2025 年 29 巻 p. 384-397
Relative sea level (RSL) changes in tectonically active areas might differ significantly from the global signal. Regional proxy data of tectonically active areas such as Taiwan are indispensable for constraining models on global sea-level change. Ostracod assemblages were examined in here to clarify paleoenvironmental and RSL changes in Dapeng Bay, Southern Taiwan. Overall, 13 genera and 16 species of ostracods were identified from two cores taken in the northern parts of the bay. Based on ostracod assemblage changes, RSL in Dapeng Bay suggested a gradual increase from about 10 to 2 m below the present sea level over the past 2000 years, although two century-scale relative highs in RSL were identified. The general trend of the increasing RSL over the past 2000 years corresponds to the subsidence rate around the study area. Two coeval RSL highs were recognized on the coasts of China and Taiwan between 500–700 CE and 1200–1400 CE, suggesting that a common factor affected the century-scale sea-level changes around Taiwan and China. Salinity in Dapeng Bay is controlled by precipitation influenced by regional and global climatic factors, including the East Asian summer monsoon (EASM) and El Niño–Southern Oscillation (ENSO).
Past sea level contains a combination of both vertical land movements and changes in ocean volume (e.g. Peltier, 1999; Lambeck et al., 2004). The most important contribution with a global signature for the relative sea level (RSL) changes during the Holocene is the mass exchange between the ice sheets and oceans—referred to as glacial isostatic adjustment (GIA)—and tectonic vertical land movements, which are important mainly on local and regional scales (e.g. Nakada et al., 1991; Lambeck et al., 2014). The eustatic sea level curve has been reconstructed based on various observations in tectonically stable regions (e.g. Woodroffe et al., 2015; Chua et al., 2021). However, RSL changes in tectonically active areas might differ significantly from the global signal (e.g. Yamada et al., 2018; Wang et al., 2020). To provide accurate models of global sea level change, regional proxy data on tectonically active areas such as Taiwan are indispensable.
Taiwan is surrounded by the East China Sea to the north, the South China Sea to the southwest, and the Philippine Sea to the south. Taiwan Island represents an exposed accretionary prism developed by the eastward subduction of the South China Sea oceanic lithosphere beneath the Philippine Sea Plate, which has been ongoing since the Middle Miocene (Huang et al., 2012). The regional tectonics related to the subduction of oceanic plates and seismic activities are strongly associated with RSL around Taiwan (e.g. Liu et al., 2021), making past sea level reconstruction difficult. Based on observations from localities far from former ice margins, the rate of global sea level rise showed a progressive decrease before 2.5 ka BP in the Late Holocene, after which ocean volumes remained nearly constant until the renewed sea level rise of 100–150 years ago (Lambeck et al., 2014). However, two high sea level intervals have been observed in the Late Holocene around Taiwan (Zhang and Huang, 1996). This sea level amplitude during 2–1.5 cal kyr BP in the Penghu Islands off western Taiwan is estimated to have been 1.3 ± 0.1 m higher than the present (Chen and Liu, 1996). Thus, RSL changes around Taiwan during the Late Holocene are not fully understood.
Because of their small size, high abundance, and excellent fossil record, ostracod shells are powerful tools for the reconstruction of the paleogeographic and paleoenvironmental histories, indicating water depth, temperature, salinity, and oxygen levels in marginal marine sediments (Hong et al., 2019). In particular, ostracod assemblages can reconstruct past sea level changes within several meter scales in bay and brackish lakes (e.g. Masuda et al., 2002; Yasuhara et al., 2002; Yamada et al., 2018). The aim of this study is to use ostracod assemblages to reconstruct bottom environments and RSL changes in Dapeng Bay, Southern Taiwan during the past 2,000 years.
