A bio- and chemostratigraphic search for the Mid-Ludfordian Carbon Isotope Excursion interval in the Ludlow of the Ohesaare core, Estonia/Kesk-Ludfordi süsiniku isotoopanomaalia taseme otsing Ohesaare puursüdamikus bio- ja kemostratigraafiliste tunnuste alusel

• Published in: Estonian journal of earth sciences 2022-03, Vol.71 (1), p.44
• Main Authors: Kaljo, Dimitri, Martma, Tonu, Marss, Tiiu, Nestor, Viiu-Kai, Viira, Viive
• Format: Article
• Language: English
• ISSN: 1736-4728
• DOI: 10.3176/earth.2022.04

For years, the Ohesaare core section and its rich fossil assemblages have enticed researchers to suggest various ideas about Silurian stratigraphy in the East Baltic despite several sedimentary gaps occurring through the Ludlow interval in particular. One of the gaps has removed from the Ohesaare record the most important event in the Palaeozoic history of carbon isotopes--the Mid-Ludfordian Carbon Isotope Excursion (MLCIE), which is partly accompanied by the Lau biotic and oceanic events. In our research, we have performed a detailed distribution analysis of chitinozoans, conodonts, ostracodes and vertebrate microremains with the aim of documenting the evidence regarding the levels of certain environmental events largely preceding the MLCIE. For this purpose, the Torgu Formation was subdivided into five working units. The first results indicate a gap within the uppermost part of the Torgu Formation as a possible level for the missing MLCIE. In order to verify this, we compared the fossil distribution pattern in Ohesaare with that on Gotland (Sweden) and in Kurzeme (Latvia), including [[delta].sup.13]C data from the Uddvide (Gotland) and Ventspils (Latvia) core sections. Both Baltic and Bohemian data show rather unanimously that the MLCIE is located below the Ozarkodina crispa conodont Biozone. However, some reports from Gotland show peak values also in higher strata; occasional records of Oz. crispa from other places may cast some doubt on these findings and raise some ecostratigraphical concerns when discussing upper Ludfordian correlation. Key words: microfossil, carbon isotope, Ludlow, Ohesaare core, Baltica. Ohesaare puursüdamiku läbilõige ja kivististe kooslused on aastaid inspireerinud uurijaid uute ideede arendamisel Siluri stratigraafia alal. Läbilõige koosneb valdavalt madalmere kivimitest tavapäraselt sagedate settelünkadega. Nendest üks langeb kokku Kesk-Ludfordi süsiniku isotoopanomaalia (ingl. akronüüm MLCIE) tasemega, mistõttu ei olnud võimalik selle asukohta analüütiliselt määrata. MLCIE kajastab Paleosoikumi suurimat isotoopgeoloogilist sündmust, millel on suur kasutusväärtus kronostratigraafias ja erinevate paleokeskkonda mõjutavate protsesside (näiteks Lau okeanoloogiline sündmus) uurimisel. Nii sai meie artikli ülesandeks leida nelja fossiilirühma (kitiinikud, konodondid, ostrakoodid ja vertebraadid) biostratigraafilise leviku ja biomitmekesisuse analüüsi abil MLCIE asukoht Ohesaare läbilõikes. Vajaliku detailsuse tagamiseks jagasime Pa