Journal of Coastal Research

SI 50

438 - 442

ICS2007 (Proceedings)

Australia

ISSN 0749.0208

History of Caspian Environmental Changes By Molluscan Stable Isotope Records H. Lahijani†, V.Tavakoli‡ and M. Hosseindoost∞ †Iranian National Centre for Oceanography Iran. [email protected]

‡Iranian National Centre for Oceanography Iran. [email protected]

∞University of Tehran University College of Sciences School of Geology, Tehran, Iran

ABSTRACT LAHIJANI, H., TAVAKOLI, V., and HOSSEINDOOST, M., 2007. History of Caspian environmental changes by molluscan stable isotope records. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 438 – 442. Gold Coast, Australia, ISSN 0749.0208. The Caspian Sea, as a land-locked basin, is more sensitive to environmental changes compared with world oceans. The Pliocene to the present sediments of the Caspian coast recorded repeated seawater fluctuations. The South Caspian coast surrounding the main water body of the sea (i.e. South Caspian sub-basin), has recorded basin-wide environmental changes, rather than local oscillations. An outcrop of the Holocene sediments in the South Caspian coast, Iran, provided new palaeoenvironmental records for this region. Field study (along with mollusc content, shell mineralogy, oxygen and carbon isotopic composition of the shells) was used to constrain the main changes in the Caspian environment during the Late Holocene. This molluscan-rich lagoonal sediment is located around four metres above the present Caspian sealevel and distanced 2 km from the present shoreline. Oxygen and carbon stable isotope data retrieved from diagenetically unaltered shells show mean values of -2.87‰ and -1.64‰, respectively. General trend of δ18O and δ13C are similar through the sedimentary sequence, except for minor changes in the mid-section. These wellpreserved isotopic records demonstrate cyclic variations along the outcrop that reflect past environmental changes. The paleotempreture curve calculated by δ18O shows that the range of seawater temperature variations was similar to last century’s instrumental records. ADDITIONAL INDEX WORDS: sea-level, climate change, Late Holocene, Iranian Coasts The Caspian Sea, with three sub-basins based on its geographical setting (VOROPAEV, 1986) demonstrates rapid INTRODUCTION exchange of water between these sub-basins (TERZIEV, 1986, As the largest land-locked water body, The Caspian Sea has experienced different sea-level changes (VARUSHCHENKO et al., 1987; RYCHAGOV, 1997, KROONENBERG et al., 2000) after 1992). Therefore, the rapid circulation contributes to the water isolation from the world ocean in the Pliocene (JONES and homogeneity of the two main sub-basins, (Middle and South SIMMONS, 1996). Lakes are more sensitive in respect to the Caspian) mainly in the distribution of stable oxygene isotope environmental changes (FRITZ et al., 2000) on global, regional and (BREZGUNOV and FERRONSKII, 2005). Hence, the isotope record local scales based on their setting. The sedimentary archives that from marine and open lagoonal deposits could be comparable with developed in the Caspian basin during its rapid water level that of the total Caspian basin. The Caspian sea-level has changes can provide valuable information for investigating the fluctuated at a range of 20 m during the Holocene (KROONENBERG past environment. The Holocene deposits are the most suitable et al., 2000). Water balance, difference between influx (mainly period for such researches as they show negligible deformation from the Volga River) and evaporation from the sea surface is and digenesis (FEDEROV, 1995). Reconstruction of past attributed to the sea-level fluctuations (RODIONOV, 1994; FROLOV, environmental changes in the Caspian Sea are mostly obtained 2003) but the mechanism that influences moisture transfer to the through sediment and pollen analysis (VARUSHCHENKO et al., Volga basin is poorly understood. Another uncertainty comes 1980; KUPRIN, 2003). Chronology of these changes is based on from evaporation, which is dependent on both meteorological radioisotope dating (VARUSHCHENKO et al., 1987; KARPYCHEV, conditions over the sea surface and seawater circulations. The 1989; RYCHAGOV, 1997; MAMEDOV, 1997), which is both higher circulation velocity provides more cool water to the contradictory and debatable (KROONENBERG et al., 2003). Many surface. On the other hand, the circulation is closely related to the previous studies using stable isotopes have focussed on reservoir Caspian sea-level changes (KOSAREV, 1975). Accordingly, analysis (K KATZ et al., 2000) and present water circulation Caspian highstand provides low salinity conditions and low (FROEHLICH et al., 1999; PEETERS et al., 2000; FERRONSKII et al., density water masses in the north Caspian that decrease the 2003; POVINEC et al., 2003). The first reliable and available stable circulations. Influence of the Volga River to the Late Holocene isotope records from the Late Holocene deposits in northwest highstands is investigated (CHEPALYGA, 1984; KARPYCHEV, 1989; Caspian provided a clear picture of the Caspian environment MAMEDOV, 1997; RYCHAGOV, 1997; MESHCHERSKAYA, 2001), (KASATENKOVA et al., 2002; VONHOF, 2002). while the stable isotopic records for the Caspian palaeoenvironment are relatively scarce (KASATENKOVA et al.,