The study area Dapeng Bay is a lagoon located on the southwestern coast of Pingtung County, Taiwan (Figure 1). The lagoon is approximately 3.5 km long, 1.8 km wide, and covers 5.32 km2. The water depth ranges from 1 m near the tidal inlet to 6 m in the inner bay, with a mean depth of 2.2 m (Hung et al., 2013). Water exchange between Dapeng Bay and Taiwan Strait is primarily driven by a semi-diurnal tide with a range <1 m, which is somewhat restricted by exchanging water through a narrow tidal inlet located in the western part of the lagoon (Hung et al., 2013). The water inflowing to the lagoon is mainly due to precipitation and river input, mostly freshwater inflows from the Linbian River, which runs on the eastern side of the lagoon. Freshwater input is low in the dry season (October–April) and high in the wet season (May–September) (Chung et al., 2011). The surface water temperature in the bay responds quickly to atmospheric temperature changes and ranges between 22°C and 32°C annually (Hung et al., 2003). Salinity, pH, dissolved oxygen, and biochemical oxygen demand are maintained at acceptable levels, at 29–32, 7.6–8.6, 4–11 mg/L, and 1.6–2 mg/L, respectively (Chen et al., 2005). In the past, Dapeng Bay was used for oyster farming, and numerous oyster racks and oyster shells form a distinctive island in the bay (Chung et al., 2011). The study cores DPW02 and DPW05 were drilled in paddy fields about 1.5 m above the present sea level.
Four cores were drilled along Dapeng Bay in 2018 by Sinotech Engineering Consultants, Ltd, Taiwan to identify tsunami deposits. Of these, DPW02 and DPW05, taken in the northern part of the bay, were examined in this study.
The core length of DPW02 (120°28′37.2″E, 22°27′46.8″N) is 15 m. The elevation of the core site is 1.42 m. The sediments of the core are mostly composed of mud. Fine-grained sand layers were found at core depths of 11.2–12.7 and 3.8–5.0 m. Further, alternations of sand and mud were recognized at core depths of 7.2–11.2 m (Figure 2). There are few shell fragments and carbonates in the core above a depth of 5 m, but they appear sporadically below that. In total, 22 1-cm-thick sediment samples were taken by spoon from middle core depths of 3.70, 3.92, 5.20, 6.10, 6.75, 7.10, 7.30, 7.75, 8.60, 8.90, 9.20, 10.30, 10.70, 11.60, 12.80, 12.81, 13.10, 13.50, 13.80, 14.10, 14.45, and 14.90 m for ostracod analysis.
The core length of DPW05 (120°28′30.0″E, 22°27′21.6″N) is 15 m. The elevation of the core site is 1.45 m. The sediments at a core depth above 5 m are back-filling and were not used in this study. The sediments at a core depth of 7.40–13.85 m are mostly composed of alternating sand and mud. Fine-grained sand layers were found at core depths of 8.80–9.05 and 13.85–15.00 m. Further, the sediments at core depths of 5.00–7.40 and 10.75–12.15 m are composed mostly of mud (Figure 2). Sporadic shell fragments and plant fragments are present throughout the core. A total of 55 1-cm-thick sediment samples were taken by spoon at core depths of 5.10, 5.30, 5.50, 5.60, 5.70, 5.90, 6.10, 6.30, 6.50, 6.70, 6.90, 7.10, 7.30, 7.50, 7.70, 7.90, 8.10, 8.30, 8.50, 8.70, 8.90, 9.10, 9.30, 9.50, 9.70, 9.90, 10.10, 10.20, 10.30, 10.45, 10.50, 10.65, 10.70, 10.90, 11.10, 11.25, 11.30, 11.50, 11.70, 11.90, 12.10, 12.30, 12.50, 12.70, 12.90, 13.10, 13.30, 13.50, 13.70, 13.90, 14.10, 14.30, 14.50, 14.70, and 14.90 m for ostracod analysis. All sample numbers were shown in terms of core names and depths.
All unprocessed samples were soaked in water and heated on a hot plate for 30 min, then washed through a 63 μm opening sieve and dried in an oven at 60°C. The resultant residues were divided into aliquots containing about 200 ostracod specimens. All ostracod specimens were hand-picked from the divided residues under a stereo microscope.
To divide samples into groups relating to environmental factors, Q-mode cluster analysis was performed on samples containing more than 30 ostracod specimens using the software PAST (Hammer et al., 2001). The analysis was based on Horn’s overlap index of similarity (Horn, 1966) and the unweighted pair group method using arithmetic average (UPGMA).
