Abstract
Evidence based on molecular clocks, together with molecular evidence/biomarkers and putative body fossils, points to major evolutionary events prior to and during the intense Cryogenian and Ediacaran glaciations. The glaciations themselves were of global extent. Sedimentological evidence, including hummocky cross-stratification (representing ice-free seas affected by intra-glacial storms), dropstone textures, microbial mat-bearing ironstones, ladderback ripples, and wave ripples, militates against a “hard” Snowball Earth event. Each piece of sedimentological evidence potentially allows insight into the shape and location, with respect to the shoreline, of ice-free areas (“oases”) that may be viewed as potential refugia. The location of such oases must be seen in the context of global paleogeography, and it is emphasized that continental reconstructions at 600 Ma (about 35 millions years after the “Marinoan” ice age) are non-unique solutions. Specifically, whether continents such as greater India, Australia/East Antarctica, Kalahari, South and North China, and Siberia, were welded to a southern supercontinent or not, has implications for island speciation, faunal exchange, and the development of endemism.
Highlights
This paper reviews the style and likely location of ice-free “oases” that existed during severeNeoproterozoic glaciations, or Snowball Earth events [1,2]
At about the same time, McKay calculated that light can be transmitted through ice up to 30 m thick, and arctic lakes with ice 5 m thick harbor microbial life [11]. These findings provided a mechanism for life to continue under a “hard” Snowball Earth, with a realization that ice must still have been comparatively thin in places for photosynthesis to continue
Possible insight into the response of late Neoproterozoic ecosystems to glaciation might be provided by the analog of the Hirnantian glaciation of North Africa, which represents the earliest expansion of major ice sheets in the Phanerozoic
Summary
This paper reviews the style and likely location of ice-free “oases” that existed during severe. The mechanisms to generate, or appear to generate, pan-glacial conditions as reviewed by Fairchild and Kennedy [9] include Snowball Earth itself, high obliquity/Earth tilt, the “Zipper Rift” model of diachronous glaciation during the rifting of Rodinia [3], and the “slushball Earth” compromise This latter model is pertinent to the present paper as pockets of unfrozen water can provide the refugia for organisms that are required by advocates of the Snowball Earth theory [7]. At about the same time, McKay calculated that light can be transmitted through ice up to 30 m thick, and arctic lakes with ice 5 m thick harbor microbial life [11] These findings provided a mechanism for life to continue under a “hard” Snowball Earth, with a realization that ice must still have been comparatively thin in places for photosynthesis to continue. Even hard snowball solutions may permit some refugia [17]
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