major et al. (2006)0.709133 Major et al. (2006)0.70865–0.70875 Major et al. (2006)0.7089 Palmer and Edmond (1989), Major et al. (2006)0.7085 Shimkus and Trimonis (1974), Palmer and Edmond (1989)0.7085 Shimkus and Trimonis (1974), Palmer and Edmond (1989)0.7089 Major et al. (2006)0.708792 Major et al. (2006)0.7082 Clauer et al. (2000), Page et al. (2003)0.70802 Clauer et al. (2000), Page et al. (2003)

Fig. 2. Local stages, polarity zones, schematic lithological column and the position of themollusc and ostracod samples used for trace elements and Sr isotopes; n.d. (no data)indicates the levels where the 87Sr/86Sr ratios were determined but, because of large Rbamounts found in the sample, interpreted as diagenetically affected. The dashed linesindicate the (interpretative) correlation of the polarity sequence to astronomicallydated polarity time scale (APTS) (Vasiliev et al., 2004) and updated to newpaleontological constraints (Stoica et al., 2007; Krijgsman et al., 2010). The age of thesamples was calculated according to their position in the magnetostratigraphic record.The sampling gap between 1200 and 1800 m is mostly related to the coarser lithologies(silts and sandstones). The initial plan to obtain a monospecific record (Cyprideis sp.) inostracods led to a major sampling gap between 1200 and 2300 m. To minimize it weselected a second specie (Thyrrenocythere filipescui) for 87Sr/86Sr analyses. In the timescale, Me1 and Me2 represent the lower and upper Meotian respectively; Odess.(Odessian), Portaf. (Portafferian) and Bosphorian are regional substages of PontianStage; Gt (Getian) and Pv (Parscovian) are substages of the Dacian Stage and Rm1represents the lower Romanian.

125I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

brackish–fresh water lake system during the upper Miocene–Pliocene(Fig. 1). Theknowledgeof itsMio-Pliocenepalaeoenvironmental historyrelies mainly on palaeoecological assessment of aquatic and terrestrialorganisms (e.g. Rögl and Daxner-Hock, 1996; Rögl, 1998), on thecorrelation to better time-constrained species from the Mediterraneanrealm (e.g. Harzhauser and Piller, 2004) and on several palynologicalstudies (Popescu, 2001; Ivanov et al., 2007; Utescher et al., 2009).

Marine connections between Paratethys and Mediterranean arecommonly considered to have ended in the Late Miocene. Since then,the Paratethys was a brackish to fresh water basin, resulting in thecomplete loss of its marine fauna (e.g. foraminifera, calcareousnannoplankton and dinocysts) (Magyar et al., 1999), and its faunalcontent became dominated by a variety of ostracods and molluscs,endemic to the Paratethys. Short periods of Mediterranean–Paratethysconnectivity are, however, suggested by some horizons containing

marine nannofossils (Clauzon et al., 2005; Snel et al., 2006). 87Sr/86Sranalyses can determine the source of Paratethys waters and mayelucidate the nature and timingofMediterranean–Paratethys exchange.

3. Analysed material and sample preparation

3.1. Analysed material

Benthic organisms like ostracods and molluscs produce carbonateshells, which can be separated from the sedimentary rock and analysedfor their isotopic composition. Twenty-five ostracod levelswere selectedfrom the Rîmnicu Sărat section, covering the interval between 6.3 and4.1 Ma. The section consists of cyclic alternations of sandstones andmudstones, deposited in a distal marine–brackish deltaic system(Panaiotu et al., 2007). The average duration (∼22 kyr) of thesedimentary cycles indicates deposition under the influence of preces-sion (Vasiliev et al., 2004). Our biogenic carbonates were extracted fromshales and siltstones because these fine-grained rocks ensured the bestpossible preservation of the shells. Single species records could not beused for the entire time interval, because the dynamically changingenvironments caused a faunal variation. The ostracod species Cyprideissp. Jones, 1857was chosen (Fig. 3a) because of its abundancewithin theselected time frame of the Romanian Carpathian foredeep (Fig. 2) andbecause this specieswas earlier successfully used for trace-elements andstable isotope studies (De Deckker et al., 1999; Anadon et al., 2002).Cyprideis is one of the most euryhaline ostracods that populate waterswith salinity ranging from 0.4% to 150%. Cyprideis torosa is a typicalshallow water species, which lives in permanent littoral marineenvironments as well as marginal marine environments such as deltas,estuaries and coastal lagoons. They have also been found in athalassicsaline lakes and have been described as anomalohaline (Van Harten,1990). During the Late Miocene, the genus Cyprideis was affected by agreat adaptive radiation in the Paratethys realm (Pipik et al., 2007),probably due to its adaptation to deeper waters (Van Harten, 1990; DeDeckker, 2001, 2002). Twenty-two of the selected levels from theRomanian Carpathian foredeep contained sufficient shells of Cyprideissp. for Sr isotope analysis (Fig. 2). Cyprideis is only scarcely present in thelower, middle and the first part of the upper Pontian at Rîmnicu Sărat(Krijgsman et al., 2010). To reduce the gap for that time period weanalysed three levels with Tyrrhenocythere filipescui (Fig. 2). Tyrrheno-cythere inhabits oligohaline to mesohaline waters, with the highestfrequency between 0 and 30 m depth (Yassini and Ghahermann, 1979).Where possible, we also used mollusc shells at the same stratigraphiclevels to compare the resultswith those obtained fromostracods (Fig. 2).The molluscs used in this study are unionids and cardiids and theirseasonal growth is recorded throughout the entire year; therefore acomplete shell records the variations in temperature, chemistry andisotopic signatures of water during the organism's lifetime.

