The Holocene environmental changes in boreal fen peatland of northern Mongolia reconstructed from diatom assemblages

Quaternary International 348 (2014) 66e81

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The Holocene environmental changes in boreal fen peatland of northern Mongolia reconstructed from diatom assemblages Yu Fukumoto a, *, Kaoru Kashima a, U. Ganzorig b a b

Department of Earth and Planetary Sciences, School of Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi Ward, Fukuoka, Japan Division of Pedology, Institute of Geography, Mongolian Academy of Sciences, Erkhuu Street, Ulaan Barilga, Ulaanbaatar 210620, Mongolia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 16 June 2014

The Holocene hydrological changes of fen peatland in northern Mongolia were reconstructed using diatom fossils with an aid of Sphagnum leaf fossils. Analyses were conducted on three cores taken across the basin and reconstructed water environments were compared to find basin-wide hydrological changes and climate anomalies. The peatland has been covered by sedges during most of its 10 thousand years history, but all cores showed common peatland hydrosere processes from fluvial environment in the early Holocene period, shallow marsh to present acidic fen peatland. The establishment of fen peatland at 6.8e6.4 cal ka BP was marked synchronously by the shift of diatom and Sphagnum taxa assemblages and this period was inferred as the onset of the mid-Holocene dry climate which is widely observed around Mongolia. The dry environment lasted until around 2.8 cal ka BP with a temporal wetter condition at 4.4e3.5 cal ka BP. Short term diatom trend shifts indicated temporal wetter environments at 8.7e8.4, 8.0e7.6, 2.4e2.1, 1.2e0.5 cal ka BP and a dry environment at 0.4e0.2 cal ka BP. These periods of hydrology changes were correlative with other studies in Mongolia and East Siberia, so they would represent climate-induced hydrological changes, and those regions might share the same response to long term insolation changes or global climate anomalies. However, disagreements of climate data even within northern Mongolian region suggest large influence of regional geography and the balance of evapotranspiration on climate behaviors as shown by recent dendrological paleoenvironment researches. Coherent differences of main taxa compositions between three cores were attributed to core site-specific local hydrology mainly in terms of relative wetness from wettest core site accompanied by abundant planktonic diatom taxa to driest site with acidic peat indicator taxa. Timings and manners of temporal diatom trend shifts were also largely different between cores suggesting significant difference in each core site sensitivity to basin-wide hydrology changes, confirming the necessity of using multiple cores and proxies in peatland sediments to detect climate-induced environmental changes. © 2014 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Peat sediment Northern Mongolia Diatom analysis Holocene climate

1. Introduction The land of Mongolia is known for its central location of Siberian High pressure and its variations affect the winter monsoon climate throughout East Asia. Mongolia is distinct from other East Asian countries in that the Asian summer monsoon rainfall generally does not reach the area (Sato, 2009) and climate pattern is more influenced by remote atmospheric behavior in North Atlantic Ocean and Arctic regions (Hurrel et al., 2003) by modulating westerly wind. There are various types of vegetation from forest to desert (Hilbig, 1995) in generally dry and cold continental climate, but they could be largely altered in the near future as the climate in

* Corresponding author. E-mail address: [email protected] (Y. Fukumoto). http://dx.doi.org/10.1016/j.quaint.2014.05.029 1040-6182/© 2014 Elsevier Ltd and INQUA. All rights reserved.

Mongolia is considered susceptible to global climate changes (Sugita et al., 2007). Climate and landscape anomalies are increasingly observed in recent decades, probably due to rapid temperature increases (Batima et al., 2005; Nandintsetseg et al., 2007), melting of permafrost (Kynický et al., 2009) and the reduction of the taiga forest and steppe zone (Dregne, 1986; Tsukuura et al., 2010). In order to understand the mechanism causing these events, knowledge on the past climate changes and subsequent response of natural environment is required. Recent climate changes in Mongolia for the last millennium are becoming clear on a yearly scale through dendrochronological research (Leland et al., 2013; and references therein), but climate records of the Holocene periods are still lacking to establish even a millennial year scale history. Wang and Feng (2013) and An et al. (2008) compiled previous climate records and suggested the occurrence of mid-Holocene drought in Mongolia. They assumed

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Fig. 1. (A) Sketch of Mongolia, the location of Khentii Mountains and the study site (Nur Sphagnum bog). Contour lines are drawn with a 1000 m interval. (B) General description of topography around Nur Sphagnum bog (painted with pattern). (C) Locations of core Kh-1, Kh-2 and Kh-3. Thick dotted lines denote river flows.

that it was caused by higher than present summer temperature and strong evaporation, and it was a peculiar phenomenon to the Mongolian plateau and East Siberia in contrast to the southern monsoon climate regions. Short term climate events in centennial scale are often suggested, but their timings are still far from consensus, probably due to regional geography and the balance of evapotranspiration. Mountain orography, for example, was suggested as a strong driver for local climate changes in Lake Baikal region (Bezrukova et al., 2013). Other responsible factors would be difficulty in sediment age control caused by the short sedimentation interval of the Holocene (Prokopenko et al., 2007) and the carbon reservoir effect in lacustrine deposits (Watanabe et al., 2009; Orkhonselenge et al., 2012). Peatlands have potential to provide high resolution records of climate history because of lesser bioturbation and reservoir effects (Blaauw et al., 2004). Biological proxies such as diatoms, pollen, testate amoebae, and plant materials are good indicators for peatland water environments, and their fossils are widely used to examine past climate changes. Diatom fossil assemblages, in particular, show sensitive responses to various hydrology and peat types such as water quality, types of Sphagnum taxa (Round et al., 2000; Klemen
ci
c et al., 2010), humidity (Beyens and Bock, 1989) and seasonality of climate (Kim et al., 2007; Liu et al., 2011). This study analyzed diatom assemblages on three cores taken in a fen peatland located near the southern limit of the East Siberian taiga forest zone in northern Mongolia. Sphagnum leaf fossils can be identified only in section level in routine counting, but are useful jek et al., 2006; Vitt, for the inferring wetness of peatlands (Ha 2006). The peat cores contain continuous sedimentary records from 10 to 9 cal ka BP. Fukumoto et al. (2012) analyzed diatom, pollen and chemical components on a single peat core and confirmed the extensive mid-Holocene dry condition and short term climate anomalies, including the Medieval Climate Anomaly and the Little Ice Age. However, analysis of a single peat core is often criticized for its low representation of regional climate changes, as autogenic peat growth rhythm and hydrological succession produce numerous site-specific, vegetation, and water environments (Bindler et al., 2004; Yu, 2006). The surrounding topographic characteristics also determine the manner of long term

hydrosere successions (Charman et al., 2006). Thus, the analysis of multiple cores and finding similar proxy changes with the same pattern and timing is necessary to remove local hydrological factors. We compared diatom inferred local hydrological and vegetation successions between three cores and tried to discover the evidence of climate changes. 2. Regional setting and study site The Nur Sphagnum bog (49
390 N, 107
480 E, 1250 m a.s.l.; Kulikovskiy et al., 2009) is located in Selenge province at the northwestern tip of the Khentii Mountains, which extend from the north of Ulaanbaatar to the Russian border (Fig. 1A). Khentii Mountains have a general elevation of 1500 m and the area is known for high diversity of vegetation types with mixed distributions of wetter Siberian taiga forests and drier Mongolian grass steppe (Dulamsuren et al., 2005). A meteorological station in Yeroo village at 600 m a.s.l., located 80 km west of Nur Sphagnum bog, documents mean air temperature of 27.1
C in January and 18.3
C in July, and annual precipitation of 288 mm. Mean air temperatures above zero are recorded only from April to September at the station but much lower temperature is expected at the study site due to its higher altitude. Pinus sibirica, Picea obovata and Abies sibirica are distributed in the upper hills surrounding the peatland (Kulikovskiy et al., 2009), corresponding to vegetation of the upper montane belt (1200e1600 m) in Mongolia (Hilbig and Knapp, 1983). Betula platyphylla widely occupies peatland margins and river banks. The Nur Sphagnum bog has a small catchment area and there are no large inflowing rivers due to its location on a mountain saddle (Fig. 1B). There is a main outflowing river from the southwest which gathers network streams of the peatland basin (Fig 1C). The peatland surface is apparently flat but the northern area is higher by a few meters, and topographical relief is slightly inclined to southwest along the outflowing river. The outflow is a tributary of Tsokh River that drains into Lake Baikal via Selenga River. The vegetation of the peatland is covered by Carex rostrata, various species of Sphagnum (mainly Sphagnum angustifolium) and brown mosses (such as Pseudobryum cinclidioides and Drepanocladus

