Response of the Intertropical Convergence Zone to Southern Hemisphere cooling during Ordovician glaciation

Difficulties in quantifying environmental conditions (particularly pCO2 and palaeobathymetry) and oversimplification in parameterization are major limitations on what deep time climate models can tell us about the dynamic behaviour of the Earth's climate. Research is still focussed at understanding climate proxy data and the best climate proxy datasets available for the Early Palaeozoic are those from palaeo-tropical latitudes. At the present day the tropics are the main source of the atmosphere's heat and water vapour; changes in tropical dynamics have a major influence on the Earth's climate. On the global scale, the position of the ITCZ affects pole-ward heat transport and latitudinal temperature gradients and is a critical feedback mechanism, sensitive to ice volume changes, during glaciation. Ocean water beneath the ITCZ is warm, has reduced salinity and is characterised by light δ18O(carb). Stable isotope records from low latitude Upper Ordovician limestone (from Nevada, Arctic Canada and Estonia) are interpreted as showing a pattern of predicted and coincident shifts in the position of subtropical and tropical water masses associated with the developing Hirnantian glaciation. We hypothesize that these changes reflect a shift in the position of the ITCZ that tracked the pattern of climate change during the Late Ordovician. We argue for a discrete palaeo-ITCZ, the location of which was controlled by ice volume changes. The repositioning of the ITCZ resulted in climatic belt re-organization resulting in complex feedbacks into the ocean-climate system that were a likely contributor to the Late Ordovician mass extinction.