• Ajoy K Bhaumik

      Articles written in Journal of Earth System Science

    • Stable carbon and oxygen isotope study on benthic foraminifera: Implication for microhabitat preferences and interspecies correlation

      Ajoy K Bhaumik Shiv Kumar Shilpi Ray G K Vishwakarma Anil K Gupta Pushpendra Kumar Kalachand Sain

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      Stable isotopes of benthic foraminifera have widely been applied in micropalaeontological research to understand vital effects in foraminifera. Isotopic fractionations are mainly controlled by ontogeny, bottom/pore water chemistry, habitat preference, kinetic effect and respiration. Discontinuous abundance of a species for isotopic analysis has forced us to select multiple species from down-core samples. Thus standardisation factors are required to convert isotopic values of one species with respect to other species. The present study is pursued on isotopic values of different pairs of benthic foraminifera from the Krishna–Godavari basin and Peru offshore to understand habitat-wise isotopic variation and estimation of isotopic correction factors for the paired species (Cibicides wuellerstorfi–Bulimina marginata, Ammonia spp.–Loxostomum amygdalaeformis and Bolivina seminuda–Nonionella auris). Infaunal species (B. marginata, Ammonia spp. and N. auris) show a lighter carbon isotopic excursion with respect to the epifaunal to shallow infaunal forms (C. wuellerstorfi, L. amygdalaeformis and B. seminuda). These lighter δ13C values are related to utilisation of CO2 produced by anaerobic remineralisation of organic matter. However, enrichment of δ18O for the deeper microhabitat (bearing lower pH and decreased CO32−) is only recorded in case of B. marginata. It is reverse in case of N. auris and related to utilisation of respiratory CO2 and internal dissolve inorganic carbon pool. Estimation of interspecies isotopic correction factors for the species pairs (δ13C of C. wuellerstorfi–B. marginata, L. amygdalaeformis–Ammonia spp., N. auris–B. seminuda) and δ18O of C. wuellerstorfi–B. marginata are statistically reliable and may be used in palaeoecological studies.

    • Characterization of organic carbon in black shales of the Kachchh basin, Gujarat, India

      Hema Srivastava Ajoy K Bhaumik Devleena Tiwari Sarada P Mohanty Dattatray J Patil

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      Thirty-three black shale samples from four locations on the onland Kachchh basin, western India were analyzed to characterize organic carbon (OC), thermal maturity and to determine the hydrocarbon potential of the basin. Upper Jurassic black shales from the Jhuran Formation (Dhonsa and Kodki areas) are characterized by the presence of chlorite, halloysite, high $T_{\mathrm{max}}$, low OC, low hydrogen index and high oxygen index. These parameters indicate the OC as type IV kerogen, formed in a marine environment. The rocks attained thermal maturity possibly during Deccan volcanism. Early Eocene samples of the Naredi Formation (Naliya-Narayan Sarovar Road (NNSR) and the Matanomadh areas) are rich in TOC, smectite, chlorite and framboidal pyrite, but have low $T_{\mathrm{max}}$. These indicate deposition of sediments in a reducing condition, probably in a lagoonal/marsh/swamp environment. Organic carbon of the Naredi Formation of NNSR may be considered as immature type III to IV kerogen, prone to generate coal. Core samples from the Naredi Formation of the Matanomadh area show two fold distribution in terms of kerogen. Organic carbon of the upper section is immature type III to IV kerogen, but the lower section has type II to III kerogen having potential to generate oil and gas after attaining appropriate thermal maturity.

    • Timing and cause of the disappearance of some elongated taxa in the Indian Ocean: Study from NGHP Hole 17A

      SANJIB BISWAS AJOY K BHAUMIK

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      Disappearance of many elongated benthic foraminifera across the Mid-Pleistocene Transition (MPT) is popularly known as ‘Stilostomella extinction’. This event is reported from several sites across the world and it is believed that most of the elongated benthic families disappeared within 0.76–0.5 Ma. It is assumed that the presence of cold bottom water triggered their disappearance. The present study is pursued on sediment cores collected from National Gas Hydrate Program Hole 17A, Andaman Sea, Indian Ocean to examine the precise timing and cause of the disappearance of elongated taxa as well as their ecological preference. Our study reveals that elongated species prefer warm and low to intermediate food condition. This study documents the disappearance of 5 major species and 13 minor species from 0.6 to 0.2 Ma which is younger than the earlier estimates. We assume that change in 41–100 kyr climatic cycle along the MPT was responsible for a major decline in their number. However, amplified glacial–interglacial cycles along Mid-Brunhes (0.5–0.2 Ma) finally played a major role in their disappearance. Both of these consecutive events extended intensified cooling and dropped the deep-ocean temperature by which the shallow infaunal elongated species disappeared and deep-infaunal species survived.

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