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revolvens). Eriophorum vaginatum and Oxycoccus palustris are also commonly distributed. Small hollow water pools are dominated by sedges (Carex limosa, Carex vesicata). A marginally drier site was inhabited by many Sphagnum species (e.g. S. riparium, S. fuscum, S. teres) and Conostomum tetragonum. These plant communities and the water pH at around 6.0 in AugusteSeptember indicate jek et al., moderately rich to poor fen status of the peatland (Ha 2006). 3. Materials and methods Three peat cores were taken in “Nur Sphagnum bog” using a Russian type sampler with lengths of 3.7 m (core Kh-1), 5.5 m (core Kh-2), and 6.0 m (core Kh-3). Coring sites are located 500 m apart (Fig. 1C). Core Kh-1 site is located in the southwestern basin at the downstream side of the outflowing river, core Kh-2 is located in the central basin, and core Kh-3 is in the northeastern basin with a few meters higher elevation. Radiocarbon dates were measured on 1 cm thick bulk sediment at five depth points in each core by accelerator mass spectrometry (AMS) at Paleo Labo Co., Ltd. in Japan. Dating layers were roughly selected at points where diatom assemblages changed (especially at 6.8e6.3 cal ka BP) found in preliminary observation of diatom assemblages. Radiocarbon ages were calibrated using the calibration program Oxcal v4.2 (Bronk Ramsey, 2009) with the calibration date set of IntCal13 (Reimer et al., 2013). Chronology of core sequence was determined by Bacon program v2.2 (Blaauw and Christen, 2011), a Bayesian age model that calculates sediment accumulation rate for every cm depth using Markov Chain Monte Carlo process and assigns calendar ages in each depth with 95.4% probability. Ages of depths shallower than the uppermost dating point was calculated by simple linear interpolation assuming the core top as present time. 0.2e0.5 g of peat sediment was subsampled for diatom analysis at 2e3 cm intervals for core Kh-1 and at 4 cm intervals for core Kh-2 and core Kh-3. These samples were treated with 15% hydrogen peroxide in a hot water bath for three hours, and the treated water aliquots were put on a cover slip and dried in room temperature before mounting on glass slide. The volume of aliquots putting on a cover slip was adjusted by repetitive trial so that the diatom density would be as high as possible. At least 300 diatom valves were identified in most samples. Diatom valves were generally suffering from severe dissolution except for the uppermost 1 m, so samples with less than 200 valves in 10 transects of a cover slip (22  22 mm) were not included in percentage data. Diatom identification and nomenclature were based on Krammer and Lange-Bertalot (1986, 1988, 1991a,b) and Watanabe (2005). A principal component analysis (PCA) was carried out using CANOCO
version 5 (Ter Braak and Smilauer, 2012) for diatom data of all depths in the three cores to clarify common diatom successional trend and taxa relationships. Only taxa that reached a 2% threshold in at least one sample were included in the analysis. Taxa compositions of Sphagnum leaf fossils were briefly analyzed to provide subsidiary data for environment interpretation. 2 cm3 of subsample was taken at 4 cm intervals in core Kh-1 and at 10 cm intervals in core Kh-2 and Kh-3. The samples were treated with 5% KOH and large organic matter was excluded by 150 mm mesh sieving. 100 Sphagnum leaves were picked in each sample and identified under a microscope of 400 magnification based on Daniels and Eddy (1985). For samples with few Sphagnum leaves, 30 leaves were identified and the main taxa were marked in a diagram. Relative volume of Sphagnum leaves in each sample was qualitatively estimated from 0 (none) to 4 (abundant). When encountered, leaves of brown mosses were identified. The data of geochemical and pollen records on core Kh-1 provided in Fukumoto et al. (2012) are shown in Fig. 8.

4. Results and interpretation 4.1. Lithology and chronology The three peat cores are lithologically homogeneous, composed mainly of herbaceous rootlets, epidermis and stems and bryophyte remains as second component (Fig. 2A). The lowermost layer of core Kh-1 is composed of inorganic silt (370e364 cm) and overlaid with silty clay (364e350 cm). Silty clay was also observed in the lowermost part of core Kh-3 (600e595 cm). Highly decomposed organic clay layers were observed above the silty clay layers. Results of radiocarbon dating (Table 1) showed the ages of organic clay layer near the bottom at 9180 (core Kh-1: depth of 350 cm), 9840 (core Kh-2: 541 cm), and 9480 cal a BP (core Kh-3: 582 cm) indicating that peatland hydrosere succession started as early as 10e9 cal ka BP. This study failed to use above ground macrofossils for dating that is often recommended in peat samples (Piotrowska et al., 2011) so the precise chronology control is difficult, but the stable trend of d13C values ranging from 28 to 25‰ and relatively smaller probability range of C14 age (60e40 years) (e.g. 400e200 years in bulk peat samples from northern Baikal ridge (Bezrukova et al., 2008)) indicate little variations of source materials. More than a thousand years of discrepancy was reported between the ages of above ground macrofossils and rootlets in peatland (Zhou et al., 2002), but it is not likely in this study considering the 14C age of 605 BP at a depth of 72 cm (core Kh-2). Based on Bayesian age model, there were averaged age uncertainties of 500e600 years with 95% confidence intervals in each depth (data not shown). A relatively negative d13C value of 28.5‰ at a depth 47 cm in core Kh-1 may indicate a deeper water table (Loisel et al., 2010). Five dating points of each core showed a rather constant sedimentation rate (Fig. 2B) with linear averaged rates of 0.38 (core Kh-1), 0.53 (core Kh-2), and 0.6 mm/y (core Kh-3). It was slightly higher near the surface and the bottom part; former part can be explained by low compaction pressure and decomposition rate, whereas the latter would be due to lithology of inorganic minerals. Table 1 Results of radiocarbon dating from bulk samples. Radiocarbon dates were calibrated using Oxcal v4.2 (Bronk Ramsey, 2009). For calibrated ages, values in brackets indicate respective confidence interval (in %). Laboratory No.

Core

Depth (cm)

d13C ( ‰)

14 C Age (a BP)

PLD-16824 PLD-14353 PLD-23948 PLD-14354 PLD-14355 PLD-23002

Kh-1 Kh-1 Kh-1 Kh-1 Kh-1 Kh-2

47 150 233 290 350 196

28.48 25.97 25.88 27.56 26.87 27.03

1135 3765 5575 7100 8205 3050

PLD-19385

Kh-2

255

24.91

3980 ± 20

PLD-23003 PLD-23949 PLD-19386 PLD-16825 PLD-16826 PLD-23950

Kh-2 Kh-2 Kh-2 Kh-3 Kh-3 Kh-3

367 446 541 72 198 349

26.84 27.53 25.83 26.43 26.79 26.27

5900 7155 8830 605 2820 4820

PLD-16827 PLD-16828

Kh-3 Kh-3

444 582

25.46 27.74

5815 ± 25 8445 ± 30

± ± ± ± ± ±

± ± ± ± ± ±

Calibrated age (cal a BP) 20 20 25 25 30 20

25 25 30 20 25 25

1024 ± 56 (93.8%) 4133 ± 50 (79.8%) 6356 ± 49 (95.4%) 7949 ± 29 (66.9%) 9175 ± 101 (89.0%) 3241 ± 35 (48.6%), 3314 ± 32 (41.7%) 4493 ± 27 (50.8), 4432 ± 18 (44.6%) 6725 ± 59 (95.4%) 7977 ± 37 (95.4%) 9844 ± 112 (69.6%) 616 ± 36 (74.0%) 2928 ± 67 (95.4%) 5507 ± 25 (54.5%), 5591 ± 13 (40.9%) 6614 ± 82 (93.1%) 9480 ± 44 (95.4%)

4.2. Diatom records Relative percentage diagrams of each diatom taxa are shown in Fig. 3 for core Kh-1 and Fig. 4 for core Kh-2 and core Kh-3. The temporal resolution of analysis was ca. 80e40 years for all cores.

