GEOCHEMICAL FINGER PRINTING OF THE LATE QUATERNARY VALLEY-FILL DEPOSITS IN THE ALAKNANDA VALLEY, CENTRAL HIMALAYA, INDIA

Valley-fill aggradation in the Himalayan Rivers is modulated by the steep monsoonal gradient and climate variability. Therefore, the alluvial sequences preserved in the Himalaya can be used to ascertain the spatial variability in the sediment flux (provenance) which in turn can help in ascertaining the past climate variability. It has been suggested that the valley-fill aggradation is contributed by three major sources these are (i) the paraglacial sediments located north of the Higher Himalayan Crystalline,(ii) the alluvial fan and debris flows dominantly which are concentrated in south of the Main Central Thrust (MCT) which acts as an orographic barrier for the Indian Summer Monsoon (ISM) and (iii) the landslides (hill slope). The above sediment sources are located in distinct lithounits, hence their contribution can be ascertained using the geochemical composition of the valley-fill sediments. This in turn can provide information about the processes responsible for the temporal changes in the sediment flux. Towards this we examined the chemical composition (major elements) of the fluvial terraces in the Alaknanda valley which span the Marine Isotopic Stage-2 (MIS-2) and early part of MIS-1 (26 ka to 8 ka), covering the Last Glacial Maximum (LGM) and early Holocene climatic optimum.

The geochemical observations suggest that during the LGM there were limited contribution from the Higher Himalayan Crystalline (HHC). This we ascribed to the expansion of the valley glaciers and weakening of the ISM . Following this, the post glacial period till the early Holocene, HHC were the major source of sediment supply in the Alaknanda valley. This we attribute to the gradual strengthening of the ISM that seems to have penetrated the longitudinal valleys into otherwise rain shadow zone of the Higher Himalaya aided with high melt water discharge associated with deglaciation. The inferences are supported by geochemical fingerprinting of the June 2013 unusual rainfall induced flood in the Mandakini valley-a tributary of the Alaknanda River suggests that high magnitude rainfall can penetrate the orographic barrier (into the paraglacial zone) and transport sediment to lower reaches in short time.

Implication of the present study in terms of sediment provenance and its implication towards past monsoon variability will be presented.