科研之友
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摘要
We present comprehensive Li-C-Sr isotopic studies on two classic marine carbonate sections (Yangdi and Dongcun) transecting the Late Devonian Frasnian-Famennian (F-F) boundary from South China to constrain the causes of the F-F mass extinction. Both sections show positive Li and C isotopic excursions in carbonate and unchanged conodont 87Sr/86Sr ratios across the F-F boundary. Low Li/Ca and Sr/Ca ratios, but heavy Li isotope composition (87Li) of the Dongcun section indicate possible diagenetic alteration on carbonate Li isotopic re-cords. By contrast, the Yangdi section records more primary seawater 87Li signal, showing a -2 %o positive excursion before the F-F boundary, although reconstructed paleo-seawater 87Li values (from 11.8 +/- 1.4%o to 13.8 +/- 1.4%o) are lower than modern times. Combined box model with Monte Carlo simulation demonstrates that low seawater 87Li requires low riverine 87Li and small effective Li isotopic fractionation by marine sinks (Delta 7Lisw-ms). The latter is likely due to high silicon concentration in the paleo-seawater and rapid formation of clay minerals. Positive 87Li excursion but invariant 87Sr/86Sr ratios in the seawater are related to attenuated continental weathering (reduced riverine fluxes but increased riverine 87Li), and enlarged effective Delta 7Lisw-ms. Based on the present data we suggest that rapid climate cooling is responsible for the weathering transition and must have exerted significant influence on the marine environment. Global climate cooling weakens seawater stratification while intensifying oceanic circulation and upwelling, resulting in enhanced primary productivity, precipitation of the Kellwasser black shales and a remarkable positive C isotopic excursion in carbonate. Addi-tionally, rapid climate cooling around the F-F boundary has significantly threatened the ecological system and finally triggered mass extinction especially for the faunas adapted to warm temperatures.
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单位中国科学院
