Holocene coastal environmental change and ENSO-driven hydroclimatic variability in East Asia

Quaternary Science Reviews 220 (2019) 75e86

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Holocene coastal environmental change and ENSO-driven hydroclimatic variability in East Asia Jaesoo Lim a, *, Jin-Young Lee a, Sei-Sun Hong a, Sujeong Park a, b, Eunmi Lee a, c, Sangheon Yi a a b c

Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, Republic of Korea Department of Geological Sciences, Pusan National University, Busan, Republic of Korea Department of Geology, Kangwon National University, Chuncheon, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2019 Received in revised form 15 July 2019 Accepted 24 July 2019

Understanding the past natural behavior of coastal hydroclimates at multi-centennial time scales is essential for planning coastal strategies in preparation for anticipated climate change. We investigated the elemental and isotopic features of 10-m-long coastal sedimentary cores from Goheung Bay located on the southern coast of Korea to extract such natural variability. Based on the relationships among total organic carbon (TOC), total sulfur (TS) content, and their isotopic values (d13CTOC and d34STS), we reconstructed the long-term coastal evolution in the area and compared this with the reported Holocene sea level change. Time series data of the titanium (Ti)/aluminum (Al) ratio, as a proxy for riverine terrigenous input, revealed a strong link to terrestrial plant contribution, as indicated by d13CTOC variability, suggesting past hydroclimatic change in the study area. The short-term variability of Ti/Al ratios correlated well with flooding events reconstructed for the Nakdong River in Korea. Moreover, periods in ~ o Southern Oscillation (ENSO) which the Ti/Al ratio was higher corresponded to those of stronger El Nin activity, suggesting ENSO-driven hydroclimatic changes in southern Korea. Especially, the periods of higher Ti/Al ratios during the late Holocene corresponded to a higher frequency of typhoon-driven overwash events along the southwestern coast of Japan. These similarities suggest that past freshwater input events along the southern coast of Korea were influenced considerably by typhoon-driven heavy rains during stronger ENSO activity. Spectral analysis of the Ti/Al ratios revealed significant periodicities of 1550, 780, 140, 120 and 105 years, suggesting centennial to millennial-timescale variability in hydroclimatic change and typhoon activity over East Asia. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: ~ o Southern Oscillation (ENSO) El Nin Flooding Holocene Tropical typhoon Carbon isotope (d13C) Sulfur isotope (d34S)

1. Introduction Global warming is expected to bring significant hydroclimatic changes in the near future. An Intergovernmental Panel on Climate Change report (Hijioka et al., 2014) predicted increased riverine, coastal, and urban flooding in Asia. However, the response of local areas in East Asia to future climate change is not well known. To eliminate uncertainties in predictions, it is essential to understand the natural variability of climate change in terms of the frequency and magnitude of events, as well as their relationship to the global climatic mean state at various timescales. Current coastal environments have evolved as the result of local/

* Corresponding author. E-mail address: [email protected] (J. Lim).

regional responses to past sea level and climate changes, and coastal sediments have therefore been used to trace past sea level and climate changes in East Asia (Stanley and Warne, 1994; Kim and Kennett, 1998; Dellwig et al., 2001; Jin et al., 2002; Lim et al., 2004; Chen et al., 2004; Hori et al., 2004; Mackie et al., 2005; Lamb et al., 2007; Nahm et al., 2008; Yang et al., 2008; Yu et al., 2011; Kim et al., 2012; Tanigawa et al., 2013). During the Holocene, present coastal areas experienced sea water transgression and highstand, although the timing depended on the regions. This long-term sea level change has influenced the long-term transportation and deposition of terrigenous materials to coastal areas. Short-term variability in the transportation and deposition of terrigenous materials to estuaries and inner bays is controlled mostly by rainfall, i.e., the hydroclimate. Current extreme hydrological events in East Asia are influenced strongly by summer monsoons and tropical cyclones. The summer monsoon is

