Radiometric dating re-evaluating the paleoenvironment and paleoclimate around the Plio–Pleistocene boundary in NE China (Changbai Mountains)

Review of Palaeobotany and Palynology 224 (2016) 134–145

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Research paper

Radiometric dating re-evaluating the paleoenvironment and paleoclimate around the Plio–Pleistocene boundary in NE China (Changbai Mountains) Andrea K. Kern a,⁎, Johanna Kovar-Eder b, Katarzyna Stachura-Suchoples c, Wei-Ming Wang d, Pujun Wang e a

University of São Paulo, Department of Sedimentary and Environmental Geology, Rua do Lago 562, São Paulo, 05508-080 SP, Brazil State Museum Natural History, Rosenstein 1, 70191 Stuttgart, Germany c Botanical Garden and Museum Berlin Dahlem, Königin Luise-Strasse 6–8, 14195 Berlin, Germany d Chinese Academy of Sciences, Nanjing Institute of Geology and Palaeontology, Key Laboratory of Economic Stratigraphy and Palaeogeography, 39 East Beijing Road, Nanjing 210008, PR China e Jilin University, College of Geoscience, Hongqi Street, Changchun 130061, PR China b

a r t i c l e

i n f o

Article history: Received 4 May 2015 Received in revised form 13 October 2015 Accepted 15 October 2015 Available online 28 October 2015 Keywords: Paleovegetation Diatom Paleoclimate Radiometric dating NE China Plio–Pleistocene transition

a b s t r a c t Today, NE China is highly affected by cold and dry winter monsoon winds, causing long and severe winters with monthly mean temperatures below −10 °C. Yet, the Neogene paleoclimatic history of this region is not well understood due to the lack of precisely dated paleontological localities. Herein, we present several Ar–Ar basalt ages round a plant-bearing diatomite sequence of Badaogou, Changbai Mountains (Jilin Province; NE China — border to N-Korea), which allow us to revise its age from Miocene to ~2.5 Ma. A paleoenvironmental reconstruction based on fossil leaves, pollen and diatoms suggests deciduous forests rich in Acer, Quercus, Tilia, Ulmus and Zelkova mixed with conifers such as Tsuga, Picea and Abies within an already established mountain region close by. These forests were growing around a nutrient rich freshwater system with a pH N 7 in a presumably deep basin indicated by the strong dominance of planktonic diatoms Stephanodiscus minutulus and Pliocaenicus changbaiensis. The presence of tree taxa known from southern China today, such as Sassafras, Nyssa, Liquidambar, Podocarpus and Cedrus, leads to significantly warmer temperature estimates by the Coexistence Approach (mean annual temperature 11.5–15.7 °C, coldest month mean temperature − 0.3–9.6 °C, warmest month mean temperature 23.0–27.8 °C). Rainfall values aren't as precise, but suggest a mean annual precipitation of 843–1577 mm with monthly extremes of 109–220 mm (wettest), 17–41 mm (driest) and 73–175 mm (warmest). Although records in Eastern and Northern Asia as well as Central China report a cooling and drying after the onset of the Northern Hemisphere glaciation, the paleovegetation in NE China was not yet or hardly affected by the intensification of the winter monsoon. This emphasizes the importance of accurately dated records to improve large-scale paleoclimatic reconstruction as well as our understanding of the complexity of climate change. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The Neogene history of China was studied since almost a century; nevertheless, paleogeographic and paleoclimatic reconstructions still remain patchy because of the large extent of the country, which covers many vegetation and climate zones. A main research concentrated on the understanding of the Asian monsoon systems and their alternation of wet and warm winds from the SE and cold and dry winds from the NW (Guo et al., 2002; Sun and Wang, 2005; Wang, 2006a,b) since they strongly characterize the country's environment today.

⁎ Corresponding author. E-mail addresses: [email protected] (A.K. Kern), [email protected] (J. Kovar-Eder), [email protected] (K. Stachura-Suchoples), [email protected] (W.-M. Wang), [email protected] (P. Wang).

http://dx.doi.org/10.1016/j.revpalbo.2015.10.002 0034-6667/© 2015 Elsevier B.V. All rights reserved.

