Changes in ammonoid fauna and palaeoceanographic environment in the late Early Jurassic Northern Hemisphere | ジュラ紀古世後期の北半球におけるアンモノイド相と古海洋環境の変遷

The late Early Jurassic time-interval (about 187-175 Ma) is attracted by the Early Toarcian Oceanic Anoxic Event (OAE), regarded as a global and dramatic paleoenvironmental change. However the paleoceanographic environments during late Early Jurassic have been analyzed vigorously in Europe, contributions from the Asian regions are still limited. The lack of these data has disturbed the discussions of global paleoceanographic changes in this time-interval. The aims of this study are to analyze the paleoceanographic changes in the West Panthalassa (present East Asia) during late Early Jurassic based on the geochemical analyses of the Early Toarcian OAE in Japan, the short-term paleoenvironmental and faunal events, and the detailed transitional patterns of the ammonoid paleobiogeography for tracing the long-term paleoceanographic change, and to discuss the global paleoceanographic environment affected to the change of marine biota (ammonoids) in the Northern Hemisphere. In this study, the revision or the establishment of ammonoid zonal schemes in the Toyora and the Kuruma Groups and the mutual/international correlation of these zonations are discussed before analyzing the geochemistry and paleobiogeography because of the necessity to propose the chronological standard. Six ammonoid zones in the Toyora Group, the Lower to Middle Jurassic epicontinental deposits distributed in southwest Japan, and three zones in the Kuruma Group, the Lower Jurassic continental shelf sediments exposed in central Japan, are proposed in this study and are corresponded with the zonal schemes established outside Japan, especially with the standard ammonoid zonation discussed in the Northwest European province. The transitional patterns of the Japanese ammonoid faunas are analyzed by utilizing the assemblages from the Toyora and the Kuruma Groups. The late to middle Late Pliensbachian ammonoid assemblage from Japan is associated with the Boreal elements and shows a high similarity with the coeval Northeast Russian ammonoid fauna. However, in latest Pliensbachian, a turnover from the Boreal fauna to the Tethyan fauna via the Tethyan-Boreal mixed fauna is distinguished mainly in the assemblage from the Kuruma Group. This turnover seems to be mainly affected by the coeval regression during latest Pliensbachian to earliest Toarcian recognized in Europe, in addition to the starting of the warming event. In contrast, a faunal mixing of the Tethyan and the Boreal faunas recognized in the middle Early Toarcian assemblage is perhaps reflected by the abrupt transgression distinguished in this time-interval. Consequently, the late Early Jurassic ammonoid transitions in the Northern Hemisphere are closely related with the changes of eustatic sea level and seawater temperature. The concentrations of trace elements and the anomalies of REE are analyzed for discussing the Early Toarcian OAE in the Lower Nishinakayama Formation of the Toyora Group.

The positive excursions of the V and Co concentrations are recognized in the middle part of the P. paltus Zone, corresponded to earliest Toarcian, with a negative shift of the Ni concentration and a positive Ce anomaly. These geochemical anomalies suggest the oxygen-depleted marine condition biostratigraphically corresponded to the coeval minor anoxic environment in Europe. In contrast, the major Early Toarcian OAE recognized in Europe and North American Cordillera is not identified in the Toyora Group. Therefore, the effect of the major Early Toarcian OAE seemed to be limited only in the West Tethys and East Panthalassa. Moreover, a diversity fall of ammonoid assemblage is also identified during the oxygen-depleted redox condition in earliest Toarcian. This diversity crisis is characterized by the extinction of East Asian endemic Harpoceratinae, and the survival of only a species of pandemic Arieticeratinae. Consequently, the ammonoid diversity crisis in the middle part of the P. palms Zone is perhaps reflected by the anoxic event in East Asia. Thus, the relationships between the marine redox conditions and the faunal associations of Hildoceratidae probably suggest the difference of the anoxic tolerances between Harpoceratinae and Arieticeratinae.