Age models for both cores were based on five accelerator mass spectrometry (AMS) radiocarbon (14C) dates of woods and charcoals taken from core depths of 7.05 and 14.70 m in DPW02 and 9.36–9.40, 12.33, and 14.90 m in DPW05. Samples taken from a core depth of 14.90 m in DPW05 were analyzed using an NEC 1.5SDH-1 0.5MV compact AMS (National Electrostatics Corporation, Wisconsin, USA) at The University Museum, The University of Tokyo in Tokyo, Japan. The other four samples were analyzed using an HVE Tandetron 1.0MV model 4110 BO-AMS (High Voltage Engineering Europa B.V., Amersfoort, The Netherlands) at the National Taiwan University AMS Laboratory (NTUAMS) in Taipei, Taiwan. Calibrations from 14C ages to calendar ages were performed by using the CALIB 8.1.0 correction program (Stuiver and Reimer, 1993) with IntCal20 data sets (Reimer et al., 2020), and the calibrated ages are represented by 2σ error.
The 14C ages of samples from DPW02 7.05 and 14.70 m were 520 ± 7 and 2292 ± 31 yr BP, respectively. The 14C ages of the 9.36–9.40, 12.33 and 14.90 m samples from DPW05 were 1137 ± 8, 1929 ± 14 and 2030 ± 27 yr BP, respectively (Table 1). Age models for both cores were established by calibrating the 14C ages to calendar years (Figure 3). DPW02 spans the interval from 307 BCE to present, whereas DPW05 spans the interval from 19 BCE to around 1400 CE.
| Sample number | LaboID | Core depth (m) | Material | Conventional age (14C BP) | Calendar years (BCE/CE) |
|---|---|---|---|---|---|
| DPW02 7.05 | NTUAMS-4432-1 | 7.05 | Wood | 520 ± 7 | 1417 ± 9 CE |
| DPW02 14.70 | NTUAMS-4433 | 14.70 | Wood | 2292 ± 31 | 307 ± 98 BCE |
| DPW05 9.36–9.40 | NTUAMS-4502 | 9.36–9.40 | Charcoal | 1137 ± 8 | 930 ± 45 CE |
| DPW05 12.33 | NTUAMS-4501 | 12.33 | Charcoal | 1929 ± 14 | 114 ± 87 CE |
| DPW05 14.90 | TKA-26869 | 14.90 | Wood | 2030 ± 27 | 19 ± 83 BCE |
At least 16 ostracod species belonging to 13 genera were present among 49 samples (Table 2). Of these samples, two in DPW02 and 19 in DPW05 contained more than 30 ostracod specimens (Table 2). These 21 samples were used to recognize vertical changes in the relative abundance of ostracods. The dominant species were Bicornucythere misumiensis, Hemicytheridea reticulata, Loxoconcha ocellata s.l., Loxoconcha zhejiangensis, Paracypria cf. inujimensis and Sinocytheridea impressa (Figure 4).
| core | depth (m) | Bicornucythere misumiensis Nakamura and Tsukagoshi | Bythocypris sp. 1 | Bythocypris sp. 2 | Candona sp. | Cyprinotus scholiosus (Sohn and Morris) | Hemicytheridea reticulata Kingma | Hemicytheridea aff. reticulata Kingma | Ilyocypris bradyi Sars | Loxoconcha ocellata s.l. Ho | Loxoconcha zhejiangensis Zhao | Neosinocythere sp | Paracypria cf. inujimensis (Okubo) | Propontocypris sp. | Sinocythere sp. | Sinocytheridea impressa (Brady) | Xestoleberis sp. | Gen et sp. indet. | miscellaneous | No. of specimens |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| DPW02 | ||||||||||||||||||||
