We studied fossil benthic foraminifera in three and seven sediment cores from the Nakdong River delta and of off Fukuoka, respectively, to assess sedimentation along the coasts of the strait between Korea and Japan during the early Holocene. Fossil benthic foraminifera in coarse-grained deposits/sandy sediments from ∼9.4–9.3 ka in cores ND-01 and KND-3 of the Nakdong River delta are characterized by a mixture of shallow-water and offshore species. These data suggest the development of transgressive deposits at ∼9.4–9.3 ka, likely associated with sea-level rise driven by the rapid retreat of glaciers at high latitudes (e.g. Laurentide Ice Sheet). Off Fukuoka, two clusters represent high-energy conditions characterized by common occurrences of attached benthic foraminifera; these clusters became dominant in the upper parts of our study intervals. Such attached species occurred commonly earlier, at around 10 ka, at deep offshore sites (> 60 m water depth) relative to shallow sites (< 60 m water depth). These findings are consistent with the development of current-dominated deposits at deep offshore sites at around 10 ka. Transgressive deposits in the Nakdong River delta and abundant attached species off Fukuoka at around 9 ka likely resulted from sea-level rise along the coasts of the strait between Korea and Japan driven by intermittent enhancement of the Tsushima Warm Current.
Introduction
The early Holocene is a period characterized by sea-level rise after the Younger-Dryas Event, such as the rise recorded in the Melt Water Pulse-1B centered at ∼11 ka (e.g. Lambeck et al., 2014). Various impacts of early Holocene sea-level rise have been recorded in shallow marine sediments of the East Asian margin (e.g. Saito, 1995; Hori et al., 2001, 2002; Tanabe et al., 2003; Tamura, 2004). In addition, changes in oceanic circulation associated with sea-level rise appear to have impacted sedimentation in coastal environments. The Taiwan/Tsushima Warm Current intensified during the early Holocene in the East Asian margin (Oba et al., 1991; Koizumi, 2008). Nishida and Ikehara (2013) studied sedimentary facies in eleven cores collected off Fukuoka, southwestern Japan (Figure 1). They argued that the Tsushima Warm Current impacted sedimentation in the strait between Korea and Japan during the late Pleistocene and Holocene, suggesting that no sedimentation occurred in the mid-Holocene (8.8–6.6 ka), possibly because the Tsushima Warm Current exhibited temporary intensification. Hsuing and Saito (2017) suggested that sediment supply sources shifted with the establishment of present-day oceanic circulation patterns in the East Asian margin during the early Holocene.
Figure 1.
a, locations of our two study areas. b, locations of core KND-3 and the published cores ND-01 (Takata et al., 2016), ND-02 (Takata et al., 2019), and KND-1 (Korean Institute of Ocean Science and Technology, 2017). The map of the Nakdong River delta was modified from the work by Cho et al. (2017). c, the locations of cores FV10-04, FV10-07-2, FV10-08, FV10-09, and FV10-10-2, as well as published cores FV10-05 and FV10-06-2 (Takata et al., 2018b). The map of the area off Fukuoka was modified from the work by Nishida and Ikehara (2013). Arrows in panel (a) show the pathway of the Tsushima Warm Current. Open stars and filled circle in panel (b) show the study cores in this study and a referred core from previous study, respectively. Open and filled circles in panel (c) show the study cores in this study and the referred cores from previous study, respectively.
Numerous sedimentological and seismic studies have been conducted out in the Nakdong River delta (Figure 1) and adjacent offshore area (Ryu et al., 2005, 2011; Kong and Park, 2007; Cho et al., 2017; Yoo et al., 2020). Borehole sediments from the present lower delta plain of the Nakdong River (Figure 1) are generally continuous, covering approximately 11–2 ka (Kim et al., 2015, 2021; Khim et al., 2019). Sandy sediments reportedly developed during the early Holocene in these boreholes on the lower delta plain and offshore area of the Nakdong River. For example, Kong and Park (2007) reported a thin sand layer (approximately 1 m thick; designated Unit II) with basal erosional contact in core SSDP 102 between 8.1 ka and 6.1 ka. They concluded that this sandy layer formed under high-energy conditions associated with an erosion event, which suggested that it had been influenced by the enhanced Tsushima Warm Current during the Holocene. Shin (2016) and Cho et al. (2017) suggested that thick sand and gravel layers representing the early Holocene in the middle part of core ND-01 formed via transgressive lag associated with sea-level rise.
Fossils have been frequently utilized to identify condensed deposits or transgressive lag (Kidwell, 1991; Kondo et al., 1998) in borehole sediments, where it is often difficult to evaluate the detailed horizontal extent of lithological features. Paleontological information has been used to distinguish specific transgressive deposits that exhibit rapid transgression in the early Holocene. Takata et al. (2016, 2019) reported the presence of diverse and nearly continuous benthic foraminifera in Holocene boreholes from the present lower delta plain of the Nakdong River. Because these sediments contain various microfossils, (e.g. diatom, foraminifera, and others) (Ryu et al., 2005, 2011; Cho et al., 2017), micro-paleontological data may provide useful information for paleoenvironmental studies in this region. Thus, micro-paleontological analysis in this area could yield insights regarding the formation of coarse-grained deposits during the sea-level rise of the early Holocene.
In this context, there is a need for detailed assessment of paleoenvironmental changes prior to approximately 8 ka in the Nakdong River delta and off Fukuoka; such assessment should include the use of new sediment cores for faunal analysis of benthic foraminifera. In this study, we investigated fossil benthic foraminifera in the newly collected sediment core KND-3 from the modern mouth of the Nakdong River; we also conducted detailed analysis of paleoenvironmental changes during the early Holocene. Furthermore, we examined benthic foraminifera in five cores (cores FV10-04, V10-07-2, FV10-08, FV10-09, and FV10-10-2) collected off Fukuoka (Nishida and Ikehara, 2013) to explore past depositional environments in the strait between Korea and Japan. Importantly, the Holocene sediment records of the Nakdong River delta are nearly continuous; they provide insights regarding possible paleoenvironmental changes along the coasts of the strait between Korea and Japan prior to intensification of the Tsushima Warm Current.