https://doi.org/10.1016/j.quascirev.2019.07.041 0277-3791/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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characterized by seasonal shifts of the rainbelt that transports heavy rain into East Asia, sometimes causing extreme flooding. Unlike summer monsoons, tropical cyclones tend to be individual events. The influence of a tropical typhoon in East Asia is determined by its track. The genesis and track of tropical typhoons in the western Pacific are considered to be linked strongly to changes in ~ o events the eastern Pacific sea surface temperature, namely, El Nin (Liu et al., 2001; Elsner and Liu, 2003; Fudeyasu et al., 2006). During ~ o years, compared with La Nin ~ a years (Williams et al., 2016), El Nin more typhoons form in the southeastern part of the western Pacific and tend to recurve farther north, resulting in more landfalls in Korea and Japan (Fig. 1) (Elsner and Liu, 2003; Camargo and Sobel, 2005; Yonekura and Hall, 2011). A recent study reported a significant link between flooding events in the middle reach of the Nakdong River in South Korea and past ENSO activity. Lim et al. (2017) found that multicentennial frequent flooding periods (2900e3400 cal yr BP, 3600e3900 cal BP, 4600e5300cal BP, and ~ o Southern 5800e6400 cal BP) corresponded to stronger El Nin Oscillation (ENSO) periods. This suggests that centennial-to millennial-scale hydrological changes in Korea are linked mainly to past ENSO-influenced typhoon activity, prompting close examination of this relationship in other areas of Korea. In this study, we attempted to reconstruct past coastal environmental changes during the Holocene using carbon (C) and sulfur (S) contents and their isotopic values as proxies for coastal environments. Temporal changes in the terrestrial input to coastal areas were traced using the titanium (Ti)/aluminum (Al) elemental ratio, measured by an X-ray fluorescence (XRF) core scanner. These longterm records from geological archives were used to compensate for the limited availability of observational records and to specify

coastal hydrological changes at centennial timescales with respect to ENSO activity.

2. Sampling site and methods The study area, Goheung, is located on the western coast of Korea and includes a small bay, Goheung Bay, as shown in Fig. 1. Since the construction of an artificial tide embankment 20 years ago, this area has been reclaimed as rice fields. Here, we recovered 10-m-long sedimentary cores in 2016 using a rotary corer, which returns a 1-m-long, 50-mm-diameter core sample in a plastic liner (STP16-20, 34
39’08.91"N, 127
14’03.45"E). After taking pictures, we subsampled the cores at 5-cm intervals. During this process, plant fragments for radiocarbon dating (n ¼ 14) were chosen (Fig. 2). Age dating was performed using the accelerator mass spectrometry facility at the Korea Institute of Geoscience and Mineral Resources (KIGAM). To analyze the TOC content and its isotope (d13C), bulk subsamples of ~500 mg were treated with 1 N HCl at ~100
C for 1 h and then rinsed with distilled water. Approximately 3e5 mg of the HCl-treated subsamples was loaded into a tin combustion cup, and TOC contents were determined using a carbon, nitrogen, and sulfur (CNS) elemental analyzer (vario Micro Cube; Elementar, Germany); simultaneously, stable carbon isotope (d13C) analyses were performed using a continuous-flow isotope ratio mass spectrometer (Isoprime 100; G.V. Instruments, Manchester, UK) coupled to the CNS elemental analyzer. The results are expressed in delta (d) notation relative to the Vienna Pee Dee belemnite standard (V-PDB). The reference material was CH-6 (sucrose, d13C ¼ 10.45 ± 0.033‰), which was obtained from the

~ o years, typhoons with recurving tracks occurred more frequently. However, during La Nin ~ a years, typhoons Fig. 1. (A) Simplified tropical typhoon tracks in East Asia. During El Nin moved straight, in the direction of southern China (Liu et al., 2001; Elsner and Liu, 2003; Camargo and Sobel, 2005; Woodruff et al., 2009; Yonekura and Hall, 2011; Williams et al., 2016). NR and KI represent the Nakdong River (core DSR09) in South Korea (Lim et al., 2017) and Kamikoshiki Island in southwestern Japan (Woodruff et al., 2009), respectively. (B) Study area along the southern coast of Korea from the Naver map. (C) Aerial view of the study area in 1984 before reconstruction of the Goheung Bay (GHB in the figure) embankment (from Google Maps). (D) Present aerial view of the area, including coring site STP16-20, after construction (1991~1998) of the embankment (from the Naver map).

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Fig. 2. Photograph of core STP16-20 from Goheung Bay, Korea, and radiocarbon dating results analyzed with plant fragments. Numbers indicate ages from plant fragments at each depth.

International Atomic Energy Agency (IAEA). Replicated analyses had a precision of >0.2‰. Analyses of the TS content and its isotope (d34S) were performed with bulk subsamples using the same continuous-flow isotope ratio mass spectrometer coupled to the CNS elemental analyzer. The results are expressed in delta (d) notation relative to the Vienna Canyon Diablo Troilite (V-CDT). The reference material used was NBS127 (seawater sulfate, d34S ¼ 20.3 ± 0.4‰) obtained from the IAEA. Replicated analyses had a precision of better than 0.6‰. The semi-quantitative elemental composition of sedimentary cores was analyzed using an XRF core scanner (Avaatech B.V., Alkmaar, Netherlands) at KIGAM. This device performs nondestructive extraction of near-continuous elemental concentra€wemark et al., tions from sediment cores (Croudace et al., 2006; Lo 2011). XRF data were collected every 0.5 cm directly at the split core surface of the archive half (downcore slit size, 1.2 cm) using generator settings of 10 and 50 kV, a current of 0.25 and 1.0 mA, and a sampling time of 30 s. The peak intensity of different elements (e.g., Al, Si, Ti, and S) are expressed as counts per second (cps). We used the elements Al, Si, and Ti as proxies for the terrigenousderived sediment component, and S as a proxy for anoxic conditions in the water mass. Al is easily transported in fine-grained clay, whereas Ti is one of the main components of heavy minerals and is easily sorted close to river mouths. Thus, we normalized Ti against Al to emphasize terrigenous input intensity. The pyrite content of sediments was determined by X-ray diffraction (XRD) analysis. The analysis was performed on finely ground samples using a Panalytical X'Pert3 powder diffractometer (Cu Ka radiation; 45 kV; 40 mA). The SIROQUANT (v3.0) software was used to quantify minerology.

