Paleo-environmental changes during the Middle-Late Ordovician transition on the Yangtze Platform, South China and their ecological implications

摘要

Rocks of Darriwilian and Sandbian age (Middle-Late Ordovician) record the extraordinary peak of the Great Ordovician Bio-diversification Event (GOBE) and significant global environmental changes. To track paleo-environmental changes through this critical time, we have analyzed iron speciation, trace elements and total organic carbon in samples of the Miaopo Formation from an intra-shelf basin in South China. The results reveal moderate primary productivity and possible fluctuations in ben-thic redox. Upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters, sustaining the biological pump. Enhanced export of organic matter would have consumed dissolved oxygen at depth, driving the system toward anoxia. However, it is also possible for deep-water renewal events to transfer more dissolved oxygen into the bottom water. Those proxies suggests high primary productivity and varying sea level (global and local), not benthic anoxia, as the driver of accumulated "black shale". Thus, the local marine ecosystem thrived in this oxic intra-shelf basin, challenging the preexisting paradox that the high-diversity community flourished in an oxygen-depleted environment.