On the correlation between oscillations of the Caspian Sea level and the North Atlantic climate

Correlations between the changes in the climate of the Caspian Sea region and in its level and the variations in the North Atlantic climate are studied. The indices of North Atlantic oscillation (NAO), Atlantic multidecadal oscillation (AMO), the intensity of Atlantic thermohaline circulation (ATHC), and the air humidity above the North Atlantic are used as basic indicators of climatic variations that influence the Caspian Sea. Results of an experiment for reproducing the World Ocean circulation and the parameterization of cyclic climate peculiarities made it possible to reveal their impact on the formation of Eurasian climatic variability and on the level regime of the Caspian Sea. This impact is studied through the variability of ATHC, the NAO index, and a composite index of moisture transport (CIMT) that is proposed as a result of the studies.     

Tidal oscillations in the Caspian Sea

Long-term hourly data from 12 tide gauge stations were used to examine the character of tidal oscillations in the Caspian Sea. Diurnal and semidiurnal tidal peaks are well-defined in sea level spectra in the Middle and South Caspian basins. High-resolution spectral analysis revealed that the diurnal sea level oscillations in the Middle Caspian Basin have a gravitational origin, while those in the South Caspian Basin are mainly caused by radiational effects: the amplitude of diurnal radiational harmonic S₁ is much higher than those of gravitational harmonics О₁, P₁, and K₁. In the North Caspian Basin, there are no gravitational tides and only weak radiational tides are observed. A semidiurnal type of tide is predominant in the Middle and South Caspian basins. Harmonic analysis of the tides for individual annual series with subsequent vector averaging over the entire observational period was applied to estimate the mean amplitudes and phases of major tidal constituents. The amplitude of the M₂ harmonic reaches 5.4 cm in the South Caspian Basin (at Aladga). A maximum tidal range of 21 cm was found at the Aladga station in the southeastern part of the Caspian Sea, whereas the tidal range in the western part of the South Caspian Basin varies from 5 to 10 cm.     

Distribution of some rare and trace elements in modern bottom sediments of the Caspian Sea

The features of the distribution of some rare and trace elements in modern bottom sediments of the Caspian Sea have been studied from samples collected during cruises 35, 39, and 41 of the R/V Rift and the cruise of the R/V Nikifor Shurekov in 2013. It was established that bottom sediments in different areas of the Caspian Sea vary to some degree in the contents of Zr, Hf, Th, V, Cr, Co, Ni, Cu, Sr, and Ba in comparison to suspended matter discharged by the Volga River and rivers originating in the Caucasus. As follows from the results of a comparison of the geochemical features of modern bottom sediments of the Volga River delta and different sedimentary subsystems of the Caspian Sea with the chemical composition of Middle Archean granitoids and Paleozoic and Mesozoic–Cenozoic basalts, which are regarded as geochemical images of such provenance areas as the basement of East European Platform, Urals, and Caucasus, none of these regions is considered the dominant provenance area for all sedimentary subsystems of the Caspian Sea region. Here, the revealed similarity between modern bottom sediments of the northern, central and southern Caspian Sea regions, the Volga River delta, and Post-Archean average Australian shale (PAAS) in some parameters, including REE spectra, assumes that the Volga River discharge plays a dominant role in the formation of the geochemical image of Caspian Sea subsystems. The role of clastics, including the fine-grained fraction, which is supplied to the Caspian Sea from the Caucasus region and Elburz Mountains, is insignificant already in the coastal area, which is determined both by influence of the marginal filter (MF) and large-scale cholestatic current circulation.     

On the nature of the Khvalynian transgression of the Caspian Sea

This paper presents a reconsideration of the hypothesis that the Khvalynian transgression of the Caspian Sea resulted from an abundant inflow of meltwaters from Siberian proglacial lakes through the Aral Sea and via the Uzboi channel and may be considered as a prototype of Noah’s Flood. A thorough analysis of the evidence suggested by the proponents of this hypothesis revealed a lack of factual substantiation; there are no facts indicating the existence of the Aral flow-through lake or of the Siberian proglacial water supply to the Caspian Sea. The spatial distribution of the mollusk fauna provides no support for the view that the Khvalynian transgression was essentially influenced by thawing of ice sheets on the Russian Plain. The Khvalynian transgression, though geologically short-term, could not be an analogue of Noah’s catastrophic flood. The sea level rose at a rate of a few centimeters per year for a few thousand years; this interval covered the lifetimes of many generations of the Caspian coast inhabitants. The Khvalynian water overflowing via the Manych Strait into the Pontian basin provided an even smaller influence. It resulted in flooding of the middle part of the Black Sea shelf, in a 30-m sea level rise, and in a water salinity increase by 5‰.     

