Lacustrine record of high magnitude flood events and climate variability during mid to late Holocene in the semiarid alluvial plains, western India

Abstract

Long-term data on flooding events are necessary for understanding the flood frequency and magnitude variations, but the available instrumental records span only a few decades. Recently, lake sediment sequences have emerged as a relatively untapped archive for high-magnitude floods over longer timescales. In the present study, multi-proxy data from a lacustrine sequence at Timbi in the Dhadhar river basin were analysed to determine high magnitude floods in this semi-arid region of western India in response to southwest monsoon variability. The results indicate the occurrence of discreet high magnitude flood events and depositional phases corresponding to different climatic conditions during the mid to late Holocene. Three large flash floods between 4830 and 2730 cal yr B.P. (Phase-I) have been identified as coeval with flood events from fluvial archives and correlated to the widespread period of aridity and weak monsoon recorded in the lakes of north western India. A shift in the depositional environment from fluvial to lacustrine occurred around 2730 cal yr B.P. as inferred from an abrupt change in the sediment character and palynofacies caused by high intensity flash floods and high sediment influx. Phase II (2730–1730 cal yr B.P.) was marked by periodic erosion and deposition in the catchment and sediment influx during pulses of higher precipitation as evident in the dominance of Glomus sp. and Alternaria sp., and relatively low values in concentration-related magnetic parameters and S-Ratio. A high magnitude flood event occurred at 1770 cal yr B.P. towards the end of phase II. During phase III (1730–880 cal yr B.P.) the monsoon was enhanced and climate was warmer as suggested by low χfd%, SIRM/χ, S-Ratio, higher Ba/Sr ratio and presence of aquatic palynomorphs. A high percentage of structured Organic Matter (OM), low fungal remains and increased Ba/Sr ratios between 880 and 360 cal yr B.P. indicate higher lake waters and better monsoon condition during phase IV. The phase V (360–0 cal yr B.P.) corresponds to lake level lowering, soil formation and eutrophication. The Timbi lake sediments demonstrate the potential for lakes in semi-arid alluvial plains of western India to preserve proxy flood records providing additional archives for generating long-term datasets. The flood record generated substantiates palaeoflood data from the fluvial archive suggestive of regional-scale southwest monsoon variability. Low frequency-high magnitude floods occurred during arid periods and low magnitude-high frequency events during relatively strengthened monsoon and thus have implication for linking the palaeoflood events to the southwest monsoon intensity in the semi-arid continental regions.