| 5.20 | 3 | 13 | 16 | |||||||||||||||||
| ● | 7.30 | 251 | 6 | 1 | 29 | 2 | 1 | 255 | 1 | 6 | 552 | |||||||||
| 7.75 | 2 | 2 | ||||||||||||||||||
| 10.30 | 1 | 1 | ||||||||||||||||||
| 12.80 | 1 | 2 | 6 | 9 | ||||||||||||||||
| ● | 12.81 | 30 | 1 | 159 | 1 | 236 | 427 | |||||||||||||
| 14.10 | 1 | 1 | ||||||||||||||||||
| 14.45 | 1 | 1 | ||||||||||||||||||
| 14.90 | 2 | 2 | ||||||||||||||||||
| DPW05 | ||||||||||||||||||||
| 5.10 | 2 | 2 | 1 | 2 | 1 | 8 | ||||||||||||||
| ● | 5.30 | 1 | 15 | 1 | 30 | 47 | ||||||||||||||
| 5.50 | 1 | 1 | 2 | 4 | ||||||||||||||||
| 5.60 | 6 | 1 | 2 | 1 | 1 | 11 | ||||||||||||||
| 5.70 | 2 | 2 | 1 | 1 | 10 | 16 | ||||||||||||||
| ● | 5.90 | 4 | 221 | 225 | ||||||||||||||||
| ● | 6.10 | 1 | 1 | 22 | 3 | 7 | 1 | 12 | 47 | |||||||||||
| 6.30 | 1 | 1 | ||||||||||||||||||
| 6.50 | 2 | 2 | ||||||||||||||||||
| 6.70 | 1 | 1 | ||||||||||||||||||
| ● | 6.90 | 2 | 33 | 35 | ||||||||||||||||
| 7.70 | 1 | 2 | 3 | |||||||||||||||||
| 7.90 | 1 | 1 | 2 | 4 | ||||||||||||||||
| 8.10 | 1 | 1 | 1 | 5 | 1 | 9 | ||||||||||||||
| 8.30 | 3 | 1 | 4 | |||||||||||||||||
| 8.50 | 1 | 1 | 8 | 1 | 11 | |||||||||||||||
| ● | 8.70 | 1 | 16 | 21 | 38 | |||||||||||||||
| ● | 8.90 | 2 | 6 | 2 | 19 | 1 | 1 | 41 | 72 | |||||||||||
| 9.30 | 1 | 1 | ||||||||||||||||||
| 9.50 | 1 | 3 | 4 | |||||||||||||||||
| 9.70 | 1 | 1 | 1 | 1 | 4 | 8 | ||||||||||||||
| ● | 9.90 | 2 | 2 | 1 | 9 | 18 | 1 | 33 | ||||||||||||
| ● | 10.10 | 48 | 4 | 1 | 7 | 170 | 1 | 231 | ||||||||||||
| ● | 10.20 | 10 | 4 | 2 | 22 | 163 | 201 | |||||||||||||
| ● | 10.30 | 26 | 3 | 64 | 241 | 334 | ||||||||||||||
| ● | 10.45 | 5 | 18 | 1 | 1 | 113 | 138 | |||||||||||||
| ● | 10.50 | 6 | 3 | 40 | 257 | 306 | ||||||||||||||
| ● | 10.65 | 12 | 15 | 295 | 322 | |||||||||||||||
| ● | 10.70 | 6 | 14 | 356 | 376 | |||||||||||||||
| ● | 10.90 | 35 | 129 | 1 | 1,475 | 1,640 | ||||||||||||||
| ● | 11.25 | 1 | 18 | 25 | 44 | |||||||||||||||
| 11.30 | 1 | 5 | 19 | 25 | ||||||||||||||||
| ● | 11.50 | 3 | 58 | 3 | 64 | |||||||||||||||
| 11.70 | 1 | 2 | 18 | 21 | ||||||||||||||||
| ● | 13.30 | 2 | 1 | 9 | 41 | 7 | 88 | 148 | ||||||||||||
| 13.90 | 1 | 1 | ||||||||||||||||||
| ● | 14.10 | 1 | 16 | 4 | 10 | 31 | ||||||||||||||
| 14.30 | 1 | 2 | 3 | |||||||||||||||||
| 14.70 | 1 | 1 | ||||||||||||||||||
| 14.90 | 1 | 1 | 2 | |||||||||||||||||
| Total no. of species: 16 | Total no. of genera: 13 | |||||||||||||||||||
Of the two samples in DPW02 that contained more than 30 ostracod specimens (Figure 5), Sinocytheridea impressa was relatively abundant (>40%) in both. In addition, B. misumiensis and L. zhejiangensis were identified at a core depth of 12.81 m, and Hemicytheridea reticulata was identified at a core depth of 7.3 m.
