Reconstruction of Holocene relative sea-level change and residual uplift in the Lake Inba area, Japan

Palaeogeography, Palaeoclimatology, Palaeoecology 441 (2016) 982–996

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Reconstruction of Holocene relative sea-level change and residual uplift in the Lake Inba area, Japan Takashi Chiba a,⁎, Shigeo Sugihara b, Yoshiaki Matsushima c, Yusuke Arai d, Kunihiko Endo e a

Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan Meiji University, 1-1 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8301, Japan Kanagawa Prefectural Museum of Natural History, 499 Iryuda, Odawara, Kanagawa 250-0031, Japan d Meiji consultant co.,ltd. 3-6-4 Rinkai-tyo, Edogawa-ku, Tokyo 134-0086, Japan e Nihon University, 3-25-40 Sakurajosui, Setagaya-Ku, Tokyo 156-8550, Japan b c

a r t i c l e

i n f o

Article history: Received 22 May 2015 Received in revised form 24 October 2015 Accepted 24 October 2015 Available online 3 November 2015 Keywords: Sea-level change Residual uplift Yayoi regression Diatom Marine terrace Holocene

a b s t r a c t We collected and analyzed fossil diatoms and volcanic ash and determined 14C ages in core samples from the lowlands around Lake Inba in the eastern Kanto Plain, central Japan, an area where late Quaternary sea-level change, fluvial erosion, and tectonic uplift have affected a region of archeological interest. We inferred Holocene paleoenvironmental changes and relative sea levels on the basis of indicators, such as high tide levels, derived from fossil diatom assemblages as well as the stratigraphic positions of 14C-dated samples and the K–Ah tephra. Our results lead to estimates of the upper half tidal range during 6800–7000 cal yr BP of 1.7 m. The Holocene highstand took place at approximately 6400–6500 cal yr BP, and mean sea level reached an elevation of 1.9 m. However, the timing of this sea-level rise is earlier in the Lake Inba area than documented in previous studies, and it is suggested that the timing of the Holocene highstand may appear at least 1000 years earlier as a result of Holocene residual uplift. After this, sea-level fell abruptly around 4000 cal yr BP. We also recognized a sealevel fall corresponding to the Yayoi regression at about 2600 cal yr BP, during which the mean relative sea level may have been as low as −2.5 m after compensating for uplift, subsidence or geomorphologic effects in this area. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Research on paleo sea-level changes due to glacial isostatic adjustment from the last glacial period to the Holocene has been carried out around the world (Woodroffe and Horton, 2005; Yokoyama and Esat, 2011; Lambeck et al., 2014). An important aspect of paleo sea-level change is whether the study site has a near-field, intermediate-field or far-field location with respect to glacial ice sheets. Relative sea level (RSL) change in the far field has been studied through coastal deposits or coral geochemistry (Yokoyama and Esat, 2011), because the effects of glacial isostasy can be ignored in the far field, leaving eustatic, local tectonic, and hydro-isostatic sea-level changes to be accounted for (Nakada et al., 1998; Sato et al., 2001; Yokoyama and Esat, 2011; Okuno et al., 2014). In Japan, which is located both in the far field and along subduction zones, researchers have been faced with the separation of tectonics and eustasy in making comparisons between relative and predicted sealevel change, as constrained by analyses of coastal sediment (Fig. 1a) (Sato et al., 2001; Tanabe et al., 2010; Yokoyama et al., 2012). In pursuing this question, there are areas of Japan where RSL records are not yet accurately known. For example, the Kanto Plain area has a complicated ⁎ Corresponding author. Tel.: +81 29 853 4304. E-mail address: [email protected] (T. Chiba).

http://dx.doi.org/10.1016/j.palaeo.2015.10.042 0031-0182/© 2015 Elsevier B.V. All rights reserved.

tectonic setting at the triple junction of the Eurasian, Pacific, and Philippine Sea plates (Fig. 1a) (e.g. Shishikura, 2003). Actively uplifting geologic structures are recognized here (Kaizuka, 1974; Sugiyama et al., 1997), although the cause of their uplift is not known in detail. Evidence of Holocene sea-level change is abundant in the eastern Kanto Plain. Masubuchi and Sugihara (2010) reconstructed changes of paleo sea-level and the paleoshoreline during the Holocene transgression and estimated a maximum sea-level elevation of 2.54 m from fossil diatom analysis in the Kinosaki area (Fig. 1b). The Editorial Committee of History in Noda-shi (2010) reported that the maximum sea level was higher than 2.0 m in Sekiyado on the basis of fossil diatoms (Fig. 1b). These sea-level changes are consistent with a sea-level maximum of 3.`0 m based on evidence from multiple sea-level proxies at Nagareyama in the central Kanto Plain (Fig. 1b) (Endo et al., 1982, 1983, 1989, 2013; Tanabe et al., 2008). In the Takagami lowland, in the lowest reach of the Tone River, Ota et al. (1985) and Kashima (1985) suggested that the maximum sea level was higher than 4.0 m from diatom assemblages. In the Kujukuri area, Masuda et al. (2001) used sediment analysis to suggest that the maximum sea-level was 4.0–6.0 m. In sum, the inferred maximum sea levels are higher in the eastern Kanto Plain than in the area to its west (Fig. 1b). The geological evidence of the sea-level maximum known as the Holocene highstand (HHS) are recognized such that the southern part

T. Chiba et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 441 (2016) 982–996

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Fig. 1. Maps showing (a) location of the study area in Japan, (b) the Kanto Plain, and (c) drilling sites and geographic setting of the study area (modified from Sugihara, 1970; Sugihara et al., 2011). Sea levels during the Holocene highstand are shown for Tokyo (Matsushima, 1988), Sekiyado (Editorial Committee of the History in Noda-shi, 2010), Kinosaki (Masubuchi and Sugihara, 2010), Nagareyama (Endo et al., 1989, 2013; Tanabe et al., 2008), Choshi (Ota et al., 1985), Kujukuri (Masuda et al., 2001), Futtsu (Kayane, 1991), and Yokosuka (Matsushima, 1996) in (b). Marine terraces formed in the Holocene highstand for Shirahama (Nakata et al., 1980) and Nobi (Kumaki, 1999) are shown in (b). Maps b and c also show contours representing the height of marine terraces formed during MIS5e (Koike and Machida, 2001).

