Gondwana platform paleoenvironment reconstruction: Insights from Ordovician black-grey shales in NW Iberia

Gondwana developed marine platforms at its northern edge with Paleozoic deposits reflecting varied paleoclimatic conditions. In the northwestern (NW) Iberian Peninsula (NW Gondwana), Ordovician black-grey shales and sandstones were deposited in these platforms at southern polar latitudes. The current research contributes to a better understanding of the recycling, climate, and redox conditions during the deposition of black-grey shales in the low/mid-Ordovician period. During the Lower-Middle Ordovician period, the black-grey shales recorded an increase in recycling (Th/Sc, Zr/Sc, Zr/Ti, La/Th), low level of chemical alteration (CIA, Th/U) and relatively low oxygen conditions (Ce/Ce*, Y/Ho). These data indicate arid-cold conditions with a seasonal glacial-periglacial environment, consistent with the location of the NW Iberian Peninsula at low latitudes close to the South Pole. Towards the Middle Ordovician, the black-grey shales recorded a more temperate climate with higher levels of chemical alteration, less recycling, and a relatively more oxygenated environment, what suggests a transition to a moderate climate with no glacial events developed. This climate evolution could have been promoted by the progressive Gondwana drift away from the south pole aided by CO ₂ input to the atmosphere from alkaline mafic intraplate volcanism linked with a previous felsic LIP event in this area. The rapid decline of this volcanism and the partial alteration of its products (fixing CO ₂ ) could have contributed to the development of the Hirnantian glacial conditions during the Upper Ordovician, which brings a new view of the secular climatic evolution of the Earth during the Ordovician.