Peculiarities of the centennial salinity regime of the Caspian Sea

Based on primary information from the Caspian Sea oceanographic database, the centennial salinity regime was analyzed. The regularities of the salinity spatial dynamics and vertical distribution were found for the characteristic periods of the sea level changes. Significant changes in the hydrological regime and the structure of the water mass for the centennial period were shown.     

Estimation of the scale of spatiotemporal variability in the lithological and geochemical characteristics of suspended particulate matter and bottom sediments at the mouth of the Volga River and the adjacent zone of the northern Caspian

Based on the data of synchronous observations of hydrophysical and biogeochemical parameters in the near-mouth and shallow-water areas of the northern Caspian in 2000–2001, the scale of spatiotemporal variability in the following characteristics of the water-bottom system was estimated (1) flow velocity and direction within vortex structures formed by the combined effect of wind, discharge current, and the presence of higher aquatic plants; (2) dependence of the spatial distribution of the content and composition of suspended particulate matter on the hydrodynamic regime of waters and development of phytoplankton; (3) variations in the grain-size, petrographic, mineralogical, and chemical compositions of the upper layer of bottom sediments at several sites in the northern Caspian related to the particular local combination of dominant natural processes; and (4) limits of variability in the group composition of humus compounds in bottom sediments. The acquired data are helpful in estimating the geochemical consequences of a sea level rise and during the planning of preventive environmental protection measures in view of future oil and gas recovery in this region.     

Role of Subsurface Brines in Salt Balance: The Case Study of the Caspian Sea and Kara Bogaz Bay

The water level of the Caspian Sea fluctuated significantly during recent history, without consensus for the cause. The varied chemistry of the Caspian, Kara Bogaz and sediment a interstitial waters provides a further insight. Element concentrations and ⁸⁷Sr/⁸⁶Sr ratios of the interstitial waters were compared to those of Caspian and Kara Bogaz open waters, and of acid-leached extractable components. The ⁸⁷Sr/⁸⁶Sr ratios of the interstitial waters are explained by addition of subterranean waters similar to nearby spring waters. These subterranean waters yield chemical characteristics such as a Cl/SO₄, ⁸⁷Sr/⁸⁶Sr, Ca/Sr and K/Rb ratios of respectively 80, 0.7086, 250 and 1,800. However, their addition does not explain the large difference in the K/Rb ratio of the Caspian and Kara Bogaz waters, respectively at 7,630 and 17,550, which implies also a leaching of salt deposits by the upward migrating subterranean waters. The sediments of the southern Caspian basin, with low Na, Cl and SO₄ in their interstitial waters, deposited apparently in an anoxic environment. The related chemical changes in the waters are also indicative of a recent change in the hydrologic regime, possibly induced by a changing morphology of the drainage basin.     

First results of investigation into the relation between magnetic properties of bottom sediments in the Northern Caspian and variations in the Caspian Sea level in the Late Neo-Pleistocene

The first data of investigation into the relation between changes in magnetic properties of the Northern Caspian sediments and variations in the Caspian Sea level in the Late Neo-Pleistocene are presented. It is shown that there is a certain correlation between magnetic characteristics of sediments and variations in the Caspian Sea level that cause changes in the lithological and faunistic composition of sediments.     

Changes in the water regime of the caspian sea

The article deals with issues of structure and dynamics of the Caspian Sea water balance. On the base of historical, paleogeomorphological and other data the evolution history of the Caspian Sea and its basin has been observed for different time intervals down to 400 thous. years ago. Presented are computerized data on water balance components in the current centenary obtained from instrumental observations, revealed are causes of the sea-level fluctuations within that time interval and anthropogenic factor contribution to this process. Based on the analysis of this material, an attempt has been undertaken to present a scenarion of a possible sea-level position of the Caspian Sea with the expected versions of climatic changes at the end of the XX and beginning of the XXI centuries.     

The driving forces behind the Caspian Sea mean water level oscillations

Closed basins such as the Caspian Sea rapidly respond to variations in atmospheric and geological events. This study has been conducted to deduce the role of natural and anthropogenic influences on the Caspian Sea mean water level fluctuations for the period of 1998–2005. It is recognized that climate is the primary mechanism of the Caspian Sea mean water level variations based on the relatively small differences of the Caspian Sea hydrologic budget residuals and the Caspian Sea mean water level fluctuations. This is further supported by the similarity in water-level variations of the Caspian Sea with those of Lake Van and Lake Urmia. On the other hand, the Caspian Sea needed to lose some of its water to attain water balance equilibrium in 2000 and 2001. The year 2000 showed anomalous seismic activity particularly in the southwestern part of the Caspian Sea. Two significant earthquakes with magnitudes of 6.8 and 6.5 M_w occurred on November 25, 2000. The focal mechanisms of these earthquakes and numerous aftershocks indicated normal faulting and, therefore, caused the Caspian Sea lake level to decline in 2000. The contribution of submarine mud volcano eruptions to the Caspian Sea lake level could be insignificant based on the comparison of water budget residuals and the mean water level variations. Neither crustal deformation (based on the GPS measurements) nor the offshore oil and natural gas production activities in the Caspian Sea are responsible for noticeable changes to the level of the Caspian Sea.     

