Abstract
The Middle-Late Ordovician transition (Darriwilian to Sandbian Age) witnessed a major pulse of the Great Ordovician Biodiversification Event (GOBE) and distinctive oceanic geochemical fluctuations, such as coeval negative C and Sr isotope excursions. In this study, investigations into geochemical variations, notably the Hg abundance (or Hg/TOC), have been carried upon the organic-rich black shale of the Mid-Upper Ordovician Saergan Formation to unravel the causes of this pulse. Based on these data, three phases were identified. Phase 1 (0 to 3 m) is characterized by rising Hg/TOC (up to 138 ppb/wt. %) and Ti/Al values as well as high CIAcorr (corrected chemical index of alteration) values (68.9-72.3) with negligible enrichment of redox sensitive elements (RSE) and nutrient elements (e.g. U ≤ 5.2 ppm, V ≤ 153 ppm, Mo ≤ 1.8 ppm, P2O5 ≤ 0.2%), suggesting intensified volcanism, which could have emitted significant amount of greenhouse gases, thereby leading to climate warming. In contrast, Phase 2 (3 to 11 m) is characterized by decreasing Hg/TOC and Ti/Al, relatively low though slightly fluctuating CIAcorr values, generally depleted in RSE (except moderately enriched U up to 14.6 ppm) and increased P/Al and Ba/Al, implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling. Phase 3 (Sandbian Age: 11-13 m) witnessed continuous decrease in Hg/TOC ratio, an increase in Ti/Al and CIAcorr values, fairly low values of RSE enrichment and P/Al and Ba/Al ratios, indicating recurrent climate warming, and the potential for slowed oceanic circulation and attenuated upwelling of nutrient-rich deep waters onto the shallow shelf. These changes could have diminished bioproductivity and organic output onto the seafloor. This study offers insights into volcanic-climatic-oceanic interactions around the Mid-Late Ordovician transition while black shales were extensively deposited and during a major pulse of the GOBE.
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