Table 1 Results of AMS Depth (m)

1.12 1.46 1.81 1.93 2.37 3.97 4.51 6.84 7.1 7.65 7.85 8.21 8.35 8.41

14

C dating and calibrated dates for core STP16-20.

14

C BP

cal BP a

(±1s)

(±2s)

810 ± 30 1210 ± 30 1800 ± 30 4830 ± 40 3410 ± 40 5080 ± 40 5830 ± 40 6770 ± 50 6930 ± 50 6860 ± 50 6950 ± 50 6910 ± 60 7360 ± 50 7340 ± 50

modern 1160 ± 100 1720 ± 100 5560 ± 90 3700 ± 130 5830 ± 90 6620 ± 120 7610 ± 90 7800 ± 130 7710 ± 110 7810 ± 120 7780 ± 150 8180 ± 140 8170 ± 140

d13C

Lab. code

Dated material

ITg160998 ITg160999 ITg161000 ITg161001 ITg161002 ITg161003 ITg161004 ITg161005 ITg161006 ITg161007 ITg161008 ITg161009 ITg161010 ITg161011

plants plants plants plants plants plants plants plants plants plants plants plants plants plants

(‰) 27.7 25.7 26.5 28.7 31.2 28 33.8 32.8 28.3 34 27.1 31.5 32.9 29

a Calibrated with Radiocarbon Calibration Program (CalPal) (http://c14.arch.ox.ac. uk/embed.php?File¼oxcal.html).

located at 8e6.6 m; Unit 3, green-gray silty grains with no clear layers corresponding to 7600e2000 cal BP and located at 6.6e1.9 m; Unit 4, gray muddy sand sediment with shell fragments, corresponding to 2000 cal BP to present, located between 1.9 and 1.0 m; and Unit 5, the uppermost layer consisting of reworked gravel and sandy deposits formed by reclamation. As shown in Table 1, a dating result at 1.93 m was reversed and suggests that old plant fragments were transported during the period between 1720 cal BP at and 3700 cal BP. This reversal age was excluded from age-depth model (Fig. 3). Based on the sedimentation rates, the depth (m) was converted to age (cal BP) (Fig. 3).

3. Results 3.1. Age dating and lithological features

3.2. Geochemical/isotopic analyses of TOC, TS, and their relationship

The sedimentary cores from Goheung Bay were formed during the Holocene period (Table 1 and Fig. 3). The cores can be divided into five units based on their lithological features: Unit 1, a gray silty sand layer overlaid on fresh bedrock, located at 9.7e8 m and formed during the early Holocene; Unit 2, clay sand with faint clay layers, corresponding to the period between 7800 and 7600 cal BP,

In the depth profiles, TOC and TS contents and their isotopic values were positively correlated (Figs. 3 and 4). TOC content varied between 0 and 1.2%, with relatively high content in the mud layer. The d13C values of TOC (d13CTOC) varied from 25 to 19‰ and were moderately correlated with TOC content (r2 ¼ 0.692). The depth profile of TS content ranging from 0 to 1.6% was similar to

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Fig. 3. Lithologic features and 14C dating, geochemical and mineral analyses results (total organic carbon (TOC %), total sulfur (TS %), isotopic values (d13CTOC and d34STS), and pyrite %) from core STP16-20, Goheung Bay, Korea. SR indicates sedimentation rate (mm/yr).

Fig. 4. Cross plots among TOC %, TS %, d13CTOC, and d34STS from core STP16-20, Goheung Bay, southern coast of Korea.

that of TOC content (r2 ¼ 0.544). The comparison between d34S values of bulk sediments (d34STS) and d13CTOC values showed a rather low correlation coefficient (r2 ¼ 0.365). Especially, Fig. 4 reveals an S-related inter-relationship among TS content and its S

isotope, in addition to pyrite content. Moderate positive correlations were observed between TS content and pyrite content (r2 ¼ 0.583) and between pyrite and d34STS values (r2 ¼ 0.546) (see Fig. 5).

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Fig. 5. Cross plots among TS %, d34STS, and pyrite % from core STP16-20, Goheung Bay, southern coast of Korea.