The monsoon origin can clearly be traced back to 22 Ma (Guo et al., 2002; Sun and Wang, 2005), although some researchers set the onset of the East Asian Monsoon to even late mid Eocene (Quan et al., 2012). During the Pliocene and Pleistocene, an alternation of dominance between dry winter and wet summer monsoon as an expression of glacial interglacial cycles and likewise orbital forcing (Shackleton, 2000; Sun and Wang, 2005; Wang et al., 2005; Lisiecki and Raymo, 2007; Wang et al., 2008) triggered vegetation changes throughout China (Sun and Wang, 2005; Passey et al., 2009). However, one problem of resolving the paleoclimatic history on a smaller scale is linked to problems in dating of certain localities and even whole regions. Since the Neogene history of China is almost entirely of fluvial and lacustrine origin (Sun and Wang, 2005), long continuous sequences are missing. Thus, the majority of paleofloras (e.g. in Sun and Wang, 2005; Wang, 2006a,b; Jacques et al., 2013) are only correlated biostratigraphically (mammals or pollen zones). Although they

A.K. Kern et al. / Review of Palaeobotany and Palynology 224 (2016) 134–145

provide a rough but valid age estimate, these biozones lack precise ages and verification by alternative dating methodologies. In Northern and North-east China in particular, absence of localities with well-preserved fossils as well as the complications to date these, made the understanding of the Neogene tricky. A potential area for investigation is the Changbai Mountains (Changbaishan, Jilin Province, NE China; Fig. 1) along the border to North Korea. A complicated Neogene volcanism and equally complex basalt flows left many questions about their origin open for a long time (Fan et al., 1999; Wei et al., 2007; Tang et al., 2014). Fossiliferous localities are known in these basalt complexes and one of them is the plant-bearing diatomite sequence of Badaogou (Fig. 1B). Some publications exist already about this locality (Li 1969; Palaeontological Atlas of Northeast China, 1980; Kovar-Eder et al., 2006, 2007; Stachura-Suchoples and Jahn, 2009; Kovar-Eder and Sun, 2009; Kovar-Eder et al., in press), in which the fossil-bearing diatomits are assigned to the Ma'anshancun Formation. So far, only a single radiometric dating existed for the lower part of this formation, placing the fossiliferous layers there into or around the Middle Miocene at ~ 13.4 Ma (Geological Bureau of Jilin, 1988). Nevertheless, the exact position of

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the dated basalt within the confusing pattern of magma flows remains unclear. Therefore, we attempted to get a better age control for the Neogene fossil-bearing diatomite sequence of Badaogou by Argon–Argon dating of the basalt flows above and beneath this sequence. Furthermore, we focus on a reconstruction of the paleoenvironment by using a combination of fossil leaves, pollen and diatoms. Additionally, we provide information on the paleotopography in the Changbai Mountains and paleoclimate estimates (Coexistence Approach), which we discuss in comparison with other records in North, North-east and East Asia. 2. Geology 2.1. Changbai Mountains' volcanism The Changbai Mountains, also known as Changbaishan, cover an area of nearly 4000 km2 of volcanic rocks in NE China (Fig. 1B) and span across the border to North Korea (Fan et al., 1999). The best known and most studied part is the Changbai Mountain (Paektu/Baitou Mountain/Shan or Baekdu san), one of the three still active volcanos in

Fig. 1. A: Maps showing the position of the Jilin Province in China; B: the position of the investigated sequence of Badaogou; C: an image giving all GPS points of the basalt samples (blue) and paleontological samples (green) (Maps from Google Earth, 2014; Table 1).

