Stalagmite evidence for East Asian winter monsoon variability and 18O-depleted surface water in the Japan Sea during the last glacial period

In the East Asian monsoon area, stalagmites generally record lower and higher oxygen isotope (δ18O) levels during warm humid interglacial and cold dry glacial periods, respectively. Here, we report unusually low stalagmite δ18O from the last glacial period (ca. 32.2–22.3 ka) in Fukugaguchi Cave, Niigata Prefecture, Japan, where a major moisture source is the East Asian winter monsoon (EAWM) that carries vapor from the warm surface of the Japan Sea. The δ18O profile of this stalagmite may imply millennial-scale changes, and high δ18O intervals that are related to Dansgaard–Oeschger (D–O) interstadials. More importantly, the stalagmite exhibits low overall δ18O values; the mean δ18O (− 8.87‰) is distinctly lower than the mid-Holocene mean of another stalagmite from the same cave (4.2–8.2 ka, − 7.64‰). An interpretation assuming a more intense EAWM and greater vapor transportation during the last glacial period, compared with the mid-Holocene, contradicts the limited inflow of the Tsushima Warm Current into the Japan Sea because of lowered sea level. Additionally, our model calculation using δ18O data from meteoric water indicated that the amount effect of winter meteoric water was insignificant (1.2‰/1000 mm). Low stalagmite δ18O for the last glacial period in Fukugaguchi Cave most likely resulted from 18O-depleted surface water, which developed in the isolated Japan Sea. The estimated amplitude of the δ18O decrease in surface water was ~ 3‰ at most, consistent with the abnormally low values for foraminifera (by ~ 2.5‰) in sediment during the last glacial period, shown by samples collected from the Japan Sea. This is the first terrestrial evidence of 18O depletion in Japan Sea surface water during the last glacial period.


Introduction
High-resolution isotopic records of well-dated stalagmites have been used as paleoclimatic archives in a terrestrial setting (e.g., Cheng et al. 2016). More specifically, the oxygen isotope (δ 18 O) records of stalagmites can provide important insight into the precipitation dynamics of the Late Pleistocene and Holocene epochs and have become a standard proxy for terrestrial climatic changes at the global scale. As a prominent example, Chinese cave records demonstrate that changes in stalagmite calcite δ 18 O (δ 18 O C ) are strongly correlated with δ 18 O records from Greenland ice cores (Wang et al. 2001) and shifts in Northern Hemisphere summer insolation on orbital timescales (Cheng et al. 2016;Wang et al. 2008).
The link between climate and δ 18 O C has been mostly attributed to variability in the δ 18 O values of local meteoric water. A popular interpretation is that this results from the amount effect on meteoric water δ 18 O (δ 18 O W ) from the intensified East Asian summer monsoon (EASM), which controls the climate in Asia (Cheng et al. 2009;Wang et al. 2001Wang et al. , 2008. Furthermore, δ 18 O C is seemingly synchronized with climatic changes in the North Atlantic Ocean during the late Quaternary (Cheng et al. 2016;Sun et al. 2012;Wang et al. 2001Wang et al. , 2008Zhao et al. 2018). Regarding the warming transition associated with deglaciation from the last glacial period to the mid-Holocene, the stalagmites generally exhibit distinct reductions in δ 18 O C values, again ascribed to the intensification of the EASM.