Y. Fukumoto et al. / Quaternary International 348 (2014) 66e81

(A) cm

0

Kh-1

Cal a BP

Kh-2

Cal a BP

Kh-3

(B) Cal a BP

1024

cm

69

Cal a BP

0

2000

4000

6000

8000

10000

0 50

620 100

100 4130

150

200

3280

2930 200

4490

250

6360

300

3.7m

7950

Kh-1

300

9180

5510 350

6730

Kh-3 h

400

400

Kh-2

6620 450

7980 500

500 9850

5.5m

550

Herbaceous peat 9480 600 600 with Brown moss with Sphagnum leaves with Brown moss and Sphagnum leaves Peaty clay

Clay silt

Silt

Fig. 2. (A) Lithology of the three peat cores from Nur Sphagnum bog. Core Kh-1 was analyzed in the previous study in Fukumoto et al. (2012). (B) Profile of age depth relationship on three cores based on Bayesian ageedepth model for 5 calibrated radiocarbon dates in each core. Area within dotted curves represents predicted age ranges of 95.4% probability.

The diagram of core Kh-1 (Fig. 3) is same as presented in Fukumoto et al. (2012). Genus of acidic water indicators of Eunotia and Pinnularia generally dominated the diatom assemblages. Diagrams of the uppermost one meter of three cores for selected taxa are shown in Fig. 5. 4.2.1. Core Kh-1 Core Kh-1 was divided into 6 diatom assemblage zones (Fig. 3). The main taxa in Zone 1-1 (370e349 cm, ~9.6e9.2 cal ka BP) were Eunotia praerupta, Navicula ignota, and Aulacoseira alpigena. E. praerupta and N. ignota are often observed in dry environments such as terrestrial moss tissues (Ito and Horiuchi, 1991; Pienitz and Smol, 1993). Moderate percentages of planktonic or tychoplanktonic A. alpigena and Staurosira construens imply the existence of a water pool. Zone 1-2 (349e313 cm, 9.2e8.4 cal ka BP) was characterized by Meridion circulare, Eunotia minor and Achnanthes lanceolata, which are exclusively observed in this zone. These species are commonly observed on substrates under flowing streams rather than standing waters (Krammer and Lange-Bertalot, 1991a; van de Vijver et al., 2003), and the existence of Navicula pupula also suggests mud substrates and high minerotrophic waters (Negro et al., 2003; Mann et al., 2004). The gradual increase of Navicula elginensis in this zone indicates the stagnation of water streams and the development of a mineral-rich marsh environment (Ando, 1990; Ellis et al., 2008). This is supported by moderate percentages of Pinnularia nodosa/subcapitata indicating acidic water (Krammer, 2000). Tabellaria flocculosa and E. minor increased

temporally at 325e317 cm (8.7e8.5 cal ka BP) probably by increase of water level. In Zone 1-3 (313e262 cm, 8.4e7.2 cal ka BP), Pinnularia borealis, Hantzschia amphioxys and Navicula mutica were found in higher abundance. These are typical aerophilic species (Beyens and Bock, 1989; Johansen, 1999), so the dry soil or moss surfaces expanded at this period. The moderate appearance of N. elginensis in lower section suggests the frequent mixture of allochthonous diatoms in depositional basin, but the dry environment would have expanded even more at 282e262 cm (7.7e7.2 cal ka BP) when dry aerophilic taxa dominated diatom assemblages. From Zone 1-4 (262e233 cm, 7.2e6.4 cal ka BP), diatom flora shifted to acidic peat indicators similar to the present one, including T. flocculosa, Eunotia hexaglyphis, Pinnularia brevicostata and Eunotia incisa. However, N. elginensis showed sporadic peaks indicating less acidic water pH (van Dam et al., 1994) and immature stage of fen peat formation. In Zone 1-5 (233e106 cm, 6.4e2.8 cal ka BP), N. elginensis disappeared and the share of E. incisa became higher, reflecting increased water acidification and lower water depth. This zone was characterized by poor preservation of diatom fossils. At the boundary with Zone 1-6 (106e0 cm, 2.8e0 cal ka BP), E. hexaglyphis increased abruptly while E. incisa declined. It is difficult to interpret this trend shift because E. hexaglyphis is associated with slightly acidic mineral rich fens (Bertrand et al., 2004) whereas E. incisa seems to be tolerant or indifferent to broad water pH range from alkaline lakes (Liu et al., 2011) to poor fen and bog environments (Rühland et al., 2000; nkova  et al., 2009; Lange-Bertalot et al., 2011). P. brevicostata Fra

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Y. Fukumoto et al. / Quaternary International 348 (2014) 66e81

c

Na a ir v n cu Cy icu the la m la s l a re b p Eu el la upu nce no n la ol at ti a a v i a c N a v pr a u r i St icu eru form au la pt i s a r o ig si no r a ta co Eu ns no tr u t Eu ia b en i s Eunot l un no ia s alis N t ia t e itz i n Br sch l app eck a ia o ii F r c h y pe ni c us sir rm a tu a i n Eu lia bre uta rh bi n Eu otia om sso bo n Eunot pa id ii e n ia lu Au oti fal dos s la a n lax a co ym se a i r a nn al i an pi a ge na

hn

ria Ta be lla

BP la a cm C 0

Eu no tia

Kh-1

River indicator

f lo cc ul os a he xa gl yp Pi hi nn s ul ar ia br ev Eu ic no os tia ta ta in cis a En cy Eu one no ma Na tia p vic gla erp ul cia us i a el lis lla St gi ne au ns Na ron is vic eis ul sp a p. Pi n n mu t ic ul a ar ia bo Ha re nt al z is Pi sc nn hia ul G ar am o m ia p ph no hiox on do ys Eu em sa no a /su tia sp bc m M p. ap in er o it a r Ac ido ta n

Marsh indicator Dry indicator

zone

(4) 1020

(4)

50

1-6

100

4130 150

1-5 200

6360

1-4

250

1-3

7950 300

(1) 1-2 9180 350

1-1 20

40

20

40

60

20

40

20

40

60

20

20

20

40

20

20

40

20

20

30

20

20

20

20

20

20 40 60 80

%

Fig. 3. Diagrams of diatom assemblages in core Kh-1 (presented in Fukumoto et al. (2012)). The grey hatched area denotes temporal diatom trend shifts mentioned in results (Section 4.2). The bracketed bold numbers indicate the major hydrological changes discussed in Section 5.2. Thick arrow indicates the onset of mid-Holocene dry climate. A detailed diagram of surface 1 m is presented in Fig. 5.

showed a clear declining trend in this zone and there was a drastic increase of planktonic A. alpigena reaching 80% at 53e45 cm (1.2e1.0 cal ka BP) indicating a temporal increase of the water level. From a depth of 27 cm (0.6 cal ka BP) upward, Eunotia paludosa, Eunotia bilunaris and Eunotia nymanniana appeared successively in small quantities (Fig. 5). These taxa inhabit highly acidic peat waters (van Dam et al., 1981; van Dam et al., 1994), and E. paludosa is one of the representative colonizers of ombrotrophic Sphagnum hummock (Aloisie et al., 2004) and resistant to periodic desiccation (Krammer and Lange-Bertalot, 1991a; Cantonati et al., 2011). It is therefore probable that moisture level has declined since 0.6 cal ka BP. At 17e11 cm (0.4e0.2 cal ka BP), various species appeared within this short interval, including Frustulia rhomboides, Navicula soehrensis, Brachysira brebissonii, Navicula subtilissima, Eunotia steineckei, Eunotia lapponica and Encyonema perpusilla. The former three taxa are associated with highly humic waters (acidobiontic) and are frequently found in mosses grown on rocks (van Dam et al., 1981; Anderson et al., 1986; Ito and Horiuchi, 1991). N. subtilissima, E. steineckei and E. lapponica are found in dry Sphagnum patches (Bertrand et al., 2004; Lange-Bertalot et al., 2011). Although some of the taxa including N. subtilissima, E. steineckei, and E. nymanniana are observed from mineral-rich, high pH waters (Watanabe, 2005; Myers-Smith et al., 2008; nkova , 2010), we infer that reduced water level and pH by drier Fra climate eliminated T. flocculosa and E. hexaglyphis and provided a vacant niche and favorable condition for various Eunotia taxa including one of the most drought tolerant, E. paludosa.

