20.05.2021 - 12:15
On 20 May at 12:15 Karin Truuver will defend her doctoral thesis “Ostracod associations of the Ordovician-Silurian boundary interval in Baltoscandia” (“Ordoviitsiumi ja Siluri piirikihtide ostrakoodid”) for obtaining the degree of Doctor of Philosophy (in Geology).
Professor Tõnu Meidla, University of Tartu
Associate Professor Oive Tinn, University of Tartu
Science Director Jean Vannier, Claude Bernard University Lyon 1 (France)
Ostracods are tiny (0.5–2 mm long) crustaceans originating from the Lower Ordovician. Due to the strong interconnection between the ostracod associations and the sedimentary environments, their fossils can be used in palaeoenvironmental reconstructions and in biostratigraphic correlations.
The Ordovician Period ended with the prominent Hirnantian glaciation and mass-extinction event, which had severe impact on ostracods. This study focussed on the composition and evolution of ostracod associations in the latest Ordovician Baltic Palaeobasin (BPB), immediately before, during and after the end-Ordovician glaciation. During the crisis once so rich ostracod associations in the BPB changed completely. The inner shelf association was replaced by an endemic low-diversity Medianella aequa ostracod association. The deeper-water ostracod association was replaced by a new, cold-water Harpabollia harparum ostracod association, a part of the cosmopolitan Hirnantia fauna sensu lato – the specific fauna that flourished during the glacial period. Post-glacial BPB was inhabited by extremely low-diversity survival fauna consisting of one or two most resilient ostracod species.
Carbon stable isotopic (δ13C - ratio of 13C/12C given in parts per thousand) analysis was included in the study to clarify the distribution of ostracod associations in the BPB. The δ13C of marine dissolved inorganic carbon is associated with carbon circulation in ocean water. A positive excursion of δ13C reflects changes in carbon circulation in ocean that cause the drawdown of atmospheric CO2 creating favourable conditions for onset of glaciation. The fast rise and following gradual decline of the uppermost Ordovician δ13C values reflect the development of the glaciation and the respective fluctuation of the sea level during the Hirnantian glaciation.
The causal connection between the glaciation and extinction events is not as clear as it might seem. Ostracod diversity started to drop before the rise of the δ13C values in most of the studied sections which is in agreement with a previous study suggesting that the Hirnantian extinction probably commenced already somewhat before the glaciation event. The very low diversity of post-glacial ostracod association suggest that this association was most likely the latest Ordovician survival fauna, not the basal Silurian recovery fauna, as traditionally considered. The results of this study confirm the need for repositioning the Ordovician–Silurian boundary upwards in the Baltoscandian area.
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