The East Asian winter monsoon (EAWM) is another meteorological system in East Asia (Fig. 1a). The EAWM is driven by the thermal contrast between the Asian continent and the North Pacific during winter, which blows dry and cold northerly winds from the Siberian High (Tada et al. 2016). The EAWM affects the climate of East Asia (Porter and An 1995) and transports heat from the Northern Hemisphere to the Southern Hemisphere as it crosses the equator (Chu et al. 2017;Yamamoto et al. 2013). EAWM variability during the late Quaternary has been reconstructed using marine sediment cores from the Sulu Sea (de Garidel-Thoron et al. 2001), South China Sea (Huang et al. 2011;Steinke et al. 2010Steinke et al. , 2011Yamamoto et al. 2013), Northwestern Pacific Ocean (Sagawa et al. 2011), and Japan Sea (Nagashima et al. 2007(Nagashima et al. , 2011, as well as from terrestrial archives (e.g., aeolian sediments from the northwestern Chinese Loess Plateau (Sun et al. 2012) and lake sediments in south China (Chu et al. 2017;Yancheva et al. 2007). The δ 18 O C rarely reflects the intensity of the EAWM because meteoric water from a dry EAWM makes only a small contribution to total precipitation in East Asia (e.g., An 2000). Exceptional findings were reported in Fukugaguchi Cave at Itoigawa in central Japan, on the coast of the Japan  Sea (Sone et al. 2013(Sone et al. , 2015, where water vapor from the Japan Sea warmed by the Tsushima Warm Current is carried by a northwesterly of the EAWM from Siberia (Fig. 1b;Hirose and Fukudome 2006). This area is wet in winter, and the snow/rainfall of the winter months (December-February) constitutes~40% of annual rainfall (Fig. 1c). Considering that the EAWM usually starts in November and ends in March, the EAWM snow/rainfall contributes a larger proportion (it falls 57% of annual rainfall during five months from November to March). Sone et al. (2013) analyzed a Holocene stalagmite (i.e., FG01) from Fukugaguchi Cave (Fig. 1d) and found that δ 18 O C variability at the top of FG01 was strongly correlated with that of winter precipitation observed from 1924 to 2010 at Takada (Fig. 1d). On the other hand, the δ 18 O C variability did not show relevance to precipitation of other seasons (Sone et al. 2013 in their Fig. 5). The amount effect apparent in δ 18 O values of winter precipitation (collected from Toyama, Fig. 1d) also implied that the δ 18 O C reflected winter precipitation variability. In addition, the δ 18 O C profile of the entire FG01, covering the past 10 kyr, demonstrated a trend similar to that of EAWM variability reported by other studies on loess and lake sediments (Sone et al. 2013). However, the variability of the EAWM beyond the pre-Holocene period has never been demonstrated based on a stalagmite record. Such data may provide insight regarding the hydroclimate during glacial periods.
Here, we present a new δ 18 O C profile spanning the last glacial period from 32.2 to 22.3 ka, obtained from another stalagmite (FG02) in Fukugaguchi Cave. A drastic shift in oceanographic conditions occurred during the last glacial period in the Japan Sea, the major moisture source in the study area. The Japan Sea is currently connected to the open ocean via four shallow straits (Fig. 1b). However, during glacial periods, the lowered sea level largely restricted entry of the Tsushima Warm Current. Freshwater influx from the surrounding land area reduced surface water salinity in the Japan Sea, as indicated by unusually low δ 18 O in foraminifers (Oba et al. 1991;Sagawa et al. 2018). Closing and opening of the Japan Sea led to glacial-interglacial contrast in the compositions and structures of core sediments recovered from the Japan Sea, which are generally laminated during glacial and bioturbated during interglacials (e.g., Irino et al. 2018;Tada et al. 2018;Seki et al. 2019). The closure of the Japan Sea may also have affected the hydrodynamics of vapor generation during glacial periods. Such changes, as well as the intensity of the EAWM and the EASM, may lead to differences in the δ 18 O C in FG02 from other cave records in Japan (Maboroshi, Ohtaki, and Kiriana; Fig. 1d), where precipitation from the EAWM is minor. Moreover, the δ 18 O C profile of FG02 exhibits millennial-scale changes that might correspond to climatic changes in the North Pacific Ocean. In this paper, the δ 18 O C records from Fukugaguchi Cave are discussed as new stalagmite evidence for the development of 18 O-depleted surface water in the Japan Sea during the last glacial period.

Study site and stalagmite sample
Fukugaguchi Cave (36°96.5′ N, 137°80.0′ E) is located in Itoigawa City, Niigata Prefecture, on the coasts of Japanese islands along the Japan Sea (Fig. 1). Itoigawa currently experiences some of the heaviest snowfall in Japan because of its geographic and altitudinal position, i.e., on a steep slope behind high mountains (e.g., Mt. Asahidake, 2418 m asl). A northwesterly of the EAWM reaches this location across the widest breadth of the Japan Sea, where the Tsushima Warm Current supplies water vapor to the initially dry northwesterly (Fig. 1a, b). The wet air mass then interacts with the mountain slope, causing heavy snowfalls. Therefore, the 5-month period (November-March) under the influence of the EAWM is generally wet in this area, accounting for 59% of the annual precipitation in Itoigawa (Sone et al. 2013).