Study areas and materials
The Nakdong River, the longest river in South Korea (Figure 1), undergoes seasonal discharges because of monsoonal precipitation (Williams et al., 2013). Core KND-3 was drilled close to the modern river mouth (35°04'08.0”N 128°54'11.0”E), located between cores ND-01 and ND-02 (Figure 1b). These cores were drilled in the lower delta plain (Figure 1; Appendix 1). The drilled length of core KND-3 is 50.58 m. Lithologies of the core sediments include massive and bioturbated mud, muddy sand, and sand with gravel (Jeong et al., 2018). Sandy sediments are dominant in the lower and uppermost parts of the core, whereas muddy sediments (usually < 10% of coarse fraction (> 63 µm)) occupy the middle–upper part of the core and contain fossils of mollusks, echinoids, and foraminifera. Jeong et al. (2018) recognized six lithologic units A–F in core KND-3. Our study interval (37–26 m) corresponds to their Units B and C (40.2–33.0 m and 33.0–24.8 m, respectively).
Off Fukuoka, the seafloor between Kyushu and Iki Islands (southwestern Japan) (Figure 1c) is relatively flat with sand waves of ∼5 m heights and scattered NNE-SSW trends under the influence of the Tsushima Warm Current (Matsumoto, 2013). The Onga and Tsuri Rivers are the main source of riverine sediments in this area. Eleven sediment cores were obtained off Fukuoka in August, 2010. Ikehara (2001) reported that sand waves with NNE-SSW lineaments, consisting of coarser sediments are developed in the offshore area of off Fukuoka due to influence of the Tsushima Warm Current, whereas finer sediments are common around off the mouth of the (paleo-) Tsuri River. Nishida and Ikehara (2013) inferred depositional environments in this area during 11.6–8.4 ka mainly as a transgressive embayment deposit (lithofacies Fb), whereas inner shelf deposits (lithofacies Fc) became dominant above the hiatus (8.4–6.6 ka). They also suggested that such the transition happened with the intensification of the Tsushima Warm Current during the early Holocene with the sea-level rise.
Analytical methods
We conducted two measurements for accelerator mass spectrometry (AMS) 14C ages of core KND-3 at AMS Laboratory, the University of Arizona ( Appendix 2 (i1880-0068-28-2-148Appendix2.pdf)). The AMS 14C ages converted to a calendar age using CALIB 8.2 (Stuiver et al., 2021). The ages were calibrated using IntCal20 for plant materials (Reimer et al., 2020) and Marine20 for molluscan shells (Heaton et al., 2020) with a ΔR value of -154 (Kong and Lee, 2005). All age calibration data, including published AMS 14C ages of cores KND-3 (Jeong et al., 2018), ND-01 (Kim et al., 2015) and ND-02 (Shin, 2016; Khim et al., 2019; Kim et al., 2021), are summarized in Appendix 2 (i1880-0068-28-2-148Appendix2.pdf). In this study, we accepted that calendar ages represented as (ka) were identical to (cal kyr BP). The same calibrations with a ΔR value of 0 were conducted for the published ages in Nishida and Ikehara (2013) for the seven cores off Fukuoka.
For foraminiferal analysis, 54 sediment samples, 2 cm in thickness, were collected from ∼37 to ∼26 m in core KND-3. After being freeze-dried and weighed, samples were washed using a 63 µm sieve. Residues were dried at 50°C, then re-weighed, and split into 1/2 to 1/1024 aliquots. More than 200 benthic foraminiferal specimens, collected from the > 63 µm fraction, were identified and counted using a binocular microscope. The number of planktonic foraminifera was also counted from the same sample aliquots. Taxonomic assignments followed Matoba (1970) and Nomura and Seto (1992), whereas generic classification followed Loeblich and Tappan (1987). Abundances of benthic and planktonic foraminifera (> 63 µm) per unit weight of sediment were calculated using the counts of specimens, the number of splits, and the weight of each sample. Planktonic/total (benthic and planktonic) foraminiferal ratio (P/T ratio) was calculated based on these abundance data from the > 63 µm fraction. The same processing and analyses were conducted for 34 samples from additional five cores FV10-04, FV10-07-2, FV10-08, FV10-09, and FV10-10-2 (4, 11, 5, 6, and 8 samples, respectively).
Cluster analysis (Q-mode) was performed to determine sample groups for the Nakdong River delta, based on the count of benthic foraminiferal taxa in each sample of cores KND-3 (21 samples), ND-01 (44 samples, 41 samples from Takata et al. (2016) and 3 samples from Takata et al. (2019)) and ND-02 (29 samples from Takata et al. (2019)). Although we do not discuss benthic foraminiferal fauna in the upper part of core ND-01, we utilized these samples, because it would be useful to compare new data of core KND-3 to the previous cluster analysis of Takata et al. (2019). The data matrix consisted of 77 taxa, among which at least three specimens occurred in each sample, and 94 samples contained more than 50 specimens. Horn's index of overlap (Horn, 1966) was applied to determine the similarity between samples. Clustering was conducted using the unweighted pair group method with the arithmetic average in a program developed by Davis (1973) and modified by Hasegawa (1988). We also conducted Q-mode cluster analysis to determine sample groups for off Fukuoka, based on the count of benthic foraminiferal taxa in each sample of cores FV10-04, FV10-07-2, FV10-08, FV10-09, and FV10-10-2 (4, 11, 3, 8, and 4 samples, respectively), FV10-05 and FV10-06-2 (19 and 18 samples from Takata et al. (2018b)), as well as the Nakdong River delta. The data matrix consisted of 88 taxa, among which at least three specimens occurred in each sample, and 67 samples contained more than 50 specimens.
Figure 2.
Downcore variations of the abundances of benthic and planktonic foraminifera (> 63 µm), ratio of planktonic foraminifera to total (benthic and planktonic) foraminifera (P/T ratio), and occurrences of selected benthic foraminifera in core KND-3. “Calc. Porc. foraminifera” indicates all the taxa belonging to calcareous porcelaneous foraminifera. Ages (ka) determined through AMS 14C dating were obtained from the work by Jeong et al. (2018) and from this study.
Results
Nakdong River delta
Based on AMS 14C age at 36.3 m depth in core KND-3 is 9665 cal yr BP (9550–9780 cal yr BP) according to woody material analysis; the AMS 14C age at 33.6 m is 9381 cal yr BP (9242–9542 cal yr BP) by shell fragments ( Appendix 2 (i1880-0068-28-2-148Appendix2.pdf)). When these AMS 14C data were combined with result of Jeong et al. (2018), our study interval of core KND-3 was presumed to comprise 9.7–7.5 ka. Specifically, the 36.3–33.6 m interval corresponds to 9.67–9.38 ka, suggesting a high sedimentation rate (93.1 cm kyr-1). In contrast, the age at 34.4 m (9.98 ka) reported by Jeong et al. (2018) is older than the age at 36.3 m; these two ages overlap in the 2-sigma age uncertainty range. The age obtained at 34.4 m presumably represents older reworked materials, whereas it was not derived from very old strata (e.g. the upper Pleistocene).