3.3. XRF core scanning results Fig. 6 shows the depth profile of semi-quantitative elemental content (Al, Si, Ti, and S). Terrigenous components (Al, Si, and Ti) were low in the sandy layer due to high pore water and vacancies;

however, a relatively high mud layer showed long-term changes. S (cps) was relatively higher in the mud layer than in the sand layer; however, its fluctuation differed from that of terrigenous components. As shown in Fig. 6. Ti/Al ratios were very high in the sandy layer and low in the mud layer, revealing significant short-term

Fig. 6. Depth profile of semi-quantitative elements (Si, Al, Ti, and S) in core STP16-20 recovered from Goheung Bay, southern coast of Korea, measured by an X-ray fluorescence core scanner (cps represents count per second).

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fluctuations. 4. Discussion 4.1. Long-term coastal change during the Holocene period based on geochemical proxies 34

4.1.1. TS % and d STS values It is important to understand water depth and sea level during the Holocene to specify past coastal environments in the study areas. Here, we attempted to constrain the Holocene sea level change using reconstructed ones from the Yellow Sea and East China Sea (Lambeck et al., 2014 and references therein). Fig. 7 shows that there may have been a >20-m change in the sea level, suggesting a long-term trend during the Holocene. Significant sea level rise occurred during 9000e7000 cal BP; this was followed by a gradual decline in sea level to present levels. Possible long-term coastal responses in the study area to the Holocene sea level change mentioned above can be tested by geochemical and isotope signals. TS (%) and its isotope values (d34STS) revealed a high correlation, indicating common factors controlling their variability (Fig. 7). Previous studies revealed that total sulfur is below ~0.3% in fluvial sediments whereas it ranges from approximately 0.3 to 3% in marine sediments (Koma and Suzuki, 1988; Ishihara et al., 2012). Regarding significant changes in d34S values, the Holocene marine sediments in Kau Bay, Indonesia, showed isotopically lighter values (d34S ¼ 20‰), whereas underlying freshwater sediments had a d34S value of þ15‰ (Middelburg (1991). Similarly, Wilkin and Arthur (2001) reported different d34S values in each deposition environment. The d34S values observed in shallow-water sedimentary cores from the Black Sea covering the past 15,000 years ranged widely, between 38.0‰ in a stratified anoxicesulfidic water column to þ11‰ in a freshwater lake. Furthermore, Chen et al. (2004) tested the d34S values of coastal sediments for inorganic (predominantly pyrite) and organic components. They found an organic d34S content of þ7.07 ± 2.69‰, whereas pyrite, which typically forms during bacterial sulfate reduction, showed a value of 8.67 ± 9.87‰ in coastal plain sediment from Taiwan (Chen et al., 2004). Furthermore, a recent study on a 300-m drill core collected on the shelf of the Gulf of Lion clearly showed remarkable glacial-interglacial fluctuations in the d34S values of sedimentary pyrite and their long-term change between 44.0‰ and þ32.3‰ has been attributed to the influences of sea-level change over the last 500,000 year (Pasquier et al., 2017). As shown in Figs. 4 and 7, the d34STS values of coastal sediment in this study area ranged between 33‰ and þ3‰. According to previous studies (e.g., Chen et al., 2004), it is likely that the observed d34STS values are combinations of those of pyrite and organic components. We observed a high correlation between TS content and pyrite content. The depleted isotope values, combined with an increase in pyrite, were likely due to seawater input from the sea level rise. Furthermore, the relatively higher TS content around 7600e6000 cal BP suggests that a ~5-m water depth, coupled with high sea levels, resulted in increased anoxic conditions responsible for the increased pyrite formation and depleted d34STS value (32.0‰) (Fig. 7). It is clear that deeper water depth following sea level rising should have provided higher potential to make anoxic condition in the study site. Based on the present tidal range (~3 m), the coring site with the water depth of 5 m around 7600e6000 cal BP would have been subtidal, suggesting more stable anoxic conditions. The subsequent reductions in TS and pyrite content may have been influenced by weakened anoxic conditions, driven by a decrease in water depth and falling sea levels, as shown in Fig. 7. This suggests that the study area was included in

