中国科学院南京地质古生物研究所机构知识库

The Hangenberg mass extinction during the Devonian-Carboniferous (D-C) transition represents one of the largest biodiversity losses of the Phanerozoic, while the underlying cause remains controversial. An improved understanding of the contemporaneous sulfur cycle can provide insights into the latest Devonian environmental changes that potentially affected marine biotas. Here, we report on a high-resolution chemostratigraphic study of the sulfur isotopic composition of carbonate-associated sulfate (CAS) through the D-C transition in the Long'an and Qilinzhai sections of South China. The delta S-34(CAS) profiles exhibit a long-term (i.e., >10(5) yr) negative excursion from +19.0 parts per thousand in the upper Lower Si. praesulcata Zone to +13.0 parts per thousand in the middle Upper Si. praesulcata Zone, and terminated with a recovery to 20.3 parts per thousand in the lower Si. sulcata - Si. duplicata zones, representing a depositional interval of similar to 0.9 Myr. In addition, this long-term negative excursion is punctuated by episodic sharp negative shifts. The negative delta S-34(CAS) excursion coincided with the end-Devonian biotic crisis, a positive shift in carbonate delta C-13, and negative shifts in bulk-sediment delta N-15 values and I/Ca ratios. Increasing organic carbon burial indicated by the positive shift in delta C-13 precludes decreased pyrite burial as an explanation for the negative shift of delta S-34(CAS), supported by intensified marine anoxia revealed by the negative shifts in delta N-15 and I/Ca. We attribute the long-term negative shift in delta S-34(CAS) to enhanced inputs of S-34-depleted riverine sulfate in conjunction with low seawater sulfate concentrations within the semi-restricted Yangtze Sea, whereas the transient negative spikes in delta S-34(CAS) were possibly caused by episodic upwelling and oxidation of H2S in expanded oceanic oxygen-minimum zones. In conjunction with the positive shift in delta C-13, the negative shift in delta S-34(CAS) supports a significant role for enhanced subaerial weathering in intensifying marine anoxia and triggering the biotic crises that occurred during the latest Devonian, the most likely driver of which was the spread of vascular (especially seed-bearing) land plants.