3.2. Sample preparation

Specimens of Cyprideis sp. and T. filipescui were separated frombulk sediment by disaggregation in sodium carbonate solution, wetsieving to retain the >250 μm fraction and hand-picking under amicroscope. The ostracods samples were cleaned to remove clayfollowing the procedure of Barker et al. (2003), followed by washingtwice in MilliQ®, five times in methanol (96%) and by 30 s cleaning inan ultrasonic bath. The washing procedure was then repeated.Cleaned samples were then evaluated for diagenetic alteration usingtrace element analysis and scanning electron microscopy (SEM).

The mollusc specimens were handpicked and embedded in raisin. Aslice fromeachmolluscwas cut andfinelypolished. The fresh surfacewassampled using laser ablation inductively coupled plasma-mass spec-trometry (LA-ICP-MS). The specimenswere subsequently re-sampled forthe strontium isotope analysis using a Merchantek micro-drill or handmini-drill avoiding contaminated external parts of the shell.

Fig. 3. (a) Scanning electron micrograph of a Cyprideis sp. from Rîmnicu Sărat Valley (from sample RMS116) with ablation craters. (b) High magnification of an ablation crater wherethe pristine structure of the ostracod shell can be observed. (c) Time resolved laser-ablation inductively coupled plasma-mass spectrometry data. Middle parts (marked by theshaded areas) represent the part of the measurement taken into consideration for the trace elements calculations (e.g. Sr/Ca ratios). Normalization to Ca was necessary to overcomethe varying response (counts per second) during ablation that generates variable quantity of removed material. Important for 87Sr/86Sr ratios isotopes is that the Sr profiles recordlittle variations indicating pristine preservation of this element within the shell structure. Other elements record higher values at the outside parts of the shells indicating thatdespite careful cleaning the outer part of the shells collected unremovable clay minerals/coating in the ornament pores. Therefore, for trace elements calculations only the middle(grey) parts of the profiles were used.

126 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

4. Methods

The ostracods and molluscs were ablated using a 193 nmwavelength laser. Such a short wave length is essential for thereproducible ablation of the fragile shells, because carbonates do notabsorb laser radiation well at higher wavelengths (Mason and Kraan,2002; Reichart et al., 2003). The system employs a Lambda Physikexcimer laser with GeoLas 200Q optics. Ablation was performed in amixture of helium and argon atmosphere at a pulse repetition rate of6 Hz. Ablation craters were 80 μm in diameter (Fig. 3a and b). Ablatedmaterial was measured with respect to time (and hence depth) usinga Micromass Platform quadrupole ICP-MS instrument (Figs. 3c and 4).Calibration was performed against U.S. National Institute of Standardsand Technology SRM 610 glass using the concentration data of Pearceet al. (1997) with 44Ca as an internal standard. A collision andreduction cell (Mason and Kraan, 2002) was used to give improvedresults by reducing spectral interferences on the minor isotopes of Ca(42Ca, 43Ca and 44Ca). Multiple isotopes were used where possible toconfirm accurate concentration determinations (Fig. 3).