4.2.2. Core Kh-2 Core Kh-2 was divided into 5 diatom assemblage zones (Fig. 4). In Zone 2-1 (550e504 cm, ~10.0e9.1 cal ka BP), fluvial taxa such as E. minor, A. lanceolata, and M. circulare showed high percentages. In Zone 2-2 (504e446 cm, 9.1e8.0 cal ka BP), N. elginensis became abundant, reaching nearly 50%, accompanied by P. nodosa/subcapitata indicating the start of marsh formation. Dry tolerant diatoms of P. borealis and H. amphioxys increased temporally in the lower part at 504e488 cm (9.1e8.8 cal ka BP), but the mixture of N. elginensis implies that these diatoms were washed in from surrounding dry soils or mosses into marsh environments. In Zone 2-3 (446e367 cm, 8.0e6.7 cal ka BP), N. elginensis continued to exist abundantly, but Stauroneis spp. (mainly Stauroneis phoenicenteron) and P. brevicostata increased rapidly. Cymbella navicuriformis that prefers alkaline water (Stenina, 2008) appeared temporally at the beginning of this zone (446e428 cm, 8.0e7.7 cal ka BP), and diatom valve density in this interval was notably high indicating a higher moisture supply. The set of present fen peat indicator taxa such as E. incisa and T. flocculosa flourish from Zone 2-4 (367e268 cm, 6.7e4.7 cal ka BP). Cymbella tynnii, a possibly rich fen taxa (Beyens and Bock, 1989), increased at the end of this zone. Zone 2-5 (268e200 cm, 4.7e3.3 cal ka BP) was distinguished by the abrupt and continuous appearance of Eunotia monodon reaching 40%. E. monodon is observed in wetter and higher trophic water (Turkia et al., 1998; Sanchez et al., 2013) and associated with river influence (Razjigaeva et al., 2008). Pinnularia rupestris also increased in Zone 2-5, but E. incisa became negligible. In the upper part of this zone

Y. Fukumoto et al. / Quaternary International 348 (2014) 66e81 71

Fig. 4. Diagrams of diatom assemblages in core Kh-2 and core Kh-3. The grey hatched area denotes temporal diatom trend shifts mentioned in results (Section 4.2.) and the bracketed bold numbers indicate the major hydrological changes discussed in Section 5.2. Thick arrow indicates the onset of mid-Holocene dry climate. A detailed diagram of surface 1 m is presented in Fig. 5.

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Fig. 5. Diagrams of selected diatom taxa for surface 100 cm depth in three cores. Hatched area denotes temporal diatom trend shifts and their estimated centennial scale ages are shown. Note that a diagram of S. construens which made peaks at 75e63 cm (0.7e0.5 cal ka BP) in core Kh-3 is not shown in this graph due to limit of space.

(232e208 cm: 4.0e3.5 cal ka BP), Diploneis ovalis, Pinnularia divergens, Encyonema mesianum, and Tabellaria fenestrata appear at the same time. D. ovalis is associated with flooding events (Kapetanovi c and Hafner, 2007) and T. fenestrata is observed in circumneutral waters (Camburn and Charles, 2000), so there the site would be under a flowing course of water during Zone 2-5 (4.7e3.3 cal ka BP) with a possible peak at 4.0e3.5 cal ka BP. Diatom assemblages observed in Zone 2-4 are also recovered in Zone 2-6 (200e0 cm, 3.3e0 cal ka BP), and P. brevicostata showed a declining trend as observed in core Kh-1, probably reflecting increasing acidification as it is often associated with the inflow of detritus materials and neutral pH water (Krammer, 1992; Rühland et al., 2000). There were slight but synchronous increases of F. rhomboides, B. brebissonii and Nitzschia perminuta at the depths of 200e180 cm (3.3e3.0 cal ka BP), 143e125 cm (2.4e2.1 cal ka BP), 47e33 cm (0.8e0.6 cal ka BP) and at near the surface. F. rhomboides and B. brebissonii are sometimes associated with humic waters and are found in mosses growing on rocks (Anderson et al., 1986; Ito and Horiuchi, 1991). However, B. brebissonii are often observed in deep water with pH higher than 6 (Battarbee et al., 2010; Cantonati et al., 2011) and N. perminuta is regarded more often as a neutralalkaliphilous taxon (van Dam et al., 1994; Watanabe, 2005; Klemen
ci
c et al., 2010). Therefore, the increase of these taxa would be signs of a wetter peat surface. This is supported by the accompanying possible rheophilic taxa, D. ovalis, E. monodon and E. mesianum at 143e125 cm, and A. alpigena at 47e33 cm and at the surface (Fig. 5). Several taxa, including E. paludosa and Eunotia fallax, increased at 20e5 cm (0.3e0.1 cal ka BP) (Fig. 5), implying drier climate. 4.2.3. Core Kh-3 Core Kh-3 was divided into five diatom assemblage zones (Fig. 4), and is distinct from other two cores that S. construens, a tychoplanktonic and alkaliphilousecircumneutral taxa, and A. alpigena appeared as main components throughout the core sequence reflecting continuous presence of deep water pools. In Zone 3-1 (600e579 cm: ~9.9e9.4 cal ka BP), deposition under a fluvial environment is shown by the appearance of A. lanceolata, M. circulare and E. minor. In Zone 3-2 (579e451 cm: 9.4e6.8 cal ka BP), P. nodosa/subcapitata, N. elginensis and P. borealis appeared continuously with moderate fluctuations. There were

distinct peaks of S. construens and A. alpigena at 543e531 cm (8.7e8.4 cal ka BP) and T. flocculosa at 507e491 cm (7.9e7.6 cal ka BP) where the share of dry indicator diatoms were high in other periods. These alternations of planktonic and benthic taxa in this zone may reflect unstable depositional environment and mixture of terrestrial sediments in a water pool. Marsh indicator diatoms such as P. brevicostata and Stauroneis spp. (mainly S. phoenicenteron) dominated in Zone 3-3 (451e413 cm: 6.8e6.3 cal ka BP), and A. alpigena and E. incisa also began to appear. Zone 3-4 (413e349 cm: 6.3e5.5 cal ka BP) was characterized by conspicuous peaks of planktonic A. alpigena with poor preservation of diatom fossils especially at 399e371 cm (6.1e5.8 cal ka BP). E. incisa and T. flocculosa increased sporadically when the share of A. alpigena was low. The diatom assemblages from Zone 3-5 (349e0 cm, 5.5e0 cal ka BP) were dominated by combinations of fen peat taxa observed at present such as E. incisa, S. construens and T. flocculosa. Increases of C. tynnii, Stauroneis spp. and P. brevicostata at 307e263 cm (4.8e4.1 cal ka BP) possibly indicate higher pH and wetter conditions. Peat acidification would have gradually decreased the share of P. brevicostata as observed in the other two cores. Short but remarkable increases of A. alpigena and S. construens occurred at 215e199 cm (3.2e2.9 cal ka BP). The peak of S. construens at 75e63 cm (0.7e0.5 cal ka BP) was accompanied by A. lanceolata. These periods indicate an increase of water level, but the coexistence of rheophilic A. lanceolata at 75e63 cm implies the influence of water flow. At 47e27 cm depth (0.4e0.2 cal ka BP), there were synchronous increases of E. nymanniana, E. fallax and E. paludosa (Fig. 5) indicating a drier environment. In the upper 10 cm, a possible wetter environment was observed by the synchronous increases of N. perminuta, E. steineckei, Navicula acidobionta and Navicula soehrensis. 4.2.4. Common trend of diatom succession on three cores The common trend of diatom succession for the three cores was clarified in a species plot profile of principal component analysis (PCA) (Fig. 6A). The first (PC1) and second (PC2) axis of principal component explained 32.9% and 13.3% of total variation, respectively. Diatom taxa on the profile were categorized into 5 groups. Taxa characteristic of a fluvial environment (Group 1) and dry soils and moss (Group 2) that appeared in the early Holocene were positioned on the right side of the PC1 axis and taxa observed in

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Late Holocene Fig. 6. (A) Plots of species scores by principal component analysis (PCA) carried out on all depth-diatom composition data of three cores. The visually categorized 5 groups showed a general transition in response to peatland hydroseral succession along a curbed arrow from right to left. (B) Taxa composition of each group and representative taxa are underlined. The transition of these groups from the early Holocene is indicated by an arrow. Zones dominated by each taxa group and brief descriptions of their water environment were noted.