Our sample analyzed in this study was a 22-cm-long stalagmite (FG02; Fig. 2a) collected from Fukugaguchi Cave in 2010. The sampling point was approximately 700 m from the cave entrance and near the location of the Holocene sample (FG01; Sone et al. 2013Sone et al. , 2015, where relative humidity (RH) is nearly 100%. FG02 has two discontinuous surfaces at 13.0 and 198.0 mm from the top (hiatus in Fig. 2a). The stalagmite lacks the regular laminae ( Fig. 2b-e) that may constitute annual bands. It mainly consists of transparent calcite mass (Fig. 2b, d), although some distal parts of the stalagmite exhibit mmscale layering (Fig. 2c, e).

U-Th age measurement
The ages of FG02 were determined by U-Th dating carried out at the National Taiwan University. The methods were described in detail by Shen et al. (2002Shen et al. ( , 2003Shen et al. ( , 2012. The stalagmite was cut along its growth axis for polishing and then drilled at nine horizons along the growth lines. Nine 0.2-0.3-g subsamples were obtained for each analysis. Because of the low uranium concentration in FG02, the amount of each sample was more than twofold greater than the amount used in ordinary U-Th dating (approximately 0.1 g; Shen et al. 2012). These powdered samples were dissolved with 5% nitric acid and spiked with an artificial radiometric tracer ( 229 Th-233 U-236 U). An Fe 3+ solution was added to this solution to remove Ca 2+ by iron co-precipitation. U and Th were purified by anion exchange chromatography.
The isotopic signature of each purified fraction was measured with a multicollector inductively coupled plasma mass spectrometer (Neptune; Thermo). An age-depth model was constructed using StalAge software, by means of a statistical algorithm based on Bayesian Monte Carlo simulation (Scholz and Hoffmann 2011).

Stable isotope analysis
Stable isotope analysis was performed with an isotope ratio mass spectrometer (DeltaPlus; Thermo Finnigan) connected to an online gas separation and introduction system (GasBench II) at Kyushu University. Subsampling was conducted down the middle of the growth band along the growth axis with a dental microdrill (Tas-35LX; Shofu) at 0.2-mm intervals. In addition, a Hendy test to examine the stability of δ 18 O values along specific growth bands (Hendy 1971) was performed for 8-10 subsamples at three horizons. Each~0.15-mg sample was enclosed in a 12-mL vial. Following replacement with He gas, these subsamples were reacted with phosphoric acid for > 5 h in a 50°C thermostat chamber. Generated CO 2 was introduced to the analysis system. δ 18 O values were normalized by using an in-house standard, which corresponded to the Vienna Pee Dee Belemnite standard. The reproducibility of the measurements of in-house standard (N = 190) was ±0.14‰ (2 SD). A typical measuring error is ±0.2‰ (2 σ). Additional details of δ 18 O measurement were described by Hori et al. (2009).

Evaluation of the amount effect of winter precipitation
For the Holocene stalagmite (FG01), Sone et al. (2013) concluded that the variability of δ 18 O C largely reflected the intensity of winter precipitation, consistent with the negative correlation between δ 18 O W and the amount of rain containing meteoric water during winter. We quantified this amount effect by using a bootstrap method and δ 18 O W data collected at Toyama, approximately 60 km southwest of the cave (Fig. 1d). The δ 18 O W data included 78 meteoric rain/snow events in winter months (December-February) from 2010 to 2012, which were collected by Sone et al. (2013). A virtual set of winter precipitation data consisted of a given number (N) of rainfall events, which were randomly selected from among the 78 events. First, the range of N was set at 24-30, which reproduced the distribution of total winter precipitation for the last 50 years at Toyama. Then, the total amount and weighted mean of δ 18 O W were calculated for each virtual set. For each of seven cases of N (24-30), the selection of virtual sets was repeated 100 times, such that the total number in the virtual set was 700.

Dating result and age-depth model
U-Th dating for FG02 yielded suitable ages from nine horizons, which ranged from ca. 13 to ca. 29 ka (Table 1). Although each age from the nine horizons exhibited relatively large uncertainty, due to the low uranium concentration (typically 6.0 ppb) and relatively high 232 Th (e.g., 3820 ppt at 9.5 mm horizon), they were generally in the correct stratigraphic order (Table 1). A distinctly younger age (ca. 13 ka) was obtained from the uppermost dated horizon (10 mm from the top; Table 1, and above the upper hiatus; Fig. 2a). An approximately 10-kyr age difference between this horizon and the next dated horizon (ca. 23 ka) at 30 mm likely signified a hiatus at an upper discontinuous surface (13.0 mm). Another hiatus was suspected at a lower discontinuous surface (198.0 mm), although no reliable age was obtained below this surface. The age model generated with StalAge (Scholz and Hoffmann 2011) indicated that the middle section between the two hiatuses was formed between ca. 32.2 ka and ca. 22.3 ka. The age-depth relation shown in Fig. 2 implies that the growth rate was fast above 130 mm (approximately 45 mm/kyr for 22-24 ka) and slow below 130 mm (approximately 7 mm/kyr for 24-32 ka).