the intertidal environment during the last 2000 years, as indicated by sandy sediment deposits with shell fragments. Consequently, the long-term TS content and its d34S value indicate that the longterm coastal evolution is driven by sea level and water depth changes. 4.1.2. TOC %, d13CTOC values, and terrestrial input (Ti/Al ratios) Other long-term coastal changes can be traced from changes in coastal sedimentary organic d13CTOC values. Several studies have reconstructed the Holocene transgression and its effects on the coastal environment using temporal changes in d13CTOC values based on the end-members for different environments (Meyers, 1994; Mackie et al., 2005; Zong et al., 2006, 2012; Lamb et al., 2006, 2007; Yu et al., 2011; Lim et al., 2015). Terrestrial and freshwater plants have d13C values between 24.9 and 32.5‰, whereas marine aquatic plants have d13C values between 17.3 and 21.7‰ (e.g., Mackie et al., 2005; Lamb et al., 2006; Lamb et al., 2007; Yu et al., 2011), forming two groups that can be clearly distinguished by their d13C values. These distinct distributions have been confirmed in different environmental settings (Zhan et al., 2011 and references therein; Williams et al., 2014). For example, modern sedimentary environments from the lower Yangtze River to the East China Sea show different d13C values at different stages in the river. The d13C values from suspended particulate matter and sediments in the river range from 28‰ to 24.4‰, and the d13C values in the mid-to lower river mouth are between 24.3‰ and 20.5‰. Shallow marine samples have a narrow range between 22.7‰ and 20‰ (Zhan et al., 2011). In Korea, modern surficial sedimentary d13C values along the Yeongsan Estuary have different ranges in different environments. The d13CTOC of modern lake surficial sediment varies from 26‰ to 24‰, inner estuary sediment ranges between 23.5‰ and 18‰, and outer estuary and coastal sediments have the highest values, varying between 18‰ and 16.5‰ (Williams et al., 2014). Sedimentary d13CTOC values in this study varied between 23.5‰ and 18‰ with a long-term change similar to that in TS % (Fig. 7). Based on the present surficial d13CTOC values along the Yeongsan Estuary in Korea (Williams et al., 2014), the increase from 24‰ to 18.5‰ during the period between 10,000 and 7,000 cal yr BP suggests a clear shift from a fluvial-influenced environment to a marine-dominant environment with 5-m water depth in response to Holocene sea-level rise. The subsequent decreasing trend in d13C values during the mid-to late Holocene indicates long-term coastal environmental change from the inner bay to tidal environments. This long-term change in the time series of d13CTOC values was superimposed by millennial to centennial fluctuations, suggesting possible significant changes in the environment or in terrestrial plant input. For example, periods of 4000, 4500, 5000, and 7700 cal BP show clear shifts in the d13CTOC values, indicating d13CTOC-depletion events. It is interesting that the long-term and short-term changes in the d13CTOC values are similar to those of Ti/Al ratios, as shown in Fig. 7. The observed d13CTOC-depletion events corresponded to remarkable peaks in the time series of Ti/Al ratios. In coastal areas, the transportation of terrigenous components from river catchment areas is usually determined by runoff and its intensity. Based on multiproxy analyses of hemipelagic sediments deposited on the Nile margin at 1389-m water depth, changes in Ti (cps) and Ti (wt%) during the last 100,000 years have been attributed to Nile River palaeohydrological fluctuations (Revel et al., 2010). Sun et al. (2008) reported that the Ti/Al ratio reflects the relative contributions of coarse- and fine-grained materials in the terrigenous components of South China Sea sediments, as influenced by sea level change and monsoon-induced fluvial input. Consequently, the similar longterm temporal changes in d13CTOC values and Ti/Al ratios were

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Fig. 7. Comparison of the time series of TS %, d13CTOC, d34STS, S intensity (cps), and Ti/Al ratios from core STP16-20 in Goheung Bay, southern coast of Korea, with an age-elevation curve of core STP16-20 (this study) and reconstructed sea level changes in the Yellow Sea and East China Sea (Lambeck et al., 2014 and references therein). The long-term change in the Ti/Al ratio is fitted with a third-degree polynomial (solid line).

mainly influenced by long-term coastal environmental changes, as discussed based on the changes in TS % and d34S values coupled with water depth and sea level changes. But it is worthy to note that there have been significant differences between sulfur-based signal (TS and d34STS) and TOC-based signal (TOC and d13CTOC). For example, at 6.8 m in depth (Fig. 3) corresponding to 7500 cal BP (Fig. 7), TS and d34STS values abruptly changed while TOC and d13CTOC values rather gradually. This suggests that TS and d34STS values have been influenced more rapidly by changes in water depth and sea level rising than TOC and d13CTOC value. It is likely that the changes in TOC content and d13CTOC values have been mainly controlled by freshwater input as supported by similar change in Ti/Al ratios. Especially, the short-term variability in the Ti/

Al ratio suggests strong freshwater input events at river mouths and in coastal areas, resulting in increased terrestrial organic matter transport.