Tsunamis in the central part of the Caspian Sea

This paper describes the geotectonics of the Caspian Sea basin and the seismicity of its central part. The seismicity analysis enables us to identify the most probable zones of tsunami generation. We also present a brief review of the historical records of tsunamis in the Caspian Sea. In order to estimate the tsunami risk, we used the method of numerical hydrodynamic simulation while taking into account the real topography of the Caspian Sea. The computation of the wave field for the possible tsunamis occurring in the central part of the Caspian Sea allowed us to estimate the maximum expected heights of the waves along the coast of the CIS countries (Russia, Azerbaijan, Kazakhstan, and Turkmenistan). On the basis of the earthquake statistics in the region and the results of numerical experiments, we show that the extreme wave heights can reach 10 m at certain parts of the coast. Such extreme events correspond to extended (up to 200 km) seismic sources with M _S ～ 8 and a recurrence period of T ≈ 1600 years. The tsunami wave heights are expected to be as high as 3 m for sources of lesser extent (<50 km) with earthquake magnitudes of M _S ～ 7 and a recurrence period of 200 years.     

Transformations in the Caspian Sea ecosystem under the fall and rise of the sea level

By the data of the surveys performed in 1976, 1983, and annually from 1995 to 2006, the successive transformations of the ecosystem were traced in the Central and Southern Caspian Sea: from the anoxic conditions in 1933–1934 to the oxic state during the low level period in 1978, then the hypoxia increase and accumulation of nutrients registered anew after 1995, and to the hydrogen sulfide appearance in the near-bottom layers of the Southern Caspian Sea. Some irreversible changes in the silicon distribution in the Central Caspian Sea were revealed (the concentration increase to 217 μM at the depth of 780 m, which considerably exceeded the absolute values for the World Ocean).     

Reproduction of Interannual Variability of the Caspian Sea Level in a High-Resolution Hydrodynamic Model

An eddy-resolving model of the thermohydrodynamics of the Caspian Sea is presented. The model reproduces the climatic (decadal) variability of the sea level. Numerical experiments were performed to reconstruct the sea circulation and the evolution of its level in the second half of the 20th century. We also investigate how variations in some natural and anthropogenic factors influencing the Caspian Sea water balance impact the long-term trends of its level.     

Composition of fragmental minerals of the surface layer of bottom sediments in the Caspian Sea

The mineral composition of heavy and light subfractions of the modern bottom sediments of the Caspian Sea is studied. The concentration of the minerals in the Caspian Sea made it possible to identify nine terrigeneous mineralogical provinces in its area. Based on the mineral composition of the heavy subfraction of the bottom sediments, the main sources of the fragmentary material supply are established. The distribution of the terrigeneous minerals over the bottom area in the modern bottom sediments of the Caspian Sea depends on the sources of the fragmentary material; the stability and flotation properties of minerals; the morphology of the bottom; and the hydrodynamic conditions.     

The Caspian Sea shelf during the pleistocene regressive epochs

During the Pleistocene regressive epochs, the Caspian Sea shelf became entirely (the Atel and Mangyshlak regressions) or partly (the Cheleken and Enotaevka regressions) exposed, which was accompanied by a change in the relief formation and sedimentation settings in the drained areas. The adjacent offshore areas were subjected to the transformation of the seawater salt composition, the temperature regime, and the biotic communities. The sea level falls during the Caspian Sea regressions were accompanied by large-scale environmental changes: the desiccation of spacious sea bottom areas, transformations of the landscapes and the relief formation and sedimentation settings, alterations of the seawater??s salt composition in the offshore areas, and the transformation of the biotic communities. The largest regressions were characteristic of the Tyurkyan, Bakuan-Khazarian, Khazarian-Khvalynian (Atel), and post-Khvalynian epochs. The shelf became entirely exposed (234 000 km²) during the maximal Atel and Mangyshlak regressions. During the Cheleken and Enotaevka regressions, the shelf was half drained. The epochs when the Caspian shelf was entirely exposed were marked by different climatic events in the Caspian Sea region and the adjacent glacier-affected areas of the Russian Plain. In general, the main regressive epochs correspond to interglacials on the Russian Plain, although this correlation was much more complex.     

Results of monitoring of the changes in the hydrochemical structure of the central and Southern Caspian Sea over 1995–2005

The results of decennial monitoring of the Caspian Sea were summarized, which allowed revealing the “pumping” of nutrients from the euphotic layer to deepwater depressions of the Central and Southern Caspian Sea. In parallel, in the deepwater depressions, growth of hypoxia to values of 0.2–0.5 ml O₂/l proceeds. In 2006, hydrogen sulfide was registered in the near-bottom layer of the South Caspian Sea Basin. It is shown that the transformation of the hydrochemical structure was directed towards the conditions observed at the times of Bruevich (1933–1934), but no complete coincidence has yet been reached.     

