The Neoproterozoic Glacial Broom
Snowball Earth — Grok via Astrobiology.com
Neoproterozoic snowball Earth events reflect globally frigid conditions thought to have stimulated changes in geochemical cycling with planetary biotic response. We investigated the impact of these events on sediment dynamics, focusing on the detrital zircon record within the Dalradian Supergroup of Scotland and Ireland.
Utilizing U-Pb detrital zircon ages, we analyzed changes in sediment provenance throughout the stratigraphy via bootstrapped variance metrics. The coefficient of variance for the detrital zircon load exhibits a first-order increase stratigraphically upward, with marked changes at glaciogenic layers.
Points of increased variance align with inputs of older detritus compared to lower, preglacial stratigraphic levels, suggesting intensified erosion and downcutting linked to wet-based glacial activity, with postglacial sediment redistribution. Average detrital zircon apparent density also changes through the stratigraphy, with implications for uranium cycling from the continents into the oceans.
While early Ediacaran shale geochemistry implies postglacial oceanic oxygenation, loss of proportionally more uranium-rich detrital zircon across glaciogenic layers suggests that modification to continental weathering also fundamentally contributed to Neoproterozoic geochemical shifts.
Schematic block diagram of icehouse vs. greenhouse conditions. Implication of enhanced variance of detrital zircon (DZ) during snowball conditions is that glaciers werewet-based and able to downcut into crystalline basement. Back-steps in detrital zircon variance on snowball exits are interpreted to reflect fluvial reworking of liberated friable grains in immediate glacial aftermath. Schematic process is given below block diagram. — Geology
C.L. Kirkland; R.A. Strachan; D.B. Archibald; J.B. Murphy
The Neoproterozoic glacial broom, Geology, (open access)
Astrobiology
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