Middle Holocene Coastal Environmental and Climate Change on the Southern Coast of Korea

: We obtained a 15 m drill core from Deukryang Bay on the southwest coast of Korea, which is now an area of reclaimed land used for agriculture. We investigated changes in the depositional environment and hydrological climate responses to sea level changes using sedimentary facies, radiocarbon ages, grain-size analysis, total organic carbon (TOC), total sulfur (TS), and stable carbon isotopes ( δ 13 C). Sediment deposition began at 12,000 cal yr BP and was divided into four stages based on changes from ﬂuvial to intertidal environments related to Holocene marine transgression events. Stage 1 (>10,000 cal yr BP) is represented by ﬂuvial sediments; Stage 2 (10,000–7080 cal yr BP) is represented by the deposition of mud facies in an intertidal zone in response to sea level rise; Stage 3 (7080–3300 cal yr BP) was a period of gradually descending sea level following the Holocene maximum sea level and is characterized by gradual changes in TOC, TS, and C/S ratios compared with the mud facies of Stage 2. Stage 4 (3300 to present) was deposited in a supratidal zone and contains low TS and an abundance of TOC. Based on our TS and C/S ratio results, the south coast of Korea was mainly affected by sea level rise between 7000 and 3000 cal yr BP, during the middle Holocene. At 3000 cal yr BP, sea level began to stabilize or gradually decrease. In addition, changes in δ 13 C values are clearly observed since ca. 5000 cal yr BP, in particular, large hydrological changes via freshwater input are conﬁrmed in 4000–3000 cal yr BP. We consider these shifts in freshwater input indicators of an increased inﬂuence of El Niño and La Niña conditions, related to the weakening of the East Asian Summer Monsoon (EASM) and changes in sea surface temperature (SST) of the Western Paciﬁc Ocean during the middle Holocene climatic optimum (between 7800 and 5000 cal yr BP). The cooling periods of SST in East Asia between 8400 and 6600 cal yr BP reported from the west coast of Korea are related closely to changes in vegetation (as evidenced by δ 13 C) from 7700 cal yrs BP to the present in the southwest coast of Korea. We interpret the freshwater input events at 4000–3000 cal yr BP to be related to changes in SST in response to the weakening of the EASM on the southwest coast of Korea. However, additional research is needed to study the southward migration effect of the westerly jet related to SST and atmospheric circulation controlling terrestrial climate in the middle Holocene.


Introduction
After the last glacial maximum (LGM), sea level rose rapidly; coastal plains were affected by the transgression and are distributed worldwide. On the western and southern coasts of the Korean Peninsula, the high tide-affected areas of intertidal zones were particularly affected by this sea level rise. Most of this intertidal zone is now used as arable land through reclamation.
The timing and altitude of sea level change around coastal Korea has been reported previously, with a focus on sea level high stands and their fluctuation after the maximum Holocene high stand [1][2][3][4][5][6][7][8]. These studies have documented sea level rise and climate change from geochemical analyses of coastal sediments.

Study Area and Methods
The study area is located on the southern coast of Korea, which has been used for agriculture for the last 100 years. In particular, sediments are sourced from the surrounding mountains and accumulate in the basin, except for the southern part, where sediments precipitate in the marine environment. Deukryang Bay is an inner bay where seawater flows from south to north, and which shallows from the southeast to the northwest. A seawall has been built in the southern part of the bay, and freshwater flows into the bay from small streams, including the Deuknyang and Joseong streams. According to Korean Meteorological Administration, the annual precipitation is 1500-2000 mm, and summer precipitation accounts for 70% of the total annual precipitation.
Using a general map of the area produced in the Japanese colonial era during the 1910s, we verified the locations of intertidal zones and coastal sands on the coastline before reclamation activity. Therefore, the sediment cores obtained include records of the depositional environment below the present sea level. Core BSL11 was obtained near the channel in the intertidal zone; the current tidal range is~3.5 m (Figure 1).

