Strontium isotope ratios of the Eastern Paratethys during the Mio ...
Strontium isotope ratios of the Eastern Paratethys during the Mio ...
Strontium isotope ratios of the Eastern Paratethys during the Mio ...
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Earth and Planetary Science Letters 292 (2010) 123–131<br />
Contents lists available at ScienceDirect<br />
Earth and Planetary Science Letters<br />
journal homepage: www.elsevier.com/locate/epsl<br />
<strong>Strontium</strong> <strong>isotope</strong> <strong>ratios</strong> <strong>of</strong> <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene<br />
transition; Implications for interbasinal connectivity<br />
Iuliana Vasiliev a, ⁎, Gert-Jan Reichart b , Gareth R. Davies c , Wout Krijgsman a , Marius Stoica d<br />
a Palaeomagnetic Laboratory ‘Fort Ho<strong>of</strong>ddijk’, Budapestlaan 17, 3584 CD, Utrecht, The Ne<strong>the</strong>rlands<br />
b Department <strong>of</strong> Geochemistry, Faculty <strong>of</strong> Geosciences, Utrecht University, Budapestlaan 4, 3584 CD, Utrecht, The Ne<strong>the</strong>rlands<br />
c Department <strong>of</strong> Petrology, Faculty <strong>of</strong> Earth and Life Sciences, Vrije University, De Boelelaan 1085, 1081 HV, Amsterdam, The Ne<strong>the</strong>rlands<br />
d Department <strong>of</strong> Palaeontology, Faculty <strong>of</strong> Geology and Geophysics, University <strong>of</strong> Bucharest, Bălcescu Bd. 1, 010041, Romania<br />
article<br />
info<br />
abstract<br />
Article history:<br />
Received 4 August 2009<br />
Received in revised form 14 January 2010<br />
Accepted 19 January 2010<br />
Available online 6 February 2010<br />
Editor: M.L. Delaney<br />
Keywords:<br />
<strong>Paratethys</strong><br />
87Sr/86Sr<br />
Pliocene<br />
Dacian basin<br />
Lago Mare<br />
<strong>Paratethys</strong> represents <strong>the</strong> large basin that extended from central Europe to inner Asia, comprising <strong>the</strong> North<br />
Alpine foreland, Pannonian and Dacian basins, <strong>the</strong> Black Sea and Caspian Sea. Connectivity between <strong>the</strong>se<br />
subbasins and <strong>the</strong> connectivity <strong>of</strong> <strong>Paratethys</strong> with <strong>the</strong> open ocean varied drastically because <strong>of</strong> pervasive<br />
tectono-climatic processes affecting <strong>the</strong> region. Here, we investigate <strong>the</strong> biogenically produced carbonates <strong>of</strong><br />
<strong>the</strong> Dacian basin for strontium analyses to monitor changes in connectivity, water geochemistry and<br />
palaeoenvironment <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition. Diagenetic evaluation showed that not all<br />
contamination could be removed, but that <strong>the</strong> strontium content <strong>of</strong> our samples was not affected by postdepositional<br />
processes. 87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> ostracods and molluscs are in good agreement and show relatively<br />
constant values <strong>of</strong> 0.70865–0.70885. These are much lower than coeval <strong>Mio</strong>-Pliocene ocean water (0.7089–<br />
0.7090), which indicates that no long-standing connection existed to <strong>the</strong> Mediterranean. The newly obtained<br />
strontium <strong>ratios</strong> for <strong>Paratethys</strong> are best explained by a mixture <strong>of</strong> Danube, Dnieper and Don river waters,<br />
implying connectivity between Dacian basin and Black Sea <strong>during</strong> <strong>the</strong> latest <strong>Mio</strong>cene–earliest Pliocene. We<br />
observed no evidence for connectivity to <strong>the</strong> Caspian Sea <strong>during</strong> this period. The 87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> <strong>the</strong> Dacian<br />
basin are similar to <strong>the</strong> ones measured in <strong>the</strong> Mediterranean “Upper Evaporites/Lago Mare” facies. The major<br />
fresh water deluge at <strong>the</strong> end <strong>of</strong> <strong>the</strong> Messinian salinity crisis could thus have been caused by drowning <strong>of</strong><br />
<strong>Eastern</strong> <strong>Paratethys</strong> waters into <strong>the</strong> Mediterranean.<br />
© 2010 Elsevier B.V. All rights reserved.<br />
1. Introduction<br />
The strontium <strong>isotope</strong> ratio <strong>of</strong> ocean waters is <strong>the</strong> same at any one<br />
time, regardless <strong>of</strong> where in <strong>the</strong> ocean it is measured, because <strong>the</strong><br />
strontium residence time is considerably longer than <strong>the</strong> mixing time<br />
<strong>of</strong> ocean water. Oceanic 87 Sr/ 86 Sr <strong>ratios</strong> fluctuate through geological<br />
time, showing a general increase from ∼50 Ma to present, and can<br />
thus be used as stratigraphic tool (McKenzie et al., 1988; Hodell et al.,<br />
1989a; Hodell et al., 1994; McArthur et al., 2001). Marine sedimentary<br />
records can be dated by measuring strontium <strong>ratios</strong> (e.g. from wellpreserved<br />
foraminifera), especially at time intervals where <strong>the</strong> 87 Sr/<br />
86 Sr curve shows steep changes (Hodell et al., 1991; Miller et al.,<br />
1991a,b; McArthur et al., 2001).<br />
In (semi)isolated inland basins strontium <strong>isotope</strong> dating is far<br />
more complicated because <strong>the</strong> 87 Sr/ 86 Sr ratio is controlled (in <strong>the</strong>se<br />
geological settings) by mixing <strong>of</strong> ocean water and river water. The<br />
⁎ Corresponding author. Tel.: +31 30 253 1361; fax: +31 30 253 1677.<br />
E-mail addresses: vasiliev@geo.uu.nl (I. Vasiliev), reichart@geo.uu.nl (G.-J. Reichart),<br />
gareth.davies@falw.vu.nl (G.R. Davies), krijgsma@geo.uu.nl (W. Krijgsman),<br />
marius@geo.edu.ro (M. Stoica).<br />
87 Sr/ 86 Sr ratio <strong>of</strong> river water reflects <strong>the</strong> regional catchments geology<br />
and may differ substantially between drainage systems (Major et al.,<br />
2006). Variations in input and mixing <strong>of</strong> different water sources will<br />
thus be reflected in <strong>the</strong> 87 Sr/ 86 Sr <strong>ratios</strong>, especially if <strong>the</strong>se sources have<br />
markedly different <strong>isotope</strong> <strong>ratios</strong>. Consequently, <strong>the</strong> strontium<br />
<strong>isotope</strong> ratio can be used to quantitatively study <strong>the</strong> influx <strong>of</strong> river<br />
water and to determine connectivity. River input, however, needs to<br />
exceed ∼50% <strong>of</strong> <strong>the</strong> total inflow and needs to have strongly<br />
contrasting 87 Sr/ 86 Sr values and high strontium concentrations to be<br />
clearly identified (Flecker et al., 2002; Flecker and Ellam, 2006).<br />
The Black Sea, or its geological precursor <strong>Paratethys</strong> (Fig. 1),<br />
represents an inland basin that is very suitable to study regional<br />
hydrological patterns and interbasinal water exchange by determining<br />
<strong>the</strong> strontium <strong>isotope</strong> values. Connectivity to <strong>the</strong> Mediterranean<br />
would result in 87 Sr/ 86 Sr <strong>ratios</strong> that are extremely close to oceanic<br />
values (0.709155), while an exclusively fresh water supply would be<br />
reflected in values typical for <strong>the</strong> rivers feeding <strong>the</strong> Black Sea basin<br />
(0.708792; Palmer and Edmond, 1989). Additionally, ocean water has<br />
∼30 times more Sr dissolved than <strong>the</strong> rivers entering <strong>the</strong> Black Sea<br />