The 87Sr/86Sr ratios were measured on both ostracods and mollusccarbonates. Fromeach sample, 0.4 to 3 mgwasdissolved in 0.5 ml of 5 Nacetic acid. Any residue was separated by centrifugation and theremaining solution was evaporated to dryness. The resulting solidresidue was dissolved in 2 drops of concentrated HNO3 to remove

organics, and again evaporated to dryness. The residue was completelyredissolved at room temperature in 0.5 ml of 3 N HNO3 and centrifugedfor four minutes. 0.4 ml of the top-most part of the samples wasintroduced to chromatographic columns composed of “ElchromSr spec”ion exchange. The Sr fractionwasdried andnitrated twicewithonedropof concentrated HNO3. The isotopic analyses were carried out at theIsotopes Laboratory from Vrije Universiteit (Amsterdam). 87Sr/86Srratios were analysed on Finningan MAT 261 and 262 mass spectro-meters, running a triple-jump routine, applying exponential fraction-ation correction and normalizing to 87Sr/86Sr=0.1194. NBS 987 runduring this study was within the long term average 0.710242±0.000012 (2σ), n>200. The blanks were less 30×10−6 havethe 87Sr/86Sr ratio marked as ‘not determined’ in Table 2 (although theywere measured).

5. Diagenetic evaluation; reliability of strontium measurements

Contamination from clay minerals, iron and manganese (hydr)oxide coatings and possibly organic material can be a problem inmicrofossil trace element analysis. Most contamination is adhered tothe outer surface of the shells post mortem and might accumulate in

Fig. 4. Trace elements ratio from a 5.5 mm profile in the mollusc shells. The profile islocated in the lower half of the picture, just below the Mn/Ca record. The well resolvedpattern of changes in Ba/Ca, Sr/Ca and Mn/Ca, most probably reflects ancient seasonalchanges, from warm to cold season (or vice-versa), marked by the grey bands.

127I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

pores and between the spines of the ostracods. Rigorous purificationprocedures have been developed to remove such extraneous phases(Boyle, 1981; Lea and Boyle, 1993). Analysis by LA-ICP-MS makes itpossible to avoid such contamination during the integration of thedata acquired during analysis of single valves. Al and U weremonitored, to evaluate the surface contamination by clay particles(Fig. 3c). Mn was used as a proxy of secondary carbonate andmanganese (hydr)oxides overgrowth. Those parts of the timeintegrated measurements having higher counts of Al, U and Mnwere excluded from integration. Some of the specimens showedcontamination throughout the profile and were excluded completely.We recorded at least two profiles for each specimen. SEM images ofrepresentative specimens were also used to examine the mineralogyof the ostracod valves. In contrast to molluscs, ostracods build theirshells more or less instantaneous. This might explain why ablationprofiles for ostracods show remarkably constant concentrations withthicknessmaking the recognition of contamination relatively straight-forward. The non-contaminated part of the ostracods shells aremarked by the grey interval (Fig. 3).

Ultimately, it is the heterogeneity between individual ostracodvalves that sets the limit to the accuracy of ostracods trace metalbased environmental reconstructions. Annual environmental andwater composition changes would result in variations betweenindividuals from the same sample. In view of the relatively fast shellbuilding of the ostracods, changes in this growth rate could easilyinfluence Sr and Mg incorporation, causing differences betweenostracods shell from the same location. The ablation profiles forostracods shows that even after the very thorough cleaning procedurenot all the contamination present at the outer parts could be removed(Fig. 3c). However, the evaluation of these profiles enabled us toobserve that the Sr/Ca ratio for the ostracods was constant throughentire ablation profiles, indicating that the Sr content was not affectedby post-deposition processes (Fig. 3c).

Ablation tracks were analysed perpendicular to the growth lines toinvestigate the possible diagenetic overprinting in the mollusc shells(Fig. 4). The excellently preserved pattern of changes in Ba/Ca, Sr/Caand Mn/Ca ratios is interpreted to reflect ancient seasonal changes,providing strong evidence for good preservation of the mollusc shellsfrom this valley. The cyclicity in the trace element ratios correlates tothe growth lines (Fig. 4) comparable to modern shells (Vonhof et al.,2003).

Because of the constant Sr/Ca ratios through the LA-ICP-MSprofiles from ostracods and the preserved seasonal patterns in themolluscs we concluded that the Sr content of the analysed shells wasnot affected by depositional processes. Therefore we used the LA-ICP-MS Sr content data (expressed as Sr molar ratios) by averaging thevalues obtained at each level (Table 2). For seven levels, the number ofostracods shells was limited to 3–4 valves and therefore we chose tonot record the trace and minor elements. We used all the availablematerial to measure the Sr isotope ratios. Based on the results ofeighteen well-preserved sites we concluded that the Sr content wasalso not affected by post-depositional processes. Two ostracod levelsout of seven gave ‘not determined’ Sr contents, but had time-equivalent molluscs data (RMS 48 M and RMS 45 M). Their Sr valuesare not deflecting from our other molluscs-based data (Table 2).