Sinocytheridea impressa was relatively common through DPW05 and was most abundant at a core depth of 10.7 m (Figure 6). Hemicytheridea reticulata was relatively abundant in the lower and upper parts of the core but less abundant in the middle. Loxoconcha ocellata s.l. was relatively abundant at core depths of 13.30, 9.90, 8.90 and 8.70 m. The vertical changes in the relative abundances of B. misumiensis and L. zhejiangensis were very similar, and both are absent in samples above a core depth of 8.90 m. Although P. cf. inujimensis was only identified from four samples, it made up over 80% of the ostracod assemblages at core depths of 11.50, 6.90 and 5.90 m.
Five biofacies (DSL, DSLB, DSP, DSH and DE) were recognized based on a Q-mode cluster analysis of 21 samples (Figure 7). The stratigraphic distribution of these biofacies in the cores is shown in Figures 4 and 5. Biofacies DSL is represented by 4 samples (DPW05 8.70, 8.90, 9.90 and 13.30). Biofacies DSLB is represented by 9 samples (DPW02 12.81; DPW05 10.10, 10.20, 10.30, 10.45, 10.50, 10.65, 10.70 and 10.90). Biofacies DSH is represented by 4 samples (DPW02 7.30; and DPW05 5.30, 6.10 and 14.10). Biofacies DE is represented by 3 samples (DPW05 5.90, 6.90 and 11.50). Biofacies DSP is represented by only one sample (DPW05 11.25).
Biofacies DSL is characterized by the dominance of S. impressa and L. ocellata s.l.. Sinocytheridea impressa was the most dominant species in the samples evaluated in this study. It is widely distributed along the eastern margin of Eurasia (Tanaka et al., 2019). In modern surface sediments, this species is dominant in oligohaline to euhaline supratidal to upper-shelf waters less than 20 m deep along the coastline of China (Zhao, 1987; Zhao and Wang, 1988) and largely inhabits the muddy bottoms in lagoon-type environments (Tanaka et al., 2009, 2019). This species was also found in Pinqing Lake (Figure 1), a typical lagoon in South China (water depths <7 m with salinity of 20–30), and was abundant (>50%) in the muddy bottoms near the river mouth in Pinqing Lake with water depths <4 m and salinity of 20–25 (Wu et al., 2024). Loxoconcha ocellata is widely distributed at water depths <50 m along the eastern coastal areas of China with salinity of 3–30 (Zhao and Wang, 1988). It is also dominant in the sandy-silt bottoms near the low tide line near river mouth in the Hiuchinada Bay, Seto Inland Sea of Japan (Yamane, 1998). According to a study of cores from the western coastal lowland of the Bohai Sea in China, S. impressa and L. ocellata are only abundant in tidal flat and lower delta plain sediments (Xue et al., 1995). Thus, the depositional environment of this biofacies is interpreted as a sandy-silt bottom of a tidal flat near a river mouth with a salinity range of 20–25.
Biofacies DSLB is characterized by the dominance of S. impressa with lower abundances (0–20%) of L. zhejiangensis and B. misumiensis. Loxoconcha zhejiangensis is abundant in the intertidal zone of muddy bottoms along the coastal areas of China (Zhao, 1984). This species is dominant in Pinqing Lake along with S. impressa and is relatively abundant (>20%) in muddy bottoms with water depths <3 m and salinity of 20–22 (Wu et al., 2024). Bicornucythere misumiensis was first reported as Form M of Bicornucythere bisanensis in the Yatsushiro Sea of Kumamoto, southwestern Japan (Abe, 1988). Nakamura and Tsukagoshi (2022) compared the morphology of various specimens of the genus Bicornucythere and revealed that Form M is distinct from B. bisanensis and reclassified it as B. misumiensis. The type locality of this species is the muddy bottom of Konoura Fishing Port in the Yatsushiro Sea at a water depth of about 5 m with a salinity of about 30 (Nakamura and Tsukagoshi, 2022). This species is also found in the muddy bottoms of the innermost part of the Yatsushiro Sea at water depths <0.5 m (Tanaka et al., 2019) and the Isahaya Bay reservoir in the Ariake Sea of southwestern Japan at water depths <1.1 m (Nakamura, 2022), cooccurring with S. impressa. Therefore, the depositional environment of this biofacies is thought to be muddy bottoms in the innermost part of a lagoon at water depths <3 m and a salinity range of approximately 20–22.
