Palaeoclimatic reconstruction by using the varvite sediments of Bharatpur, Upper Lahaul Valley, NW Himalaya, India

摘要

A 6 m thick varvite profile exposed at Bharatpur (77 degrees 27' E: 32 degrees 48' N, altitude 4692 m) in the NW Himalaya was investigated to understand the past climatic signatures during the Late Quaternary period. The radiocarbon chronology brackets the varvite sedimentation between similar to 24.3 and 7.2 ka BP and the sedimentary sequence is bounded by river born gravel at the top and glacial outwash at the bottom. The proxies used were lithology, chronology (AMS/radiocarbon dating), geochemical analysis, loss on ignition (LOI), and mineral magnetism. Other parameters, e.g., Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Al2O3/SiO2, Fe2O3, CaO, MgO and Al/Na ratio were also used. Based on the combined data set, six phases of warm and cold climatic conditions are reconstructed. The mean CIA, CIW and Al/Na ratios reveal that the sediments were incorporated from sedimentary rocks by moderate silicate chemical weathering process. Based on the available chronology, the basal most glacial outwash event can be estimated as ca. 24.3 ka BP and the laminated section shows continuity of sedimentation from the Late Quaternary to Holocene. The period between similar to ca. 24.3 and 22.6 ka can be regarded as an arid phase. From similar to ca. 22.6-20.5 ka BP, a slight increase in monsoonal precipitation is observed, followed by decreased precipitation from similar to ca. 20.5 to 18 ka BP. The period from ca. similar to 18 to 15.7 reveals warm and humid climatic conditions. We suggest that a cold event from ca. similar to 15.7 to 11.5 may correspond to the Younger Dryas (YD) which in turn is followed by a phase of maximum moisture of the Holocene Climate Optimum (HCO) (ca. similar to 11.5-7.2 ka BP) in the study area. The increased CIA during deglaciation and HCO suggest high chemical weathering, whereas, the LGM (Last Glacial Maximum) and YD are represented by the low CIA. The observed climatic phases/events are in agreement with the records from other parts of the Himalaya. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.