Morphoevolution of the Seymareh landslide-dam lake system (Zagros Mountains, Iran. Implications for Holocene climate and environmental changes

The Seymareh landslide is the largest rock slope failure (44 Gm3) ever recorded on the exposed Earth surface. It detached at ~10 ka from the northeastern flank of the Kabir-Kuh anticline (Zagros Mountains, Iran) creating a natural dam responsible for the formation of a lake system that persisted for ~3 kyr during the early and mid- Holocene before its emptying phase caused by overflow. The chronology of its demise and connection to the Holocene climate variations have been barely studied. To reconstruct the influence of local vs. regional envi- ronmental changes upon the lake history and unravel their contribution on the lake overflowing phase, we analyzed 13 samples extracted from a 30-m-thick lacustrine succession belonging to Seymareh Lake. We per- formed grain size analysis, carbon and oxygen stable isotopes of carbonate-bearing sediments, and X-ray diffraction analysis of clay minerals, in addition to the geomorphological analysis to reconstruct the lake system morphoevolution. Main results highlight that the lake infilling phase was characterized by a first period (from 9.73 ± 0.13 to 7.37 ± 0.73 ka) with a median sedimentation rate of ~2 cm yr− 1 and a second period (from 7.37 ± 0.73 to 6.98 ± 0.39 ka) marked by an increased sedimentation rate up to ~10 cm yr− 1. The age of 6.98 ± 0.39 ka is here considered as the termination of lacustrine sedimentation and the beginning of lake emptying and new valley entrenchment. Stable isotope and X-ray diffraction analyses confirm a stable humid interval after the early-mid Holocene arid to humid transition, consistent with other regions of the Middle East. During this transition, rainfall seasonality and its inter-annual irregularity may have prevented the development of a widespread vegetation cover that likely favored the increase of denudation rates on hillslopes and sediment supply along connected channels network. Therefore, the sedimentation rate increased within the downstream lake basin. However, in the late infilling phase from ~7.6–7.32 ka, local high energy hydroclimatic events, characterized by high sand content and low δ18O concentration, were recorded more frequently. This could justify a sudden increase in the tributaries' discharge that culminated at ~7.3 ka because of the complete overflow of Jaidar Lake into Seymareh Lake likely causing the overflowing of the entire Seymareh landslide-dam lake system.