| The largest mass extinction event of the Phanerozoic occurred during the PermianTriassic transition,and the following Early Triassic was marked by a 4-8 million-year interval of protracted marine biotic recovery.Ocean redox perturbations may have played a critical role in the mass extinction event and the delayed recovery.Carbon and nitrogen are essential bio-organic elements,carbon isotope and nitrogen isotope have emerged as powerful tools for reconstructing environmental changes in the ancient ocean.To better understand carbon-nitrogen cycle perturbations and marine redox evolution during the Permian-Triassic transition,we conduct systematic analyses ofδ13Ccarb,δ13Corg,and δ15N values of the Dajiang section.To reconstruct a highresolution Early-Middle Triassic δ13Ccarb curve and its relationship with the prolonged biotic recovery,the Lower and Middle Triassic strata of the Tianshengqiao section have been sampled to undertake the δ13Ccarb study.The main results are shown as follows:(1)Three progressive δ13Ccarb negative excursions were observed within the microbialite layer in the Dajiang section.We suggest that the episodic incursion of sulfidic waters and enhanced weathering lead to the first negative δ13Ccarb excursion,while the second and the third negative δ13Ccarb excursion were caused by the expansion of 13C-depleted anoxic deep waters.(2)Significant variations in δ15N around the extinction horizon may be best explained by the highly dynamic water column redox condition with fluctuations between oxic and euxinic states,which may have been driven by the episodic incursion of sulfidic waters.The low δ15N values are likely to result from ammonium assimilation under anoxic conditions,while the high δ15N values are attributed to a combination of incomplete nitrification and incomplete denitrification during transient shifts from anoxic to oxic conditions.The following stabilization in δ15N and the corresponding decline in δ13Ccarb may imply two episodes of expansion of anoxic deep waters.Deep euxinic water masses have a large 13C-depleted dissolved organic carbon and ammonium pool.Oxidation of the 13C-depleted organic carbon would result in the negative δ13Ccarb excursions,and a large amount of nitrate produced by nitrification could lead to the stabilization of δ15N.Finally,the δ15N varying around+1.2‰ indicates that the bioavailable nitrogen was mainly derived from nitrogen fixation.The deficiency of the bioavailable nitrogen in the ocean might be caused by denitrification and anaerobic ammonium oxidation under protracted anoxic conditions.Oscillations between sulfidic and oxic conditions,the following expansion of anoxic environment,and the widespread nitrogen famine may have accounted for the mass extinction event.(3)The highly dynamic water column redox conditions driven by the episodic incursion of sulfidic waters is further confirmed by the instantaneous and large variations in δ13Corg and Δ13Ccarb-org values below the extinction horizon.Both δ13Corg and Δ13Ccarb-org display a slight negative excursion in the lower part of the microbialite layer,indicating an increase in marine primary productivity directly after the mass extinction.The following Δ13Ccarb-org excursions may be ascribed to changes in microbial communities caused by perturbed ocean conditions.(4)Based on well-constrained conodonts,ammonoids,and bivalves biostratigraphy,the Early-Middle Triassic δ13Ccarb curve reported in the Tianshengqiao section is well correlated with those in the cotemporaneous sections globally.We suggest that negative shifts in δ13Ccarb are associated with enhanced organic carbon oxidation induced by the expansion of anoxic deep waters,and the ensuing positive shifts are related to the enhanced primary productivity caused by high nutrient availability.The temporal coincidence between carbon cycle perturbation and the delayed biotic recovery suggests a causal link between the expansion of anoxic deep waters and prolonged biological recovery during the Early Triassic. |
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