JOURNAL OF QUATERNARY SCIENCE (2005) 20(5) 485–492 Copyright ß 2005 John Wiley & Sons, Ltd. Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.938
A 20-ka climate record from Central Himalayan loess deposits R. K. PANT,1 N. BASAVAIAH,2,4 N. JUYAL,3 N. K. SAINI,1 M. G. YADAVA,3 E. APPEL4 and A. K. SINGHVI3* 1 Wadia Institute of Himalayan Geology, 33, Gen. Mahadeo Singh Marg, Dehradun, 248001, India 2 Indian Institute of Geomagnetism, Kalamboli, New Panvel, Navi Mumbai, 410218, India 3 Physical Research Laboratory, Navrangpura, Ahmedabad, 380009, India 4 Institut fu¨r Geowissenschaften, Universita¨t Tu¨bingen, Sigwartstrasse 10, 72076 Tu¨bingen, Germany Pant, R. K., Basavaiah, N., Juyal, N., Saini, N. K., Yadava, M. G., Appel, E. and Singhvi, A. K. 2005. A 20-ka climate record from Central Himalayan loess deposits. J. Quaternary Sci., Vol. 20 pp. 485–492. ISSN 0267-8179. Received 29 March 2004; Revised 21 March 2005; Accepted 13 April 2005
ABSTRACT: The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well-developed soil that was bracketed around 9 to >4 ka. This was followed by the third phase of loess accretion that occurred around 4 to >1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright ß 2005 John Wiley & Sons, Ltd. KEYWORDS: loess; Central Himalayas; mineral magnetism; geochemistry; luminescence dating; monsoon.
Introduction Loess deposits have been considered as sensitive recorders of past climatic changes (Kukla, 1977; Pant, 1993; Porter and An, 1995; Singhvi et al., 2001). In periglacial areas, loess deposition is controlled by meteorological parameters (Pant, 1993). Dust plumes generated by anticyclonic circulation at the glacier front move downslope by gravity or by katabatic winds, and are deposited as aeolian silt (loess) that blankets the regional (periglacial) topography (Pant, 1993). The resultant deposits are homogeneous, porous and well sorted, and satisfy the criteria of being designated as loess. Typical loess exposures form vertical bluffs that are strengthened by diffused carbonate (Kukla, 1977). In India, Pant et al. (1978) were the first to identify loess– palaeosol sequences in the Kashmir valley where loess blankets the lacustrine Karewa deposits of Pleistocene age. Subsequently, Williams and Clarke (1984) reported loessic silts in the Son valley of central India. The present contribution reports a new occurrence of loess–palaeosol deposits on the high tablelands and ridges between 1800 m and 2500 m altitude, in Central Himalaya. Absence of tectonically stable surfaces has limited the spatial extent of loess deposition and preservation. Detailed field survey enabled identification of patchy deposits in a narrow zone lying between Dhakuri (Bageshwar district,
* Correspondence to: A. K. Singhvi, Physical Research Laboratory, Navrangpura, Ahmedabad, 380 009, India. E-mail:
[email protected]
NE) in the Pindar river basin and Chopta (Chamoli district, NW) in the Alaknanda river basin (Fig. 1). In the present study, three loess profiles in the Alaknanda and Pindar river basins were examined and more detailed investigation on a profile at Dhakuri Dhar in the Pindar river basin was carried out (Fig. 2).
Stratigraphic details In the field, loess appears as a homogeneous and structureless silt deposit with a yellowish brown colour. The grain size analyses on unweathered loess are given in Table 1, showing dominance of fine silt (23–47%). The deposits were low in calcium carbonate possibly due to (a) freezing and thawing of winter snow cover (Pant and Dilli, 1985) and (b) leaching due to high precipitation in the region ( 1500 mm a1). The stratigraphy of an exposed, 2 m section at the DhakuriDhar is provided in Fig. 2. The section has 30 cm thick pale yellow loessic silt at the base (200–170 m). This is overlain by a moderately weathered reddish-yellow loess horizon ( 30 cm) at 170–140 cm. Following this, unweathered loess continues up to about 90 cm and is succeeded by a 40 cm thick palaeosol designated as S1 (Fig. 2). S1 has a well-developed 20 cm thick dark brown humus rich horizon (Ah). In its lower part, this horizon shows the presence of translocated clay coatings in the ped faces and hence can be designated as Ah(t) horizon. This horizon in turn
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Figure 1 Map of the Central Himalaya region showing distribution of loess deposits south of the glaciated terrain
grades down into a 20 cm thick clay illuviated horizon (Bt) in the lower part. Absence of a nodular carbonate (Cca) horizon at the base of the Bt horizon of S1 suggests leaching of carbonate from the profile. The presence of specks of charcoal in S1 indicates forest fire and physical examination indicates that the charcoal was derived from grasses. S1 is overlain by a moderately weathered loess horizon ( 10 cm). Following this, a weakly developed soil S2 (35 cm) appears towards the top. Compared to soil S1, the Ah horizon of S2 ( 15 cm) has lower humus content and this horizon grades into a 20 cm thick light brown B-horizon. This soil also contained disseminated specks of charcoal. A freshly exposed section at Dhakuri Dhar was sampled for geochemical, mineral magnetic and chronometric analyses to understand the environmental conditions during loess accretion and soil formation. In all, 98 samples were collected at 2 cm intervals for mineral magnetic analyses and, of these, 60 samples were analysed for geochemical studies. For radiocarbon dating two samples from the humus-rich Ah horizon were taken and four samples from loess horizons were collected for luminescence dating (Fig. 2).
Environmental magnetism
Figure 2 Detailed stratigraphy of Dhakuri-Dhar loess–palaeosol sequences. Radiocarbon ages (dark rectangle) and infrared luminescence ages (solid circles) are shown alongside Copyright ß 2005 John Wiley & Sons, Ltd.
Environmental magnetism involves the use of magnetic properties of naturally occurring minerals. Variability in magnetic mineralogy depends on the ambient environmental conditions and geomorphic processes (Verosub and Roberts, 1995). Parameters such as magnetic susceptibility () and isothermal remanent magnetisation (IRM) provide a measure of the concentration of magnetic particles. Similarly, the coefficient of frequency-dependent susceptibility (fd) provides an estimate of the proportion of ultra-fine super-paramagnetic (SP) ferrimagnetic particles (63 mm) (%)
Coarse silt (63–31 mm) (%)
Medium silt (31–16 mm) (%)
Fine silt (16–8 mm) (%)
5.5 1.0 2.8
8.1 7.79 6.75
28.1 26.0 30.2
47.4 30.8 23.2
180 140 90
Very fine silt (8–4 mm) (%) 2.5 14.4 13.2
Clay (15 ka, 12 ka to >9 ka and 4 ka to >1 ka, suggesting these to be phases of weaker southwest monsoon. 3 Enhanced southwest monsoon existed between 16 ka and 12 ka and even stronger monsoon conditions existed during 9 ka to >4 ka. These periods facilitated pedogensis of loess.
Acknowledgements This study was supported by the Department of Science and Technology, Government of India (ESS/CA/A3-17/95). The Director, Wadia Institute of Himalayan Geology, Dehradun, kindly provided the facilities. N. Basavaiah thanks the Alexander von Humboldt Foundation, the Deutsche Forschungsgemeinschaft, Germany, for support of work at Tu¨bingen University under the HimLakes project (AP 34/15). We thank Professor F. Sirocko for providing the Arabian Sea core data and Professor S.C. Porter for valuable suggestions.
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