Carbon- and sulfur-isotope geochemistry of the Hirnantian (Late Ordovician) Wangjiawan (Riverside) section, South China: Global correlation and environmental event interpretation

摘要

Detailed geochemical analyses (δ ~(13)C _(carb), δ ~(13)C _(org), δ ~(34)S _(sulfide)), and abundance of sulfide, carbonate and organic carbon) were performed on samples from the Wangjiawan (Riverside) section, close to the GSSP (Global Stratotype Section and Point) for the Hirnantian stage of the Ordovician. New data show two increases in carbonate content coincident with two glacial pulses that reduced detrital input. The new δ ~(34)S _(sulfide) data show distinct changes in this section, with relatively high values in the Kuanyinchiao Formation, a pattern observed throughout the Yangtze Platform. However, there is no consensus on the cause of these changes. The new δ ~(13)C _(carb) data show a sharp rise and peak in the extraordinarius zone, below the previously published δ ~(13)C _(org) peak in the persculptus zone. A compilation of the new results with other sections indicates the Hirnantian carbon-isotope excursion starts near the pacificus-extraordinarius boundary and elevated values remain until the end of the excursion in the persculptus zone for both δ ~(13)C _(org) and δ ~(13)C _(carb). The controversy over correlating Hirnantian graptolite zones with chitinozoan zones can also be addressed. The new δ ~(13)C _(carb) data also allow direct comparison with Hirnantian δ ~(13)C _(carb) data from Anticosti Island and the Baltic region, which are zoned by chitinozoan fossils. This comparison favors a correlation of the taugourdeaui and scabra chitinozoan zones with the extraordinarius graptolite zone.