Geochemical evidence for a catastrophic biotic event at the Frasnian/Famennian boundary in south China

A strong δ¹³C anomaly coincides with a weak Ir anomaly at the Frasnian/Famennian (F/F) boundary exposed at Xiangtian, Guangxi, south China. The maximum whole-rock Ir abundance is 0.23 ppb (0.35 ppb on a carbonate-free basis) compared with averages of 0.016 and 0.044 ppb above and below the boundary interval. The δ¹³C in carbonate abruptly shifts from a late Frasnian level of about +1‰ to -2.49‰in the boundary interval and then abruptly returns to pre-boundary levels, suggesting a temporary reduction of surface-water biomass. A widely distributed (over at least several hundred kilometres) limestone breccia unit of possible wave-deposit origin is present directly below the boundary in south China. Enrichments of Al, V, Cr, As, and U, and a dip in the Mn abundance in the boundary interval indicate that reducing conditions might have been associated with the element enrichment. However, an alternative hypothesis is that oceanic impact(s) at the F/F boundary near south China provided the excess Ir and caused the biotic crisis and the formation of breccia deposits observed in the Late Devonian South China Sea.