Fossil benthic foraminifera were obtained from all 21 samples that had been taken from ∼37–26 m in core KND-3 (Figure 2). The abundances of benthic and planktonic foraminifera were 39–6395 and 4–10394 individuals per unit weight (# g – 1), respectively (Figure 2); the highest abundances were present in the ∼30–26 m interval. Preservation of benthic foraminifera was generally good, with no signs of major destruction or abrasion; these results suggested that no significant transportation or carbonate dissolution of foraminiferal tests had occurred, similar to observations in cores ND-01 and ND-02 described by Takata et al. (2016, 2019). Although we cannot dismiss the possibility that foraminiferal tests were transported, our observations suggest that benthic foraminifera were unlikely to be biased by significant transportation from another area or reworking. Buccella frigida (Cushman), Pseudoparrella naraensis Kuwano, Elphidium somaense Takayanagi, Haynesina sp. A, and Uvigerinella glabra (Millett) were common constituents of the cores (Figure 2; Appendix 3 (i1880-0068-28-2-148Appendix3.xls)). Bolivina robusta Brady, Eilohedra nipponica (Kuwano), Rosalina spp., Gavelinopsis spp., and Elphidium advenum (Cushman) were also present partly (Figure 2). These faunal associations are generally identical to the associations reported in previous studies of this area (Ryu et al., 2005, 2011; Takata et al., 2016, 2019, 2022).
Table 1.
Characteristics of the four sample clusters identified in cores KND-3, ND-01, and ND-02.
Figure 3.
Stratigraphic changes in the percentage of coarse fraction, ratio of planktonic foraminifera to total (benthic and planktonic) foraminifera (P/T ratio), and stratigraphic distributions of four clusters (An, Bn, Cn, and Dn) in cores ND-01, KND-3, and ND-02.
The percent of sediment in the coarse fraction was high in the ∼37–32 m interval of core KND-3. The planktonic to total foraminifera (P/T) ratio increased rapidly above 36.4 m in this core (Figure 2). Some common species show specific downcore variations. Ammonia “beccarii” forma 1 and Haynesina sp. A were common at ∼36–33 m, whereas E. nipponica increased continuously above ∼33 m (Figure 2). In addition, B. frigida showed two peaks at 36.6 m and 33 m; E. advenum was also common at 33 m. Thus, the foraminiferal fauna generally shows two transitions in core KND-3 (at 37–36 m and 33 m).
Based on Q-mode cluster analysis, four clusters were recognized in cores KND-3, ND-01 and ND-02 (Table 1; Figure 3; Appendix 4 (i1880-0068-28-2-148Appendix4.pdf)). The characteristics of each cluster are outlined in Table 1. Nine samples from the upper- and lowermost parts of core ND-01, as well as the lowermost part of core ND-02 (Figure 3), belong to cluster An, which includes B. frigida, E. somaense, Haynesina sp. A, A. “beccarii” forma 1, and A. “beccarii” forma 2. Three samples from the upper part of core ND-01 (Figure 3) belong to cluster Bn, containing Pseudorotalia gaimardii compressiuscula, Quinqueloculina spp., E. somaense, E. advenum, and B. frigida. Twenty-two samples from the middle to upper parts of core ND-01 (Figure 3) belong to cluster Cn, which includes Haynesina sp. A, E. somaense, E. advenum, B. frigida, Nonionella stella, P. naraensis, and U. glabra. Sixty samples from the majority of cores KND-3 and ND-02, as well the lower parts of cores ND-01 (Figure 3), belong to cluster Dn. In this cluster, P. naraensis, E. advenum, and E. nipponica were the common taxa. Sample clusters An, Bn, Cn, and Dn in this study generally correspond to sample clusters C, A, B, and D, respectively, defined for cores ND-01 and ND-02 by Takata et al. (2019). Although small differences in clusters An and Bn were found in the upper part of core ND-01, the stratigraphic distributions of clusters An, Cn, and Dn in the lower parts of the three cores are consistent with the results and interpretation of cluster analysis reported by Takata et al. (2019).
Off Fukuoka
Fossil benthic foraminifera were obtained from most samples of cores collected off Fukuoka, although they were very rare in some samples from the lower part of the study interval in cores FV10-08 and FV10-10-2. Preservation of benthic foraminifera was generally good, with no signs of serious destruction or abrasion; these results suggested that no significant transportation or carbonate dissolution of foraminiferal tests had occurred (Figures 4, 5), similar to those in cores FV10-05 and FV10-06-2 (Takata et al., 2018b). Although we could not dismiss the possibility that foraminiferal tests were transported, their features suggest that these benthic foraminifera were unlikely to be biased because of significant transportation from another area or reworking. Faunal associations, including abundant Quinqueloculina spp., Gavelinopsis spp., Cibicides lobatulus, and Hanzawaia nipponica, are similar to the associations described by Takata et al. (2018b) ( Appendix 5 (i1880-0068-28-2-148Appendix5.xls)). Cibicides spp., Quinqueloculina spp. and Gavelinopsis spp. are common in cores from the western area (cores FV10-04, FV10-05, FV10-06-2, FV10-07-2, and FV10-08) and from the eastern area (cores FV10-09 and FV10-10-2) ( Appendix 5 (i1880-0068-28-2-148Appendix5.xls)). No significant spatial differences were detected in the faunal associations of benthic foraminifera in the study area.
The abundances of benthic and planktonic foraminifera generally increased upward in cores FV10-07-2 and FV10-09 ( Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)), as well as core FV10-05 (Takata et al., 2018b). In contrast, the timings of the increasing are variable among these cores. In core FV10-07-2, C. lobatulus and calcareous porcelaneous taxa (largely Quinqueloculina seminulum and Quinqueloculina akneriana) are common in association with Gavelinopsis spp., H. nipponica, and Elphidium advenum ( Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)). In core FV10-09, calcareous porcelaneous taxa (largely Q. seminulum and Q. akneriana) are generally dominant in association with Gavelinopsis spp. and Cibicides spp. ( Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)). Thus, the faunal associations in the deeper depth of the western area (core FV10-07-2) and the eastern area (core FV10-09) are partly different from that of the shallower depths of the western area (cores FV10-05 and FV10-06-02; Takata et al., 2018b).