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China and the highest peak in these mountains with a height of 2749 m (Tian et al., 2009). Its latest eruption happened in the year 1702, but its activity is well recorded during the last millennia (Simkin and Siebert, 1994; Chu et al., 2011). One is known as the so-called Millennium eruption (approx. around 1000 BP), which counts as one of the most violent eruptions during the last 2000 years and shows the power and threat of this volcano (e.g. Sun et al., 2014). Additionally, the crater is nowadays filled by a lake (Tianchi Lake or Cheonji Caldera Lake), which floods pose another threat in a case of a new volcanic event (Lee et al., 2013). The Changbai Volcano basalt rocks directly overlay the Archean and Mesozoic granite basement rocks (Wei et al., 2007 and therein). For a long time, it was unclear, what causes the ongoing volcanic activity as this zone is 1300 km away from the Japan Trench subduction. Current theories are based on a subducted Pacific slab, that stretches below the Changbai Mountains at a depth of 660 km, where it is stagnant in the Mantle transition zone (Li and Yuan, 2003; Ai et al., 2003; Tian et al., 2009). A new model based on wide-spread seismic data by Tang et al. (2014) suggests Changbai volcanism to be caused by mantle upwelling. According to Tang et al. (2014), there exists a gap in the transition zone above the deep stagnant slab and the Changbai region, which allows melted magma to reach up to the surface. The presence of the gap, however, cannot be proven yet (Tang et al., 2014). Furthermore, details about the underlying magma chambers are still unclear and require further studies (Tian et al., 2009). Despite the amount of studies on origin and risks of the Changbai Volcano, the history of the Changbai Mountains is less well documented. According to the Geological Bureau of Jilin (1988), the volcanism started in the Oligocene (28.4 Ma), which is documented by the oldest volcanic rocks around the Maanshan Volcano (NW of the Changbai Mountain) (Wei et al., 2007). The plateau-forming eruptions lasted until approx. 1.66 Ma (Wei et al., 2007), overlapping with other active volcanic phases. From 21.58 Ma until 10.39 Ma, most activity was recorded in the NE and NW Changbai Mountain, e.g., Zhengengshan, Naitoishan, Maanshan and Cuocaodingzi. Other plateau-forming phases are much younger, such as the cones of Cuocaodingzi, Touxi, Wangtian'e and Hongtoushan, dated to 5.56–2.47 Ma (Wei et al., 2007). The volcanic zone closest to our studied paleoflora of Badaogou is Wangtian'e (Fig. 1B). Its age is either set to ~5 Ma ago into the Pliocene (Fan et al., 1999; Fan, 2008; Tian et al., 2009) or further back in time to the Late Miocene (~7 Ma; Chen and Liu, 2012). Wangtian'e Mountain is also the southernmost and highest volcano in the Changbai mountain chain (2,051 m), a shield volcano, from which large lava flows originated (Fan et al., 1999; Chen and Liu, 2012). Its basalts can be found as far to the West as Linjiang and across the Yalu river (border to North Korea) (Fan et al., 1999). Drainage from these mountains later on formed valleys, which were numbered — Badaogou therefore represents the eighth valley and the surrounding basalts most likely derived from the Wangtian'e volcanic zone. Unfortunately, a clear chronology of the development of this volcano is still missing; nevertheless, its activity was still reported during the last millennium (Chu et al., 2011). 2.2. The Badaogou section The Badaogou valley (Fig.1B, Fig. 1C) is characterized by different massive basalt flows reaching a thickness of more than 200 m (Fig. 2A). The basalt sequences can be distinguished by their basalt type, which most likely represent different volcanic eruption phases. The fossil-bearing strata are an intercalated diatomite sequence of approximately 20 m thickness (Fig. 2A), which is mined throughout the valley (Kovar-Eder and Sun, 2009). Our field work was focused around the village of Xidapo (also translated as Xidape; Kovar-Eder and Sun, 2009; Kovar-Eder et al., in press). The outcrop situation is rather poor; main outcrops can be found along a small stream (Fig. 1C), where mainly less than two meters of sediment thickness can be studied at one point. To try to understand the whole section around Xidapo village based on our own observation and

