BIOGEOCHEMICAL CYCLING THROUGH THE NEOPROTEROZOIC-CAMBRIAN TRANSITION IN CHINA: AN INTEGRATED STUDY OF REDOX-SENSITIVE ELEMENTS

The Neoproterozoic – Cambrian transition was characterized by several major events including global glaciations, the break-up of the supercontinent Rodinia and the subsequent reassembly into Gondwana, biological innovations, exemplified in the Ediacara biota and the Cambrian Explosion and, last but not least, the last significant step in the oxygenation of the Earth atmosphere and ultimately the ventilation of the deep ocean. Whereas the previous Great Oxygenation Event in the Paleoproterozoic is much better constrained and relatively undisputed, the timing and extent of the Neoproterozoic Oxygenation Event is still matter of much debate. Well preserved black shale successions on the Yangtze Platform, South China, offer a unique opportunity to study the changes in the biogeochemical cycling of redox-sensitive trace-elements during the Neoproterozoic – Cambrian transition and we collected over 320 samples from 15 black shale and organic rich carbonate sections spanning from the Late Cryogenian to the Early Cambrian. X-ray fluorescence, inductively coupled plasma mass spectrometry and iron speciation analysis yielded a wide range of elemental concentrations which allows us to treat the questions surrounding the Oxygenation of the Earth’s surface with an extensive multi proxy approach. Focussing on the trace-metals molybdenum and vanadium, their behaviour with respect to total organic carbon and paleoredox proxies such as Fe_HR/Fe_T and Th/U, we demonstrate the complex situation on the Yangtze Platform during this biogeochemical ‘revolution’ which allowed the Earth to cross irreversible thresholds towards the modern Earth system characterised by the presence of metazoans, soil biota, more equable climates, and persistently high levels of oxygen in the oceans and atmosphere.