6. 87Sr/86Sr values for the carbonates of the Dacian basin duringthe Mio-Pliocene transition

Biological and inorganic precipitates record ambient water Srisotope compositions. Unlike oxygen isotopes, the measurementtechniques for strontium isotope ratios rule out measurable massdependent fractionation from biological effects, temperature, or otherphysical environmental changes (Major et al., 2006). This potentiallyprovides valuable information on the isotopic composition of thewater, reflecting both connectivity of the basin to the open ocean, andchanges in regional climate and hydrography. The resultant isotopicwater composition depends on the Sr concentration of river water andon the isotopic contrast between oceans and rivers. Strontium isotoperatios can thus be used to test whether salinity fluctuations resultedfrom changes in fresh water supply (precipitation plus river runoff),from variations in evaporation, or from both.

The Sr content in biogenic carbonates depends upon temperatureand on the biological and physical environment, but these parametersdo not influence 87Sr/86Sr (Faure, 1998). The values for Sr content ofthe Dacian basin, expressed as Srmolar ratio, are ∼2.2 times higher forthe ostracods than for the molluscs (Table 2). The decreasing trend ofSr incorporated over time is similar for both ostracods and molluscs.

Our results show that the 87Sr/86Sr ratios of ostracod valves fromthe Rîmnicu Sărat section (Fig. 5, Table 2) range from 0.708664 to0.708768. The 87Sr/86Sr ratios in mollusc carbonates range are in goodagreement ranging from 0.708683 to 0.708882 (Table 2 and Fig. 5).Only at one level (RMS116, Table 2), the 87Sr/86Sr ratio obtained frommolluscs (0.708865±26) differ substantially from the one obtainedfrom the ostracod valves (0.708664±6). The latest Miocene–earliestPliocene strontium values of the Dacian Basin carbonates aremarkedly different from coeval global ocean values (Henderson etal., 1994; McArthur et al., 2001) being significantly lower than themarine waters at that time (Fig. 5).

Two samples show 87Sr/86Sr ratio that significantly differ from themean values of the Dacian basin record. Sample RMS 96 O, located atthe Portaferrian/Bosphorian boundary has a value of 0.708964±9,which is very close to the oceanic curve (Fig. 2). A possibleexplanation is that this level corresponds to a very short marineinflux into the Dacian basin. It concerns one of the twomeasurementsbased on T. filipescui, but Rb content, monitored by default for all theanalyses, is within the limits describing it as non-diageneticallyaffected. The second sample (RMS114 O) has the lowest strontiumratio of the dataset (0.708511±11). This low value may be related to

Table 287Sr/86Sr ratios, 2σ error, stratigraphic position (in m), age (in Ma) and local stages from Rîmnicu Sărat Valley section. Sr concentrations (mmol/mol) are also reported; n.d. (no data)indicates the levels where the values were not determined.

Sample name 87Sr/86Sr 2(10−6)

Ag(Ma)

Level(m)

Local stage Sr(mmol/mol)

