Increases in Anaerobic Photosynthetic Bacteria in the End-Permian Ocean: Empirical Evidence of Photic-Zone Euxinia in the Pelagic Realm

Oceanic anoxia is a key factor in the Permian–Triassic mass extinction. Analyses of organic molecular fossils from anaerobic green sulfur bacteria, primarily in sediments from paleo-continental shelves, have indicated that this anoxia led to euxinic conditions within the photic zone, creating unique photosynthetic communities (e.g., Grice et al., 2005). However, it has remained unclear whether similar changes occurred in the pelagic open ocean, which covers the majority of Earth's surface. Permian–Triassic pelagic deep-sea sedimentary rocks are preserved within accretionary complexes, such as those in Japan and New Zealand. The high diagenetic maturity of these rocks makes it difficult to extract intact organic molecular fossils of photosynthetic organisms. To overcome this problem, this study employed the maleimide analysis method proposed by Asahina et al. (2022). This method targets maleimide molecules derived from the side chains of chlorophyll and bacteriochlorophyll, extracted through the degradation of insoluble organic matter (kerogen). This approach allows for the detection of organic molecules from anaerobic photosynthetic bacteria as fossilized bacteriochlorophyll, even in highly thermally mature rocks. We applied this approach to three different Permian–Triassic boundary sections: the Meishan section in China (shallow marine deposits), the Akkamori section in Japan (low-latitude pelagic chert and claystone), and the Waiheke section in New Zealand (mid-latitude pelagic chert and claystone). The results revealed an increase in maleimides from anaerobic bacteriochlorophyll (2-methyl-3-n-propyl maleimide) relative to those from general photosynthesis (3-ethyl-4-methyl maleimide; likely derived from cyanobacteria, algae, etc.) in the end-Permian strata of all studied sections. These findings strongly suggest that anoxic-euxinic surface water and blooms of anaerobic photosynthetic bacteria developed even in open ocean settings during the end-Permian mass extinction event. Such conditions would require a sufficient nutrient supply to the pelagic surface waters to sustain high primary productivity and subsequent sulfate reduction within the photic zone.