River inflow and salinity changes in the Caspian Sea during the last 5500 years

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Abstract

Pollen, spores and dinoflagellate cysts have been analysed on three sediment cores (1.8–1.4 m-long) taken from the south and middle basins of the Caspian Sea. A chronology available for one of the cores is based on calibrated radiocarbon dates (ca 5.5–0.8 cal. ka BP). The pollen and spores assemblages indicate fluctuations between steppe and desert. In addition there are some outstanding zones with a bias introduced by strong river inflow. The dinocyst assemblages change between slightly brackish (abundance of Pyxidinopsis psilata and Spiniferites cruciformis) and more brackish (dominance of Impagidinium caspienense) conditions. During the second part of the Holocene, important flow modifications of the Uzboy River and the Volga River as well as salinity changes of the Caspian Sea, causing sea-level fluctuations, have been reconstructed. A major change is suggested at ca 4 cal. ka BP with the end of a high level phase in the south basin. Amongst other hypotheses, this could be caused by the end of a late and abundant flow of the Uzboy River (now defunct), carrying to the Caspian Sea either meltwater from higher latitudes or water from the Amu-Daria. A similar, later clear phase of water inflow has also been observed from 2.1 to 1.7 cal. ka BP in the south basin and probably also in the north of the middle basin.

Introduction

The Caspian sea (CS) levels have changed dramatically over various timescales both erratically and cyclically (Kazancı et al., 2004), causing vast modifications in both the volume and the area of the water body. During the 20th Century, CS water levels have fluctuated rapidly and suddenly, causing serious environmental and economic damages and adversely affecting oil and gas exploration, agriculture and fishing as well as areas contaminated by nuclear wastes. The instrumental record (since 1837) indicates a 3 m change with a sharp drop between 1930 (26 m below sea level or bsl) and 1977 (29 m bsl), followed by a sharp increase until 1995 (26.5 m bsl), and followed by a present-day possible stabilization at 27 m bsl (Cazenave et al., 1997; Giralt et al., 2003; Leroy et al., 2006; USDA, 2006). The precise causes of these changes are not well deciphered yet. Climate, human impact and tectonic events (in sequence of decreasing likelihood) are believed to have interacted to influence CS levels (e.g. Shiklomanov et al., 1995; Froehlich et al., 1999). The sea-level fluctuations during the Holocene have been reconstructed in different investigations, but no consensus has arisen: amplitude and timing of the events vary dramatically between authors (e.g. Mamedov, 1997; Rychagov, 1997; Hoogendoorn et al., 2005).

The vegetation history of the area around the CS is equally poorly known. Moreover the few works that exist are published mainly in Russian. This vast region remains a gap in the Global Pollen Database and makes past climate modelling less robust. Palaeovegetation reconstructions in this region are therefore ground breaking.

Organic-walled dinoflagellate cysts (dinocysts) found in the CS have been described in Marret et al. (2004). These microfossils are nowadays increasingly used to reconstruct salinity changes, as the tolerance range of many species is relatively well known (e.g. Wall et al., 1977; Dodge and Harland, 1991; de Vernal et al., 1994; Marret and Zonneveld, 2003). Furthermore, quantitative reconstructions of salinity have been obtained for a number of marine records from the Northern Hemisphere (de Vernal et al., 2005). The dinocyst record of the CS has remained poorly known until recently and is mostly used for the stratigraphy of oil wells. No Holocene diagrams of the dinocyst assemblages have been published yet. Many forms, species and even genera had not been described before the recent works of Marret et al. (2004). Since the taxonomy has been established, it has become possible to investigate sedimentary sequences to derive palaeoenvironmental changes. Dinocyst assemblages, cyst morphology and endemism may provide information about the impact of various environmental parameters and a basis for palaeoenvironmental reconstructions.

Based on sediment cores from the second half of the Holocene in the south and the middle basins of the CS, the aims of this investigation are to use (1) new data from pollen, spores and other microfossils to reconstruct palaeovegetation and taphonomy and (2) new dinocyst assemblages to reconstruct changes in lake levels, mostly via salinity changes.

Section snippets

Study area

The Black, Caspian and Aral Seas constitute relics of the Paratethys basin. Their relative isolation from each other after the closing of the Tethys led to different physical and biological conditions in each basin.

Pollen

Pollen analyses on Holocene CS sediment have remained very scarce until now. As part of an INCO-Copernicus project, some pollen analyses have been performed on long cores (ca 10 m) from the south and the middle basins covering from ca 24 to ca 5.5 ka BP. The very low time resolution reached for the Holocene, the very low pollen counts and the probable lack of core overlap with the sequences presented here do not enable comparisons (Kuprin and Rybakova, 2003; Seret G., pers. comm.). An earlier

Coring

Cores (140–182 cm long) have been taken in the south basin, the middle basin and the northern part of the middle basin during a French–Russian oceanographic cruise (August 1994), on board a Russian military ship, rented for the sea cruise (Fig. 1). Core locations were in deep water, and were chosen to avoid direct river influence (CP14, 330 m; CP18, 480 m; and CP21, 460 m depth, Fig. 1 and Appendix A). Three coring techniques were combined to recover complete sections for the Late Pleistocene and

Lithology, mineralogy and age-depth model

A brief lithological description of relatively homogenous sediment of each core is provided along with the carbonate content.

The core sediment CP14 (140 cm) consists of fine-grained carbonate mudstone, with alternation of black and grey layers, often scarcely visible all along the sequence. Between 122 and 100 cm, the sediment is sometimes laminated. The high carbonate values fluctuate mildly between 42% and 53%.

Core CP18 (182 cm-long) consists of a mud, texturally very homogeneous, with faint

Comparison of the three cores

Based on the pollen assemblages of the Lower Volga River (Bolikhovskaya, 1990), it is likely that the Volga delta and other river deltas have had little direct impact on the composition of the pollen diagrams presented here, even on the northernmost core (CP21). Most of the pollen grains are wind-transported from a large area around the CS (Hooghiemstra et al., 2006; Mudie and McCarthy, 2006). To identify more clearly where from the wind-transported pollen originated, present-day meteorological

Conclusion

As the river input to the CS is one of the main factors influencing the sea level, if not the main one, it is crucial to demonstrate that palynology (both pollen-spores and dinocysts) is able to reconstruct past fluctuations of river inflow and salinities. Nowadays the Volga is by far the main river inflow. In the Holocene until a still undetermined time, the Uzboy River brought a large amount of water as well.

Despite difficulties with the chronological framework and the ecological

Acknowledgements

This study has been conducted with the help of the European Contract INCO-Copernicus “Understanding the Caspian Sea erratic fluctuations” no. IC15-CT96-0112. This was funded by the Centre National de la Recherche Scientifique in the frame of the CNRS-INSU-DYTEC (DYnamique de la Terre et du Climat) Programme (France). Our gratitude goes to F. Gasse, the project leader. Thanks are due to the French and Russian colleagues who organized and participated in the coring and the hydrological sea

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