Post-Hunnebergian Ordovician carbon isotope trend in Baltoscandia, its environmental implications and some similarities with that of Nevada
Abstract Carbon isotope changes during post-Hunnebergian Ordovician time were investigated. As the corresponding sequence of rocks is stratigraphically nearly complete in Baltoscandia, an attempt was made to use it to elaborate the general pattern of carbon isotope changes in most of the Ordovician. Complications caused by several local hiatuses, condensed sections and facies changes were mitigated by the study of overlapping sections. A total of 326 new whole-rock analyses and published data from 18 drill cores and 4 outcrops in Estonia, Latvia and Sweden were considered. The following positive carbon isotope events were observed: (1) the mid-Darriwilian excursion (peak δ13C value 1.9‰) in the Aseri Stage; (2) the mid-Caradoc excursion (2.2‰) in the uppermost part of the Keila Stage; (3) the first late Caradoc excursion (2.3‰) in the lower part of the Rakvere Stage; (4) the second late Caradoc excursion (2.4‰) in the upper part of the Nabala Stage; (5) the early Ashgill excursion (2.5‰) in the lowermost part of the Pirgu Stage; (6) the mid-Ashgill excursion (2.0‰) in the upper part of the Pirgu Stage; (7) the widely known large Hirnantian excursion (in Estonia the peak value reaches 6.7‰) in the Porkuni Stage. The study interval comprises a long (∼ 19 Ma) period characterized by low-magnitude carbon isotope changes and a following much briefer (∼ 12 Ma) interval with often medium-scale shifts and a major event at the end. The latter interval reflects onset in the mid-Caradoc of a period of variable climate (alternation of arid and humid episodes), which ended with a serious cooling and glaciation. The Baltoscandian and North American (Nevada) δ13C trends are in general lines very similar, providing good prospects for global chemostratigraphical correlation and understanding of role of oceanic processes in carbon cycling.