Lithofacies and ichnofacies distribution in offshore sediments of the Langarud–Rudsar region of Iran,southern Caspian Sea

Abstract

Innovative Computerized co-axial tomography (CAT)-scan methods have been applied to two short cores collected in the southern Caspian Sea, offshore from the Langarud–Rudsar region of Iran, to the east of the Sefidrud delta. Magnetic susceptibility (MS) readings, in association with other lithological criteria, are used to correlate the cores and calibrate to real time. The cores provide a sedimentary record from this largely unstudied region covering the last 200+ years (~1784 to 2008 CE). Three principal lithofacies types were recognized. The more distal locality (T2-3, water depth 117 m) consists entirely of poorly fossiliferous silty muds, whereas the more proximal locality (T2-2, water depth 51 m) contains beds with rich ostracod, gastropod and bivalve assemblages. Peak MS readings occur between ~1872 and 1918 CE and are linked to erosion of sediments deposited during the ‘Little Ice Age’ (LIA) highstands of the Caspian Sea. CAT-scan results reveal the presence of the non-marine Mermia ichnofacies type, which is reported for the first time in the Caspian Sea. Horizontal burrows (feeding structures) by Treptichnus ichnofauna coincide with periods of LIA highstand in Caspian Sea level and suggest low-energy conditions on the marine shelf. Vertical burrows (feeding structures) occur at times of relatively stable, but variable Caspian water levels. Interbedded dark coloured, organic-rich muds and lighter silt-rich layers within the last ~100 years show potential cyclicity which may be linked to climatic and/or river discharge events.

Keywords

CAT-scan analysis ichnofauna ‘Little Ice Age’sedimentology South Caspian Sea

Introduction

Depositional environments can be defined in terms of physical, biological and chemical variables; thus, changes in environmental conditions and their instability during sedimentation are often followed by changes in diversity, distribution and abundance of planktonic and benthic organisms as well as by the nature of their activity and, consequently, changes in biogenic (i.e. ichnofacies) structures (Aller, 1989; Michaud et al., 2003; Pearson and Rosenberg, 1978; Rhoads and Boyer, 1982; Taylor et al., 2003). Such studies on ancient deposits, in particular marine sediments, have been carried out extensively. However, the number of studies undertaken on recent, unconsolidated sediments (e.g. Dashtgard et al., 2008; Genise et al., 2009; Gérino et al., 1999; Martin et al., 2005; Michaud et al., 2005) is limited, partly due to the restricted research methods available. Computerized co-axial tomography (CAT)-scan images are now used to analyse sedimentary structures, in particular biogenic structures. These developments in research methodology have improved the quality and precision of ichnofacies and related studies (e.g. Boespflug et al., 1995; Gagnoud et al., 2009; Mermillod-Blondin et al., 2003; Michaud et al., 2003).

In this paper, we present the results of a lithofacies and biofacies study of two short cores collected from the southern shelf of the Caspian Sea, offshore from the Langarud–Rudsar region of Iran. The cores were taken at a distance between 10 and 14 km from the shoreline in water depths of 51 and 117 m and provide a sedimentary record for the last 200+ years (from ~1784 to 2008 CE). CAT-scan techniques were used to recognize patterns of burrowing by ichnofauna, the first time these techniques have been applied to ichnofacies studies in the Caspian Sea. In addition, core samples were examined for various lithological and sedimentological parameters. Results obtained are considered in relation to changes in Caspian Sea level, climate and sediment input.

Geographical and hydrodynamic setting

The Caspian Sea is a remnant of the Tethys Sea, which was separated into two principal basins during the late Mesozoic and early Cenozoic (Allen et al., 2003; Brunet et al., 2003). The southern basin formed the Mediterranean Sea and the northern basin formed Paratethys (Golonka, 2004, 2007), which was primarily brackish. With the uplift of the Alps, Carpathians, Balkans, Caucasus and Anatolian mountains, Paratethys was again separated into three smaller western, central and eastern basins; eastern Paratethys included the Black, Caspian and Aral seas (Brunet and Cloetingh, 2003; Brunet et al., 2003) that are referred to as ‘Ponto-Caspian basins’ (Yanina, 2014). Water level fluctuations due to eustatic variations and tectonics have repeatedly opened or closed pathways between individual Paratethyan basins and sub-basins (Palcu et al., 2017; Popov et al., 2010, 2006; Van Baak et al., 2016; Yanina, 2014).

The present-day Caspian Sea is the world’s largest inland water body, both by area and by volume. Three principal sub-basins are present that are characterized in terms of bathymetry, bottom morphology and hydrodynamic conditions (Dumont, 1998; Kosarev and Yablonskaya, 1994; Figure 1a). The North Caspian Sea Basin has a gentle slope with a maximal depth of 25 m and is supplied by major rivers, particularly the Volga and the Ural. The Middle Caspian Sea Basin is bordered by a narrow shelf to the west and a wide shelf to the east and has a maximal depth of 788 m (Kroonenberg et al., 2000; Lahijani et al., 2009). The South Caspian Sea Basin is situated to the south of the Apsheron Sill and has undergone significant subsidence during the Neogene (Abdullayev et al., 2012; De la Vara et al., 2016). It has a maximum water depth at present of ca. 1025 m (Kosarev, 2005). The southern and southwestern coasts of the Caspian Sea are located adjacent to Iran and Azerbaijan. Around 130 rivers flow into the Caspian Sea from the southern and western coasts which are mainly sourced from the Elborz and Caucasus Mountains. Sediments along the southern and western shorelines are mainly redistributed by wave-induced currents (Kakroodi et al., 2012; Koshinskii, 1975).

Figure 1.

(a) Caspian Sea bathymetry and neighbouring countries, showing North Caspian Sea Basin (NCSB), Middle Caspian Sea Basin (MCSB) and South Caspian Sea Basin (SCSB). AP = Apsheron Sill, on the boundary between the Middle and South Caspian Sea Basins (modified after Voropaev, 1986). The study area is indicated by the red rectangle. (b) Map of the study area. Locations of cores are shown as yellow dots. (c) Cross-shelf profile showing water depth and slope variation at the sampling sites.

