The Early Silurian Valgu Glaciation and Its Effects on Ocean Chemistry and Conodont Faunal Turnover

The Telychian Age (Llandovery, Silurian) is marked by significant radiation among marine organisms, a faunal turnover, and palaeoenvironmental changes, collectively known as the Valgu Bioevent (Valgu BE), Valgu Carbon Isotope Excursion (Valgu CIE), and the Valgu Oxygen Isotope Excursion (Valgu OIE). However, the environmental drivers of these changes remain debated. Here we investigate middle and upper Telychian strata on the Yangtze platform of South China, as a case study to explore the mechanisms linking climate variability, ocean biogeochemistry, and ecosystem response, based on a robust bio- and chemostratigraphic framework. The Pterospathoduseopennatus Superzone is recognized and linked with the Valgu CIE in the studied area. Significant levels of extinction (Datum points 1, 2, 4, and 6) of the Valgu BE are identified within the rising limb and peak interval of the Valgu CIE, corresponding to the rising limb of the Valgu OIE, the latter reflecting a concurrent sea-surface temperature decline of 4 °C due to global climate change and glaciation. The marine geochemistry data show that the Valgu BE and CIE occurred in a well oxygenated setting in the studied area, likely affected by the upwelling of deeper cool waters enforced by the Valgu Glaciation. This study suggests that both the rapid evolution of conodont faunas, and their faunal turnover, during the Valgu BE were accelerated by a strong global climate cooling.