9:45 AM - 10:00 AM
[BCG04-04] Paleovegetation changes reconstructed by terrestrial plant biomarker analyses in the sediments deposited across the Cenomanian/Turonian boundary in the Great Valley Sequence, northern California, USA
Keywords:Paleovegetation, biomarker, Great Valley Sequence, C/T boundary, Cretaceous
The Cretaceous is known as the extreme greenhouse period attributed to the elevated atmospheric CO2 concentration triggered by the superplume activity. The Oceanic Anoxic Events (OAEs) caused by the occurrences of the Large Igneous Plateaus (LIPs) in several times, especially in the mid-Cretaceous. Global environmental disturbance events have been extensively investigated in the ocean, but there have been few reports for terrestrial environmental variation during the OAE periods (e.g. Heimhofer et al., 2018). In the present study, we analyzed biomarkers of terrestrial plants in the sediments deposited across the Cenomanian/Turonian boundary (CTB) from the northern California, USA to reconstruct paleovegetation changes and to evaluate paleoecology of plants during environmental disturbance event.
We analyzed sedimentary rocks across the CTB in the Budden Canyon Formation, Great Valley Sequence (GVS), were collected from the North Fork Cottonwood Creek sections in northern California, USA. The sediments used correspond to the OAE2 intervals (1st build-up, Trough, 2nd build-up, Plateau, and Recovery phases), determined by δ13C stratigraphy (Takashima et al., 2011). For the biomarker analysis, the extractions of crashed sediments were fractionated using silica-gel column and analyzed by GC-MS.
By sterane and hopane ratios, we confirmed the high contribution of terrigenous matter input and low maturity of organic matter (vitrinite reflectance ca. 0.4% level; lignite to subbituminous coal rank) in the North Fork Cottonwood Creek section. The aromatic terpenoid-based angiosperm/gymnosperm index (ar-AGI) values tended to increase gradually until the Plateau phase, although it decreased temporarily during environmental disturbances (1st build-up and 2nd build-up). The coniferous vegetation index as retene / cadalene ratio-based higher plant parameter (HPP), consistently low until the Plateau phase, and then increased rapidly in the Recovery phase. The ratio of perylene to plant-derived aromatic terpenoids (Pery / arT), which is tentatively defined as an indicator of fungal biomass in terrestrial flora i.e. fungal activity, also declined during the Recovery phase. These results suggest that, in the western margin of the north America continent during the CTB, the environmental disturbance had a negative effect on the angiosperm, causing a temporary increase in gymnosperms, and that the vegetation changed drastically around the end of OAE2 due to the expansion of coniferous vegetation.
We analyzed sedimentary rocks across the CTB in the Budden Canyon Formation, Great Valley Sequence (GVS), were collected from the North Fork Cottonwood Creek sections in northern California, USA. The sediments used correspond to the OAE2 intervals (1st build-up, Trough, 2nd build-up, Plateau, and Recovery phases), determined by δ13C stratigraphy (Takashima et al., 2011). For the biomarker analysis, the extractions of crashed sediments were fractionated using silica-gel column and analyzed by GC-MS.
By sterane and hopane ratios, we confirmed the high contribution of terrigenous matter input and low maturity of organic matter (vitrinite reflectance ca. 0.4% level; lignite to subbituminous coal rank) in the North Fork Cottonwood Creek section. The aromatic terpenoid-based angiosperm/gymnosperm index (ar-AGI) values tended to increase gradually until the Plateau phase, although it decreased temporarily during environmental disturbances (1st build-up and 2nd build-up). The coniferous vegetation index as retene / cadalene ratio-based higher plant parameter (HPP), consistently low until the Plateau phase, and then increased rapidly in the Recovery phase. The ratio of perylene to plant-derived aromatic terpenoids (Pery / arT), which is tentatively defined as an indicator of fungal biomass in terrestrial flora i.e. fungal activity, also declined during the Recovery phase. These results suggest that, in the western margin of the north America continent during the CTB, the environmental disturbance had a negative effect on the angiosperm, causing a temporary increase in gymnosperms, and that the vegetation changed drastically around the end of OAE2 due to the expansion of coniferous vegetation.