Gong and Xu question whether the positive excursions in conodont apatite δ18O during the Late Frasnian and Frasnian-Famennian transition are the effect of climatic cooling. Instead, they favor changes in surface-water salinity in order to account for the observed shifts in δ18O of +1‰ to +1.5‰. Gong and Xu do not provide any data that may support higher surface-water salinity during the late Frasnian, but speculate on hypersalinity, red tides, and anoxia as potential causes of the Frasnian-Famennian extinction event.

To our understanding, a change in surface-water salinity is not plausible. The studied sections were situated...

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