Marine Biomass Changes after the Neoproterozoic Marinoan Global Glaciation Evidenced by Organic Geochemistry

概要

Most of the Earth was covered by ice during the late Neoproterozoic Marinoan glaciation (ca. 635 Ma). The global glaciation has probably influenced the subsequent biosphere. However, the relationship between glaciation and biotic evolution has not been clarified. To elucidate changes in the biosphere at that time, organic geochemical analyses were carried out for Marinoan-associated deposits and the overlying post-glacial sediments in two sections in the Yangtze Platform, south China, and one section in the Kimberley region, northwestern Australia. The extract from every sample was carefully checked its indigenousness or migrating level by system blank experiments and maturity comparison between kerogen and bitumen; this assessment revealed that sedimentary organic molecules from the Jiulongwan samples are partly autochthonous and can be discussed for reconstruction of paleoenvironment. In the Jiulongwan Section, analyses show barren of biota in the cap dolostone overlying the Marinoan diamictite but presence of photosynthetic bacteria and/or the other bacteria at the top of the cap dolostone followed by presence of eukaryote and bacteria, which are based on n-C17 and n-C19 alkanes (a proxy for biomass of algae), pristane and phytane (a proxy for biomass of photosynthetic organisms), C29-C31 hopanes (bacterial biomarkers), and C27-C29 regular steranes (eukaryotic biomarkers). These changes indicate that recovery of bacteria started at the end of the cap dolostone precipitation, and then recovery of eukaryota followed. This biosphere recovery seems to have been controlled by a mass input of continental weathering into the ocean and/or a fall of temperature with the escape from the post-Marinoan greenhouse climate.