Hydrological evolution during the last 15 kyr in the Tso Kar lake basin (Ladakh, India), derived from geomorphological, sedimentological and palynological records

Abstract

The Tso Kar lake basin in Ladakh, northwestern Himalayas (33°18′N, 78°E, 4527 m a.s.l.) is located close to the modern boundary of effective moisture transport from the Indian summer monsoon. In order to reconstruct the possible monsoonal impact on the hydrology of the lake basin during the last 15 kyr, we studied the geomorphology of the basin and combined the results with multi-proxy records from Tso Kar lake sediments. Major analyses comprise geochemical data (high-resolution X-ray fluorescence, XRF), mineral composition (XRD), aquatic pollen and non-pollen palynomorphs (NPP). Although the chronological framework based on 41 ¹⁴C AMS dates remains speculative for the Lateglacial and late Holocene periods, our results indicate major hydrological phases linked to regional climate variations and tectonic activity. The maximum glacier advance falls within the period between the global LGM and ca 15 kyr BP, while the basin remained open and may have drained along the paleo Tso Kar River. Tectonic pulses may have triggered terrace development and the closure of the lake. The estimated mean vertical displacement rate accounts for 7–9 mm/yr. Considerable summer monsoon moisture supply occurred for the periods around 12.5 kyr BP and between ca 11.5 and 8.6 kyr BP, indicated by deposition of more profundal lake facies and the development of aquatic fauna and flora. The lake level rise about 11.8 kyr BP was due to intensified glacier melt and summer monsoon effective moisture supply. The lake reached its maximum level and extent between 8.5 and 7 kyr BP under warm-moist climate conditions. Permafrost was absent from the basin. Summer monsoon influence weakened after 8 kyr BP with a short reverse trend at ca 5–6 kyr BP. The lake shrank gradually after 7 kyr BP towards its lowest stand at about 4.2 kyr BP, with return of permafrost activity. Subsequent terrace formation was probably amplified by tectonic movement. The lake remained at a low level with minor fluctuations. The results indicate that the orbitally driven regional climate variability is well reflected in the Tso Kar basin by hydrological variations. Considerable summer monsoon moisture seems to have influenced the area only during the early Holocene. The late Quaternary evolution of the Tso Kar was controlled by the interplay of climate forces, local morphodynamics and tectonic impact.

Introduction

It is widely accepted that the strength of the Asian summer monsoon depends on the seasonal migration of the Intertropical Convergence Zone (ITCZ), its latitudinal position during summer and the resulting pressure gradient between the heat low over the Tibetan Plateau and the high pressure cell over the Indian Ocean. The high mountain ranges of the Himalaya, however, constitute a considerable barrier to the northward migration of moist summer monsoon air masses, resulting in a strong latitudinal gradient of increasing aridity towards the central parts of the Tibetan Plateau. To date, it remains unclear to what extent monsoonal moisture has affected the interior of the Tibetan Plateau and the Himalayas especially during the Lateglacial and Holocene.

Multi-proxy studies from lake sediments and peats in India and adjacent areas of the Tibetan Plateau (e.g. Bhattacharyya, 1989, Gasse et al., 1996, Enzel et al., 1999, Phartiyal et al., 2005, Prasad and Enzel, 2006) reveal the rather early onset of the Indian summer monsoon (ISM) during the Lateglacial, a full development during the early and mid-Holocene time, and a gradual weakening thereafter. Its onset at ca 13 kyr BP (Gasse et al., 1996) coincided with increasing wind strength over the Arabian Sea (e.g. Overpeck et al., 1996), indicating an early northward shift of effective moisture. However, most of the data reflect early to mid-Holocene moist conditions across the ISM trajectories, as deduced from speleothem and marine records (Schulz et al., 1998; Gupta et al., 2003, Dykoski et al., 2005; Wang et al., 2005; Fleitmann et al., 2007;), lake records in India (Prasad and Enzel, 2006 and references therein) and the south-western Tibetan Plateau (e.g. Gasse et al., 1996), and from runoff behavior of the Indus and Ganges–Brahmaputra systems (Kudrass et al., 2001, Staubwasser et al., 2003). Staubwasser and Weiss (2006), however, point out that ISM rain was not uniformly enhanced. Evidence from glaciers (Thompson et al., 2006, Owen, 2009) further indicates the spatio-temporal asynchrony of the summer monsoon moisture distribution and may point to considerable impact of westerly air flow. It is suggested that the relationship between the Rossby wave pattern of the upper-level westerly flow and the tropical convection intensity has an important influence on the precipitation pattern over subtropical Asia and may induce widespread droughts (Staubwasser and Weiss, 2006 and references therein). The gradual decline of summer monsoon moisture during the mid and late Holocene after about 5 kyr BP seems to be a general phenomenon that affected the entire monsoon domains (e.g. Gupta et al., 2003, Prasad and Enzel, 2006) and which is explained by the progressive southward shift of the ITCZ due to orbital forcing (Fleitmann et al., 2007).

The intermontane Tso Kar basin in the Zanskar range of the western Himalaya is believed to represent a valuable archive for palaeoclimatic reconstructions related to monsoon moisture transport and impact by westerly disturbances during the Late Quaternary. The particular significance of the Tso Kar basin results from its rain-shadow position relative to monsoonal effective moisture supply, its location close to the glaciated mountain ranges and the considerable influence of westerly air flow over this part of the region.

However, little is known about climatic responses in the high-elevated region of Ladakh, although the area under consideration is located close to the modern boundary of summer monsoon influence and thus may reflect gradual and abrupt shifts in monsoon intensity during the past 10–20 millennia. Previously, several lake studies have used sediment records from lakes or peatbogs (e.g. Gasse et al., 1996, Prasad et al., 1997, Phadtare, 2000, Demske et al., 2009) for climate reconstructions. However, it is very important to supplement this work by considering the potential impact of catchment processes in regions like the Tso Kar basin, since they may have affected the sedimentary environment in the basin more intensively than previously expected. Consequently, we aim to combine geomorphologic evidence of former glacier extent and fluctuating lake levels throughout the Lateglacial and Holocene and to integrate these with multi-proxy records from Tso Kar sediments.