Late Cretaceous-Eocene marginal seas in the Black Sea-Caspian region: Paleotectonic reconstructions

A series of seven reconstructions is presented to illustrate the evolution of marginal seas in the Black Sea-South Caspian segment of the margin of the Tethys Ocean from the Late Jurassic to the middle Eocene. After Middle Jurassic inversion and until the Aptian Age, no marginal (backarc) basins were formed in the region, while the Pontides-Rhodope margin developed in the passive regime. The retained relict of the Late Triassic-Early Jurassic backarc basin includes the southeastern part of the Greater Caucasus, the northern part of the South Caspian Basin, and the shallow-water Kopetdagh Basin. The basins of the southern slope of the Greater Caucasus, Balkans (Nish-Trojan Trough), and Dobrogea developed as flexural foredeeps in front of the Middle Jurassic fold systems. The next, Aptian-Turonian epoch of opening of marginal seas was related to the origination of subduction zones at the Pontides-Rhodope margin and to the incipient consumption of the Vardar Basin lithosphere with formation of the West Black Sea Basin and its western continuation in the Bulgarian Srednogorie. The backarc rifting in the Greater Caucasus resulted in transformation of the foredeep into the backarc basin. Two basins approximately 2000 km in total extent were separated by the bridge formed by the Shatsky and Andrusov rises. The last, late Paleocene-middle Eocene epoch of the formation of backarc basins was associated with the newly formed subduction zone south of the Menderes-Taurus Terrane that collided with the active margin in the early Paleocene. The Greater Caucasus Basin widened and deepened, while to its south the East Black Sea Basin, the grabens in the Kura Depression, and the Talysh Basin, all being separated by a chain of uplifts, opened. The Paleogene South Caspian Basin opened in the course of the southward motion of the Alborz volcanic arc at the late stage of closure of the Iranian inner seas.     

Intrasecular variability in the level of the largest lakes of Russia

The features of the intrasecular changes in the level of the largest lakes of Russia (Baikal, Ladoga, Onego, and the inland Caspian Sea) are jointly considered for the first time on the basis of the long-term data measured in the 20th–21st centuries. The influence of climate and anthropogenic factors is considered to improve the forecast of the level of lakes.     

Amplitude,frequency and drivers of Caspian Sea lake-level variations during the Early Pleistocene and their impact on a protected wave-dominated coastline

The Caspian Sea, the largest isolated lake in the world, witnessed drastic lake-level variations during the Quaternary. This restricted basin appears very sensitive to lake-level variations, due to important variations in regional evaporation, precipitation and runoff. The amplitude, frequency and drivers of these lake-level changes are still poorly documented and understood. Studying geological records of the Caspian Sea might be the key to better comprehend the complexity of these oscillations. The Hajigabul section documents sediment deposited on the northern margin of the Kura Basin, a former embayment of the Caspian Sea. The 2035 m thick, well-exposed section was previously dated by magneto-biostratigraphic techniques and provides an excellent record of Early Pleistocene environmental, lake-level and climate changes. Within this succession, the 1050 m thick Apsheronian regional stage, between ca 2·1 Ma and 0·85 Ma, represents a particular time interval with 20 regressive sequences documented by sedimentary and palaeontological changes. Sequences are regressing from offshore to coastal, lagoonal or terrestrial settings and are bounded by abrupt flooding events. Sediment reveals a low energy, wave-dominated, reflective beach system. Wave baselines delimiting each facies association appear to be located at shallower bathymetries compared to the open ocean. Water depth estimations of the wave baselines allow reconstruction of a lake-level curve, recording oscillations of ca 40 m amplitude. Cyclostratigraphic analyses display lake-level frequency close to 41 kyr, pointing to allogenic forcing, dominated by obliquity cycles and suggesting a direct or indirect link with high-latitude climates and environments. This study provides a detailed lake-level curve for the Early Pleistocene Caspian Sea and constitutes a first step towards a better comprehension of the magnitude, occurrence and forcing mechanisms of Caspian Sea lake-level changes. Facies models developed in this study regarding sedimentary architectures of palaeocoastlines affected by repeated lake-level fluctuations may form good analogues for other (semi-)isolated basins worldwide.     

