Variations of paleoclimate and paleoenvironment during the last 40 kyr recorded in clay minerals in the Kathmandu Basin sediments

Abstract

...... The Asian monsoon system and its evolution are known to be closely linked to the Himalayan-Tibetan orogen. The Kathmandu Basin is the best target for clarifying the variability of the Asian monsoon climate and its linkage to the uplifting of the Himalayan-Tibetan orogen, since it is located on the southern slopes of the central Himalayas and filled with thick sediments from late Pliocene to Quaternary (Sakai 2001). Unfortunately, previous studies could not critically and completely resolve on the paleoclimatic and paleoenvironmental changes in the Kathmandu Basin because of samples from discontinuous surface exposures. Our group has proceeded on a Japan-Nepal collaborative project “Paleo-Kathemandu Lake (PKL) Project”. In the project, we have carried out core drilling within the Kathmandu Basin and have investigated the cores and surface exposures from various viewpoints and methods. The reconstruction of paleoclimatic and paleoenvirnmental variations recorded in the Kathmandu Basin sediments is one of many purposes of our project. We have already reported the results of fossil pollen analysis and characteristics of sediments for a drilled core sample obtained from the Kathmandu Basin and surface geological survey in the southern part of the Kathmandu Basin. Clay minerals represent useful markers of successive climates, since they formed through weathering or hydrolysis processes during successive periods of the geological history and basically express the intensity of weathering or hydrolysis in the land masses adjacent to sedimentary basin. The information provided from such clay minerals fundamentally integrates the combined effects of temperature and precipitation. Detrital clay minerals can also be used as tracers of sediment transport processes, dispersal and provenance. Clay minerals in the Kathmandu Basin sediments do contain good information on the paleoclimate and paleoenvironment in this area, because they are directly formed through the weathering or hydrolysis process only from the parent minerals, feldspars and micas, in both gneiss and granite of the Shivapuri injection complex and weakly metamorphic rocks of the Phulchauki Group. In order to reconstruct the paleoclimatic and paleoenvironmental changes during the last 40 kyr recorded in the Kathmandu Basin, we examined the estimation of the amount of the clay size fraction, the relative amounts of individual clay minerals, and the crystallinity of illite in the drillcore sediments by using the decomposition procedure of X-ray diffraction (XRD) patterns and the mineral reference intensity (MIF) method. In this paper, we report the paleoclimatic and paleoenvironmental information in the Kathmandu Basin deduced from the clay mineral data. For clay mineral analysis, we used core sediment samples collected at 10 cm interval between 7 m and 40 m in depth of the RB core, which was drilled at Rabibhawan in the western central part of the Kathmandu Basin and is 218 m long (Sakai et al. 2001). The topmost part of the RB core from 7 m to 12.15 m in depth is generally composed of medium-to very coarse-grained micaceous granitic sand. The core sediments between 12.15 m and 40 m in depth are organic black or dark gray mud called “Kalimati Clay”. A 14 C age of the Kalimati clay at 38.3 m in depth of the RB core is 44690±360 yr. B.P. and the mean sedimentation rate between 7 m and 40 m in depth of the RB core is about 900 mm/kyr.