A history of glaciations: the perks of combining recent and ancient morphostratigraphic archives
Résumé
It is undoubtedly the observation of modern European Alps glaciers along with their erosive, transportational and depositional actions shaping the landscapes that first led scientists to reveal the existence of a ‘past ice age’ during which glaciers not so long ago covered northern Europe and North America. Based on similar observations, evidence for much more ancient (Permian, ca. 300 Ma) glaciers were simultaneously discovered in Wales. Since then, others glacial episodes punctuating the Earth history were successively discovered (Eyles, 2008), the oldest of which being the ‘Barberton diamictites of South Africa, dated back to 3.5 Ga (deWit & Furnes, 2016).
The modern glaciation may serve as the basis to decipher past ice ages and associated climate dynamics that remain obscure as growing evidences indicate that these ancient glacial epochs share similarities, but also discrepancies, with the Cenozoic one, in term of tempos (ice ages encompassing periods of contraction-dilatation of ice) or forcing parameters (e.g., Ghienne et al., 2014; Kochhann et al., 2021; Montanez, 2021). Past ice dynamics may therefore be unraveled by the integration in time and space of punctual glacial processes whose interpretation is made on the basis of their modern and recent equivalent. The glacio-isostatic adjustment, for example, is a process well-known for the ultimate glacial cycle, as marked by widespread evidences such as the raised beaches of Scandinavia and Canada. Given its time span of completion however (a few tens of thousands of years), this process is hardly decipherable for ancient epochs, for which temporal resolution is intrinsically too low, therefore hindering our ability to constrain ancient ice-sheet dynamics. A stratigraphic model built upon recent glacial strata has been successfully extrapolated to both the Ordovician and Carboniferous-Permian ice ages, providing clues about pattern of glacial retreat of postglacial relative sea level changes. Similarly, the geomorphic and stratigraphic imprints of fjords nowadays dissecting high-latitude continental margins were used as an analog that permitted the characterization of fossil fjords and associated glacial dynamics tied to the Carboniferous-Permian glaciation in Namibia (Dietrich et al., 2021). On the other hand, strata related to ancient ice ages may provide novel insights into the understanding of modern glacial processes that remain obscure by granting access to sectors otherwise ‘locked’ such as the ice-substrate interface or sediments nowadays on continental margins, covered in great water depths and buried under younger sediments. Finally, the window into deep and long times offered by sedimentary basins hosting deposits tied to ancient glacial epochs may provide clues on the impact of repeated or long-lasting glaciation on the earth surface evolution (Jaeger & Koppes, 2016). The presentation will briefly review how mutual benefits can be obtained from combining the investigation of ancient and recent glacial deposits (Dowdeswell et al., 2019).
