Luminescence chronology of fluvial archives from Kaveri basin: Implications to late-Quaternary climate changes

Goswami, Kartika (2020) Luminescence chronology of fluvial archives from Kaveri basin: Implications to late-Quaternary climate changes. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Kartika Goswami (14RS043)) 14RS043.pdf - Submitted Version Restricted to Repository staff only Download (8MB)

Abstract

The study uses optically stimulated luminescence (OSL) dating to reveal the chronology of the flood and deltaic sediments from the Kaveri basin, south India, and investigates the relationship of the sediment depositional history with the late-Quaternary climate changes. This objective is addressed by collecting flood deposit samples from the upper Kaveri basin, and the coastal/deltaic sediments from the coastal part of Kaveri delta, southeast India. The OSL ages are correlated with independent age controls such as radiocarbon (¹⁴C) dating and young ages with the river gauge records, whenever possible. The quartz grains from southern India generally have bright OSL signal. This was advantageous in the dating of extremely young sediments. However, the river Kaveri flows mostly through rocky terrain in its upper reaches and is also joined by various tributaries. It therefore carries locally weathered sediments. Hence, it was interesting to explore the extent of bleaching, and thus the feasibility of OSL dating young flood sediments (~ few tens of hundreds of years only). Heterogeneous bleaching was observed in the flood sediments from the upper parts of Kaveri. Age estimates were made using existing age models, like the minimum age model (MAM) and the central age model (CAM). Additionally, grain size analysis, micropaleontological studies (of deltaic sediments), and palaeodischarge analysis using the slope-area method (from flood deposits) are applied to further reach our research goals. The study of the flood deposits, provided insights into the monsoon flood-climate relationship. The study revealed that high-magnitude flooding events have occurred during major climatic shifts, from fluvial dormancy to sudden outburst of monsoons (~2 ka), from warmer to colder periods (onset of 'Little Ice Age' (LIA) ~14th century), from colder to warmer periods (end of LIA ~19th century) and ~20th century. This also indicates that not all wet phases are associated with large floods and not all dry phases with low floods or droughts. The study also demonstrated that the occurrence of the 20th century floods in the upper reaches of Kaveri is a result of highintensity, short-duration storms. Brief precipitation analysis of the recent 2018 Kerala/Karnataka floods suggests that it is not only the increased amount of rainfall but also the temporal variability in the downpour that affects the occurrence of floods in the study area. Additionally, the paleodischarge analysis points out that the floods in recent times (post 1950) are occurring at a higher magnitude than the paleofloods. The OSL ages, and the micropaleontological results obtained from the four deltaic subsurface sediment cores of 25 m depth each, revealed a close correlation of the sedimentary dynamics of the Kaveri river with the late-Quaternary sea-level and accompanied warmer conditions. The river has attempted to re-establish its equilibrium profile with the rise in the sealevel during ~140-143 ka, ~121 ka, ~95-100 ka, ~81-89 ka, ~73-78 ka, ~57- 60 ka, ~40-45 ka, 6-9 ka, ~5 ka, and ~3 ka. This is majorly achieved by the river’s vertical aggradational deposition as a result of a balanced influx of fluvial sediment with sea-level rise. However, a gradual progradation of the coastline towards the sea since ~30-15 ka is also observed, suggesting the effect of gradually falling sea-level in the coastal stratigraphy during this period. At ~6-9 ka, ~45 ka, ~75 ka, and ~121 ka, the presence of foraminifers is also observed in these coastal shores, suggesting marine influence. However, ~6-9 ka transgression is seen as the most pronounced and long-lived in the study area out of them all. Luminescence chronology of the cores also hints towards a fault movement post ~70 ka, indicating the role of Pleistocene tectonics, sea-level changes, climate, river dynamics in the late-Quaternary evolution of the Kaveri delta. Therefore, the findings of present work contribute to the understanding of the regional palaeo-climate and its implications to the fluvial responses from the Kaveri basin, southeast India.

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