Geosciences Division Seminar
Title : Nitrogen loss processes in aquatic ecosystems
Date : 05-08-2025
Time : 16:00:00
Speaker : Dr. KM Ajayeta Rathi
Area : Geosciences Division
Venue : Ground Floor Lecture Hall
Abstract
Nitrogen, though abundant in the atmosphere, often limits primary productivity in aquatic ecosystems. These ecosystems play a crucial role in regulating the global nitrogen cycle both by converting inert atmospheric nitrogen into bioavailable forms through dinitrogen fixation and by removing excess reactive nitrogen via microbially mediated loss processes, including denitrification, anaerobic ammonium oxidation (anammox), and, to some extent, dissimilatory nitrate reduction to ammonium (DNRA). However, with increasing anthropogenic nitrogen inputs and climate-driven environmental changes, the efficiency and dominance of these pathways are shifting, with significant implications for ecosystem health, biogeochemical feedbacks, and nitrogen budgets. In this seminar, I will discuss the mechanisms, environmental controls, and experimental approaches used to study nitrogen loss processes in aquatic ecosystems.
Title : Evolution of the Laxmi Basin, Arabian Sea, during the Deccan volcanism
Date : 12-08-2025
Time : 16:00:00
Speaker : Dr. Sibin Sebastian
Area : Geosciences Division
Venue : Ground Floor Lecture Hall
Abstract
The Laxmi Basin is a prominent geomorphic feature and a marginal depression in the Northwest Indian Ocean (Arabian Sea). This ~300 km wide basin separates the western Indian continental margin from the Laxmi Ridge (LR), which is believed to be continental. The precise nature of the basin's basement remains controversial, with differing views suggesting it could be either a stretched continental crust with magmatic intrusions related to continental rifting (contemporaneous with Deccan volcanism) or a pre-Paleogene oceanic crust. Additionally, a geochemical study of the basin's igneous basement indicates that these rocks formed in a subduction zone setting. Understanding the crust's nature has implications for the geodynamic events of continental breakup and the formation of the Indian Ocean during the Late Cretaceous. To address this controversy, we conducted geochemical and isotopic studies on basaltic lava samples from this basement, recovered during IODP 355. In this talk, I will present the results of our study and our inferences regarding the crustal nature of the Laxmi Basin.
Title : Uranium in the ocean: Inferences on bottom water anoxia during the Last Glacial Period
Date : 19-08-2025
Time : 16:00:00
Speaker : Prof. Manmohan Sarin
Area : Geosciences Division
Venue : Ground Floor Lecture Hall
Abstract
The paleo-oceanographic studies suggest that deep ocean was depleted in dissolved oxygen (O2) during the last glacial period (LGP, ~18 kyr BP). Therefore, paleoceanographers have sought a direct tracer for studying the changes in dissolved O2 in the deep ocean during the LGP. Geochemical proxies like redox-sensitive trace elements (Vanadium, Molybdenum, Uranium, Manganese) in bulk sediments have been used to reconstruct the past bottom water environment. Example, in oxic seawater, uranium is present in relatively high concentration as uranyl-carbonate complexes which are highly soluble. Under O2 deficient (anoxic) conditions, uranium can be reduced from U(VI) to a less soluble tetravalent state U(IV), which is particle reactive and gets fixed into the sediments. A geochemical study validating this concept was published in 1993 (‘Geochemical evidence for anoxic deep water in the Arabian Sea during the last glaciation’; Sarkar, Bhattacharya & Sarin; Geochimica et Cosmochimica Acta). The purpose of this seminar is to discuss two recent articles (Geochemical Perspectives Letters 2024 and Marine Geology 2025) using the very same concept/approach published more than three decades ago (PRL study).