Following the Smithian thermal maximum (STM) of the Olenekian stage (the second of the Early Triassic), a rapid cooling event occurred at the Smithian-Spathian boundary (SSB) (Sun et al., 2012). This period was marked by carbon cycle perturbations, as evidenced by carbonate carbon isotope records (δ¹³Ccarb) reaching a negative peak during the STM and shifting positively at the SSB (e.g., Song et al., 2019). We examined the SSB from the Taho Formation (Maekawa et al., 2018) in Seiyo City, Ehime Prefecture, analyzing δ¹³Ccarb, δ¹³Corg, total organic carbon (TOC), sedimentary textures, and biomarkers. Petrographic analysis revealed most horizons as packstone and wackestone, with black intraclasts and erosional discontinuities present at the SSB. Polycyclic aromatic hydrocarbons, particularly coronene—an indicator of high-temperature heating events from volcanic activity—were detected in all horizons, with highest abundances coinciding with negative δ¹³Ccarb peaks, suggesting volcanic activity contributed to the STM. The subsequent cooling at the SSB likely resulted from enhanced organic carbon burial due to increased primary productivity, supported by the positive correlation between TOC and δ¹³Ccarb. This productivity increase may have been associated with enhanced continental weathering triggered by the STM. Additionally, sea-level decline associated with cooling at the SSB may have contributed to black intraclast formation and seafloor erosion. These findings indicate volcanic activity potentially played a significant role in influencing the carbon cycle during this critical period.

Triassic,Geochemical Analysis