(Table 1). During <strong>the</strong> last glacial, when <strong>the</strong> Black Sea became a lake by<br />
complete isolation from <strong>the</strong> open ocean, 87 Sr/ 86 Sr values (Table 1;<br />
0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.epsl.2010.01.027
124 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
Fig. 1. Schematic palaeogeographic map <strong>of</strong> <strong>Paratethys</strong> region, comprising Lake Pannon, <strong>the</strong> Dacian basin (DB), Black Sea and Caspian Sea. The star indicates <strong>the</strong> position <strong>of</strong> <strong>the</strong><br />
Rîmnicu Sărat section in <strong>the</strong> Dacian basin as a <strong>Paratethys</strong> sub-basin. DS indicates <strong>the</strong> location <strong>of</strong> <strong>the</strong> Dobrogea sill and MS <strong>the</strong> position <strong>of</strong> <strong>the</strong> Marmara Sea.<br />
Major et al., 2006) were indeed close to a weighted average <strong>of</strong> <strong>the</strong><br />
major rivers entering <strong>the</strong> basin (Major et al., 2006). It has also been<br />
suggested that <strong>the</strong> <strong>Paratethys</strong> was temporarily isolated from <strong>the</strong> open<br />
ocean <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition (Hsü and Giovanoli, 1979;<br />
Popov et al., 2006), especially when <strong>the</strong> Mediterranean water level<br />
dropped because <strong>of</strong> <strong>the</strong> Messinian salinity crisis (MSC). There is little<br />
information so far on strontium <strong>isotope</strong> changes in <strong>the</strong> <strong>Paratethys</strong><br />
<strong>during</strong> this period.<br />
In this paper, 87 Sr/ 86 Sr analyses are applied to assess <strong>the</strong> changes in<br />
basin water geochemistry and palaeoenvironment <strong>of</strong> <strong>the</strong> Dacian basin<br />
(Romania) <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition. We selected <strong>the</strong><br />
magnetostratigraphically well-dated successions <strong>of</strong> <strong>the</strong> Focşani<br />
Depression (Vasiliev et al., 2004; Vasiliev et al., 2007), and focussed<br />
on <strong>the</strong> interval between 6.3 and 4.1 Ma (Fig. 2), when <strong>Paratethys</strong><br />
water level may have dropped because <strong>of</strong> <strong>the</strong> MSC. During this period,<br />
<strong>the</strong> Dacian basin formed, toge<strong>the</strong>r with <strong>the</strong> Black Sea and Caspian Sea,<br />
<strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> (Fig. 1). The three subbasins were separated by<br />
shallow sills at Dobrogea and <strong>the</strong> Caucasus, and minor changes in<br />
tectonic uplift, sea level or hydrological budgets could have seriously<br />
influenced interbasinal connectivity. We selected both ostracod<br />
valves and mollusc shells for strontium analyses, because <strong>of</strong> <strong>the</strong>ir<br />
omnipresence throughout <strong>the</strong> succession and excellent state <strong>of</strong><br />
preservation. Prior to Sr analyses we determined trace and minor<br />
elements to assess <strong>the</strong> preservation state <strong>of</strong> <strong>the</strong> biogenic carbonates.<br />
The results will be used to analyse <strong>the</strong> hydrological balance <strong>of</strong> <strong>the</strong><br />
Dacian basin, and to determine <strong>the</strong> interbasinal connectivity within<br />
<strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> and <strong>the</strong> Mediterranean. The data will fur<strong>the</strong>r<br />
serve as crucial constraints for ongoing strontium models <strong>of</strong><br />
Messinian evaporites (Flecker et al., 2002) and may help to decipher<br />
<strong>the</strong> alleged <strong>Paratethys</strong> influx into <strong>the</strong> Mediterranean in <strong>the</strong> final (Lago<br />
Mare) stage <strong>of</strong> <strong>the</strong> MSC (Hsü et al., 1973; Orszag-Sperber, 2006).<br />
2. Geological setting<br />
The <strong>Paratethys</strong> has been a semi-enclosed basin since <strong>the</strong> beginning<br />
<strong>of</strong> <strong>the</strong> Oligocene, when it extended from central Europe to inner Asia<br />
(Rögl, 1996; Ramstein et al., 1997; Popov et al., 2006). Alpine<br />
continental collision <strong>during</strong> <strong>the</strong> Neogene caused restricted circulation<br />
and progressive rearrangement <strong>of</strong> individual subbasins. <strong>Paratethys</strong><br />
gradually transformed into a restricted marine and finally into a giant<br />
Table 1<br />
87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> various rivers and open marine domains around <strong>the</strong> <strong>Paratethys</strong>. The Average Ocean Waters (AOW) is also reported.<br />
Water body Remarks Sr<br />
(ppm)<br />
87 Sr/ 86 Sr Reference<br />
Present-day ocean water 7.62 0.709155 Henderson et al. (1994)<br />
Global river water 0.712 Palmer and Edmond (1989)<br />
AOW <strong>during</strong> Lower Evaporites 0.708999 Howarth and McArthur (1997)<br />
AOW <strong>during</strong> Upper evaporites 0.709012 Howarth and McArthur (1997)<br />
Messinian sea water 0.708983–0.709028 Howarth and McArthur (1997)<br />
Marmara Sea (modern) 0.70915 Major et al. (2006)<br />
Aegean Sea (modern) 0.709157 Major et al. (2006)<br />
Black Sea (modern) 0.709133 Major et al. (2006)<br />
Black Sea (Last Glacial Maximum) 0.70865–0.70875 Major et al. (2006)<br />
Danube 53% <strong>of</strong> freshwater run<strong>of</strong>f to Black Sea 0.24 0.7089 Palmer and Edmond (1989), Major et al. (2006)<br />
Dnieper 14% <strong>of</strong> freshwater run<strong>of</strong>f to Black Sea 0.22 0.7085 Shimkus and Trimonis (1974), Palmer and Edmond (1989)<br />
Don ∼16% <strong>of</strong> freshwater run<strong>of</strong>f to Black Sea 0.22 0.7085 Shimkus and Trimonis (1974), Palmer and Edmond (1989)<br />
Sakarya ∼4% <strong>of</strong> freshwater run<strong>of</strong>f to Black Sea 0.7089 Major et al. (2006)<br />
River average Danube, Dnieper, Don and Sakarya 0.24 0.708792 Major et al. (2006)<br />
Caspian Sea (modern) 0.48 0.7082 Clauer et al. (2000), Page et al. (2003)<br />
Volga 82% <strong>of</strong> freshwater run<strong>of</strong>f to Caspian Sea 9.92 0.70802 Clauer et al. (2000), Page et al. (2003)
I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
125<br />
marine nann<strong>of</strong>ossils (Clauzon et al., 2005; Snel et al., 2006). 87 Sr/ 86 Sr<br />
analyses can determine <strong>the</strong> source <strong>of</strong> <strong>Paratethys</strong> waters and may<br />
elucidate <strong>the</strong> nature and timing <strong>of</strong> Mediterranean–<strong>Paratethys</strong> exchange.<br />
3. Analysed material and sample preparation<br />
3.1. Analysed material<br />
Fig. 2. Local stages, polarity zones, schematic lithological column and <strong>the</strong> position <strong>of</strong> <strong>the</strong><br />
mollusc and ostracod samples used for trace elements and Sr <strong>isotope</strong>s; n.d. (no data)<br />
indicates <strong>the</strong> levels where <strong>the</strong> 87 Sr/ 86 Sr <strong>ratios</strong> were determined but, because <strong>of</strong> large Rb<br />
amounts found in <strong>the</strong> sample, interpreted as diagenetically affected. The dashed lines<br />
indicate <strong>the</strong> (interpretative) correlation <strong>of</strong> <strong>the</strong> polarity sequence to astronomically<br />
dated polarity time scale (APTS) (Vasiliev et al., 2004) and updated to new<br />
