PALEOGEOGRAPHIC AND PALEOCLIMATIC RECONSTRUCTION ALONG THE MAHANADI DELTA

Abstract

The present study on Mahanadi delta has two broad objectives: (i) to define implications of sea-level fluctuations on channel morphologic changes in deltaic environment and, (ii) to reconstruct Indian Summer Monsoon variability from lake sediment records. Contrary to previous laboratory-based investigations lacking empirical evidence on fluvial response to base-level changes, the present work is focused on correlating concepts of base-level controlled channel morphologic changes with fieldbased observations. Various paleo-fluviogeomorphic features such as anastomosingmeandering transition, paleo-dendritic channels, and lateral shift of river mouths, which are indicative of base-level change have been studied and correlated with past marine transgression and regression events. Past studies on borehole cuttings indicate that the Mahanadi delta has experienced several episodes of marine transgression and regression events. Paleo-barrier spits, relict ridges, and paleo-marine terraces in the inner Mahanadi deltaic region are evidence of past coastline positions. Ground Penetrating Radar study along paleo-strandlines of the Mahanadi delta reveals several stratal termination units in the sub-surface sediment layers. These subsurface depositional units may have developed due to repeated marine transgression/regression and sequential deposition of sediments. Major rivers in the Mahanadi delta e.g. Mahanadi, Devi, Brahmani, and Baitarani, show episodic anastomosing and meandering characteristics corresponding to transgressive and regressive events, respectively. The paleo-anastomosing branches along the paleo-strandline positions can be observed in satellite imagery. Dendritic drainage patterns formed due to flow accumulation along the coast have been observed along the paleo-strandlines. Based on OSL ages and affinity to strandline positions, two generations of Holocene dendritic channels can be distinguished in the Mahanadi deltaic region. Paleo-dendritic channels with age > 5 ka BP correspond to Early-Mid Holocene strandline position and, with age group < 5 ka BP correspond to the Late Holocene strandline position. The Early to Mid and Mid to Late Holocene paleo-dendritic channels developed along the transgressive coasts and later were abandoned when the coastline regressed. Lateral shifting of river mouths in response to sea-level changes have been observed in the lower deltaic plain. The delta distributaries such as Bhargavi, Kushavadra, Brahmani and Baitarani rivers ii show lateral shift of river mouth before meeting the Bay of Bengal. Age of Kushabhadra River paleochannel indicating paleo-flow direction dates back to ~ 7 ka BP. Similarly, the paleo-flow paths of the Brahmani River date back to ~ 6 ka BP. The river mouth shift directions of both the rivers are parallel to the Early Holocene strandline. Major rivers in the delta have migrated up to the present coastline with base-level adjustments to changing coastline, while the small channels ended abruptly with a retreat in sea level. Paleochannels along the paleo-strandlines indicate last fluvial activity along the paleo-coastlines. When compared to fluvial morphological patterns along paleostrandlines of major deltas around the world, the Mahanadi delta shows similar paleofluvial morphology with changing coastline. Two sediment cores from Anshupa Lake and Chilka Lake were analyzed to study paleoclimatic changes and their effect on sedimentation in the Mahanadi delta region. Sediment core from Anshupa Lake dates back (14C) to 1400 AD, provides evidence of the Little Ice Age (LIA). The average sedimentation rate during LIA was 0.12 cm/yr and it drastically changed to 0.45 cm/yr in the post LIA period. During the LIA period, the sedimentation rate was highest in the 16th century (0.35 cm/yr) and lowest in the 18th century (0.09 cm/yr). Down core variation of mineral magnetic, organic carbon and stable isotope record suggest that LIA extended from 1450 AD to 1850 AD. The 𝜒𝑙𝑓 values vary from ~ 5× 10− 8 m3 kg− 1 