PRECAMBRIAN-CAMBRIAN CARBONATE-PHOSPHORITE AND CHERT-PHOSPHORITE ASSEMBLAGES FROM THE YANGTZE PLATFORM, CHINA

The Precambrian-Cambrian (PcC) boundary represents one of the most pivotal intervals in Earth’s history, as global changes in tectonics, climate and chemistry of the Earth’s atmosphere and oceans induced the worldwide Cambrian Radiation and a concomitant ecosphere revolution, correlating with the occurrence of the first widespread “giant phosphorites”. In this respect, China’s Yangtze Platform offers unique sedimentary successions of terminal Neoproterozoic and Early Cambrian age documenting phosphorite formation amid a rapid biodiversification event.

This contribution aims to elucidate the relation between the simultaneous appearance of giant phosphorites and environmental changes at the PcC-boundary. Samples from three different sections distributed along a platform to basin transect were investigated for their mineralogical (XRD, IR-, Raman-spectroscopy), facial (optical and fluorescence microscopy) and geochemical (main, trace and RE elements, TCNS, TOC) composition.

The Yangtze sedimentary apatites represent carbonate fluorapatites (CFA), which are mainly crypto- to microcrystalline and appear pseudo-isotropic under x-nicols. We found phosphate mineralization to occur in three major forms: (1) phosphate rocks of granular appearance being the dominant phosphate facies in platform settings, (2) stratiform and nodular phosphate fabrics, and (3) biological and microbial-mediated phosphate fabrics. The variability in trace metal enrichment (e.g. As, Cr, Mo, Ni, V) suggests changes of redox-conditions and related temporal cycling of Ba at the seawater-sediment interface. Rare earth patterns display mainly normal seawater patterns, with only basinal phosphate concretions indicating mediation by microbial activity in the more and more enclosed microenvironment of the rapidly forming concretion. Furthermore, both P_total/N_total- and P_total/C_org-ratios indicate “excess phosphorus” during the time of deposition of the Zhongyicun Mb (inner to outer platform) and the main part of the Lijiatuo Fm (slope to basin). To summarize, our findings support the hypothesis of a dramatic increase in biological productivity within a stratified ocean and thus the redox-dependent regeneration of P under anoxic conditions leading to the accumulation of large reservoirs of dissolved phosphorus.