communication with local mine workers, we can divide the diatomite into four different stages (Fig. 2B). Directly overlying the basalt, the sediment is a gray silty clay/clayey silt, which only partly shows fine laminations (Fig. 2C bottom). The transition to the other diatomite stages could not be directly observed; therefore we assume a slow increase of the amount of diatoms in the sediment composition. Except for the lowermost part, a fine lamination is present. Along the section, the sediment color changes from dark gray to almost alternations of white and whitish gray (Fig. 2C top). Occasionally, brown layers can be found. The uppermost part (stage 4) further includes much iron, which partly precipitates in single laminae, forms congregations or runs along small chasms (Fig. 2C top). Another method of observing the diatomite sequence is inside the mines (Fig. 1B; Table 1). However, the soft diatomite requires constant stabilization, which limits the accessibility of the sediments inside the hundred or more meter long tunnels. Additionally, most of these mines (all during the field work 2013) were closed for safety reasons and only big heaps of sediment remained. 3. Material and methods 3.1. Basalt samples The samples were collected along the stream in the Badaogou valley close to Xidapo village in 2012 (CB4 and CB6) and 2013 (BDG-3, BDG-4 and BDG-6) (Fig. 1C; Table 1). The distance between the lowest sample CB4 and the uppermost sample BDG-6 is more than 100 m of elevation and they originate from different basalt types and likewise lava flows (Fig.2A). The samples were analyzed in two turns at the Argon–Argon Research Laboratory at the Open University (Milton Keyense, UK). The samples were crushed, sieved and washed before the remaining clay minerals were removed. Whole rock basalt pieces were picked under a binocular and further prepared for irradiation which was performed at the McMaster Nuclear Reactor (McMaster University, Canada). Standards were packed for irradiation, either side of the unknown samples and analyzed using the single grain fusion method using a 1059 CSI fiber laser and a MAP215-50 mass spectrometer. The J Values for each sample were then calculated by linear extrapolation between the 2 measured J values. Results were furthermore corrected 37Ar decay and neutron-induced interference reactions. 3.2. Pollen samples The pollen samples were collected during two different field trips (2006 and 2013). Samples NEC-15-15 and NEC-15-16 were taken inside a mining tunnel by Dr. Angela Bruch during the earlier field trip close to the GPS point X2006 (Table 1; Fig. 1C). Inside the tunnel, the wall showed regular lamination of a whitish diatomite with marks of iron (from the diatomite stages 2–4; Fig. 2B). Both samples are each from approx. 5 cm thick part of section with a vertical distance of 30 cm (NEC-15-15 is the lower sample). All remaining samples were taken in 2013, along the river close to the town of Xidapo (Fig. 1C). Analyzed samples from the point X27-2 (Fig. 1C; Table 1) showed a low amount of pollen and were therefore not counted. The samples from X27-4 originated from a heap of a freshly closed mine (Fig. 1C). According to a local mine worker, the samples represent 3 different stages in the diatomite evolution (stage 1: X27-4/1 (non-laminated) and X27-4/2 (laminated); stage 2: X27-4/4; state 4: X27-4/6; Fig. 2B). All the samples were prepared with HCl and HF to remove carbonates and silicates (Wood et al., 1996; Green, 2001). Afterwards, the residue was stained with Safranin O and sieved with a 6 μm nylon sieve. From each sample more than 400 pollen grains were counted. The pollen diagrams were created using the software C2 (Juggins, 2007). The sample residues are kept in distilled water at the State Museum Natural History Stuttgart (P24301).

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Fig. 2. A: Stratigraphic section of Badaogou valley; lithological column gives information and thickness of each basalt layer and the position of the basalt samples; B schematic changes along the Badaogou diatomite section; C: comparing pictures about the diatomite lamination from the upper stage (top; white diatomite) and the lowermost stage (bottom gray diatomite).

Table 1 Overview of all samples points giving GPS coordinated, measured elevation, sample type and sample numbers as well as some remarks. Points match Fig. 1. A summary of cuticle samples is not included. GPS coordinates Point (map)

Sample numbers

Sample type

Coordinates

Elevation Remarks

X2006 X26-2 X27-2 X27-4 BDG6 BDG3 CB6 CB4

NEC15-15, NEC15-16 CB 2013/1, CB 2013/12 X27-2-1 to 2-14 X27-4-1 to 4-4 BDG-6 BDG-3, BDG-4 CB-6 CB-4

Cuticles, pollen cuticles Pollen, diatoms Diatoms, pollen, cuticles Basalt Basalt Basalt Basalt

N 41°32′54.06″ E 127°18′39.35″ N 41°32′53.10″ E 127°18′38.90″ N 41°33′10.40″ E 127°18′39.50″ N 41°33′06.20″ E 127°18′38.30″ N 41°33′13.62″ E 127°18′42.06″ N 41°33′04.01″ E 127°18′34.59″ N 41°32′57.65″ E 127°18′30.64″ N 41°32′48.79″ E 127°18′26.82″

765 m 764 m 790 m 763 m 784 m 748 m 723 m 666 m

Mining tunnel, additional samples from K Kovar-Eder and Sun (2009) Heap samples from the same mine as in the year 2006 Samples taken along the river Mining tunnel, samples originate approx. 180 m to the East Top basalt layer 8 in Fig. 2A Basalt layer 6 in Fig. 2A, samples originate from the same horizont Taken in the year 2012, represents basalt layer 4 in Fig. 2A Taken in the year 2012, represents basalt layer 3 in Fig. 2A

