[B-CG04] Decoding the history of Earth: From Hadean to the present

[BCG04-P10] Land plant burial event in deep-sea chert during the Aptian Oceanic Anoxic Event 1a (Cretaceous)

*Yuki Nakagawa¹, Julien Legrand^1,2, Rie Hori, S.³, Junichiro Kuroda⁴, Masayuki Ikeda² (1.Department of Earth Science, Faculty of Science, Shizuoka University, 2.University of Tokyo, 3.Department of Earth Science, Ehime University, 4.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:OAE 1a, land plants, global warming, Cretaceous

Aptian Oceanic Anoxic Event (OAE 1a; ~120 Ma) was accompanied by a massive eruption of the Ontong Java Plateau, doubling atmospheric pCO₂ from 800-1000 ppm to 2000 ± 1200 ppm, and increasing sea surface temperature by 4-8 °C (Naffs et al., 2016; Naafs and Pancost, 2016). OAE 1a is characterized as up to 4 ‰ negative and positive shifts of δ¹³C on several thousands year scale, but <100-kyr scale fluctuations up to 5 ‰ was observed in the pelagic deep-sea bedded chert succession from the Goshikinohama section in the Yokonami mélange of the Shimanto Belt, Kochi, Japan (Ihoriya et al., 2009; Kuroda et al., 2015). One possible cause of such local δ¹³C fluctuations could be changes in the mixing ratio of organic matters with different δ¹³C values, for example land plants. In this study, we performed a palynological analysis of Hauterivian to Cenomanian bedded chert from the Goshikinohama section. We analysed 44 samples and found more than 4000 palynomorphs from 34 samples, mainly represented by cuticles, filamentous algae and amorphous organic matter. Exclusively from Aptian and Hauterivian samples, we identified tracheids and resins belonging to land plants. Of particular interest is the abundance of tracheids up to 65 % during the short-term negative δ¹³C peaks, suggesting that some of these fluctuations could be due to an increase in proportions of ¹²C-rich land plants. Nevertheless, the negligible amount of land plants from the long-term negative peak of δ¹³C supports the carbon isotope correlation previously proposed by Kuroda et al. (2015). The occurrence of wood tracheids within the negative and positive δ¹³C excursions can be correlated with an interval of increased CO₂ and global warming in the Tethys region (Naffs et al., 2016; Naafs and Pancost, 2016). Therefore, an increase of land plant burial in Aptian deep-sea chert might be linked with a massive plant discharge due to enhanced hydrological cycles, such as intensified storms and/or sea-level changes up to ~100 m during the OAE 1a.