Study Area and Methods
The study area is located on the southern coast of Korea, which has been used for agriculture for the last 100 years. In particular, sediments are sourced from the surrounding mountains and accumulate in the basin, except for the southern part, where sediments precipitate in the marine environment. Deukryang Bay is an inner bay where seawater flows from south to north, and which shallows from the southeast to the northwest. A seawall has been built in the southern part of the bay, and freshwater flows into the bay from small streams, including the Deuknyang and Joseong streams. According to Korean Meteorological Administration, the annual precipitation is 1500-2000 mm, and summer precipitation accounts for 70% of the total annual precipitation.
Using a general map of the area produced in the Japanese colonial era during the 1910s, we verified the locations of intertidal zones and coastal sands on the coastline before reclamation activity. Therefore, the sediment cores obtained include records of the depositional environment below the present sea level. Core BSL11 was obtained near the channel in the intertidal zone; the current tidal range is ~3.5 m (Figure 1). Core BSL11 comprises 15 m of sediments; samples for grain-size and geochemical analyses were collected every 10 cm, while radiocarbon dating samples were collected at specific depth where plant or shell fragments appear (Table 1). Radiocarbon dating of samples was conducted using an accelerator mass spectrometer (AMS) at the Korean Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, Korea. For grain-size analysis, the samples were treated with 35% H 2 O 2 to decompose organic matter and then boiled in 1 N HCl for 2 h to remove carbonates and iron oxides. Samples were then rinsed three times using distilled water, then treated with an ultrasonic vibrator for 30 s to keep them in suspension and facilitate dispersion. We then performed grain-size analysis using a Mastersizer 2000 laser analyzer (Malvern Instruments, Worcestershire, UK). For TOC and TS analyses, samples were treated with 1 N HCl for 1 h to remove carbonates and iron oxides then rinsed three times using distilled water. Approximately 3-5 mg of the treated sample was then loaded into a tin combustion cup. The TOC and TS were determined using a CNS elemental analyzer (vario Micro cube; Elementar, Langenselbold, Germany). The C/S ratio was calculated using TOC and TS. Stable carbon isotope (δ 13 C TOC ) analyses of HCl-treated samples were analyzed using an isotope ratio mass spectrometer (Isoprime100; G.V. Instruments, Manchester, UK) connected to a CNS elemental analyzer. Results are expressed in delta (δ) notation of the Vienna Pee Dee belemnite standard (VPDB). The standard CH6 (sucrose, δ 13 C = −10.45 ± 0.033 ) was obtained from the International Atomic Energy Agency (IAEA) and has a precision of 0.2 or more for replicate measurements.

Lithology
Core BSL11 was divided into units 1 and 2 ( Figure 2). Unit 1 (13-14.9 m depth; −0.9 to −12.7 m a.s.l.) consists mainly of gravel and sand, which overlie basement rock. Grains in the gravels are mainly pebble-sized to 3 cm in diameter and sub-angular. Yellowish brown fine-grained sand occurs between 13.5 and 13.8 m, bluish gray clay with shell fragments between 13 and 13.5 m. ments between 13 and 13.5 m.
Unit 2 (1.2-13 m depth; −0.9 to −12.7 m a.s.l.) is composed mainly of clay intercalated with occasional sandy clay and fine-grained sand layers. A yellowish brown zone from 1.2 to 1.5 m, indicating oxidation, is observed in the upper part of Unit 2. Unit 2 is divided two sub units; Unit 2a (1.2-7 m) is distinguished from Unit 2b (7-13 m) by a high number of shell fragments. The uppermost part of the core (0-1.2 m) is interpreted as a disturbance layer caused by reclamation and agricultural cultivation.

Age Model
Radiocarbon ages are plotted as a depth-age curve ( Figure 3). The age model was generated based on 3960 years at a depth of 2.41 m except for a disturbance layer at a depth of 0-1.2. Several ages at depths of 12.19, 12.22, and 7.05 m, which were clearly disturbed during the drilling process, were excluded from our final results. Results from shells, based on comparison with plants and shells from the same depth interval in the Yeongsan Estuary in southwestern Korea, were influenced by the marine reservoir effect and were biased by 470 years [4,8]. After applying these corrections, the radiocarbon ages from core BSL11 yielded ages of ca. 3400 cal yr BP at a depth of 1.2 m and ca. 12,000 cal yr BP at a depth of 13.52 m, indicating the entire core was deposited during the Holocene. In this study, we focused on the middle Holocene, from ca. 8000 to 3000 cal yr BP.

