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Doctoral defence: Bilal Gul „Palaeotemperature reconstruction based on oxygen stable isotopic trends from the Ordovician-Silurian brachiopods of Baltoscandia“
On 29 November at 12.15 Bilal Gul will defend his doctoral thesis „Palaeotemperature reconstruction based on oxygen stable isotopic trends from the Ordovician-Silurian brachiopods of Baltoscandia“ for obtaining the degree of Doctor of Philosophy (in Geology).
Supervisors:
Professor Tõnu Meidla, University of Tartu
Professor Leho Ainsaar, University of Tartu
Opponent:
Professor Oliver Lehnert, Friedrich-Alexander-Universität Erlangen-Nürnberg (Germany)
Summary
Over the last two decades, extensive research has focused on climate fluctuations during the Ordovician-Silurian period. Baltoscandia is considered an ideal region for such studies due to well-preserved Palaeozoic sediments and minimal post-depositional alteration. The current study is focusing on the environmental and climatic changes reflected in the Ordovician-Silurian Baltoscandian Palaeobasin, using the stable oxygen isotopic data from the brachiopod shell calcite as a proxy for paleoclimate, providing a comprehensive record of palaeotemperature trends in this region. Despite certain limitations in absolute temperature precision, the results show that brachiopod isotope data can be used for interpretation of climatic trends. The study identifies that more significant global δ¹³C excursions in the Ordovician-Silurian periods, including the Mid-Darriwilian Carbon Isotope Excursion (MDICE), the Hirnantian Isotope Carbon Excursion (HICE) and Ireviken Event, are also reflected in the brachiopod shell composition. The oxygen isotope data from brachiopods shows an overall trend of cooling from the Early to Middle Ordovician, with a significant temperature decrease of about 8 °C. This cooling likely reflects global climate trends rather than local environmental disturbances. Following a relatively warm late Katian age, a significant global cooling occurred during the Hirnantian glaciation, with a subsequent rise of temperatures during the early Silurian. The study also highlights that, despite the plate tectonic drift of Baltica towards tropical latitudes, the regional palaeoenvironmental changes were primarily influenced by the global climatic shifts towards cooler climate. The δ¹⁸O values from brachiopods of the Baltoscandian region are consistent with those from other continents. Brachiopods are therefore considered a reliable proxy for understanding palaeoclimate during the Ordovician-Silurian periods.