Species

RMS116 O 0.708664 6 6.21 1099 Upper Meotian (Me2) 2.15 Cyprideis sp.RMS114 O 0.708511 11 6.20 1112 Upper Meotian (Me2) 2.07 Cyprideis sp.RMS110 O 0.708782 4 6.13 1142 Upper Meotian (Me2) 2.95 Cyprideis sp.RMS096 O 0.708964 9 5.52 1808 Bosphorian (Po3) n.d. Tyrrhenocythere filipescuiRMS094 O n.d. n.d. 5.50 1815 Bosphorian (Po3) n.d. Tyrrhenocythere filipescuiRMS088 O 0.708823 10 5.32 1993 Bosphorian (Po3) n.d. Tyrrhenocythere filipescuiRMS084 O 0.708779 8 5.12 2243 Bosphorian (Po3) 1.95 Cyprideis sp.RMS074 O 0.708722 11 4.98 2483 Bosphorian (Po3) 1.26 Cyprideis sp.RMS067 O 0.708763 13 4.95 2739.5 Bosphorian (Po3) 1.09 Cyprideis sp.RMS065 O 0.708774 10 4.90 2841.5 Getian (Dc1) 1.59 Cyprideis sp.RMS060 O 0.708853 16 4.68 3001 Getian (Dc1) n.d. Cyprideis sp.RMS055 O 0.708821 10 4.60 3252 Getian (Dc1) 1.29 Cyprideis sp.RMS053 O 0.708786 9 4.52 3341 Getian (Dc1) 1.02 Cyprideis sp.RMS051 O 0.708768 8 4.50 3395.5 Getian (Dc1) 1.53 Cyprideis sp.RMS048 O n.d. n.d. 4.45 3441 Getian (Dc1) n.d. Cyprideis sp.RMS045 O n.d. n.d. 4.40 3561 Getian (Dc1) n.d. Cyprideis sp.RMS044 O 0.708825 8 4.36 3594 Getian (Dc1) 1.43 Cyprideis sp.RMS038 O 0.708812 7 4.35 3598.5 Getian (Dc1) 0.82 Cyprideis sp.RMS033 O 0.708786 10 4.34 3619 Getian (Dc1) 1.04 Cyprideis sp.RMS030 O 0.708777 7 4.33 3697 Getian (Dc1) 1.15 Cyprideis sp.RMS029 O 0.708804 10 4.32 3716 Getian/Parscovian 0.86 Cyprideis sp.RMS028 O 0.708810 10 4.31 3725.5 Getian/Parscovian 1.27 Cyprideis sp.RMS019 O 0.708844 15 4.17 3912 Parscovian (Dc2) n.d. Cyprideis sp.RMS018 O 0.708817 9 4.16 3914 Parscovian (Dc2) 0.88 Cyprideis sp.RMS007 O 0.708826 8 4.12 4011 Parscovian (Dc2) 1.17 Cyprideis sp.RMS116 M 0.708865 26 6.21 1099 Upper Meotian (Me2) 0.72 Unio (Psilunio) sp.RMS110 M 0.708776 9 6.13 1142 Upper Meotian (Me2) 1.23 Unio (Psilunio) sp.RMS1 styllo 0.708831 10 4.50 3395.5 Getian (Dc1) 0.59 Stylodacna stylodacnaRMS3 styllo 0.708803 7 4.49 3410 Getian (Dc1) 0.73 Stylodacna hebertiRMS048 M 0.708776 10 4.45 3441 Getian (Dc1) 0.64 Prosodacna sp.RMS045 M 0.708811 8 4.40 3561 Getian (Dc1) 0.65 Stylodacna hebertiRMS043 M 0.708772 8 4.36 3579 Getian (Dc1) 0.54 Unio (Rumanounio) rumanusRMS039 M 0.708795 10 4.35 3594 Getian (Dc1) 0.67 Prosodacna (Psilodon) neumayriRMS033 M 0.708778 6 4.34 3619 Getian (Dc1) 0.45 Prosodacna (Psilodon) neumayriRMS031 M 0.708683 7 4.34 3691 Getian (Dc1) 0.47 Prosodacna (Psilodon) neumayriRMS2 proso 0.708815 5 4.21 3875 Parscovian (Dc2) 0.57 Prosodacna (Psilodon) neumayriRMS4 styllo 0.708834 8 4.20 3877 Parscovian (Dc2) 0.37 Stylodacna hebertiRMS018 M 0.708743 9 4.16 3925 Parscovian (Dc2) 0.60 Prosodacna (Psilodon) neumayri

128 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

diagenesis that can be evaluated from the trace elements record(Supplementary Fig. 1).

7. Discussion

7.1. The isolation of the Eastern Paratethys during the Mio-Plioceneboundary interval

The present-day situation of the Black Sea, having only a very smallconnection to the Mediterranean through the Bosporus strait (2 kmwide and 30 m deep), results in sufficient radiogenic Sr supply toinduce oceanic 87Sr/86Sr ratios in the Black Sea, due to the highcontent of radiogenic Sr of sea water (Major et al., 2006). The majorParatethys rivers carry relatively low amounts of radiogenic Sr,ensuring a mean fluvial input with lower 87Sr/86Sr than ocean water(Palmer and Edmond, 1989; Muller and Mueller, 1991; Henderson etal., 1994; Flecker and Ellam, 1999). Paleoisolation of Paratethys wouldthus induce a tendency towards less radiogenic strontium, while anexclusively continental supply would be reflected in 87Sr/86Sr valuestypical for the rivers feeding the basin (Flecker and Ellam, 1999;Majoret al., 2006). This has been observed during the last glacial period,when strontium values (87Sr/86Sr∼0.70879) were close to a weightedaverage of the major rivers entering the Black Sea (Table 1 and Fig. 5).