Biofacies DSH is characterized by the dominance of Hemicytheridea reticulata and S. impressa. H. reticulata has been reported from the very shallow waters of the Indo-Pacific region (Zhao and Whatley, 1989; Hong et al., 2019). This species prefers brackish water conditions along the salinity gradient and exhibits a high tolerance to significant salinity variations (Barik et al., 2022). In Chilika Lake (mean water depth of 1.4 m with a maximum depth of 6.0 m), a lagoon on the northeastern coast of India, H. reticulata is most abundant ostracod in the interior region of the lagoon, away from both fluvial and seawater influence (Barik et al., 2022). According to Zhao (1987), this species lives at salinity greater than 25. Thus, the depositional environment of this biofacies is interpreted as a muddy bottom in the central part of a lagoon at water depths <4 m and a salinity range of 25–30.
Biofacies DE is characterized by more than 90% of P. cf. inujimensis. Paracypria inujimensis is a phytal species and is abundant in the intertidal zone of stony shores where algae grow (Okubo, 1980). The valves of this species can easily be transported from original habitats to greater depths postmortem, and the resultant residues of core samples contain almost no algal fragments. Therefore, the particularly high population density of P. cf. inujimensis in this biofacies may indicate strong influence from open sea that transported the valves of P. cf. inujimensis from a habitat with abundant seaweed. This biofacies is thought to represent event deposits, which could be the result of storms or tsunamis. However, our results cannot provide a detailed explanation of these events, and further studies are needed.
Biofacies DSP is characterized by the dominance of S. impressa and P. cf. inujimensis. Paracypria inujimensis is tolerant of marine and brackish conditions and is also dominant on silty bottoms of an artificial, rectangular channel parallel to the coastline of Kinkai Bay in Ushimado, Okayama, Japan. That habitat is influenced by the tide, and its salinity varies considerably (Smith and Kamiya, 2003). Considering these points, the depositional environment of this biofacies is interpreted to be the muddy bottom of the intertidal zone near a habitat with abundant seaweed with a salinity range of 20–30.
Vertical changes of the depositional environment in Dapeng BaySinocytheridea impressa was the most dominant species in all samples except DPW05 5.90, 6.90, and 11.50. This species largely inhabits the muddy bottoms of lagoon-type environments (Tanaka et al., 2009, 2019). Thus, the study site is interpreted to have been a lagoon environment throughout all depositional intervals. The Q-mode cluster analysis of ostracods suggests that the vertical changes of depositional environments in the cores DPW02 and DPW05 were defined as follows.
The site of DPW02 was the muddy bottom of the innermost part of a lagoon (biofacies DSLB) at around 120 CE. The paleo-water depth at the core site at that time was less than 3 m and the salinity was approximately 20–22. The period around 1350 CE is represented by biofacies DSH, suggesting that the core site became the muddy bottom of the central part of lagoon at water depths < 4 m and a salinity range of 25–30.
