Middle Holocene environmental reconstruction and climatic inferences through multi-proxy records from Seymareh lake sediments (Zagros Mts., Iran)

The Seymareh landslide is the largest rock slope failure (44 Gm3) ever recorded on the exposed Earth surface. It detached ∼10 ka BP from the northeastern flank of the Kabir-Kuh anticline (Zagros Mts., Iran) originating the natural dam responsible for the formation of a three-lake system (Seymareh, Jaidar, and Balmak lakes, with an area of 259, 46, and 5 km2, respectively). The lake system persisted for ∼3000 yr during the Holocene before its emptying phase due to overflow. A sedimentation rate of 21 mm yr−1 was estimated for the Seymareh lacustrine deposits, which increased during the early stage of lake emptying because of enhanced sediment yield from the lake tributaries. To reconstruct the climatic and environmental impact on the lake infilling, we reviewed the geomorphology of the basins and combined the results with multi-proxy records from a 30 m thick lacustrine sequence in Seymareh Lake. Major analyses comprise grain size analysis, carbon and oxygen stable isotopes of carbonate-bearing sediments, and X-ray diffraction analysis of clay minerals. Lake overflowing is largely accepted as the main response to variations in water discharge and sediment supply since the alternation from dry to wet phases enhances sediment mobilization along hillslopes decreasing the accommodation space in the downstream sedimentary basins. In this regard, during the early-middle Holocene, the Seymareh area, as well as the entire Middle East, was affected by short-term climate changes at the millennial-scale, as testified by both paleoecological and archaeological evidence. Indeed, several records from Iranian lakes (i.e., Mirabad, Zeribar, Urmia) well documented the temperature and the moisture conditions of the western Zagros Mountains during the Holocene. During the early Holocene, the precipitation remained low up to 6 ka BP, reaching the driest condition around 8-8.2 ka BP. The impact of this abrupt climate change is evident across West Asia, where the first large villages with domesticated cereals and sheeps disappeared, converting to small hamlets and starting habitat-tracking. As regards the Seymareh area, a more irregular distribution of rainfalls and their increasing seasonality may support rhexistasy conditions, during which the scarce vegetation cover enhances both the hillslope erosion and sedimentation rate in the basins, most likely contributing to the overflow of Seymareh Lake.