The Permian-Triassic boundary sections in northern Vietnam (Nhi Tao and Lung Cam sections): Carbon-isotope excursion and elemental variations indicate major anoxic event

Two Permian-Triassic boundary sections (Lung Cam and Nhi Tao) in shallow marine carbonates from northern Vietnam, near the border between Vietnam and China, were studied for major and trace elemental variations as well as carbon and oxygen isotopic compositions. At the Nhi Tao section, we observe a continuous transition from Late Permian limestones, siliceous limestones and microbial limestones (Dong Dang Formation) to Early Triassic limestones and dolomitic limestones (Hong Ngai Formation). At the Lung Cam section, a thin layer of shale (the uppermost part of the Dong Dang Formation-Late Permian) separates carbonate layers of the Dong Dang Formation (Late Permian) from those of the Hong Ngai Formation (Early Triassic). is a clear negative shift of about 2.5–3‰ in the δ13C values at both sections, beginning about 10 cm below the Permian-Triassic boundary as defined by paleontological and lithological data. The existence and extent of these excursions is compatible with introduction of 12C-rich deep ocean water onto the shallow carbonate platform. In the Nhi Tao section, this negative shift in δ13C starts at about the same level as a change in magnetic susceptibility, indicating some input from terrestrial sources. The δ13C curves shows evidence for a rapid reduction in biomass, which may be related to the extinction event, and the long-term reduction in δ13C in the Early Triassic suggests long-term perturbation of the environment and slow recovery. The shift towards negative values indicates a reduction in carbon burial. ape of the Ce/Ce* curve suggests that the redox conditions of depositional environments in both sections fluctuated, but that there was a decline in oxygenation with time, possibly into the anoxic range. Distinct negative anomalies coincide with the onset of the carbon isotope shifts, which might indicate additional influx of anoxic waters. The Th/U ratios in both sections are mainly less than 2, reinforcing this possibility. Upwelling of anoxic deep-ocean water onto the shallow carbonate platform at Nhi Tao and Lung Cam at the end of the Permian, with related disastrous consequences for the biosphere, is indicated by our data.