Ordovician carbonate factory turnover tracked by its depositional, diagenetic, and carbon and oxygen isotope record

Abstract

Throughout Earth's history, carbonate factories have been production systems which appeared, declined and re-appeared (in modified form) in function of biotic evolution and systemic stability. Reconstructing the history of these intricate systems requires a combined assessment of sedimentological, paleoecological, geochemical and diagenetic criteria. Here, the Ordovician of the western Tarim Basin (Darriwilian to Katian, ramp-platform to basin settings) is used as the case example to test a multiproxy approach, including statistical analysis of proxy data. We identify stratigraphic segmentation of these strata in terms of facies/mineralogy, diagenesis and a first comprehensive data set of component-specific carbon and oxygen isotope values. The published bulk carbon and oxygen isotope data are analysed in the second step to extract marine diagenetic versus burial/meteoric signatures (virtual bulk values, Monte-Carlo simulation). Finally, a synoptic chemo-chronostratigraphic plot for the Ordovician of the western Tarim Basin with segments lasting from tens of Myrs down to several 100 kyrs is proposed. A long-term trend of increasing δ13C values culminates in the early Katian at 3.2 ‰ followed by a steady decrease. A baseline shift, arguably due to the initiation of a bahamite-type factory, blurs the globally reported Guttenberg carbon-isotope excursion (GICE). A medium-term (Darriwilian to earliest Sandbian) negative δ13C isotope excursion coincides with the demise of the Darriwilian sponges-pelmatozoa factory (suecicus Event, term proposed in this study) and a positive δ18O excursion. This event might represent the effects of volcanism/SO2 outgassing during the switch from a passive to an active continental arc. The suecicus Event masquerades the middle Darriwilian carbon-isotope excursion (MDICE).