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3.3. Leaf material Fossil leaves were collected during several field campaigns. All material derives from heaps of mainly two mines (X2006 equals X26-2; X274; Fig. 1C; Table 1) because work in the mine was impossible. Both mines are the same as for the palynological samples. To the extent possible the plant fossils were collected from material immediately after it was brought out from the mine in Xidapo. In the fresh material most plant remains are preserved organically allowing cuticular studies. The cuticles are often very well preserved yielding even details of the indumentum. For details of preparation we refer to Kovar-Eder and Sun (2009) and Kovar-Eder et al. (in press). 3.4. Diatomite samples All samples for the diatom analyzes were collected along the small stream during the field work in 2013. The two localities (X27-2 and X27-4) are between BDG-3 and BDG-4 (Fig. 1C; Table 1), although the samples from X27-4 originate from inside the mine (identical samples with the palynological samples). X27-2 is the highest outcrop along the lithological column, which was sampled in higher resolution. All diatom samples were cleaned in 30% H202 solution and then washed several times with distilled water. The permanent diatom slides were mounted in Naphrax. Light microscope (LM) observations were made using a Zeiss Axioplan microscope. For scanning electron microscope (SEM) observations, specimens were mounted on aluminum stubs and sputter-coated with gold-palladium. The SEM observations were made with SEM Hitachi 810 at the BGBM, Berlin-Dahlem. In each sample, a minimum of 300 diatom valves were counted.

age slightly older than 2.5 Ma (especially BDG-4), but a more precise interpretation is not possible. As the dating strongly overlap, a rather fast deposition of the diatomite can be assumed (Sarah Sherlock, personal communication). 4.2. Pollen flora From all samples containing pollen, the preservation of the pollen grains and spores is very good. Pollen assemblages collected during different field trips do not differ. In two samples (NEC-15-16 and X27-4-4), the amount of gymnosperms is significantly higher, exceeding 65% (Fig. 3, Table 2). On the contrary, X27-4-1 and X27-4-2 are the richest in angiosperms with 63.72% and 64.38%, respectively. One of the major differences is caused by the fluctuating amount of Tsuga. In sample NEC-15-15 (5.99%), X27-4-1 (6.19%) and X27-4-2 (7.51%) it is less frequent, but exceeds more than 13% in the remaining samples (Fig. 3). Another dominant conifer varying in these samples is Picea, reaching up to 22.09% (NEC-15-16) and 23.7% (X27-4-4), while it appears only with less than 14% in the other samples. Abies and Cedrus are less common, but follow the same trend. The opposite is true for the Cupressaceae pollen grains (Fig. 3), which are most abundant in samples X27-4-1 (4.25%), X27-4-2 (2.79%) and X27-4-6 (3.4%). Among the angiosperms, Ulmus is the most frequent, but it shows a rather different distribution frequency, with its highest percentages in sample NEC-15-15 (19.59%) and its lowest abundance in sample X27-4-4 (8.48%). The other common angiosperms (Quercus, Fagus, Betula, Poaceae) show the same distribution pattern with highest abundance in samples X27-4-1 and X27-4-2 (Fig. 3; Table 2). The pollen diversity is highest in sample X27-4-1 with 37 different taxa, but rather similar to the other samples (31 to 33 taxa; Table 2).

3.5. Coexistence Approach 4.3. Leaf assemblage Climatic estimates in this paper are generated by using the Coexistence Approach (CA; Mosbrugger and Utescher, 1997; Utescher et al., 2014). This method is based on the link of every fossil plant to its nearest living relative (genus level in this paper). From this recent plant genus, information about the global distribution (e.g. Greenwood, 2005; Huang et al., 2008; Fang et al., 2011) and the climatic boundaries are collected from climatic stations and summarized in the Palaeoflora Database (Utescher and Mosbrugger, 1990–2014). Thus, from each plant in the paleoflora, its nearest living relative outlines the climatic interval, in which the plant can survive today and thus presumably in the past. The interval of each taxa commonly overly, resulting in one climatic range in which all (or almost) all plants can live. This is called Coexistence Intervals (CI) and represents the paleoclimatic estimate of the CA. 4. Results 4.1. Basalts Already Wei et al. (2007) showed the complexity of the basalt flows around the Changbai Mountains. Commonly, phases of lava flow overly each other and thus one valley can have a complicated mix of older and younger volcanic rocks. All our basalt samples gave a very good age indication and are concurrent with each other (Fig. 2A). The sample CB4 is the lowermost sample (Fig. 2A); its Ar–Ar dating also shows the oldest age recorded of 2.79 +/− 0.14 Ma. CB6 from more than 50 m above was dated to a distinct younger age of 2.47 +/− 0.16 Ma.BDG-3 and BDG-4 were taken within short vertical distance just below the plantbearing sediments. Their ages vary between 2.463+/− 0.075 Ma and 2.649 +/− 0.006 Ma, respectively. The only age overlying the plantbearing diatomite was from sample BDG-6 with 2.563+/−0.069 Ma. Including all the errors, the deposition of the Badaogou diatomite took place in between 2.388 and 2.655 Ma, spanning a time frame crossing the Plio–Pleistocene boundary. Most ages point towards a depositional