The assimilation of stratigraphic records illustrates that several regressions and transgressions have occurred in the Caspian Sea during the Quaternary (e.g. Chen et al., 2017; Naderi Beni et al., 2013a; Rychagov, 1997; Svitoch, 2014; Yanina, 2014). These are documented from the deep middle and southern basins by Kuprin et al. (2003, 2002), Leroy et al. (2014) and Levchenko and Roslyakov (2010), and in the shallow southern basin by Kakroodi et al. (2015), Kazancı and Gulbabazadeh (2013), Kazancı et al. (2004), Lahijani et al. (2009) and Svitoch et al. (2016), among others. Transgressions of wave-dominated facies are evident in the lowland and coastal areas in the southern Caspian Sea with landward advances of barrier lagoons and by spit-lagoon formation through littoral drift and the formation of beach-ridge complexes (e.g. Haghani and Leroy, 2016; Haghani et al., 2016b; Leroy et al., 2011; Naderi Beni et al., 2013b). Regressions in wave-dominated facies are associated with increased density of gravitational streams on the slope, formation of alluvial fans and soil formation and deposition of loess on the coastal plain (Figure 2). During long-term regressions, deltaic progradation occurs and coastal deposits typically include low beach bars without lagoons (Kakroodi et al., 2012, 2015; Kazancı et al., 2004; Kazancı and Gulbabazadeh, 2013; Kroonenberg et al., 2007; Svitoch et al., 2016). Deeper water facies in the middle and southern Caspian Sea show laminated sediments preserved because of rhythmic oxygenation, related to alternating relative highstands and lowstands of water level (Kuprin et al., 2002, 2003; Tudryn et al., 2014).

Figure 2.

Schematic model showing main depositional features during transgressions and regressions in the southern Caspian Sea.

The study area

The sediment cores were taken from the southern Caspian Sea offshore from the Langarud–Rudsar region of Iran, to the east of the Sefidrud Delta (Figure 1b). The salinity is approximately 13‰ (Kosarev and Yablonskaya, 1994) and the waters categorized as persistently hypoxic at moderate to severe levels (Diaz and Rosenberg, 1995). This area is a humid zone within an otherwise largely dry region, typically receiving ca. 1450 mm of rainfall per annum and with a mean annual temperature of ca. 16ºC (Molavi-Arabshahi et al., 2016). In the coastal areas, October and November are the wettest months and summer is the driest time of the year (Amini and Shabani, 2000). Anticyclonic processes play a key role in causing stormy winds and waves in the southern Caspian Sea, with wind-induced waves of up to 3 m occurring frequently along the Iranian coastline (Koshinskii, 1975). Anticyclonic movement of water masses (eddies) with a diameter of 28–30 km has also been documented from the Langarud–Rudsar region (Shipilova, 2000). The coastal plains to the north of the Elborz Mountains are narrow, and the continental shelf is steeply sloped, with water depths of 100 m reached ca. 14 km from the coastline (Figure 1c). Weak longshore currents in the area have led to a configuration of subaqueous mouth bars and lateral lagoons but have minimal impact on the positions of the river mouths (Alizadeh et al., 2008). Waves and wave-induced currents are the main factors that influence delta configuration (Azimov et al., 1986; Lahijani, 1997).

Rivers in the regions surrounding the southern Caspian Sea mainly begin from densely vegetated hills at an average elevation of 200 to 300 m, with local topography and climate having influences on factors such as fluvial discharge, coastal slope, wave-induced currents and configuration of the river mouth (Azimov et al., 1986; Mikhailov, 1997; Voropaev et al., 1986). The study area receives freshwater input from two principal rivers, the Polrud and the Shalmanrud (Table 1; Figure 1b). The Polrud River is of moderate size and its maximum discharge occurs in April and May (Figure 3a). The headwaters of the Polrud are located in a mountainous area with little vegetation cover that gradually changes to a densely forested area before passing through to the south Caspian coastal plain (Alizadeh et al., 2008; Amini and Shabani, 2000). The climate is typically one with short, severe showers and prolonged rains in spring, accompanied by melting of snow. The Shalmanrud River is a small river with maximum discharge taking place in the spring and autumn months (Figure 3a). The annual discharge of the Shalmanrud showed significant variations during the period 1975 to 2008 CE (Figure 3b). Between ~1975 and 1990 CE, water discharge values were typically within the range of ~4 to 7 m³/s. An increase in Shalmanrud discharge then occurred from ~1992 to 2008 CE, with values mostly within the range of ~8 to 10 m³/s. Additional water and sediment input may also occur from the larger Sefidrud River (Figure 1b; Table 1). Most of the Sefidrud catchment is poorly vegetated and precipitation over the upstream region is around 300 mm per annum (Alizadeh et al., 2008).

Table 1.

Characteristics of Shalmanrud, Polrud and Sefidrud rivers.

River	Catchment area (km²)	Length of river (km)	Water discharge (m³/yr)	Sediment discharge (tonnes/yr)
Polrud	1650	80	476,000,000	1,000,000
Shalmanrud	390	54	222,000,000	3729.6
Sefidrud	67,000	800	4,037,000,000	26,000,000

Data were taken from the database of Water Research Institute, Iran and Alizadeh et al. (2008).

Figure 3.

(a) Monthly average discharge for the Polrud and Shalmanrud Rivers. (b) Average annual discharge of Shalmanrud during the period 1975 to 2008 CE. Data are taken from the database of Water Research Institute, Iran.

Methods

Our investigation concentrates on two short sediment cores, T2-2 and T2-3, which were collected in the offshore region of the Langarud–Rudsar, Iran, Caspian Sea, during the Iranian research vessel cruise in 2008, conducted by the Iranian National Institute for Oceanography and Atmospheric Science. The sampling was carried out in order to investigate the lithofacies, biofacies and associated depositional processes in this little studied region of the Caspian Sea. The location and core data are summarized in Table 2 and Figure 1b.

Table 2.

Location, water depth and length of cores examined for this study.

Core	Latitude	Longitude	Length (cm)	Water depth (m)
T2-2	37° 15.03 N	50° 22.38 E	140	51
T2-3	37° 16.53 N	50° 24.93 E	132	117

To investigate sedimentary structures and biofacies, including ichnofacies, CAT-scans of cores T2-2 and T2-3 were performed on a total core section of 1.6 m, using a medical (GE Hi-Speed Qx/i) CAT-scanner at the Tooska Medical Imaging Centre, Tehran. A source radiation of 120 keV, 45 mA was used for longitudinal sections and 120 keV, 210 mA used for transversal sections, with obtained images measuring 916 pixels in length and 512 pixels in width, respectively. Transversal sections were obtained at intervals of 80 mm, resulting in 10 slices per core. A continuous image of both cores was then constructed using eFilm Lite (TM) software, with Grey scale values expressed as computed tomography (CT) numbers, or Hounsfield Units, which give an indication of the bulk density, mineralogy and porosity of the sediments (e.g. Boespflug et al., 1995). CT numbers are obtained by comparing the attenuation coefficient (µ) to that of water (µw) as follows:

CT (Hounsfield Units) = (μ / μ w - 1) \times 1000

The packing density of burrowing was then calculated based on the method of Blatt et al. (1980) and expressed as the total space occupied by burrows in each transversal section.