Age Model
Radiocarbon ages are plotted as a depth-age curve ( Figure 3). The age model was generated based on 3960 years at a depth of 2.41 m except for a disturbance layer at a depth of 0-1.2. Several ages at depths of 12.19, 12.22, and 7.05 m, which were clearly disturbed during the drilling process, were excluded from our final results. Results from shells, based on comparison with plants and shells from the same depth interval in the Yeongsan Estuary in southwestern Korea, were influenced by the marine reservoir effect and were biased by 470 years [4,8]. After applying these corrections, the radiocarbon ages from core BSL11 yielded ages of ca. 3400 cal yr BP at a depth of 1.2 m and ca. 12,000 cal yr BP at a depth of 13.52 m, indicating the entire core was deposited during the Holocene. In this study, we focused on the middle Holocene, from ca. 8000 to 3000 cal yr BP.
pl. Sci. 2021, 11, x FOR PEER REVIEW 6 of Figure 3. Stratigraphic section, age-depth curve, sedimentation rates, and sedimentary proxies for core BSL11 (MGS: median grain size, TOC: total organic carbon, TS: total sulfur, C/S: carbon/sulfur, and SR: sedimentation rate). Horizontal bars and blue dots in the stratigraphic section indicate grain size and shell locations.

Depositional Environments
The results of age dating, lithology, and geochemical analyses from core BSL11 ind cate paleoenvironmental changes in the study area. Based on our TS and C/S ratio dat we divided the core into four stages during the Holocene (Figure 3). 4.1.1. Stage 1 (prior to ca. 10,000 cal yr BP) Stage 1 (elevation −12.52 to −14.52 m) was deposited at ca. 10,000 cal yr BP, and com prises gravelly sand overlain by a layer of fining-upward clay ( Figure 3). During th stage, clay contents and δ 13 CTOC values increase simultaneously, TS was close to 0%, an the C/S ratio is not shown owing to low TS. The δ 13 CTOC values of Stage 1 could be inte preted as an increase in organic matter from marine sources (relative to terrestri sources). The TS value in the lowermost portion of this interval is <0.3%, indicating a te restrial origin for these sediments ( Figure 3). Considering the well-documented global s level transgression during the Holocene, we surmise the gravelly sand in the lower pa which includes both angular and well-rounded gravel, was not affected by the marin transgression. The decreasing grain size toward the upper part of the interval is inte preted to indicate an environment in which the water level of the terrestrial stream i  The δ 13 C TOC values from core BSL11 ranged from −22.5 to −28.1 , with an average of −23.9 . The lowest δ 13 C TOC values were observed at 2.6 and 7.65 m and were −26.0 and −25.6 , respectively. The maximum values for TOC and TS were 0.95% and 0.8%, and the average values were 0.58% and 0.26%, respectively.

