Browsing by Author "R.C. Mehrotra"

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Paleo-wildfire signatures revealing co-occurrence of angiosperm-gymnosperm in the early Paleogene: Evidences from woody charcoal and biomarker analysis from the Gurha lignite mine, Rajasthan, India
(Elsevier B.V., 2023) Anumeha Shukla; Andre Jasper; Dieter Uhl; Runcie P. Mathews; Vikram P. Singh; Kajal Chandra; Rimpy Chetia; Samiksha Shukla; R.C. Mehrotra
Charcoal, a common product of wildfire, is abundant in many sedimentary rocks deposited in a wide range of environments, from terrestrial to marine. The analysis of paleofloras and the related paleoecological conditions is of great importance for the understanding of past environmental and paleoclimatic events. Woody charcoal is identified from the early Paleogene sediments of the Gurha lignite mine (Palana Formation), Bikaner, Rajasthan. This evidence is in the form of fragments of tracheids and vessels that show homogenized cell walls and well-preserved anatomical details, like pitting on cell walls, characteristic features of charcoal. These charcoal remains, most of which belong to angiosperms, represent the first verified occurrence of paleo-wildfires in this region during the early Paleogene. Moreover, a gymnospermous taxonomic affinity can be established for some of the charred woods and a relationship with conifers is likely, thus providing additional evidence for the taxonomic composition of early Paleogene floras in this region. Consecutively, the presence of diterpenoid compounds together with the occurrence of (mega-)sporinite in the lignite samples also shows the presence of gymnosperm vegetation (conifers). Furthermore, the petrographic composition reveals that the lignite samples are dominated and sub-dominated by the huminite (av. 43.2 vol%) and inertinite (av. 39 vol%) groups, respectively, followed by liptinite (av. 13.9 vol%) group of macerals. Fusinite, semifusinite (and inertodetrinite) macerals are commonly considered as fossil charcoal. The relatively higher content of these macerals in the studied lignite indicates the regular events of palaeofire during the deposition. Subsequently, the presence of unsubstituted polycyclic aromatic hydrocarbon (PAH) compounds identified in samples further suggests the occurrence of fire. Additionally, the inertinite (fusinite) reflectance indicates that the charcoal/inertinite was formed at a temperature above 300 °C, while the highest reflectance value suggests a burning temperature of ∼800 °C. © 2022 Elsevier B.V.
Physico-chemical behaviour of hydrated aluminium oxides and associated clay minerals occurring around Pipra, district Sidhi, India
(Kluwer Academic Publishers, 1979) R.C. Mehrotra; S.N. Varma; B.K. Singh
DTA techniques were employed to study the thermal and structural characteristics of hydrated aluminium oxides and aluminous clays of the Pipra pelitic rocks from district Sidhi, India. Detailed microscopic investigations, X-ray and chemical analyses reveal that these clays were derived by the localized weathering of arkosic metasediments. The chemical and normative behaviours have confirmed their formation by the isochemical metamorphism of arkose, aided by a little granitization and followed by minor retrogression. A tentative correlation between the thermal and structural changes of these oxides and clay minerals at various transition temperatures has nicely displayed the presence of kaolinite, diaspore and gibbsite. The exothermic curves of kaolinite confirm the recrystallization. The presence of kaolinite in the clay fractions indicates the detrital origin. © 1979 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.
Recent advances in understanding neogene climatic evolution: Indian perspective
(Indian National Science Academy, 2020) A.D. Singh; A.K. Ghosh; R.C. Mehrotra; R. Patnaik; M. Tiwari
This article presents an overview of the research work done during 2015-2019 on the South Asian/Indian monsoon climate variability, based on the Neogene terrestrial and marine proxy records. The paleoclimatic and paleoceanographic records provide better insights into our current understanding of timing of initiation/intensification of the Indian monsoon system; its evolution and seasonal variability pattern through time and underlying mechanisms; and impacts on paleobiogeography of terrestrial fauna and flora and physico-chemical and biological processes in the northern Indian Ocean. The Neogene upliftment history of the Himalaya was reconstructed based on detailed analysis of fossil flora from Himalayan sequences and a link between step wise uplift of Himalaya-Tibetan plateau and Indian summer monsoon intensification was established. The rainfall pattern in the northeast India during the Neogene period was quantified; and the new data suggest the presence of rainfall in the region since 11.6 Ma. δ13C record of pedogenic nodules in Siwalik succession provides evidence of major shift in vegetation between ~2.8 Ma and 1.1 Ma due to large variation in temperature and precipitation. New results of the multiproxy based paleoceanographic studies carried out on sediment cores collected during recent IODP expeditions in the Arabian Sea and Bay of Bengal, have major implications on the evolution of the Indian monsoon and dynamics of its seasonal variations in the Neogene. The sedimentological and geochemical records from carbonate platform of the Maldives suggest an abrupt onset of modern monsoon circulation at ~ 12.9 Ma, which cannot be entirely explained by the Himalayan tectonics as many believed earlier. It appears that the global climate also controls Indian monsoon circulation, in addition to the tectonics. Record of past changes in the Arabian Sea denitrification driven mainly by monsoon-wind induced productivity and water column ventilation condition, reveals that the Indian monsoon intensified during ~2.8-3.2 Ma. Recently, based on the integrated multiple proxy records it was inferred that seasonal monsoon circulation was weak during 1.85-2.7 Ma, subsequently followed by the intensification of winter monsoon between ~1.65 and 1.85 Ma, attributed to the development of strong zonal and meridional circulations due to enhanced E-W Pacific temperature gradients. © 2020 Indian National Science Academy. All rights reserved.