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Earth and Planetary Science Letters 292 (2010) 123–131
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j ourna l homepage: www.e lsev ie r.com/ locate /eps l
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: [email protected] (I. Vasiliev), rei
[email protected] (G.R. Davies),
[email protected]@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 [email protected] (G.-J. Reichart),l (W.
Krijgsman),
ll rights reserved.
© 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:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.epsl.2010.01.027http://www.sciencedirect.com/science/journal/0012821X
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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)
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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.
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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
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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
-
130 I. Vasiliev et al. / Earth and Planetary Science Letters 292
(2010) 123–131
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