Depositional Environments
The results of age dating, lithology, and geochemical analyses from core BSL11 indicate paleoenvironmental changes in the study area. Based on our TS and C/S ratio data, we divided the core into four stages during the Holocene (Figure 3). 4.1.1. Stage 1 (Prior to ca. 10,000 cal yr BP) Stage 1 (elevation −12.52 to −14.52 m) was deposited at ca. 10,000 cal yr BP, and comprises gravelly sand overlain by a layer of fining-upward clay ( Figure 3). During this stage, clay contents and δ 13 C TOC values increase simultaneously, TS was close to 0%, and the C/S ratio is not shown owing to low TS. The δ 13 C TOC values of Stage 1 could be interpreted as an increase in organic matter from marine sources (relative to terrestrial sources). The TS value in the lowermost portion of this interval is <0.3%, indicating a terrestrial origin for these sediments (Figure 3). Considering the well-documented global sea level transgression during the Holocene, we surmise the gravelly sand in the lower part, which includes both angular and well-rounded gravel, was not affected by the marine transgression. The decreasing grain size toward the upper part of the interval is interpreted to indicate an environment in which the water level of the terrestrial stream increased gradually in response to sea level rise.  (Figure 4). Therefore, Stage 2 in our study area was likely affected by marine transgression.  [11]; green squares on blackline), (c) the Beolgyo area (core BSL18 [4] purple diamonds on black line), and (d) simplified relative sea level curve of the Toyooka basin [6] turquoise inverted triangles on blue line). Other points refer to sea level fluctuations in the Yellow sea between China and around the coastal area of Korea [3,7] and global relative sea level [1].
In our record, the most pronounced effect from the marine transgression is the change in grain size. In the clay layer from −12.52 to −10.82 m in elevation, which is part of Unit 2b, TOC values range from 0.3% to 0.8%, and TS values exhibit two peaks from 0.14% to 0.53%. These changes may be related to sea level rise, but they are not consistent with known temporal changes in global sea level. In addition, the clay layer, which was likely deposited in the intertidal zone, has a relatively high age uncertainty due to the low number of radiocarbon dating analyses. Therefore, it is very difficult to estimate the age of the marine transgression from a specific elevation in the core. In addition, the sedimentation rate during sea level rise in this area was relatively low at 0.6 mm/year, compared with the high value of 0.8 mm/year observed at the same time in other studies [4]. However, the highest TS value occurs at an elevation of −9.42 m, which is near the sample that was age dated from this interval. This TS peak (0.8%) occurs at ca. 8000 cal yr BP, which may be due to the marine transgression, since most studies indicate a sea level rise of 6 m. TOC is also the highest during this interval (0.8-0.9%), and the C/S ratio is very low (1.0). It is difficult to provide the exact timing of the transgression in this area, but it is clear that by ca. 8000 cal yr BP the area had been affected by the Holocene transgression. Therefore,  [11]; green squares on blackline), (c) the Beolgyo area (core BSL18 [4] purple diamonds on black line), and (d) simplified relative sea level curve of the Toyooka basin [6] turquoise inverted triangles on blue line). Other points refer to sea level fluctuations in the Yellow sea between China and around the coastal area of Korea [3,7] and global relative sea level [1].
In our record, the most pronounced effect from the marine transgression is the change in grain size. In the clay layer from −12.52 to −10.82 m in elevation, which is part of Unit 2b, TOC values range from 0.3% to 0.8%, and TS values exhibit two peaks from 0.14% to 0.53%. These changes may be related to sea level rise, but they are not consistent with known temporal changes in global sea level. In addition, the clay layer, which was likely deposited in the intertidal zone, has a relatively high age uncertainty due to the low number of radiocarbon dating analyses. Therefore, it is very difficult to estimate the age of the marine transgression from a specific elevation in the core. In addition, the sedimentation rate during sea level rise in this area was relatively low at 0.6 mm/year, compared with the high value of 0.8 mm/year observed at the same time in other studies [4]. However, the highest TS value occurs at an elevation of −9.42 m, which is near the sample that was age dated from this interval. This TS peak (0.8%) occurs at ca. 8000 cal yr BP, which may be due to the marine transgression, since most studies indicate a sea level rise of 6 m. TOC is also the highest during this interval (0.8-0.9%), and the C/S ratio is very low (1.0). It is difficult to provide the exact timing of the transgression in this area, but it is clear that by ca. 8000 cal yr BP the area had been affected by the Holocene transgression. Therefore, the sedimentary environment had changed from a fluvial to an intertidal zone in response to sea level rise at ca. 8000 cal yr BP.
A distinct peak where the grain size increases and TOC decreases markedly is observed at an elevation of −7.22 m at ca. 7280 cal yr BP, which records a dramatic decrease in δ 13 C TOC values to −25.3 ( Figure 5). This value indicates that the organic matter formed in a terrestrial environment, whereas the TOC, TS, and C/S ratios indicate a marine depositional environment. Therefore, we interpret this interval as an event in which sediments from terrestrial environments were temporarily supplied to the marine environment. the sedimentary environment had changed from a fluvial to an intertidal zone in response to sea level rise at ca. 8000 cal yr BP. A distinct peak where the grain size increases and TOC decreases markedly is observed at an elevation of −7.22 m at ca. 7280 cal yr BP, which records a dramatic decrease in δ 13 CTOC values to −25.3‰ ( Figure 5). This value indicates that the organic matter formed in a terrestrial environment, whereas the TOC, TS, and C/S ratios indicate a marine depositional environment. Therefore, we interpret this interval as an event in which sediments from terrestrial environments were temporarily supplied to the marine environment. Figure 5. Comparison of sedimentary environmental proxies (δ 13 CTOC, TS, C/S ratios, and red color intensity) from the Deukryang area with reconstructed climate change on the southwestern coast of Korea (Goheung area from core STP18-20 [11], and Beolgyo area from core BSL18 [4]).

Figure 5.
Comparison of sedimentary environmental proxies (δ 13 C TOC , TS, C/S ratios, and red color intensity) from the Deukryang area with reconstructed climate change on the southwestern coast of Korea (Goheung area from core STP18-20 [11], and Beolgyo area from core BSL18 [4]).