The oceanic strontium ratios are well-determined for the Mio-Pliocene interval, showing a significant increase from 0.70895 to0.70904 (Hodell et al., 1991; Miller et al., 1991a,b; McArthur et al.,2001). The 87Sr/86Sr ratios measured from the Dacian basin are

significantly lower (ranging 0.708511 to 0.708768 in ostracods and0.708683 to 0.708882 in molluscs). These low 87Sr/86Sr ratios arecompatible with very limited input or even with complete isolationfrom the open ocean waters. Our 87Sr/86Sr values are rather constantwhen compare to the noticeable globally increasing trend of the lateNeogene seawater 87Sr/86Sr (Farrell et al., 1995), implying that theDacian Basin was not connected to the Mediterranean during thelatest Meotian (6.5–6.0 Ma). This is in good agreement with seismicsequence stratigraphic interpretations of the western Dacian basin(Leever et al., 2010) and the biochronological data from the FocşaniDepression (Krijgsman et al., 2010) that suggested a major trans-gression in the Dacian basin at the Meotian–Pontian boundary inmarine waters from the Mediterranean. Unfortunately, we have nostrontium data of the lower Pontian (6.0–5.6 Ma), to evaluate thepresence and the duration of this marine connection.

Our data further indicate that the Dacian basin did not receivemarinewaters from theMediterraneanduring the Bosphorian substage,which corresponds in time to the latest Messinian–early Pliocene(Krijgsman et al., 2010). Based on the strontium results, the only periodthat marine waters entered the Dacian basin was the Portaferrian/Bosphorian boundary interval (Fig. 2). The relatively low resolution ofour Paratethys data, however, still leaves room for other short marineincursions that are not yet resolved. Futureworkwill therefore focus onobtaining a higher resolution Sr isotope ratio record to establish possibletransient changes in sea level that cause marine incursions. The higherSr concentrations of seawater compared to brackish water, makes thebasin highly sensitive to such incursions.

Fig. 5. 87Sr/86Sr ratios for the Miocene–Pliocene samples of Rîmnicu Sărat Valley plotted against the ocean Sr isotope curve in grey between 3.5 and 6.5 Ma (Farrell et al., 1995;McArthur et al., 2001). The values are listed in Table 1. The open circles (Hodell et al., 1991), open triangles (Hodell et al., 1989b), open squares (Beets, 1991) and × (Richter andDePaolo, 1988) are individual Sr isotope data used for construction of the reference Ocean Sr isotope curve. Filled circles (squares) indicate ostracods (molluscs) from this study. Theerror for individual Romanian samples is plotted and the age is derived from the magnetostratigraphic correlation of Rîmnicu Sărat magnetostratigraphy to the APTS (Vasiliev et al.,2004). The other values represent all the published data for the 3.5–6.5 Myr time interval from the Mediterranean realm: Gavdos (Flecker et al., 2002), southern Turkey (Flecker andEllam, 1999), Eastern Italy (Montanari et al., 1997), Sicily (Lower and Upper Evaporites) (McKenzie et al., 1988; Muller and Mueller, 1991; Keogh and Butler, 1999), the TyrrhenianSea (Muller et al., 1990; Muller and Mueller, 1991) and the Balearic, Levantine and Ionian basins (Muller and Mueller, 1991). Age data for this compilation is according to (Fleckeret al., 2002). Blue dashed lines indicate the values from the four major rivers feeding the Black Sea (one of the remnants of the old Paratethys domain). The Caspian Sea (otherremnant of the Paratethys) and the values for the main rivers (Volga and Ural) feeding it are much lower (87Sr/86Sr=0.7082) than any of the values and are not included in thegraphic representation. In the left hand side data for the last glacial times (Major et al., 2006) are very similar to those obtained for the Mio-Pliocene transition of the Dacian basin.Note the different scale of the time axes.

129I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131

7.2. Interbasinal connectivity during the Mio-Pliocene transition

To investigate the interbasinal connectivity of the EasternParatethys domain we use the present day 87Sr/86Sr ratios of thedominant rivers that fed the Dacian, Black Sea and Caspian basins(Table 1). This assumption is justified because the palaeographicconfiguration of the source region had been relatively stable since theMio-Pliocene. The most important mountain ranges surrounding theParatethys, the Alps. Carpathians and Caucasus, were already formedand the drainage areas of the Danube, Don, Dniepr and Volgaremained roughly the same (Popov et al., 2006).