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geomorphology data retrieved from the Larim and other coastal outcrops were analysed (Lahijani et al., 2006). Bivalve-rich samples were used for the present study. The bivalve shells were manually gathered from samples, then cleaned and identified. Stained thin section and X-ray diffraction (XRD) show early stage digenesis, with presence of calcite in some shells. The rate of calcification is estimated to be less than 4%. The oxygen and carbon stable isotope analysis was carried out on the nondigenetic and negligible digenetic shells by mass spectrometer in the INSTAR Lab of Colorado University.

RESULT AND DISCUSSION Lithologycally the Larim outcrop can be classified into two parts (Fig. 2). The basal section (1.8 m) mainly comprises sands rich in molluscan bivalves. The top section (2.2m) represented by the slightly lithified mud, organic mud and soil scarcely contains air-breathing gastropods. Convention radiocarbon dating revealed that the base of the Larim outcrop has an age of 2480 ± 50 yr BP (LAHIJANI et al., 2006). Combined sedimentological and paleontological investigations show that the base section represents the open lagoon environment deposited during the Late Holocene Caspian Sea-level highstand (LAHIJANI et al., 2006). The most abundant articulated shells were cerostoderma lamarcki, Didacna trigonoides, Didacna pyramidata, Didacna Figure 1. Location map of the study area: a) the Caspian Sea and its parallela, Dreissena rostriformis, Dreissena polymorpha, three sub-basins - North Caspian, Middle Caspian and South Dreissena elata, Dreissena andrusovi, Hypanis caspia and Caspian b) Iranian Coast on the South Caspian c) study area in the Hypanis colorata. As the world’s largest brackish water body, the Caspian Sea is characterised by high endemic fauna (DUMONT, Eastern part of the Iranian Coast. 2000). Since isolation of the Caspian basin, its water has experienced dramatic oscillations in salinity and surface level 2002; VONHOF, 2002). The rapid and large fluctuations in the (ZENKEVICH, 1963; VARUSHCHENKO et al., 1980). Caspian sea-level and its molluscan-rich sediments offer an During the great highstands the Caspian Sea temporarily excellent opportunity for paleoenvironmental reconstruction. This connected with the peripheral basins, Black Sea and Arctic Ocean paper illustrates the stable isotope compositions of the different (GRIGOROVICH et al., 2003). Fauna interchange occurred among molluscan shells retrieved from the south Caspian coast for the basins, and they adapted to the new hosting environment. The reconstruction of the Late Holocene Caspian environment. Caspian basin played the role of donor and target until the Holocene but during the Holocene no dispersion from the Caspian Sea to other basins is recognised (NIKOLAEV, 1979). Humanmediated introduction of non-indigenous species has been STUDY AREA recorded since the 20th century (GRIGOROVICH et al., 2003). The The studied site is located along the south Caspian coast, Iran Caspian Holocene deposits could be identified by the presence of (Fig. 1). The Iranian coast of the Caspian Sea is extended around Cerastoderma lamarcki which were introduced from the Black 800 km in the southern part of the Sea. The south coast of the Sea, probably during the Middle Holocene (CHEPALYGA, 1984) or south Caspian sub-basin with its sub-tropical humid and rainy even earlier, during the Late Khvalyn highstand (RASS, 1978). The climate (annual average 1200 mm and maximum in west part 2000 molluscan bivalves adapted to the Caspian seawater salinity which mm) differs from the remainder of the Caspian coast experienced negligible fluctuation during the Holocene (FEDEROV, (KHALEGHIZAVAREH, 2005). Combined impact of the last sea-level 1995, KASATENKOVA et al., 2002). The Caspian seawater salinity rise and alongshore sediment transport developed coastal lagoons varies between 1‰ in the North Caspian deltaic area and in the with good connection to the Sea water (VOROPAEV et al., 1998). West and South Caspian coastal lagoons up to 14‰ in the east The Caspian coast has experienced lagoon evolution during its coast (TERZIEV, 1986). Some molluscan bivalves (Cerastoderma long history of water level fluctuations (ZENKOVICH, 1957; and Didacna) in the present conditions are abundant in coastal LEONTIEV et al., 1977; KROONENBERG et al., 2000). They have waters with salinity of 6-12‰, where some others (Dreissena and relatively low energy environment that could archive the past Hypanis) can tolerate fresher waters (SVITOCH, 1981). The bivalve environmental change (KARPYCHEV, 1989). The investigated shells along the Larim outcrop demonstrate periodical domination outcrop in Larim River valley (hereafter Larim outcrop and Larim of seawater (presence of Cerastoderma and Didacna) and Lagoon) with the Late Holocene age in base (LAHIJANI et al., freshwater (Dreissena and Hypanis) in the lagoon. Good exchange 2006) distanced 2 km from the present shoreline and situated 4 m of water between the Larim lagoon and the sea provided the above the present sea-level. tolerance limit for the bivalves. The studied bivalve shells from Cardidate and Dreissenidae families composed of aragonite METHODS (KASATENKOVA, 2002; PATHY and MACKIE, 1992). During the field work, 22 samples were collected from the base Bays, lagoons and endorheic deposits could be used for section of the Larim outcrop. The sedimentary and reconstructing past environmental changes because a small

