Corresponding author: Artem Lyubas (
Academic editor: Yuliya V. Bespalaya
The article provides information on the outcrops of the Neogene-Quaternary riverine sediments of the North-Western Black Sea coastal area. A description of five outcrops of fluvial deposits located on the territory of the modern basins of the Dniester and Prut rivers is given. Based on the granulometric composition of the sediments and the presence in them of the fossil shells of freshwater bivalve mollusks (
Determining the age of Pliocene and Pleistocene riverine sediments is often problematic due to various reasons, including the age limitations of some dating methods or the inability to apply the method to a specific material. Numerous works are devoted to the geological characteristics of the sediments of the rivers of the Russian Plain which affect, among other things, the region of the North-Western Black Sea region (
In this paper, we tested the method of amino acid racemization in relation to subfossil shells of bivalve mollusks found in uneven-aged sediments of the Dniester and Prut rivers. The territory of these riverine basins is interesting in terms of paleogeography, because in the Pliocene and Pleistocene riverine sediments there is often found fossil material represented by the bones of various animals, mollusk shells and many other artifacts (Adamenko 1996). This material can be used to reconstruct paleoenvironments in different geological periods and to determine the age of the enclosing sediments. The use of new dating methods, such as the analysis of racemization of amino acids, can solve some problems of the stratigraphic division of heterochronous riverine sediments in the region. This approach to dating riverine deposits in the studied area may have an advantage when comparing with other dating methods for the determination of a geological age of the sediments. Its ranges of age determination make it possible to clarify information about the stratigraphy of the riverine deposits in this region. The mollusks were chosen by us as the material for amino acids racemization dating, because in this case the sample preparation proved to be very simple. The use of subfossil shells of bivalve mollusks allows solving the problems of stratigraphic dissection of both continental and marine sediments. There are examples of dating such a material using the amino acids method and even from locations in the Russian High Arctic (
The aim of this work was to study the D/L ratio of some amino acids (amino acid racemization (or
The collecting of the subfossil shells of freshwater bivalve mollusks was made from five outcrops, located on the territory of the Dniester and Prut riverine basins (Fig.
Location map of the field study areas: 1 – Sucleia, 2 – Gura Bîcului, 3 – Brînza, 4 – Slobozia Mare, 5 – Giurgiuleşti
The Brînza outcrop is located between the Brînza village and the Văleni village, between the Prut river and the highway passing here. It is a sand and gravel pit in which the river sediments of the end of the Early Pliocene are outcropping (
Published data on the numerical age of sediments with freshwater bivalve shells in the studied localities
Locality | Geological age (dating’s method) | Geological epoch | Reference |
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Brînza | 3855–4045 ka (biometric + paleomagnetic) | Early Pliocene |
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Sucleia | 710–800 ka (thermoluminescent) | Middle Pleistocene | |
Gura Bîcului | 140±35 ka (thermoluminescent) | Middle Pleistocene |
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The Giurgiuleşti outcrop is located in a small sand pit to the west of the Giurgiuleşti village. Riverine deposits are outcropped here. The geological age of this locality may vary widely, for example in the Late Pliocene age of the outcrop located near Reni village (
The Sucleia locality is a famous outcrop, where one can find subfossil mollusk shells belonging to the Tiraspol faunal complex (
The Gura Bîcului outcrop is located in the north of the village near the Dniester River. The composition of the sediments is in many ways similar to the Middle Pleistocene locality of Sucleia. These are sands with a pebble layer, and those include numerous bivalve shells belonging to the genus
In the ravine, located north of the village of Slobozia Mare, sediments are exposed, represented by a pebble layer with sand, and including numerous mollusk shells (
Samples for amino acid racemization dating were collected from the bivalve shells
Materials sampling scheme for analysis: 1 – The area from which the shell’s fragments were cut for analysis
List of analytical samples
Nomenclature | Locality | Analytical samples |
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GUBI | Gura Bîcului | LEB 12161- LEB 12170 |
SLMA | Slobozia Mare | LEB 12171- LEB 12174, LEB 12185 - LEB 12190 |
GIUR | Giurgiuleşti | LEB 12175 - LEB 12184 |
SUCL | Sucleia | LEB 13191 - LEB 13195, LEB 13206 - LEB 13210 |
BRIN | Brînza | LEB 13211-LEB 13220 |
During the fieldwork, a layered description of ancient alluvial deposits from top to bottom and the collecting of subfossil material together with the fixing of its position in the outcrop were undertaken. The fieldwork was carried out during May 2013.
Ten (10)
As with several studies (
Amino acid concentrations and ratios were quantified using HPLC following the sample preparation protocol described in
Samples were injected into an Agilent HPLC-1100, equipped with a fluorescence detector. Excitation and emission wavelengths were programmed at 335 nm and 445 nm, respectively. A Hypersil BDS C18 reverse-phase column (5 μm; 250 × 4 mm i.d.) was used for the analysis.