To investigate lithology and sediment type, 68 samples were taken at 4 cm intervals. Samples were oven dried at 60°C and crushed and pulverized using an agate mortar and pestle. Percentage carbonate, percentage organic matter and faunal content were analysed for all samples. Percentage counts of grain size were made in three size classes of >63, 10–63 and 0–10 μm. The carbonate and organic matter-free sediments samples were characterized granulometrically by wet sieving, using ASTM standard sieves for the >63 µm fraction and by a Laser-Particle-Sizer (Analysette 22) for the <63 µm fraction. Calcium carbonate content was determined using a Bernard calcimeter (Lewis and McConchie, 1994). Measurements were carried out in triplicate using 1 g of finely crushed sediment. Organic matter content was measured by heating sediment samples at 325°C for 24 h and treated with 30% high-purity hydrogen peroxide according to the methodology of Heiri et al. (2001). The mineralogy of fine-grained samples (<4 µm) was determined by x-ray diffraction (XRD) analysis and the sand to medium silt fraction studied optically using a Zeiss Axio Lab microscope. For the microfaunal analysis, samples were wet sieved using a mesh size of 125 µm and then dried. The fauna were examined microscopically using a Scanning Electron Microscope (Zeiss DSM 962) at Tehran University. The magnetic susceptibility (MS) of the cores was measured using a Bartington Magnetic Susceptibility Meter MS2C at the Institute of Geophysics, Tehran University. Ages and sedimentation rates for the cores were estimated on the basis of 210Pb and 137Cs activities in the Caspian sediments in the studied area as described by Sharmad et al. (2012). These measurements are based primarily on the T2-2 core, where a sedimentation rate of 8.9 mm per year was calculated. Ages for the T2-3 core in this study are estimated based on the correlation of MS and other sedimentary parameters between T2-2 and T2-3.

Results

Lithology and stratigraphy

Three lithofacies types have been recognized on the basis of sediment texture; all three are made up of fine-grained, hemipelagic deposits with interbeds of silty mud (Figure 4). Lithofacies I occurs at a depth of 140–90 cm in the lower portion of the T2-2 core. Based on Folk’s (1974) textural classification, sediments of this section consist of mud with a high percentage of silt. The mean grain size varies between 5 and 7 (phi). Calcium carbonate content is relatively high, ranging from 16% to 19%, and amount of organic matter also fairly high, within a range of 2–4%. The principal fauna are ostracods, as well as foraminifera which are seen in some horizons.

Figure 4.

Sediment parameters of Core T2-2 (top) and Core T2-3 (bottom). From left to right: sedimentary classification (Folk, 1974) of samples, lithostratigraphy of the cores, mean grain size (phi), magnetic susceptibility (MS), calcium carbonate (CaCO₃ %), organic matter content (OM%) and sorting (phi). Three lithofacies types have been identified on the basis of sediment textures and distributions of benthic faunal assemblages: Lithofacies I (ostracod-rich silty mud), Lithofacies II (gastropod-rich mud) in core T2-2 and Lithofacies III (fossil-poor mud) in core T2-3.

Lithofacies II occurs at a depth of 90–0 cm in the upper portion of the T2-2 core. Sediments of this facies are located in the mud zone (Folk, 1974) with more or less equal percentages of silt and clay. Mean grain size is within a range of 6 to 7.5 (phi). Calcium carbonate content varies between 14.5% and 19%, with amounts of organic matter variable but within a range of 2.5–4%. The dominant fauna are gastropods, with assemblages of diatom frustules (Coscinodiscus radiatus) present in the interval 76–56 cm. Bivalve shells also occur, mainly at intervals where there are thin, light beds, for example, between 32–28 cm and 52–48 cm (Figure 4). The sorting range in Lithofacies I and II is fairly constant at around 1.5 (phi).

Lithofacies III includes all sediments in the T2-3 core. Based on textural classification (Folk, 1974), the grain size is predominantly mud with a high percentage of clay. In comparison to Lithofacies I and II, Lithofacies III has slightly less organic content, varying within a range of 1–3%. Content of calcium carbonate varies from 12.5% to 20%. Mean grain size within Lithofacies III is within a range of 7–8.5 (phi) and sorting range variable but within a range of 1.5–2.0 (phi). Lithofacies III is poorly fossiliferous.

The overall mineral composition of the sediments in XRD graphs is similar, consisting of quartz, dolomite, calcite, plagioclase, alkali feldspar, amphibole, haematite, illite and smectite. Optical microscope analyses show volcanic and calcareous lithoclasts and shell fragments (Figure 5).

Figure 5.

(a) Siliceous frustule of Coscinodiscus radiatus in sub-sample of Core T2-2 at depth of 56–67 cm. (b) Bivalve shell fragment in sub-sample of core T2-2 at depth of 28–32 cm. (c and d) Calcareous and volcanic lithoclasts, respectively, in sub-samples of core T2-2.

In this study, we have used the ages calculated by Sharmad et al. (2012) for the T2-2 core in order to calibrate the MS signature and curves for other mineralogical data in real time. These have then been used to correlate between the T2-2 and T2-3 and to provide age information for the latter. MS changes throughout T2-2 and T2-3 allowing identification of three units within each of the cores. MS Unit 1 extends from 140 to 130 cm in core T2-2 and from 132 to 85 cm in core T2-3, where MS readings are close to 14 × 10⁻³ m³/kg. MS Unit 2 is assigned from 130 to 90 cm in T2-2 and 85 to 55 cm in T2-3 and is characterized by high values within a range of 13–17 × 10⁻³ m³/kg. MS Unit 3 extends from 90 to 0 cm in T2-2 and 55 to 0 cm in T2-3 and is characterized by lower values, within a range of 10–14 × 10⁻³ m³/kg (Figure 4). Most variations in MS in marine systems are due to the influx of terrigenous paramagnetic grains (Crick et al., 2001). The most important of these include clay minerals, particularly chlorite, smectite and illite, ferromagnesian silicates such as amphiboles, iron carbonates such as siderite and ankerite, and other iron- and iron-bearing lithogenic or detrital fractions (Ellwood et al., 2006). The XRD analysis in this study illustrates that amphibole, in association with smectite and illite in the clay fraction, are the main paramagnetic components.