Stage 3 (7080 to ca. 3300 cal yr BP)
Stage 3 was deposited during 7080-3300 cal yr BP and has an elevation of −6.42 m to −1.52 m. This period corresponds to a stagnant or gradually descending sea level following the sea level maximum in the middle Holocene, during which water depths were >6 m greater than present levels at 8000 cal yr BP and 1 m greater at 3000 cal yr BP. There is almost no change during this interval in the grain size of intertidal mud, and the changes in TOC, TS, and the C/S ratio are relatively small compared with the changes observed during Stage 2. Stage 3 corresponds to Unit 2a and is represented by abundant shell fragments, which suggests that the sedimentary environment was different from that during Stage 2 ( Figure 3). We therefore interpret the sedimentary environment during Stage 3 to be an intertidal zone in which biological activity increased gradually as the water depth decreased via a lowering of sea level or by increased sediment supply. In particular, the fine-grained sand layers observed during the transition to the intertidal zone temporarily changed, owing to the supply of coarser sediments, which might have occurred during typhoons or coastal storms.
The C/S ratios in Stage 3 indicate a marine depositional environment, but this differs from the sudden change in C/S ratios related to sea level rise in Stage 2. The highest TS value in this interval occurred at 5680 cal yr BP, whereas the lowest occurred at 3960 cal yr BP. The TOC value was lowest at 6270 cal yr BP and highest at 3800 cal yr BP. The δ 13 C TOC value was lowest at 3800 cal yr BP, when TOC was the highest (Figure 3). This indicates a large fresh water input during Stage 2, which supplied a large amount of organic matter. (Figure 3). Increases in grain size, TOC, and C/S ratios and a decrease in TS indicate this interval was influenced by terrestrial freshwater environments. The large fluctuation in δ 13 C TOC values indicates an unstable environment that is influenced by marine and freshwater inputs and is interpreted as an upper intertidal zone temporarily affected by freshwater.