Caspian Sea Level Fluctuations in the Neopleistocene (Was There an Atelian Regression?)

The article deals with deposits exposed in sections along the rivers of the Caspian lowland and on the eastern slope of the Ergenei. The conclusion is made about the lagoonal origin of chocolate clays lying on high-capacity subaerial and alluvial–deltaic deposits. The impact of the Caspian Sea level rise on coastal processes is examined. Taking into account the regularities in the behavior of the coastal zone during sea level rise, subaerial deposits with such thickness in the outcrops could not have been preserved during the early Khvalynian transgression from –100 m. Another curve of Caspian Sea level fluctuations is proposed, where there is no deep Atelian regression between the Khazarian and Khvalynian transgressions. It is concluded that the Khazarian transgression was one of the largest in the history of the Caspian Sea. Its level was slightly less than that of the Khvalynian transgression. The rise of the level of the latter began not from –100 m, but from about 5–15 m; i.e., this transgression was in fact an oscillation of the Caspian Sea against its gradual regression after the Khazarian transgression. Sea level oscillations contributed to the formation of lagoonal-transgressive terraces, in which chocolate clays accumulated.

     

Evolution of the Northern Caspian Sea Region and the Volga delta in the late Pleistocene-Holocene

On the basis of geologic-geomorphologic studies, an alternative concept regarding the evolution of the Northern Caspian Sea Region and the adjacent areas in the boundary of the Pleistocene-Holocene is presented. A supposition is made about the existence of a large lagoon in the North Caspian Region that the Volga River emptied into forming a delta. Later, water from the lagoon flowed as a “stratum” stream into the Black Sea through the Manych Strait. A conclusion is reached that the Baer mounds were formed on the bottom of this stream.     

Seasonal variability of climatic currents in the Caspian Sea reconstructed by assimilation of climatic temperature and salinity into the model of water circulation

We present the results of an analysis of the seasonal variability of current fields in the Caspian Sea, reconstructed by assimilation of climatic temperature and salinity into the primitive-equation model of water circulation on the basis of an algorithm for adaptive statistics of prediction errors. The sources in heat and salt transfer-diffusion equations depend on the spatial and temporal variability of the variances of prediction errors and one-dimensional (in the vertical coordinate) variances of measurement errors for temperature and salinity. The variances of prediction errors are adjusted at the moments of data assimilation in accordance with a simplified Kalman filter. The climatic circulation of waters in the Caspian Sea is shown to be highly varying. The maximum of its intensity over the entire depth is reached in February. The minimum of kinetic energy is observed in April. The currents in deep-sea areas are determined by the balance between wind and baroclinic factors of the formation of circulation with wind currents prevalent.     

A comparative analysis of the Harpacticoida (Copepoda) faunas from the northern and southern seas of Russia

Based on extensive voluminous literary data, a comparison of the Harpacticoida faunas of the Black Sea; the Sea of Azov; and the Caspian, Baltic, Barents, White, Kara, Laptev, and East Siberian seas, as well as of the Spitsbergen and Franz Josef Land areas, was performed. The degree of community and specificity of the faunas of different regions was estimated and the general patterns of the latitudinal variability in the species compositions were revealed. It is shown that the Harpacticoida faunas of geographically separated areas and even those isolated from each other such as those of the Black Sea, the Sea of Azov, and the Caspian Sea are rather similar; an hypothesis is put forward that this is caused by the common history of the seas’ formation. In contrast, the faunas of the Arctic seas (Barents, White, Kara, Laptev, and East Siberian), whose water areas are closely connected, are considerably different, being related to their different temperature conditions.     

Impact of a new invasive ctenophore (Mnemiopsis leidyi) on the zooplankton community of the Southern Caspian sea

The invasive ctenophore Mnemiopsis leidyi (Agassiz), which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001–2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (four Copepoda and one Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of Mnemiopsis leidyi into the Caspian Sea. Cladocera species seemed to be highly affected by the invasion of Mnemiopsis leidyi; only one species, Podon polyphemoides, remained in the study area, whereas 24 Cladocera species were found in the study carried out in 1996. Whereas among the Copepoda Eurytemora minor, Eurytemora grimmi, Calanipeda aquae dulcis and Acartia tonsa that were abundant before the Mnemiopsis leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maximum in zooplankton abundance (22,088 ± 24,840 ind·m^?3) and biomass (64.1 ± 56.8 mg·m^?3) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001–2006 was in the range of 3361–8940 ind·m^?3; this was two‐ to five‐fold less than the zooplankton abundance in 1996. During 2001–2006, the highest abundance and biomass of Mnemiopsis leidyi were observed during summer‐autumn months coincident with warm temperatures and generally when the abundance of other zooplankton organisms was low.     