Present-day 87Sr/86Sr ratios of the major Paratethys rivers arebetween 0.7085 and 0.7089 (Table 1; Fig. 5). This range overlaps withthe low Sr isotope ratios in the Rîmnicu Sărat section (Table 2; Fig. 5).We thus interpret these Mio-Pliocene Sr isotope ratios of the Dacianbasin to be highly dominated by river input. Themain river that drainsinto the basin, the Danube, has a 87Sr/86Sr ratio of 0.7089, much lowerthan the ocean water during the Mio-Pliocene transition time(Shimkus and Trimonis, 1974; Palmer and Edmond, 1989) but stillhigher than all our data. Hence, the Danube cannot account for themeasured 87Sr/86Sr ratio on its own, indicating that an additional freshwater source should have been present. The best candidates for thesource of lower 87Sr/86Sr are Dnieper and Don, rivers located to theeast and draining now into the Black Sea. The Danube currentlyprovides ∼60% of the freshwater runoff to the Black Sea, while theother ∼40% comes from the Dnieper and Don. The 87Sr/86Sr data fromthe Dacian basin are similar to the values obtained for the Black Sea inthe last glacial times (Major et al., 2006) and suggests that the Dacianand the Black Sea basins were also connected during the latestMiocene–earliest Pliocene. Therefore, we conclude that the strontiumisotope ratio of the Eastern Paratethys (comprising at least the Dacian

Basin and Black Sea) during the Mio-Pliocene transition wasdominated by a mixed inflow from the Danube, Dnieper and Donrivers, having a relatively constant value ranging 0.70865–0.70885.Similar to our Dacian basin data are the five 87Sr/86Sr values obtainedfrom the lower part of the Alçıtepe Formation at Yenimahalle in theMarmara sea region (0.708656–0.708836) (Çagatay et al., 2006). Themagneto-biostratigraphic data from Yenimahalle indicated that theAlçıtepe Formation was deposited during chron C3r (6.04–5.24 Ma),partly corresponding in time to our Dacian basin record. Thus, Srisotope ratios from the Dacian basin are sustaining the conclusion ofÇagatay et al. (2006) that during the deposition of the lowerYenimahalle section the area was connected to the Eastern Paratethys(Çagatay et al. 2006).

When compared to the Danube, Don and Dnieper, the present-dayCaspian Sea has even lower 87Sr/86Sr values (∼0.7082), similar to theVolga river (Clauer et al., 2000; Page et al., 2003) that supplies 82% ofthe total amount of fresh water into the Caspian basin (Table 1).Connectivity between Black Sea and Caspian Sea is thus expected toimprint a low 87Sr/86Sr ratio signature. We conclude that during theMio-Pliocene transition the Caspian basin was probably isolated fromthe Black Sea, becausewe do not see any evidence for Volga signaturesin our 87Sr/86Sr data. This is in agreement with the late Miocenepaleogeographic reconstructions of the Eastern Paratethys thatindicate a subdivision into a Dacian/Euxinian basin system and aCaspian basin (Popov et al., 2006).

7.3. The possible Paratethys Sr signature in MSC waters

An extensively studied, but still poorly understood, major episodeof freshwater influx into amarine basin concerns the final phase of theMediterranean MSC. A major deluge of low salinity waters was

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proposed to have diluted the hypersaline environment of theMediterranean, generating wide-spread brackish-water conditions inthe latest Messinian and transforming the basin into a large LagoMare(Lake Sea) (Hsü et al., 1973). 87Sr/86Sr ratios measured in theMediterranean domain for those times reached mean values of0.70874 (McKenzie et al., 1988; Muller and Mueller, 1991; Montanariet al., 1997; Flecker and Ellam, 1999; Flecker et al., 2002) while theocean had a much higher 87Sr/86Sr ratio, of 0.709012 (Howarth andMcArthur, 1997). These highly deflected values for the Lago Marefacies must have been generated by a massive water influx of verydifferent Sr isotopic composition. Potential sources of distinctlydifferent isotopic composition are the Rhône and Nile rivers. Anotherhypothesis infers that fresh–brackish waters came from Paratethys, inagreement with the common presence of caspo-brackish faunalelements (ostracods, molluscs and dinoflagellates) in the Lago Maresediments (Hsü et al., 1973). The inflow from Paratethys into theMediterranean is difficult to ascertain since there is little informationon late Miocene connectivity (Çagatay et al., 2006).