4.2. Short-term hydroclimatic variability in southern coastal areas during the Holocene Fig. 7 shows the time series of Ti/Al as a proxy for the intensity of terrestrial mineral transportation to the coring site; the data revealed opposite trends in long-term and short-term changes in TS content and S intensity. This indicates that the long-term change in the Ti/Al ratio, represented by a third-degree polynomial (solid line) may have been influenced by water depth at the coring site

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Fig. 8. Spectral analysis results of the time series of Ti/Al ratios in core STP16-20 from Goheung Bay, Korea. Possible periodicities were estimated using the REDFIT spectral analysis program (Schulz and Mudelsee, 2002). Peaks above the false-alarm level (80%) are labeled with their cycles.

and sea level changes. The detrended short-term variability of Ti/Al ratios, realized by subtracting the long-term change, may provide information on past freshwater input variability at centennial to millennial timescales, as supported by its similarity to d13CTOC values. In this study we tried to test potential cycles in the time series of the Ti/Al ratios by using two different methods (spectral analysis and curve fitting method). As shown in spectral analysis results (Fig. 8), the variability in the freshwater input along the southern coast of Korea exhibits significant periodicities of 1550, 780, 140, 120, and 105 years. It is worthy to note that we chose 80% of falsealarm level to trace millennial-scale variability, which is supported by the following curve fitting method. Waveform curve fitting using a nonlinear regression method provides a possible statistical description for this centennial to millennial scale variability. The waveform curve-fitting process revealed millennial-scale oscillation with a 1580-yr cycle superimposed on the time series of the detrended Ti/Al ratios, as shown in Fig. 9. After subtracting this millennial oscillation signal from the detrended Ti/Al ratios, the second application of the curve-fitting process revealed a multicentennial oscillation with a 780-yr cycle. Through the same

Fig. 9. Waveform curve fitting-derived long-term and short-term oscillations using the time series of Ti/Al ratios (a proxy for freshwater input change) in core STP16-20 from Goheung Bay, Korea. Waveform curve fitting using a nonlinear regression method included in SigmaPlot software provides a possible statistical description for centennial (140 yr)-, multi-centennial (780 yr)-, and millennial (1580 yr)-scale variability superimposed on the Ti/Al ratios. (Millennial scale variability (M): y ¼ 0.3511 þ 0.6516  sin(2p(t)/3150.5231.3404)2, Multi-Centennial scale variability (MC): y ¼ 0.3142 þ 0.5529  sin(2p(t)/1492.0531 þ 1.0243)2, Centennial scale variability (C): y ¼ 0.1988 þ 0.3987  sin(2p(t)/ 275.908 þ 6.28)2).

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process, a centennial oscillation with a 140-yr cycle was estimated. These two tests showed quite similar results at cycles of ~1550, 780 and 140 years, suggesting that these cycles have been dominant frequencies in the time series of the Ti/Al ratios or freshwater input. Thus the sum of the three sine-curve functions (M þ MC þ C) from Fig. 9 can be considered a rough model of the past Ti/Al ratio change (or hydroclimate change) during the Holocene. 4.3. ENSO-driven hydroclimatic change in southern Korea over the past 9000 years If changes in freshwater input are responsible for changes in the Ti/Al ratio, the time series of the detrended Ti/Al ratios should have been influenced by regional hydrological circulation changes. In a previous study, Lim et al. (2017) reconstructed past flooding events in the Nakdong River, located in southern Korea. The changes in the grain size and carbon isotope values recorded in the sediments of the distant floodplain and swamp revealed multi-centennial variability in past flooding events. The change in high-resolution Ti/Al ratios from the coastal sediments appeared to be partly correlated with low-resolution change in the sand content, a proxy for inland flooding events in the Nakdong River (Fig. 10). For example, the frequent flooding events around 8000e7500 cal BP, as shown by an increased sand percentage, correspond to higher Ti/Al ratios at that time. Additionally, the subsequent decrease in sand percentage indicates a decrease in flooding events associated with the Nakdong River between 7500 and 6500 cal BP, which is correlated with decreased Ti/Al ratios along the southern coast of Korea. Patterns of increased Ti/Al ratios at ~5900 cal BP are similar to those for the sand percentage in the Nakdong River. These similar patterns in two widely separated regions indicate that common factors may have been controlling hydrological cycles in inland and coastal areas. This suggests that, in the past, the fluctuations in the Ti/Al ratio observed in coastal areas were influenced mainly by the frequency of severe rainfall events.

Fig. 10. Comparison of past freshwater input variability (detrended Ti/Al ratios) reconstructed from Goheung Bay, south coast of Korea, during the early to middle Holocene (this study) with past flooding frequency change represented by the time series of sand (%) from core DSR09 in the middle reach of the Nakdong River, South ~ o Southern Oscillation (ENSO) activity index (Moy Korea (Lim et al., 2017), and El Nin et al., 2002); aef indicate possible corresponding peaks between detrended Ti/Al ratios and the ENSO index.

