Did a Katian large igneous province trigger the Late Ordovician glaciation?: A hypothesis tested with a carbon cycle model

Abstract

The Ordovician is known as a period with high temperatures and high sea levels and a cooling event at the end of the period, leading to the Hirnantian glaciation and the first of the ‘big five’ mass extinctions of marine life. The cause of this cooling event remains unclear. Several authors correlate it with a drop in atmospheric pCO 2 to a threshold permitting the ice accumulation on the Gondwana supercontinent. However, what are the reasons of the atmospheric pCO 2 decrease? Here, we follow the hypothesis initiated by Barnes (2004) that an Ordovician superplume event occurred during the Middle to Late Ordovician. Such an event would not only have a large impact on the Ordovician biodiversification (Barnes, 2004) but it would also be responsible for the climatic upheaval during the Late Ordovician by the emplacement of a low latitudinal continental basaltic province that had an impact on the atmospheric pCO 2. There is no direct evidence of a superplume event or of basaltic traps and the present study is therefore a hypothetical modelling approach where we demonstrate with a numerical box model, including carbon, alkalinity and phosphorus cycles coupled with a 1D climate model (energy balance model-EBM) (François and Walker,1992), that such a scenario allows to explain both the mid-Ashgill (Katian) global warming event, known as the Boda Event (Fortey and Cocks, 2005), and the subsequent Late Ordovician (Hirnantian) glaciation. Because silicate weathering is enhanced upon warm and wet climate, we try to constrain the size and the latitudinal emplacement of the basaltic province leading to a sufficient consumption of atmospheric pCO 2 to the threshold proposed by Herrmann et al. (2004 a, b) to initiate a glaciation on Gondwana.