2:45 PM - 3:00 PM
[BCG06-17] A molecular approach to the Early Cretaceous fluvial plain of the Tetori Group: depositional environment and wildfires
Keywords:Early Cretaceous, biomarker, fluvial deposit, seasonality, wildfire
During the Cretaceous, atmospheric CO2 levels were elevated and global climate conditions were notably warm. In particular, the Aptian–Turonian interval is recognized as a “supergreenhouse”. Moreover, latitudinal aridity belts, vegetation components, and wildfire regimes are thought to have differed significantly from those of the present, yet many details remain unclear. In the Kitadani Dinosaur Quarry (Katsuyama City, Fukui Prefecture), the Lower Cretaceous Kitadani Formation of the Tetori Group is continuously exposed, preserving meandering fluvial deposits with well-documented vertical and lateral facies changes (Suzuki et al., 2015). In this study, we combined sedimentary facies information with biomarker and combustion-derived polycyclic aromatic hydrocarbon (PAH) analyses to reconstruct micro-scale environmental diversity in an Early Cretaceous fluvial plain.
Sedimentary samples were pulverized and extracted with organic solvent, then fractionated using silica-gel column chromatography. Biomarkers and PAHs were measured by GC-MS. Rock-Eval pyrolysis was also performed to obtain the bulk composition of organic matter.
Our biomarker analyses detected pristane and phytane (derived from plant pigments), perylene (fungal origin), bacterial hopanes, and 2α-methylhopanes associated with cyanobacteria. Perylene was relatively abundant in the mudstone layers of abandoned-channel and overbank facies, suggesting that the fluvial plain was moderately humid at that time. In contrast, the basal sandstone of the abandoned channel-fill facies displayed higher value of homohopane index, an indicator of reducing conditions, and conspicuously high 2α-methylhopane contents. Modern abandoned channels undergoing reduced water levels during dry seasons can foster cyanobacterial blooms (Bovo-Scomparin et al., 2008), implying that seasonal fluctuations in water level may also have characterized this Early Cretaceous setting.
The Kitadani Formation belongs to the Aptian–Albian Akaiwa Subgroup, where the presence of plant fossils and pedogenic carbonate nodules suggests progressive warming and aridification (Sakai et al., 2018), whereas stable isotope data from vertebrate fossils indicate locally increased precipitation (Amiot et al., 2015). Based on these lines of evidence, we explored whether seasonal variability in precipitation could reconcile the diverse paleoclimatic indicators. The inferred low-water phases in the abandoned channel-fill facies may illustrate how marked contrasts in seasonal rainfall became evident in local riverine environments.
Furthermore, the Kitadani Formation contains high molecular weight PAHs (e.g., coronene) interpreted as high-temperature combustion products, reported in greater abundance compared to underlying strata (Hasegawa & Hibino, 2011; Yano et al., 2023. JpGU). However, because this formation lies within the oil-generation window, it is required to differentiate diagenetically produced PAHs from true combustion PAHs is necessary (e.g., Xu et al., 2019), and simple classification by aromatic ring number has shown insufficient (Yano et al., 2024. JpGU). To address this, we applied principal component analysis (PCA) to segregate PAHs strongly affected by diagenesis from those reflecting genuine combustion, thereby establishing improved indicators for combustion-derived PAH contributions and fire intensity. As a result, overbank mudstones showed high contributions of combustion PAHs along with elevated fire-intensity indices, whereas bonebed layers and abandoned-channel facies displayed relatively lower pyrolytic signatures. This distribution pattern suggests that wildfire frequency and strength were closely linked to local geomorphological and vegetational settings along the river system. Furthermore, the increasing seasonality with extreme wet and dry seasons may have contributed to the widespread wildfires inferred from the Kitadani Formation.
Sedimentary samples were pulverized and extracted with organic solvent, then fractionated using silica-gel column chromatography. Biomarkers and PAHs were measured by GC-MS. Rock-Eval pyrolysis was also performed to obtain the bulk composition of organic matter.
Our biomarker analyses detected pristane and phytane (derived from plant pigments), perylene (fungal origin), bacterial hopanes, and 2α-methylhopanes associated with cyanobacteria. Perylene was relatively abundant in the mudstone layers of abandoned-channel and overbank facies, suggesting that the fluvial plain was moderately humid at that time. In contrast, the basal sandstone of the abandoned channel-fill facies displayed higher value of homohopane index, an indicator of reducing conditions, and conspicuously high 2α-methylhopane contents. Modern abandoned channels undergoing reduced water levels during dry seasons can foster cyanobacterial blooms (Bovo-Scomparin et al., 2008), implying that seasonal fluctuations in water level may also have characterized this Early Cretaceous setting.
The Kitadani Formation belongs to the Aptian–Albian Akaiwa Subgroup, where the presence of plant fossils and pedogenic carbonate nodules suggests progressive warming and aridification (Sakai et al., 2018), whereas stable isotope data from vertebrate fossils indicate locally increased precipitation (Amiot et al., 2015). Based on these lines of evidence, we explored whether seasonal variability in precipitation could reconcile the diverse paleoclimatic indicators. The inferred low-water phases in the abandoned channel-fill facies may illustrate how marked contrasts in seasonal rainfall became evident in local riverine environments.
Furthermore, the Kitadani Formation contains high molecular weight PAHs (e.g., coronene) interpreted as high-temperature combustion products, reported in greater abundance compared to underlying strata (Hasegawa & Hibino, 2011; Yano et al., 2023. JpGU). However, because this formation lies within the oil-generation window, it is required to differentiate diagenetically produced PAHs from true combustion PAHs is necessary (e.g., Xu et al., 2019), and simple classification by aromatic ring number has shown insufficient (Yano et al., 2024. JpGU). To address this, we applied principal component analysis (PCA) to segregate PAHs strongly affected by diagenesis from those reflecting genuine combustion, thereby establishing improved indicators for combustion-derived PAH contributions and fire intensity. As a result, overbank mudstones showed high contributions of combustion PAHs along with elevated fire-intensity indices, whereas bonebed layers and abandoned-channel facies displayed relatively lower pyrolytic signatures. This distribution pattern suggests that wildfire frequency and strength were closely linked to local geomorphological and vegetational settings along the river system. Furthermore, the increasing seasonality with extreme wet and dry seasons may have contributed to the widespread wildfires inferred from the Kitadani Formation.