present fen peatland (Group 4) and at temporal periods (Group 5) were positioned on the left side. The axis of PC1 represents the chronological taxa transition from positive to negative side of PC1 axis (Fig. 6A) and this taxa ordering would generally be the result of autogenic hydrosere succession. Group 1 consists of fluvial environment indicators observed at near bottom peaty clay layers lasted until 8.4, 9.1 and 9.4 cal ka BP in core Kh-1, Kh-2 and Kh-3, respectively (Fig. 6B). Diatoms of the lowermost inorganic silt layer in core Kh-1 (~9.6e9.2 cal ka BP) showed static water pool environments as a precursor for these fluvial environments. Dry indicator diatoms of Group 2 appeared probably after damming and cessation of water currents. Marsh indicators of Group 3 appeared by the swamping of depositional basins, but they often coexisted with the earlier Group 2, so frequent inflows of allochthonous terrestrial sediments or desiccation of sedimentary surface might have occurred. Within the taxa of Group 3, N. elginensis appeared earlier than Stauroneis spp. in core Kh-2 and Kh-3, probably reflecting a distinct pattern of hydrosere processes. Group 4 represented by E. incisa, T. flocculosa and E. hexaglyphis appeared approximately at 6.8e6.4 cal ka BP in all cores and continued to the present. Taxa of Group 5 such as N. perminuta, B. brebissonii, and E. paludosa showed episodic appearances from surface to one meter depth of each core. Taxa distinction within Group 5 is not clear in species plots, but taxa including E. paludosa, E. fallax, E. nymanniana and E. bilunaris were inferred to indicate drier conditions, whereas taxa such as N. perminuta and B. brebissonii indicate wetter conditions in this fen peatland. The meaning of PC2 axis is not clear, but the taxa at both extremes of the axis are characterized by isolated and sudden appearances, suggesting their unique opportunistic

ecological character. Taxa at the positive phase of PC2 were planktonic A. alpigena and S. construens. A. alpigena appeared abruptly in core Kh-1 and Kh-3, and it dominated Zone 3-4 at the transitional phase from marsh to fen, with no similar events in other cores. It is possible that the dying of vascular plants and increased open water during the marsh to fen transition (Kokfelt et al., 2009) or episodic water flooding (Weilhoefer et al., 2008) would have favored the blooming of A. alpigena. Main taxa in the negative axis of PC2 were E. monodon and C. tynnii, and appearance of these taxa was almost limited to core Kh-2 (Zone 2-5). P. brevicostata accompany these two taxa, and it is characteristically observed at early periods of fen peatland development. 4.3. Sphagnum leaves Fossils of Sphagnum leaves were generally composed of Sphagnum sect. Squarrosa (mainly Sphagnum teres), Sphagnum sect. Subsecunda (mainly Sphagnum subsecundum) and Sphagnum sect. Cuspidata (Fig. 7). Leaves of brown mosses were often found represented by Drepanocladus aduncus, Meesia triquetra and Warnstorfia exannulata. These taxa indicate a generally minerotrophic fen environment, and few ombrotrophic peat indicators were observed. 4.3.1. Kh-1 Core Kh-1 was characterized by abundance of S. sect. Subsecunda (Fig. 7) compared to other cores. S. sect. Subsecunda, especially for S. subsecundum, has tendency to appear in mineral rich and more aquatic environment compared to other Sphagnum taxa (Ellis and

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Fig. 7. Diagram of estimated volume of Sphagnum leaves (0e4) and counts of each Sphagnum taxa. Arrow mark denotes the common trend shift at 6.7e6.4 cal ka BP. Bracketed numbers denote temporal hydrological changes discussed in Section 5.3. Main taxa of observed brown moss are noted on the lithology column. Wa exa: Warnstorfia exannulata, Dr ad: Drepanocladus aduncus, Me tri: Meesia triquetra.

Rochefort, 2004; Atherton et al., 2010), and its frequent cooccurrence with M. triquetra suggests a generally wet rich fen environment with high pH (Joan and Montagnes, 1990). Sphagnum leaves were scarce from Zone 1-1 to 1-3, but S. sect. Squarrosa appeared temporally at 319e315 cm (8.5 cal ka BP) in Zone 1-2 implying mild summer rainfall and stable water levels. S. sect. Squarrosa was dominant in Zone 1-4 and replaced by S. sect. Subsecunda in Zone 1-5, but there is a temporal increase of S. sect. Cuspidata at the boundary of these two zones (235e230 cm: 6.4e6.3 cal ka BP). This rapid transition is shown by a arrow mark in Fig. 7. Sphagnum leaves became scarce in the middle of Zone 1-5 (215e135 cm) probably due to stronger flooding or seasonal water level changes. S. sect. Subsecunda appeared again from a depth of 140 cm continuing to 47 cm in Zone 1-6. Sphagnum palustre appeared at 47 cm reaching 13% of total leaf counts. This taxon has widespread ecological tolerance, but some environment change promoted its growth. S. sect. Cuspidata was observed at the core surface, probably due to mild climate. 4.3.2. Kh-2 Core Kh-2 was characterized by the frequent appearances of S. sect. Squarrosa. Sphagnum leaves were scarce in Zone 2-1 and 2-2, but S. sect. Squarrosa became successively dominant in Zone 2-3. S. teres (S. sect. Squarrosa) is often observed as the first colonizer of a
 et al., 2008), and the co-appearance peatland succession (Stechov a of W. exannulata in Zone 2-3 suggests that the core site lies in the vicinity of river flow with neutraleslightly acidic waters (Atherton et al., 2010). S. sect. Squarrosa was replaced by S. sect. Subsecunda in Zone 2-4, but S. sect. Cuspidata increases temporally at the boundary of these two zones (375e355 cm; 6.9e6.5 cal ka BP) as

similarly observed in core Kh-1. S. sect. Subsecunda was replaced by S. sect. Squarrosa at the end of Zone 2-4, probably due to lesser water inundation (Andrus, 1986; Nungesser, 2003) and continued to be prominent until 155 cm in Zone 2-6. There was a period with few Sphagnum leaves at 225e215 cm (3.9e3.7 cal ka BP) in Zone 25. At 155e135 cm depths in Zone 2-6, there were large alternations of several Sphagnum taxa and there were also abundance of twigs and leaves of O. palustris, a highly acidic water indicator, and leaves of D. aduncus and D. revolvens at this depth. The mixture of these various ecological indicators implies a flooding event. Sphagnum leaves were scarce in the near-surface 1 m layers. 4.3.3. Kh-3 S. sect. Cuspidata is generally abundant in core Kh-3 (Fig. 7). S. sect. Cuspidata is capable of adapting to a lower pH, trophic waters, and water tables than are S. sect. Subsecunda and S. teres (S. sect. jek et al., Squarrosa) (Nungesser, 2003; Ellis and Rochefort, 2004; Ha 2006) which were more abundant in core Kh-1 and Kh-2. Sphagnum leaves were scarce in Zone 3-1 and Zone 3-2, but there was an increase of S. sect. Squarrosa at a single depth of 538 cm (8.6 cal ka BP). S. sect. Cuspidata increased at the beginning of Zone 3-3 (448 cm: 6.7 cal ka BP) and continued to be dominant also in Zone 3-4 probably indicating drier and stable water conditions. The brown mosses favored by slightly acidic and mineral-rich fen (W. exannulata, Helodium blandowii) also appeared in Zone 3-3. S. sect. Cuspidata was replaced by S. sect. Squarrosa at the beginning of Zone 3-5, but Sphagnum poor layers continued from 350 to 190 cm. D. aduncus appeared sporadically (310e250 cm) in these Sphagnum poor layers. D. aduncus is indicative of meso-eutrophic water pools with neutral pH (Li and Vitt, 1994; Camill, 1998) so water level