Based on Q-mode cluster analysis, six clusters were recognized in the cores collected off Fukuoka (Table 2; Appendix 6 (i1880-0068-28-2-148Appendix6.pdf)). The characteristics of each cluster are presented in Table 2 and Figure 6. Thirty-nine samples from the upper parts of cores FV10-05, FV10-06-2, FV10-04, FV10-07-2, FV10-08, FV10-09, and FV10-10-2 belong to cluster Af, and Quinqueloculina spp., Cibicides spp., Gavelinopsis spp., E. advenum, and Hanzawaia nipponica are common taxa. Four samples from 41–51 cm and 70.5–80.5 cm in core FV10-07-2 belong to cluster Bf, in which Cibicides spp., Quinqueloculina spp., E. advenum, H. nipponica, and Gavelinopsis spp., are common. Three samples from 61 and 90.5 cm in core FV10-07-2 and 60 cm in core FV10-08 belong to cluster Cf, in which Quinqueloculina spp., Cibicides spp., Gavelinopsis spp., E. advenum, and Rosalina spp. are common. Fifteen samples from the middle part of core FV10-05 and the lower parts of cores FV10-06-2 and FV10-10-2 belong to cluster Df, in which Gavelinopsis spp., Quinqueloculina spp., E. advenum, Bolivina spp., and Rosalina spp. are the common. Five samples from the lower part of our study interval in core FV10-05 belong to cluster Ef, in which Quinqueloculina spp., E. advenum, Gavelinopsis spp., and Rosalina spp. are common. One sample from 61 cm in core FV10-04 belongs to cluster Ff, in which Gavelinopsis spp., Ammonia “beccarii” forma 2, E. advenum, Quinqueloculina spp., Cibicides spp., and Rosalina spp. are common. Cluster Af is only present in the core top sediments among all the seven cores, whereas the other five clusters (Bf, Cf, Df, Ef, and Ff) occurred temporally in the older sediments. In addition, stratigraphic distributions of the clusters tend to be variable in the lower parts of both cores FV10-05 and FV10-07-2 (western area), whereas these are generally uniform in the upper portion of these cores and throughout the study interval of core FV10-09 (eastern area) (Figure 6; Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)).
Discussion
Benthic foraminiferal fauna in cores KND-3, ND-01 and ND-02
The paleoenvironment in the Nakdong River delta represented by each cluster was inferred primarily from ecological information regarding modern benthic foraminifera, as described by Matoba (1970), Akimoto and Hasegawa (1989), Inoue (1989), Oki (1989), Kosugi et al. (1991), Nomura and Seto (1992), and Woo et al. (1997).
Figure 4.
Light micrographs of selected benthic foraminifera from the area off Fukuoka. Scale bar is 100 µm. 1a, b, Sahulia conica from 61 cm in core FV10-07-2; 2a, b, Sahulia kerimbaensis from 60 cm in core FV10-08; 3a, b, Textularia sp. E from 90.5 cm in core FV10-07-2; 4, Spiroplectinella wrighiti from 70.5 cm in core FV10-07-2; 5a, b, Quinqueloculina lamarckiana from 62 cm in core FV10-10-2; 6a, b, Quinqueloculina akneriana from 1 cm in core FV10-09; 7a, b, Quinqueloculina seminulum from 1 cm in core Fv10-10-2; 8a, b, Bolivina robusta from 31 cm in core FV10-08; 9a, b, Bolivina humilis from 1 cm in core FV10-08; 10, Bolivina cf. decussata from 1 cm in core FV10-10-2; 11, Bolivina pseudoplicata from 1 cm in core Fv10-08; 12a, b, Loxostomina karreriana from 1 cm in core FV10-08; 13a–c, Pseudorotalia gaimardii compressiuscula from 1 cm in core FV10-07-2.
Figure 5.
Light micrographs of selected benthic foraminifera from the area off Fukuoka. Scale bar is 100 µm. 1a–c, Ammonia ketienziensis angulata from 1 cm in core FV10-08; 2a–c, Cibicides lobatulus from 1 cm in core FV10-07-2; 3a–c, Cibicides cf. lobatulus from 1 cm in core FV10-08; 4a–c, Cibicides cf. refulgens from 1 cm in core FV10-07-2; 5a–c, Hanzawaia nipponica from 1 cm in core FV10-08; 6a–c, Gavalinopsis sp. from 62 cm in core FV10-10-2; 7a–c, Discorbinella convexa from 1 cm in core FV10-08; 8a–c, Rosalina sp. A from 81 cm in core FV10-04; 9a, b, Elphidium advenum from 1 cm in core FV10-09; 10a, b, Elphidium kushiroense from 81 cm in core FV10-04.
Table 2.
Characteristics of the six sample clusters identified off Fukuoka.
Figure 6.
Stratigraphic changes in lithologic facies, the abundance of benthic foraminifera and the occurrences of clusters Af to Ff. Columnar sections are modified from Figure 3 in the work by Nishida and Ikehara (2013).
In cluster An, Ammonia “beccarii” forma 1 has been reported from brackish-water lagoons and inner bays around the Japanese Islands, which are characterized by salinity fluctuations and may have seasonal low-oxygen conditions (Matoba, 1970; Kosugi et al., 1991; Nomura and Seto, 1992). Cluster An is therefore considered representative of an inner bay environment. Components of cluster Bn, including Buccella frigida and Elphidium advenum, have been reported in the inner sublittoral zone around the Japanese Islands (Akimoto and Hasegawa, 1989). Pseudorotalia gaimargii compressiuscula has been reported in the inner sublittoral zone (Akimoto and Hasegawa, 1989; Takata et al., 2016), especially in areas influenced by the Kuroshio Current (Inoue, 1989). These taxa have been reported in bay environments (e.g. Oki, 1989). Thus, cluster Bn is presumed to indicate an environment with outer bay and inner shelf characteristics. In cluster Cn, Elphidium somaense has been reported from the middle to outer portions of bays along the Japanese coasts (Matoba, 1970; Kosugi et al., 1991). Haynesina sp. A has been reported from enclosed lagoons and bays, sometimes with organic-rich substrates (Kosugi et al., 1991; Takata et al., 2006). Thus, cluster Cn also likely corresponds to an outer bay to inner shelf environment. In cluster Dn, Pseudoparrella naraensis is common in water depths of 42 m and 85 m off the Tsugaru Peninsula, northern Japan (Matoba and Honma, 1986) and at depths of 50–130 m off the southeast coast of Korea (Woo et al., 1997). This cluster also contains Eilohedra nipponica, Bolivina decussata and Angulogerina ikebei, which are considered indicator species of the upper bathyal zone (Akimoto and Hasegawa, 1989). Accordingly, cluster Dn is presumed to represent an outer bay to outer shelf environment. The P/T ratio was generally high in the ∼33–29 m interval of core ND-01 (Figure 7), suggesting that this interval developed in a far offshore depositional environment. Thus, cluster Dn likely represents the most offshore position among the four clusters.