paleontological constraints (Stoica et al., 2007; Krijgsman et al., 2010). The age <strong>of</strong> <strong>the</strong><br />
samples was calculated according to <strong>the</strong>ir position in <strong>the</strong> magnetostratigraphic record.<br />
The sampling gap between 1200 and 1800 m is mostly related to <strong>the</strong> coarser lithologies<br />
(silts and sandstones). The initial plan to obtain a monospecific record (Cyprideis sp.) in<br />
ostracods led to a major sampling gap between 1200 and 2300 m. To minimize it we<br />
selected a second specie (Thyrrenocy<strong>the</strong>re filipescui) for 87 Sr/ 86 Sr analyses. In <strong>the</strong> time<br />
scale, Me 1 and Me 2 represent <strong>the</strong> lower and upper Meotian respectively; Odess.<br />
(Odessian), Portaf. (Portafferian) and Bosphorian are regional substages <strong>of</strong> Pontian<br />
Stage; Gt (Getian) and Pv (Parscovian) are substages <strong>of</strong> <strong>the</strong> Dacian Stage and Rm 1<br />
represents <strong>the</strong> lower Romanian.<br />
brackish–fresh water lake system <strong>during</strong> <strong>the</strong> upper <strong>Mio</strong>cene–Pliocene<br />
(Fig. 1). The knowledge <strong>of</strong> its <strong>Mio</strong>-Pliocene palaeoenvironmental history<br />
relies mainly on palaeoecological assessment <strong>of</strong> aquatic and terrestrial<br />
organisms (e.g. Rögl and Daxner-Hock, 1996; Rögl, 1998), on <strong>the</strong><br />
correlation to better time-constrained species from <strong>the</strong> Mediterranean<br />
realm (e.g. Harzhauser and Piller, 2004) and on several palynological<br />
studies (Popescu, 2001; Ivanov et al., 2007; Utescher et al., 2009).<br />
Marine connections between <strong>Paratethys</strong> and Mediterranean are<br />
commonly considered to have ended in <strong>the</strong> Late <strong>Mio</strong>cene. Since <strong>the</strong>n,<br />
<strong>the</strong> <strong>Paratethys</strong> was a brackish to fresh water basin, resulting in <strong>the</strong><br />
complete loss <strong>of</strong> its marine fauna (e.g. foraminifera, calcareous<br />
nannoplankton and dinocysts) (Magyar et al., 1999), and its faunal<br />
content became dominated by a variety <strong>of</strong> ostracods and molluscs,<br />
endemic to <strong>the</strong> <strong>Paratethys</strong>. Short periods <strong>of</strong> Mediterranean–<strong>Paratethys</strong><br />
connectivity are, however, suggested by some horizons containing<br />
Benthic organisms like ostracods and molluscs produce carbonate<br />
shells, which can be separated from <strong>the</strong> sedimentary rock and analysed<br />
for <strong>the</strong>ir isotopic composition. Twenty-five ostracod levels were selected<br />
from <strong>the</strong> Rîmnicu Sărat section, covering <strong>the</strong> interval between 6.3 and<br />
4.1 Ma. The section consists <strong>of</strong> cyclic alternations <strong>of</strong> sandstones and<br />
mudstones, deposited in a distal marine–brackish deltaic system<br />
(Panaiotu et al., 2007). The average duration (∼22 kyr) <strong>of</strong> <strong>the</strong><br />
sedimentary cycles indicates deposition under <strong>the</strong> influence <strong>of</strong> precession<br />
(Vasiliev et al., 2004). Our biogenic carbonates were extracted from<br />
shales and siltstones because <strong>the</strong>se fine-grained rocks ensured <strong>the</strong> best<br />
possible preservation <strong>of</strong> <strong>the</strong> shells. Single species records could not be<br />
used for <strong>the</strong> entire time interval, because <strong>the</strong> dynamically changing<br />
environments caused a faunal variation. The ostracod species Cyprideis<br />
sp. Jones, 1857 was chosen (Fig. 3a) because <strong>of</strong> its abundance within <strong>the</strong><br />
selected time frame <strong>of</strong> <strong>the</strong> Romanian Carpathian foredeep (Fig. 2) and<br />
because this species was earlier successfully used for trace-elements and<br />
stable <strong>isotope</strong> studies (De Deckker et al., 1999; Anadon et al., 2002).<br />
Cyprideis is one <strong>of</strong> <strong>the</strong> most euryhaline ostracods that populate waters<br />
with salinity ranging from 0.4% to 150%. Cyprideis torosa is a typical<br />
shallow water species, which lives in permanent littoral marine<br />
environments as well as marginal marine environments such as deltas,<br />
estuaries and coastal lagoons. They have also been found in athalassic<br />
saline lakes and have been described as anomalohaline (Van Harten,<br />
1990). During <strong>the</strong> Late <strong>Mio</strong>cene, <strong>the</strong> genus Cyprideis was affected by a<br />
great adaptive radiation in <strong>the</strong> <strong>Paratethys</strong> realm (Pipik et al., 2007),<br />
probably due to its adaptation to deeper waters (Van Harten, 1990; De<br />
Deckker, 2001, 2002). Twenty-two <strong>of</strong> <strong>the</strong> selected levels from <strong>the</strong><br />
Romanian Carpathian foredeep contained sufficient shells <strong>of</strong> Cyprideis<br />
sp. for Sr <strong>isotope</strong> analysis (Fig. 2). Cyprideis is only scarcely present in <strong>the</strong><br />
lower, middle and <strong>the</strong> first part <strong>of</strong> <strong>the</strong> upper Pontian at Rîmnicu Sărat<br />
(Krijgsman et al., 2010). To reduce <strong>the</strong> gap for that time period we<br />
analysed three levels with Tyrrhenocy<strong>the</strong>re filipescui (Fig. 2). Tyrrhenocy<strong>the</strong>re<br />
inhabits oligohaline to mesohaline waters, with <strong>the</strong> highest<br />
frequency between 0 and 30 m depth (Yassini and Ghahermann, 1979).<br />
Where possible, we also used mollusc shells at <strong>the</strong> same stratigraphic<br />
levels to compare <strong>the</strong> results with those obtained from ostracods (Fig. 2).<br />
The molluscs used in this study are unionids and cardiids and <strong>the</strong>ir<br />
seasonal growth is recorded throughout <strong>the</strong> entire year; <strong>the</strong>refore a<br />
complete shell records <strong>the</strong> variations in temperature, chemistry and<br />
isotopic signatures <strong>of</strong> water <strong>during</strong> <strong>the</strong> organism's lifetime.<br />
3.2. Sample preparation<br />
Specimens <strong>of</strong> Cyprideis sp. and T. filipescui were separated from<br />
bulk sediment by disaggregation in sodium carbonate solution, wet<br />
sieving to retain <strong>the</strong> >250 μm fraction and hand-picking under a<br />
microscope. The ostracods samples were cleaned to remove clay<br />
following <strong>the</strong> procedure <strong>of</strong> Barker et al. (2003), followed by washing<br />
twice in MilliQ®, five times in methanol (96%) and by 30 s cleaning in<br />
an ultrasonic bath. The washing procedure was <strong>the</strong>n repeated.<br />
Cleaned samples were <strong>the</strong>n evaluated for diagenetic alteration using<br />
trace element analysis and scanning electron microscopy (SEM).<br />
The mollusc specimens were handpicked and embedded in raisin. A<br />
slice from each mollusc was cut and finely polished. The fresh surface was<br />
sampled using laser ablation inductively coupled plasma-mass spectrometry<br />
(LA-ICP-MS). The specimens were subsequently re-sampled for<br />
<strong>the</strong> strontium <strong>isotope</strong> analysis using a Merchantek micro-drill or hand<br />
mini-drill avoiding contaminated external parts <strong>of</strong> <strong>the</strong> shell.
