5:15 PM - 6:45 PM
[MIS12-P29] Paleoenvironmental reconstruction of the Japan Sea using radiolarian fossils from the Miocene Uchisugawa Formation, Tainai City, Niigata Prefecture, Japan.
Keywords:Biostratigraphy, Japan Sea, Miocene, Paleoenvironmental reconstruction, Radiolaria, Uchisugawa Formation
The Japan Sea is a marginal sea of the northwestern Pacific Ocean, connected to the open ocean through four shallow straits, Tsushima Strait, Tsugaru Strait, Soya Strait, Mamiya Strait, and is a semi-enclosed sea exchanging only surface water. The Japan Sea is considered to be a back-arc basin that opened from the Late Oligocene to the Middle Miocene (ca. 28–13 Ma) and has experienced various environmental changes under the influence of tectonic changes of the Japanese Islands and glacial sea-level changes.
In the Natsui Section on the right bank of the Tainai River, which flows through central Tainai City, Niigata Prefecture, the Middle Miocene Shimoseki Formation, the Middle to Upper Miocene Uchisugawa Formation, and the Pliocene to Pleistocene Kuwae Formation are continuously exposed on the terrace cliffs along the river. Biostratigraphic and paleoenvironmental studies have been conducted in this section using various microfossils, among which diatoms and radiolarians are reported to be abundant in the Uchisugawa Formation of the Middle to Upper Miocene. In this section, a rough radiolarian stratigraphy was studied by Motoyama et al. (2017) using 12 samples, but no paleoenvironmental reconstructions were made. In this study, 53 samples were collected from the Uchisugawa Formation of the Natsui Section in order to determine the age based on radiolarian stratigraphy and to reconstruct the paleoenvironment of the Japan Sea during the Middle to Late Miocene.
As a result, the basal to middle part of the Uchisugawa Formation can be correlated with the Eucyrtidium inflatum Zone (15.3–11.8 Ma) and the middle to upper part of the Uchisugawa Formation with the Lychnocanoma magnacornuta Zone (11.8–9.1 Ma), similar to Motoyama et al. (2017). The disappearance of Cyrtocapsella japonica, which marks the boundary between the Cyrtocapsella japonica Subzone and Collosphaera reynoldsi Subzone, is recognized in the upper part of the Uchisugawa Formation. The upper to uppermost part of the Uchisugawa Formation can be correlated with the Lipmanella redondoensis Zone (9.1–7.3 Ma), similar to Motoyama et al. (2017).
Q-mode cluster analysis was conducted on raw data using PAST (Hammer et al., 2001), a free statistical analysis software, and 11 clusters were identified from the drawn dendrogram. And then, four paleoenvironmental stages (Stages I, II, III and IV) were identified based on the temporal trends of these clusters. Based on modern radiolarian ecology, paleoenvironments were reconstructed as follows; during Stages I and II (ca. 15.2–13.0 Ma, 13.0–11.8 Ma, respectively), warm and calm environments with inflows of both warm and cold currents, and the reduction in the abundance of cool water species suggest that Stage II was a warm current dominated environment compared to Stage I. In Stage III (ca. 11.8–10.0 Ma), the lack of warm, intermediate, and deep-water species suggests changes in the amount and path of warm current and sea-level or sill depth changes. During Stage IV (ca. 10.0–8.9 Ma), a significant decrease in the diversity of warm water species and the continuous occurrence of deep-water species suggest the progression of cooling and an increase in the strait depth, which allowed deep water exchange between the Japan Sea and the Pacific.
In the Natsui Section on the right bank of the Tainai River, which flows through central Tainai City, Niigata Prefecture, the Middle Miocene Shimoseki Formation, the Middle to Upper Miocene Uchisugawa Formation, and the Pliocene to Pleistocene Kuwae Formation are continuously exposed on the terrace cliffs along the river. Biostratigraphic and paleoenvironmental studies have been conducted in this section using various microfossils, among which diatoms and radiolarians are reported to be abundant in the Uchisugawa Formation of the Middle to Upper Miocene. In this section, a rough radiolarian stratigraphy was studied by Motoyama et al. (2017) using 12 samples, but no paleoenvironmental reconstructions were made. In this study, 53 samples were collected from the Uchisugawa Formation of the Natsui Section in order to determine the age based on radiolarian stratigraphy and to reconstruct the paleoenvironment of the Japan Sea during the Middle to Late Miocene.
As a result, the basal to middle part of the Uchisugawa Formation can be correlated with the Eucyrtidium inflatum Zone (15.3–11.8 Ma) and the middle to upper part of the Uchisugawa Formation with the Lychnocanoma magnacornuta Zone (11.8–9.1 Ma), similar to Motoyama et al. (2017). The disappearance of Cyrtocapsella japonica, which marks the boundary between the Cyrtocapsella japonica Subzone and Collosphaera reynoldsi Subzone, is recognized in the upper part of the Uchisugawa Formation. The upper to uppermost part of the Uchisugawa Formation can be correlated with the Lipmanella redondoensis Zone (9.1–7.3 Ma), similar to Motoyama et al. (2017).
Q-mode cluster analysis was conducted on raw data using PAST (Hammer et al., 2001), a free statistical analysis software, and 11 clusters were identified from the drawn dendrogram. And then, four paleoenvironmental stages (Stages I, II, III and IV) were identified based on the temporal trends of these clusters. Based on modern radiolarian ecology, paleoenvironments were reconstructed as follows; during Stages I and II (ca. 15.2–13.0 Ma, 13.0–11.8 Ma, respectively), warm and calm environments with inflows of both warm and cold currents, and the reduction in the abundance of cool water species suggest that Stage II was a warm current dominated environment compared to Stage I. In Stage III (ca. 11.8–10.0 Ma), the lack of warm, intermediate, and deep-water species suggests changes in the amount and path of warm current and sea-level or sill depth changes. During Stage IV (ca. 10.0–8.9 Ma), a significant decrease in the diversity of warm water species and the continuous occurrence of deep-water species suggest the progression of cooling and an increase in the strait depth, which allowed deep water exchange between the Japan Sea and the Pacific.