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fluctuation in lake level could lead to an essential change in depositional conditions (KARPYCHEV, 1989). Despite the reliability of these deposits, if exposed for long enough to freshwater it could alter the shells and disturb isotope record (KARPYCHEV, 1989). Stained thin section and XRD analysis indicate early calcification in the shells retrieved from the basal section. The mean rate of calcification in the shells was estimated to be around 3.4%, with a maximum of 4%. Oxygen isotope values of bivalve shells in the Larim outcrop range from -5.06‰ PDB to -1.64‰ PDB, and carbon isotope varies between -10.32‰ PDB up to -2.87‰ (Fig. 2). The mean values are around -2.87‰ and -1.56‰ for oxygen and carbon respectively. The carbon isotope value of the bivalve shells show a shift from negative to positive value in that one cycle could be identified along the outcrop. All samples show a negative value for oxygen-stable isotope, which demonstrates more fluctuation in the mid-basal section in comparison to the δ13C values. Averages of δ13C and δ18O in slightly digenetic shells fall very close to the nondiagentic samples. Duplicate samples (3, 8, 9, 15, 18, 19) show maximum differences of 0.03‰ in δ13C and 0.04‰ for δ18O. The mean values of duplicate samples are used in Fig 2. Statistical analysis indicates that δ18O positively correlates with δ13C (r=0.85, n=22) (Fig 3). The δ18O in bivalve shells strongly connected with the temperature and salinity of the seawater (GROSSMAN and KU, 1986). The δ18O in the Caspian seawater ranges between +0.5 to 5.7‰, assuming the fluctuation of salinity between 18‰ to 8‰ (BREZGUNOV et al., 1987). The mean composition of δ18O in the

