Response of the Inter-tropical Convergence Zone to Southern Hemisphere cooling during Upper Ordovician glaciation

Abstract

Stable isotope ratios of whole rock carbonates and faunas from three low latitude Upper Ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts. We suggest that tropical water beneath the Inter-tropical Convergence Zone (δ 18O = − 10‰ to − 15‰) was isotopically light reflecting high sea-surface temperatures (SST) and reduced sea-surface salinity (SSS). Subtropical water in both the northern and southern hemispheres is characterised by δ 18O values from − 4 to − 6‰. Our interpretation is consistent with the published results from physical–chemical modeling of Ordovician seawater composition and climate models. Deriving absolute temperature and salinity values from Ordovician δ 18O carb data is problematic as they could reflect SST, SSS, ice volume and potentially a variable diagenetic overprint, and currently no independent constraints exist for any of these variables. The ITCZ was positioned at ~− 10°N of the Equator during the early Katian consistent with the likely position of the region of peak sea-surface temperature (SST). During the late Katian Boda Event the ITCZ moved across the Equator to ~− 10°S. By analogy with the Pliocene this resulted in a symmetrical distribution of sea-surface currents about the Equator. This led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients, potentially explaining the migration of warm water faunas to higher southern latitudes during the Boda Event. During the Hirnantian the ITCZ shifted north of the Equator, resulting in a reduced poleward heat transport. ITCZ position has been proposed as a likely forcing mechanism of Neogene glaciation and the evidence from the Ordovician suggests a discrete palaeo-ITCZ whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and Hadley Cell dynamics. We hypothesize that the position of the ITCZ was sensitive to the growing Hirnantian ice sheet during the Late Ordovician. We propose that latitudinal movement of the ITCZ during the Late Ordovician resulted in a major atmosphere–ocean circulation re-organization. Higher pCO 2 estimates for the Ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction. If the monsoon is considered an off-equator ITCZ then we provide direct evidence of an Ordovician monsoonal climate.