The changes in depositional environments, associated sea level and salinity changes from DPW05 are shown in Figure 8. The core site was the muddy bottom of the central part of a lagoon (biofacies DSH) at around 1 CE. The paleo-water depth at that time was less than 4 m, and the salinity range was 25–30. The core site changed to the sandy-silt bottom of a tidal flat near a river mouth with a salinity range of 20–25 (biofacies DSL) at around 60 CE. Thus, RSL fell (Figure 8) and salinity decreased between 1 CE and 60 CE. The core site became the muddy bottom of an intertidal zone near a habitat with abundant seaweed with a salinity range of 20–30 (biofacies DSP) between 300–450 CE, but there was no significant change in water depth from the previous period. However, the decline of L. ocellata, which is abundant in river mouths, indicated a decrease in river inflow. This led to a relative increase in seawater influence and increased salinity during this period. Furthermore, biofacies DE appeared at around 350 CE (Figure 6), indicating a strong influence from the open sea at that time. Between 500–800 CE, the core site was the muddy bottom of the innermost part of a lagoon at water depths < 3 m and a salinity range of approximately 20–22 (biofacies DSLB). Hence, RSL rose from the previous period. However, the relatively high abundance of L. zhejiangensis suggests that salinity was relatively low during that period, which indicates that freshwater input increased at that time. Furthermore, a decline in the relative abundance of L. zhejiangensis was accompanied by an increase in the relative abundance of B. misumiensis between 700 and 800 CE (Figure 6). Bicornucythere misumiensis was found in the water depths <1.1 m accompanying occurrence of S. impressa (Nakamura, 2022; Tanaka et al., 2019). Thus, the area of the lagoon was reduced, and water depth decreased again from 700 CE (Figure 8). The core site became the sandy-silt bottom of a tidal flat near a river mouth with a salinity range of 20–25 (biofacies DSL) again between 800–1050 CE, which indicates that RSL fell again. Between 1200–1400 CE, the core site became the muddy bottom of the central part of a lagoon at water depths < 4 m and a salinity range of 25–30 (biofacies DSH) again, suggesting an enlargement of the lagoon and an increase in water depth. Thus, RSL rose, and salinity increased from the previous period. Furthermore, the influence of the open sea was more than twice as strong (biofacies DE) between around 1230 and 1370 CE.
As mentioned, RSL gradually increased between 1 CE and 1500 CE. Furthermore, based on the analysis of ostracod assemblages, at least two RSL changes were identified from the core DPW05. RSL fell between 1–60 CE and at 800 CE and rose between 450–500 CE and 1050–1200 CE, although the amplitudes of these changes were estimated within a few meters (Figure 8).
RSL changes in Dapeng BayRSL changes during the Holocene were affected by glacio-hydro-isostatic adjustment in addition to eustasy and regional tectonics (Lambeck et al., 2004). Overall, based on ostracod assemblage changes, RSL in Dapeng Bay increased gradually from approximately 10 to 2 m below the present sea level over the past 2000 years. Based on radiocarbon dates from marine terraces and cores, the average subsidence rate in the study area was about 4 mm/yr (Lai et al., 2002). This subsidence rate corresponds to the general trend of RSL change in the study area. Thus, the general trend of RSL in Dapeng Bay is believed to have been affected by regional tectonics over the past 2000 years.
In addition to the general trend, there are two century-scale RSL changes in the study area (Figure 9b). Zhang and Huang (1996) reconstructed the Holocene sea level along the coast of Taiwan according to elevation and 14C age data for uplifted coral reefs. According to these data, two sea level highs occurred at about 600 and 1300 CE, and the amplitude of the two sea level changes was about 4 m (Figure 9a). Yu et al. (2023) showed similar Holocene sea level trends along the southern coast of Fujian in China, which lies to the northwest of Taiwan across the Taiwan Strait. Based on elevation and 14C age data for sea level indicators, they found two sea level highs at about 550 and 1300 CE, and the amplitude of the two sea level changes was about 3 m. A comparison of geological records and glacio-hydro-isostatic adjustment modeling shows that hydro-isostatic influence has dominated the RSL change in that region since 7000 cal yr BP, although this mechanism remains poorly understood (Wang et al., 2022). The sea level records around Taiwan also suggest that at least two RSL high stand intervals of similar scale (3–4 m) occurred over the past 2000 years (Figure 9a). The coincidence of two high RSL intervals and sea level amplitudes around the coasts of China and Taiwan, including the study area, implies that the century-scale RSL changes recognized in Dapeng Bay were caused by global sea level change.
The salinity in Dapeng Bay was relatively low at around 60 CE and between 500–1000 CE and relatively high at around 1 CE and between 300–450 CE and 1200–1400 CE. In the present, Dapeng Bay is a lagoon, and freshwater inflow comes mainly from precipitation and terrestrial water, which are caused by dry and wet seasonal changes (Chung et al., 2011). Based on recent records, the annual precipitation of southern Taiwan is mainly controlled by precipitation in June (rainy season) and July and August (cyclonic storms and typhoons) (Li et al., 2015). Thus, salinity in Dapeng Bay might be influenced by precipitation. Furthermore, recent precipitation in southern Taiwan appears to have been affected by the East Asia summer monsoon (EASM) and surface oceanic conditions such as the El Niño Southern Oscillation (ENSO) (Li et al., 2015).
