Late Holocene glacier change in the eastern Nyainqêntanglha Range, SE Tibet

Loibl, David; Lehmkuhl, Frank (Thesis advisor); Bräuning, Achim (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis

Abstract

The eastern Nyainqêntanglha Range is located at the southeastern fringe of the Tibetan Plateau, representing one of the major pathways of monsoonal moisture from the foreland to the plateau. Owing to the combination of a rugged high mountain topography and abundant moisture, more than 8000 km² of the eastern Nyainqêntanglha Range are covered by monsoonal temperate glaciers. Previous studies have shown that these glaciers are particularly sensitive to climate change. The eastern Nyainqêntanglha Range’s glaciers thus provide an important archive of past and present climate dynamics. The interplay of glaciers, topography and climate in this region is, however, hardly constrained to date. Furthermore, a consistent late Holocene glacial chronology, combining dating results with geomorphological and sedimentological evidence, is still lacking. The research synthesized in this dissertation aims to improve the understanding of spatial and temporal patterns of late Holocene glacier fluctuations in the eastern Nyainqêntanglha Range and to identify relevant climatic forcing mechanisms. In this context, focus was put on the Little Ice Age (LIA) because its maximum glacier advance represents the last major turning point from an advancing to a retreating glacier regime, implying a transient steady-state. Aiming to achieve a consistent and well-constrained overall result, the study was conducted by application of a multi-proxy approach including glaciological, geomorphological, sedimentological and dendrochronological methods, aided by optically stimulated luminescence and radiocarbon dating. Starting with remote sensing, 1964 glaciers were mapped from a Landsat ETM+ scene and subsequently parameterized by DEM-supported measurements. Analysis of the geomorphological evidence provided the basis to investigate patterns of modern glaciation and post-LIA changes quantitatively, which revealed substantial retreat. An evaluation of different methods to calculate equilibrium line altitudes (ELAs) showed that an optimized toe-to-ridge altitude method was more suitable than other methods in settings with complex topography, a lack of mass balance measurements, and adequate hypsographic data. The results of these studies provided insights into the complex topography–climate–glacier coupling and revealed the underlying patterns. These included a combination of distinct insolation effects, channeling of moisture by the valley system, foehn effects in lee positions, orographic precipitation in luv positions, and large-scale hygric and thermal gradients. Additionally, analysis of glacier length and ELA changes showed that the sensitivity of the glacier’s reaction to climatic changes is positively correlated to both size of the glacier and the grade of continentality. Field-work-based work focused on the creation of a consistent morphosequence. The results yielded similar configurations and numbers of moraines at all studied settings, implying that similar climatic events affected the whole region since the LIA. The geomorphological settings of the glacier forelands were, however, remarkably different, showing the substantial impact of the local topography-climate-glacier configurations. A critical evaluation of different relative and numerical dating methods provided constraints for a conceptual chronosequence. They suggest that the LIA comprised several distinct glacier advances, particularly an early advance before ∼1500 CE and a succession to the maximum advance from the mid-17th to mid-18th century. After the LIA maximum, continued retreat that was only interrupted by short phases of stability followed, as evidenced by 2–3 recessional moraines in the investigated settings.