In general, the range of the P/T ratio in core ND-01 among clusters An and Bn, Cn, and Dn supports more seaward location in that order (Figure 7). As reported by Takata et al. (2019), these sample clusters likely reflect a landward–seaward gradient of environmental conditions. The lateral distributions of cluster Dn throughout core KND-3 and in most samples from core ND-02 are consistent with the offshore positions of these cores relative to core ND-01.
Cluster Dn represents the most offshore fauna; this cluster is dominant throughout our study interval in core KND-3, while the P/T ratio in that core exhibits significant increases both at 37–36 m and 33 m (Figure 2). The P/T ratio significantly increases at 37–36 m and drops briefly around 33 m, and then increases upward continuously. These features suggest that the influence of pelagic water increased twice, at 37–36 m and 33 m, in core KND-3. Takata et al. (2016) reported an overall deepening upward trend in the lower part of core ND-01 at ∼42–32 m, representing ∼9 ka, based on P/T ratios and benthic foraminiferal fauna. The increase in P/T ratio at 37–36 m in core KND-3 corresponds to 9.67 ka, while the increase at 33 m represents 9.38 ka ( Appendix 2 (i1880-0068-28-2-148Appendix2.pdf)). Although coring gaps in the middle part of core ND-01 prevented continuous assessment of paleoenvironmental changes, the results from core KND-3 reveal rapid sedimentation associated with deepening for approximately 0.3 kyr.
Song et al. (2018) reconstructed past sea-level fluctuations along the west coast of Korea during the Holocene. No significant starvation driven by deepening water was apparent at ∼9.7–9.4 ka in that study. The smaller variations of P/T ratio in the ∼36–33 m interval might be explained by rapid sedimentation over ∼0.3 kyr (9.7–9.4 ka). As noted by Song et al. (2018), the high sedimentation rate in core KND-3 can be reasonably attributed to rapid sea-level rise during the early Holocene, corresponding to the rapid retreat of the Laurentide Ice Sheet at 10–8.5 ka (Carlson et al., 2008).
Benthic foraminiferal fauna in seven cores collected off Fukuoka
The paleoenvironment of each cluster off Fukuoka was primarily inferred from ecological information regarding modern benthic foraminifera, as well as the Nakdong River delta. In cluster Af, Cibicides lobatulus and Hanzawaia nipponica are regarded as species that attach to hard substrates or calcareous algae (Kitazato, 1988) in the inner sublittoral zone around the Japanese Islands (Akimoto and Hasegawa, 1989). Thus, cluster Af is presumed to represent a shelf environment under relatively high-energy conditions driven by wave/current processes. Cluster Bf contains abundant Cibicides spp. (mean 25%) with Quinqueloculina spp. Thus, this cluster is likely related to a shelf environment under generally high-energy conditions driven by wave/current processes. In cluster Cf, Quinqueloculina spp., Cibicides spp. and Gavelinopsis spp. are common, in addition to E. advenum. Because this cluster is observed in only three samples, its ecological characteristics are difficult to determine; based on the common occurrence of Cibicides spp., cluster Cf may be associated with a shelf environment with generally high-energy wave/current processes. In cluster Df, Gavelinopsis spp., sometimes designated Rosalina spp., are the most common taxa. This group has been reported from the inner sublittoral zone of Japanese coastal areas (Akimoto and Hasegawa, 1989); it has plano-convex tests or biconvex tests with a nearly flat spiral side. These test shapes are presumed to indicate species that attach to hard substrates (Kitazato, 1988); thus, cluster Df likely corresponds to a shelf environment under generally high-energy conditions driven by wave/current processes. Cluster Ef is represented by Elphidium advenum and Quinqueloculina spp. which have been reported from the inner sublittoral zone around the Japanese Islands (Akimoto and Hasegawa, 1989). This cluster is considered indicative of a shelf environment. In cluster Ff, Gavelinopsis spp. are highly abundant (39%); this taxon is also present in cluster Df in association with E. advenum and Ammonia “beccarii” forma 2. Because cluster Ff is present in only at one sample, its ecological characteristics are difficult to determine.
The four clusters (Af, Bf, Cf, and Df) seem to relate to the shelf environments with high-energy wave/current processes, except for cluster Ef. Because cluster Af is only present in the surface sediments and the other clusters occurred temporally in the past, it is hard to determine the difference of environmental characteristics among clusters Af, Bf, Cf, and Df. In contrast, the spatiotemporal distributions are different among clusters Bf, Cf, and Df. Occurrences of clusters Bf and Cf are limited in the deeper sites, cores FV10-07-2 and FV10-08 at ∼9 ka. By contrast, cluster Df mainly occurred in the shallower sites, cores FV10-05 and FV10-06-02 in ∼8–6 ka. Thus, the four clusters relate to the shelf environments with high-energy wave/current processes, whereas it is reasonable to suppose clusters Bf, Cf, and Df exhibit the temporal differences possible on the past local environmental setting between the shallower (more neritic) and deeper (more offshore) areas. Because of such background, we argue the Holocene faunal transitions of benthic foraminifera off Fukuoka, focusing on three major sites, cores FV10-05, FV10-07-2, and FV10-09 which represent the shallower and deeper sites of the western area, and the eastern area, respectively.
Figure 7.
Stratigraphic changes in schematic lithological type, coarse fraction (%), benthic foraminiferal abundance, ratio of planktonic foraminifera to benthic and planktonic foraminifera (P/T ratio), and the occurrences of selected taxa of benthic foraminifera in the lower part of core ND-01; stratigraphic changes in coarse fraction (%), benthic and planktonic foraminiferal abundance, P/T ratio, and the occurrence of selected taxa of benthic foraminifera in the lower part of core KND-3.
In the western area, Takata et al. (2018b) reported a faunal transition in core FV10-05 across the hiatus (Nishida and Ikehara, 2013) based on cluster analysis. The upper part of this core is occupied by cluster Af (equivalent to cluster A in Takata et al., 2018b), which contains attached species. In contrast, the lower part (∼11–6 ka) is characterized by alternation of three clusters that are equivalent to our clusters Af, Df and Ef. In contrast, the whole studied interval of core FV10-09 (∼11 ka to present) was occupied by cluster Af. Thus, depositional environments in the western area off Fukuoka in the period before 8.4 ka appear to be more complicated than depositional environments in the eastern area off Fukuoka.