Potential map of gas hydrate formation in Caspian Sea based on physicochemical stability evaluation of methane hydrate

Preliminary studies of Caspian Sea have shown the possibility of gas hydrate accumulations, because of suitable physicochemical conditions, existence of clayey deposits, and high concentrations of organic matter. Studies have indicated that gas hydrates are mainly composed of methane. Therefore, based on physicochemical equations for methane hydrate stability in different pressure, temperature, and salinity, this study was designed to calculate the potential of gas hydrate formation in the Caspian Sea basin. For this, data of more than 600 locations were analyzed and in each location, upper and lower limits of methane hydrate formation zone were calculated. Then, the zoning maps of upper and lower limits were prepared which can be useful for exploring the gas hydrate as an energy source or predicting gas hydrate hazards. According to the calculations and maps, methane hydrate formation in Caspian Sea, theoretically, can take place from near the seabed to 4000 and 2500 m beneath the sea surface when low and high geothermal gradient are supposed, respectively. By comparing the results with gas hydrate zones revealed in geophysical profiles, it has been shown that, in Caspian Sea, gas hydrates probably accumulate near the lower limit when a high geothermal gradient is assumed.     

Macrophytobenthos of the Caspian Sea: Diversity,distribution, and productivity

In the Russian sector of the northern and middle Caspian Sea, 36 species of macroalgae have been identified. The green and red algae from the mesosaprobic group are dominant. An increase in the number of green algae species is revealed. The distribution of macroalgae is inhomogeneous. It is confined to the solid substrate and epiphyton. The biomass of seaweeds reaches 1.5 kg/m². Climate change has little influence on the appearance of new species in the northern Caspian Sea, but new invaders can appear in the Middle and Southern Caspian. The distribution of aquatic and coastal hygrophytic vegetation shows considerable spatial dynamics due to fluctuations in the level and salinity of the Caspian Sea. The biomass of aquatic vegetation varies in a wide range from 0.5 to 10.0 kg/m₂. Spatially detailed mathematical models adequately reflect the changes in key species of aquatic plants in space and time. It is shown that expansion of the zone of the seagrass Zostera noltii to shallow water areas is occurring at present, as well as shrinkage of the range of the dominant littoral aquatic plant Phragmites australis.     

Metal Concentrations in the Coastal Fauna of the Caspian Sea

Ocean Science Journal - Metal concentrations were measured in predominant coastal fauna of the Caspian Sea including six macro-invertebrates (Mnemiposis leidyi, Balanus improvisus, Pontogammarus...     

Satellite observations of surface manifestations of internal waves in the Caspian Sea

Results of satellite observations of surface manifestations of internal waves in the Caspian Sea are presented. It is proposed that the possible cause of generation of the revealed internal waves is uninodal seiches with a nodal line located in the vicinity of the Apsheron Sill. The basic parameters of internal waves in the Caspian Sea, having the form of classical soliton trains, are determined. Seasonal variability of surface manifestations of the internal waves is revealed. The horizontal current velocity of the solitons is assessed. According to the estimation, velocity is about 0.2 m/s, which is sufficient for wind ripple modulation.     

Genealogical concordance,comparative species delimitation,and the specific status of the Caspian pipefish Syngnathus caspius (Teleostei: Syngnathidae)

To evaluate the specific validity of the Caspian pipefish Syngnathus caspius, we used a comparative molecular species delimitation method on a COI barcode library of Syngnathus, as well as principles of genealogical concordance. Comparative species delimitation allowed us to delineate putative species without a priori assignment of individuals to nominal species, while genealogical concordance extended our species delimitation results to multiple genes, multiple codistributed species, and comparisons with biogeographic evidence. All species delimitation analyses including two topology‐based, one network‐based, and one distance‐based analysis showed genetically isolated lineages of pipefish in the Black and Caspian Sea, corresponding to S. abaster and S. caspius, respectively. Mean evolutionary divergence between the two lineages (0.029) was within the range separating species of Syngnathus (0.024–0.217). The interclade/intraclade ratio of variation was comparable to the operational criterion of divergence between clades greater or equal to 10 × the level within clades to recognize separate species. Our argument on taxonomic validity of S. caspius is also supported by the principles of genealogical concordance as a conceptual basis for recognition of biological species. As a second objective, using a limited number of S. caspius specimens from two semi‐confined water bodies along the Caspian Sea south coastal zone (i.e., Anzali Wetland in the west and Gorgan Bay in the east), we searched for a possible matrilineal structure. The retrieved phylogeographic pattern was characterized by a shallow genealogy and lineage distributions varied, most probably caused by low to modest contemporary gene flow between populations of S. caspius across the southern Caspian Sea that are linked tightly through history.