The effect of Caspian Sea water level fluctuations in the late Pleistocene and Holocene periods on the dispersion of prehistoric settlements in Mazandaran

Human societies have always been influenced by climatic and geographical factors of their ecosystems throughout history, so interdisciplinary studies to identify these influences will help to understand the cultural changes of ancient societies. One of the geographical factors affecting the ancient sites of the northern regions of the country is the Caspian Sea. The separation of this lake from the high seas has caused the sea level of this sea to increase slowly and when it reaches its maximum height, it decreases with the same rate. In this study, an attempt has been made to combine the information obtained from marine sediment survey studies with archaeological evidence and findings, as far as possible, to investigate the effects of Caspian Sea water regressions and advances on the location of prehistoric settlements in the Mazandaran region of The Paleolithic period to the Iron Age. For this purpose, studies on the altitude range of these changes were reviewed and the results obtained in the field of paleontology of Caspian Sea level fluctuations were combined with the results of archaeological studies in the region. Simultaneously with the fluctuations of the Caspian Sea water during the Pleistocene and Holocene, the location of prehistoric sites and populations on the southern shores of the Caspian Sea changed relative to seawater, and with the advance of seawater, the remains of previous evidence are buried under sediments.

The Caspian Sea is the largest lake in the world and has nothing to do with open waters, and its water is supplied by rivers that flow into it. The water level of this lake is a dynamic phenomenon and its oscillating amplitude has different intensities in time scales. Fluctuations in the water level of the Caspian Sea are subject to various factors such as changes in the runoff of rivers leading to the sea and the intensity of evaporation and undoubtedly affect the coasts and the distribution of existing settlements in them. In each of the advances, the water of the Caspian Sea rose to such an extent that it submerged parts of the lowlands of northern Iran. The morphology of the coast is very different in response to these fluctuations and has a wide range of features based on the slope of the coast and the sea to land. It is natural that the advancement of water level will have the greatest impact in the parts of the Caspian Sea that have less depth and less in the areas that have more depth. 800 km south coastline of the Caspian Sea is divided into 4 categories based on the type of response to fluctuations. It is a steep beach. While the changes in coastal morphology in the western coasts of Mazandaran have not been very noticeable, its southeastern regions are one of the areas sensitive to water level. Studies and measurements of marine sediments and their combination with archaeological evidence and findings are effective in recognizing the effects of backwaters and advances of the Caspian Sea water on ancient sites. These effects can be traced back to the Khvalyian and Neo-Caspian cycles, which archaeologically continue from the Paleolithic, Late Paleolithic, Neolithic, and prehistoric and historical periods to the present. The Old Paleolithic period coincides with the Caspian cycle, in which the advance of sea water with a level of -20 to -15 meters occurred about 300 to 250 thousand years ago.

The Middle Paleolithic period coincides with the Khvalyian cycle, which itself coincides with the last temperate glaciation and the dissolution of ice sheets at higher latitudes. Since the largest Quaternary advance in the Khvalyian cycle occurred with a water level of 48 to 47 meters, so the southeastern shores of the Caspian Sea due to its gentle slope during these advances below The water is gone and it seems that the Middle Paleolithic sites, which were located at low altitudes and close to the sea, were buried during the advance of the water. Noshahr dam area is located at an altitude of 400 meters above sea level in the western part of Mazandaran with a steep coastal slope and therefore has the least impact from the fluctuations of the Caspian Sea water level. The Neolithic period coincides with the period between the Lower and Upper Khavalyian, the water level of the Caspian Sea has receded by about 110 to 120 meters, and evidence for this period may have been buried following further advances. Towards the end of the Pleistocene and before the beginning of the Holocene (11550-10550 years ago), the water level of the Caspian Sea decreased by 100 meters, which coincides with the abundance of extra-Paleolithic settlements on the southeastern coast. With the beginning of the Holocene period in 10,000 years ago, the water level of the Caspian Sea reached an altitude of -22 meters. The first Neolithic settlements of the region were formed in low plains near the coast and at a distance of less than two kilometers of surface water. Around 7000 BC, the water level of the Caspian Sea reached -25 meters. And in the millennium 6000 BC, the water level of the Caspian Sea was -28 meters and was at the lowest level since the beginning of this period. During the Chalcolithic, in the fifth millennium BC, the water level of the Caspian Sea increased to -20 meters above sea level, and during one millennium, at 4000 BC, it decreased by one meter and reached -21 meters. The most important settlements of this period have been identified in the mountainous plains. Around 3000 BC, the water level of the Caspian Sea was about -35 meters above sea level, so the coastline, especially in the south and southeast of the Caspian Sea, was far behind today. At this date, which coincides with the Bronze Age, the greatest decline in seawater occurred during the Holocene, and of course the position of many human settlements relative to the coastline has changed. However, the greatest advance of the Caspian Sea water level was during the Holocene period - 20 meters, which occurred in 1300 AD, and with the advent of water, the evidence of many ancient settlements was washed away and buried under sediments. Perhaps this is why the remains of Iron Age sites in the low plains of Mazandaran are small. In general, humans at different times tried to choose the most accessible place to the sea for settlement, and with the retreat and advance of sea water, the location of settlements changed. Previous settlements were buried following the advance of seawater. Undoubtedly, in order to fill information gaps about different prehistoric periods in the field of research, the effects of these fluctuations must be considered. The study of water fluctuations in the Caspian Sea is important in analyzing the behaviors of ancient populations and their settlement patterns.