Coupling relationship of multiple events and enrichment of organic matter during Ordovician-Silurian transition period in Yangtze region

Key words:  Ordovician-Silurian transition period  geological events  coupling relationship  environmental response  enrichment of organic matter  development of shale

Abstract:

From the perspective of geologic events, this study explores the interconnections between various geological occurrences, their environmental repercussions, and their influence on the development of organic-rich shale during the Ordovician-Silurian transition period in the Yangtze area. The findings indicate that continental silicate weathering intensified by tectonic movements and increased carbon burial due to volcanic events both contributed to a decline in atmospheric CO₂ concentration. This decrease in CO₂ concentration triggered the Hirnantian glaciation when it fell below a certain threshold. Sulfur released from volcanic eruptions formed sulfate aerosols in the stratosphere, increasing albedo and promoting the Hirnantian glaciation. The Hirnantian glaciation amplified the latitudinal temperature gradient, fostering upwelling. The abrupt cooling during the Hirnantian glaciation, the expansion of euxinia environments in the water column, and the introduction of toxic heavy metals like mercury and arsenic from volcanic events were the primary mechanisms leading to the Late Ordovician Mass Extinction (LOME). Volcanic ash input and upwelling from volcanic events brought substantial nutrients to the ocean surface. The rapid decline in sea level during the Hirnantian glaciation increased the concentration of dissolved inorganic phosphate (DIP), boosting oceanic primary productivity and organic matter production. The consumption of oxygen by abundant organic matter during subsidence resulted in the formation of an oxygen minimum zone (OMZ) in deeper layers. The swift uptake of dissolved oxygen by volcanic ash and the rapid rise in sea level at the end of the Hirnantian glaciation expanded anoxic conditions in the water column, aiding organic matter preservation. The primary sedimentation process was suspended sedimentation, although locally, gravity slumping, turbidity currents, clastic flows and storm surge deposits were also developed due to tectonic movement, shallow water columns, and increased terrigenous input.