126 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
Fig. 3. (a) Scanning electron micrograph <strong>of</strong> a Cyprideis sp. from Rîmnicu Sărat Valley (from sample RMS116) with ablation craters. (b) High magnification <strong>of</strong> an ablation crater where<br />
<strong>the</strong> pristine structure <strong>of</strong> <strong>the</strong> ostracod shell can be observed. (c) Time resolved laser-ablation inductively coupled plasma-mass spectrometry data. Middle parts (marked by <strong>the</strong><br />
shaded areas) represent <strong>the</strong> part <strong>of</strong> <strong>the</strong> measurement taken into consideration for <strong>the</strong> trace elements calculations (e.g. Sr/Ca <strong>ratios</strong>). Normalization to Ca was necessary to overcome<br />
<strong>the</strong> varying response (counts per second) <strong>during</strong> ablation that generates variable quantity <strong>of</strong> removed material. Important for 87 Sr/ 86 Sr <strong>ratios</strong> <strong>isotope</strong>s is that <strong>the</strong> Sr pr<strong>of</strong>iles record<br />
little variations indicating pristine preservation <strong>of</strong> this element within <strong>the</strong> shell structure. O<strong>the</strong>r elements record higher values at <strong>the</strong> outside parts <strong>of</strong> <strong>the</strong> shells indicating that<br />
despite careful cleaning <strong>the</strong> outer part <strong>of</strong> <strong>the</strong> shells collected unremovable clay minerals/coating in <strong>the</strong> ornament pores. Therefore, for trace elements calculations only <strong>the</strong> middle<br />
(grey) parts <strong>of</strong> <strong>the</strong> pr<strong>of</strong>iles were used.<br />
4. Methods<br />
The ostracods and molluscs were ablated using a 193 nm<br />
wavelength laser. Such a short wave length is essential for <strong>the</strong><br />
reproducible ablation <strong>of</strong> <strong>the</strong> fragile shells, because carbonates do not<br />
absorb laser radiation well at higher wavelengths (Mason and Kraan,<br />
2002; Reichart et al., 2003). The system employs a Lambda Physik<br />
excimer laser with GeoLas 200Q optics. Ablation was performed in a<br />
mixture <strong>of</strong> helium and argon atmosphere at a pulse repetition rate <strong>of</strong><br />
6 Hz. Ablation craters were 80 μm in diameter (Fig. 3a and b). Ablated<br />
material was measured with respect to time (and hence depth) using<br />
a Micromass Platform quadrupole ICP-MS instrument (Figs. 3c and 4).<br />
Calibration was performed against U.S. National Institute <strong>of</strong> Standards<br />
and Technology SRM 610 glass using <strong>the</strong> concentration data <strong>of</strong> Pearce<br />
et al. (1997) with 44 Ca as an internal standard. A collision and<br />
reduction cell (Mason and Kraan, 2002) was used to give improved<br />
results by reducing spectral interferences on <strong>the</strong> minor <strong>isotope</strong>s <strong>of</strong> Ca<br />
( 42 Ca, 43 Ca and 44 Ca). Multiple <strong>isotope</strong>s were used where possible to<br />
confirm accurate concentration determinations (Fig. 3).<br />
The 87 Sr/ 86 Sr <strong>ratios</strong> were measured on both ostracods and mollusc<br />
carbonates. From each sample, 0.4 to 3 mg was dissolved in 0.5 ml <strong>of</strong> 5 N<br />
acetic acid. Any residue was separated by centrifugation and <strong>the</strong><br />
remaining solution was evaporated to dryness. The resulting solid<br />
residue was dissolved in 2 drops <strong>of</strong> concentrated HNO 3 to remove<br />
organics, and again evaporated to dryness. The residue was completely<br />
redissolved at room temperature in 0.5 ml <strong>of</strong> 3 N HNO 3 and centrifuged<br />
for four minutes. 0.4 ml <strong>of</strong> <strong>the</strong> top-most part <strong>of</strong> <strong>the</strong> samples was<br />
introduced to chromatographic columns composed <strong>of</strong> “Elchrom Sr spec”<br />
ion exchange. The Sr fraction was dried and nitrated twice with one drop<br />
<strong>of</strong> concentrated HNO 3 . The isotopic analyses were carried out at <strong>the</strong><br />
Isotopes Laboratory from Vrije Universiteit (Amsterdam).<br />
87 Sr/ 86 Sr<br />
<strong>ratios</strong> were analysed on Finningan MAT 261 and 262 mass spectrometers,<br />
running a triple-jump routine, applying exponential fractionation<br />
correction and normalizing to 87 Sr/ 86 Sr=0.1194. NBS 987 run<br />
<strong>during</strong> this study was within <strong>the</strong> long term average 0.710242±<br />
0.000012 (2σ), n>200. The blanks were less 30×10 −6 have<br />
<strong>the</strong> 87 Sr/ 86 Sr ratio marked as ‘not determined’ in Table 2 (although <strong>the</strong>y<br />
were measured).<br />
5. Diagenetic evaluation; reliability <strong>of</strong> strontium measurements<br />
Contamination from clay minerals, iron and manganese (hydr)<br />
oxide coatings and possibly organic material can be a problem in<br />
micr<strong>of</strong>ossil trace element analysis. Most contamination is adhered to<br />
<strong>the</strong> outer surface <strong>of</strong> <strong>the</strong> shells post mortem and might accumulate in
I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
127<br />
Ablation tracks were analysed perpendicular to <strong>the</strong> growth lines to<br />
investigate <strong>the</strong> possible diagenetic overprinting in <strong>the</strong> mollusc shells<br />
(Fig. 4). The excellently preserved pattern <strong>of</strong> changes in Ba/Ca, Sr/Ca<br />
and Mn/Ca <strong>ratios</strong> is interpreted to reflect ancient seasonal changes,<br />
providing strong evidence for good preservation <strong>of</strong> <strong>the</strong> mollusc shells<br />
from this valley. The cyclicity in <strong>the</strong> trace element <strong>ratios</strong> correlates to<br />
<strong>the</strong> growth lines (Fig. 4) comparable to modern shells (Vonh<strong>of</strong> et al.,<br />
2003).<br />
Because <strong>of</strong> <strong>the</strong> constant Sr/Ca <strong>ratios</strong> through <strong>the</strong> LA-ICP-MS<br />
pr<strong>of</strong>iles from ostracods and <strong>the</strong> preserved seasonal patterns in <strong>the</strong><br />
molluscs we concluded that <strong>the</strong> Sr content <strong>of</strong> <strong>the</strong> analysed shells was<br />
not affected by depositional processes. Therefore we used <strong>the</strong> LA-ICP-<br />
MS Sr content data (expressed as Sr molar <strong>ratios</strong>) by averaging <strong>the</strong><br />
values obtained at each level (Table 2). For seven levels, <strong>the</strong> number <strong>of</strong><br />
ostracods shells was limited to 3–4 valves and <strong>the</strong>refore we chose to<br />
not record <strong>the</strong> trace and minor elements. We used all <strong>the</strong> available<br />
material to measure <strong>the</strong> Sr <strong>isotope</strong> <strong>ratios</strong>. Based on <strong>the</strong> results <strong>of</strong><br />
eighteen well-preserved sites we concluded that <strong>the</strong> Sr content was<br />
also not affected by post-depositional processes. Two ostracod levels<br />
out <strong>of</strong> seven gave ‘not determined’ Sr contents, but had timeequivalent<br />
molluscs data (RMS 48 M and RMS 45 M). Their Sr values<br />
are not deflecting from our o<strong>the</strong>r molluscs-based data (Table 2).<br />
6. 87 Sr/ 86 Sr values for <strong>the</strong> carbonates <strong>of</strong> <strong>the</strong> Dacian basin <strong>during</strong><br />
<strong>the</strong> <strong>Mio</strong>-Pliocene transition<br />
