DETECTION OF COSMOGENIC MATERIAL IN DEEP-SEA SEDIMENTS BASED ON PLATINUM GROUP ELEMENT (PGE) ABUNDANCES

Snowball Earth events are widely recognized to have occurred in both the Palaeoproterozoic and Neoproterozoic. All present-day animal phyla appeared following the Marinoan-Snowball Earth, which is the last recorded global glaciation. The primary objective of this research is to determine the cause of this Snowball-Earth event, which is likely associated with the evolution of life. Three models that attempt to explain the onset and termination of Snowball-Earth events are: (1) episodic decrease of greenhouse gases, (2) changes of the albedo of the Earth accompanied by the arrangement of the continents, and (3) an increase in cosmic-ray bombardment to the Earth due to Starbursts in the Milky Way Galaxy or transects of Earth through nebula.

Based on recent investigations, the effects of extra-terrestrial forcing on the Earth are estimated here through the measurement of the abundances of platinum group elements (PGE) in sediments. Pelagic sediments composed of interlayered shale and/or mudstones are optimal for PGE-abundance analysis because of their low sedimentation rate. Pelagic sediments used for PGE-abundance analyses in this investigation are composed of bedded shales and/or mudstones collected from the accretionary complex of Anglesy-Llyen, U.K. The Anglesy-Llyen reportedly was formed by an accretionary orogeny in the Neoproterozoic. With respect to shale parts, we can identify the relatively high PGE concentration and a Cr/Ir ratio, which indicates a layer containing cosmogenic materials (up to 0.03 wt.%). In addition, correlation between Eu anomaly and Ir abundance in a sequence of black mudstones indicates that a fluctuation of redox conditions is one of the processes to change PGE-abundance.