is higher in distribution of the elevation than the northern part of Kanto (Fig. 1b), and tectonic activity clearly differs between the northern and southern parts of the Kanto Plain, an effect of variations in the plate subduction regime. Data for Tokyo Bay indicate HHS sea levels of 4.4 m (Matsushima, 1988). The sea-level maximum was around 8.0 m in the Futtsu area in southern Chiba Prefecture (Kayane, 1991), 14–20 m in Nobi on the Miura Peninsula (Kumaki, 1985, 1999), and about 23 m as indicated by marine terraces at Shirahama at the southern tip of the Boso Peninsula (Nakata et al., 1980). The topography of the southern Kanto Plain is also strongly influenced by Quaternary tectonics and seismicity (Sugimura and Naruse, 1954; Yonekura et al., 1968; Matsuda et al., 1978; Nakata et al., 1980; Kaizuka, 1987; Horiguchi, 1997; Kumaki, 1999; Shishikura, 2003; Takahashi, 2006; Shimazaki et al., 2011). On the other hand, the distribution of marine and fluvial terraces at 20–30 m elevation, dating from marine isotope stages MIS5a–e, attests to gradual uplift during late Pleistocene and Holocene time in the lower reach of the Tone River including the Lake Inba area in central Japan (Fig. 1b,c) (e.g. Sugihara, 1970; Machida, 1973; Kaizuka, 1987). From the elevation of the MIS5e marine terrace, the vertical uplift rate is estimated to be about 0.1–0.3 mm/yr during the late Pleistocene (Kaizuka et al., 2000). The RSL curve for the Kanto area is poorly known in detail; HHS heights are known only for a few localities. On the other hand, recently, new insight into the timing of the HHS in far-field localities have come from reexamination of ice models (Nunn and Peltier, 2001), geophysical models (Okuno et al., 2014), as well as geomorphologic, micropaleontological and geochemical data (Yokoyama et al., 2012), on the basis of glacial isostatic adjustment considerations. Holocene sea-level changes have been studied using many methods and records from many localities (e.g. Yokoyama and Esat, 2011; Horton and Sawai, 2010). In earlier studies, for example, Shennan (1982) and Tooley (1982) used the boundary between terrestrial and marine sediments as a proxy of sea level, the “sea-level index point,” and used it for reconstruction of sealevel during the Holocene. Since then, various ways to obtain the sealevel index point have been devised (Maeda et al., 1982; Sato et al.,

1983, 2001; Eronen et al., 1987; Denys and Baeteman, 1995; Yokoyama et al., 1996; Shennan and Horton, 2002; Tanabe et al., 2010; Stattegger et al., 2013; Tanigawa et al., 2013; Reynolds and Simms, 2015). To investigate the details of RSL changes in central Japan during the Holocene and reexamine the timing of the HHS, we obtained six drill core samples from the Lake Inba area including the Kashima river area of the eastern Kanto Plain, for which the residual uplift during MIS5e is known, and analyzed them using fossil diatom assemblages as a sea-level proxy as well as 14C dates from the cores.

2. Materials and methods 2.1. Core samples and 14C dating In 2006–2009, six cores designated SK-3 (elevation 2.6 m), SK-4 (elevation 3.7 m), SK-6 (elevation 4.7 m), SK-7 (elevation 6.3 m), SK-8 (elevation 8.5 m) and SK-9 (elevation 11.6 m) were obtained from the lowland south of Lake Inba at localities considered to be at the upper limit of the coastline during the HHS (Fig. 1c; Sugihara et al., 2011). These cores penetrated the entire succession of valley-fill deposits laid down during the Holocene transgression. Sedimentary facies in these cores were described and their radiocarbon ages were reported by Sugihara et al. (2011); however, we found it necessary to reexamine the lower limits of the postglacial deposits and these facies (Fig. 2). In this paper, we report new 14C dates from core samples and tephra ages, along with lithostratigraphic data, from the six cores of Sugihara et al. (2011). We selected shell materials from sediments that represented as much as possible the main habitat of that species. New radiocarbon ages were obtained using accelerator mass spectrometry by Paleo Labo Co., Ltd., and all ages, including previously reported age data (Sugihara et al., 2011), were calibrated by the program Calib 6.0 (Stuiver and Reimer, 2010). The reservoir effect ΔR (the difference between regional and global marine 14C ages; Stuiver and Braziunas, 1993) was taken as 0 for marine samples such as shell

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Fig. 2. Cross section along line A–B in Fig. 1 showing sedimentary facies and 14C ages in the six cores used in this study.

material. We selected materials by considering the main habitat of that species in the selection of shell material. 2.2. Tephra analysis The Kikai–Akahoya (K–Ah) tephra, from Kikai caldera, is a widespread stratigraphic marker in Japan (Machida and Arai, 2003). Its age of about 7165–7303 calibrated radiocarbon years before present (cal yr BP) represents the age of the K–Ah volcano eruption (Smith et al., 2013). We determined the presence of the K–Ah tephra in the cores by the following procedure. Volcanic glass shards were identified by stereo microscopy and picked from core subsamples taken at 0.1 m intervals. Refractive indices of the glass shards were determined by the RIMS 86 system using the thermal immersion method (Takemura and Danhara, 1994). The lower depth limit of sediment containing K–Ah tephra glass was assigned the age of 7165–7303 cal yr BP(Smith et al., 2013). 2.3. Diatom analysis Samples for diatom analysis were collected from the six cores at 0.1 to 0.5 m intervals (47 samples from SK-3, 25 from SK-4, 36 from SK-6, 36 from SK-7, 35 from SK-8, and 21 from SK-9). The samples were treated with H2O2, then the cleaned diatom frustules were mounted in Pleurax. Relative abundances of diatom species were determined by counting at least 300 tests under an optical microscope at 1000 × using oil immersion. We also calculated the total diatom abundance in each horizon (diatom valves/g dry sediment). Diatom identifications

were made and environmental conditions were interpreted by reference to Krammer and Lange-Bertalot (1986, 1988, 1991a,b), Kosugi (1988), Round et al. (1990), Tanimura and Sato (1997), Witkowski et al. (2000), Sawai (2001), Nagumo (2003), Sawai and Nagumo (2003), Ohtsuka (2005), Idei et al. (2012) and Chiba and Sawai (2014). 2.4. Paleo sea-level reconstruction by diatom assemblage Holocene sea-level reconstructions have relied extensively on diatom analysis. Denys and Baeteman (1995) suggested mean high water spring tide (MHWSP) and mean high water (MHW) as sea-level index points that can be specified by diatom assemblages. In Japan, Maeda et al. (1982) suggested the marine limit (the boundary between terrestrial and marine sediments) as a proxy for sea level corresponding to MHW. Marine limits and records of sea-level change have been reconstructed using sea-level index points suggested by diatom assemblages in Kyushu (Yokoyama et al., 1996), by sulfur ratios in the Kinki area (Sato et al., 2001), and by fossil shells in the Hokuriku area (Tanigawa et al., 2013). Sawai and Mishio (1998) reconstructed detailed sea-level changes in Akkeshi, Hokkaido, during the last 3000 year using the diatom Pseudopodosira kosugii (Sato et al., 1996: Tanimura and Sato, 1997) as an indicator of the high tide level and Planothidium haukianum (Acknanthes haukiana; in Kosugi, 1988) as an indicator of mean sea level (mean tide level). When indicators of high tide were recognized in peaty deposits the paleo-environment was considered salt marsh, and when the indicators were found in muddy deposits the paleoenvironment was considered tidal flat. Diatoms can complement other