Stable isotope analysis
The δ 18 O C values of the middle section of FG02 ranged from − 7.48‰ to − 10.68‰, with a mean value of − 8.87‰. This was clearly lower than the range observed in Holocene FG01 (mainly − 8.5‰ to − 7.0‰; Sone et al. 2013Sone et al. , 2015. The δ 18 O C values of FG02 indicated millennial-scale changes resembling the changes in the paleoclimatic records of the last glacial period. From 32.2 ka (i.e., the bottom) to ca. 26 ka, the δ 18 O C variability showed sawtooth-shaped fluctuation on a millennial scale (Fig. 3). The fluctuation amplitude was 0.5‰-1.0‰, clearly greater than the δ 18 O C measurement deviation (± 0.14‰). The upper δ 18 O C profile in the interval from 26 to 24 ka was nearly flat around − 9‰; this was followed by a positive shift to − 8‰ at 23.1 ka, which rapidly recovered at ca. 22.8 ka (Fig. 3).

Relationship between δ 18 O and winter precipitation at Toyama
A weak negative correlation between the total winter precipitation and weighted mean of δ 18 O was observed upon analysis of 78 meteoric water samples during winter months (December-February) at Toyama (Fig. 5) (Sone et al. 2013). In the past 50 years , 395.5-994.5 mm of rain and snow (mean, 696.4 ± 135 mm) fell each winter (Fig. 5a). First, we reproduced this observed distribution of total winter precipitation by random selection of a range of N (number of rain/snow events) from among the 78 actual events. The closest distribution was obtained with the range of N from 24 to 30 (mean, 687.9 ± 132 mm) (Fig. 5a). The calculation was repeated 100 times for each case of N (24-30; 7 cases), and the 700 virtual sets of winter precipitation data yielded weighted δ 18 O C values ranging from − 10.74‰ to − 7.81‰ (mean, − 9.34‰ ± 0.49‰). The precipitation amount and weighted mean (Fig. 5b) indicated a weak but significant negative correlation (R = − 0.33, p = 6 × 10 -19 ), with a slope of − 1.2‰/1,000 mm. We use this slope as the amount effect of modern winter precipitation in the following section.

Millennial-scale changes in FG02
The FG02 stalagmite recorded millennial-scale changes according to the characteristic features in the paleoclimatic records during the last glacial period. Figure 6 compares the FG02 δ 18 O C with the δ 18 O and Ca 2+ concentration of a Greenland ice core Seierstad et al. 2014), as well as a reconstructed EAWM derived from grain size variation in the Chinese Loess Plateau (Sun et al. 2012), and a reconstructed EASM derived from stalagmites in Sanbao and Hulu Caves in China (Wang et al. 2001) and stalagmites in Kiriana Cave on the Pacific Ocean side of Japan (Mori et al. 2018). The FG02 stalagmite shows millennial-scale changes in δ 18 O C . Because the low uranium content of FG02 yields U-Th ages with relatively large uncertainty (Table 1), two possible relationships exist between the δ 18 O C and Dansgaard-Oeschger (D-O) events (cases 1 and 2; Fig. 6). In order to determine which case is true, stalagmite samples with small uncertainty of U-Th ages are needed.