The leaf assemblage is dominated by woody deciduous angiosperm taxa while conifers are rare and their diversity is low. Evergreen woody angiosperms are scarce if present at all. Main components are maples (Acer rotundatum, Acer trifoliatum), oak (Quercus changbaiensis), linden (Tilia sp.) and Zelkova. Less common are three more oaks (Quercus maii, Quercus waltheri, Q. sp.), birch (Betula sp.), elm (Ulmus sp.), Juglandaceae, the laurel Sassafras paratsumu, pine (Pinus sp.) and Cupressaceae (Cupressoideae, 2 species) (Table 1) as well as yet unidentified taxa possibly representing Berberidaceae, Ericaceae and Vitaceae. Neither remains indicative of riparian forests, of reeds and sedges or aquatic plants have been detected in the macro record. Regarding most similar living relatives of the fossil taxa, the maples and Sassafras paratsumu show clear affinities to modern Asian species – Acer rotundatum to A. mono and A. cappadocicum, A. trifoliatum to A. triflorum, and Sassafras paratsumu to Sassafras tzumu – while the most similar living relatives of the fossil oaks from Badaogou remain highly ambiguous (Kovar-Eder and Sun, 2009; Kovar-Eder et al., in press) 4.4. Diatom flora The diatom flora is well preserved; planktonic diatoms account for 96–98%. They are dominated by well-preserved Stephanodiscus minutulus (80 to 96% of the whole assemblage; Table 3), followed by Pliocaenicus changbaiensis (2–18%), which is well preserved or fragmented. Other planktonic taxa belong to Aulacoseira spp., Cyclotella pseudostellingera and Stephanodiscus spp., present in low abundances (below 1%) and their valves are often fragmented. Benthic and periphytic diatoms only make up (2–4%), represented by valves and fragments of valves of Achnanthes spp., Cymbella spp., Eunotia, spp. Fragilaria sensu lato spp., Gomphonema spp., Nitzschia spp. From the outcrop X27-2, the samples X27-2-1, X27-2-3, X27-2-5, X27-2-7 and X27-2-9 are dominated by Stephanodiscus minutulus

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Fig. 3. Pollen diagrams created with the C2 software (Juggins, 2007) of the 28 most frequent taxa; gray background indicates the samples originate from inside the mining tunnel (2006).

(96%), the abundances of the remaining species were below 3%. The samples X27-2-11 and X27-2-13 were dominated by S. minutulus (94% and 80% respectively), in which the frequency of Pliocaenicus changbaiensis reached 4% and 18%, respectively (Table 3). From the outcrop X27-4, the sample X27-4-1 is dominated by Stephanodiscus minutulus (90%), the abundances of the remaining species are below 3%. The samples X27-4-5 and X27-4-6, S. minutulus (92% and 94%, respectively) and Pliocaenicus changbaiensis (6% and 4%, respectively) are the most frequent. Overall, the species diversity was low; it oscillates between 8 and 16 species.

4.5. Paleoclimate estimation The Coexistence Approach suggests a lower Coexistence Interval (CI) border for mean annual temperature due to the presence of Liquidambar and Cedrus to 11.5 °C opposed by the herb Patrinia with a global distribution below 15.7 °C (Fig. 4). Coldest month mean temperatures (CMT) did not drop much below zero (e.g. Cedrus −0.3 °C, Podocarpus − 0.6 °C, Liquidambar − 1.0 °C) while the mean temperature of the warmest month (WMT) ranged between 23.0 °C (Liquidambar and 21.6 °C for Sassafras) and 27.8 °C (Fig. 4).