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would be high and stable at this period. S. sect. Cuspidata recovered thereafter, but S. sect Subsecunda became dominant at 178e158 cm (2.6e2.2 cal ka BP). This interval corresponds to the appearance of M. triquetra and may indicate an unstable water environment as M. triquetra is possibly more tolerant of flooding conditions than are other Drepanocladus taxa (Zibulski et al., 2013). Sphagnum leaves were poor at the surface 80 cm of the core, probably due to an unstable water level, but there was a single peak of S. palustre at 58 cm (0.5 cal ka BP), and the uppermost surface sample was dominated by S. sect. Cuspidata. 5. Discussion 5.1. Core site-specific hydrology and local environmental changes The three cores were obtained from apparently same peat depositional basins, but the permanent hydrologic character peculiar to each core site was inferred from differences in the main diatom and Sphagnum taxa. All cores contained acidophilic E. incisa and T. flocculosa as main components, but core Kh-3 was distinct from the other cores by abundant alkaliphilous and tychoplanktonic taxa such as S. construens and A. alpigena. This indicates that the core Kh-3 site, although its highest elevation of the three sites, is highly water saturated and could be a water lag basin during summer rainy days due to the topography (Blaauw and Mauquoy, 2012) or the existence of thick humic peat layers with low hydraulic conductivities (Baird et al., 2008). The most dominant taxa of core Kh-1 and Kh-2 were E. hexaglyphis and E. incisa, respectively. It is difficult to infer hydrological character of the two core sites as these two taxa seems to share close habitat preferences, but the negligibly smaller amount of E. hexaglyphis in core Kh-3 and its disappearance at flooding periods in cores Kh-1 and Kh-2 imply its preference for acidic and drier habitat compared to E. incisa. Therefore, the core Kh-1 site would be generally more arid and acidic than the other core sites, as indicated by the shorter core length of core Kh-1 (3.7 m) than core Kh-2 (5.5 m) and core Kh-3 (6.0 m). It is possible that core Kh-1 site lies on sloping topography, as it is located at the downstream location of the outflowing river compared to other sites, so the decomposition of plant materials would be promoted in core Kh-1 by stronger water level fluctuations (Freeman et al., 1997; Whittington and Price, 2006). The depositional environment of core Kh-2 site is likely stable, as the general diatom succession from Group 1 to 5 is clearly represented. The relatively wet core Kh-2 site might be positioned between the dry core Kh-1 site and wet core Kh-3. However, this site was more likely influenced by water currents, as implied by occurrences of large flooding events and the generally higher abundance of S. sect. Squarrosa. In core Kh-3, we cannot reconcile the generally larger numbers of S. sect. Cuspidata, a possible lower water level indicator, with the abundance of planktonic and alkaliphilous diatoms, but this might be related to oligotrophic waters in a static and deep water pool. The local hydrological character inferred above could be related to numerous core-specific trend shifts of diatom and Sphagnum taxa. The three cores share actually no diatom trend shifts that are synchronous in timing and manner except for the expansion of dry and acidic peat at 6.8–6.4 cal ka BP. The difficulty of finding synchronous hydrological changes in multiple peat cores is often the €, 2002; Bauer case even between cores of high proximity (Seppa et al., 2003), and an increasing number of studies show the delicate responses of diatom assemblages to local environments (Kingston, 1982; Cantonati et al., 2011). Thus, some of the corespecific responses could represent climate changes while others reflect autogenic peat growth processes or only accidental local environment changes. We identified an increase of dry indicator

75

diatoms in core Kh-1 at 7.7e7.2 cal ka BP (282e262 cm), but no corresponding trend shift was observed in other cores. This section matched with the rapid decreasing trend of mineral materials, so it might reflect autogenic sedimentation processes. In core Kh-3, there were abrupt peaks of planktonic A. alpigena at 3.2e2.9 cal ka BP (215e199 cm) and brown moss M. triquetra at 3.3e3.1 cal ka BP (218e208 cm). They could be correlated with appearance of wet indicators in core Kh-2 at 3.3e3.0 cal ka BP (200e180 cm), but we cannot discern this environmental change as an independent event from the preceding large flooding phase in Zone 2-5 (4.7e3.3 cal ka BP). Including this event, frequent abrupt increases of planktonic diatoms in core Kh-3 suggests that small hydrological changes could be pronounced (apparently in the diagram) by opportunistic blooming of planktonic diatoms in the water effluent site. Another shift that we suspect as not being climate-driven is the increase of dry indicator diatoms at 9.1e8.8 cal ka BP (504e488 cm) in core Kh-2 and at 9.2e9.0 cal ka BP (571e559 cm) in core Kh-3. At this time, a marsh indicator N. elginensis was observed, and these periods were preceded by fluvial environments with inorganic clay layers, so water stagnation after damming of streams, and sedimentation of surrounding terrestrial materials were likely occurring. Mixing of allochthonous diatoms is observed at the onset of peat formation (Florin, 1970), so it is difficult to immediately associate this event with temporal climate change. 5.2. Mid-Holocene dry period Pollen and geochemical records of core Kh-1 presented in Fukumoto et al. (2012) showed an extensive mid-Holocene dry period starting from ~6 cal ka BP probably driven by higher summer temperature. Core Kh-2 and core Kh-3 in this study reconfirmed this climate anomaly with higher age precision. In pollen records (Fig. 8), Pinus sylvestris-type began to increase from 6.4 cal ka BP (233 cm), reaching highest peak at 5.5 cal ka BP (249 cm) and remained high until 2.8 cal ka BP (106 cm) when it was replaced by Betula. A higher share of organic materials relative to mineral particles was observed during this period. Diatom records of three cores showed clear replacement from marsh environment indicators (Group 3) to present acidic fen peat indicators (Group 4) at the onset of Zone 1-5 (6.4 cal ka BP), Zone 2-4 (6.7 cal ka BP) and Zone 3-4 (6.8 cal ka BP) (Figs. 3 and 4). At exactly the same time, a similar transition of Sphagnum taxa was observed in all cores from S. sect. Squarrosa, S. sect. Cuspidata to S. sect. Subsecunda (Fig. 7), although S. sect. Subsecunda was not present in core Kh-3. Comparing of pollen records with our data suggest more gradual and slower response of the terrestrial forest community compared to aquatic micro organisms. The water table of the peatland should be low during the midHolocene dry period, but this seems not to be the case in this fen peatland. Poor preservation of diatom fossils was observed for all cores at approximately 6e3 cal ka BP, and this implies high water levels because the dissolution of diatom frustules can occur in anoxic, stagnant, and near neutral pH waters in peat deposits (Bennett et al., 1991; Vos and Wolf, 1994). In this period, Sphagnum fossil records showed the dominance of S. sect. Subsecunda, a wet indicator, in core Kh-1 and Kh-2, and Sphagnum fossil became scarce in core Kh-1 and Kh-3. These phenomena also imply excessive high water levels necessary for the growth of Sphagnum. The reason for the high water level during the mid-Holocene is difficult to explain, but it is conceivable that earlier deposition of highly decomposed and low hydraulic conductivity peat became the substrate for the retention of a water pool (Hughes et al., 2000). These layers might correspond to the S. sect. Cuspidata dominant layers at the beginning of the mid-Holocene dry period

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Fig. 8. Diagrams of dry weight percentages of chemical components and relative abundance of each pollen taxa in core Kh-1 (presented in Fukumoto et al. (2012)). The grey hatched area with bracketed numbers denotes temporal hydrological changes discussed in Section 5.3.

(6.4e6.3 cal ka BP (core Kh-1), 6.9e6.5 cal ka BP (core Kh-2), 6.8e5.5 cal ka BP (core Kh-3)). This is exemplified by the regeneration of sedge peat above S. sect. Cuspidata dominant layers (Charman, 2007). It is also likely that thicker accumulation of S. sect. Cuspidata layers in Zone 3-3 and Zone 3-4 produced large water basins at the Kh-3 site and promoted the growth of planktonic and alkaliphilous diatoms. The termination of the mid-Holocene dry period was marked by pollen and geochemical records in core Kh-1 at 2.8 cal ka BP (106 cm) and diatom record shows the increase of E. hexaglyphis at the expense of E. incisa. E. hexaglyphis was inferred as drier and more acidic indicator compared to E. incisa in previous section, so the climate might became cooler which induced slower peat decomposition and water acidification. The appearance of Sphagnum leaves from 2.8 cal ka BP in core Kh-1 and core Kh-3 supports this cooler inference. Core Kh-2 and Kh-3, on the other hand, did not show distinct changes at this period. These two core sites may not be sensitive enough to record temperature changes due to the permanent water saturation, as implied by the linear sedimentation rate. Brown mosses appear continuously at around 2.8 cal ka BP, but distribution is not generally coherent with diatom trends, so their response to hydrological changes could be muted (Gaiser and Rühland, 2010) or indicate unknown hydrological changes. 5.3. Temporal hydrology changes We found temporal proxy shifts that were common in all three cores at the following four periods: (1) wet environment at 8.7e8.4 and 8.0e7.6 cal ka BP; (2) increased water flooding and wet environment at 4.4e3.5 cal ka BP; (3) increased water flooding and wet environment at 2.4e2.1 cal ka BP; (4) wet environment at