Middle Holocene Paleoclimate and δ 13 C Variability
Changes in the strength of the EASM have been considered an important factor in climate change on the Korean Peninsula. From 8000 to 3000 cal yr BP, the results from core BSL18 of the Beolgyo area and core STP16-20 from the Goheung area show C/S ratios from 0 to 4, whereas core BSL11 from the Deukryang area has C/S ratios of 0-9 ( Figure 5). As shown in Figure 5, the average of the C/S values in the Goheung and Beolgyo areas is <2, whereas the Deukryang area C/S average values are >2. The C/S ratios in these three areas indicate a marine environment after 7000 cal yr BP. Compared with the Goheung region, where the entire core was deposited in a marine environment except for the peak at 7000 cal yr BP, the Bulgyo and Deukryang areas have C/S ratios of ≥3 before 7000 cal yr BP, indicating a marine environment affected by freshwater until the Holocene climatic optimum. These results indicate that the Beolgyo and Deukryang areas were largely affected by freshwater, and in particular, the Deukryang area was more affected by freshwater than the Beolgyo area. As seen from C/S ratios, the Beolgyo area underwent early changes from a stable marine environment to an unstable environment of freshwater and seawater input since at 7760 cal yr BP, whereas the Goheung area was in a relatively unstable environment until ca. 6000 cal yr BP. The Deukryang area was continuously unstable until ca. 7000 cal yr BP. This period is known to be the period during which global sea level rose rapidly and then gradually stabilized. Therefore, the data indicate that the Beolgyo, Goheung, and Deukryang areas were simultaneously affected by sea level rise and associated changes in regional sedimentation.
A recent study reported that the Goheung area (STP16-20) had changed to a subtidal environment at ca. 7500 cal yr BP and shifted to an environment dominated by brackish marine input after 6000 cal yr BP [11]. This means that the local environmental changes in the region are not identical, despite the fact that cores BSL11 and STB16-20 were deposited on the same coast of Deukryang Bay. Conversely, providing that the same change is observed in different sedimentary environments, this means that it is at least possible to interpret more widely. Given the above information, we assume that the effect of overall sea level rise was complete by ca. 7000 cal yr BP, and the southwest Korean coast remained a relatively stable environment in regard to changes in sea water and freshwater for the remainder of the Holocene. Unlike the changes in C/S ratios, changes in TS values tend to be very similar in the Deukryang (BSL11) and Goheung (STP16-20) areas located in the Deukryang Bay, whereas the TS values in the Beolgyo (BSL18) area of Yeoja Bay varied. This suggests that TS values, which are highly dependent on local salinity changes, represent regional differences in sea level and freshwater inputs. Therefore, it is reasonable that sea level rise and the changes in the sedimentary environment that appear in the Deukryang area (core BSL11) are similar to those in the Goheung area (core STP16-20).
The unusually large variations in TS values identified in the Deukryang area (core BSL11) indicate that the study area was relatively sensitive to fluvial and marine input. Thus, the change in TS values between around 6300 and 5200 cal yr BP is observed only in the Deukryang area, unlike the Goheung and Beolgyo areas. These changes in TS values coincide with low δ 13 C TOC values at 6300, 5800, and 5200 cal yr BP ( Figure 5), with decreasing TS and δ 13 C TOC values at 5200 and 6300 cal yr BP. These decreases in TS and δ 13 C TOC are consistent with an influx of organic matter from freshwater from terrestrial sources. However, TS values increase at ca. 5800 cal yr BP, while the δ 13 C TOC values are constant or decreasing. Based on these results, we regard the changes observed at 5200 and 5300 cal yr BP as being caused by events related to freshwater input from terrestrial environments, but that the changes observed at 5800 cal yr BP were caused by changes in the marine environment. These results were not observed for the Beolgyo or Goheung areas (cores BSL18 and STP16-20, respectively). Instead, freshwater input from the Beolgyo area occurred simultaneously in the Deukryang area since ca. 5000 cal yr BP. Lim et al. reported freshwater input events at 4300 and 3600 cal yr BP from the Beolgyo area (core BSL18) [4]. Given the error range of the age-depth model, this freshwater input event may be the same as that identified at 3600 cal yr BP in the Deukryang area. This change is not identified in the Goheung area, and further research is therefore required to understand this event. An important similarity in the freshwater input sources of the Beolgyo and Deukryang areas is that both watersheds are located in the inland area of the Korean Peninsula. Conversely, the Goheung area has a relatively strong influence from freshwater from the very small catchments in the Goheung Peninsula. This geographic location means that climate change in the Goheung area is mainly affected by the marine environment, and similar changes in the Goheung area and Deungryang area may have been attributed to changes in the marine environment. Therefore, we interpret the freshwater input events in the Beolgyo area at ca. 4300 cal yr BP to have been caused by hydrological climate change in the southern coast of Korea. In addition, the freshwater input at ca. 3600 cal yr BP, which is recorded in both the Beolgyo and Deukryang areas, is interpreted as a more severe impact of hydrological climate change (relative to that observed at ca. 4300 cal yr BP; Figure 6).

Climate Change and Driving Forces in the Beolgyo Area during the Middle Holocene
As discussed in Section 1, changes in the δ 13 CTOC values are important for freshwater input interpretation in this area. In order to interpret the factors causing changes in the sedimentary environment, the origin of sediment and changes in vegetation must be considered simultaneously. Lee et al. reported a rapid change in vegetation and environment in the Goheung area on the southern Korean coast at ca. 7500 cal yr BP and surmised that the influence of the EASM prevailed until ca. 6000 cal yr BP, before the influence of the EASM weakened, affecting climate change associated with SST in the southwestern Pacific and typhoons related to El Niño conditions [11]. This coincides with the period of decreasing SST in the southwestern coast of the Korean Peninsula from 6600 to 8400 cal yr BP [29]. Therefore, changes in the vegetation environment (as evidenced by carbon isotopes) reported at the same time as sea level change on the west coast are interpreted to be related to the effect of atmospheric circulation due to climate change. This period can be interpreted as indicative of climate change and the southward migration of the westerly jet, which was suggested as the cause of the decrease in SST [29].  [11], BSL18 [4], and BSL11 (this study) with the Asian monsoon index [30], sea surface temperature data [31], and alkenone temperature [29].

