No evidence of extraterrestrial noble metal and helium anomalies at Marinoan glacial termination

Abstract

High concentrations of extraterrestrial iridium have been reported in terminal Sturtian and Marinoan glacial marine sediments and are used to argue for long (likely 3–12 Myr) durations of these Cryogenian glaciations. Reanalysis of the Marinoan sedimentary rocks used in the original study, supplemented by sedimentary rocks from additional terminal Marinoan sections, however, does not confirm the initial report. New platinum group element concentrations, and 187Os/188Os and 3He/4He signatures are consistent with crustal origin and minimal extraterrestrial contributions. The discrepancy is likely caused by different sample masses used in the two studies, with this study being based on much larger samples that better capture the stochastic distribution of extraterrestrial particles in marine sediments. Strong enrichment of redox-sensitive elements, particularly rhenium, up-section in the basal postglacial cap carbonates, may indicate a return to more fully oxygenated seawater in the aftermath of the Marinoan snowball earth. Sections dominated by hydrogenous osmium indicate increasing submarine hydrothermal sources and/or continental inputs that are increasingly dominated by young mantle-derived rocks after deglaciation. Sedimentation rate estimates for the basal cap carbonates yield surprisingly slow rates of a few centimeters per thousand years. This study highlights the importance of using sedimentary rock samples that represent sufficiently large area–time products to properly sample extraterrestrial particles representatively, and demonstrates the value of using multiple tracers of extraterrestrial matter.