Fig. 4. Trace elements ratio from a 5.5 mm pr<strong>of</strong>ile in <strong>the</strong> mollusc shells. The pr<strong>of</strong>ile is<br />
located in <strong>the</strong> lower half <strong>of</strong> <strong>the</strong> picture, just below <strong>the</strong> Mn/Ca record. The well resolved<br />
pattern <strong>of</strong> changes in Ba/Ca, Sr/Ca and Mn/Ca, most probably reflects ancient seasonal<br />
changes, from warm to cold season (or vice-versa), marked by <strong>the</strong> grey bands.<br />
pores and between <strong>the</strong> spines <strong>of</strong> <strong>the</strong> ostracods. Rigorous purification<br />
procedures have been developed to remove such extraneous phases<br />
(Boyle, 1981; Lea and Boyle, 1993). Analysis by LA-ICP-MS makes it<br />
possible to avoid such contamination <strong>during</strong> <strong>the</strong> integration <strong>of</strong> <strong>the</strong><br />
data acquired <strong>during</strong> analysis <strong>of</strong> single valves. Al and U were<br />
monitored, to evaluate <strong>the</strong> surface contamination by clay particles<br />
(Fig. 3c). Mn was used as a proxy <strong>of</strong> secondary carbonate and<br />
manganese (hydr)oxides overgrowth. Those parts <strong>of</strong> <strong>the</strong> time<br />
integrated measurements having higher counts <strong>of</strong> Al, U and Mn<br />
were excluded from integration. Some <strong>of</strong> <strong>the</strong> specimens showed<br />
contamination throughout <strong>the</strong> pr<strong>of</strong>ile and were excluded completely.<br />
We recorded at least two pr<strong>of</strong>iles for each specimen. SEM images <strong>of</strong><br />
representative specimens were also used to examine <strong>the</strong> mineralogy<br />
<strong>of</strong> <strong>the</strong> ostracod valves. In contrast to molluscs, ostracods build <strong>the</strong>ir<br />
shells more or less instantaneous. This might explain why ablation<br />
pr<strong>of</strong>iles for ostracods show remarkably constant concentrations with<br />
thickness making <strong>the</strong> recognition <strong>of</strong> contamination relatively straightforward.<br />
The non-contaminated part <strong>of</strong> <strong>the</strong> ostracods shells are<br />
marked by <strong>the</strong> grey interval (Fig. 3).<br />
Ultimately, it is <strong>the</strong> heterogeneity between individual ostracod<br />
valves that sets <strong>the</strong> limit to <strong>the</strong> accuracy <strong>of</strong> ostracods trace metal<br />
based environmental reconstructions. Annual environmental and<br />
water composition changes would result in variations between<br />
individuals from <strong>the</strong> same sample. In view <strong>of</strong> <strong>the</strong> relatively fast shell<br />
building <strong>of</strong> <strong>the</strong> ostracods, changes in this growth rate could easily<br />
influence Sr and Mg incorporation, causing differences between<br />
ostracods shell from <strong>the</strong> same location. The ablation pr<strong>of</strong>iles for<br />
ostracods shows that even after <strong>the</strong> very thorough cleaning procedure<br />
not all <strong>the</strong> contamination present at <strong>the</strong> outer parts could be removed<br />
(Fig. 3c). However, <strong>the</strong> evaluation <strong>of</strong> <strong>the</strong>se pr<strong>of</strong>iles enabled us to<br />
observe that <strong>the</strong> Sr/Ca ratio for <strong>the</strong> ostracods was constant through<br />
entire ablation pr<strong>of</strong>iles, indicating that <strong>the</strong> Sr content was not affected<br />
by post-deposition processes (Fig. 3c).<br />
Biological and inorganic precipitates record ambient water Sr<br />
<strong>isotope</strong> compositions. Unlike oxygen <strong>isotope</strong>s, <strong>the</strong> measurement<br />
techniques for strontium <strong>isotope</strong> <strong>ratios</strong> rule out measurable mass<br />
dependent fractionation from biological effects, temperature, or o<strong>the</strong>r<br />
physical environmental changes (Major et al., 2006). This potentially<br />
provides valuable information on <strong>the</strong> isotopic composition <strong>of</strong> <strong>the</strong><br />
water, reflecting both connectivity <strong>of</strong> <strong>the</strong> basin to <strong>the</strong> open ocean, and<br />
changes in regional climate and hydrography. The resultant isotopic<br />
water composition depends on <strong>the</strong> Sr concentration <strong>of</strong> river water and<br />
on <strong>the</strong> isotopic contrast between oceans and rivers. <strong>Strontium</strong> <strong>isotope</strong><br />
<strong>ratios</strong> can thus be used to test whe<strong>the</strong>r salinity fluctuations resulted<br />
from changes in fresh water supply (precipitation plus river run<strong>of</strong>f),<br />
from variations in evaporation, or from both.<br />
The Sr content in biogenic carbonates depends upon temperature<br />
and on <strong>the</strong> biological and physical environment, but <strong>the</strong>se parameters<br />
do not influence 87 Sr/ 86 Sr (Faure, 1998). The values for Sr content <strong>of</strong><br />
<strong>the</strong> Dacian basin, expressed as Sr molar ratio, are ∼2.2 times higher for<br />
<strong>the</strong> ostracods than for <strong>the</strong> molluscs (Table 2). The decreasing trend <strong>of</strong><br />
Sr incorporated over time is similar for both ostracods and molluscs.<br />
Our results show that <strong>the</strong> 87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> ostracod valves from<br />
<strong>the</strong> Rîmnicu Sărat section (Fig. 5, Table 2) range from 0.708664 to<br />
0.708768. The 87 Sr/ 86 Sr <strong>ratios</strong> in mollusc carbonates range are in good<br />
agreement ranging from 0.708683 to 0.708882 (Table 2 and Fig. 5).<br />
Only at one level (RMS116, Table 2), <strong>the</strong> 87 Sr/ 86 Sr ratio obtained from<br />
molluscs (0.708865±26) differ substantially from <strong>the</strong> one obtained<br />
from <strong>the</strong> ostracod valves (0.708664±6). The latest <strong>Mio</strong>cene–earliest<br />
Pliocene strontium values <strong>of</strong> <strong>the</strong> Dacian Basin carbonates are<br />
markedly different from coeval global ocean values (Henderson et<br />
al., 1994; McArthur et al., 2001) being significantly lower than <strong>the</strong><br />
marine waters at that time (Fig. 5).<br />
Two samples show 87 Sr/ 86 Sr ratio that significantly differ from <strong>the</strong><br />
mean values <strong>of</strong> <strong>the</strong> Dacian basin record. Sample RMS 96 O, located at<br />
<strong>the</strong> Portaferrian/Bosphorian boundary has a value <strong>of</strong> 0.708964±9,<br />
which is very close to <strong>the</strong> oceanic curve (Fig. 2). A possible<br />
explanation is that this level corresponds to a very short marine<br />
influx into <strong>the</strong> Dacian basin. It concerns one <strong>of</strong> <strong>the</strong> two measurements<br />
based on T. filipescui, but Rb content, monitored by default for all <strong>the</strong><br />
analyses, is within <strong>the</strong> limits describing it as non-diagenetically<br />
affected. The second sample (RMS114 O) has <strong>the</strong> lowest strontium<br />
ratio <strong>of</strong> <strong>the</strong> dataset (0.708511±11). This low value may be related to
128 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
Table 2<br />
87 Sr/ 86 Sr <strong>ratios</strong>, 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)<br />
indicates <strong>the</strong> levels where <strong>the</strong> values were not determined.<br />
Sample name<br />
