Late Holocene flooding history of a tropical river in western India in response to southwest monsoon fluctuations: A multi proxy study from lower Narmada valley

Abstract

A multi-proxy study on the sediments from lower Narmada valley, western India records large flooding events and high sediment influx linked to southwest monsoon variations during late Holocene. On the basis of proxy data and chronology of preserved sediments, three major environmental perturbations at 1809, 1487, and 1187 cal BP have been identified that corroborate well with the regional fluctuations in southwest monsoon variability. A phase of dominant marine influence (between 2185 and 1809 cal BP) is inferred based on the presence of marine palynomorphs, mangrove pollen, and amorphous organic matter and is coincident with the phase of weak monsoon in the region. A transition from marine marsh environment to a phase of high fluvial influx occurred around 1809 cal BP. The fluvial influence is inferred based on the replacement of amorphous organic matter with structured organic matter, presence of fresh water algae, decrease in the δ13C values and dominance of kaolinite clay. The fluvial regime stabilized between 1487 and 1187 cal BP as recorded in the overbank sediments, palynomorphs, and geochemical indices, and sedimentation occurred under a highly oxidizing and high energy condition with long distance transport of sediments. The later incision of the sediment sequence, which post-dates 1187 cal BP, can be attributed to enhanced precipitation. The response of the lower Narmada valley to short term southwest monsoon fluctuations is manifest in the shifting deposition environments and a major role of terrestrial sediment influx variability is surmised, leading to relative sea level variations in this part of the Indian subcontinent. A temporal and process-based interplay between flooding periods and sediment discharge in river systems and tidal conditions is suggested for the tropical estuarine environments.