The derivatization takes place before injection by mixing the sample (2 μl) with the pre-column derivatization reagent (2.2 μl), which comprised 260 mM isobutyryl-L-cysteine (chiral thiol) and 170 mM o-phtaldialdehyde, dissolved in 1.0 M potassium borate buffer solution at pH 10.4. Eluent A consisted of 23 mM sodium acetate with 1.5 mM sodium azide and 1.3 micro M EDTA, adjusted to pH 6.00 with 10 M sodium hydroxide and 10% acetic acid. Eluent B was HPLC-grade methanol and eluent C consisted of HPLC-grade acetonitrile. A linear gradient was performed at 1.0 ml/min and 25 °C, from 95% eluent A and 5% eluent B upon injection to 76.6 eluent A, 23% eluent B, and 0.4% eluent C at 31 min.
According to
To compare the D/L values in the shells from localities of different ages, the Wilcoxon signed-rank test was used. Statistical data processing was performed using StatSoft, Inc. software (2011), STATISTICA (data analysis software system), version 10.
We obtained mean D/L-values for each of the 5 locations. The values for aspartic acid were from 0.606±0.054 to 0.773±0.042, and for glutamic acid were from 0.340±0.040 to 0.919±0.067. For leucine, phenylalanine and isoleucine, D/L-values greater than 1 are marked. The results obtained allow us to compare the relative geological age of the sediments containing subfossil shells. The average values of five amino acid ratios for the Pleistocene outcrops do not exceed one, which is important for the interpretation of these results in the geochronological aspect.
Aminostratigraphy consists in placing sets of geological, paleontological or archaeological localities in a stratigraphical order according to the measured D/L ratios from the same group of fossils (genera), which were preserved under similar environmental conditions, inorganic geochemistry and thermal histories. Accordingly, each group of levels with the same amino acid racemization values constitutes an almost-isochronous event.
The amino acid racemization/epimerization ratios (Table
Mean amino acid racemization ratios obtained in
Locality | N | D/L |
D/L |
D/L |
D/L |
D-aiLe/L- |
---|---|---|---|---|---|---|
GUBI | 10 | 0.630 ± 0.049 | 0.370 ± 0.037 | 0.636 ± 0.084 | 0.763 ± 0.080 | 0.293 ± 0.032 |
SLMA | 10 | 0.606 ± 0.054 | 0.340 ± 0.040 | 0.563 ± 0.081 | 0.711 ± 0.098 | 0.293 ± 0.042 |
GIUR | 10 | 0.771 ± 0.055 | 0.899 ± 0.077 | 0.957 ± 0.072 | 1.243 ± 0.068 | 1.059 ± 0.100 |
SUCL | 10 | 0.695 ± 0.050 | 0.539 ± 0.056 | 0.762 ± 0.051 | 0.900 ± 0.068 | 0.530 ± 0.057 |
BRIN | 10 | 0.773 ± 0.042 | 0.919 ± 0.067 | 1.008 ± 0.087 | 1.202 ± 0.079 | 1.014 ± 0.090 |
The age of the Pleistocene localities was interpreted based on the racemization rate of amino acids in subfossil shells, as described in what follows. Slobozia Mare is the youngest locality, as the D/L values in the shells from this site are the lowest.
Mean D/L-values of amino acids for Pleistocene localities:
Pairwise comparison of these three localities across the D/L ratios and across all five amino acids using the Wilcoxon signed-rank test, allow us to conclude that the D/L ratios are statistically significantly different between Slobozia Mare and Sucleia (p=0,042) and between Gura Bîcului and Sucleia (p=0,043). When comparing the Slobozia Mare and Gura Bîcului localities, no statistically significant differences were found for these indicators (p> 0.05).
Dating of Middle and Late Pleistocene paleontological sites with mollusk fauna using the amino acid racemization method is a promising area of paleogeographical research on the territory of the Russian plain. Many paleontological sites located in the southern part of the Russian plain provide the necessary material for such studies.
A comparison of the results of the analysis of amino acid racemization in the subfossil shells of freshwater bivalve mollusks with the numerical age of riverine sediments, shows that the determinated geological age of shells using D/L ratio of five amino acids can reach up to 800 thousand years. This is consistent with modern concepts of time constraints which use the method of aminostratigraphy. Further studies of the fluvial deposits of Dniester and Prut rivers should aim to form databases on amino acid racemization in subfossil mollusk shells, because this may be useful for the stratigraphic division of sedimentary strata.
The study was carried out by a grant from the Russian Science Foundation (project № 18-77-00058).
List of studied localities.
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1 | Brînza (Cahul District, Moldova, |
Layer thickness in studied area is 7.5 m, |
2 | Giurgiuleşti (Cahul District, Moldova, |
Layer thickness in studied area is 4 m, |
3 | Sucleia (Slobozia District of Transnistria, Moldova, |
Layer thickness in studied area is 4.6 m. |
4 | Gura Bîcului (Anenii Noi District, Moldova, |
Layer thickness in studied area is 6.5 m. |
5 | Slobozia Mare (Cahul District, Moldova, |
Layer thickness in studied area is 8.7 m. |
Brînza (Cahul District, Moldova)
Giurgiuleşti (Cahul District, Moldova)
Sucleia (Slobozia District of Transnistria, Moldova)
Gura Bîcului (Anenii Noi District, Moldova)
Slobozia Mare (Cahul District, Moldova)