Similar transitions from cluster Df to Af in core FV10-06-2 were observed in this study and in the work by Takata et al. (2018b). A similar position of cluster Af is present in cores FV10-04 and FV10-10-2, with intercalation of cluster Df or Ff, characterized by abundant Gavalinopsis spp. Thus, high-energy conditions related to wave/current processes are dominant in the upper parts (prior to ∼8 ka) of these cores from the shallow sites (< 60 m water depth).
In core FV10-09 of the eastern area, cluster Af occupied our study interval across the hiatus described by Nishida and Ikehara (2013) (Figure 6; Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)). Although some common taxa, such as C. lobatulus, Gavelinopsis spp., and E. advenum show small stratigraphic variations in this core ( Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)), like those of core FV10-05, continuous occurrence of this cluster throughout the core is likely due to the dominance of calcareous porcelaneous taxa ( Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)). This spatial difference of the clusters between the eastern and western areas may be explained by different environmental setting, such as the riverine influence from the Onga River in the eastern area.
In contrast, cluster Af also occupied large parts of cores FV10-07-2 and FV10-08 that are the deeper sites of the western area (Figure 6). In addition, clusters Bf and Cf were present in the middle part of our study interval in core FV10-07-2; these clusters are also characterized by high abundances of Cibicides spp. (mean 23% and 16%, respectively). Cluster Cf was identified sporadically in these cores and may be related to common substrates based on the abundance of E. advenum (mean 10%) (Table 2). Thus, the stratigraphic distributions of the clusters are largely characterized by clusters Af, Bf, and Cf in cores FV10-07-2 and FV10-08, which were collected from deeper water depths (62 and 66 m, respectively); these findings suggest generally high-energy conditions related to wave/current processes in our study intervals. According to the ages reported by Nishida and Ikehara (2013), the lower parts of the study intervals in these cores correspond to ∼10 ka. The presence of clusters Af, Bf, and Cf in cores FV10-07-2 and FV10-08 is presumably associated with deeper water depths and older ages relative to the shallower coring sites.
The lower portion of the study interval in the western area is characterized by the frequent faunal changes. These faunal successions seem to be explained by increasing water depth and more wave/current energy with the Tsushima Warm Current. In contrast, the eastern area is characterized by the relatively smaller fluctuations in benthic foraminiferal fauna, such as continuous cluster Af. It may be due to the influence of riverine terrestrial materials from the Onga River. Thus, the marked faunal succession prior to ∼8 ka in the western area may be explained by increasing water depth and more wave/ current energy with the Tsushima Warm Current under the limited supply of riverine siliciclastic fractions.
Coarse-grained deposit/sandy sediments in the Nakdong River delta during the early Holocene
In cores ND-01, KND-3 and ND-02, coarse-grained deposits/sandy sediments are present in the lower parts (∼39–34 m, ∼37–32 m, and ∼37–33 m, respectively) (Figure 3). According to their ages, these sediments formed within the period between 10.4 ka and 8.1 ka in core ND-01, between 9.7 and 9.3 ka in core KND-3, and between 9.7 ka and 8.4 ka in core ND-02 (Figure 3); such formation was associated with rapid sea-level rise during the early Holocene (e.g. Lambeck et al., 2014; Song et al., 2018). In particular, coarse-grained deposits characterized by > 80% coarse fraction were found at ∼38 m in core ND-01 (Figure 7). Such coarse-grained deposits/ sandy sediments from Holocene shallow-marine systems may be important clues to the development of the Nakdong River delta (e.g. Kong and Park, 2007; Cho et al., 2017).
According to the concept of sequence stratigraphy in siliciclastic sediments, some coarse-grained deposits of Quaternary coastal sediments with a transgressive phase have been attributed to transgressive lag with rapid transgression (e.g. Saito, 1995; Shirai and Tada, 2000) that is mediated by shoreface erosion along with landward movement of the shoreline. Kidwell (1991) described the characteristics of paleontological features in four types of condensed deposits based on sequence stratigraphy with third-order (scale of millions of years) sea-level fluctuations. She proposed that onlap condensed deposits formed during rapid transgression; she also summarized the paleontological characteristics specific to such deposits, including the presence of basal erosional contact and mixing of fossils with various preservation statuses. Kitamura et al. (1997), Kitamura (1998), and Kondo et al. (1998) presented that the characteristics of paleontological features within the fifth-order depositional sequences of the Pleistocene successions formed by glacio-eustatic sea-level changes. Coastal sediments with a transgressive phase consist of shell bed with erosional basements, and the overlying deposits are characterized by upward increase of fine-grained and biogenic grains. Paleontological information is helpful for inferring the transgressive lag, especially for borehole sediments, in which the detailed horizontal extent of lithological features may be difficult to evaluate.
Benthic foraminiferal fauna in core ND-01 generally exhibits a rapid shift from cluster An to cluster Dn across the coarse-grained deposit at ∼38 m (Figures 3 and 7). According to ecological information regarding benthic foraminifera, such faunal transitions suggest deepening associated with rapid sea-level rise in the early Holocene (Figure 7). In addition, large abraded fragments of oysters (several millimeters to one centimeter) were present at 37.9 m, 37.7 m, and 37.6 m. Several 1-cm-thick oyster shell beds are intercalated in the ∼42–24 m interval of core KND-1 (∼6 km northeast and ∼9 km northward of cores ND-01 and KND-3, respectively; Figure 1), which corresponds to ∼10.0–8.6 ka (Korean Institute of Ocean Science and Technology, 2017). Thus, the oyster shell fragments at ∼38 m in core ND-01 may have originated from the upper reach of the paleo-Nakdong River estuary. In Quaternary shallow-marine sediments, such coarse-grained deposits with basal erosional contact surfaces that contain bioclasts (including fragments) have occasionally been recognized as onlap condensed deposits or transgressive lags (e.g. Kidwell, 1991; Kondo et al., 1998; Kitamura et al., 1997). Thus, the coarse-grained deposit at ∼38 m in core ND-01 presumably formed during rapid sea-level rise.
In core ND-01, benthic foraminiferal fauna from the coarse-grained deposit between 38.0 m and 37.7 m and the overlain sandy sediment between 37.5 m and 37.3 m are characterized by a mixture of species associated with clusters An and Dn; such species are also characteristic of muddy sediments from the intervals below and above those deposits (40.0–39.2 m and 34.0–30.0 m, respectively) (Figure 7). The faunal associations in the coarse-grained deposit and the overlain sandy sediment are therefore regarded as mixed fauna that include both shallow-water and offshore taxa. Notably, the estimated paleo-depth of core ND-01 at ∼9 ka was ∼18 m, based on the sea-level curve finding by Song et al. (2018) and the assumption that no tectonic uplift or subsidence has strongly impacted the core site. We suggest that these deposits containing shallow-water taxa were transported to the core site. In addition, the transition from the coarse-grained deposit to the overlying sandy sediments at 37.5–37.3 m exhibits an upward decreasing percentage of coarse fraction and slow increases in the abundance of benthic foraminifera per unit weight and P/T ratio (Figure 7).