In case 1, the δ 18 O C values of FG02 are presumed to be heavier during the D-O interstadials. Three positive δ 18 O C excursions, at 23.1, 27.3, and 28.7 ka, are correlated with D-O events 2, 3, and 4, respectively (Fig. 6c). Another positive excursion around 30.8 ka is also recognized in the Greenland ice core and Chinese loess  6) and could correspond to the Greenland Interstadial 5.1 (at 30.84 ka) described by Rasmussen et al. (2014). Furthermore, Sone et al. (2013Sone et al. ( , 2015 concluded that the variability of δ 18 O C in Holocene FG01 reflects the amount of winter precipitation. According to this interpretation, the millennial-scale positive excursions of δ 18 O C in FG02 (with an amplitude of 0.5‰-1‰) should have been associated with less winter precipitation and the EAWM during short-term interstadials in Greenland and the North Atlantic Ocean. There might be a linkage between the D-O events and the weakened EAWM, which enriched 18 O of meteoric water and FG02. The δ 18 O C values of FG02 are presumed to be lighter during the D-O interstadials in case 2. In this scenario, four low-δ 18 O C intervals at 22.4, 27.6, 29.0, and 31.4 ka correspond to D-O events 2, 3, 4, and 5.1, respectively (Fig. 6c). This scenario fully accords with the stalagmites from China (e.g., Wang et al. 2008), whereby the amount effect due to the intensified EASM reduced δ 18 O C values during D-O interstadials. However, this is inconsistent with the findings of Sone et al. (2013), suggesting that the variability of the EASM did not strongly affect the stalagmite δ 18 O C . In addition, the δ 18 O C reduction during these D-O interstadials is unclear even in Kiriana Cave on the Pacific Ocean side of the Japanese islands ( Fig. 6f; Mori et al. 2018), where the majority of meteoric water originated from the EASM.
The relationship in case 1 is likely to be more plausible than the relationship in case 2; the δ 18 O C values of FG02 increased during the D-O events, which contrasts with the findings in Chinese caves. Stalagmite FG02 exhibits an overall decreasing trend of δ 18 O C (Fig. 6c), which also contrasts with the Chinese stalagmites (Fig. 6e) and findings in Kiriana Cave (Fig. 6f). The case 1 scenario is consistent with the overall trend of δ 18 O C , as well as the  (Table 1), solid circles, and bars for the smoothed ages and the error ranges (Fig. 2), respectively conclusions of Sone et al. (2013Sone et al. ( , 2015, whereby the variability of δ 18 O C reflects the amount of winter precipitation. However, considering the calculated amount effect of the modern meteoric water (− 1.2‰/1000 mm; Fig. 5 were also associated with other factors, such as the evaporation of water in the soil or cave, which increases both δ 18 O W and δ 18 O C . Our data do not rule out an evaporation effect during water infiltration from the soil. However, any such evaporation effect in the cave was presumably unimportant at the sampling site deep in the cave (700 m from the entrance), because it only appears to be robust where RH is unstable (Deininger et al. 2012).   (Sone et al. 2013(Sone et al. , 2015, and EASM weakening is likely to cause negative Δ 18 O H-G . Alkenone-based reconstruction studies for nearshore sediments from the Japan Sea suggested that the sea surface temperature during 17-32 ka was 1-2°C higher than in the mid-Holocene (Fujine et al. 2006(Fujine et al. , 2009Ishiwatari et al. 2001;Xing et al. 2011). Ishiwatari et al. (2001) ascribed these enigmatic results in the Japan Sea to the retention of solar energy on the surface water layer that was stratified with  (Sone et al. 2013). b Maboroshi Cave in Hiroshima Prefecture (Hori et al. 2014;Shen et al. 2010). c Ohtaki Cave in Gifu Prefecture (Mori et al. 2018). d Kiriana Cave in Mie Prefecture (Mori et al. 2018) heavier, deeper layers. Other studies have suggested that diminished salinity during the last glacial period might have led to a change in alkenone production, resulting in a higher reconstructed temperature (Fujine et al. 2006(Fujine et al. , 2009. Although this interpretation cannot be ruled out, the air temperature on the land was generally lower during the glacial period than the interglacial period, as indicated by pollen assemblages from Lakes Suigetsu and Biwa (Nakagawa et al. 2006(Nakagawa et al. , 2008. Similar to other Japanese cave sites, the lower air temperature during the last glacial period would increase δ 18 O C , thus making Δ 18 O H-G negative.

