Globally synchronous Marinoan deglaciation indicated by U-Pb geochronology of the Cottons Breccia, Tasmania, Australia
C.R. Calver et al., Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australia. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34568.1.
The apparently global distribution of Marinoan glacial deposits inspired the "snowball Earth" hypothesis, and prompted designation of the top of the type Marinoan glacials in South Australia as the Global Stratotype Section and Point ("golden spike") for the base of the terminal Proterozoic, Ediacaran System. However, horizons suitable for radioisotopic dating are lacking in the stratotype section and correlated sequences on mainland Australia. Ash beds suitably placed to directly and precisely date the Cryogenian-Ediacaran transition have so far been found only in Namibia (635.5 plus or minus 0.8 million years ago) and south China (635.2 plus or minus 0.8 million years ago). In this paper, C.R. Calver and colleagues show that a probable reworked volcaniclastic horizon at the very top of the Cottons Breccia, a Marinoan glacial correlative on King Island, Tasmania, has yielded an abundant population of juvenile zircons dated (by U-Pb on zircon, using chemical abrasion-thermal ionization mass spectroscopy) at 636.4 plus or minus 0.5 million years ago. Equivalence to the ash bed dates from Namibia and China supports correlation of those strata to the Australian type sections, and globally synchronous deglaciation at the beginning of the Ediacaran Period, and is consistent with the "snowball Earth" hypothesis.
A hematite-bearing layer in Gale Crater, Mars: Mapping and implications for past aqueous conditions
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