1.2e0.5 cal ka BP and dry environment at 0.4e0.2 cal ka BP. There might be episodic basin wide hydrological changes as consequences of regional climate changes in these periods rather than the local hydrological changes. A diatom inferred hydrological change was often recorded by different diatom taxa in another core exemplifying core-specific hydrological response to climate change. These events are discussed below and marked by bracketed numbers in the diagrams (Figs. 3, 4, 7 and 8). 5.3.1. Wet environment at 8.7e8.4 and 8.0e7.6 cal ka BP Increases of planktonic and marsh diatoms were consistently observed at 8.7e8.5 cal ka BP (325e317 cm) in core Kh-1 and at 8.7e8.4 cal ka BP (543e531 cm) in core Kh-3. Sphagnum leaves (S. sect. Squarrosa) appeared in both cores and the chemical data showed rapid drops of mineral material (Fig. 8). These proxy changes could be interpreted as swamping processes or drier climate, but the isolated peak of planktonic diatoms in core Kh-3 and the slight increase of Betula pollens in core Kh-1 probably reflect a mild and wetter climate at around 8.7e8.4 cal ka BP. The diatom record in core Kh-2 showed a temporary wetter condition at 8.0e7.7 cal ka BP (446e428 cm) when Sphagnum leaves began to appear. This matched with peaks of tychoplanktonic T. flocculosa at 7.9e7.6 cal ka BP (507e491 cm) in core Kh-3. These similar increases of planktonic diatoms and Sphagnum leaves may indicate generally wetter climate and stable annual rainfall at 8.0e7.6 cal ka BP. 5.3.2. Increased water flooding and wet environment at 4.4e3.5 cal ka BP In core Kh-1, temporal recovery of wetness during the midHolocene dry interval at 4.4e3.9 cal ka BP (160e142 cm) was

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noted in Fukumoto et al. (2012) when there were slight decline of P. sylvestris-type pollen and increases of Betula and Cyperaceae pollen (Fig. 8). Diatom assemblages of core Kh-1 in this period were suffering severe dissolution, but the higher share of tychoplanktonic T. flocculosa suggests high water level. In core Kh-2, diatom and Sphagnum records showed long duration of frequent flooding at the entire period of Zone 2-5 (4.7e3.3 cal ka BP) with a peak strength at 4.0e3.5 cal ka BP (232e208 cm) when river indicator diatoms increased and Sphagnum leaves disappeared. This is reflected in core Kh-3 by the alternating appearance of C. tynnii, Stauroneis spp. and P. brevicostata at 4.8e4.0 cal ka BP (307e263 cm). 5.3.3. Increased water flooding and wet environment at 2.4e2.1 cal ka BP Diatom assemblages in core Kh-2 marked wetter conditions with possible water flooding at 2.4e2.1 cal ka BP (143e125 cm), and S. sect. Subsecunda appeared at 2.4 cal ka BP (145 cm). In core Kh-1, E. incisa increased and E. hexaglyphis decreased at 2.3e2.0 cal ka BP (89e81 cm) implying declined water acidity. This is in good agreement with the increase of wet indicator diatoms, S. sect. Subsecunda and M. triquetra in core Kh-3 at 2.6e2.2 cal ka BP (178e158 cm). 5.3.4. Wet environments at 1.2e0.5 cal ka BP and dry environment at 0.4e0.2 cal ka BP Although age determination of the last two millennia is uncertain due to the lack of radiocarbon dates and low peat compaction, clear temporal trend shifts were observed. Wet indicator diatoms increased at 1.2e1.0 cal ka BP in core Kh-1, 0.8e0.6 cal ka BP in core Kh-2 and 0.7e0.5 cal ka BP in core Kh-3 (Fig. 5). These periods at around 1.2e0.5 cal ka BP are associated with the Medieval Climate Anomaly (MCA) and match the increase of Cyperaceae pollen (Fig. 8) and negative d13C at 1.0 cal ka BP in core Kh-1 (Table 1). The increase of S. palustre was also observed in core Kh-1 and Kh-3 at this time. Dry indicator diatoms increased at 0.4e0.2 cal ka BP in core Kh-1, 0.3e0.1 cal ka BP in core Kh-2 and 0.4e0.2 cal ka BP in core Kh-3, respectively. These periods at around 0.4e0.2 cal ka BP may correspond to the Little Ice Age (LIA). Although obscure in core Kh-1, it is notable that a return to wetter conditions was observed at the uppermost surfaces. 5.4. Comparison with previous studies The periods of hydrological changes inferred in this study are correlative with other climatic records on the Mongolian plateau and East Siberia, implying the existence of common climate dynamics (Fig. 9) although chronological uncertainties remain for discussing centennial scale climate events because of the bulk sediments measured for radiocarbon dates. The mild and wetter climate at 8.7e8.4 and 8.0e7.6 cal ka BP are probably the part of wetter climate referred to as the Holocene optimum period in the early Holocene (Prokopenko et al., 2007; Watanabe et al., 2009; Bezrukova et al., 2011b) when zonal atmospheric circulation was strong, but average air temperature was still low due to extensive snow cover in Eurasian (Bush, 2005). These periods are most correlative with the appearance of a paleosol in the loess deposits in Shaamar, located 100 km northwest of Nur Sphagnum bog, at 8.7e7.0 14C ka BP (Feng et al., 2007) and the temporal humid condition in the Selenge River valley at 8.2e7.7 cal ka BP (White et al., 2012). Pollen record from Ugii Nuur, in central Mongolia, was out of phase with our results: dry climate at 8660e8350 and wet climate at 8350e8250 cal BP (Wang et al., 2009), probably reflecting regional climate behavior. The period between these two wet phases at 8.4e8.0 cal ka BP is reminiscent of one of the sudden

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cooling Bond events in the northern Atlantic at 8.2 cal ka BP (Bond et al., 1997). Drier climate at this period was found in peat deposits in Lake Baikal (Bezrukova et al., 2008) and in Gun Nuur (Wang et al., 2004; Zhang et al., 2012, Fig. 9), 100 km northwest from our site, but the evidence of dry climate was not clear in this study. The mid-Holocene dry climate is a widely observed climate signal on the Mongolian plateau and the East Siberian region, as reviewed in An et al. (2008) and Wang and Feng (2013). This is mainly caused by the strong evaporation due to the warm summer temperature and higher CO2 concentration (Tarasov et al., 1999; Bush, 2005) in conjunction with reduced boreal snow-cover compared to the early Holocene. The present study showed the onset of this period by synchronous changes of multi-proxies at 6.8e6.4 cal ka BP. This matched well with the pollen records of the Lake Baikal region represented by the increase of P. sylvestris type pollens and decrease of Betula pollens at around 7e6 cal ka BP (Takahara et al., 2000; Demske et al., 2005; Mackay, 2007; Shichi et al., 2009). Exact matching is seen with peat archives of the northern Baikal where pine forest expanded at 6800 cal BP and was replaced by Cyperaceae at 2.7 cal ka BP (Bezrukova et al., 2011a) (Fig. 9). In the proximity of our study site, Gun Nuur showed lower lake level from 7.0 cal ka BP (Zhang et al., 2012) (Fig. 9). The fluvial sediments of the Selenge River valley showed the disappearance of molluscan shells by 6.8 cal ka BP during increasing aridity from 7.7 cal ka BP (White et al., 2012), and drier vegetation was observed at 7e3 cal ka BP in Shaamar (Ma et al., 2013) and from 6860 cal a BP in Ugii Nuur (Wang et al., 2009). In Lake Hovsgol, rainfall decreased from 6.6 cal ka BP (Prokopenko et al., 2005) or 6 cal ka BP (Prokopenko et al., 2007) (Fig. 9). The rapid shifts of diatom and Sphagnum proxies in this study at 6.8e6.4 cal ka BP imply rapid climate aridification on this period. However, paleosols in eolian and fluvial deposits near the Orkhon River were dated at 7e6 cal ka BP (Lehmkuhl et al., 2011, 2012) in opposite to our results. The attenuation of the mid-Holocene dry climate centered at 4.4e3.5 cal ka BP corresponds to wetter and cooler conditions at 5.5e3.5 cal ka BP in Lake Kotokel near Lake Baikal (Fedotov et al., 2012) and a moisture increase at 4.8e4.5 cal ka BP in Lake Borsog near Lake Hovsgol (Orkhonselenge et al., 2012). Pollen records from Ugii Nuur showed slightly wetter conditions at 3910e3430 cal BP (Wang et al., 2009). In other records, this wetter period is sometimes represented as an end of mid-Holocene drought instead of a temporary event (e.g. Feng, 2001; Fowell et al., 2003) (Fig. 9), and Gun Nuur showed lower lake level at 4.1e3.6 cal ka BP (Zhang et al., 2012). This opposite trends with Gun Nuur in spite of the proximity of Nur Sphagnum bog might be attributed to the significant difference of altitude, as much as 1000 m, and the large influence of mountainous geography on regional moisture balance, as the annual amount of precipitation in the Khentii Mountains region varies significantly depending upon the altitude and orography (Broccoli and Manabe, 1992; Dulamsuren and Hauck, 2008). Mayewski et al. (2004) suggested stronger westerlies at this period from the compilation of global climate data, but this climate anomaly seems to be relatively minor and regionally varied in Mongolia. The end of the mid-Holocene drought at around 2.8 cal ka BP was well-observed in other records in Mongolia (Fig. 9) and reminiscent of the cooling and increased rainfall in northern Europe at SubborealeSubatlantic period boundary (Wanner et al., 2008) and the Bond event in the North Atlantic (Bond et al., 1997). In nearby Gun Nuur, lake level started to increase at 2.5 cal ka BP (Zhang et al., 2012) and taiga forest expanded in Shaamar from 3 cal ka BP (Ma et al., 2013). Loess and fluvial deposits in Darkhan (Lehmkuhl et al., 2012) and Khyaraany (Feng, 2001) also showed wet conditions at 3 cal ka BP. Correlative climate events were also observed in

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Fig. 9. A summary scheme of wet (W) and dry (D) conditions inferred in this study and previous studies around Mongolia (modified from Fukumoto et al. (2012)). Bracketed bold numbers denote the temporal climate changes discussed in Section 5.3. The 14C dates presented in Feng (2001) were calibrated into calendar years using CALIB 6.0.1 (Stuiver and Reimer, 1993). The period of the end of mid-Holocene dry period at ~2.8 cal ka BP is correlated with the results of previous studies by dotted lines.