Factors responsible for low δ 18 O C in glacial FG02
With respect to the Holocene δ 18 O C variability at Fukugaguchi Cave (FG01), Sone et al. (2013Sone et al. ( , 2015 considered the intensity of the EAWM and winter precipitation to be dominant factors. The correlation between δ 18 O C and winter (December-February) precipitation at Takada (Fig. 1d) is evident in high-resolution δ 18 O C analysis of the uppermost FG01, with a 1‰ difference between the dry (1000 mm/3 months) winters in the late 2000s and wet (1500 mm/3 months) winters in the early 1940s at Itoigawa (Sone et al. 2013). Additionally, our analysis of data from Toyama, which is a drier area, revealed a negative correlation of δ 18 O W with the amount of precipitation (slope of − 1.2‰/1,000 mm (Fig. 5b)). In this scenario, a stronger EAWM during the last glacial period (Huang et al. 2011;Steinke et al. 2010;Tian et al. 2010) substantially increased winter precipitation (thus decreasing δ 18 O W ) and generated an approximately 3‰ difference in Δ 18 O H-G between the Japan Sea side (+ 1.23‰ at Fukugaguchi Cave) and the Pacific Ocean side of the islands (− 1.81‰ at Kiriana Cave; Table 2). When the slope of − 1.2‰/1000 mm is regarded as the amount effect of the winter precipitation, the~3‰ difference requires 2500 mm of winter precipitation during the glacial period, which is more than fourfold greater than the present amount of winter precipitation at Toyama. However, such a large amount of winter precipitation is considered unrealistic because the pollen records from Lakes Suigetsu (Nakagawa et al. 2006) and Biwa (Hayashi et al. 2010) suggest drier winter conditions during the same period. Furthermore, Schlolaut et al. (2014) suggested that the winter monsoon on the Japan Sea coast had less moisture during the glacial period because of the limited inflow of the Tsushima Warm Current.
If conditions on the sea surface were drier, the enhanced fractionation from water to vapor could generate lower δ 18 O in both water vapor and stalagmites (Lachniet 2009). The effect of RH on the δ 18 O in water vapor has been estimated in several studies; a 10% reduction in RH was posited to decrease the vapor δ 18 O by 1.3‰ (Gonfiantini 1986) and, in another study, by 0.6‰ (Merlivat and Jouzel 1979). Considering these contrasting estimates, the~3‰ difference corresponds to a 25-50% decrease in RH on the Japan Sea side, whereas the RH has remained stable on the Pacific Ocean side. However, this scenario might reduce winter precipitation, causing the meteoric water at Itoigawa to be dominated by summer precipitation that is more 18 O-rich than winter precipitation (Sone et al. 2013). Therefore, the RH effect is unlikely to be equivalent to an approximately 3‰ difference.
A factor likely to be important in the positive Δ 18 O H-G at Fukugaguchi Cave is the change in δ 18 O sw on the surface of the Japan Sea (i.e., the dominant vapor source for the cave) (Sone et al. 2013). Currently, δ 18 O SW is comparable between the Japan Sea and the Pacific Ocean. The Japan Sea is a landlocked marginal sea that connects with the East China Sea and the Pacific Ocean through four narrow straits (Fig. 1b). Because all four straits are shallow, water exchange (e.g., inflow of the Tsushima Warm Current) was largely limited during the last glacial maximum when sea level was reduced by~130 m (Matsui et al. 1998). Under this restricted condition, the surface seawater may have been diluted by riverine water from the Russian mainland and Japanese islands. Sediment cores from the Japan Sea indicate strong stratification during the last glacial period (Oba et al. 1991;Sagawa et al. 2018), which consisted of oxygen-deficient deep water and low-salinity surface water (Tada et al. 1992(Tada et al. , 1999. 18 O depletion of the Japan Sea surface was first proposed because of the unusually low δ 18 O in planktonic foraminifera during the last glacial period, recorded in sediment cores recovered from a depth of 935 m at Oki Ridge (Oba et al. 1991) and 800-750 m offshore of Akita (Okumura et al. 1996). By using δ 18 O records of planktonic foraminifera (Globigerina umbilicate; Oba et al. 1995) and assuming δ 18 O W in the modern freshwater inflow to the Japan Sea (-7.6‰), Matsui et al. (1998) estimated that the δ 18 O W in the Japan Sea surface water fell to 20‰ during the last glacial maximum. This drastic decrease in salinity is conceivable if the significant sea-level fall during the last glacial maximum led to reduced seawater inflow through the Tsushima Strait (Matsui et al. 1998 The magnitude of the salinity reduction at the sea surface can be estimated using the δ 18 O W in freshwater entering the Japan Sea