Sedimentary structures

CAT-scan images obtained from the mid portions of each core clearly show darker and lighter zones, which represent lower and higher x-ray attenuations, respectively, according to previous work by Boespflug et al. (1995). The paler intervals are labelled X1–X6 (core T2-2) and X7–X10 (core T2-3; Figure 6). The frequencies of medium silt and coarse silt increase in light (X) beddings, whereas the average percentage of organic matter and mean (phi) both decrease. Conversely, the frequencies of medium and coarse silt decrease in the dark (V) beddings, whereas average percentage of organic matter and mean (phi) both increase.

Figure 6.

(a) Lithological analyses of cores T2-2 and T2-3, showing variations in organic matter content, mean (phi) and grain size analyses. The grain size results are shown in three size classes of >63, 10–63 and 0–10 µm. Thickness of light laminations and packing density of burrowing have been calculated in terms of (mm) and (%), respectively. The types of burrowing are determined on the basis of obtained transversal sections (shown in Figure 7). The ages are assigned in years CE. The longitudinal tomographic sections and the CT number series are shown on the left (modified after Abbasian et al., 2010). The grey column on the left side indicates the scanned section analysed within the core. Light (X) and dark (V) layers are marked along the tomographic image by the dashed lines. The longitudinal tomographic sections were obtained from depths of 28 to 105 cm in core T2-2 and 40 to 115 cm in core T2-3. The depths of tomographic sections are determined by CT number graphs. (b) Trends of laminations and burrowing structures correlated with Caspian water level. The trends are numbered on the graphs in both (a) and (b). Caspian water level modified from Chen et al. (2017) after 1840 CE and Naderi Beni et al. (2013a) prior to 1840 CE.

Medium light bedded and very thinly bedded laminations reoccur in the interval 110–75 cm (X10 to X8) in core T2-3, which corresponds with a time period of approximately 1821–1880 CE. The predominance of sortable silt (10–63 µm) increases in light interval X9 (~1838 to 1864 CE) and, together with cross lamination of silt bodies (Figure 6a, no. 5), suggests relatively strong current activity (McCave, 2008; McCave et al., 1995) and reworking of sediments on the middle and outer shelf, as recognized by Kuprin et al. (2003). Bed X9 is of similar appearance to pale, laminated beds in the Klaus lagoon, illustrated by Haghani and Leroy (2016). Those beds have a median radiocarbon age of 1842 to 1847 ca. BP (>70% probability), although the authors of the study prefer an older age interpretation. In core T2-2, higher CT values in the core interval 70 to 30 cm coincide with light coloured laminations (X5 to X1) in the time period of approximately 1938 to 1978 CE (Figure 6a, no. 1).

Ichnofacies

The CAT-scan techniques, by quantifying the packing density of the sediments, permit recognition of burrowing patterns by ichnofauna (Figures 6 and 7). Small, horizontal burrows were encountered in this study, formed by Cochlichnus (core T2-2) and Treptichnus (core T2-3), which are types of a non-marine ichnofacies known as Mermia, as defined by Buatois and Mángano (1995, 2007). The Mermia ichnofacies is characteristic of fine-grained sediments and occurs in low energy, permanently subaqueous zones of lacustrine environments. These are reported from the Caspian Sea for the first time (Figure 7).

Figure 7.

Photographs of ichnofacies traces in transversal sections obtained from depths of 28 to 105 cm of core T2-2 (top) and 40 to 115 cm of core T2-3 (bottom). Transversal sections were scanned at intervals of 80 mm, resulting in 10 slices per core (Abbasian et al., 2010). The burrowing types are interpreted based on methodology of Buatois and Mángano (1995, 2007).

In sections obtained from the T2-3 core, a high density of Treptichnus horizontal burrowing is visible at a depth of 116 cm, and this remains present but progressively decreases in sections from depths of 108 to 100 cm (Figure 7t-r). A lower density of burrowing is subsequently noted, meaning that sediment laminations (e.g. in pale interval X9 in the T2-3 core) are mostly preserved. A re-appearance of burrowing is evident from the CAT-scan data in core T2-3 above 60 cm. These, however, are vertical burrows (feeding structures; Figure 7k–l). Transversal sections obtained from depths of 95 to 80 cm in the T2-2 core also show a low density of similar small, vertical burrows within the same time interval (Figure 7h). Burrowing traces re-appear in the T2-2 core between ca. 70 and 50 cm where horizontal grazing trails, formed by Cochlichnus (Figure 7e), and vertical feeding structures (Figure 7d) both occur. Vertical burrows are again noted at 36 cm. At depth of 28 cm in core T2-2, burrowing by a bivalve is also visible (Figure 7a).

Discussion

Caspian sea level and sedimentation

Caspian water level according to Naderi Beni et al. (2013a) and Chen et al. (2017) shows six principal phases of change within the last 250 or so years. Maximum water levels in the region of −22 m bgsl (below global sea level) were reached during several decades prior to 1805 CE, associated with the ‘Little Ice Age’ (LIA). At this time, extensive beach ridges formed on the present-day Iranian coastal plain (Haghani et al., 2016a; Leroy et al., 2011), and these beach sediments subsequently will have become available for erosion (Kaplin and Selivanov, 1995). A sea level fall of ca. 4.5 m then occurred in the years between 1805 and ca. 1845 CE, with water levels dropping to around 26.5 m bgsl. Caspian water levels subsequently remained fairly stable, fluctuating within a range of ca. −25.5 to −26.5 m bgsl between ca.1845 and 1929 CE, before falling by a further 3 m during a 50-year period until 1977. A subsequent rise of 2.5 m occurred between 1977 and 1995, which caused significant damage to livelihoods and infrastructure around the Caspian Sea coasts (Kroonenberg, 2017).

