1:45 PM - 3:15 PM
[HQR03-P12] Paleoenvironmental shifts based on assemblage changes of fossil ostracods in Dapeng Bay, Southern Taiwan in the past 2500 years
Keywords:Ostracod, Late Holocene, Southern Taiwan, Paleoenvironment, Sea level
1. Introduction
Taiwan is situated at the frontal edge of the East Asian Monsoon range, and surrounded by the East China Sea, the South China Sea, and the Philippine sea. This location makes Taiwan suitable for paleoenvironmental research. The past climate changes in Taiwan were identified based on the records from land (e.g., Wang et al., 2015). However, paleoceanographic records in and around Taiwan are still insufficient. This study reconstructed paleoenvironment in the past 2500 years of the Dapeng Bay, southern Taiwan, based on assemblage changes of fossil ostracods by using two drilling cores.
2. Study area and cores
The study area is Dapeng Bay, which is a lagoon located in the southwestern coast of Taiwan. Two cores DPW02 and DPW05 drilled in the northern part of the Bay, were examined in this study. Both cores were composed of mostly mud and fine-grained sand. Total 77 samples were used for the ostracod analysis. The age model for the cores based on four AMS 14C dates suggest that the cores were deposited in the past 2500 years.
3. Results and discussion
At least fifteen ostracod species belonging to thirteen genera were present in 49 samples. Of them, 20 samples contained more than 30 ostracod individuals. Based on Q-mode cluster analysis with these 20 samples, two biofacies (Da and Db) were recognized. Biofacies Da was divided into two sub-biofacies (Da-1 and Da-2). Biofacies Da is characterized by dominance of Sinocytheridea impressa, which is especially abundant in the brackish waters influenced by freshwater inflows along the Chinese coasts (Irizuki et al., 2005). Thus, the depositional environment of this biofacies is interpreted as a lagoon. The subordinate species of sub-biofacies Da-1 are Bicornucythere bisanensis s.l. and Loxoconcha zhejiangensis. Bicornucythere bisanensis is tolerant at water depth at 5–10 m in the inner bay around Japan (Ikeya and Shiozaki, 1993), and the water depth of the Xiangshan harbour, where L. zhejiangensis distributes, is 10–15 m (Cao et al., 1995). Therefore, the depositional environment of sub-biofacies Da-1 is interpreted near the central of lagoon with muddy bottom at water depths about 10 m. The subordinate species of sub-biofacies Da-2 is Hemicytheridea reticulata. This species has been reported in the Chilika Lagoon (water depth: 2–6 m), northeastern coast of India (Barik et al., 2022). Sub-bioifacies Da-2 also contains a few Ilyocypris bradyi, which is found in rivers of weak flow (Wilkinson et al., 2005). Therefore, this sub-biofacies is thought to be the river mouth of lagoon at water depths of about 2–6 m. Biofacies Db is characterized by high abundance of Paracypria inujimensis. This species is abundant at 21 in salinity around Yakushima, Southern Japan at water depth of below 5 m (Smith and Kamiya, 2006). Therefore, the depositional environment of this biofacies is thought to be the coast of lagoon at water depths of below 5 m with relatively high salinity.
Based on assemblage changes of fossil ostracods, relative sea level in the Dapeng bay was reconstructed. The RSL had increased gradually from -10 to 1 m above the present sea-level during the past 2500 years, although at least two RSL changes in centennial scale was identified. The increasing rate of the RSL corresponds to subsidence rate around the study area. The RSL rose at BC 400–200 and AD 300–700 and fell at BC 200–AD 300 and AD 700–1200. The amplitude of the two RSL changes was about 5 m. It showed maximum sea level at about AD 700, which was about 1.5 m higher than the present sea-level. Since the two RSL changes are identical in the coast of China and around Taiwan coastal area, it is possible that the common RSL changes occurred in these areas.
Taiwan is situated at the frontal edge of the East Asian Monsoon range, and surrounded by the East China Sea, the South China Sea, and the Philippine sea. This location makes Taiwan suitable for paleoenvironmental research. The past climate changes in Taiwan were identified based on the records from land (e.g., Wang et al., 2015). However, paleoceanographic records in and around Taiwan are still insufficient. This study reconstructed paleoenvironment in the past 2500 years of the Dapeng Bay, southern Taiwan, based on assemblage changes of fossil ostracods by using two drilling cores.
2. Study area and cores
The study area is Dapeng Bay, which is a lagoon located in the southwestern coast of Taiwan. Two cores DPW02 and DPW05 drilled in the northern part of the Bay, were examined in this study. Both cores were composed of mostly mud and fine-grained sand. Total 77 samples were used for the ostracod analysis. The age model for the cores based on four AMS 14C dates suggest that the cores were deposited in the past 2500 years.
3. Results and discussion
At least fifteen ostracod species belonging to thirteen genera were present in 49 samples. Of them, 20 samples contained more than 30 ostracod individuals. Based on Q-mode cluster analysis with these 20 samples, two biofacies (Da and Db) were recognized. Biofacies Da was divided into two sub-biofacies (Da-1 and Da-2). Biofacies Da is characterized by dominance of Sinocytheridea impressa, which is especially abundant in the brackish waters influenced by freshwater inflows along the Chinese coasts (Irizuki et al., 2005). Thus, the depositional environment of this biofacies is interpreted as a lagoon. The subordinate species of sub-biofacies Da-1 are Bicornucythere bisanensis s.l. and Loxoconcha zhejiangensis. Bicornucythere bisanensis is tolerant at water depth at 5–10 m in the inner bay around Japan (Ikeya and Shiozaki, 1993), and the water depth of the Xiangshan harbour, where L. zhejiangensis distributes, is 10–15 m (Cao et al., 1995). Therefore, the depositional environment of sub-biofacies Da-1 is interpreted near the central of lagoon with muddy bottom at water depths about 10 m. The subordinate species of sub-biofacies Da-2 is Hemicytheridea reticulata. This species has been reported in the Chilika Lagoon (water depth: 2–6 m), northeastern coast of India (Barik et al., 2022). Sub-bioifacies Da-2 also contains a few Ilyocypris bradyi, which is found in rivers of weak flow (Wilkinson et al., 2005). Therefore, this sub-biofacies is thought to be the river mouth of lagoon at water depths of about 2–6 m. Biofacies Db is characterized by high abundance of Paracypria inujimensis. This species is abundant at 21 in salinity around Yakushima, Southern Japan at water depth of below 5 m (Smith and Kamiya, 2006). Therefore, the depositional environment of this biofacies is thought to be the coast of lagoon at water depths of below 5 m with relatively high salinity.
Based on assemblage changes of fossil ostracods, relative sea level in the Dapeng bay was reconstructed. The RSL had increased gradually from -10 to 1 m above the present sea-level during the past 2500 years, although at least two RSL changes in centennial scale was identified. The increasing rate of the RSL corresponds to subsidence rate around the study area. The RSL rose at BC 400–200 and AD 300–700 and fell at BC 200–AD 300 and AD 700–1200. The amplitude of the two RSL changes was about 5 m. It showed maximum sea level at about AD 700, which was about 1.5 m higher than the present sea-level. Since the two RSL changes are identical in the coast of China and around Taiwan coastal area, it is possible that the common RSL changes occurred in these areas.