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Table 2 Pollen abundances in percentages next to the number of identified taxa and pollen taxa diversity and a list of all determined macro-fossils. Badaogou samples

NEC_15-15

NEC_15-16

X27_4-1

X27_4-2

X27_4-4

X27_4-6

Abies Cedrus Cupressaceae Ephedra ?Ginkgo Larix Picea Pinus Podocarpus Tsuga Acer Alnus Artemisia Asteraceae Betula Carpinus Carya Castanea Celtis Chenopodiaceae Cyperaceae Corylus Ericaceae ?Euphorbiaceae Fagus Juglans Juglandaceae Liquidambar Nyssa ?Salix ?Sparganium Oleaceae Patrinia Poaceae Pterocarya Quercus Rutaceae Sassafras Tilia Ulmus Zelkova Gymnosperms Angiosperms Plant diversity Pollen identified

2.53 5.30 1.84 0.23 0.00 0.46 13.59 12.90 0.92 5.99 1.15 3.00 1.61 0.23 2.07 2.07 2.53 0.23 0.00 0.00 0.92 0.46 0.00 0.00 3.92 0.92 0.69 0.23 1.15 0.00 0.00 0.00 0.69 2.07 0.23 9.45 0.46 0.00 1.84 19.59 0.69 43.78 56.22 32 434

3.94 3.60 1.20 0.17 0.00 0.00 22.09 18.32 0.68 16.44 0.34 1.03 0.51 0.34 1.20 2.05 0.51 0.17 0.00 0.17 0.00 0.68 0.34 0.00 2.23 1.20 0.00 0.51 0.86 0.00 0.00 0.17 0.00 1.03 0.68 5.14 0.34 0.00 1.03 12.50 0.51 66.44 33.56 31 584

3.89 1.42 4.25 0.53 0.00 0.00 9.20 10.44 0.35 6.19 1.24 0.71 4.42 0.35 3.89 3.72 0.18 1.42 0.53 1.59 0.35 1.77 0.18 0.35 3.54 2.30 0.00 0.53 1.59 0.35 0.18 0.71 0.18 2.12 0.53 14.34 0.53 0.00 1.06 10.09 4.96 36.28 63.72 37 565

1.72 3.65 2.79 0.21 0.00 0.21 10.30 8.37 0.86 7.51 0.64 1.72 2.36 0.43 4.08 5.15 0.86 0.86 0.00 0.21 1.72 1.93 0.00 1.72 6.44 1.93 0.00 0.00 1.72 0.64 0.00 0.86 0.00 3.43 0.21 11.80 1.07 0.00 1.07 11.37 2.15 35.62 64.38 33 466

3.30 2.20 1.57 0.31 0.16 0.16 23.70 19.15 0.47 17.43 0.47 0.00 0.78 0.31 0.78 0.78 1.41 0.31 0.31 1.10 0.31 0.47 0.16 0.00 4.24 0.47 0.00 0.31 0.47 0.00 0.00 0.00 0.00 2.35 0.47 4.55 0.47 0.00 1.41 8.48 1.10 68.45 31.55 33 637

3.63 2.49 3.40 0.23 0.00 0.00 13.83 10.20 0.23 13.83 1.13 1.36 1.81 0.23 1.81 2.27 0.91 0.45 0.68 1.13 0.23 1.36 0.00 0.00 3.40 2.27 0.00 0.23 0.91 0.00 0.00 0.45 0.23 3.40 0.23 8.62 0.91 0.00 2.27 14.97 0.91 47.85 52.15 33 441

CI for mean annual precipitation (MAP) and seasonal values (mean precipitation of the wettest (MPwet), driest (MPdry) and warmest (MPwarm)) are supported less clearly by multiple taxa (Fig. 4). MAP higher than 843 mm is indicated by the presence of Sassafras while Cedrus limits the upper border (1577 mm) although an annual rainfall of less than 1800 mm is suggested by many plants (Fig. 4). A wet month is indicated with rainfall values of 109–220 mm (Liquidambar– Patrinia) as well as a dry month with rainfall amount of less than 41 mm due to the presence of Cedrus and Zelkova (less than 67 mm). Although the MPwarm shows a quite large CI of 73–175 mm, its stable intervals (Sassafras/Patrinia–Cedrus/Juglans) point towards higher precipitation during the warmer season.