T. Chiba et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 441 (2016) 982–996

methods of inferring past coastal environments, such as sedimentary facies analysis and studies of sulfur in peaty deposits. In this study, we determined the upper limit of the tidal range as a sea-level index point corresponding to the high tide level primarily by use of the brackish-water benthic diatoms Pseudopodosira kosugii

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(Sato et al., 1996; Tanimura and Sato, 1997), Pinnunavis elegans, and P. yarrensis (Sawai and Nagumo, 2003) (Fig. 3). The living Pseudopodosira kosugii is most abundant at high tide level in Tokyo Bay (Sawai, 2001), and Pinnunavis elegans and P. yarrensis are most abundant in lower salinity brackish water at salt marshes (Sawai and

Fig. 3. Photomicrographs of diatoms used as lower salinity or high tide level indicators. 1. Hyalodiscus sp., 2. Hydrosera triquetra, 3. Actinocyclus aquae-dulsis, 4. Pseudopodosira kosugii, 5. Pseudostaurosira brevistriata, 6. Cosmioneis pusilla, 7. Navicula cryptotenella, 8. Navicula peregrina, 9. Pinnunavis elegans, 10. Pinnunavis yarrensis, 11. Rhopalodia gibberula, 12. Tryblionella littoralis.

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Nagumo, 2003). We recognized tidal flat sediment by use of indicator diatoms such as Diploneis smithii, Giffenia cocconeiformis, Planothidium delicatulum and Tryblionella granulata (Kosugi, 1988; Chiba and Sawai, 2014) (Fig. 4). These are useful for determining sea-level index points representing mean sea level. Together, these diatoms also enabled us to estimate paleo tidal range from the difference in elevation between mean high tide level and mean tide level, as described below. We corrected the obtained paleo sea-level curve using vertical uplift values (0.02–0.14 mm/yr) calculated from the height of the MIS5e marine terrace in the Lake Inba area (Koike and Machida, 2001) (Fig. 1). In the calculation of uplift rates since MIS5e time, we used the following values: the elevation of the geomorphic surface was 20–30 m (Sugihara, 1970), the thickness of the eolian Loam covering the surface of MIS5e terrace was about 5 m (Sugihara, 1970; Arai, 2012), the RSL highstand due to glacial isostatic adjustment for the last Interglacial period (LIG) was 6–8 m above sea level (Kopp et al., 2009) and the magnitude of RSL highstand due to GIA along the Shimokita peninsulan for LIG was 1–3 m higher than the employed ESL value in Yokoyama et al. (2012).

3. Results and paleoenvironmental reconstruction Our results indicate that the sedimentary succession in the study area covers the entire postglacial period. Columnar sections of the six cores are shown in Fig. 2, 14C ages and tephra ages are listed in Table 1, and the succession of diatom assemblages in the cores are summarized in Figs. 5–10. For the most part, the diagrams (Figs. 5–10) show relative abundance of diatom species with more than 5% of total diatom valves mainly. Each core is described in detail below.

3.1. Core SK-3 Core SK-3 contained a record of freshwater, brackish and marine sediments in its top 20.0 m above the Pleistocene sediments (Fig. 2). Artificial fill occupied the uppermost 1.3 m. The natural sediments beneath consisted of peat from 1.3 to 1.6 m depth, sandy silt from 1.6 to 2.0 m depth, silt containing fossil shells from 2.0 to 18.1 m depth, peat from 18.1 to 19.0 m depth, fine sand from 19.0 to 19.2 m depth, and peat from 19.2 to 20.0 m depth. A gravel layer and late Pleistocene deposits formed the bottom part of the core (20.0–21.0 m depth). We divided core SK-3 into seven “diatom zones” on the basis of the diatom assemblages (Fig. 5). The oldest zone 1 (19.0–20.0 m depth) is characterized by freshwater species and represents a freshwater environment. Zone 2 (18.5–19.0 m depth) is characterized by freshwater, brackish and marine diatoms representing brackish environment such as salt marsh. Zone 3 (17.8–18.5 m depth) is characterized by brackish-marine and marine diatoms such as Planothidium delicatulum representing a tidal flat environment. Zone 4 (3.0–17.8 m depth) is characterized by marine planktonic species such as Cyclotella litoralis and Thalassionema nitzschioides and represents an inner bay environment. Zone 5 (2.6–3.0 m depth) is characterized by brackish and marine diatoms Tryblionella granulata, T. compressa and Planothidium delicatulum, representing a tidal flat. Zone 6 (2.2–2.6 m depth) is characterized by freshwater and brackish diatoms. The brackish diatoms Pseudopodosira kosugii, Pinnunavis elegans, P. yarrensis, and Thalassiosira lacustris signify that this environment was a tidal flat or salt marsh at the high tide level. Zone 7 (1.3–2.2 m depth) is characterized by freshwater species such as Staurosira construens and Diploneis yatukaensis and represents a freshwater environment.

Fig. 4. Photomicrographs of diatoms used as tidal flat indicators. 1. Amphora holsatica, 2. Diploneis smithii, 3. Diploneis suborbicularis, 4. Giffenia cocconeiformis, 5. Navicula digitoradiata, 6. Opephora sp., 7. Planothidium delicatulum, 8. Tryblionella compressa, 9. Tryblionella granulata, 10. Tryblionella hyalina.

Table 1 List of 14C age and K–Ah ash data in this study. All elevations and calibrated dates are rounded to the nearest 10 cm and 10 years, respectively. Tidal range showing (F) Freshwater, (HTL) High tide level, (MTL) Mean tide level and (ST) Subtidal. Site

Depth (m)

Elevation (m)

Sk-3 Sk-3 Sk-3 Sk-3 Sk-3 Sk-3 Sk-3 Sk-3 Sk-4 Sk-4 Sk-4 Sk-4 Sk-4 Sk-4 Sk-4 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-6 Sk-7 Sk-7 Sk-7 Sk-7

1.4 1.8 2.8 4.2 10 11 15.7 17.9 3.4 5.6 5.8 8.5 10.7 11 11.5 2.2 2.8 3.7 4 6 6 7.1 7.8 8 8.1 8.6 9.9 10.9 11.4 13 13.7 16.1 2 2.9 3.4 4

1.2 0.8 -0.2 -1.6 -7.4 -8.4 -13.1 -15.3 0.3 -1.9 -2.1 -4.8 -7 -7.3 -7.8 2.5 1.9 1 0.7 -1.3 -1.3 -2.4 -3.1 -3.3 -3.4 -3.9 -5.2 -6.2 -6.7 -7.6 -9 -11.4 4.3 3.4 2.8 2.3

IAAA-100097 IAAA-100098 Tka-14707 Tka-14708 Tka-14709 Tka-14710 Tka-14711 Tka-14712 IAAA-100115 IAAA-100116 IAAA-100117 PLD-24092 PLD-24093 PLD-24093 IAAA-100119 IAAA-100120 IAAA-100121 IAAA-100122 IAAA-100123 IAAA-100124 IAAA-100125 IAAA-100127 IAAA-100128 IAAA-100130 IAAA-100131 IAAA-100132 IAAA-100133 PLD-24094