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Changes in the flooding event frequency and magnitude in the Nakdong River have been attributed to ENSO activity (Lim et al., 2017). The variability in the detrended Ti/Al ratios in the coastal sediments is similar to that in ENSO activity (Fig. 10). Especially, they displayed nearly synchronous variability from 8000 to 5500 cal yr BP, despite different time controls. Remarkable peaks followed by significant increases in the detrended Ti/Al ratio, as shown by arrows and labels aef in Fig. 10, are nearly consistent in timing with significant peaks in the red color index, which indicates strong El Nino-related event beds preserved in lake sediments from the equatorial Andes (Moy et al., 2002). The relationships among Ti/Al ratios in the Goheung coastal area, sand percentages in the inland Nakdong River, and ENSO activity covering the whole Holocene period are shown in Fig. 11; despite the persistent connections among them, there are several mismatches in peaks and troughs due to different timing control and sampling resolutions. Interestingly, regarding the last cooling and warming periods, our data suggest that strong freshwater input occurred more frequently during the medieval warm period (MWP) (1200e800 cal BP) than during the Little Ice Age (600e300 cal BP), as shown in Fig. 11. Woodruff et al. (2009) traced the typhoon activity across southwestern Japan over the past 6500 years. They reconstructed the time series of Sr intensity change (a proxy for storm surgedriven coastal inundation), using coastal lake sediments from the island of Kamikoshiki (Lake Namakoike and Lake Kaiike). Their data indicated periods of barrier breaching concurrent with strong ENSO activity, indicating that ENSO activity potentially played a key role in governing typhoon variability during the mid-to late Holocene (Fig. 11). Recently, a 2000-yr sedimentary record of extreme coastal flooding was reconstructed from coastal lakes in Daija, Japan (Woodruff et al., 2015) (Fig. 11). These two studies clearly showed that periods of increased overwash activity in Daija and Kamikoshiki coastal lakes were correlated with strong ENSO activity, supporting the results of previous studies suggesting a higher frequency of the recurving track of typhoons toward Japan and Korea during stronger ENSO periods (Liu et al., 2001; Elsner and Liu, 2003; Camargo and Sobel, 2005; Yonekura and Hall, 2011; Williams et al., 2016). A comparison of the time series of the detrended Ti/Al ratios from the southern coast of Korea and past typhoon activity records of southern Japan (Woodruff et al., 2009, 2015) provides a rare chance to test possible spatial changes in the typhoon track and ENSO influences. As shown in Fig. 11, during the late Holocene, changes in the detrended Ti/Al ratios in Korea are similar to Sr intensity changes in Kamikoshiki and the overwash events per century in Daija, as well as to ENSO activity. These similarities suggest that freshwater input events along the southern coast of Korea may have been influenced to some extent by typhoon-driven heavy rains during increased El Nino activity. It is worth noting that in this study, it was impossible to separate freshwater input events driven by typhoon activity from summer heavy rainfall-driven freshwater input with no influence of typhoon activity. Despite this limitation, based on similar changes between the detrended Ti/Al ratios in Korea and storm surges and overwash events in western coastal Japan, it appears likely that southern Korea and southwestern Japan were strongly influenced by recurving tracks during the periods of ~ o-like stage). increased ENSO activity (El Nin Finally, if we accept the rough modeling of past freshwater input variability in the study area (Fig. 9), and apply this to the subsequent 1000 years, the present period, covering the past 100 years (based on the dating error) appears to be situated at the peak of 1550-yr and 780-yr cycles. The modeled hydroclimate shows a decreasing trend in freshwater input for the next 300 years (Fig. 12), which suggests a reduction in typhoon activity having recurving

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Fig. 11. Past freshwater input variability in Goheung Bay, southern coast of Korea (this study) and other climatic indices. (A) Past flooding frequency change represented by the time series of sand (%) from core DSR09 in the middle reach of the Nakdong River, South Korea (Lim et al., 2017). (B) Stack of wave-form fitting results comprising 1580-yr, 780-yr, and 140-yr cycles from detrended Ti/Al ratios in core STP16-20 (Goheung Bay, Korea). Waveform curve fitting using the nonlinear regression method included in SigmaPlot software provides a possible statistical description for centennial (140 yr)-, multi-centennial (780 yr)-, and millennial (1580 yr)-scale variability superimposed on Ti/Al ratios (see Fig. 9 for the details). (C) Time series of Ti/Al ratios (a proxy for freshwater input change) from core STP16-20, Goheung Bay, Korea (this study). (D) ENSO activity index (Moy et al., 2002). (E) Time series of Sr intensity, a proxy for storm surge-driven coastal inundation change using coastal lake sediments on the island of Kamikoshiki, southwest Japan (Woodruff et al., 2009). (F) Overwash frequency data from a coastal lake Daija, southwest Japan (Woodruff et al., 2015). Interestingly, regarding the last cooling and warming periods, our data suggest that strong freshwater input occurred more frequently during the medieval warm period (MWP, 1200e800 cal BP) than during the Little Ice Age (LIA, 600e300 cal BP).