to ~ 60× 10− 8 m3 kg− 1. The 𝜒𝑙𝑓 values are lower at the Dalton (~ 5× 10− 8 m3 kg− 1) and Munder Minimums (~ 9× 10− 8 m3 kg− 1), while the 16th century showed an increasing trend. The χfd % value varies from ~0 to ~13%. The χARM value varies from 0.785 × 10− 5 m3 kg− 1 to 0.021 × 10− 5 m3 kg− 1. The saturation isothermal remnant magnetization value varies from 627.0126× 10− 5 Am2 kg− 1 to 33.094× 10− 5 Am2 kg− 1. χfd %, χARM and SIRM show similar trend as shown by 𝜒𝑙𝑓 values. The 𝜒𝑙𝑓 shows a positive correlation with reconstructed sunspot numbers. Spectral analysis of χlf values shows significant periodicity of 74, 64, 44 and 11 years. These periodicities suggest a solar influence on Indian Summer Monsoon. Organic carbon and nitrogen percentage vary from 1.5 to 6 % and 0.07 to 0.7 %, respectively. The δ13C value of organic carbon fluctuates from – 21.028 to − 26.528 ‰. The downcore variation of δ13C and TOC reflects two phases of climate during LIA. Phase-I (1450 to 1700 AD) reflects relatively high content of TOC and more negative values δ13C indicating relatively warmer climatic condition. During Phase-II (1700-1850 AD), TOC deposition relatively decreased and δ13C iii became comparatively positive reflecting relatively cold climate. The inter-parametric ratio χARM/SIRM is > 200 around 1600 AD, suggesting the presence of bacterial magnetism. Bacterial magnetite develops due to a high influx of nutrients into the lake. The bacterial magnetism correlates with high nutrient supply to the lake ecosystem due to a relatively warm period during the 16th century, as suggested by an increasing trend in 𝜒𝑙𝑓 values. The organic carbon percentage, C/N ratio and stable isotope record indicate comparatively increased rainfall during the 16th century than the 17th and 18th centuries. From these geochemical parameters, it is inferred that the 17-18th century corresponding to Dalton Minimum was the coldest period with reduced precipitation and the 16th century was a relatively warmer period during LIA. Several events of drought, high rainfall, and the onset of aridity can be correlated with similar events documented in speleothems from different parts of India. The sediment core from Chilka Lake dates back to 9039 cal yr. BP. Sedimentation rate fluctuates from ~ 0.2 cm/year to ~ 0.007 cm/year, with maximum sedimentation from 8,000 cal yr BP to 6,000 cal yr BP and minimum sedimentation from 6,000 cal yr BP to 2,000 cal yr BP. Down core variation of mineral magnetic parameters reveals wide fluctuation in 𝜒𝑙𝑓 values. The 𝜒𝑙𝑓 values range from 6.4× 10− 8 m3 kg− 1 to 43× 10− 8 m3 kg− 1. The 𝜒𝑙𝑓 values are minimum from 6,000 cal yr BP to 2,000 cal yr BP, suggesting mid-Holocene weakening of Indian Summer Monsoon. χARM varies from 0.71 × 10− 5 m3 kg− 1 to 0.021 × 10− 5 m3 kg− 1. The saturation isothermal remnant magnetization values vary from 636.33× 10− 5 Am2 kg− 1 to 44.905× 10− 5 Am2 kg− 1. The downcore variation of 𝜒𝑙𝑓, χARM, and SIRM show a similar trend. An increasing trend in these mineral magnetic parameters are observed during early to mid (8,000 cal yr BP to 6,000 cal yr BP) and late Holocene period (>2,000 cal yr BP). The study of Chilka Lake core suggests Holocene warm period extended from 9,000 cal yr BP to 6,000 cal yr BP, and the mid-Holocene period was relatively cold. Regional records on flourishing and extinction of river valley civilizations are found to be correlated with climatic records obtained from Chilka Lake core. Spectral analysis of 𝜒𝑙𝑓 values shows significant periodicities of 1058, 690, 484, 396, 288, 212, 207, 183 and 158 years, suggesting a possible solar influence on Holocene variation of Indian Summer Monsoon. Paleoclimatic study from two sediment cores suggests that magnetic susceptibility (χlf) data is highly correlating with regional climatic records. Spectral analysis of χlf in both the cores indicates that periodicities in ISM can be obtained from high-resolution magnetic susceptibility data. We have thus explored the potential of using magnetic susceptibility (χlf) as a proxy for paleorainfall variations in a tropical region.