Geochemical and sedimentary evidences of a cold and dry climate occurred in around 4000 BP in the Southern Caspian Sea

Studying the past climate changes of the earth can have an important role to understand how similar changes will likely occur and which parameters control it. In addition, based on it, we will be able to predict more accurately the environmental changes of the earth in future. One of the main climate changes reported in the Holocene is a period of cold and dry climate that so called 4.2 climate event. This event that reported around the world was associated with many civilization collapses. Some studies showed the effect of this event in the Middle East which for example claimed that the Akkadian Empire in Mesopotamia collapsed under the effect of this climate change. In spite of the past studies, more knowledge of the 4.2 event and its effect on the sedimentary basins will be very useful to understand the similar changes in future of this region of the earth.

In the current study, two sediment cores retrieved from the depths of 400 and 600 m of the eastern part of the south Caspian Sea, near to the coastline of Babolsar, were used to assess the 4.2 event. Different geochemical and sedimentological laboratory tests were implemented to assess every change that may represent the effects of the mentioned climate event. The tests including magnetic susceptibility (MS) with 1 cm increment, continuous XRF measurements and also sedimentary tests to measure the grain size, calcium carbonate, and organic matter (OM) contents for every 2 cm were performed in INIOAS laboratory (Iran) and CEREGE laboratory of Aix-Marseille University (France). In addition, two bulk samples taken from different horizons of the cores were dated in Poznan Radiocarbon Laboratory by 14C method.

Based on the observation in the studied cores, sedimentological and geochemical anomalies likely affected by the 4.2 event were observed in 3900 to 4400 BP and 4400 to 4700 BP in cores taken from depths 600 and 400 m, respectively. The concentrations of Li and Zr and their ratio showed an increased anomaly during these periods. The increase probably caused due to relative upraise of physical weathering of the igneous rocks in the Alborz Mountains which itself occurred because of lower vegetation coverage as a result of the cold and dry weather. The concentrations of Ca and Sr also were decreased which probably represents lower bioproductivity during the discussed periods. The fall in bioproductivity caused likely because the biological activities deceased under the lower temperature of water and also lower nutrients resulted by less vegetation coverage. In addition, relative increase of magnetic susceptibility and also grain size shows higher volume of detrital sediments that confirms the predominance of physical erosion in the surrounding areas.

Considering the accuracy of the chronological tests and also the previous experiences around the world, it seems that the event, so called 4.2, occurred around 4000 BP in the Caspian region and associated with significant environmental effects. Some studies showed that the migration of Indo-European peoples coincided with this climatic event. So it seems that the cold climate that was more powerful in the northern attitudes forced Indo-European peoples to migrate to the southern attitudes with warmer climate.