Impacts of the Deccan Traps Volcanism on the Environmental Conditions: A Stable Isotope and Biomolecular Perspective

Roy, Sohom (2022) Impacts of the Deccan Traps Volcanism on the Environmental Conditions: A Stable Isotope and Biomolecular Perspective. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sohom Roy (15RS024)) 15RS024.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

The Deccan Traps (DT) in Western India is hypothesized to have caused significant fluctuations in climatic condition and organic matter (OM) production across the Cretaceous-Paleogene Boundary (KPB). It has been argued that multiple short pulses and high extrusion rates within a short time-span caused enormous amount of CO₂ and SO₂ accumulation in the atmosphere right before the boundary, which along with oceanic acidification eventually led to global biotic stress prior to the Chicxulub impact and the extinction that followed. However, disparities in geochronological models regarding timing and tempo of the episodic eruptions, difficulty in delineating effects of DT from the contemporaneous extra-terrestrial Chicxulub impactor, as well as the potential (or the lack of potential) for the DT to have released significantly higher amounts of CO₂ from sources other than mantle degassing (like passive and metamorphic degassing) have led to considerable uncertainties regarding the environmental impacts of the Deccan volcanism and it’s relation to the end-Cretaceous ecological crisis. Large igneous provinces like the DT are usually accompanied by perturbations in the regional and global carbon cycles by regulating atmospheric CO₂ levels through active or passive degassing processes and silicate weathering feedbacks. Biosphere forms an integral component of the carbon pool, and during massive volcanic eruptions, magmatic interaction with pre-existing OM within sedimentary layers can lead to the release of unprecedented volumes of CO₂ potentially capable of causing global climatic changes. Tracing changes in the carbon reservoir between pre- and post-volcanic sequences is therefore important to understand impact of the DT and the ecological stress that ensued after volcanism. Yet, direct impact of Deccan volcanism on the carbon cycle and ecosystem remains relatively unconstrained, and it still remains unclear how much CO₂ might have been liberated from thermal interactions with sedimentary OM. In this study, for the first time, multi-proxy investigation is carried out to trace changes in both marine and terrestrial carbon reservoirs and organic matter composition from pre- and inter-volcanic sedimentary trappean sequences (infra- and inter-trappeans, respectively). Rajahmundry, lying ~1500 km SE of the last basalt outcrops from the main volcanic province in Western India, is chosen as our study location. The basalt flows and trappean sequences are observed southwest of the city of Rajahmundry in basalt excavation quarries, marking exposures for the longest lava flow episodes in the world. Molecularlevel characterization of OM and stable isotope composition of carbonates (δ¹³3Ccarb and δ¹⁸Ocarb), bulk OM (δ¹³Corg), and n-alkane (δ¹³Calk and δDalk) have been analysed to provide a chemo-stratigraphic framework. Lipid compound classes (biomarkers) have been measured to constrain OM sources and identify traces of thermal alterations in hydrocarbons. Observations from biomarkers and stable isotopes has been combined with major and trace element data for providing better constrains on the paleo-environmental conditions. Atmospheric CO₂ concentrations have also been estimated from across the initial and main volcanic episode in the study area. In summary, this thesis provides a detailed overview of the various impacts of the Deccan volcanism on the environmental condition and the associated with numerous and complex feedbacks, and underscores the necessity for multi-proxy geochemical investigations from within the volcanic province to better constrain the role of the massive flood basalt eruption in relation to the biological and environmental crises. This study also provides the first evidence of deposition of thermally degraded OM in significant quantities, highlighting the potential for Deccan volcanism to have liberated higher amounts of CO₂ via metamorphic degassing. Findings from this work therefore has implications for future research on understanding the various mechanisms by which a large igneous province emplacement can drive climatic and ecological transitions.

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