present Caspian seawater is around -1.68‰, with a standard deviation of 0.17 (n=137) (BREZGUNOV and FERRONSKII, 2005). Considering the negligible fluctuation in salinity of the South Caspian sub-basin (KOSAREV, 1975), the change in δ18O could be attributed both to the variations in seawater temperature and sealevel fluctuations. Paleotempreture of the seawater can be estimated based on δ18O values for seawater and its counterpart in aragonite (GRASSMAN and KU, 1986): T (oC) = 20.6- 4.34 (δ18Oar – δ18Ow) where T is paleotemperature; δ18Oar is value for aragonite; and δ18Ow is the value for seawater. The estimated paleotemperature shows two warm periods in the base and top of the studied section. The basal part corresponds with Late Holocene (around 2000-2500 yr BP). The mid-section shows cyclic variation in seawater temperature. The South c Caspian Sea surface temperature in coastal areas varies from 0o in c extreme cold of winter up to 30o in the warm period of summer (TERZIEV, 1992). The paleotemperature calculated on the basis of oxygen isotopic composition shows a range similar to those obtained during instrumental measurements. The present composition of δ18O in the seawater (FERRONSKII et al., 2003) demonstrates that the South Caspian sub-basin has a more homogeneous value of stable oxygen isotope with a slight increase from surface to the deeper layers. Uniform distribution of salinity and δ18O along the water depth do not provide enough

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Figure 2. The stable isotope (δ O and δ C) records in bivalve shells and reconstructed paleotemperature curve along the Larim Outcrop.

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The contribution of riverine influxes, evaporation to the Caspian sea-level fluctuations and how environmental changes (in global, regional and local scales) during the Late Holocene influenced this basin, are the main unanswered questions that need further investigations using multiproxy data with reasonable geographical distributions.

ACKNOWLEDGEMENTS The present research was supported by the Iranian National Centre for Oceanography under grant 381-2-05 in the framework of ‘Investigation of South Caspian Holocene Deposits’. We would like to thank Dr H. Rahimpour-Bonab and A Sharifi for help in the field work and stable isotope analysis.

Figure 3. Plot of δ18O vs. δ13C of bivalve shells in the Larim Outcrop. sensibility for tracing sea-level fluctuation, even for the last sealevel rise since 1979. Sea-level rise since 1979 (at a range of 2.3 m) was forced by 1000 km3 excess freshwater influx (FROLOV, 2003), which is comparable with that which occurred during the Late Holocene. The δ13C value in bivalve shells related mainly to the carbon isotope composition of dissolved inorganic carbon (DIC) within the hosting aquatic environment, when vital factors have a small influence (BURCHARDT and FRITZ, 1980). However, the stable carbon isotope in DIC is controlled by the lake’s hydrological balance and productivity (TALBOT, 1990; LI and KU, 1997). As there is no available data about δ13C in DIC of Caspian Sea water, it is assumed that δ13C shows a similar trend to δ18O during the Caspian sea-level oscillation in a range of around 3 m. It is well known that highstand in the Caspian Sea is accompanied with lower vertical circulation of water (KOSAREV, 1975; TERZIEV, 1992). Therefore, it causes lower bioproductivity for a few years after rising sea-level (TERZIEV, 1986) because of the lower exchange of oxygen and nutrients. During the lowstands, Caspian deep water has more dissolved oxygen than those in the times of highstand. Photosynthesis causes enrichment of δ13C in DIC of water by absorbing 12C (MCKENZIE, 1985) but higher productivity could lead to negative value for δ13C, where vertical circulation is faster. This creates an oxic environment at the bottom which releases 12C from decomposed organic matter into the seawater. It seems that during Caspian sea-level rise with lower vertical circulation and lower productivity, the δ13C in bivalve shells could increase.

CONCLUSIONS The investigation of the Late Holocene sediments in the South Caspian coast utilising field study, mollusc content, shell mineralogy and oxygen-carbon isotope records showed significant fluctuations in the isotopic records of this area, representing significant changes in the Caspian environment during the Late Holocene. Changes in the molluscan assemblages indicate salinity variations in the coastal waters. The δ18O and δ13C curves show similar trends through sedimentary sequence, except for some minor anomalies in the mid-section. The calculated Late Holocene seawater temperatures are comparable with the instrumental records of the last century. The δ18O values demonstrate two warm periods in the base and top and some cyclic variations in the mid-sections.

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