Because sediment samples in ∼39–38 m and ∼37–34 m were missing in core ND-01, we did not observe the basal contact of the coarse-grained deposit at 38.0–37.7 m with the lower muddy sediment (40.0–39.2 m) and the upward transition of overlain sandy sediment (37.5–37.3 m) toward the upper muddy sediment (above ∼34 m). In core KND-3, sandy sediment (maximum ∼50% coarse fraction) is present at ∼33 m (9.38 ka) (Figure 2). The percentage of coarse fraction in core KND-3 showed frequent fluctuations around ∼37–33 m (Figures 2, 7), whereas this core showed gradual fining above ∼33 m. These sedimentological characteristics are similar to those of the coarse-grained sediment and overlain sandy sediment in core ND-01 (38.0–37.7 m). In addition, the age of the coarse-grained deposits in core ND-01 (9.3 ka) is similar to the ages of sandy sediments (∼33–32 m) in core KND-3 (∼9.4 ka). Furthermore, the transitions of the percentage of coarse fraction and P/T ratio at 33.6 m are sharp; a deepening upward trend is present above 33 m in core KND-3 (Figure 7). Thus, the sandy sediment at ∼33–32 m in core KND-3 presumably represents as a seaward extension of the coarse-grained deposit at ∼38 m in core ND-01, based on their ages, along with lithological and paleontological features.
These features are consistent with the inference of upward deepening immediately after sedimentation of the coarse-grained deposit at 38.0–37.7 m in core ND-01; they are also consistent with the presumption that sandy sediment at ∼33–32 m in core KND-3 is an onlap condensation deposit, according to criteria established by Kidwell (1991). Although the sandy sediment at ∼33–32 m in core KND-3 has been presumed to comprise sediments deposited under an Estuary Complex (Unit B in Jeong et al., 2018), a portion of the sandy sediment might have formed under conditions of rapid transgression during the early Holocene.
No similar equivalent coarse-grained deposit is present in core ND-02 that could be characterized by as an onlap condensed deposit (Figure 3). A sandy sample was obtained at 35.4 m in core ND-02 with 73% coarse fraction; however, abundant shallow-water benthic foraminifera associated with downward transportation were not observed (Figure 2 in Takata et al. (2019)). This result is likely attributable to the more seaward position of core ND-02 relative to cores ND-01 and KND-3 (collected ∼5 km and ∼3 km landward of core ND-02, respectively), as demonstrated by the presence of cluster Dn (Figure 7). Therefore, the deposition of coarse-grained materials driven by a landward shift of shoreface erosion would not be apparent at the more offshore location of core ND-02.
We cannot fully dismiss other possible sources of the coarse-grained deposit; for example, the observed features of sediments, benthic foraminifera, and grading might be also explained by a single storm event, rather than deposition with transgressive lag. Nevertheless, these coarse-grained deposits formed under conditions of rapid deepening based on benthic foraminiferal data. Although sedimentological evidence should be the primary criteria used for such assessment, the information provided by fossil foraminiferal fauna is useful for interpretation of the transgressive lag and overlain transgressive deposit from the early Holocene in the Nakdong River delta.
Comparison of sedimentation along the strait between Korea and Japan and surrounding areas in the early Holocene
Nishida and Ikehara (2013) described the depositional environments off Fukuoka at 11.6–8.4 ka as mainly a transgressive embayment deposit (lithofacies Fb), whereas inner shelf deposits (lithofacies Fc) became dominant above the hiatus (8.4–6.6 ka). They suggested that this transition occurred with the intensification of the Tsushima Warm Current during the early Holocene, particularly current-dominated deposition after ∼6.6 ka. Takata et al. (2018b) supported this interpretation of current-dominated deposition after ∼6.6 ka, based on the presence of typical attached species of benthic foraminifera, including Cibicides spp. and/or Hanzawaia nipponica. In contrast, that study reported frequent alternation among three clusters in the lower part of core FV10-05 (154.5–86.5 cm). This pattern suggests complicated sedimentation prior to ∼8.4 ka; therefore, it is difficult to attribute the depositional mechanism to either fluctuations of the Tsushima Warm Current or local sedimentary environmental changes, based on only a single core.
All cores collected off Fukuoka in this study generally show the presence of cluster Af, which contains frequent occurrences of typical attached species (e.g. Cibicides spp. and/or H. nipponica) in the upper portions of the cores (Figure 6), as do cores FV10-05 and FV10-06-2. The attached species increased earlier, around 10 ka, in the deeper (> 60 m water depth) offshore cores FV10-07-2 and FV10-08, relative to the shallower cores FV10-04, FV10-05, FV10-06-2, FV10-09, and FV10-10-2 (< 60 m water depth) (Figure 6). Nishida and Ikehara (2013) reported that the current-dominated deposits (lithofacies Fc) developed earlier at deeper offshore sites; such deposits were present in cores FV10-07-2 and FV10-08 beginning at ∼10 ka. They also demonstrated temporal and spatial variation of the “TWC influenced deposits” among cores collected at different water depths (their Figure 6), suggesting an influence of sea-level rise during the early Holocene on these spatiotemporal variations. Thus, our suggested timings of cluster Af appearance are generally consistent with the spatiotemporal distribution of lithofacies Fc in the scheme of Nishida and Ikehara (2013) in relation to the water depths of the cores. On the contrary, the eastern area off Fukuoka is characterized by the less significant fluctuations in benthic foraminiferal fauna. It may relate to the supply of riverine terrestrial materials from the Onga River.
Detailed assessment of the relationship between the coarse-grained deposits in the Nakdong River delta and the current-influenced deposits off Fukuoka is difficult because limited age data are available. In contrast, the presence of cluster Bf, characterized by abundant Cibicides spp. (mean 26%), in core FV10-07-2 at 9.3 ka (Figure 6) suggests high-energy conditions driven by the current/wave processes in both areas at that time. This cluster appears to arise off Fukuoka with similar timing to the transgressive lag in the Nakdong River delta at ∼9.7–9.3 ka. In addition, the lower part of core ND-02 in ∼10–9 ka is intercalated by several sandy layers (Khim et al., 2019; Yoo et al., 2020), despite no marked feature of the transgressive lag, whereas Gavelinopsis spp. are also relatively common during 10–9.8 ka in this core (Takata et al., 2022). These features imply intermittent high-energy conditions driven by the current/wave processes in core ND-02, as well. Thus, although further detail study is highly preferable with better age constraint, rapid sea-level rise might be related to the sedimentation of both coarse-grained deposit/sandy sediments (Nakdong River delta) and Tsushima Warm Current-influenced deposits (off Fukuoka).
