[H-CG23] Earth surface processes related to deposition, erosion and sediment transport

[HCG23-03] Possible changes in weathering process through the Aptian Oceanic Anoxic Event (OAE 1a) recorded in the Cretaceous deep-sea bedded chert

*Yuki Nakagawa¹, Tenichi Cho², Masayuki Ikeda¹ (1.The University of Tokyo, 2.Waseda University)

Keywords:OAE 1a, Global warming, Weathering , Low latitude

Enhanced chemical weathering due to global warming is an important negative feedback system in the carbon cycle. Tropical weathering contributes three quarters of global silicate weathering, yet their response to global warming is still a matter of debate due to their transport-limited weathering. Hyperthermal events such as Cretaceous oceanic anoxic events (OAEs) may be a clue to examine changes in physical weathering in response to global warming. Nakagawa et al. (2022) reported an increase in terrestrial organic matters from deep-sea chert during the Aptian OAE 1a, suggesting the hyperthermal-related massive plant discharge. However, associated changes in weathering conditions are still unknown.Here we examined changes in chemical weathering intensity of deep-sea chert by RW index (Cho and Ohta, 2022) which can be applied to biogenic Si, Ca, and P-rich sediments, including chert. Calculated RW is between 8 and 35 with their lower values during the OAE 1a interval. Additionally, the mafic/felsic ratio of Cho and Ohta (2022) varies between 0.5 and 0.8, implying changes in terrigenous sources in deep-sea chert. FE-EPMA observation revealed um-scale K-rich aluminosilicates and silty Na-rich aluminosilicates, of which grain size corresponds to eolian and fluvial material, respectively. K/Al clearly shows an inverse-relationship with Na/Al and RW. These relations imply that K-feldspars are mainly of relatively-weathered eolian origin, whereas Na-rich feldspars are likely transported from less-weathered fluvial materials in oceanic islands, possibly by turbidity and/or bottom-water current. An increase in Na-rich and low RW values at the onset of OAE 1a CIE and following an increase in RW up to ~35 imply the increased contribution of fresh mafic rocks for deep-sea chert. Coincident increase in terrestrial plant and mm-scale occurrence of silty cross-laminated layers indicate the plant flooding, potentially by millennium-scale changes in tropical storm, earthquake, and/or bottom water current. Exceptionally coarse mafic fragments within the OAE 1a interval may have originated from the massive flooding in volcanic islands, implying the enhanced millennium-scale physical weathering during the hyperthermal event at OAE 1a.