A Smooth Negative Shift of Organic-Carbon Isotope Ratios at an End-Permian Mass Extinction Horizon in Central Pelagic Panthalassa

The most significant mass extinction of the Phanerozoic occurred at the end of the Permian. Evidence of global oceanic anoxia during this event has been reported by previous investigators (e.g., Isozaki, 1997; Wignall and Twitchett, 1996). Increasing volumes of euxinic water at the end of the Permian are suggested by an increase in sulfur isotope ratios (δ³⁴S) of sulfates (Newton et al., 2004; Kaiho et al., 2002). Previous works suggested that end-Permian euxinic water reached the photic zone on the strength of pronounced blooms of anaerobic green sulphur bacteria (Cao et al., 2009; Riccardi et al., 2007; Grice et al., 2005). Increased cyanobacterial biomass is suggested in the photic zone of the eastern Paleotethys immediately after the mass extinction (Xie et al., 2007, 2005). However, the global extent of such algal and bacterial increases is uncertain.

The end-Permian mass extinction is also associated with a global, stratigraphically sharp negative carbon isotope ratio (δ¹³C) excursion (Corsetti et al., 2005; Sephton et al., 2002; Jin et al., 2000; Holser et al., 1989). Although the δ¹³C of carbonate (δ¹³C_carb) declines by 3‰ to 5‰ (Kaiho et al., 2009), the δ¹³C of organic matter (δ¹³C_org) values of marine organic matter increases during the negative shift of δ¹³C_carb in the shallow-water Paleotethys (Kaiho et al., 2009; Riccardi et al., 2007). Riccardi et al. (2007) discussed that the ephemeral increases of δ¹³C_org across the mass extinction horizon at several Permian/Triassic boundary (PTB) sections were interpreted as an increase in isotopic fractionation by phototrophic sulphur bacteria in continental shelf environments. Schwab and Spangenberg (2004) attributed increased δ¹³C_org in a Slovenian shallow-marine PTB section to an influx of terrigenous material and/or a bloom in primary productivity. In contrast, Riccardi et al. (2007) suggested that no green sulphur bacterial increase occurred in pelagic Panthalassa, based on the absence of significant increase in δ¹³C _org values from Japanese PTB sections on pelagic Panthalassic seamounts (Musashi et al., 2001). However, such carbon isotope variations are not well documented for pelagic deep-sea sediment because complete deep-sea PTB sections are rare.

Permian to Triassic pelagic deep-sea sediments of the Panthalassic Ocean are preserved in accretionary complexes in Japan. The stratigraphic succession records environmental changes at low latitude in central pelagic Panthalassa (Ando et al., 2001; Matsuda and Isozaki, 1991). Accretionary complexes generally have structural complexities that compromise the continuity of stratigraphic sections. The absence of stratigraphic continuity is commonly complicated by poor biostratigraphic control. However, the newly discovered Akkamori-2 (Am-2) Section of Northeast Japan is among the most continuous of PTB sections, as indicated by both biostratigraphic and lithologic evidence (Takahashi et al., 2009).

New data for δ¹³C _org were obtained from section Am-2. The δ¹³C _org excursion curve exhibits a negative shift of 2.0‰ in the low-latitude, pelagic Panthalassic Ocean at the end of the Permian, which coincides with a radiolarian demise (Takahashi et al., 2009). The δ¹³C _org values of pelagic, deep-sea Panthalassic sections and those of shallow-water sections from Panthalassic seamounts exhibit a smooth, negative shift that lacks temporary increases like those reported from Paleotethyan PTB sections. Absence of temporary δ¹³C_org increases at the PTB in Panthalassa may reflect less algal and bacterial blooming in pelagic Panthalassa compared to the shallow-water Paleotethys.