Ostracod associations of the Ordovician–Silurian boundary interval in Baltoscandia
Kuupäev
2021-04-16
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Ajakirja pealkiri
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Abstrakt
Ostrakoodid on Alam-Ordoviitsiumist pärinevad pisikesed, ~0.5–2 mm pikkused koorikloomad. Kuna nad on tugevasti seotud settekeskkonnaga, on nende fossiilid heaks abivahendiks paleokeskkondade rekonstrueerimisel, kuid siiski kasulikud ka biostratigraafilisetes korrelatsioonides.
Ordoviitsiumi ajastu lõpus toimus suur Hirnanti jääaeg ja leidis aset ka samaaegne väljasuremissündmus, mis avaldas ostrakoodidele väga tugevat mõju. Käesoleva doktoritöö eesmärgiks oli uurida Balti Paleobasseini (BPB) ostrakoodikoosluste taksonoomilist koosseisu ja muutusi Hilis-Ordoviitsiumis, vahetult enne Hirnanti jääaega, selle kestel ning peale selle lõppu. Ökoloogiline kriis tõi BPB ostrakoodikooslustes kaasa suured muutused. Basseini madalaveelist osa asustanud kooslus asendus endeemse, küllaltki liigivaese Medianella aequa ostrakoodikooslusega. Basseini sügavama osa ostrakoodikoosluse vahetas välja uus, külmaveeline Harpabollia harparum’i ostrakoodikooslus, mida peetakse laiemas mõttes kosmopoliitse Hirnantia fauna – spetsiifilise, Hirnanti jääajale iseloomuliku koosluse – osaks. Jäätumisjärgsel ajal BPB-i asustanud ostrakoodikooslus oli äärmiselt liigivaene, koosnedes vaid mõnest kõige kõrgema taluvusega ostrakoodiliigist.
BPB-i ostrakoodikoosluste leviku ajaliste raamide täpsustamiseks kasutati käesolevas doktoritöös ka stabiilse süsiniku isotoopanalüüsi (δ13C ehk 13C/12C suhe, mida väljendatakse promillides). Merevees lahustunud anorgaanilise süsiniku δ13C kajastab muutusi mere süsinikuringluses. δ13C väärtuste tõus viitab muutustele, mis on tingitud CO2 taseme langusest atmosfääris. Sellised muutused loovad soodsad tingimused liustike arenguks. Kiire δ13C väärtuste tõus ja nende järk-järguline langus Hilis-Ordoviitsiumis peegeldab otseselt jäätumise arengut ja vastavaid meretaseme kõikumisi Hirnanti jääajal.
Hirnanti jääaja ja väljasuremissündmuse vahel ei ole nii selget põhjuslikku seost, kui esmapilgul tundub. Enamikus käesoleva töö raames uuritud läbilõigetes hakkasid ostrakoodikooslused vaesustuma juba enne δ13C väärtuste järsku tõusu. See tähelepanek on kooskõlas varasema uurimusega, mille kohaselt Hirnanti massväljasuremine algas tõenäoliselt juba veidi enne jäätumise algust. Jäätumisjärgse ostrakoodikoosluse ülim liigivaesus viitab sellele, et tõenäoliselt on see pigem Hilis-Ordoviitsiumi nn. jäänukkooslus kui Vara-Siluri taastumiskooslus, nagu seni arvatud.
Töö tulemused kinnitavad vajadust nihutada Ordoviitsiumi-Siluri piiri Baltoskandia piirkonnas ülespoole.
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.
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.
Kirjeldus
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Märksõnad
ostracodes, Late Ordovician, Silurian, biostratigraphy, Baltoscandia