Rhabdosphaera clavigera (calcareous nannofossils) as paleotemperature indicators: a comparison with isotopic data

Abstract

The genera Nuculites and Cucullella have originated in different geographic areas, with the oldest record found in the Llandovery (Lower Silurian). The former was reported in Gondwana with two subgenera, Nuculites and Trilobonuculites, while the latter is found in Avalonia. Due to environmental and tectonic factors, those taxa became associated and distributed throughout Gondwana, Avalonia, Kazakhistan and Laurussia from the Late Silurian to the Late Devonian. The subgenus Trilobonuculites appeared firstly at the Pitinga Formation, Amazon Basin, Brazil, and the subgenus Nuculites is only recorded in the Silurian from the Tabuk Formation, Saudi Arabia. Cucullella was found in the Ross Brook Formation of New Scotia, Canada. Since the Late Silurian (Ludlow), which is marked by a marine transgression, Trilobonuculites and Cucullella started to occur associated, invading other paleocontinents, confirmed by the presence of Cucullella in the Gondwana (Bolivia) and Trilobonuculites in Avalonia (New Scotia). This suggests that the environment in these areas became more uniform due to the rise of the sea level and the narrowing of the ocean Rheic. The Silurian-Devonian boundary is marked by a marine regression followed by a new transgression, with maximum in the Frasian, and the collision of Avalonia/Baltica and Laurentia forming Laurussia. At this point, the subgenus Nuculites reappeared. Still in the Lower Devonian (Pragian-Emsian), the genus Nuculites diversified, dominating by the subgenus Trilobonuculites, while Cucullella was only represented by a sole species from Kazakhistan. In the Middle Devonian (Eifelian-Givetian) the subgenus Nuculites predominated while Trilobonuculites was restricted to the Maecuru Formation (Brazil). The only species of Cucullella dispersed to the oriental Laurussia and North and Occidental Gondwana. In the Late Devonian (Frasnian), the occurrence of these taxa was restricted to the Armorican Massif, Northeastern of Brazil and some doubtful occurrences in Bolivia. At that time, the oceans deepened, dysaerobic and anoxic environments spread out and Gondwana and Laurussia were approaching. These factors, associated with thermal stress, led to extinction of several taxa at the Frasnian/Fammenian boundary, including those studied here. The paleogeographic distribution of Nuculites and Cucullella agrees with the sea level curves and the movement of the continents suggested to have taken place during the Silurian and Devonian.-(