Friday, September 30, 2016

Explaining the Iron Drop Stones of the Sturtian Glaciation (Snowball Earth)


Lechte et al


The Neoproterozoic Sturtian glaciation is considered to be among the most severe glaciations in Earth history, possibly encompassing the entire planet and lasting for more than 50 m.y. Iron formations are globally associated with Sturtian glacial successions, although the influence of glaciation on the genesis of these iron formations remains contentious. Here we examine the Sturtian iron formations of Namibia and Australia that feature finely laminated ironstones containing up to 55% total iron. These ironstones are repeatedly interbedded with massive diamictites, yet dropstones and other clastic input are nearly absent in the laminated ironstone facies. Intercalated diamictites are variably ferruginous and characterized by a strong glacial influence with evidence of glaciotectonism. The ferruginous facies are laterally discontinuous and commonly occupy paleobathymetric depressions. Rare earth element signatures from these iron formations are similar to those from modern seawater but lack cerium anomalies. The paradox of dropstone-free, laminated sediments intimately interlaminated with massive ice-proximal diamictites can be resolved by deposition under an ice shelf. Polynya activity and the mixing of cold, oxygenated glacial fluids with ferruginous seawater via an ice pump mechanism may explain the deposition of these iron formations and their restriction to Sturtian glacial successions globally

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