Unravelling the Pleistocene glacial history of the Pamir mountains, Central Asia

Highlights

• Large Middle Pleistocene (≥220 ky) glaciers shaped the major west Pamir valleys.

• During the penultimate glacial cycle, major valleys were largely ice free.

• Paraglacial destabilization led to enormous (≥0.05 km³) debris flows.

• Warming and/or aridisation at the end of MIS-5 led to regional glacial melting.

• We propose a new agorithm to detemine a moraine age from a boulder age distribution.

Abstract

Several hundred thousand year old moraines preserved in the semi-arid environment of High Mountain Asia attest to Middle Pleistocene glaciations, but the regional correlation of glacial stages and the spatial extent of the glacial advances remain poorly constrained. We examined glacial landforms and Quaternary sediments in the Bartang valley, northwestern Pamir, a region with no previous quantitative glacial chronology. Using cosmogenic ¹⁰Be exposure ages, we dated glacially polished bedrock, moraines, and mass wasting deposits. Our data show that the northwestern Pamir was heavily glaciated in the Middle Pleistocene (≥220 ky) with large valley glaciers occupying some of the major valleys in the western Pamir. During the penultimate glacial cycle (191–130 ky) these valleys may have been largely ice free. Catastrophic mega debris flows with volumes ≥0.05 km³ occurred after the ice retreat and reflect paraglacial destabilization of glacial sediments. The age of the best-dated mega debris flow (81 ± 4 ky) is similar to moraine ages ∼70–80 ky documented throughout the Pamir, demonstrating that remobilized sediments may provide valuable age constraints on glacial histories. In order to facilitate regional comparison of glacial chronologies, we developed a Gaussian separation algorithm, which determines a moraine age from a distribution of boulder exposure ages based on the assumption that post-depositional processes prevail over inheritance, and that the oldest boulder ages best represent the timing of moraine formation. We compiled moraine boulder exposure ages from the Pamir and adjacent regions and provide a summary of Middle and early Late Pleistocene glacial cycles of western High-Mountain Asia.

Introduction

High Mountain Asia contains the highest concentration of glaciers outside of the polar regions (Yao et al., 2012). The monsoon-influenced eastern and southern margins of High Mountain Asia mostly record glacial advances ≤30 ky, older glacial landforms are preserved in the semi-arid orogenic interior (Amidon et al., 2013; Owen and Dortch, 2014). Quantifying the age and extent of old glacial stages is challenging because in degraded moraines, cosmogenic exposure ages of boulders underestimate the age of moraine formation (e.g. Heyman et al., 2011). Moreover, glacial sediments may be similar to mass wasting deposits, and distinguishing landslides and rock avalanches from moraines is difficult if the diagnostic morphologies are poorly preserved (Hewitt, 1999; Benn and Owen, 2002). Middle and Late Pleistocene glacial advances in High Mountain Asia have been related to global climate oscillations, to changes in the strengths of regional climate systems, and to topographic controls (e.g., Derbyshire, 1996; Benn and Owen, 1998; Amidon et al., 2013; Dortch et al., 2013). However, the interpretation and regional correlation of glacial stages depend on accurate determination of moraine ages, which is often compromised by highly dispersed boulder ages.

A long history of Pleistocene glacial advances has been documented in the Pamir mountains at the western end of High Mountain Asia (Fig. 1A). The western Pamir is a high-relief mountain range with peak altitudes ∼7000 m above sea level (asl) and deeply incised valleys (∼1500–2500 masl). Glacier snowlines (equilibrium-line altitude, ELA, from the Randolph Glacier Inventory, RGI Consortium, 2017) range from <4000 masl west of the NW Pamir massif to >5000 masl east of the massif (Fig. 1B). The ELA variation reflects moisture availability: the Pamir receives precipitation from the mid-latitude westerlies resulting in higher annual precipitation and wetter winters in the western Pamir compared to the east (Fig. 1C). An orogenic plateau (median elevation 4350 masl) exists east of ∼73°E. The peaks of the “Chinese” Pamir east of the plateau are part of the uplifted footwall of the Muji–Tashkurgan graben system and include the Kongur Shan and Muztagh Ata ice caps (Fig. 1A-B). Pleistocene glacial records have been studied in the Chinese Pamir (Seong et al., 2009; Owen et al., 2012; Hedrick et al., 2017), in the central Pamir (Zech et al., 2005; Abramowski et al., 2006; Röhringer et al., 2012) and in the SW Pamir (Stübner et al., 2017, Fig. 1A). Despite the abundance of moraine boulder ages, the spatial extent and temporal correlation of glacial advances across the Pamir remain elusive. Here, we provide new age constraints on glacial landforms and mass wasting deposits in the NW Pamir, a region with no previous quantitative glacial chronology. We document extensive Middle Pleistocene valley glaciation consistent with similar results from the SW Pamir (Stübner et al., 2017) and with Middle Pleistocene glacial sediments in the Chinese Pamir (Seong et al., 2009; Owen et al., 2012). To facilitate the regional comparison of Pleistocene glacial chronologies, we developed an algorithm that determines probable moraine ages from dispersed boulder exposure ages based on the assumption that post-depositional processes prevail over cosmogenic nuclide inheritance, and that the oldest boulder ages best represent moraine formation.