Material Peat Peat Bivalve Bivalve Balk K-Ah ash Bivalve Bivalve Wood Wood Wood Gastropod Bivalve K-Ah ash Gastropod Peat Plant fossil Plant fossil Plant fossil K-Ah ash Plant fossil Bivalve Bivalve Bivalve Bivalve Bivalve Bivalve Wood Bivalve Plant fossil Plant fossil Plant fossil Plant fossil Plant fossil Wood Plant fossil

14

δ13C(‰)

Method

C (yr BP±1σ)

cal yr BP (1σ)

AMS AMS AMS AMS AMS

-23.14 -25.59 -2.09 -0.45 -30.21

± ± ± ± ±

0.33 0.31 0.37 0.31 0.13

1470 2440 4610 4960 6080

± ± ± ± ±

30 40 20 20 30

1330 - 1380 2360 - 2670 4810 - 4850 5270 - 5320 6890 - 6960

AMS AMS β β β β β

-2.24 0.98

± ± -30.4 -31.6 31.6 -7 -4.4

0.39 0.39

7000 7880 1650 2350 2510 4920 5750

± ± ± ± ± ± ±

30 30 30 30 30 40 40

7460 - 7540 8320 - 8380 1530 - 1590 2330 - 2430 2500 - 2720 5200 - 5310 6130 - 6240

β AMS AMS AMS AMS

-26.42 -27.85 -26.98 -29.6

-5 ± ± ± ±

0.32 0.32 0.31 0.18

8000 2950 1730 1360 6210

± ± ± ± ±

40 30 30 20 30

8410 - 8500 3070 - 3200 1570 - 1690 1280 - 1300 7030 - 7170

AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS

-1.65 -1.65 -2.87 -4.13 -1.87 -3.35 -0.7 -27.22 -2.43 -30.72 -31.49 -29.85 -30.92 -28.78 -29.76 -29.51

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.2 0.2 0.31 0.34 0.34 0.39 0.34 0.32 0.36 0.42 0.34 0.33 0.3 0.3 0.28 0.16

7640 7640 7780 7770 7680 7850 7930 7430 7880 7830 7700 10470 3520 3180 3640 4850

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

30 30 30 30 30 30 30 30 30 30 30 40 30 30 30 20

8400 - 8430 8050 - 8140 8200 - 8300 8200 - 8290 8090 - 8190 8300 - 8360 8360 - 8420 8200 - 8320 8320 - 8380 8560 - 8640 8430 - 8540 12240 - 12540 3720 - 3840 3380 - 3440 3910 - 3980 5590 - 5600

cal yr BP(2σ) 1310 - 1390 2360 - 2700 4780 - 4890 5250 - 5410 6810 - 7000 7170 - 7300 7440 - 7560 8280 - 8410 1420 - 1690 2330 - 2460 2490 - 2740 5060 - 5390 6030 - 6270 7170 - 7300 8370 - 8550 3010 - 3210 1570 - 1700 1260 - 1320 7010 - 7240 7170 - 7300 8380 - 8510 8010 - 8160 8170 - 8330 8170 - 8320 8030 - 8240 8230 - 8340 8320 - 8470 8180 - 8340 8280 - 8410 8540 - 8720 8410 - 8550 12150 - 12580 3700 - 3860 3370 - 3450 3880- 4080 5490 - 5640

Tidal range F F MTL ST ST ST ST MTL HTL MTL MTL ST ST ST ST F F MTL MTL ST ST ST ST ST ST ST ST ST ST MTL MTL F F F HTL MTL

T. Chiba et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 441 (2016) 982–996

Lab. Cord IAAA-100093 IAAA-100094 IAAA-100095 IAAA-100096 PLD-24091

(continued on next page)

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988

Table 1 (continued) Site

Depth (m)

Elevation (m)

IAAA-100134 IAAA-100135

Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-7 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 SK-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-8 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9 Sk-9

4.5 6.2 6.5 7.8 9 9.1 9.1 9.5 9.6 10 11.6 11.8 0.9 2.2 2.5 3.8 4 6.3 6.3 7 7.4 7.9 8.1 8.8 9.9 10.7 11.3 0.9 1.9 1.9 3.4 3.5 3.9 4.6 5.7 8.3 8.3 9.1 9.5 11.5

1.8 0 -0.3 -1.5 -2.7 -2.8 -2.8 -3.2 -3.3 -3.8 -5.4 -5.5 7.6 6.3 6 4.7 4.5 2.2 1.6 1.5 1.1 0.6 0.4 -0.4 -1.4 -2.2 -2.8 10.7 9.7 9.7 8.2 8.1 7.7 7 5.9 3.3 3.3 2.5 2.1 0.1

IAAA-100136 IAAA-100137 IAAA-100138 IAAA-100139 IAAA-100140 IAAA-100141 IAAA-100142 IAAA-100143 IAAA-100144 IAAA-100145 IAAA-100146 IAAA-100147 IAAA-100148 PLD-24095 IAAA-100150 IAAA-100151 IAAA-100152 IAAA-100153 IAAA-100154 IAAA-100155 IAAA-100156 IAAA-100157 IAAA-100158 IAAA-100159 IAAA-100160 IAAA-100161 IAAA-100162 IAAA-100163 IAAA-100164 IAAA-100165 IAAA-100166 IAAA-100168 IAAA-100169 IAAA-100170 IAAA-100171

Material Plant fossil Wood K-Ah ash Wood Wood Bivalve Plant fossil Bivalve Bivalve Bivalve Wood Wood Peat Peat Peat Plant fossil Plant fossil Wood Wood K-Ah ash Wood Wood Wood Plant fossil Plant fossil Plant fossil Plant fossil Wood Plant fossil Wood Wood Wood Wood Wood Wood Plant fossil Wood Wood K-Ah ash Plant fossil

14

δ13C(‰)

Method

C (yr BP±1σ)

cal yr BP (1σ)

AMS AMS

-29.28 -30.1

± ±

0.27 0.27

5330 6120

± ±

30 30

6020 - 6180 6950 - 7150

AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS

-32.64 -26.78 -5.07 -29.98 -7.44 -2.93 -4.21 -25.88 -31.75 -27.65 -26.59 -25.93 -33.48 -27.18 -32.25 -24.49

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.43 0.34 0.36 0.38 0.41 0.37 0.31 0.35 0.36 0.31 0.3 0.31 0.37 0.18 0.3 0.34

6340 7700 7630 7020 7790 7790 7790 12930 11220 2250 3490 2990 4540 5670 6320 6180

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

30 30 30 30 30 30 30 40 40 30 30 30 30 25 30 30

7250 - 7310 8450 - 8540 8040 - 8140 7840 - 7930 8210 - 8310 8210 - 8310 8210 - 8310 15150 - 15610 13080 - 13210 2180 - 2340 3720 - 3830 3080 - 3240 5070 - 5310 6410 - 6470 7180 - 7290 7020 - 7160

AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS AMS

-27.67 -28.1 -30.29 -32.31 -30.65 -33.19 -28.89 -30.05 -31.03 -29.58 -29.26 -29.49 -29.75 -31.66 -30.56 -30.98 -30.54 -26.36

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.29 0.29 0.43 0.3 0.3 0.44 0.3 0.33 0.33 0.38 0.33 0.46 0.33 0.36 0.31 0.35 0.33 0.3

6560 6630 6920 7060 8280 12800 8380 450 2890 3900 3330 3930 3810 4690 5230 8640 6060 12860

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

30 30 30 30 40 50 40 20 30 30 30 30 30 30 30 40 30 50

7430 - 7480 7490 - 7570 7700 - 7790 7860 - 7940 9140 - 9400 15020 - 15470 9330 - 9470 500 - 520 2980 - 3060 4300 - 4410 3490 - 3610 4300 - 4420 4150 - 4240 5330 - 5470 5940 - 5990 9540 - 9620 6880 - 6970 15110 - 15510

AMS

-30.39

±

0.42

12660

±

40

14870 - 15170

cal yr BP(2σ) 6000 - 6200 6930 - 7160 7170 - 7300 7170 - 7410 8420 - 8550 8000 - 8170 7790 - 7940 8180 - 8340 8180 - 8340 8180 - 8340 15060 - 16070 12940 - 13270 2160- 2340 3690 - 3840 3080 - 3320 5050 - 5310 6400 - 6500 7170 - 7310 6980 - 7170 7170 - 7300 7420 - 7560 7440 - 7580 7680 - 7830 7840 - 7960 9130 - 9410 14900- 15800 9300 - 9480 490 - 530 2930 - 3140 4250 - 4420 3480 - 3640 4260 - 4440 4090 - 4290 5320 - 5580 5920 - 6170 9540 - 9680 6800 - 7000 15000 - 15880 7170 - 7300 14590 - 15240

Tidal range MTL ST ST ST ST ST ST ST ST MTL F? F F F F HTL HTL MTL MTL MTL ST ST MTL MTL F F F F F F F F F F F HTL HTL MTL MTL F?

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Lab. Cord

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Fig. 5. Stratigraphy, diatom assemblages, and 14C ages in core SK-3.

The lithostratigraphic and diatom evidence suggests that mean tide level was at 17.9 m depth (elevation –15.3 m) in 8280–8410 cal yr BP, during the transgression. The mean tide level was at 2.8 m depth (elevation –0.2 m) in 4780–4890 cal yr BP. The high tide level was at 2.2–2.6 m depth (elevation 0.4 to 0 m) and the freshwater environment was at 1.3–2.2 m depth (elevation 1.3 to 0.4 m) in 1310–2700 cal yr BP (Fig. 11).

3.2. Core SK-4 Core SK-4 contained fill sediment from the surface to 2.8 m depth (Fig. 2). Under the fill, brackish or marine muddy sediment extended from 2.8 m to 12.3 m depth and was underlain by a thin layer of gravelly

sediment. Under the gravel was late Pleistocene sediment (12.3–15.0 m depth). We divided core SK-4 into four diatom zones (Fig. 6). The oldest zone 1 (7.7–11.9 m depth) is characterized by marine species such as Cocconeis scutellum, Cyclotella litoralis and Thalassionema nitzschioides and represents an inner bay environment. Zone 2 (5.5–7.7 m depth) is characterized by marine benthic species such as Achnanthes brevipes and Cocconeis scutellum, a gradual increase in brackish diatoms such as Pleurosira laevis and Thalassiosira lacustris and a gradual decrease in marine planktonic species such as Thalassionema nitzschioides and Cyclotella litoralis. Furthermore, the tidal-flat indicators Diploneis smithii, Giffenia cocconeiformis, Planothidium delicatulum and Tryblionella granulata are abundant, so the ensemble of species suggests a tidal flat environment at mean tide level. Zone 3

Fig. 6. Stratigraphy, diatom assemblages, and 14C ages in core SK-4.

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Fig. 7. Stratigraphy, diatom assemblages, and 14C ages in core SK-6.

(3.4–5.5 m depth) is characterized by an increase in the freshwater species Eunotia praerputa and Staurosira construens and the presence of the freshwater-brackish diatoms Pseudopodosira kosugii and Pinnunavis elegans and the brackish-marine diatom Tryblionella littoralis, therefore this zone represents a tidal flat or salt marsh environment at high tide level. Zone 4 (3.0–3.4 m depth) is dominated by freshwater species such as Staurosira construens and Staurosirella pinnata, so this zone represents a freshwater environment. This diatom-based environemntal change consisted with the paleoenvironmental reconstruction from fossil molluscan assemblages in SK-4 (Matsushima, 2011). The lithostratigraphic and diatom evidence suggests that mean sea level was reached at 5.5–7.7 m depth, particularly in 2490–2740 cal yr BP (5.6 m depth, elevation –1.9 m) and 2330–2460 cal yr BP (5.8 m

depth, elevation −2.1 m) during the regression. The high tide level estimated from diatoms was at 3.4 to 5.5 m depth (elevation 0.3 to −1.8 m) in 1420–1690 cal yr BP (Fig. 11).

3.3. Core SK-6 Core SK-6 consisted of fill from the surface to 1.9 m depth (Fig. 2). Below this was peaty sediment from 1.9 to 2.9 m depth, sandy silt from 2.9 to 3.1 m depth, and fine sand from 3.1 to 15.5 m depth, interrupted by an interval of fine sand 0.9 m thick at 5.0 m depth and another one 1.7 m thick at 7.9 m depth. From 15.5 to 18.0 m depth was late Pleistocene silty sediment, at the base of which was a gravelly layer of an unidentified

Fig. 8. Stratigraphy, diatom assemblages, and 14C ages in core SK-7.

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Fig. 9. Stratigraphy, diatom assemblages, and 14C ages in core SK-8.

tephra. Late Pleistocene sandy sediments were beneath the tephra (18–22 m depth). We divided core SK–6 into six diatom zones (Fig. 7). The oldest zone 1 (14.5–16.1 m depth) is characterized by freshwater species such as Ulnaria ulna, Staurosira construens and Hantzschia amphioxys, representing a freshwater environment. Zone 2 (11.7–14.5 m depth) is characterized by marine and brackish diatoms such as Tryblionella granulata, T. lanceola and Planothidium delicatulum, representing a tidal flat environment. Zone 3 (4.4–11.7 m depth) is characterized by marine planktonic species such as Cyclotella litoralis, Thalassionema nitzschioides and Cymatotheca weissflogii, and represents an inner bay environment. Zone 4 (3.1–4.4 m depth) is characterized by brackish and marine diatoms Tryblionella granulata, T. lanceola and Planothidium delicatulum, representing a tidal flat environment. Zone 5 (2.9–3.1 m depth) is characterized by freshwater and brackish diatoms, in particular the brackish diatoms Pseudopodosira kosugii, Pinnunavis yarrensis, Pseudostaurosira brevistriata and Rhopalodia gibberula, so it represents a tidal flat or salt marsh environment at the high tide level. Zone 6 (2.1–2.9 m depth) is characterized by freshwater species such as Staurosira construens and Staurosirella pinnata, representing a freshwater marsh environment. The lithostratigraphic and diatom evidence suggests that mean sea level was from 14.5 to 11.7 m depth (elevation − 9.2 to − 7.0 m) in 8410–8550 cal yr BP during the transgression and from 3.1 to 4.4 m depth (elevation 1.6–0.3 m) in 1260–7240 cal yr BP during the