Global change and the biogeochemistry of the North Sea: the possible role of phytoplankton and phytoplankton grazing

Phytoplankton plays a dominant role in shelf biogeochemistry by producing the major part of organic matter. Part of the organic matter will reach the sediment where diagenetic processes like denitrification, apatite formation or burial will remove nutrients from the biogeochemical cycle. In this article current knowledge on the decadal plankton variability in the North Sea is summarized and possible implications of these changes for the biogeochemistry of the North Sea are discussed. Most of the observed interdecadal dynamics seem to be linked to large-scale oceanographic and atmospheric processes. Prominent changes in the North Sea ecosystem have taken place around 1979 and 1988. In general, the phytoplankton color (CPRS indicator of phytoplankton biomass) reached minimum values during the end of the 1970s and has increased especially since the mid 1980s. Changes with a similar timing have been identified in many time series from the North Sea through the entire ecosystem and are sometimes referred to as regime shifts. It is suggested that the impact of global change on the local biogeochemistry is largely driven by the phyto- and zooplankton dynamics during spring and early summer. At that time the extent of zooplankton–phytoplankton interaction either allows that a large part of the new production is settling to the sediment, or that a significant part of the new production including the fixed nutrients is kept within the pelagic system. The origin of the extent of the phytoplankton–zooplankton interaction in spring is probably set in the previous autumn and winter. In coastal areas, both large-scale atmospheric and oceanographic changes as well as anthropogenic factors influence the long-term dynamics. Due to eutrophication, local primary production nowadays still is up to five times higher than during pre-industrial conditions, despite a decreasing trend. Recently, introduced species have strengthened the filter feeder component of coastal ecosystems. Especially in shallow coastal seas like the Wadden Sea, this will enhance particle retention, shift organic matter degradation to the benthic compartment and enhance nutrient removal from the biogeochemical cycle by denitrification or apatite formation.     

Structure of the crust and upper mantle of the Black and Caspian Seas

Many geophysical characteristics of the Caspian and Black Seas' deep basins are similar, having: suboceanic type of the crust, low average seismic velocity, absence of earthquakes and relatively small variation of magnetic anomalies. However, the sediments in the Caspian Sea deep basin are folded whereas in the Black Sea they are approximately horizontal. The Caspian Sea also has a far greater thickness of sediment accumulation.

The deep basins of the Caspian, Black and Mediterranean seas represent a sequence having similar crustal structures but with a decreasing thickness of sediments and consolidated layer, in that order. It is possible that the intensive sinking and accumulation of sediments began earliest in the Caspian Sea and spreaded continuously to the Black Sea and then the Mediterranean Sea. The Caspian and Black Sea deep basins have existed for long time (perhaps from Paleozoic time or even earlier) as areas with a specific and related type of evolution.     

The eastern flank of the Caspian Basin: Sedimentary complexes and sedimentation conditions in the Early-Middle Carboniferous

Carboniferous and Lower Permian Carbonate and terrigenous rocks with the total thickness of >4000 m serve as the productive units in the Paleozoic subsalt complex at the eastern flank of the basin surrounding the northern area of the present-day Caspian Sea (hereafter, Caspian Basin in the broad sense). In recent years, several large oil and gas-condensate fields were discovered in these rocks. The complexity of geological evolution of this region, which is situated at the junction between the East European Platform and the Ural orogen, as well as multiple changes of sedimentation conditions during the Middle and Late Paleozoic, are reflected in the diversity of types of terrigenous and carbonate sediments and their facies alterations. Reconstruction of these environments makes it possible to elucidate specific features of the location of reservoir rocks in vertical and horizontal sections, as well as regularities of variations in their filtration-capacitive properties.     

Modeling the effects of environmental variables on short-term spatial changes in phytoplankton biomass in a large shallow lake,Lake Taihu

Understanding the short-term response of phytoplankton biomass on environmental variables is needed for issuing early warnings of harmful algal blooms in aquatic ecosystems. Predicting harmful algal blooms are particularly challenging in large shallow lakes due to their complex mixing patterns. This study used a two-dimensional hydrodynamic–phytoplankton model to evaluate the effects of environmental variables on short-term changes in the horizontal distribution of phytoplankton biomass in a large shallow lake, Lake Taihu, China. Two simulations were performed using daily and hourly average wind condition and water temperature data collected in 2009. Other model inputs were identical for these two simulations. The response of phytoplankton to wind conditions, light intensity, water temperature, and total dissolved phosphorus and nitrogen concentrations were examined based on a sensitivity analysis using the hourly data. Hourly simulation achieved a more realistic distribution of phytoplankton biomass than the daily simulation. This finding implies that data with a higher temporal resolution are more useful for short-term prediction of phytoplankton biomass in this lake. Sensitivity analysis indicated that water temperature and light intensity dominate short-term changes in phytoplankton biomass in this lake. Wind conditions also affect phytoplankton biomass distribution by causing advective water movement.     