The newly obtained strontium isotope ratios from the EasternParatethys can be compared with data from the Mediterranean MSCfacies (Fig. 5). The 87Sr/86Sr ratio of Paratethyswaters are similar to thevalues measured in Upper Evaporites (McKenzie et al., 1988; MullerandMueller, 1991; Keogh and Butler, 1999), and distinctly lower thanthe Lower Evaporites that still reflect the oceanic water ratios. Thisimplies that a major dilution of the Mediterranean brine took placeafter the “Lower Evaporites” (after 5.55 Ma), when the Mediterraneanbecame isolated from the Atlantic (Hilgen et al., 2007; Krijgsman andMeijer, 2008; Roveri et al., 2008). The similar Sr isotope ratios from theDacian basin, make the Eastern Paratethys a reasonable candidate forthe source of low Sr isotope waters of the Lago Mare facies (Fig. 5).However, an additional source is required to lower the Mediterraneanratios to the lowest 87Sr/86Sr values registered for theUpper Evaporites(0.70852). The best candidates for the low 87Sr/86Sr ratios, as proposedbefore (e.g. Muller et al., 1990; Muller and Mueller, 1991; Flecker andEllam, 1999, 2006; Flecker et al., 2002), are the Rhône (0.7087) andespecially the Nile (0.706).

8. Conclusions

We have observed a clear relation between the Sr concentrationsincorporated in the biogenic carbonates from the Carpathiansforedeep and 87Sr/86Sr. Different, but consistent, Sr partition coeffi-cients for the two groups of organisms, implies a general decreasingbasin water Sr/Ca ratio. The 87Sr/86Sr values are rather constant whencompare to the noticeable globally increasing trend of the lateNeogene seawater 87Sr/86Sr (Farrell et al., 1995). Both independentproxies show that relatively little Sr was supplied to the basin throughweathering of the local mountains and that exchange with the openoceanwas very limited or non-existent. Diagenetic evaluation showedthat even after very thorough cleaning, not all the contamination atthe outer parts of the ostracod shells could be removed. Nevertheless,the Sr/Ca ratio was constant in the ablation profiles, indicating thatthe Sr content was not affected by post-deposition processes.

The first reported 87Sr/86Sr ratios record for the Eastern Paratethysduring the Mio-Pliocene transition indicate much lower values thanthose in the coeval ocean waters. This indicates that the basin wasisolated from the Mediterranean and mainly fed by riverine waters.The Sr isotope ratios are consistent with a mixture of Danube, Dnieperand Don rivers, indicating connectivity between the Dacian basin andBlack Sea. The strongly contrasting 87Sr/86Sr signature of the Volga(0.70802) river, is not observed. Therefore, we suggest that theCaspian Sea was disconnected from the rest of Paratethys andbehaved as a separate entity. We further conclude that during latePontian–Dacian times (5.3–4.0 Ma) the Eastern Paratethys wasdisconnected from the Mediterranean.

The newly obtained 87Sr/86Sr ratios from the Dacian basin can beused to unravelwater exchange patterns in the circum-Mediterraneanregion during Pliocene times. The Sr ratios are similar to the onesmeasured in the Mediterranean “Upper Evaporites/Lago Mare”,indicating that the distinctly lower Sr isotope ratios of the latest MSCphase in theMediterraneanmay have been caused by waters from theEastern Paratethys.

Acknowledgements

I.V. thanks toMarinWaaijer and Richard Smeets (Vrije Universiteit)for help in the clean lab and during the 87Sr/86Sr measurements, toMartin Ziegler for help with ostracods cleaning procedures and to PaulMason for facilitating the access in the LA-ICP-MS laboratory. This workwas financially supported by the Netherlands Research Centre forIntegrated Solid Earth Sciences (ISES) and the Netherlands GeosciencesFoundation (ALW)with support from the Netherlands Organization forScientific Research (NWO). We thank Rachel Flecker and twoanonymous reviewers for their thorough and constructive reviewsthat significantly improved the manuscript.

Appendix A. Supplementary Data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2010.01.027.

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Strontium isotope ratios of the Eastern Paratethys during the Mio-Pliocene transition; Implicat.....IntroductionGeological settingAnalysed material and sample preparationAnalysed materialSample preparation

MethodsDiagenetic evaluation; reliability of strontium measurements87Sr/86Sr values for the carbonates of the Dacian basin during the Mio-Pliocene transitionDiscussionThe isolation of the Eastern Paratethys during the Mio-Pliocene boundary intervalInterbasinal connectivity during the Mio-Pliocene transitionThe possible Paratethys Sr signature in MSC waters

ConclusionsAcknowledgementsSupplementary DataReferences