Late Ordovician carbon isotope trend in Estonia, its significance in stratigraphy and environmental analysis

Carbon isotope changes during most of Late Ordovician time (from the mid-Caradoc Kinnekulle K-bentonite until the beginning of the Silurian) were investigated. As the corresponding sequence of rocks is stratigraphically nearly complete in Estonia, an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the Late Ordovician. Complications were caused by several local or regional hiatuses in the middle and late Caradoc and Hirnantian. A total of 385 whole rock samples were studied from eight drill cores in northern and central Estonia. The following positive carbon isotope events were observed: (1) the mid-Caradoc excursion (peak δ¹³C value 2.2‰) in the uppermost part of the Keila Stage, also known in Sweden; (2) the first late Caradoc excursion (1.9‰) in the lower part of the Rakvere Stage; (3) the second late Caradoc excursion (2.4‰) in the upper part of the Nabala Stage; (4) the early Ashgill excursion (2.5‰) in the lowermost part of the Pirgu Stage; (5) the widely known large Hirnantian excursion (in Estonia the peak value reaches 6.7‰) in the Porkuni Stage. The study interval comprises a long (10 Ma) period characterized by low-magnitude carbon isotope changes and a following brief (2 Ma) interval with large changes. No obvious lithological preference for hosting the positive shifts was recorded. In principle, the δ¹³C values exceeding the background values may occur in all types of rocks present in a sedimentary basin. Several δ¹³C positive excursions (values 1.5‰ to 3‰) in the Mohawkian of North America are evidence that the minor Caradoc and early Ashgill δ¹³C positive shifts in Baltoscandia may have counterparts in Laurentia. If correctly correlated, these shifts may have global significance. The Hirnantian excursion is usually linked to a major glacial event, even if some carbon cycling mechanisms are not completely understood. The environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level. Our study supports the primary role of climatic or climatically triggered oceanic processes.     

Pattern of some Silurian bioevents

The best‐known Silurian bioevent occurred at the end of the Wenlock: the lundgreni event, together with the nassa‐ludensis crisis, was established among planktic graptolites. The East Baltic data show several peaks of high diversity (especially triangulatus, turriculatus, scanicus zones) and three levels of low diversity of graptolites (antennularius, radians, ludensis zones). These are more or less coincident with sea‐level changes. Energetic innovation of the shallow shelf corals started in the early Silurian. The late Wenlock regression seems not to affect them seriously but the late Silurian decline coincides with the aridization of the climate and a regression of the shelf seas. The Agnatha had strong radiations in the Wenlock (ludensis Zone) and Ludlow (leintwardinensis Zone), the fishes in the Pridoli. Many Silurian vertebrates were long‐ranging and extinction rate was relatively low; only at the late leintwardinensis level and in the latest Ludlow did considerable extinctions occur.