The occurrence of warm-water plankton often provides information regarding the past behavior of the Tsushima Warm Current (e.g. Kitamura et al., 2001; Gallagher et al., 2017). Takata et al. (2019) investigated planktonic foraminiferal fauna at ∼33–28 m in core ND-02; the findings suggested that the Tsushima Warm Current may influence the Nakdong River delta (Takata et al., 2019). However, warm-water planktonic foraminifera below ∼33 m (prior to ∼9 ka) were not discussed in that work because few specimens (> 125 µm) were available. Takata et al. (2018a) reported the occurrence of warm-water planktonic foraminifera in core GH87-2-308, which was collected off Tottori (southwestern Japan; Figure 1). Planktonic foraminiferal data from core GH87-2-308 are available for assessment of the past influence of the Tsushima Warm Current on the strait between Korea and Japan; such data may be more useful than data from core ND-02. The timing of the coarse-grained deposit/ sandy sediments (transgressive lags) in cores ND-01 and KND-3 at ∼9.7–9.3 ka corresponds to the onset of increasing abundance of Globigerinoides ruber, a subtropical species, in core GH87-2-308 at 9.5 ka (Takata et al., 2018a). Domitsu and Oda (2008) reported the onset of “TC fauna” in core GH87-2 K-B from 9.3 ka. The intensity of the Tsushima Warm Current depended on sea level during the early Holocene (e.g. Tada et al., 1999). The coincidence of transgressive lag in the Nakdong River delta and the onset of increasing warm-water planktonic foraminiferal abundance off Tottori (southwestern Japan) at ∼9.7–9.3 ka may provide important information regarding the impacts of rapid sea-level rise and intermittent development of the Tsushima Warm Current on sedimentation in coastal areas along the strait between Korea and Japan. In particular, these events can be used to interpret the coarse-grained deposits in the Nakdong River delta, prior to significant enhancement of the Tsushima Warm Current.
Hsuing and Saito (2017) argued that a shift in sediment supply sources was associated with the establishment of present-day oceanic circulation in the East Asian margin around ∼7–6 ka, in addition to the rapid sea-level rise during 9.0 ka and 8.2 ka. They emphasized that the onset of the Taiwan Warm Current since ∼7.3 ka has played an important role not only in the sedimentation of the East China Sea, but also in the coastal areas along the strait between Korea and Japan through development of the Tsushima Warm Current. There is a need to identify sedimentation processes along the strait between Korea and Japan prior to the intensification of the Tsushima Warm Current. Takata et al. (2018b) reported that occurrences of clusters Af, Df, and Ef alternated in core FV10-05 over the period of 11.4–6.5 ka (Figure 6; Appendix 7 (i1880-0068-28-2-148Appendix7.pdf)). Such alternation suggests intermittent changes between high and low current/wave energy in the strait between Korea and Japan prior to intensification of the Tsushima Warm Current. Globigerinoides ruber was sporadically observed in core FV10-05 over 11.4–6.5 ka, despite low numbers in some portions (Figure 2 of Takata et al., 2018b). It remains challenging to attribute the changes in sedimentation and benthic foraminiferal fauna prior to significant intensification of the Tsushima Warm Current to either intermittent changes of sand wave migration or the variations in the intensity of the Tsushima Warm Current. In contrast, the Tsushima Warm Current may intermittently affect sedimentation off Fukuoka (Takata et al., 2018b). Although it is difficult to obtain a continuous record of fossil benthic and planktonic foraminifera from shallow marine sediments at a single site along the strait between Korea and Japan during the early Holocene, further studies are needed to fully elucidate the impacts of rapid sea-level rise and development of the Tsushima Warm Current on sedimentation using composite records from multiple core sites in this area.
Conclusions
We studied fossil benthic foraminifera in three and five sediment cores from the Nakdong River delta and off Fukuoka, respectively; our conclusions were as follows.
(1) Fossil benthic foraminifera from early Holocene coarse-grained deposit/sandy sediments in cores ND-01 and KND-3 are characterized by mixed fauna, including both shallow-water and offshore species, during ∼9.4–9.3 ka. This result suggests the development of transgressive lags at the site of cores ND-01 and KND-3 over the period of ∼9.4–9.3 ka, which is likely related to sea-level rise associated with the rapid retreat of the Laurentide Ice Sheet.
(2) Off Fukuoka, two clusters (Af and Bf) represented generally high-energy conditions and were characterized by common attached taxa of benthic foraminifera, which became dominant in the upper parts of the studied core intervals. The abundance of attached species increased earlier around 10 ka, at deep offshore sites (> 60 m water depth), compared to shallow cores (< 60 m water depth). These results are consistent with current-dominated deposits that developed earlier at deep offshore sites, beginning around 10 ka.
(3) The transgressive deposits in the Nakdong River delta and the appearance of cluster Bf off Fukuoka generally coincided at ∼9 ka. These occurrences likely resulted from the impacts of rapid sea-level rise and intermittent development of the Tsushima Warm Current, which drove sedimentation in coastal areas along the strait between Korea and Japan prior to significant enhancement of the Tsushima Warm Current.
Acknowledgements
We thank the Korean Institute of Ocean Science and Technology and Kangwon National University for their efforts in collecting borehole samples from the Nakdong River delta. Study materials and data off Fukuoka were obtained by AIST Research Project “Investigations on Geology and Active Faults in the Coastal Zone of Japan.” We also thank Kawasaki Geological Engineering Co. Ltd. and the crew of R/V Choyo-maru for their efforts in collecting the cores off Fukuoka, Japan. We are indebted to Yong-Un Chae, Jihun Kim, and Marine Core Center of KIOST for their kind support for our core subsampling of core KND-3. We thank Takuya Itaki, Hajime Katayama, and Atsuko Amano for their arrangements to subsampling of cores, off Fukuoka. We thank David L. Dettman for his kind arrangements to AMS 14C dating for core KND-3. We also thank Akihisa Kitamura (Editor-in-Chief) and two anonymous reviewers for their constructive comments to improve the manuscript. This study was supported by 2022 post-Doc. Development Program (HT) of Pusan National University, and a KIOST program (Grant No. PE99775).
© by the Palaeontological Society of Japan