tracks toward Korea and Japan during this time frame. However, if we consider the increases in freshwater input and typhoon activity during the medieval warm period (MWP), a recent warm period, the anticipated global warming may attenuate this natural decreasing trend to some extent. This scenario should be tested using reconstructed records of past coastal flooding events from various coastal areas in East Asia to elaborate on future predictions of hydroclimatic change and typhoon activity. 5. Conclusions In this study, we attempted to reconstruct past coastal evolution during the Holocene by testing the relationships among TOC and TS content and their isotopic values (d13CTOC and d34STS), as possible indicators of the past coastal environment. Based on a comparison with past sea level changes, the long-term changes in TS content and its d34S value indicate that long-term coastal evolution was driven by sea level and water depth changes. The time series of d13CTOC values showed similar long-term changes in TS % and d34STS values, suggesting sea-level influences. The millennial variability of these values was attributed to changes in terrestrial plant input. For

example, clear shifts in d13CTOC values occurred during periods around 4000, 4500, 5000, and 7700 cal BP, indicating d13CTOCdepletion events driven by an increase in terrestrial plant input. Time series of the Ti/Al ratio obtained using an XRF core scanner showed a strong link to the d13CTOC variability, a proxy for terrestrial plant input, suggesting past hydroclimatic change in the study area. Short-term variability in the Ti/Al ratio was well correlated with flooding events reconstructed in the middle reach of Nakdong River in Korea. Furthermore, periods of higher Ti/Al ratios corresponded to those of stronger ENSO activity, suggesting ENSO-driven hydroclimatic changes in southern Korea. Especially, periods of higher Ti/Al ratios during the late Holocene correspond to the increased frequency of typhoon-driven overwash events along the southwest coast of Japan. These similarities suggest that past freshwater input events along the southern coast of Korea have been influenced considerably by typhoon-driven heavy rains during stronger ENSO activity. Spectral analysis with the time series of Ti/Al ratios revealed significant periodicities of 1550, 780, and 120 years, suggesting centennial-to millennial-timescale variability in hydroclimatic change and typhoon activity over Far East Asia. Additionally, these

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Fig. 12. Future predictions using waveform curve fitting-derived long-term and short-term oscillations in the time series of Ti/Al ratios (a proxy for freshwater input change) from core STP16-20, Goheung Bay, Korea. Waveform curve fitting using a nonlinear regression method included in SigmaPlot software provides a possible statistical description for the centennial (140 yr)-, multi-centennial (780 yr)-, and millennial (1580 yr)-scale variability superimposed on the Ti/Al ratios (see the Fig. 9 for the details). MWP and LIA represent the medieval warm period and Little Ice Age, respectively.

cycles were found in the results of waveform (sine) curve fitting, providing a possible statistical description for the centennial-to millennial-scale variability. Future prediction modeling based on the sum of three sine-curve fitting functions showed a decreasing trend in the freshwater input for the next 300 years. Although this rough model was based on cycle-based statistical fitting from one site (one core), future studies based on multiple sites and multiple time series from various coastal areas in East Asia are expected to provide further insight into the past natural behavior of coastal hydroclimates in preparation for anticipated climate change. Acknowledgments This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM, 193414). We are grateful to two anonymous reviewers for their helpful comments and suggestions on the manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.quascirev.2019.07.041. References Camargo, S.J., Sobel, A.H., 2005. Western north pacific tropical cyclone intensity and ENSO. J. Clim. 18, 2996e3006. Chen, Y.-G., Liu, J.C.L., Shieh, Y.-N., Liu, T.-K., 2004. Late Pleistocene to Holocene environmental changes as recorded in the sulfur geochemistry of coastal plain sediments, southwestern Taiwan. J. Asian Earth Sci. 24, 213e224. Croudace, I.W., Rindby, A., Rothwell, R.G., 2006. ITRAX: description and evaluation of a new multi-function X-ray core scanner. In: Rothwell, R.G. (Ed.), New techniques in sediment core analysis. Geological Society of London, London, UK, pp. 51e63. https://doi.org/10.1144/GSL.SP.2006.267.01.04. Geological Society Special Publication 267. Dellwig, O., Watermann, F., Brumsack, H.-J., Gerdes, G., Krumbein, W.E., 2001. Sulphur and iron geochemistry of Holocene coastal peats (NW Germany): a tool for palaeoenvironmental reconstruction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 167, 359e379. Elsner, J.B., Liu, K.B., 2003. Examining the ENSO-typhoon hypothesis. Clim. Res. 25, 43e54. Fudeyasu, H., Iszuka, S., Matsuura, T., 2006. Impact of ENSO on landfall characteristics of tropical cyclones over the western North Pacific during the summer monsoon season. Geophys. Res. Lett. 33, L21815. https://doi.org/10.1029/ 2006GL027449.

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