水库水文情势与浮游植物群落结构

综述和讨论了当前水库水文情势对浮游植物群落的影响研究,主要是水文情势对浮游植物数量、种类组成、物种多样性和群落的空间格局包括垂直和水平分布以及群落演替的影响.     

Changes in nutrient uptake of phytoplankton under the interaction between sunlight and phosphate in the Changjiang （Yangtze） River Estuary

We conducted ship-board incubation experiments to investigate changes in nutrient uptake of phytoplankton under different phosphate concentrations and irradiances in the Changjiang River Estuary and its adjacent waters in China. Under 100% natural irradiance the uptake rates of phosphate, silicate, and nitrate were accelerated at high phosphate levels （1.84 μM）, while under low irradiance （about 50% natural irradiance） their uptake rates were restrained at the high but stimulated greatly at the intermediate phosphate concentrations （1.26 μM）, as the growth of phytoplankton, changes in nitrite and ammonium uptake didn＇t follow an obvious pattern. Our results also showed that there were linear relationships between nitrate, silicate and phosphate uptake at different phosphate concentrations under low and high irradiances, and the growth period of phytoplankton was prolonged both at the high phosphate concentrations under high irradiance and at the intermediate concentrations under low irradiance, suggesting that the limitation of phytoplankton growth mainly reflected changes in its growth period, and because no such environment （low irradiance and low phosphate concentrations） actually existed in a high turbidity zone, phytoplankton blooms hardly occurred there. In the absence of irradiance, denitrification occurred readily and phytoplankton was kept decreasing, which resulted in phosphate regeneration.     

The Lower Jurassic of the Eastern Caspian region and the Middle Caspian Basin: Lithology,facies, taphonomy

Lithofacies of the productive Upper Triassic-Lower Jurassic deposits of the Eastern Caspian region, studied in wells on the Caspian coast and exposed in the outcrops of the Mountainous Mangyshlak, are described and analyzed. The similarity of the structure of the Mesozoic sedimentary beds of the Middle Caspian Basin and of those of the land adjacent to its eastern coast is confirmed. Comparative analysis of lithofacies allowed the reconstruction of the paleogeographic setting and depositional environments in the studied region during the Early Jurassic. A unique fossil plant occurrence is discovered in the upper part of the Lower Jurassic series (in the lower subformation of the Kokala Formation; Eastern Caspian region). Fossil plant taphonomy and the lithology of host rocks in the occurrence resulted from unusual paleogeographic settings that existed in the Middle Caspian Basin at the time of the Early-Middle Jurassic boundary.     

Contour Currents and Contourites of the Middle Caspian

Experimental data have shown that the cyclonic circulation in the Middle Caspian is a seasonal contour current surrounding the Middle Caspian megabasin along the slope during the cold half of the year. This current and the enormous influx of suspended particulate matter on the western slope of the Middle Caspian Basin provide conditions for the formation of contourites near the slope foot. In addition, the joint action of the contour current and slope gravity flows led to the formation of specific accumulative structures, sedimentary waves, on the western slope of the Middle Caspian.

     

Schematic zoning of the region of the Caspian artesian basin with respect to the formation of the chemical composition of underground waters in the upper hydro-geodynamic zone

The hydrogeochemical zoning of the region of the Caspian artesian basin was considered on the basis of the corresponding revealed parameters. The consideration was based upon the natural-historic analysis of the landscape-climatic, geological, hydrogeological, and hydrogeochemical data. The subject of the study was the upper hydrogeodynamic zone of the Caspian artesian basin.     

Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.     

Geological evolution and petroleum systems in the North Caspian Region

The structure and formation evolution of the principal North Caspian zones (North Caspian Basin, Karakul-Smushkovskii Foredeep, South Emba Foredeep, Karpinsky Range Fold Zone, Mangyshlak-Central Ustyurt Fold Zone, etc.) are discussed. Drilling data and seismic profiles were utilized in the study, and an analysis of hydrocarbon systems has been performed.