from the surrounding land area. Assuming that δ 18 O SW and δ 18 O MW were + 1‰ (Schrag et al. 2002) and − 9‰, respectively, a 2.5-3.0‰ reduction in the Japan Sea surface water (δ 18 O JS = − 1.5 to − 2.0‰) indicates f MW of 0.25-0.3. Assuming glacial period ocean water salinity of 35, the Japan Sea surface salinity ranged from 24.5 to 26.3. This is in the lower part of the salinity range (24.0-32.5) estimated for 22-32 ka by Matsui et al. (1998); their estimate was based on δ 18 O in planktic foraminifers (Globigerina umbilicata), while ours was based on δ 18 O in the stalagmite. In the closed Japan Sea during the glacial period, the salinity at the sea surface, where vapor is generated, was probably lower than the salinity at the foraminifer habitat depth. In addition to freshwater inflow, sea ice production in the Northern Japan Sea may have contributed to 18 O-depletion of the surface water, because frozen water becomes relatively enriched with 18 O. Based on the occurrence of dropstones and ice-rafted debris in sediment cores, Ikehara (2003) reconstructed that the sea ice in the northern Japan Sea approached the southern end of Hokkaido during the last glacial maximum. However, ice melting during summer releases 18 Oenriched water, and the effect of sea ice development on δ 18 O SW is not sustainable. Thus, seasonal waxing and waning of sea ice were unlikely to cause a marked reduction in δ 18 O SW. In addition, oxygen isotopic enrichment from water to ice is only~3‰ (O'Neil 1968). Sustainable freshwater influx of − 9‰ was presumably a major factor in 18 O depletion in the Japan Sea surface water.
A similar linkage between marine and δ 18 O C records was reported in the eastern Mediterranean ( Bar-Matthews et al. 2003), where δ 18 O SW is sensitive to climatic conditions. Two stalagmite records in Israel extending to 250 ka are consistent with the δ 18 O C in Globigerinoides ruber in the eastern Mediterranean (Fontugne and Calvert 1992).
Both marine and stalagmite δ 18 O showed minimum values during sapropel events (i.e., humid intervals in interglacial period marine isotope stages 5 and 7), which developed under enhanced low-latitude hydrological activity. Currently, the δ 18 O SW in the eastern Mediterranean is + 1.6‰ (Pierre 1999), although this has been reduced by additional meteoric water during humid periods (Kallel et al. 1997). While Bar-Matthews et al. (2003) partly ascribed the low δ 18 O C values of the stalagmites to the amount effect observed in the area, the reduction of δ 18 O SW was also an essential factor involved in the low δ 18 O C . Moisture source δ 18 O W is an essential factor governing stalagmite δ 18 O C .

Conclusions
Stalagmite FG02 during the last glacial period (32.3-22.3 ka) in Fukugaguchi Cave on the Japan Sea side of the Japanese islands shows unique δ 18 O C trends, which have not been previously described in other caves in East Asia. Due to the relatively large uncertainty of U-Th ages, we examined two possible relationships (Fig. 6). The scenario in case 2 contrasts with the findings of a previous Japanese cave study, suggesting that EASM variability did not generate clear δ 18 O C peaks during D-O interstadials. According to the scenario in case 1, four positive excursions of the δ 18 O C profile probably correspond to three D-O events and one interval of high δ 18 O in the Greenland ice sheet (Fig. 6). We considered that case 1 correlation was more likely. An important finding of this study was that the glacial FG02 had distinctively lower δ 18 O C than the Holocene stalagmite from the same cave (FG01). Its mean value (− 8.87‰) is 1.23‰ lower than the mid-Holocene mean value (4.2-8.2 ka, − 7.64‰; Sone et al. 2013). This feature is unique to the Fukugaguchi Cave, i.e., is inconsistent with the δ 18 O C records from other Japanese and Chinese caves (Fig. 7). The factor making the largest contribution to reduced δ 18 O C in FG02 was presumably the development of low-salinity water in the semi-isolated Japan Sea, consistent with foraminifera δ 18 O findings (Oba et al. 1991;Sagawa et al. 2018). The amount effect of the intensified EAWM was a potential factor in the seduction in δ 18 O C in FG02. However, our model calculation based on δ 18 O W at Toyama indicated that the amount effect of winter precipitation (1.2‰/1000 mm; Fig. 5) was insufficient to explain the observed 2.5-3.0‰ depletion. In addition, the generation of water vapor and winter precipitation likely decreased during the last glacial period due to the blocking of the Tsushima Warm Current. Our stalagmite record provides insight regarding 18 O depletion and salinity in the Japan Sea surface water during the last glacial period.