regression. During the regression, the very low sedimentation rate produced a hiatus in the core. High tide level was from 2.9 to 3.1 m depth (elevation 1.1–0.9 m) after 1260 cal yr BP (Fig. 11). 3.4. Core SK-7 Core SK-7 contained fill from the surface to 1.8 m depth (Fig. 2). This was underlain by freshwater or brackish peaty sediment from 1.8 to 4.5 m depth with two beds of fine sand 0.1 m and 0.2 m thick intercalated within the peat. Fine sand extended from 4.5 to 5.0 m depth, then silt extended from 5.0 to 9.1 m depth. Fine sand extended from 9.1 to 14.9 m depth, and a thin late Pleistocene unidentified tephra layer was found at 14.0 to 14.2 m depth. Late Pleistocene sediment also made up the remainder of the core (14.9–16.0 m depth). We divided core SK-7 into seven diatom zones (Fig. 8). Diatoms were absent below 11.9 m depth. The oldest zone 1 (11.8 m depth) is characterized by freshwater diatoms such as Ulnaria ulna and Cocconeis placentula, representing a freshwater environment. Zone 2 (11.4–11.8 m depth) is characterized by freshwater diatoms such as Ulnaria ulna and Cocconeis placentula and abundant brackish–marine diatoms Tryblionella granulate and Cyclotella litoralis, representing a tidal flat or salt marsh environment. Zone 3 (9.8–11.4 m depth) is characterized by brackish-marine and marine diatoms such as Opephora sp., Planothidium delicatulum, Tryblionella granulate and Cyclotella litoralis

Fig. 10. Stratigraphy, diatom assemblages, and 14C ages in core SK-9.

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Fig. 11. Age–depth model along line A–B in Fig. 1c during the postglacial period using 14C ages, the K–Ah tephra age and core elevations. The age data are shown as mean values within the 2σ error range.

representing a tidal flat environment. Zone 4 (4.5–9.8 m depth) is characterized by marine planktonic species such as Cyclotella litoralis and Thalassionema nitzschioides and the benthic species Cocconeis scutellum, representing an inner bay environment. Zone 5 (3.8–4.5 m depth) is characterized by brackish-marine and marine diatoms Opephora sp., Planothidium delicatulum, Navicula digitoradiata and also freshwaterbrackish species Pseudostaurosira brevistriata and Rhopalodia gibberula, representing a tidal flat environment. Zone 6 (3.4–3.8 m depth) is characterized by freshwater and brackish diatoms, in particular the brackish diatoms Pseudopodosira kosugii, Pinnunavis elegans and P. yarrensis, so it represents a tidal flat or salt marsh environment at the high tide level. Zone 7 (2.0–3.4 m depth) is characterized by freshwater species such as Luticola mutica and Hantzschia amphioxys, representing a freshwater marsh environment.

The lithostratigraphic and diatom evidence places mean sea level at 9.8–11.4 m depth (elevation −3.5 to −5.1 m) around 8180–8340 cal yr BP, during the transgression, and from 3.8 to 4.5 m depth (elevation 2.5–1.8 m) in 5490–6200 cal yr BP during the regression. The high tide level was at 3.4 m depth (elevation 2.8 m) in 3880–4080 cal yr BP (Fig. 11). 3.5. Core SK-8 Core SK–8 contained fill from the surface to 0.8 m depth (Fig. 2). This was underlain by freshwater or brackish peaty sediment from 0.8 to 5.0 m depth with two beds of fine sand 0.5 m thick intercalated within the peat. Fine sand sediment from 5.0 to 6.0 m depth, then silt extended from 6.0 to 10.0 m depth. Fine sand extended from 8.0 to 12.5 m depth

Fig. 12. Sea-level changes during the last 8000 years in the Lake Inba area, in Nagareyama (Endo et al., 2013), in Shimokita Peninsula (SZG; lower mantle viscosity: 1 × 1022 Pa s and Lithosphere thickness: 50 km in Yokoyama et al., 2012), in Tokyo (Okuno et al., 2014) and ESL in Lambeck et al. (2014). Sea-level curves adjusted for uplift rates of 0.14 mm/yr and 0.03 mm/yr are also shown. Reworked 14C samples are excluded. (a) RSL and corrected curves in this study. (b) Comparison of the sea-level changes between the previous studies and this study. (c) Interpretation of Holocene residual uplift in this study.

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above thin muddy 1.0 m thick layer, at the base of which was a sandy layer consisting of an unidentified tephra. Late Pleistocene sediments were beneath the tephra (12.5–15 m depth). We divided core SK-8 into six diatom zones (Fig. 9). The oldest zone 1 (9.2–11.0 m depth) is characterized by freshwater species such as Ulnaria ulna and Planothidium lanceolatum, representing a freshwater environment. Zone 2 (8.0–9.2 m depth) is characterized by marine and brackish-marine diatoms such as Tryblionella hyalina, T. granulata and Planothidium delicatulum, representing a tidal flat environment. Zone 3 (7.3–8.0 m depth) is characterized by marine planktonic species such as Cyclotella litoralis, Thalassiosira spp. and Tryblionella lanceola, representing an inner bay environment. Zone 4 (5.0–7.2 m depth) is characterized by the brackish and marine diatoms Tryblionella granulata, Planothidium delicatulum and Rhopalodia gibberula, representing a tidal flat environment. Zone 5 (3.8–5.0 m depth) is characterized by freshwater-brackish and brackish-marine diatoms. Tryblionella granulata, Pseudostaurosira brevistriata and Rhopalodia gibberula are dominant and the freshwaterbrackish diatoms Pseudopodosira kosugii and Pinnunavis yarrensis are present, representing a salt marsh environment at the high tide level. Zone 6 (1.0–3.8 m depth) is characterized by freshwater species such as Staurosira construens and Planothidium laceolatum, representing a freshwater environment. The lithostratigraphic and diatom evidence suggest that mean sea level was around 8.0–9.2 m depth (elevation 0.5 to − 0.7 m) in 7680–7960 cal yr BP, during the transgression, and around 5.0–7.3 m depth (elevation 3.5 to 1.2 m) in 6980–7300 cal yr BP, during the regression. High tide level was at 4.0 m depth (elevation 4.5 m) in 6400–6500 cal yr BP (Fig. 11). 3.6. Core SK-9 Core SK-9 contained fill from the surface to 0.9 m depth (Fig. 2). Beneath this was peaty sediment to 3.3 m depth, brackish or marine sandy mud or sand from 3.3 to 10.2 m depth, and sandy gravel at 10.3 m depth. Beneath the sandy gravel was late Pleistocene sediment. We divided core SK-9 into four diatom zones (Fig. 10). The oldest zone 1 (9.0–9.5 m depth) is characterized by freshwater, marine and brackish species such as Cocconeis disculus, C. placentula, Planothidium lanceolatum, P. delicatulum, Rhopalodia gibberula, and Opephora sp. and represents a tidal flat environment at mean tide level. Zone 2 (8.2–9.0 m depth) is characterized by an upward increase in freshwater and freshwaterbrackish diatoms such as Cocconeis disculus, C. placentula, Planothidium lanceolatum, and Pseudostaurosira brevistriata and Pseudopodosira kosugii. Moreover, marine diatoms decrease upward in this zone, so this zone represents a tidal flat environment at high tide level. Zone 3 (3.5–8.2 m depth) is characterized by freshwater and brackish diatoms, particularly Eunotia spp., Cocconeis placentula, Planothidium lanceolatum and Pseudostaurosira brevistriata, along with an upward increase in Staurosira construens and Staurosirella pinnata, and therefore represents a freshwater environment. Zone 4 (1.0–3.5 m depth) is characterized mainly by freshwater diatoms. Staurosira construens, Staurosirella pinnata and Eunotia praerupta increased and peat was deposited, so in this zone the freshwater environment also changed the conditions of plant growth. The lithostratigraphic and diatom evidence suggests that mean sea level was around 9.0–9.5 m depth (elevation 2.6 to 2.1 m) in 7165–7300 cal yr BP, and high tide level was at 8.2–9.0 m depth (elevation 3.4 to 2.6 m) in around 6800–7000 cal yr BP (Fig. 11). 4. Discussion 4.1. Holocene tidal range, relative sea-level and geographic changes inferred from diatoms Coastal paleoenvironments at a single core site respond chiefly to the position of the shoreline and the effect of salinity changes. For detailed environmental reconstructions, many 14C dates and tephra ages