The fingerprints of these water level oscillations have been recorded in the sedimentary environments of the Caspian basin during recent decades, for example, in the Dagestan area, where low-amplitude terraces formed during a period of water level fall between 1929 and 1977 CE, and lagoons isolated by barrier bars formed during water level rise up until 1995 (Kroonenberg et al., 2000). During the same period prior to 1977, Hassan Gholi Bay, a large inland lagoon in north of the Gorgan delta in the Gomishan area of Iran, dried out and the coastline shifted several kilometres seaward and narrow parallel sand barriers accreted to the coast. The sand barriers subsequently breached and a new lagoon formed during the period of water level rise after 1977 (Kakroodi et al., 2012). On the whole, the Caspian Sea water level during the last two centuries has shown a long-term decline from about −22 to −28 m bgsl, with a continued decline expected because of increasing northern hemisphere temperatures (Chen et al., 2017).

The highest MS values both in core T2-2 and T2-3 (MS Unit 2) equate with the period ~1872 to 1918 CE, when Caspian water levels were around 2 m higher than the present day, fluctuating at around −26 m bgsl (Chen et al., 2017), although these levels were still ca. 3 m below the maximum water levels reached (−22.7 m bgsl) several decades earlier at the end of the LIA (Naderi Beni et al., 2013a). The high MS values (up to 17 × 10⁻³ m³/kg) in MS Unit 2 (~1872 to 1918 CE) are probably the result of increased terrigenous sediment supply, and higher sediment load occurring, with erosion of beach ridges and coastal sediments by rejuvenated rivers, supplying more terrigenous sediment, including heavy minerals, into the marine system. This is supported by an increase in grain size in core T2-2. MS Unit 1 (~1784 to 1872 CE) corresponds to a time of maximum Caspian Sea levels (LIA) but has somewhat lower MS values at around 14 × 10⁻³ m³/kg. At the time of deposition, sedimentation is likely to have been predominantly retrogradational (i.e. in a landward direction) or aggradational (i.e. in equilibrium) under maximum highstand conditions. Sediment flux and, as a result, MS readings are therefore likely to have been reduced, as is the case in MS Unit 1. MS Unit 3 (~1918 to 2008 CE) has variable MS responses that show no clear link with observed Caspian Sea levels. During this period, anthropogenic activities are also significant, with dam structures, increased deforestation and desertification having led to reduced river discharge (Haghani et al., 2016b) and potentially decreased the available sediment load.

The distribution and thicknesses of the light laminations in core T2-2 suggest that rhythmic events occurred periodically at around 1903 (X6), 1943 (X5), 1953 (X4), 1963 (X3), 1973 (X2) and 1978 (X1) CE (Figure 6a, no. 1). These layers are probably linked to annual or seasonal climatic oscillations or storm deposition (Boespflug et al., 1995; Kuprin et al., 2003) and coincide approximately with periods of sea level fall. Dark, clay-rich sediment layers typically have massive and homogeneous appearances, which are indications of primary preservation due to the absence or low density of trace fossils. They are likely to have been deposited in quasi-steady and low-energy conditions.

Distribution of trace fossils and microfauna

Treptichnus horizontal burrowing in the T2-3 core between 116 and 100 cm (Figure 6, no. 6) equates to a time period of ~1821 to 1838 CE when Caspian Sea levels were high and gradually falling. Above 50 cm in core T2-3, vertical burrows occur during a time interval of ~1923 to 1932 CE with very low density (Figure 6, no. 4). In core T2-2, vertical burrows appear at 85 cm with high density around 1923 CE (Figure 6, no. 3). These intervals with vertical burrowing occur during a time period when Caspian Sea levels were high (around 2 m higher than at present), although relatively stable. Burrowing traces re-appear in the T2-2 core between ca. 70 and 50 cm (~1938 to 1958 CE) where horizontal grazing trails, formed by Cochlichnus, and vertical feeding structures both occur (Figure 6, no. 2). This period (~1940 to 1973 CE) coincides with an extended phase of lowering Caspian Sea levels after 1932. Very low density of vertical burrows at 36 cm (~1973 CE) in core T2-2 (Figure 7b) coincides with a period when Caspian Sea level continued to fall (between 1940 and 1973 CE). Water levels were, however, relatively stable, within the range of −27.5 to−28.5 m bgsl (Chen et al., 2017), which is comparable with the present day.

It is noted in this study that intervals with a common presence of horizontal Treptichnus burrowing as a feeding structure occur at times when Caspian Sea level was at a highstand level during the LIA and with low-energy conditions (i.e. ~1821 CE) and burrowing gradually decreases with falling water level and increased occurrence of currents (X9) (Figure 6, no. 5, 6). Periods of water rise may be linked with reduced amounts of oxygen in the bottom waters, causing the ichnofauna to feed horizontally without oxygen deprivation. Conversely, vertical burrows tend to occur at times of fairly stable Caspian Sea levels, either during relative highstand under low-energy conditions (e.g. 1923 to 1932 CE; Figure 6, no. 4) or relative lowstand phases subject to current activity (e.g. 1938 to 1958 and 1973 CE; Figure 6, no. 2). Horizontal burrows of Cochlichnus (grazing trails) are visible in one section around 1948 CE alongside vertical burrows (Figure 7e). They are likely to have formed during lowering phases (e.g. 1948 CE), perhaps in response to increased oxygenation and an abundance of deposited food, meaning that the organisms can consistently occupy similar stratigraphic levels (and therefore feed laterally). This is consistent with the observations of Miller (2007) who notes that vertical burrows and horizontal structures tend to occur in oxygenated waters and with good availability of food, respectively, and subject to strong currents. Indeed, the vertical burrows are prevalent at times of consistent but variable sea levels. Additional factors which impact burrowing type with increasing water depth are decrease in grain size and decrease in organic matter, each of which in turn can control trace fossil distribution (Miller, 2007). In core T2-2, mud with high amounts of organic matter and silt host neoichnogenera dominated by feeding structures (vertical burrows) and some horizontal grazing trails (Cochlichnus). In core T2-3, the Treptichnus horizontal feeding structures are recognizable in clayey mud sediments where percentages of organic matter are relatively low.

It is therefore suggested that Caspian Sea level and depositional environment exert controls on burrowing patterns and ichnofacies preservation (Figure 8). Indeed, hemipelagic deposits with silty mud interbeds have been associated with vertical burrows and horizontal structures (Cochlichnus, Treptichnus) as part of the Mermia ichnofacies. Buatois and Mángano (1995) confirm that the Mermia ichnofacies is characteristic of permanently subaqueous environments but punctuated episodic deposition (e.g. turbidity currents and density underflows) may occur.

Figure 8.

