13:45 〜 14:00
[BCG05-13] Extreme continental weathering in the northwestern Tethys during the Triassic–Jurassic transition
キーワード：ストロンチウム同位体比、炭素同位体比、多変量解析、三畳紀/ジュラ紀境界
The end-Triassic mass extinction (ETE) is thought to have been triggered by widespread eruption of the Central Atlantic Magmatic Province (CAMP) flood basalts. Although palynological studies of shallow-marine deposits in the Danish and German basins suggest that deforestation and catastrophic soil loss occurred during the CAMP volcanism and ETE interval, the degree of intensity and timing of continental weathering in the northwestern Tethys Ocean remains unclear. Here we present the results of strontium (Sr), carbon and oxygen isotope analyses as well as multivariate statistical analyses to major element contents from the carbonate-clastic deposits of the Kardolína section, Slovakia. This section consists of a shallow marine carbonate sequence of the Rhaetian Fatra Formation and the overlying Hettangian Kopieniec Formation. The Fatra Formation represents a ramp facies deposited in a restricted pull-apart basin of the Fatric Zone on the southern margin of the Bohemian Massif in the NW Tethys.
Carbon isotope analysis of the limestones revealed two negative carbon isotope excursions (NCIE) in the uppermost part of the Fatra Formation. These two NCIEs occurred in the latest Rhaetian and can be compared to the "precursor" and "initial" NCIEs reported in the NW Tethys. Sr isotope analysis of the limestones revealed a sharp increase in Sr isotope ratios between the precursor and initial NCIEs, which indicates that continental weathering of the Bohemian Massif increased rapidly in the latest Rhaetian. Multivariate analysis of major element contents in carbonate rocks also shows that intense chemical weathering of hinterland was accelerated after the precursor NCIE, supporting the results of Sr isotope analysis. Furthermore, this study reveals that the carbonate depositional environments of the Fatra Formation shifted toward the formation of specific iron-enriched oolite with the increase of continental weathering after the precursor NCIE. The concentrations of redox-sensitive major elements (e.g., manganese and iron) and multivariate analysis of major element data suggest that the iron-enriched oolites were formed by the influx of oxygen-depleted water masses into the shallow water depositional environment of the Fatra Formation. A possible origin of such oxygen-depleted water masses was the oxygen minimum zone (OMZ) that formed in the intermediate water-depth in the European basins (e.g., German and Eiberg basins) during the latest Rhaetian. These results suggest that the marine environment in the European basins may have changed to develop the OMZ with the increase of continental weathering during the latest Rhaetian, and these environmental changes may have played an important role in the marine ETE event.
Carbon isotope analysis of the limestones revealed two negative carbon isotope excursions (NCIE) in the uppermost part of the Fatra Formation. These two NCIEs occurred in the latest Rhaetian and can be compared to the "precursor" and "initial" NCIEs reported in the NW Tethys. Sr isotope analysis of the limestones revealed a sharp increase in Sr isotope ratios between the precursor and initial NCIEs, which indicates that continental weathering of the Bohemian Massif increased rapidly in the latest Rhaetian. Multivariate analysis of major element contents in carbonate rocks also shows that intense chemical weathering of hinterland was accelerated after the precursor NCIE, supporting the results of Sr isotope analysis. Furthermore, this study reveals that the carbonate depositional environments of the Fatra Formation shifted toward the formation of specific iron-enriched oolite with the increase of continental weathering after the precursor NCIE. The concentrations of redox-sensitive major elements (e.g., manganese and iron) and multivariate analysis of major element data suggest that the iron-enriched oolites were formed by the influx of oxygen-depleted water masses into the shallow water depositional environment of the Fatra Formation. A possible origin of such oxygen-depleted water masses was the oxygen minimum zone (OMZ) that formed in the intermediate water-depth in the European basins (e.g., German and Eiberg basins) during the latest Rhaetian. These results suggest that the marine environment in the European basins may have changed to develop the OMZ with the increase of continental weathering during the latest Rhaetian, and these environmental changes may have played an important role in the marine ETE event.