87 Sr/ 86 Sr 2<br />
(10 − 6 )<br />
Ag<br />
(Ma)<br />
Level<br />
(m)<br />
Local stage<br />
Sr<br />
(mmol/mol)<br />
RMS116 O 0.708664 6 6.21 1099 Upper Meotian (Me2) 2.15 Cyprideis sp.<br />
RMS114 O 0.708511 11 6.20 1112 Upper Meotian (Me2) 2.07 Cyprideis sp.<br />
RMS110 O 0.708782 4 6.13 1142 Upper Meotian (Me2) 2.95 Cyprideis sp.<br />
RMS096 O 0.708964 9 5.52 1808 Bosphorian (Po3) n.d. Tyrrhenocy<strong>the</strong>re filipescui<br />
RMS094 O n.d. n.d. 5.50 1815 Bosphorian (Po3) n.d. Tyrrhenocy<strong>the</strong>re filipescui<br />
RMS088 O 0.708823 10 5.32 1993 Bosphorian (Po3) n.d. Tyrrhenocy<strong>the</strong>re filipescui<br />
RMS084 O 0.708779 8 5.12 2243 Bosphorian (Po3) 1.95 Cyprideis sp.<br />
RMS074 O 0.708722 11 4.98 2483 Bosphorian (Po3) 1.26 Cyprideis sp.<br />
RMS067 O 0.708763 13 4.95 2739.5 Bosphorian (Po3) 1.09 Cyprideis sp.<br />
RMS065 O 0.708774 10 4.90 2841.5 Getian (Dc1) 1.59 Cyprideis sp.<br />
RMS060 O 0.708853 16 4.68 3001 Getian (Dc1) n.d. Cyprideis sp.<br />
RMS055 O 0.708821 10 4.60 3252 Getian (Dc1) 1.29 Cyprideis sp.<br />
RMS053 O 0.708786 9 4.52 3341 Getian (Dc1) 1.02 Cyprideis sp.<br />
RMS051 O 0.708768 8 4.50 3395.5 Getian (Dc1) 1.53 Cyprideis sp.<br />
RMS048 O n.d. n.d. 4.45 3441 Getian (Dc1) n.d. Cyprideis sp.<br />
RMS045 O n.d. n.d. 4.40 3561 Getian (Dc1) n.d. Cyprideis sp.<br />
RMS044 O 0.708825 8 4.36 3594 Getian (Dc1) 1.43 Cyprideis sp.<br />
RMS038 O 0.708812 7 4.35 3598.5 Getian (Dc1) 0.82 Cyprideis sp.<br />
RMS033 O 0.708786 10 4.34 3619 Getian (Dc1) 1.04 Cyprideis sp.<br />
RMS030 O 0.708777 7 4.33 3697 Getian (Dc1) 1.15 Cyprideis sp.<br />
RMS029 O 0.708804 10 4.32 3716 Getian/Parscovian 0.86 Cyprideis sp.<br />
RMS028 O 0.708810 10 4.31 3725.5 Getian/Parscovian 1.27 Cyprideis sp.<br />
RMS019 O 0.708844 15 4.17 3912 Parscovian (Dc2) n.d. Cyprideis sp.<br />
RMS018 O 0.708817 9 4.16 3914 Parscovian (Dc2) 0.88 Cyprideis sp.<br />
RMS007 O 0.708826 8 4.12 4011 Parscovian (Dc2) 1.17 Cyprideis sp.<br />
RMS116 M 0.708865 26 6.21 1099 Upper Meotian (Me2) 0.72 Unio (Psilunio) sp.<br />
RMS110 M 0.708776 9 6.13 1142 Upper Meotian (Me2) 1.23 Unio (Psilunio) sp.<br />
RMS1 styllo 0.708831 10 4.50 3395.5 Getian (Dc1) 0.59 Stylodacna stylodacna<br />
RMS3 styllo 0.708803 7 4.49 3410 Getian (Dc1) 0.73 Stylodacna heberti<br />
RMS048 M 0.708776 10 4.45 3441 Getian (Dc1) 0.64 Prosodacna sp.<br />
RMS045 M 0.708811 8 4.40 3561 Getian (Dc1) 0.65 Stylodacna heberti<br />
RMS043 M 0.708772 8 4.36 3579 Getian (Dc1) 0.54 Unio (Rumanounio) rumanus<br />
RMS039 M 0.708795 10 4.35 3594 Getian (Dc1) 0.67 Prosodacna (Psilodon) neumayri<br />
RMS033 M 0.708778 6 4.34 3619 Getian (Dc1) 0.45 Prosodacna (Psilodon) neumayri<br />
RMS031 M 0.708683 7 4.34 3691 Getian (Dc1) 0.47 Prosodacna (Psilodon) neumayri<br />
RMS2 proso 0.708815 5 4.21 3875 Parscovian (Dc2) 0.57 Prosodacna (Psilodon) neumayri<br />
RMS4 styllo 0.708834 8 4.20 3877 Parscovian (Dc2) 0.37 Stylodacna heberti<br />
RMS018 M 0.708743 9 4.16 3925 Parscovian (Dc2) 0.60 Prosodacna (Psilodon) neumayri<br />
Species<br />
diagenesis that can be evaluated from <strong>the</strong> trace elements record<br />
(Supplementary Fig. 1).<br />
7. Discussion<br />
7.1. The isolation <strong>of</strong> <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene<br />
boundary interval<br />
The present-day situation <strong>of</strong> <strong>the</strong> Black Sea, having only a very small<br />
connection to <strong>the</strong> Mediterranean through <strong>the</strong> Bosporus strait (2 km<br />
wide and 30 m deep), results in sufficient radiogenic Sr supply to<br />
induce oceanic 87 Sr/ 86 Sr <strong>ratios</strong> in <strong>the</strong> Black Sea, due to <strong>the</strong> high<br />
content <strong>of</strong> radiogenic Sr <strong>of</strong> sea water (Major et al., 2006). The major<br />
<strong>Paratethys</strong> rivers carry relatively low amounts <strong>of</strong> radiogenic Sr,<br />
ensuring a mean fluvial input with lower 87 Sr/ 86 Sr than ocean water<br />
(Palmer and Edmond, 1989; Muller and Mueller, 1991; Henderson et<br />
al., 1994; Flecker and Ellam, 1999). Paleoisolation <strong>of</strong> <strong>Paratethys</strong> would<br />
thus induce a tendency towards less radiogenic strontium, while an<br />
exclusively continental supply would be reflected in 87 Sr/ 86 Sr values<br />
typical for <strong>the</strong> rivers feeding <strong>the</strong> basin (Flecker and Ellam, 1999; Major<br />
et al., 2006). This has been observed <strong>during</strong> <strong>the</strong> last glacial period,<br />
when strontium values ( 87 Sr/ 86 Sr∼0.70879) were close to a weighted<br />
average <strong>of</strong> <strong>the</strong> major rivers entering <strong>the</strong> Black Sea (Table 1 and Fig. 5).<br />
The oceanic strontium <strong>ratios</strong> are well-determined for <strong>the</strong> <strong>Mio</strong>-<br />
Pliocene interval, showing a significant increase from 0.70895 to<br />
0.70904 (Hodell et al., 1991; Miller et al., 1991a,b; McArthur et al.,<br />
2001). The 87 Sr/ 86 Sr <strong>ratios</strong> measured from <strong>the</strong> Dacian basin are<br />
significantly lower (ranging 0.708511 to 0.708768 in ostracods and<br />
0.708683 to 0.708882 in molluscs). These low 87 Sr/ 86 Sr <strong>ratios</strong> are<br />
compatible with very limited input or even with complete isolation<br />
from <strong>the</strong> open ocean waters. Our 87 Sr/ 86 Sr values are ra<strong>the</strong>r constant<br />
when compare to <strong>the</strong> noticeable globally increasing trend <strong>of</strong> <strong>the</strong> late<br />
Neogene seawater 87 Sr/ 86 Sr (Farrell et al., 1995), implying that <strong>the</strong><br />
Dacian Basin was not connected to <strong>the</strong> Mediterranean <strong>during</strong> <strong>the</strong><br />
latest Meotian (6.5–6.0 Ma). This is in good agreement with seismic<br />
sequence stratigraphic interpretations <strong>of</strong> <strong>the</strong> western Dacian basin<br />
(Leever et al., 2010) and <strong>the</strong> biochronological data from <strong>the</strong> Focşani<br />
Depression (Krijgsman et al., 2010) that suggested a major transgression<br />
in <strong>the</strong> Dacian basin at <strong>the</strong> Meotian–Pontian boundary in<br />
marine waters from <strong>the</strong> Mediterranean. Unfortunately, we have no<br />
strontium data <strong>of</strong> <strong>the</strong> lower Pontian (6.0–5.6 Ma), to evaluate <strong>the</strong><br />
presence and <strong>the</strong> duration <strong>of</strong> this marine connection.<br />
Our data fur<strong>the</strong>r indicate that <strong>the</strong> Dacian basin did not receive<br />
marine waters from <strong>the</strong> Mediterranean <strong>during</strong> <strong>the</strong> Bosphorian substage,<br />
which corresponds in time to <strong>the</strong> latest Messinian–early Pliocene<br />
(Krijgsman et al., 2010). Based on <strong>the</strong> strontium results, <strong>the</strong> only period<br />
that marine waters entered <strong>the</strong> Dacian basin was <strong>the</strong> Portaferrian/<br />
Bosphorian boundary interval (Fig. 2). The relatively low resolution <strong>of</strong><br />
our <strong>Paratethys</strong> data, however, still leaves room for o<strong>the</strong>r short marine<br />
incursions that are not yet resolved. Future work will <strong>the</strong>refore focus on<br />
obtaining a higher resolution Sr <strong>isotope</strong> ratio record to establish possible<br />
transient changes in sea level that cause marine incursions. The higher<br />
Sr concentrations <strong>of</strong> seawater compared to brackish water, makes <strong>the</strong><br />
basin highly sensitive to such incursions.
