3:45 PM - 4:00 PM
[BCG06-14] Origins of aromatic compounds in the Lower Cretaceous Kitadani Formation, Tetori Group: assessing evidence of wildfires
Keywords:polycyclic aromatic hydrocarbons, wildfire, Tetori Group, Cretaceous
Fossil charcoals, mesofossils, and polycyclic aromatic hydrocarbons (PAHs) preserved in sedimentary records are crucial for reconstructing paleo-wildfires, representing remnants of incomplete combustion of organic matter (OM) [1]. Last year, our study identified a significant presence of high-molecular-weight (HMW) PAHs, represented by coronene, in the fluvial sediments of the Kitadani Formation of the Tetori Group exposed at the Kitadani Dinosaur Quarry in Katsuyama City, Fukui Prefecture [2]. Compared with a previous record on other formations in the Tetori Group [3], this finding suggests an increased activity of high-temperature wildfires towards the end of the Tetori Group's depositional period, likely driven by climatic warming and aridification. However, PAHs can arise from diagenetic processes and thermal maturation of OM [4]. Considering the thermal history of the Tetori Group [5], it is necessary to carefully discern the origins of PAHs to accurately estimate the contribution of combustion-derived organic matter to understand the fire history recorded in the PAH composition of the Kitadani Formation. In this study, we conducted organic geochemical and petrological analyses on a broad spectrum of sedimentary rocks and plant fossil samples collected from the quarry outcrops, aiming to decode wildfire signatures in their aromatic hydrocarbon profiles.
The PAH composition in the fluvial sediments within the Kitadani Formation exhibited variation by lithofacies. In the silt/mudstone of floodplain deposits, a higher proportion of 4-ring or larger HMW PAHs was observed than other lithofacies. In contrast, the bonebed-1 horizon, consisting of poorly sorted, very coarse-grained granular sandstone characterized by vertebrate and plant fossils and containing allochthonous pedogenic carbonate nodules, showed a unique composition rich in alkylated naphthalenes and alkylated phenanthrenes, such as 1,7-dimethylphenanthrene (1,7-DMP). The predominance of 1,7-DMP, combined with the biomarker composition of plant fossils, suggests that alkylphenanthrenes in the analyzed sediments are primarily diagenetic products derived from plant diterpenoids (with pimarane-type skeletons). Meanwhile, the semi-quantitative ratio of several HMW PAHs (including fluoranthene, benzo[b/k/j]fluoranthenes, benzo[e]pyrene, perylene, indeno[cd]pyrene, benzo[ghi]perylene, and coronene) showed positive correlations with each other and an inverse correlation with lower molecular weight PAHs including diagenetic 1,7-DMP. This suggests that HMW PAHs originate from a process different from diagenetic PAHs. The Tmax values obtained from Rock-Eval analysis varied significantly, ranging 450–530 °C, with higher values observed in the silt/mudstone of floodplain. Originating from a sedimentary thickness of several tens of meters within the Kitadani Formation, the Tmax value variations likely reflect differences in organic matter quality rather than thermal maturation effects. Preliminary kerogen observations indicated that the silt/mudstone of floodplain are rich in opaque plant fragments, such as inertinites, containing combustion-derived fusains. Such combustion-affected OM serves as a source of HMW PAHs while lacking the most volatile and thermally labile components at the time of deposition, contributing to relatively higher Tmax values compared to unaffected OM. Our findings underscore persistent wildfire activity in floodplain of meandering river basin during the deposition of the quarry outcrop and affirm the role of HMW PAHs as effective tracers for combustion-derived OM in its thermally matured sediments, supported by comprehensive PAH composition analysis and corroborative evidence.
References
[1] Brown et al. (2012). Cret. Res., 36, 162-190.
[2] Yano et al. (2023). JpGU Meeting 2023.
[3] Hasegawa & Hibino. (2011). Isl. Arc, 20(1), 23-34.
[4] Xu et al. (2019). Mar. Pet. Geol., 102, 402-425.
[5] Suzuki et al. (1994). Jour. Geol. Soc. Japan, 100, 302-311
The PAH composition in the fluvial sediments within the Kitadani Formation exhibited variation by lithofacies. In the silt/mudstone of floodplain deposits, a higher proportion of 4-ring or larger HMW PAHs was observed than other lithofacies. In contrast, the bonebed-1 horizon, consisting of poorly sorted, very coarse-grained granular sandstone characterized by vertebrate and plant fossils and containing allochthonous pedogenic carbonate nodules, showed a unique composition rich in alkylated naphthalenes and alkylated phenanthrenes, such as 1,7-dimethylphenanthrene (1,7-DMP). The predominance of 1,7-DMP, combined with the biomarker composition of plant fossils, suggests that alkylphenanthrenes in the analyzed sediments are primarily diagenetic products derived from plant diterpenoids (with pimarane-type skeletons). Meanwhile, the semi-quantitative ratio of several HMW PAHs (including fluoranthene, benzo[b/k/j]fluoranthenes, benzo[e]pyrene, perylene, indeno[cd]pyrene, benzo[ghi]perylene, and coronene) showed positive correlations with each other and an inverse correlation with lower molecular weight PAHs including diagenetic 1,7-DMP. This suggests that HMW PAHs originate from a process different from diagenetic PAHs. The Tmax values obtained from Rock-Eval analysis varied significantly, ranging 450–530 °C, with higher values observed in the silt/mudstone of floodplain. Originating from a sedimentary thickness of several tens of meters within the Kitadani Formation, the Tmax value variations likely reflect differences in organic matter quality rather than thermal maturation effects. Preliminary kerogen observations indicated that the silt/mudstone of floodplain are rich in opaque plant fragments, such as inertinites, containing combustion-derived fusains. Such combustion-affected OM serves as a source of HMW PAHs while lacking the most volatile and thermally labile components at the time of deposition, contributing to relatively higher Tmax values compared to unaffected OM. Our findings underscore persistent wildfire activity in floodplain of meandering river basin during the deposition of the quarry outcrop and affirm the role of HMW PAHs as effective tracers for combustion-derived OM in its thermally matured sediments, supported by comprehensive PAH composition analysis and corroborative evidence.
References
[1] Brown et al. (2012). Cret. Res., 36, 162-190.
[2] Yano et al. (2023). JpGU Meeting 2023.
[3] Hasegawa & Hibino. (2011). Isl. Arc, 20(1), 23-34.
[4] Xu et al. (2019). Mar. Pet. Geol., 102, 402-425.
[5] Suzuki et al. (1994). Jour. Geol. Soc. Japan, 100, 302-311