Nano particles as the primary cause for long-term sunlight suppression at high southern latitudes following the Chicxulub impact - evidence from ejecta deposits in Belize and Mexico
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Life on Earth was sharply disrupted 66 Ma ago as an asteroid hit the sea-floor in what is today Yucatan Peninsula, Mexico. Approximately 600 km(3) of sedimentary rock were vapourized, ejected into the atmosphere and subsequently deposited globally as an ejecta apron and fallout layer. Proximal ejecta deposits occur in Belize and southern Mexico where the so called Albion island spheroid bed is superimposed on the target rock (the Barton Creek Formation). We analysed the spheroid bed via Mossbauer spectroscopy, petrology. XRD, and palynology at several sites similar to 350-500 km distance from the crater centre. Our results show that the relative concentrations of Fe in nano-phase goethite (alpha-FeOOH) are very high in the spheroid bed samples from Albion Island (Belize) and from Ramonal South (Mexico), but are low to absent in the spheroid bed at Ramonal North, and in the Cretaceous target rock. Moreover, our study shows that goethite and haematite are the dominant Fe-oxide nano-phases and the XRD results show that the target rock consists of both calcite and dolomite. We suggest that the heterogeneous composition of the spheroid bed between the various sites reflects the different types of target rocks that were dispersed within the rapidly expanding vapour plume and the complex sorting processes involved in the formation of the ejecta blanket. The distribution of the vapourized target rock strongly influenced life on Earth at the close of the Mesozoic. However, the comparatively thin K-Pg boundary clay in high-latitude Gondwanan successions combined with evidence of catastrophic changes to the biota in this region implies that the long-term sunlight suppression in the Southern Hemisphere was mainly governed by the large quantities of hydrous aerosols nucleated around sulphuric acid droplets or nano-sized particles, such as the nano-phase Fe-oxides. (C) 2014 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research.