Marine sulfate sulfur isotopic evidence for enhanced terrestrial weathering and expansion of oceanic anoxia during the Devonian-Carboniferous transition

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 δ³⁴S_CAS profiles exhibit a long-term (i.e., >10⁵ yr) negative excursion from +19.0‰ in the upper Lower Si. praesulcata Zone to +13.0‰ in the middle Upper Si. praesulcata Zone, and terminated with a recovery to 20.3‰ in the lower Si. sulcata – Si. duplicata zones, representing a depositional interval of ∼0.9 Myr. In addition, this long-term negative excursion is punctuated by episodic sharp negative shifts. The negative δ³⁴S_CAS excursion coincided with the end-Devonian biotic crisis, a positive shift in carbonate δ¹³C, and negative shifts in bulk-sediment δ¹⁵N values and I/Ca ratios. Increasing organic carbon burial indicated by the positive shift in δ¹³C precludes decreased pyrite burial as an explanation for the negative shift of δ³⁴S_CAS, supported by intensified marine anoxia revealed by the negative shifts in δ¹⁵N and I/Ca. We attribute the long-term negative shift in δ³⁴S_CAS to enhanced inputs of ³⁴S-depleted riverine sulfate in conjunction with low seawater sulfate concentrations within the semi-restricted Yangtze Sea, whereas the transient negative spikes in δ³⁴S_CAS were possibly caused by episodic upwelling and oxidation of H₂S in expanded oceanic oxygen-minimum zones. In conjunction with the positive shift in δ¹³C, the negative shift in δ³⁴S_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.