11:00 〜 13:00
[MIS18-P20] イギリス北東部および西南日本における大陸地殻風化作用とトアルシアン期海洋無酸素事変の関係
キーワード：海洋無酸素事変、風化仮説、化学組成、粘土鉱物、W値、ジュラ紀
The Toarcian Oceanic Anoxic Event (T-OAE) was characterized by active volcanism, climate changes, and widespread oceanic anoxia. It is marked stratigraphically by the global deposition of organic-rich black shales and a negative carbon isotope excursion (δ13C).
Many models have been advocated to explain the cause of OAEs, and one of them is the “weathering hypothesis” of Weissert et al. (1998). In this hypothesis, enhanced continental weathering is the trigger of OAEs, and results in oceanic eutrophication and excessive primary productivity.
Previous studies on paleoenvironments during the T-OAE have mainly been performed within the Tethys realm. In contrast, paleoenvironmental information from the Panthalassa realm is limited. Therefore, the global response to the T-OAE and the detailed mechanisms are not fully understood.
The present contribution aims to reconstruct the degree of continental weathering during the T-OAE in the European epicontinental seaway and the northwestern Panthalassa margin. The successions investigated are located in Yorkshire, NE UK and Toyora, SW Japan. These two successions can be readily correlated based on well-established ammonite biostratigraphy and δ13C chemostratigraphy (Hirano, 1973; Nakada and Matsuoka, 2011; Kemp and Izumi, 2014). Mudstone samples were collected from both successions, and geochemical and mineralogical compositions were measured by XRF and XRD, respectively.
In both successions, hinterland chemical weathering indices (W value; Ohta and Arai, 2007 and K2O/Na2O) increase synchronously coeval with the onset of the δ13C negative excursion. The W values obtained from the T-OAE successions range from 60 to 90 in Yorkshire, and 50 to 80 in Toyora. These values are comparable to values from recent soils developed under temperate or tropical rainforest climates. Nevertheless, the highest chemical weathering index values occur above the T-OAE interval in Yorkshire. The clay mineralogy of Yorkshire indicates a warm and humid paleoclimate throughout the T-OAE and post-T-OAE intervals. A proxy for grain size (TiO2/Al2O3) in the Yorkshire succession suggests that terrestrial sedimentary supply increased exclusively during the T-OAE interval. In Toyora, the results suggest that both paleoweathering and terrestrial sedimentary supply were intensified only during the T-OAE interval. From these results, we suggest that the post-T-OAE enhanced paleoweathering revealed in Yorkshire may reflect a regional signal. However, enhanced hinterland chemical weathering and terrestrial sediment fluxes during the T-OAE were induced by a global event, because these two phenomena are detected from both regions and our data support previous work indicating a global acceleration of chemical weathering in the T-OAE. Overall, the present result is consistent with the “weathering hypothesis” since the T-OAE was associated with enhanced continental weathering. However, there are several problems which need to be assessed in future studies for validating the “weathering hypothesis.” For example, further work is required to determine the significance of the difference in W values from each succession, and to more precisely establish the timing and pace of weathering changes.
References
Hirano, 1973, Trans. Proc. Palaeont. Soc. Japan, N. S., 90, 45-71.
Kemp and Izumi, 2014, Palaeogeogr. Palaeoclimatol. Palaeoecol., 414, 332-341.
Nakada and Matsuoka, 2011, Newsl. stratigr., 44, 89-111.
Ohta and Arai, 2007, Chem. Geol., 240, 280-297.
Weissert et al., 1998, Palaeogeogr. Palaeoclimatol. Palaeoecol., 137, 189-203.
Many models have been advocated to explain the cause of OAEs, and one of them is the “weathering hypothesis” of Weissert et al. (1998). In this hypothesis, enhanced continental weathering is the trigger of OAEs, and results in oceanic eutrophication and excessive primary productivity.
Previous studies on paleoenvironments during the T-OAE have mainly been performed within the Tethys realm. In contrast, paleoenvironmental information from the Panthalassa realm is limited. Therefore, the global response to the T-OAE and the detailed mechanisms are not fully understood.
The present contribution aims to reconstruct the degree of continental weathering during the T-OAE in the European epicontinental seaway and the northwestern Panthalassa margin. The successions investigated are located in Yorkshire, NE UK and Toyora, SW Japan. These two successions can be readily correlated based on well-established ammonite biostratigraphy and δ13C chemostratigraphy (Hirano, 1973; Nakada and Matsuoka, 2011; Kemp and Izumi, 2014). Mudstone samples were collected from both successions, and geochemical and mineralogical compositions were measured by XRF and XRD, respectively.
In both successions, hinterland chemical weathering indices (W value; Ohta and Arai, 2007 and K2O/Na2O) increase synchronously coeval with the onset of the δ13C negative excursion. The W values obtained from the T-OAE successions range from 60 to 90 in Yorkshire, and 50 to 80 in Toyora. These values are comparable to values from recent soils developed under temperate or tropical rainforest climates. Nevertheless, the highest chemical weathering index values occur above the T-OAE interval in Yorkshire. The clay mineralogy of Yorkshire indicates a warm and humid paleoclimate throughout the T-OAE and post-T-OAE intervals. A proxy for grain size (TiO2/Al2O3) in the Yorkshire succession suggests that terrestrial sedimentary supply increased exclusively during the T-OAE interval. In Toyora, the results suggest that both paleoweathering and terrestrial sedimentary supply were intensified only during the T-OAE interval. From these results, we suggest that the post-T-OAE enhanced paleoweathering revealed in Yorkshire may reflect a regional signal. However, enhanced hinterland chemical weathering and terrestrial sediment fluxes during the T-OAE were induced by a global event, because these two phenomena are detected from both regions and our data support previous work indicating a global acceleration of chemical weathering in the T-OAE. Overall, the present result is consistent with the “weathering hypothesis” since the T-OAE was associated with enhanced continental weathering. However, there are several problems which need to be assessed in future studies for validating the “weathering hypothesis.” For example, further work is required to determine the significance of the difference in W values from each succession, and to more precisely establish the timing and pace of weathering changes.
References
Hirano, 1973, Trans. Proc. Palaeont. Soc. Japan, N. S., 90, 45-71.
Kemp and Izumi, 2014, Palaeogeogr. Palaeoclimatol. Palaeoecol., 414, 332-341.
Nakada and Matsuoka, 2011, Newsl. stratigr., 44, 89-111.
Ohta and Arai, 2007, Chem. Geol., 240, 280-297.
Weissert et al., 1998, Palaeogeogr. Palaeoclimatol. Palaeoecol., 137, 189-203.