10:45 AM - 12:15 PM
[BPT04-P02] Early Pleistocene deep-sea paleoenvironments in the Kazusa forearc basin based on benthic foraminifera
Keywords:Paleoceanographic environment, Benthic foraminifera, Kiwada Formation, Kazusa Group, Boso peninsula, Early Pleistocene
The two sea-surface currents exist off the eastern coast of the Japanese archipelago, the Kuroshio Current, a part of the North Pacific subtropical gyre, and the Oyashio Current, a part of the North Pacific subpolar gyre. These are the western boundary currents of the subtropical and subarctic gyres in the North Pacific and form the transition water eastward off the Boso Peninsula. These surface currents in the North Pacific Ocean have been strongly related to climate changes between low and high latitudes. Therefore, paleoceanographic studies in the North Pacific are important to understand climatic change on a global scale.
The Kazusa Group in the Boso Peninsula, situated on the Pacific side in the middle part of the Japanese archipelago, can be one of the good geological records in order to investigate past climatic changes in the North Pacific. Moreover, many stratigraphic records and high accumulation rates of the Kazusa Group also enable us to perform high-resolution paleoceanographic analysis. Recently, a high-resolution age model was established in the upper part of the Kiwada Formation, a lower formation in the Kazusa and the environmental changes in sea-surface conditions were reconstructed (Kuwano et al., 2021, 2022). On the basis of the analyses of calcareous nannofossils and oxygen isotopes of foraminifera in this formation, glacial-interglacial oceanographic changes were extracted from sea-surface proxies. Here, we present benthic foraminiferal assemblages in the Kiwada Formation to reconstruct deep-sea environmental changes in the Kazusa basin during 1310–1204 ka.
This study examined 22 samples from the upper part of the Kiwada Formation, and at least 81 genera and 100 benthic foraminifera species were recognized. During 1310–1204 ka, the depositional depth of the study area is estimated to be the middle bathyal zone (approximately 1000–2000 m deep) because of the continuous occurrence of Stilostomella spp. throughout the section. The deep-sea environments of the study area were similar to those under the Kuroshio Current based on co-occurrences of Globocassidulina spp., Cassidulina ubcarinate, and other characteristic taxa dominated in the Kuroshio region. Time-progressive changes of several benthic foraminiferal taxa generally coincide with the glacial-interglacial cycles, suggesting the influence of the global ice volume oscillation on deep-sea environment in the basin.
Additionally, the following characteristic environmental changes are inferred from benthic foraminiferal occurrences; 1) characteristic occurrence of Melonis pompilioides and Bulimina rostrata during 1274–1248 ka suggests the invasion of by high oxygen water mass and the low supply of organic matter fluxes, 2) abundant occurrences of Pseudoparrella naraensis seem to relate with episodic supplies of fresh phytodetritus from sea-surface during 1248–1243 ka, corresponding to a southward shift of nutrient-rich Oyashio Current (Kuwano et al., 2022), 3) abundant supply of organic matters from the sea-surface and dysoxic deep-sea environments were inferred during 1230–1204 ka because of occurrences of Bolivina robusta and Bulimina aculeata. In addition, the presence of cool-water taxa, Cassidulina delicata, and Cassidulina norcrossi characterizes during this interval. It means that the bottom waters in the Kasusa basin were composed of not only the Kurosio origin but also the Oyashio origin waters.
Kuwano et al., Stratigraphy, 18(2), 103-121, 2021.
Kuwano et al., Palaeogeography, Palaeoclimatology, Palaeoecology, 592, 110873, 2022.
The Kazusa Group in the Boso Peninsula, situated on the Pacific side in the middle part of the Japanese archipelago, can be one of the good geological records in order to investigate past climatic changes in the North Pacific. Moreover, many stratigraphic records and high accumulation rates of the Kazusa Group also enable us to perform high-resolution paleoceanographic analysis. Recently, a high-resolution age model was established in the upper part of the Kiwada Formation, a lower formation in the Kazusa and the environmental changes in sea-surface conditions were reconstructed (Kuwano et al., 2021, 2022). On the basis of the analyses of calcareous nannofossils and oxygen isotopes of foraminifera in this formation, glacial-interglacial oceanographic changes were extracted from sea-surface proxies. Here, we present benthic foraminiferal assemblages in the Kiwada Formation to reconstruct deep-sea environmental changes in the Kazusa basin during 1310–1204 ka.
This study examined 22 samples from the upper part of the Kiwada Formation, and at least 81 genera and 100 benthic foraminifera species were recognized. During 1310–1204 ka, the depositional depth of the study area is estimated to be the middle bathyal zone (approximately 1000–2000 m deep) because of the continuous occurrence of Stilostomella spp. throughout the section. The deep-sea environments of the study area were similar to those under the Kuroshio Current based on co-occurrences of Globocassidulina spp., Cassidulina ubcarinate, and other characteristic taxa dominated in the Kuroshio region. Time-progressive changes of several benthic foraminiferal taxa generally coincide with the glacial-interglacial cycles, suggesting the influence of the global ice volume oscillation on deep-sea environment in the basin.
Additionally, the following characteristic environmental changes are inferred from benthic foraminiferal occurrences; 1) characteristic occurrence of Melonis pompilioides and Bulimina rostrata during 1274–1248 ka suggests the invasion of by high oxygen water mass and the low supply of organic matter fluxes, 2) abundant occurrences of Pseudoparrella naraensis seem to relate with episodic supplies of fresh phytodetritus from sea-surface during 1248–1243 ka, corresponding to a southward shift of nutrient-rich Oyashio Current (Kuwano et al., 2022), 3) abundant supply of organic matters from the sea-surface and dysoxic deep-sea environments were inferred during 1230–1204 ka because of occurrences of Bolivina robusta and Bulimina aculeata. In addition, the presence of cool-water taxa, Cassidulina delicata, and Cassidulina norcrossi characterizes during this interval. It means that the bottom waters in the Kasusa basin were composed of not only the Kurosio origin but also the Oyashio origin waters.
Kuwano et al., Stratigraphy, 18(2), 103-121, 2021.
Kuwano et al., Palaeogeography, Palaeoclimatology, Palaeoecology, 592, 110873, 2022.