Summary diagram showing changes in various recorded parameters between 1784 CE and the present day in relation to Caspian water level. From left to right: biofacies, lithofacies, magnetic facies, ichnofacies and sedimentary environments in cores T2-2 and T2-3. Caspian water level modified from Chen et al. (2017) after 1840 CE and Naderi Beni et al. (2013a) prior to 1840 CE. For lithofacies legend, see Figure 4.

Differences in microfauna observed between cores T2-2 and T2-3 are not clearly explained, although they are probably largely due to the difference in water depths of the two cores (51 and 117 m, respectively). The deeper water site (T2-3) is poorly fossiliferous, whereas the shallower site has rich ostracod, gastropod and bivalve associations. Ostracods occur frequently during the time interval of ~1868 to 1918 CE (Lithofacies I) when Caspian Sea levels were higher than at the present day (Figure 8). They inhabit mud facies with a high percentage of silt and likely high sediment influx. Gastropods and bivalves predominate during the time interval of ~1918 to 2008 CE. The gastropods occur mainly in muddy facies (low sediment influx), whereas the bivalve shell fragments occur in the thin, lighter coloured beds with increasing silt percentages (higher sediment influx). The bivalve shells are of similar sizes and appear to be well-sorted, suggesting that they may be transported. Diatom frustules (Coscinodiscus radiatus) are common in core T2-2 during the period 1938 to 1957 CE, a time of falling Caspian Sea levels. The reason for their increased numbers is not certain but is perhaps related to an increase in available silicates.

Conclusion

MS, in conjunction with other lithological criteria, has been used to correlate two short cores taken from the marine shelf, offshore Iran, in the southern Caspian Sea. Correlations and age calibrations show that the records date back to ~1784 CE and therefore provide a sedimentary record for the last 200+ years (up to 2008, the year of collection). The highest MS readings occurred during the time period of ~1872 to 1918 CE. These are linked to increased sediment supply, including heavy minerals, sourced from LIA highstand beach ridges and barrier bars, and transported to the marine shelf by river action.

CAT-scan methodology has revealed the presence of interbedded light (silt-rich) and dark (organic-rich) coloured sediments. The light coloured beds have an approximately decadal cyclicity since ~1903 CE and may be storm deposits or linked to climate or river outflow cycles.

A non-marine Mermia ichnofacies type is reported for the first time in the Caspian Sea based on burrowing patterns revealed by the CAT-scan method. Horizontal burrows formed by Treptichnus and Cochlichnus (both part of the Mermia ichnofacies) occurred in the periods ~1821 to 1838 CE and ~1948 CE, at times when Caspian Sea level was rising and falling, respectively. Treptichnus is probably indicative of relatively low-energy deposition within poorly oxygenated waters. Vertical burrows occurred in the periods ~1906 to 1932 CE, ~1940 to 1958 CE and 1973 CE, which coincide with fairly stable but variable Caspian Sea levels and maybe linked to more oxygenated waters.

Ichnofacies show a probable link with Caspian Sea levels within the past 200 years and should be investigated further. Additional controls such as climate cyclicity are also worthy of further study.

Footnotes

Acknowledgements

The authors would like to thank the crew of the research vessel for their assistance in the sampling procedure and the x-ray department at the Tooska Medical Imaging Centre in Tehran for access to their CAT-scan, as well as the operator of the SEM laboratory and Iranian Geophysical Institute of Tehran University. Most sedimentology analyses were carried out at Iranian National Institute for Oceanography laboratory in Tehran. The authors would also like to thank Salomon Kroonenberg (Delft University of Technology) for valuable comments and Abdolhossein Amini (Tehran University) and Hamid Lahijani (INIOAS) for project guidance. The anonymous reviewers induced us to think further on the paper, we are grateful for their guidance.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

References

Abbasian

Amini

Alizadeh

(2010) Study on controlling depositional processes and their effects on distribution of organisms and trace fossils in offshore environments of the South Caspian Sea (Langarud-Rudsar Coasts). Iranian Journal of Geology 4(15): 99–112 [In Persian].

Abdullayev

Riley

Bowman

(2012) Regional controls on Lacustrine sandstone reservoirs: The Pliocene of the South Caspian Basin. In: Baganz

Bartov

Bohacs

et al . (eds) Lacustrine Sandstone Reservoirs and Hydrocarbon Systems: AAPG Memoir 95. Tulsa, OK: American Association of Petroleum Geologists, pp. 71–98.

Alizadeh

Tavakoli

Amini

(2008) South Caspian river mouth configuration under human impact and sea level fluctuations. Environmental Sciences 5(2): 65–86.

Allen

Vincent

Ian Alsop

et al . (2003) Late Cenozoic deformation in the South Caspian region: Effects of a rigid basement block within a collision zone. Tectonophysics 366: 223–239.

Aller

(1989) Quantifying sediment disturbance by bottom currents and its effects on benthic communities in a deep-sea western boundary zone. Deep Sea Research 36(6): 901–934.

Amini

Shabani

(2000) Sedimentology and geological depositional processes in Polrud River (Guilan province). MSc Thesis. University of Tehran, Iran.

Azimov

Kerimov

Steinman

(1986) River Delta Formation Processes along the West Caspian Coast and Rational Use of Natural Resources in River Mouths. Leningrad: Gidrometeoizdat [In Russian].

Blatt

Middleton

Murray

(1980) Origin of Sedimentary Rocks. Upper Saddle River, NJ: Prentice Hall Inc.

Boespflug

Long

Occhietti

(1995) CAT-Scan in marine stratigraphy: A quantitative approach. Marine Geology 122: 281–301.

10.

Brunet

Cloetingh

(2003) Integrated Peri-Tethyan basins studies (Peri-Tethys Programme). Sedimentary Geology 156(1–4): 1–10.

11.

Brunet

Korotaev

Ershov

et al . (2003) The South Caspian Basin: A review of its evolution from subsidence modeling. Sedimentary Geology 156(1): 119–148.

12.

Buatois

Mángano

(1995) The paleoenvironmental and paleoecological significance of the Mermia ichnofacies: An archetypical subaqueous nonmarine trace fossil assemblage. Ichnos 4: 151–161.

13.

Buatois

Mángano

(2007) Invertebrate ichnology of continental freshwater environments. In: Miller

(ed.) Trace Fossils: Concepts, Problems, Prospects. Amsterdam: Elsevier, pp. 285–232.

14.

