Late Holocene history of the Shkhelda Glacier, Northern Caucasus, according to remote sensing, dendrochronology and cosmogenic (¹⁰Be) dating of moraines

The position of the Shkhelda valley glacier front (Elbrus region, 43.18N, 42.64 E) for the period from the 1880s to 2022 was reconstructed based on interpretation of aerial and satellite images and old maps. For the first time, the age of the Late Нolocene moraines was determined using cosmogenic isotopes (¹⁰Be) and the results of dendrochronological dating. Judging from historical and cartographic data, the Shkhelda Glacier was advancing in the 1880–1910, when most glaciers in the region gradually decreased in size after reaching their maximum during the Little Ice Age. In the 1880–1920, the front of the glacier was located at an altitude of about 2207 m asl. In the 1920s, the glacier began to retreat, and by 2022 had shrunk by 1.9 km; the altitude of its terminus was 2430 m asl. Left lateral moraines of the glacier, overgrown with pine forest, is indicative of 4 stages of its advance (or stationary positions), which, according to dendrochronological data, are dated to the middle and second half of the XIX century. The terminal moraine corresponding to these stages is dated by ¹⁰Be to 0.16±0.02 ka. Similar date (0.16±0.02 ka) was previously determined for the neighboring Kashkatash Glacier. Two older moraines at the Shkhelda Glacier with the cosmogenic dates of 0.5±0.08 ka and 0.89±0.22 ka apparently had been formed synchronously with the moraines of the Kashkatash Glacier (cosmogenic dates of 0.5 and 0.7–0.8 ka). Evidences of the glacier advance occurred in about 0.7– 0.8 ka were also revealed for the glaciers Donguz-Orun and Chalaati. The older (outer) moraine of the Shkhelda Glacier was formed 1.4–1.6 ka, i.e. approximately simultaneously with the moraine of the Irik Glacier, dated earlier by the same method of the cosmogenic isotope analysis. All cosmogenic isotope dates, determined for the forefield of the Shkhelda Glacier, need to be confirmed, as they are still single, sporadic and isolated. Despite this, they are in a good agreement with other moraine dates; the similarity of the late Holocene fluctuations of the Shkhelda Glacier with the neighboring Kashkatash Glacier is especially significant, notwithstanding the fact that the Shkhelda Glacier is covered with a dense debris cover of the supraglacial deposits, and the Kashkatash Glacier is practically free of it. The anomalous behavior (advancing) of the Shkhelda Glacier in the 1880–1910 is apparently explained by rockfall that occurred in the 1860s, which caused the glacier to be covered by debris and protected it from melting that decreased its ablation.