I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
129<br />
Fig. 5. 87 Sr/ 86 Sr <strong>ratios</strong> for <strong>the</strong> <strong>Mio</strong>cene–Pliocene samples <strong>of</strong> Rîmnicu Sărat Valley plotted against <strong>the</strong> ocean Sr <strong>isotope</strong> curve in grey between 3.5 and 6.5 Ma (Farrell et al., 1995;<br />
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 and<br />
DePaolo, 1988) are individual Sr <strong>isotope</strong> data used for construction <strong>of</strong> <strong>the</strong> reference Ocean Sr <strong>isotope</strong> curve. Filled circles (squares) indicate ostracods (molluscs) from this study. The<br />
error for individual Romanian samples is plotted and <strong>the</strong> age is derived from <strong>the</strong> magnetostratigraphic correlation <strong>of</strong> Rîmnicu Sărat magnetostratigraphy to <strong>the</strong> APTS (Vasiliev et al.,<br />
2004). The o<strong>the</strong>r values represent all <strong>the</strong> published data for <strong>the</strong> 3.5–6.5 Myr time interval from <strong>the</strong> Mediterranean realm: Gavdos (Flecker et al., 2002), sou<strong>the</strong>rn Turkey (Flecker and<br />
Ellam, 1999), <strong>Eastern</strong> Italy (Montanari et al., 1997), Sicily (Lower and Upper Evaporites) (McKenzie et al., 1988; Muller and Mueller, 1991; Keogh and Butler, 1999), <strong>the</strong> Tyrrhenian<br />
Sea (Muller et al., 1990; Muller and Mueller, 1991) and <strong>the</strong> Balearic, Levantine and Ionian basins (Muller and Mueller, 1991). Age data for this compilation is according to (Flecker<br />
et al., 2002). Blue dashed lines indicate <strong>the</strong> values from <strong>the</strong> four major rivers feeding <strong>the</strong> Black Sea (one <strong>of</strong> <strong>the</strong> remnants <strong>of</strong> <strong>the</strong> old <strong>Paratethys</strong> domain). The Caspian Sea (o<strong>the</strong>r<br />
remnant <strong>of</strong> <strong>the</strong> <strong>Paratethys</strong>) and <strong>the</strong> values for <strong>the</strong> main rivers (Volga and Ural) feeding it are much lower ( 87 Sr/ 86 Sr= 0.7082) than any <strong>of</strong> <strong>the</strong> values and are not included in <strong>the</strong><br />
graphic representation. In <strong>the</strong> left hand side data for <strong>the</strong> last glacial times (Major et al., 2006) are very similar to those obtained for <strong>the</strong> <strong>Mio</strong>-Pliocene transition <strong>of</strong> <strong>the</strong> Dacian basin.<br />
Note <strong>the</strong> different scale <strong>of</strong> <strong>the</strong> time axes.<br />
7.2. Interbasinal connectivity <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition<br />
To investigate <strong>the</strong> interbasinal connectivity <strong>of</strong> <strong>the</strong> <strong>Eastern</strong><br />
<strong>Paratethys</strong> domain we use <strong>the</strong> present day 87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> <strong>the</strong><br />
dominant rivers that fed <strong>the</strong> Dacian, Black Sea and Caspian basins<br />
(Table 1). This assumption is justified because <strong>the</strong> palaeographic<br />
configuration <strong>of</strong> <strong>the</strong> source region had been relatively stable since <strong>the</strong><br />
<strong>Mio</strong>-Pliocene. The most important mountain ranges surrounding <strong>the</strong><br />
<strong>Paratethys</strong>, <strong>the</strong> Alps. Carpathians and Caucasus, were already formed<br />
and <strong>the</strong> drainage areas <strong>of</strong> <strong>the</strong> Danube, Don, Dniepr and Volga<br />
remained roughly <strong>the</strong> same (Popov et al., 2006).<br />
Present-day 87 Sr/ 86 Sr <strong>ratios</strong> <strong>of</strong> <strong>the</strong> major <strong>Paratethys</strong> rivers are<br />
between 0.7085 and 0.7089 (Table 1; Fig. 5). This range overlaps with<br />
<strong>the</strong> low Sr <strong>isotope</strong> <strong>ratios</strong> in <strong>the</strong> Rîmnicu Sărat section (Table 2; Fig. 5).<br />
We thus interpret <strong>the</strong>se <strong>Mio</strong>-Pliocene Sr <strong>isotope</strong> <strong>ratios</strong> <strong>of</strong> <strong>the</strong> Dacian<br />
basin to be highly dominated by river input. The main river that drains<br />
into <strong>the</strong> basin, <strong>the</strong> Danube, has a 87 Sr/ 86 Sr ratio <strong>of</strong> 0.7089, much lower<br />
than <strong>the</strong> ocean water <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition time<br />
(Shimkus and Trimonis, 1974; Palmer and Edmond, 1989) but still<br />
higher than all our data. Hence, <strong>the</strong> Danube cannot account for <strong>the</strong><br />
measured 87 Sr/ 86 Sr ratio on its own, indicating that an additional fresh<br />
water source should have been present. The best candidates for <strong>the</strong><br />
source <strong>of</strong> lower 87 Sr/ 86 Sr are Dnieper and Don, rivers located to <strong>the</strong><br />
east and draining now into <strong>the</strong> Black Sea. The Danube currently<br />
provides ∼60% <strong>of</strong> <strong>the</strong> freshwater run<strong>of</strong>f to <strong>the</strong> Black Sea, while <strong>the</strong><br />
o<strong>the</strong>r ∼40% comes from <strong>the</strong> Dnieper and Don. The 87 Sr/ 86 Sr data from<br />
<strong>the</strong> Dacian basin are similar to <strong>the</strong> values obtained for <strong>the</strong> Black Sea in<br />
<strong>the</strong> last glacial times (Major et al., 2006) and suggests that <strong>the</strong> Dacian<br />
and <strong>the</strong> Black Sea basins were also connected <strong>during</strong> <strong>the</strong> latest<br />
<strong>Mio</strong>cene–earliest Pliocene. Therefore, we conclude that <strong>the</strong> strontium<br />
<strong>isotope</strong> ratio <strong>of</strong> <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> (comprising at least <strong>the</strong> Dacian<br />
Basin and Black Sea) <strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition was<br />
dominated by a mixed inflow from <strong>the</strong> Danube, Dnieper and Don<br />
rivers, having a relatively constant value ranging 0.70865–0.70885.<br />
Similar to our Dacian basin data are <strong>the</strong> five 87 Sr/ 86 Sr values obtained<br />
from <strong>the</strong> lower part <strong>of</strong> <strong>the</strong> Alçıtepe Formation at Yenimahalle in <strong>the</strong><br />
Marmara sea region (0.708656–0.708836) (Çagatay et al., 2006). The<br />
magneto-biostratigraphic data from Yenimahalle indicated that <strong>the</strong><br />
Alçıtepe Formation was deposited <strong>during</strong> chron C3r (6.04–5.24 Ma),<br />
partly corresponding in time to our Dacian basin record. Thus, Sr<br />
<strong>isotope</strong> <strong>ratios</strong> from <strong>the</strong> Dacian basin are sustaining <strong>the</strong> conclusion <strong>of</strong><br />
Çagatay et al. (2006) that <strong>during</strong> <strong>the</strong> deposition <strong>of</strong> <strong>the</strong> lower<br />
Yenimahalle section <strong>the</strong> area was connected to <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong><br />
(Çagatay et al. 2006).<br />
When compared to <strong>the</strong> Danube, Don and Dnieper, <strong>the</strong> present-day<br />
Caspian Sea has even lower 87 Sr/ 86 Sr values (∼0.7082), similar to <strong>the</strong><br />
Volga river (Clauer et al., 2000; Page et al., 2003) that supplies 82% <strong>of</strong><br />
<strong>the</strong> total amount <strong>of</strong> fresh water into <strong>the</strong> Caspian basin (Table 1).<br />
Connectivity between Black Sea and Caspian Sea is thus expected to<br />
imprint a low 87 Sr/ 86 Sr ratio signature. We conclude that <strong>during</strong> <strong>the</strong><br />
<strong>Mio</strong>-Pliocene transition <strong>the</strong> Caspian basin was probably isolated from<br />
<strong>the</strong> Black Sea, because we do not see any evidence for Volga signatures<br />
in our 87 Sr/ 86 Sr data. This is in agreement with <strong>the</strong> late <strong>Mio</strong>cene<br />
paleogeographic reconstructions <strong>of</strong> <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> that<br />
indicate a subdivision into a Dacian/Euxinian basin system and a<br />
Caspian basin (Popov et al., 2006).<br />
7.3. The possible <strong>Paratethys</strong> Sr signature in MSC waters<br />
An extensively studied, but still poorly understood, major episode<br />
