Climatic vs. Seismic Controlled Rockglacier Advances in Northern Tien Shan - Insights from Lichenometry

Abstract:

Glaciers and permafrost landforms in high mountain areas are of major importance for storing and providing fresh water for the surroundings, especially in arid or semi-arid areas as Central Asia. Rockglaciers have been traditionally used as landform proxies of the distribution of alpine permafrost. In the northern Tien Shan mountains, the most distinct lobes of >200 rockglaciers that we mapped from satellite imagery occur at minimum elevations between 2500 and 3700 m. However, individual and particularly low-lying lobes extend down to well below 3000 m, and seem difficult to reconcile with regional paleoclimatic fluctuations. To support ground based and satellite imagery estimates on rockglacier advances (1 to 10 m/yr) and to gain information on their morphological characteristics, we present results of terrestrial LiDAR measurements on six rockglaciers in four steep mountain valleys in the Kyrgyz and Kazakh Tien Shan. Having the ability to destabilize, and thus provide massive sediment input from hillslopes, we hypothesize that strong earthquakes may also have influenced the formation or advance of some of the region’s rockglaciers. This hypothesis is based on the observation that the tectonically active area was affected by a series of major earthquakes in the late 19th and earliest 20th centuries, which have subsequently triggered numerous landslides and rock falls. Using lichenometry, we aim to resolve age patterns on lobes of different rockglaciers located both, in the vicinity and distal from large young historic earthquakes. This method has been used successfully to reconstruct glaciation histories, and paleoseismicity. To gain information about and to compare advance histories of the rockglaciers from the relative ages of their lobes, we compiled a dataset of several thousand lichen diameter measurements of different species (e.g., Rhizocarpon geographicum, Aspicilia tianshanica, Lecanora muralis, and Xanthoria elegans). Results show that lichen age patterns vary between the different locations and support the notion that the analyzed Tien Shan rockglaciers do not record climate-driven advances exclusively. We conclude by highlighting a number of constraints that may limit the use of lichenometry for dating rockglacier advances, and scope for future research on seismic triggers.