End-Ordovician ostracod faunal dynamics in the Baltic Palaeobasin/Muutused Balti paleobasseini Hilis-Ordoviitsiumi ostrakoodikooslustes

• Published in: Estonian journal of earth sciences 2021-03, Vol.70 (1), p.51
• Main Authors: Truuver, Karin, Meidla, Tonu, Tinn, Oive
• Format: Article
• Language: English
• Subjects: Mass extinction theory
• ISSN: 1736-4728
• DOI: 10.3176/earth.2021.02

The Late Ordovician Baltic Palaeobasin (BPB) offered a favourable environment for a diverse and abundant ostracod fauna to thrive across the basin. A short period of unstable environmental conditions at the end of the Ordovician, the Hirnantian glaciation, and the concurrent extinction event completely rearranged the ostracod associations. Statistical analyses reveal temporal (diverse pre-glacial, more uniform glacial and poorly diverse post-glaciation faunas) and geographical (upper shelf and middle shelf faunas) separation of ostracod associations in the BPB. The geographical division applies to the pre-glacial and glacial faunas only; the scanty post-glacial fauna found in deeper-water sections is uniform. Ostracod associations are distinct both in a temporal and geographical sense. Juxtaposing ostracod data with [[delta].sup.13]C curves shows that the typical Hirnantian Harpabollia harparum fauna in the BPB appeared near the peak of a [[delta].sup.13]C excursion. This might explain why the H. harparum fauna seems to appear first in deeper-water sections and is absent or appears later in shallower-water sections. The majority of the nearshore sections in Estonian and Lithuanian shelves lack most or all of the Hirnantian strata. The lowermost Hirnantian is preserved in only a few sections (e.g. Männamaa and Puhmu in Estonia). The appearance of the post-glacial ostracod fauna, which most probably was a survival fauna rather than a recovery fauna, is related to the falling limb of the [[delta].sup.13]C curve. The appearance of this fauna was previously considered as a marker of the Ordovician-Silurian boundary in the region but, according to the present understanding, took place in the late Hirnantian. Hilis-Ordoviitsiumi-aegne Balti paleobassein (BPB) oli rikkalike ja mitmekesiste ostrakoodikoosluste arenguks soodne keskkond. Ajastu lõpul toimunud järsk kliimamuutus, luhiajaline Hirnanti jääaeg ja sellega samaaegselt aset leidnud globaalne väljasuremissundmus tõid kaasa suured muutused ka siinses ostrakoodifaunas. Levikuandmete statistiline analuus näitab, et Balti paleobasseini ostrakoodifaunas võib eristada mitmekesiseid jäätumiseelseid, liigivaesemaid jäätumisaegseid kooslusi ja tugevasti vaesunud jäätumisjärgset ostrakoodikooslust. Geograafiliselt eristuvad selfi ulemise ja keskmise osa kooslus, kuid selline jaotus kehtib vaid jäätumiseelsete ning -aegsete koosluste kohta. Jälgi jäätumisjärgsest kooslusest leiti vaid uksikutest avaselfi läbilõigetest.