<strong>of</strong> fresh water influx into a marine basin concerns <strong>the</strong> final phase <strong>of</strong> <strong>the</strong><br />
Mediterranean MSC. A major deluge <strong>of</strong> low salinity waters was
130 I. Vasiliev et al. / Earth and Planetary Science Letters 292 (2010) 123–131<br />
proposed to have diluted <strong>the</strong> hypersaline environment <strong>of</strong> <strong>the</strong><br />
Mediterranean, generating wide-spread brackish-water conditions in<br />
<strong>the</strong> latest Messinian and transforming <strong>the</strong> basin into a large Lago Mare<br />
(Lake Sea) (Hsü et al., 1973).<br />
87 Sr/ 86 Sr <strong>ratios</strong> measured in <strong>the</strong><br />
Mediterranean domain for those times reached mean values <strong>of</strong><br />
0.70874 (McKenzie et al., 1988; Muller and Mueller, 1991; Montanari<br />
et al., 1997; Flecker and Ellam, 1999; Flecker et al., 2002) while <strong>the</strong><br />
ocean had a much higher 87 Sr/ 86 Sr ratio, <strong>of</strong> 0.709012 (Howarth and<br />
McArthur, 1997). These highly deflected values for <strong>the</strong> Lago Mare<br />
facies must have been generated by a massive water influx <strong>of</strong> very<br />
different Sr isotopic composition. Potential sources <strong>of</strong> distinctly<br />
different isotopic composition are <strong>the</strong> Rhône and Nile rivers. Ano<strong>the</strong>r<br />
hypo<strong>the</strong>sis infers that fresh–brackish waters came from <strong>Paratethys</strong>, in<br />
agreement with <strong>the</strong> common presence <strong>of</strong> caspo-brackish faunal<br />
elements (ostracods, molluscs and din<strong>of</strong>lagellates) in <strong>the</strong> Lago Mare<br />
sediments (Hsü et al., 1973). The inflow from <strong>Paratethys</strong> into <strong>the</strong><br />
Mediterranean is difficult to ascertain since <strong>the</strong>re is little information<br />
on late <strong>Mio</strong>cene connectivity (Çagatay et al., 2006).<br />
The newly obtained strontium <strong>isotope</strong> <strong>ratios</strong> from <strong>the</strong> <strong>Eastern</strong><br />
<strong>Paratethys</strong> can be compared with data from <strong>the</strong> Mediterranean MSC<br />
facies (Fig. 5). The 87 Sr/ 86 Sr ratio <strong>of</strong> <strong>Paratethys</strong> waters are similar to <strong>the</strong><br />
values measured in Upper Evaporites (McKenzie et al., 1988; Muller<br />
and Mueller, 1991; Keogh and Butler, 1999), and distinctly lower than<br />
<strong>the</strong> Lower Evaporites that still reflect <strong>the</strong> oceanic water <strong>ratios</strong>. This<br />
implies that a major dilution <strong>of</strong> <strong>the</strong> Mediterranean brine took place<br />
after <strong>the</strong> “Lower Evaporites” (after 5.55 Ma), when <strong>the</strong> Mediterranean<br />
became isolated from <strong>the</strong> Atlantic (Hilgen et al., 2007; Krijgsman and<br />
Meijer, 2008; Roveri et al., 2008). The similar Sr <strong>isotope</strong> <strong>ratios</strong> from <strong>the</strong><br />
Dacian basin, make <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> a reasonable candidate for<br />
<strong>the</strong> source <strong>of</strong> low Sr <strong>isotope</strong> waters <strong>of</strong> <strong>the</strong> Lago Mare facies (Fig. 5).<br />
However, an additional source is required to lower <strong>the</strong> Mediterranean<br />
<strong>ratios</strong> to <strong>the</strong> lowest 87 Sr/ 86 Sr values registered for <strong>the</strong> Upper Evaporites<br />
(0.70852). The best candidates for <strong>the</strong> low 87 Sr/ 86 Sr <strong>ratios</strong>, as proposed<br />
before (e.g. Muller et al., 1990; Muller and Mueller, 1991; Flecker and<br />
Ellam, 1999, 2006; Flecker et al., 2002), are <strong>the</strong> Rhône (0.7087) and<br />
especially <strong>the</strong> Nile (0.706).<br />
8. Conclusions<br />
We have observed a clear relation between <strong>the</strong> Sr concentrations<br />
incorporated in <strong>the</strong> biogenic carbonates from <strong>the</strong> Carpathians<br />
foredeep and 87 Sr/ 86 Sr. Different, but consistent, Sr partition coefficients<br />
for <strong>the</strong> two groups <strong>of</strong> organisms, implies a general decreasing<br />
basin water Sr/Ca ratio. The 87 Sr/ 86 Sr values are ra<strong>the</strong>r constant when<br />
compare to <strong>the</strong> noticeable globally increasing trend <strong>of</strong> <strong>the</strong> late<br />
Neogene seawater 87 Sr/ 86 Sr (Farrell et al., 1995). Both independent<br />
proxies show that relatively little Sr was supplied to <strong>the</strong> basin through<br />
wea<strong>the</strong>ring <strong>of</strong> <strong>the</strong> local mountains and that exchange with <strong>the</strong> open<br />
ocean was very limited or non-existent. Diagenetic evaluation showed<br />
that even after very thorough cleaning, not all <strong>the</strong> contamination at<br />
<strong>the</strong> outer parts <strong>of</strong> <strong>the</strong> ostracod shells could be removed. Never<strong>the</strong>less,<br />
<strong>the</strong> Sr/Ca ratio was constant in <strong>the</strong> ablation pr<strong>of</strong>iles, indicating that<br />
<strong>the</strong> Sr content was not affected by post-deposition processes.<br />
The first reported 87 Sr/ 86 Sr <strong>ratios</strong> record for <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong><br />
<strong>during</strong> <strong>the</strong> <strong>Mio</strong>-Pliocene transition indicate much lower values than<br />
those in <strong>the</strong> coeval ocean waters. This indicates that <strong>the</strong> basin was<br />
isolated from <strong>the</strong> Mediterranean and mainly fed by riverine waters.<br />
The Sr <strong>isotope</strong> <strong>ratios</strong> are consistent with a mixture <strong>of</strong> Danube, Dnieper<br />
and Don rivers, indicating connectivity between <strong>the</strong> Dacian basin and<br />
Black Sea. The strongly contrasting 87 Sr/ 86 Sr signature <strong>of</strong> <strong>the</strong> Volga<br />
(0.70802) river, is not observed. Therefore, we suggest that <strong>the</strong><br />
Caspian Sea was disconnected from <strong>the</strong> rest <strong>of</strong> <strong>Paratethys</strong> and<br />
behaved as a separate entity. We fur<strong>the</strong>r conclude that <strong>during</strong> late<br />
Pontian–Dacian times (5.3–4.0 Ma) <strong>the</strong> <strong>Eastern</strong> <strong>Paratethys</strong> was<br />
disconnected from <strong>the</strong> Mediterranean.<br />
The newly obtained 87 Sr/ 86 Sr <strong>ratios</strong> from <strong>the</strong> Dacian basin can be<br />
used to unravel water exchange patterns in <strong>the</strong> circum-Mediterranean<br />
region <strong>during</strong> Pliocene times. The Sr <strong>ratios</strong> are similar to <strong>the</strong> ones<br />
measured in <strong>the</strong> Mediterranean “Upper Evaporites/Lago Mare”,<br />
indicating that <strong>the</strong> distinctly lower Sr <strong>isotope</strong> <strong>ratios</strong> <strong>of</strong> <strong>the</strong> latest MSC<br />
phase in <strong>the</strong> Mediterranean may have been caused by waters from <strong>the</strong><br />
<strong>Eastern</strong> <strong>Paratethys</strong>.<br />
Acknowledgements<br />
I.V. thanks to Marin Waaijer and Richard Smeets (Vrije Universiteit)<br />
for help in <strong>the</strong> clean lab and <strong>during</strong> <strong>the</strong> 87 Sr/ 86 Sr measurements, to<br />
Martin Ziegler for help with ostracods cleaning procedures and to Paul<br />
Mason for facilitating <strong>the</strong> access in <strong>the</strong> LA-ICP-MS laboratory. This work<br />
was financially supported by <strong>the</strong> Ne<strong>the</strong>rlands Research Centre for<br />
Integrated Solid Earth Sciences (ISES) and <strong>the</strong> Ne<strong>the</strong>rlands Geosciences<br />
Foundation (ALW) with support from <strong>the</strong> Ne<strong>the</strong>rlands Organization for<br />
Scientific Research (NWO). We thank Rachel Flecker and two<br />
anonymous reviewers for <strong>the</strong>ir thorough and constructive reviews<br />
that significantly improved <strong>the</strong> manuscript.<br />
Appendix A. Supplementary Data<br />
Supplementary data associated with this article can be found, in<br />
<strong>the</strong> online version, at doi:10.1016/j.epsl.2010.01.027.<br />
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