Compared with standardized δ18O stalagmite values from Heshang Cave in central China (Figure 9c) (Hu et al., 2008; Sagawa et al., 2014) and the Southern Oscillation Index (SOI) (Figure 9d) (Yan et al., 2011), high-salinity periods in the study area correspond to intervals of a strong EASM and El Niño conditions, whereas low-salinity periods correspond to intervals of a weak EASM and La Niña conditions. Climate records from previous studies show that a weak EASM enhances precipitation along the Meiyu front (or Baiu front), which leads to a rainy season in June, whereas a strong EASM suppresses the Meiyu front over East Asia, which reduces precipitation in June (Wang et al., 2001; Li et al., 2015). Under El Niño conditions, the Western Pacific warm pool and Walker circulation shift eastward such that cyclonic storms and typhoons tend to vanish or turn away northward before reaching Taiwan, which reduces precipitation in July and August (Woodruff et al., 2009; Chen et al., 2012). La Niña conditions increase the frequency of cyclonic storms and typhoons, which enhances precipitation in July and August in Taiwan (Chen et al., 2012). Thus, a strong EASM and El Niño conditions reduce precipitation in June, July, and August, which leads to a dry climate in southern Taiwan. The dry climate reduces water inflow and precipitation into Dapeng Bay, which increases its salinity. A high-resolution stalagmite record from the past 1300 years from southern Taiwan shows that such a drying trend occurred between 1200–1400 CE (Li et al., 2015). On the other hand, a weak EASM and La Niña conditions enhance precipitation in June, July, and August, which leads to a wetter climate, and decreases salinity in the lagoon.
Thus, high-salinity occurred in Dapeng Bay under dry conditions caused by a strong EASM and El Niño conditions, whereas low-salinity occurred under wet conditions caused by a relatively weak EASM and La Niña conditions. These findings suggest that water salinity in this shallow lagoon is controlled by precipitation influenced by both regional and global factors such as EASM and ENSO.
(1) Sixteen ostracod species belonging to 13 genera were identified in cores DPW02 and DPW05 from Dapeng Bay, Southern Taiwan.
(2) Based on our paleoenvironmental reconstruction of DPW05, Dapeng Bay has been a lagoon for the past 2000 years. The base of DPW05 was deposited at around 1 CE in the center of the lagoon at a water depth less than 4 m and salinity of 25–30. The area and water depth of the lagoon decreased, and the site became a tidal flat near a river mouth by around 60 CE. Between 300–450 CE, the inflow of freshwater decreased, and the site became an intertidal zone near a habitat with abundant seaweed. Thereafter, the lagoon enlarged and deepened between 450–500 CE before its area and water depth decreased again between 700–800 CE. The environment changed to a tidal flat near a river mouth again between 800–1050 CE. Between 1050–1200 CE, the water depth of the lagoon increased again, and the site became the center of the lagoon at a water depth less than 4 m and a salinity of 25–30 between 1200–1400 CE.
(3) Long-term increases in RSL from approximately 10 m to 2 m below the present sea level in Dapeng Bay were affected by regional tectonics, whereas millennial-scale RSL changes were a common phenomenon throughout eastern China and Taiwan.
(4) High-salinity environments in Dapeng Bay occurred under dry conditions caused by a strong East Asian summer monsoon (EASM) and El Niño, whereas low-salinity environments occurred under wet conditions caused by a relatively weak EASM and La Niña.
We would like to express our deep gratitude to Koichi Hoyanagi (Shinshu University) for his helpful support. We thank Takayuki Omori (The University of Tokyo) for his help with radiocarbon dating. We also deeply acknowledge two anonymous reviewers and editor Atsushi Yabe for improving our manuscript.
S.T. was primarily responsible for the ostracod analysis and its interpretation, drafted the manuscript and compiled all figures. K.Y. and A.T.L. initiated the study. A.T.L and N.H. drilled the cores, prepared the samples and provided basic data such as lithofacies description. S.T., R.K. and K.Y. identified ostracod specimens. K.Y. revised the manuscript. N.H., C.T., Kuo-wei, S. and Kenta, S. carried out AMS 14C analysis. All authors contributed to the writing of the paper.
