Did a Katian Large Igneous Province trigger the Late Ordovician glaciation ? A hypothesis tested with a carbon cycle model

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 !rst of the ‘big !ve’ mass extinctions of marine
life. The cause of this cooling event remains unclear. Several authors correlate it with a drop in atmospheric
pCO2 to a threshold permitting the ice accumulation on the Gondwana supercontinent. However, what are
the reasons of the atmospheric pCO2 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 biodiversi!cation (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 pCO2. 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 suf!cient consumption of atmospheric pCO2 to the
threshold proposed by Herrmann et al. (2004 a, b) to initiate a glaciation on Gondwana.