Climate Change and Driving Forces in the Beolgyo Area during the Middle Holocene
As discussed in Section 1, changes in the δ 13 C TOC values are important for freshwater input interpretation in this area. In order to interpret the factors causing changes in the sedimentary environment, the origin of sediment and changes in vegetation must be considered simultaneously. Lee et al. reported a rapid change in vegetation and environment in the Goheung area on the southern Korean coast at ca. 7500 cal yr BP and surmised that the influence of the EASM prevailed until ca. 6000 cal yr BP, before the influence of the EASM weakened, affecting climate change associated with SST in the southwestern Pacific and typhoons related to El Niño conditions [11]. This coincides with the period of decreasing SST in the southwestern coast of the Korean Peninsula from 6600 to 8400 cal yr BP [29]. Therefore, changes in the vegetation environment (as evidenced by carbon isotopes) reported at the same time as sea level change on the west coast are interpreted to be related to the effect of atmospheric circulation due to climate change. This period can be interpreted as indicative of climate change and the southward migration of the westerly jet, which was suggested as the cause of the decrease in SST [29].
Between 5100 and 4900 cal yr BP, the intensity of EASM tended to decrease, and carbon isotopes increased in the Deukryang, Beolgyo, and Goheung areas ( Figure 6). This period corresponds to a rapid decrease in summer SST in the Yellow Sea. In addition, the red color intensity trend shows that the TS peak of Deukryang area (core BSL11) corresponds to the time period from 5000-3000 cal yr BP ( Figure 5). The climate change induced by the weakening EASM reported by Lee et al. coincides with decreases in the summer SST, and sea level decreased gradually with temperature after the Holocene optimum period [11]. This is also consistent with the interpretation that the influence of El Niño and La Niña conditions was caused by changes in SSTs in the South Pacific, which became stronger as the EASM weakened [4,[9][10][11]. Since the interpretation of the westerly jet migration during this period was suggested by Bae et al. [29], further study is needed to relate the influences of El Niño and La Niña conditions with the westerly jet migration. Nevertheless, it is possible to infer that changes in land hydrology caused by the influences of El Niño and La Niña conditions resulted in increased freshwater input, because changes in land hydrological events, such as typhoons and freshwater input, have been reported in many areas between 5100 and 3000 cal yr BP. However, it is not clear how the westerly jet migration relates to both hydrological change in the southern coast and the weakening of the EASM. If the cause of the weakening of the EASM on the west coast is due to the migration of the westerly jet, it is possible to better understand the changing hydrology on the west and south coast of the Korean Peninsula in the middle Holocene. In addition, further research is needed to understand the difference in the intensity of the hydrological changes recorded by the sediments in the area, which is dependent on the pathway of the typhoon.

Conclusions
The study area in Deukryang Bay is on the southwest coast of Korea, which is now used as reclaimed land for agricultural purposes. We analyzed the sedimentary facies, radiocarbon isotope ages, grain size, TOC, TS, and δ 13 C from a 15 m drill core. We determined that sedimentary deposition began prior to 12,000 cal yr BP and can be divided into four stages based on fluvial to intertidal depositional environments influenced by Holocene marine transgressions. Stage 1 began before 12,000 cal yr BP and is represented by fluvial deposits exhibiting a fining-upward trend that begins with gravelly sand. Stage 2 represents the period from 10,000 to 7080 cal yr BP and consists of intertidal mud deposits and a rapid change in TOC and TS that reflect a rapid rise in sea level. Stage 3 is the period from 7080 to 3300 cal yr BP and records a gradual decrease in sea level after the Holocene maximum. During Stage 3 there is no change in the intertidal mud deposit facies, but the TOC, TS, and C/S ratio changes are relatively small. Stage 4 is from 3300 cal yr BP to the present and records a change from an intertidal zone with very low TS values to a supratidal zone with a pronounced change in TOC values.
During the period from 8000 to 3000 cal yr BP (middle Holocene), the area was primarily affected by sea level rise until 7000 cal yr BP. Changes in vegetation (as evidenced by δ 13 C values) are apparent at ca. 5000 cal yr BP, when sea level was either stabilized or gradually decreasing. In particular, large hydrological changes attributed to freshwater input are indicated at 7200 and 3800 cal yr BP, with even greater changes related to freshwater input observed between 4000 and 3000 cal yr BP.
The cooling periods of SST in East Asia from 8400 to 6600 cal yr BP reported for the western coast of Korea are closely related to changes in vegetation (from δ 13 C TOC values) at 7700 cal yr BP in the southwestern coast of Korea. Freshwater input events between 4000 and 3000 cal yr BP are interpreted to be related to changes in SST associated with the weakening of the EASM in the southwest Korean coast. Additional research is needed to study the southward migration effect of the westerly jet related to SST and atmospheric circulation that control terrestrial climate in the middle Holocene.