5:15 PM - 6:45 PM
[BCG06-P14] Multiproxy evidence that changing atmospheric composition terminated the Early Jurassic oceanic anoxia
★Invited Papers
Keywords:Toarcian, Toyora Group, charcoal, carbon isotope
The Toarcian oceanic anoxic event (T-OAE; ~183 Ma) caused a major disturbance in the global carbon cycle, depleting oxygen in oceans and resulting in a mass extinction among marine organisms. An increase in charcoal abundance at the end of the T-OAE is generally interpreted as a result of an increase in atmospheric oxygen that ultimately terminated the OAE in the Tethys Ocean, but studies of the T-OAE remain rare outside the Tethyan region and it is therefore unclear whether this change occurred globally.
In this study, we conducted a geological survey of the Lower Jurassic Nishinakayama Formation at Sakuraguchidani in Toyota-cho, southwestern Japan and analyzed the sedimentology, organic carbon stable isotopic composition , and charcoal abundance throughout the T-OAE. Our field survey revealed new outcrops at the onset of the negative carbon isotope excursion interval and at an interval marked by a fault breccia. These outcrops have not been sampled by previous studies and therefore fill gaps in the established chemostratigraphy at Sakuraguchidani.
The δ13Corg data from the new outcrops suggests that the negative shift of δ13Corg at the onset of the T-OAE occurred in three steps, in contrast to the two-step shift recognized in previous studies. Similar three-step changes in δ13Corg are reported in T-OAE sections in both Tethyan and Panthalassic regions, suggesting that multiple rapid carbon emissions and global warming may have triggered the T-OAE.
Charcoal abundance in the Sakuraguchidani section increased throughout the T-OAE interval, peaking at the beginning of the recovery interval and decreasing thereafter. This pattern is consistent with those observed in northern Europe as well as numerical models. Global increase in atmospheric O2 level associated with decreased atmospheric CO2 level, or changes in the atmospheric composition in short, could therefore have accelerated the termination of the T-OAE.
In this study, we conducted a geological survey of the Lower Jurassic Nishinakayama Formation at Sakuraguchidani in Toyota-cho, southwestern Japan and analyzed the sedimentology, organic carbon stable isotopic composition , and charcoal abundance throughout the T-OAE. Our field survey revealed new outcrops at the onset of the negative carbon isotope excursion interval and at an interval marked by a fault breccia. These outcrops have not been sampled by previous studies and therefore fill gaps in the established chemostratigraphy at Sakuraguchidani.
The δ13Corg data from the new outcrops suggests that the negative shift of δ13Corg at the onset of the T-OAE occurred in three steps, in contrast to the two-step shift recognized in previous studies. Similar three-step changes in δ13Corg are reported in T-OAE sections in both Tethyan and Panthalassic regions, suggesting that multiple rapid carbon emissions and global warming may have triggered the T-OAE.
Charcoal abundance in the Sakuraguchidani section increased throughout the T-OAE interval, peaking at the beginning of the recovery interval and decreasing thereafter. This pattern is consistent with those observed in northern Europe as well as numerical models. Global increase in atmospheric O2 level associated with decreased atmospheric CO2 level, or changes in the atmospheric composition in short, could therefore have accelerated the termination of the T-OAE.