Ugii Nuur (Schwanghart et al., 2009; Wang et al., 2009), Lake Telmen (Fowell et al., 2003) and even in Uvs Nuur of western Mongolia (Grunert et al., 2000). In line with our data, this wetter condition was promoted mainly by a suppressed evaporation rate due to cooler summer temperature (Meeker and Mayewski, 2002; Bezrukova et al., 2008). Increased flooding and wetter climate at 2.4e2.1 cal ka BP is also correlative with other data supporting the likelihood of this short term climate anomaly although this period is not obvious on a global scale (Mayewski et al., 2004; Wanner et al., 2008). Periods of higher wetness were observed in Lake Telmen at 2210e2070 cal a BP (Peck et al., 2002) and in Ugii Nuur at 2340e2010 14C BP (Wang et al., 2011). In fluvial sediments in the Khentii Mountains located 70 km southwest of our study site, the pollen record implied stronger river activities at 380e100 BC (2330e2050 cal BP) (Schlütz et al., 2008). Higher water level was observed at 2300e2000 cal BP in Lake Gun (Zhang et al., 2012) and at 2.5 cal ka BP in Lake Baikal (White and Bush, 2010). The wet and dry environment at 1.2e0.5 and 0.4e0.2 cal ka BP may represent the well known climatic periods of Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). Higher water level was inferred at 920e680 cal BP in Lake Telmen (Peck et al., 2002) and at 1390e1020 cal BP in Ugii Nuur (Wang et al., 2009). Higher sedimentation was observed in Lake Borsog at 1.0e0.9 cal ka BP (Orkhonselenge et al., 2012) and warmer and wetter conditions were inferred at AD 850e1200 in Lake Baikal (Mackay, 2007), so the wetter MCA would be widespread in the Mongolian plateau, although not always accompanied by higher air temperatures (Tian et al., 2013). The LIA climate associated with weaker insolation (Maunder minimum) was observed in Lake Baikal at AD 1775e1200 as an increased thickness of winter ice cover (Mackay, 2007), and the pollen record in the Khentii mountains showed dry climate lasting from 1000 AD to the present (Schlütz et al., 2008). However,

a wetter LIA seems to be the main trend in central-southern Mongolia and northern China (Tian et al., 2013) including Lake Telmen (Fowell et al., 2003), so the southward shift of moisturebearing westerlies could be one explanation for the drier LIA in Nur Sphagnum bog. As for the climate changes in the recent several hundred years, Leland et al. (2013) and Pederson et al. (2013) found different regional decadal-scale rainfall patterns from dendrological analyses within north central Mongolia, and between the Selenge and Yeroo river basins showing more stable trend in the latter, so the local climate would be regulated by mountain topography, wind directions, and distance from lakes. Possible wetter condition observed in the surface samples may reflect an unusually wet condition of the 20th century in north-central Mongolia (Pederson et al., 2013). The last century was also recorded as humid at Lake Telmen (Fowell et al., 2003) and Lake Baikal (Tarasov et al., 2007). Although the influence of recent human activities on diatom assemblages was inferred in surface sediments of western Mongolian lakes (Shinneman et al., 2010), their influence seems much smaller in the north Khentii mountain regions (Schlütz et al., 2008). 6. Conclusions This study analyzed three peat cores taken from ‘Nur Sphagnum bog’ in northern Mongolia and tried to find climate-induced basin wide hydrological changes through the analyses of diatom and Sphagnum fossils. Differences in main diatom taxa and existence of core specific trend shifts suggested a large influence of local hydrology from the driest site (core Kh-1) to the most water effluent site (core Kh-3), but comparisons of three set of core data showed synchronous hydrosere successions as clarified by species plots of PCA from fluvial, marsh, to fen peat. The appearance of acidic fen peat diatoms at 6.8e6.4 cal ka BP is one of these successions, but

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observation of synchronous changes in three cores indicate the start of drier climate. The mid-Holocene dry climate ended at 2.8 cal ka BP, probably due to declined summer temperature, which promoted peat acidification. The sediment age control is poor in this study due to five dating points and bulk dated samples, but we could make robust correlations of temporal proxy shifts between the three cores, and common hydrological changes were inferred at 8.7e8.4, 8.0e7.6, 4.4–3.5, 2.4–2.1, 1.2e0.5 cal ka BP, and in recent years. All these periods were inferred as wetter climate, and drier climate was recorded only for the period of 0.4e0.2 cal ka BP, suggesting that a water effluent fen peatland is less sensitive to dryness compared to flooding events. These climate changes are correlative with previous data on the Mongolian plateau and Siberian region, so it is probable that those hydrological changes are induced by climate changes, but we may need to consider local climate behaviors to reconcile disagreements between other data. It is difficult to determine which core was the most suitable sensor for peatland hydrology changes, but one finding was that possible opportunistic blooming of distinct diatom taxa (e.g. A. alpigena, E. monodon) in deep water basin site leads to large differences in the diagram. Sphagnum leaf fossils were useful as subsidiary data and estimating the precise timing of hydrological changes, but intercore differences were observed as in diatom data. We suggest, therefore, that detection of temporal climate changes from peat archives is possible only using a combination of cores across peat basins. Acknowledgements We are grateful to D. Dorjgotov and O. Batkhishig at the Institute of geography, Mongolian Academy of Sciences for their suggestion to start this study, and we also appreciate the members of the institute for their support on field survey. Publication of this manuscript could not be realized without comments and elaborate corrections made by two reviewers. This study was financially supported by Japan Society for the Promotion of Science KAKENHI Grant Number 24$3060. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quaint.2014.05.029. References Aloisie, P., Petra, H., Pavla, K., Michal, H., 2004. Distribution of diatoms and bryophytes on linear transects through spring fens. Nova Hedwigia 78 (3e4), 411e424. An, C.B., Chen, F.H., Barton, L., 2008. Holocene environmental changes in Mongolia: a review. Global and Planetary Change 63, 282e289. Anderson, D.S., Davis, R.B., Berge, D.F., 1986. Relationship between diatom assemblages in lake surface-sediments and limnological characteristics in southern Norway. In: Smol, J.P., Battarbee, R.W., Devis, R.B., Meril€ ainen, J. (Eds.). Diatoms and Lake Acidity, Dordrecht, pp. 97e113. Ando, K., 1990. Environmental indicators based on freshwater diatom assemblages and its application to reconstruction of paleo-environments. Annals of the Tohoku Geographical Association 42, 73e88 (in Japanese, with English abstract). Andrus, R.E., 1986. Some aspects of Sphagnum ecology. Canadian Journal of Botany 64 (2), 416e426. Atherton, I., Bosanquet, S., Lawley, M., 2010. Mosses of Liverworts of Britain and Ireland e a Field Guide. British Bryological Society. Baird, A.J., Eades, P.A., Surridge, B.W.J., 2008. The hydraulic structure of a raised bog and its implications for ecohydrological modeling of bog development. Ecohydrology 1, 289e298. Batima, P., Natsagdorj, L., Gombluudev, P., Erdenetsetseg, B., 2005. Observed Climate Change in Mongolia. Assessments and Adaptations to Climate Change (AIACC) Working Paper 12, pp. 1e26. Battarbee, R.W., Simpson, G.L., Bennion, H., Curtis, C., 2010. A reference typology of low alkalinity lakes in the UK based on pre-acidification diatom assemblages from lake sediment cores. Journal of Paleolimnology 45 (4), 489e505.

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