Chen

Pekker

Wilson

et al . (2017) Long-term Caspian water level change. Geophysical Research Letters 44: 6993–7001.

15.

Crick

Ellwood

Hladil

et al . (2001) Magnetostratigraphy susceptibility of the Přídolian-Lochkovian(Silurian-Devonian) GSSP (Klonk, Czech Republic) and a coevalsequence in Anti-Atlas Morocco. Palaeogeography, Palaeoclimatology, Palaeoecology 167: 73–100.

16.

Dashtgard

Gingras

Pemberton

(2008) Grain-size controls on the occurrence of bioturbation. Palaeogeography, Palaeoclimatology, Palaeoecology 257(1): 224–243.

17.

De la Vara

Van Baak

CGC

Marzocchi

et al . (2016) Quantitative analysis of Paratethys sea level change during the Messinian Salinity Crisis. Marine Geology 379: 39–51.

18.

Diaz

Rosenberg

(1995) Marine benthic hypoxia: A review of its ecological effects and the behavioral responses of benthic macrofauna. Oceanography and Marine Biology: An Annual Review 33: 245–303.

19.

Dumont

(1998) The Caspian Lake: History, biota, structure and function. Limnology and Oceanography 43(1): 44–52.

20.

Ellwood

Gracía-Alcalde

Hassani

et al . (2006) Stratigraphy of the Middle Devonian boundary: Formal definition of the susceptibility magnetostratotype in Germany with comparisons to sections in the Czech Republic, Morocco and Spain. Tectonophysics 418: 31–49.

21.

Folk

(1974) Petrology of Sedimentary Rocks. Austin, TX: Hemphil Publishing.

22.

Gagnoud

Lajeunesse

Desrosiers

et al . (2009) Litho and biofacies analysis of postglacial marine mud using CT-scanning. Engineering Geology 103(3–4): 106–111.

23.

Genise

Melchor

Archangelsky

et al . (2009) Application of neoichnological studies to behavioural and taphonomic interpretation of fossil bird-like tracks from lacustrine settings: the Late Triassic-Early Jurassic? Santo Domingo Formation, Argentina. Palaeogeography Palaeoclimatology Palaeoecology 272: 143–161.

24.

Gérino

Stora

Weber

(1999) Evidence of bioturbation in the Cap-Ferret canyon in the deep northeastern Atlantic. Deep Sea Research II 46: 2289–2307.

25.

Golonka

(2004) Plate tectonic evolution of the southern margin of Eurasia in the Mesozoic. Tectonophysics 381: 235–273.

26.

Golonka

(2007) Geodynamic evolution of the South Caspian Basin. In: Yilmaz

Isaksen

(eds) Oil and Gas of the Greater Caspian Area: AAPG Studies in Geology, vol. 55. Tulsa, OK: AAPG, pp. 17–41.

27.

Haghani

Leroy

SAG

(2016) Differential impact of long-shore currents on coastal geomorphology development in the context of rapid sea level changes: The case of the Old Sefidrud (Caspian Sea). Quaternary International 408: 78–92.

28.

Haghani

Leroy

SAG

Khdir

et al . (2016a) An early ‘Little Ice Age’ brackish water invasion along the south coast of the Caspian Sea (sediment of Langarud wetland) and its wider impacts on environment and people. The Holocene 26(1): 3–16.

29.

Haghani

Leroy

SAG

Wesselingh

et al . (2016b) Rapid evolution of coastal lagoons in response to human interference under rapid sea level change: A south Caspian Sea case study. Quaternary International 408: 93–112.

30.

Heiri

Lotter

Lemcke

(2001) Loss on ignition as a method for estimating organic and carbonate content in sediments: Reproducibility and comparability of results. Journal of Paleolimnology 25(1): 101–110.

31.

Kakroodi

Kroonenberg

Hoogendoorn

et al . (2012) Rapid Holocene sea-level changes along the Iranian Caspian coast. Quaternary International 263: 93–103.

32.

Kakroodi

Leroy

SAG

Kroonenberg

et al . (2015) Late Pleistocene and Holocene sea-level change and coastal paleoenvironment evolution along the Iranian Caspian shore. Marine Geology 361(1): 111–125.

33.

Kaplin

Selivanov

(1995) Recent coastal evolution of the Caspian sea as a natural model for coastal responses to the possible acceleration of global sea-level rise. Marine Geology 124(1–4): 161–175.

34.

Kazancı

Gulbabazadeh

(2013) Sefidrud delta and Quaternary evolution of the southern Caspian lowland, Iran. Marine and Petroleum Geology 44: 120–139.

35.

Kazancı

Gulbabazadeh

Leroy

SAG

et al . (2004) Sedimentology and environmental characteristics at the Gilan–Mazandaran plain, northern Iran; influence of long- and short-term Caspian level fluctuations on geomorphology. Journal of Marine Systems 46(1–4): 145–168.

36.

Kosarev

(2005) Physico-geographical conditions of the Caspian Sea. In: Kostianoy

Kosarev

(eds) The Handbook of Environmental Chemistry. Berlin: Springer, pp. 5–31.

37.

Kosarev

Yablonskaya

(1994) The Caspian Sea. The Hague: SPB Academic Publishing, p. 259.

38.

Koshinskii

(1975) Characteristics of the Strong Waves Over the Soviet Seas: Part 1: The Caspian Sea. Leningrad: Gidrometeoizdat [In Russian].

39.

Kroonenberg

(2017) 25 years of Caspian sea research. In: Proceedings of the Where East meets West: Pontocaspia, the historical dimension of the evolution of a unique biodiversity. Abstracts of the international youth school conference, Rostov-on-Don, Russia, 21 August–2 September, pp. 38–40. Rostov-on-Don, Russia: SSC RAS Publishers.

40.

Kroonenberg

Abdurakhmanov

Badyukova

et al . (2007) Solar-forced 2600 BP and Little Ice Age highstands of the Caspian Sea. Quaternary International 173–174: 137–143.

41.

Kroonenberg

Badyukova

Storms

JEA

et al . (2000) A full sea-level cycle in 65 years: Barrier dynamics along Caspian shores. Sedimentary Geology 134(3): 257–227.

42.

Kuprin

Ferronsky

Popovchak

et al . (2003) Bottom sediments of the Caspian sea as an indicator of changes in its water regime. Water Resources 30(2): 154–172.

43.

Kuprin

Zolotaya

Kalisheva

(2002) Magnetic susceptibility of deep-water sediments of the South and Middle Caspian Sea. Lithology and Mineral Resources 37(2): 419–430.

