Early Palaeozoic cooling events: peri-Gondwana and beyond

Abstract

The short-lived end-Ordovician Hirnantian glaciation allied to marine mass extinction is variously considered as a short-lived event or as the peak of long-drawn-out climatic cooling through at least late Ordovician–early Silurian times. Evidence from Early Palaeozoic facies, faunas and stable isotope excursions used to interpret climatic cooling events ranges farther, from late Mid-Cambrian to late Silurian times. Glacigenic sediments, structures and geomorphology provide direct evidence of glacial episodes. Cool-water carbonate deposition, which is particularly widespread during the late Ordovician Boda event in high-latitude peri-Gondwana–Gondwana, and beyond into mid–low palaeolatitudes, is interpreted as indicating global cooling, not warming as has been proposed. Such carbonates also characterize mid-latitude continents widely at horizons earlier in the Ordovician, and more locally in the mid-Silurian in high-latitude Gondwana. Cool-water carbonate mounds have distinctive facies-controlled mound faunas across palaeocontinents. Other facies evidence for palaeoclimates includes black shale deposition, including deglacial organic-rich ‘hot shales’, which indicate transgression in epeiric seas, and sea-level curves interpreted from facies and faunal successions. Correlation is shown between facies evidence and positive C isotope excursions, from which cyclicities are apparent. The possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography, and changes in global sea level and in pCO2. Facies and faunal evidence from peri-Gondwanan terranes (Armorica, Central Europe, Alborz) is assessed with that from Gondwana (mostly North Africa, South America) and correlatives in Avalonia, Baltica and Laurentia to establish a wider picture of early Palaeozoic cooling events.

An in situ shelly fauna from the lower Paleozoic Zapla diamictite of northwestern Argentina: Implications for the age of glacial events across Gondwana

“…This age has been uncritically accepted in some global-scale Early Paleozoic paleoclimatic discussions (i.e. Brand et al, 2006;Eyles, 2008;Cherns and Wheeley, 2009;Lehnert et al, 2010;Loi et al, 2010). However, an increasing amount of micro and macropaleontological evidence, including the fauna described in the present work, tends to constrain the main glacial advance in the Central Andean Basin essentially to the Hirnantian in accordance with the timing of the glaciation peak in northern Gondwana and other regions of the world.…”

Chapter 10 Biogeography of Ordovician linguliform and craniiform brachiopods

“…Ephippelasma, Rhysotreta, Spinilingula, Undiferina) spread to Baltica and South China. A possible explanation is that this is related to the suggested cooling episode during the early to mid Darriwilian (Ghobadi Pour et al 2007;Cherns & Wheeley 2009), which is most evident from the regions of temperate-latitude peri-Gondwana, within the so-called 'overlap zone' (see also Fortey & Cocks 2003). The subsequent warming, which probably occurred synchronously, as indicated by the mid-Darriwilian carbon isotope excursion (Bergström et al 2008), resulted in a southern shift of the climatic belts and the migration of some characteristic taxa from the tropics to higher latitudes (Ghobadi Pour & Popov 2009).…”

Could land‐based early photosynthesizing ecosystems have bioengineered the planet in mid‐Palaeozoic times?

“…). Based on a variety of facies indicators, including black shale horizons, sea‐level curves and carbon stable isotope records, Cherns and Wheeley () interpreted multiple climatic cooling events from Late Cambrian through to late Silurian. Testing of the periodicity of these events against long‐term eccentricity predictions indicated that orbital forcing of climate was probable (Cherns et al .…”