The versatility of point bars in paleohydrology and paleoclimatic studies in Southern West Bengal, India

Shivsager, Vibhuti (2024) The versatility of point bars in paleohydrology and paleoclimatic studies in Southern West Bengal, India. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Vibhuti Shivsager (17RS022)) 17RS022.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

Fluvial systems respond swiftly to climatic changes and tectonic disturbances and are important geomorphic archives of such events for continental records. One such parameter is the discharge directly manifested by precipitation and thus climate. For modern systems, various measurements, including stage height, velocity, and cross-section, help estimate a river's discharge. However, understanding the amount and fluctuations of past discharges remains a challenge. There has yet to be a direct method available to get fluvial discharges in the past. Point bars in a meandering fluvial system are sinuous accretionary wedges of sand and finer sediments deposited by laterally migrating rivers. The dimension of the meander loops/oxbow lakes is directly proportional to the river's discharge and the sediment load type. They provide an excellent means to study past environments due to their ability to preserve the sedimentary structures corresponding to alteration in energy conditions. The central West Bengal, India, has a complex network of numerous oxbow lakes/paleo meander loops as a remnant of river Hooghly, a major distributary of the River Ganga. The present work revisits Schumm’s classic work to estimate river discharges from the meander loops and shows these point bars as a potential proxy for estimating past discharges. Thus, it is extended to reconstruct past climate using grain size data, facies analysis, and the dimension of the meander loops coupled with the luminescence chronology of oxbow lakes. The study reveals that the region received good rainfall during reported periods of intense monsoon, and the increased discharge at different time intervals has given rise to numerous oxbow lakes. The discharge values have shown considerable fluctuations, rising to 15 to 20 times the current value and very few anomalously high, even up to 250 to 300 times, which might be lost and left unrecorded from other archives. The study shows the preservation of systematic growth of discharges and the meander loops with climate revealing events with anomalously high (and so highly disastrous) discharges at the scale of tens of thousands of years, usually missed from other archives. Results from Ichhamati and Hooghly River meander loops in West Bengal, India, indicate a manifold increase of discharges from the current during 1-1.5 ka (known as the Medieval Warm Period); around 2.2 ka and around 3.9 ka with low or inactive phases or gaps of enhanced monsoon, e.g., Little Ice Age (LIA) and during 2.5 - 3.5 ka. Apart from that, we failed to observe a few anomalously high discharge events (250 times the current), but they may occur in the future and must be dealt with cautiously. This is an effort to show the potential of past meander loops, which needs to be explored well for comprehensive records. Various geochemical indices (CIA, PIA, CIW, ICV, Mobile/immobile elements ratio) were calculated from XRF data to assess the feasibility of using point bar sediments to trace past climatic fluctuations. While initial evidence indicates that point bar sediments are sensitive to climatic fluctuations, more rigorous studies are needed for definitive conclusions. This research highlights the potential of point bar sediments as a valuable tool for understanding past climatic and hydrological conditions. It emphasizes the importance of exploring point bars further to develop comprehensive records of past discharges and climatic events.

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