44.

Lahijani

(1997) Riverine sediments and stability of Iranian coast of the Caspian Sea. PhD Thesis, Russian Academy of Sciences, Moscow, Russia.

45.

Lahijani

Rahimpour-Bonab

Tavakoli

et al . (2009) Evidence for late Holocene highstands in Central Gilan–East Mazandaran, South Caspian coast, Iran. Quaternary International 197: 55–71.

46.

Leroy

SAG

Lahijani

HAK

Djamali

et al . (2011) Late Little Ice Age palaeoenvironmental records from the Anzali and Amirkola Lagoons (south Caspian Sea): Vegetation and sea level changes. Palaeogeography Palaeoclimatology Palaeoecology 302(3–4): 415–434.

47.

Leroy

SAG

Lopez-Merino

Tudryn

et al . (2014) Late Pleistocene and Holocene palaeoenvironments in and around the middle Caspian basin as reconstructed from a deep-sea core. Quaternary Science Reviews 101: 91–110.

48.

Levchenko

Roslyakov

(2010) Cyclic sediment waves on western slope of the Caspian Sea as possible indicators of main transgressive/regressive events. Quaternary International 225: 210–220.

49.

Lewis

McConchie

(1994) Analytical Sedimentology. New York and London: Chapman & Hall.

50.

McCave

(2008) Size sorting during transport and deposition of fine sediments: Sortable silt and flow speed. Developments in Sedimentology 60: 121–142.

51.

McCave

Manighetti

Robinson

(1995) Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography. Paleoceanography 10(3): 593–610.

52.

Martin

Boes

Goddeeris

et al . (2005) A qualitative assessment of the influence of bioturbation in Lake Baikal sediments. Global and Planetary Change 46(1–4): 87–99.

53.

Mermillod-Blondin

Marie

Desrosiers

et al . (2003) Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: Importance of fine-scale heterogeneity. Experimental Marine Biology and Ecology 287: 193–208.

54.

Michaud

Desrosiers

Blondin

et al . (2005) The functional group approach to bioturbation: The effects of biodiffusers and gallery-diffusers of the Macoma balthica community on sediment oxygen uptake. Journal of Experimental Marine Biology and Ecology 326(1): 77–88.

55.

Michaud

Desrosiers

Long

et al . (2003) Use of axial tomography to follow temporal changes of benthic communities in an unstable sedimentary environment (Baie des Ha! Ha!, Saguenay Fjord). Journal of Experimental Marine Biology and Ecology 285–286: 265–282.

56.

Mikhailov

(1997) River Mouths of Russia and Adjacent Countries. Moscow, Russia: GEOC [In Russian].

57.

Miller

(2007) Trace Fossils: Concepts, Problems, Prospects. Amsterdam: Elsevier.

58.

Molavi-Arabshahi

Arpe

Leroy

SAG

(2016) Precipitation and temperature of the southwest Caspian Sea region during the last 55 years: Their trends and teleconnections with large-scale atmospheric phenomena. International Journal of Climatology 36: 2156–2172.

59.

Naderi Beni

Lahijani

Mousavi Harami

et al . (2013a) Caspian Sea level changes during the last millennium: Historical and geological evidences from the south Caspian Sea. Climate of the past 9(4): 1645–1665.

60.

Naderi Beni

Lahijani

Moussavi Harami

et al . (2013b) Development of spit-lagoon complexes in response to Little Ice Age rapid sea-level changes in the central Guilan coast, South Caspian Sea, Iran. Geomorphology 187: 11–26.

61.

Palcu

Golovina

Vernyhorova

et al . (2017) Middle Miocene paleoenvironmental crises in Central Eurasia caused by changes in marine gateway configuration. Global and Planetary Change 158: 57–71.

62.

Pearson

Rosenberg

(1978) Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanography and Marine Biology Annual Review 16: 229–311.

63.

Popov

Antipov

Zastrozhnov

et al . (2010) Sea-level fluctuations on the northern shelf of the Eastern Paratethys in the Oligocene–Neogene. Stratigraphy and Geological Correlation 18(2): 200–224.

64.

Popov

Shcherba

Ilyina

et al . (2006) Late Miocene to Pliocene palaeogeography of the Paratethys and its relation to the Mediterranean. Palaeogeography, Palaeoclimatology, Palaeoecology 238: 91–106.

65.

Rhoads

Boyer

(1982) The effects of marine benthos on physical properties of sediments: A successional perspective. Topics in Geobiology 2: 3–52.

66.

Rychagov

(1997) Holocene oscillations of the Caspian sea and forecasts based on paleogeographical reconstructions. Quaternary International 41–42: 167–172.

67.

Sharmad

Nabi Bidhendi Gh

Karbassi

et al . (2012) Historical changes in distribution and partitioning of natural and anthropogenic shares of heavy metals in sediment core from the southern Caspian Sea. Environmental Earth Science 67(3): 799–811.

68.

Shipilova

(2000) Eddy formation in the Caspian sea. In: Lulla

Dessinov

(eds) Dynamic Earth Environment Remote Sensing Observation from Shuttle-Mir Missions. New York: John Wiley & Sons, pp. 211–219.

69.

Svitoch

(2014) The Great Caspian Sea: Structure and History. Moscow, Russia: Lomonosov Moscow State University Press, p. 272 (In Russian with English summary).

70.

Svitoch

Badyukova

Yanina

et al . (2016) Biostratigraphy of the marine Holocene on the Iranian coasts of the Caspian Sea. Quaternary International 409: 8–15.

71.

Taylor

Roland

Growland

(2003) Analysis and application of ichnofabrics. Earth-Science Reviews 60(3–4): 227–259.

72.

Tudryn

Giannesini

Guichard

et al . (2014) The role of iron minerals in laminae formation in Late Pleistocene sediments of the Caspian Sea. Quaternary International 345: 68–76.

73.

Van Baak

CGC

Stoica

Grothe

et al . (2016) Mediterranean-Paratethys connectivity during the Messinian salinity crisis: The Pontian of Azerbaijan. Global and Planetary Change 141: 63–81.

74.

Voropaev

(1986) The Caspian Sea: Hydrology and Hydrochemistry. Moscow, Russia: Nauka.

75.

Yanina

(2014) The Ponto-Caspian region: Environmental consequences of climate change during the Late Pleistocene. Quaternary International 345: 88–99.