Reconstruction and study of late Quaternary climate change in southeastern part of Caspian Sea

Introduction Reconstruction of past climate change can provide us valuable information about the time, scale, and details of climate changes. For reconstruction of late quaternary climatic conditions, various methods are used, as well as the use of losses, paleomagnetism, paleontology, etc.One of these methods is to use stable isotopes of carbon and oxygen, which is particularly valuable information on the temperature of the depositional environment, digenesis temperature, the temperature changes in different digenesis environments, separating different areas carbonates offers Considering the importance of Gorgan Bay from different aspects and the fact that not many studies have been done in this region regarding long-term climate change, in this research we have tried to use different data obtained from sedimentary cores harvested from the southeastern part. Caspian Sea (Gorgan Bay) Temperature changes and environmental conditions of the region in the late Quaternary to be studied and reconstructed. Gorgan Bay with an area of more than 400 km is located in the southeastern part of the Caspian Sea (36°48’N, 53°35’E and 36°55’N, 54°03’E, 60 km ×12 km, maximum depth of 4 m ). Gorgan Bay formed during the Holocene period by a sandy spit which is named Miankaleh coastal barrier system. It is connected to the Caspian Sea through the inlet of Ashoradeh-Bandar Torkaman (Chapaghli) which is located in the northeastern part of the Bay This bay is mainly influenced by processes that are operating within the basin. Water balance in the Gorgan Bay is influenced by freshwater inflow from a number of small rivers and streams, including Gorgan-roud from the north and QarehSu from the east. These two rivers drain runoffs from residential and agricultural areas into the bay.   Materials and methods In this for reconstruction of late Quaternary climatic conditions a sedimentary core (N1) were collected from Gorgan Bay using a gravity corer. The core was first examined for magnetic susceptibility (MS). After slicing the core and subsampling with certain distances, Laser particle size analyzer used (Horbia LA-950) for grain size analysis. Also to specify total organic matter content (TOM), loss of ignition method (L.O.I) was used. For this purposes, a portion of each sample (about 3 gr) was placed inside a crucible and heated at 550 ° C for 5 hours and after that the weighing percentage of the organic material was reported. For measurement of carbonate content (CaCO3%), coarser grain particles were manually removed and remaining sediments were placed in a muffle furnace (Excitation, EX.1200-12L) for 1 hour at 950 °C . For 14C dating, totally 3 organic matter samples were sent to the Poznan Laboratory, Poland, The oxygen and carbon stable isotope analysis was carried out on the non-digenetic and negligible digenetic benthic foraminifera (Ammonia Beccari) by mass spectrometer in the Winsor Laboratory, Canada. Paleotempreture of the seawater estimated based on δ18O values for seawater and its counterpart in aragonite.   Results and discussion Grain size, was very variable from sand to clay. N1 core consists of 39 sandy silt facies, 28 silty facies, 9 silty sand facies, 9 facies of gravelly sandy mud , 3 facies gravelly muddy sand and 1 facies gravelly sand. The maximum and minimum ​​of sand were abstained at 617 and 690 cm (80.27% and 0%), respectively, while the highest content of silt and clay were at depth 138 and 416 cm (99.45% and 8.95%), respectively. The lowest amount of silt content was observed at 626 cm (7.23%). The average of sand, silt and clay in this core was 25.13%, 72.52% and 1.56%, respectively. Result of magnetic susceptibility showed a close relationship between particle size distribution and MS magnitude due to variation in terrestrial influx, which is caused by sea-level fluctuations in different times. This process increases with particle size and ranges of MS simultaneously with sea level fall and decreases during sea-level rise. The range of oxygen (δ18O) and carbon isotope (δ13C) was between -4.22 to -1.17‰ and 2.66 to‌‏ -0.94‰ with a mean of 3.34 and -2.11 ‰ correspondingly. According to the results of carbon dating (14C Dating), age of sediment in the N1 achieved 22070 cal. The rate of sedimentation in the cores varied from 0.57 to 3 mm. Reconstruction of paleotemperature with stable isotopes showed that the maximum temperature recorded in the core was acquired at 144 and 686 cm (27.4 and 27.1°C) and the minimum temperature was 450 cm (13.7°C). The mean temperature in this core was 22.2°C.   Conclusion 1-Caspian sea sediments as one of the best region in the world ,record Quaternary climate change in northern IRAN. 2- Accordingly, seven warm and cold periods were identified, which the coldest period being the last glacial maximum (Wurm) with a temperature of 13.7 °C. Furthermore, the highest temperatures related to the pre-glacial and the present periods (about 27°C). Moreover the sedimentology and magnetic susceptibility data confirmed the above results.   3-The degree of magnetic susceptibility in different core subsamples indicates environment conditions and low sediment rate. 4 - The rate of sedimentation in the cores varied from 0.57 to 3 mm. 5 - Reconstruction of paleotemperature with stable isotopes showed that the maximum temperature recorded in the core was acquired at 144 and 686 cm (27.4 and 27.1°C) and the minimum temperature was 450 cm (13.7°C). The mean temperature in this core was 22.2°C. 6 - Based on the results of 14C age and stable isotopes 15 centigrade changes in temperature have occurred during about 22,000 years.