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provide the better constraints on these environmental changes. However, it is known that the tidal range also responds to changes in the sea depth, that is, sea-level change (Hinton, 1992, 1995; Yokoyama et al., 1996). In addition, it is sometimes difficult to apply a modern tidal range to the inner part of a paleo bay that has subsequently emerged (Ando and Fujimoto, 1990). Here, we accept diatom-based index points for mean tide level and high tide level as proxies of mean sea level and high tide level, respectively. The difference in elevation between these two levels thus represents the upper half of the tidal range during the time period within the range of the calibrated 14C age, as defined by its measurement error (Fig. 11). The example in Fig. 11 shows that the upper half of the paleo-tidal range was 1.7 m during 7000–7500 cal yr BP. This value is consistent with an estimate from the Arakawa lowland in the central Kanto Plain (Ando and Fujimoto, 1990). We also used this value for estimating previously undetermined mean tide levels in the HHS and regression periods. These index points were determined by subtracting the upper half of the tidal range, 1.7 m, from the high tide level (Figs. 11, 12a). We used these values of tidal range in sea-level reconstruction, although they may possibly be overestimated because of the effects of Holocene uplift, as described below. The diatom assemblages in this area suggest that during 9000–16,000 cal yr BP, a freshwater environment prevailed and that the Younger Dryas cold event and meltwater pulse 1-B (e.g. Bard et al., 2010) had little or no effect. The freshwater environment changed to a brackish tidal flat environment as sea level rose in the Holocene transgression by 15 mm/yr during 7800–8500 cal yr BP (Fig. 12a). This rate is similar to coeval estimates in the Nagareyama area (Endo et al., 2013) and the Toyooka basin, Niigata prefecture, Japan (Tanigawa et al., 2013) and the rate obtained from dating of corals in Tahichi (Bard et al., 1996, 2010). We did not find effects of the 8.2 ka climate event (e.g. Alley et al., 1997) in the study area. During 6500–7800 cal yr BP, the rate of sea-level rise decreased from 15 to 4 mm/yr, and the tidal flat environment changed to a marine inner bay environment, except at core site SK-9. During these two periods, a salt marsh environment either did not arise or had its sedimentary record eroded, perhaps by regressive denudation. This pattern may be common in Japan during the transgression period (Chiba, 2014). Around 7000 cal yr BP, a tidal flat was formed that established an index for the high tide level at the site of core SK-9. The high-tide level here during 6400–6500 cal yr BP was at 4.5 m (Fig. 2), and given the upper half of tidal range of 1.7 m (Fig. 10), the resulting mean sea level was 2.8 m. These estimated rates are a reasonable match to those previously recorded in the Kanto Plain (Endo et al., 1982, 1983, 2013; Ota et al., 1990; Matsushima, 1996). We also obtained a corrected HHS value of 1.9 m by using the vertical uplift (0.9 m) estimated in this study. This transgression was caused mainly by melting of Northern Hemisphere ice sheets (Yokoyama et al., 2012). Sea level then began to fall until 4000 cal yr BP at a rate of 0.7 mm/yr (Fig. 11). During this regressive period, salt marshes formed at lower elevations (as seen in core SK-7) and freshwater environments formed at higher elevations (as seen in cores SK-8 and SK-9). Chiba (2014) reported that salt marsh formed during this period in Obukai, just west of the site of core SK-3, which is consistent with our results. Shortly after 4000 cal yr BP, sea level declined at a rate of 3 mm/yr, corresponding to the Yayoi regression (Ota et al., 1990; Endo et al., 2013; Tanabe and Ishihara, 2013), and a freshwater environment formed in the study area except at the site of core SK-4. This increase in the rate of sealevel decline is probably related to the end of melting of the Antarctic ice sheet (Nunn and Peltier, 2001; Yokoyama et al., 2012). On the other hand, our evidence shows the possibility that the mean sea level fell to the maximum − 2.5 m, although this evidence was recognized only in core SK-4. Therefore this sea-level fall event may correspond to a geomorphologic event such as the closing or opening of a sand barrier. After 2600, sea level probably rose again and salt marshes formed in parts of the study area (core SK-6). This sea-level history is consistent with the discussion of Ota et al. (1990).

