科学前沿

The end-Permian mass extinction was the most catastrophic event for life in the Phanerozoic eon because it impacted numerous organisms, from micro-sized photosynthetic organisms to large (meter-long) animals and fundamentally altered marine and terrestrial ecosystems. C₃₃ n-alkyl cyclohexane (C₃₃-ACH), an angstrom-size molecular fossil of phytoplankton, has been widely found in Permian–Triassic (P-Tr) marine sediments, associated with the collapse of the marine ecosystems at the end-Permian mass extinction. Here, we describe multiple C₃₃-ACH spikes in the Lower Triassic succession at the Chaohu section of the South China Block, which imply that phytoplankton blooms occurred repeatedly during the early–middle Early Triassic. Comparison with previous studies shows that C₃₃-ACH was not only globally enriched at the P-Tr boundary, but also abundant at the Induan–Olenekian boundary and middle Smithian in both the South China and Boreal seas. In addition, the Chaohu section record reveals a C₃₃-ACH peak at the Smithian–Spathian boundary. Moreover, the C₃₃-ACH spikes were synchronous with the peaks of mercury and the pristane/phytane (Pr/Ph) ratio. Since the peaks in mercury and the Pr/Ph ratio indicate increased volcanic activity and large influxes of terrestrial source material into the ocean, the correspondence between the high abundance of C₃₃-ACH with mercury and Pr/Ph ratio peaks implies that volcanism and riverine nutrient input fertilized the surface phytoplankton, which triggered the expansion of anoxia that in turn delayed the benthic metazoan recovery.