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4.2. Residual uplift and cause of uplift in the Lake Inba area during the last 8000 years For studies of tectonic movements, the residual uplift after accounting for sea-level changes is an important value for calculating tectonic uplift rates (e.g. Nakada et al., 1991; Sato et al., 2001). Residual uplift is determined by comparison between the detailed RSL measured during the Holocene and the sea-level curve predicted by geophysical modeling (Yokoyama et al., 1996, 2012; Sato et al., 2001; Okuno et al., 2014). Recently, Yokoyama et al. (2012) derived RSL curves for coastal areas of northern Japan using sea-level changes determined by using a glacial isostatic adjustment model in which a three-layer rheological model consisted of an elastic lithosphere with thicknesses of 30 and 50 km, an upper mantle viscosity of 2 × 1020 Pa s, and three different values of lower mantle viscosity (5 × 1021, 1 × 1022 and 3 × 1022 Pa s). They modeled and compared three patterns of melting ice: (A) cessation of major Antarctic melting 6000 years ago, (B) cessation of major melting about 4000 years ago, and (C) continuous melting with a melt-related sea-level rise during the mid to late Holocene of 3 m. They concluded that model B best fit the geological and micropaleontological data and that the HHS occurred around 4000 years ago in northern Japan. On the other hand, Okuno et al. (2014) derived sea-level curves for coastal areas of Japan using RSL changes based on (1) a glacial isostatic adjustment model developed at Australian National University (ANU model; Lambeck et al., 1998) with a melt-related rise during the mid to late Holocene of 2 m and (2) a revised ANU model (ANUr) with a melt-related rise of 4 m, using a three-layer rheological model consisting of an elastic lithosphere with a thickness of 40 km, an upper mantle viscosity of 2 × 1020 Pa s, and a lower mantle viscosity of 1 × 1022 Pa s. Their calculation of sea-level was based on the HHS having occurred 6000 years ago. Furthermore, Lambeck et al. (2014) conducted inversions of ~1000 global observations for the past 35,000 years under various lithospheric assumptions for the far field. In Fig. 12a we compare our RSL curve, corrected for uplift around Lake Inba, with estimates for Nagareyama (Endo et al., 1989, 2013), Shimokita (Yokoyama et al., 2012), and Tokyo (Okuno et al., 2014) along with the ESL curve of Lambeck et al. (2014). The maximum Holocene sea levels were 1.9 m around Lake Inba and 3.0 m at Nagareyama, so the uncorrected values of sea-level changes (from HHS at 2.8 m in the Lake Inba area) are similar at both sites. The corrected Holocene sealevel curve around Lake Inba is similar to the predicted model in Tokyo (Okuno et al., 2014). However, the sea-level curve in the Lake Inba area during the regression period differs greatly from the curve for Tokyo around 4000 years ago and afterward, which mainly reflects the end of Antarctic ice sheet melting (Yokoyama et al., 2012). The two curves from this study and RSL in Okuno et al. (2014), shown in Fig. 12b, differ in the timing of the maximum sea level, but when we compared the maxima as if they were not simultaneous, these two curves were similar during 4000–8000 years ago. At first glance, these two curves appear to be in good agreement. The rate of sea-level rise during the transgression period is similar for each ESL curve (Yokoyama et al., 2012; Lambeck et al., 2014; Okuno et al., 2014) and for RSL in Nagareyama (Endo et al., 1989, 2013), but the timing of the rise is earlier in the Lake Inba area. The MIS5e marine terrace elevations become abruptly higher between the northern river mouth and the southern inland area of the Kashima river (Fig. 1c). In other words, this study suggests that around Lake Inba, sea-level index points are progressively higher than elsewhere with increasing age. Even if the RSL curve is corrected by the uplift rate obtained from MIS5e marine terrace elevations, a residual uplift of 3 m remains as an effect of subduction of the Philippine Sea plate beneath the southern Kanto area, especially before 6500 cal yr BP in SK-8 and SK-9 (Fig. 12c). Therefore, our results suggest that the timing of the HHS in this study may be at least 1000 years early. What caused the uplift in SK-8 and SK-9? Masuda et al. (2001) suggested, from analyses of beach sand deposits, that the mean sea level of

4.0–6.0 m during the HHS at Kujukuri beach (Fig. 1b) was higher than that at Nagareyama (3.0 m; Endo et al., 1989), and they made a connection with great earthquakes in the Kanto section of the subduction zone, such as the 1703 Genroku earthquake. For instance, the Shimousa upland in the Sakura area near Lake Inba was uplifted more than 0.04 m by the 1923 Great Kanto earthquake (Geospatial Information Authority of Japan, 2015). Therefore, the uplift in the Lake Inba area may reflect tectonic movements due to these great earthquakes. However, by itself, it doesn't help explain the whole of Holocene residual uplift in terms of the recurrence interval (Earthquake Research Committee, 2014), so there might be unknown tectonic events with the uplift. It is not likely that the uplift rate has been steady since 100 ka. Instead, the possible recognition of the Yayoi regression event in this study suggests the possibility of some hydro-isostatic or tectonic movement confined to this area (Ota et al., 1990; Tanabe and Ishihara, 2013). On the other hand, small-scale fluctuations in RSL may correspond to global glacial advances between 2000 and 3000 years ago (Denton and Karlén, 1973; Haug et al., 2001; Mayewski et al., 2004). More investigations, including paleogeographic (Tanabe et al., 2014) and local tectonic reconstructions, are required to address this question over the coast of Japan. 5. Conclusions We inferred the following sequence of Holocene sea-level and paleogeographic changes in the Lake Inba area from a diatom analysis of six cores. The shoreline reached the Lake Inba lowland during the Holocene transgression. At that time, maximum mean sea level reached about 1.9 m elevation at approximately 6400–6500 cal yr BP, corrected for a 0.14 mm/yr uplift rate. Sea-level rises averaged 15 mm/yr in 7800–8500 cal yr BP and 4 mm/yr in 6500–7800 cal yr BP during the Holocene transgression. These rates are similar to those of previous studies; however, it is possible that Holocene uplift in this area may make the timing of shoreline indicators appear early. These results also suggest that the timing of the HHS in this study may be at least 1000 years early. The decline of sea-level after 4000 cal yr BP may correspond to the end of melting of the Antarctic ice sheet. We also found evidence of a sea-level fall event corresponding to the Yayoi regression in the Lake Inba area, where sea level fell to the maximum 4 m during 2600–4000 cal yr BP. This regression may not be related only to hydro-isostatic adjustment effect but also local geomorphologic and tectonic effects. Acknowledgment We thank Mr. Tokuo Fujimori, Mr. Takeji Toizumi, Mr. Kensuke Tsurumaki, Dr. Taiji Kurozumi, and Dr. Kunio Yoshida for assistance in core sampling, and Dr. Junichi Okuno and Dr. Masao Nakada for reference of the data predicted sea-level changes in that used as reference in this discussion. We thank Dr. Susumu Tanabe, Dr. Arata Momohara, Dr. Kazuo Masubuchi and Dr. Shigehiro Fujino for suggesting improvements to this paper. We also thank Dr. Akira Ono, Dr. Masashi Nagai and Mr. Taro Kannari for the using measuring instruments and helping. Finally, we thank our editor in chief, Thierry Corrège and two anonymous reviewers for their useful comments. This work was supported, in part, by Program to Disseminate Tenure Tracking System and Academic Frontier Project for private universities, MEXT, Japan. References Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25, 483–486. Ando, K., Fujimoto, K., 1990. Paleo-environmental history and sea-level records based on the diatom assemblages in the middle Part of the Arakawa Lowland, Central Japan. Quat. Res. (Daiyonki kenkyu) 29, 427–437 (in Japanese with English abstract). Arai, Y., 2012. MIS5e marine terrace in southern Inbanuma, Japan (HQR22-P05). Japan Geoscience Union Meeting 2012 abstract, May 2012, Chiba.

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