Macro-fossils

2 species

x

A. trifoliatum, A. rotundatum

x ? x

x

4 species S. paratsumu x x x

17 species

5. Discussion 5.1. Paleoenviromental reconstruction 5.1.1. Paleovegetation of Badaogou All studied plant fossils were collected along one small stream next to Xidapo village and mining tunnels close by (Fig. 1C; Table 1). Although more mining tunnels are/were operated throughout the Badaogou valley, we concentrated our study on this spot as it is known for its well-preserved macro paleoflora and the basalt layers were easily assessable for radiometric dating. The records from leaves and pollen complement each other. In the pollen record

Table 3 Diatom abundances in percentages; + represents the counts below 3%. Diatoms [%]

27_2-1

27_2-3

27_2-5

27_2-7

27_2-9

27_2-11

27_2-13

27_4-1

27_4-5

27_4-6

Planktonic Stephanodiscus minutulus Pliocaenicus changbaiense Non-planktonic

98 96 + +

98 96 + +

98 96 + +

98 96 + +

98 96 + +

98 94 4 +

98 80 18 +

96 90 + 4

98 92 6 +

98 94 4 +

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Fig. 4. Climatic ranges of significant plants are selected to represent the Coexistence Intervals (CI) for mean annual temperature, coldest month mean temperature, warmest month mean temperature, mean annual precipitation as well as the mean precipitation of the wettest, the driest and the warmest month. Red (gray) dashed line shows the CI limits, range are given in numbers at the bottom of each interval.

Chenopodiaceae, Cyperaceae and Poaceae, present in low percentages (less than 5.5%), are the only possible representatives of shoreline plants. Based on the rich leaf material Quercus changbaiensis, Acer rotundatum, Acer trifoliatum, Tilia and Zelkova may have grown in closer surroundings to the depositional site. Further components in the lake surroundings probably were Ulmus, Betula and Alnus. The diversity of these deciduous forests is reflected further by Carya, Quercus maii, Quercus waltheri, Quercus sp., Sassafras paratsumu, as well as by ?Berberidaceae, ?Ericaceae and ?Vitaceae in the leaf record and additionally by Fagus, Castanea, Carpinus, Juglans, Nyssa and Corylus in the pollen record. Cupressaceae and Pinus present in the pollen record, but not abundant among the macro remains, may also have been components of these forests though farther transport for the robust vegetative plant organs cannot be excluded. Rutaceae, Ericaceae, Poaceae, Chenopodiaceae or Asteraceae may represent the shrub and herb layer inside the forest. One aspect of the pollen flora of Badaogou is the strong presence of typical elements of higher altitudes, e.g., Cedrus, Fagus, Picea, Tsuga and Abies (Brach and Song, 2006; Flora of China, 'eFloras, 2008). A higher relief in the surrounding of the Badaogou is supported by the fact that the Changbai volcanism started during the Oligocene (Geological Bureau of Jilin, 1988; Wei et al., 2007) and the origin of Wangtian'e volcano at ~ 5 Ma (Fan et al., 1999; Fan, 2008; Tian et al., 2009) also predates the deposition of our herein studied paleoflora of Badaogou. Based on our results from the pollen and leaf record, steppe-like vegetation adapted to dry conditions was not yet established in this region. Still, many taxa are similar to the modern vegetation in the Changbai Mountains. Today in this region, Pinus (mainly Pinus koreanensis) and

Picea (several species) mix with deciduous trees (Quercus, Ulmus, Tilia, Acer, Fraxinus) at elevations between 720 and 1100 m (Sun et al., 2003). Picea, Abies, Larix and Pinus occur further below 1700 m. The lowland forest vegetation today (below 720 m), although secondary forest, still shows several similarities with the Badaogou paleoflora. Characterized as a broad-leaved deciduous forest, Quercus mongolica and Corylus heterophylla are the most common elements, along with several species of Acer, Tilia amurensis, Juglans mandshurica, Ulmus propinqua and Populus ussuriensis (Sun et al., 2003). Other elements as Cedrus, Fagus, Liquidambar, Nyssa, Podocarpus, Sassafras and Tsuga do not occur in NE China anymore today, but are still present in other regions of China (Fang et al., 2011). Fagus, Cedrus and Tsuga mainly occur in the mountains of southern China (Brach and Song, 2006; Flora of China, 'eFloras, 2008). Modern representatives of Liquidambar, Nyssa and Podocarpus occur in southern China, where they are more frequent in higher elevations; similar patterns apply to Celtis and Nyssa, as well as for Zelkova and Carya, which are documented by leaves and pollen in the paleoflora of Badaogou. 5.1.2. Pliocene/Pleistocene flora and vegetation of East Asia The Pliocene is characterized by the expansion of arid vegetation, stretching from central China further towards East and South-east (Sun and Wang, 2005; Passey et al., 2009; Jacques et al., 2013; Wang and Shu, 2013). Forest vegetation is supposed to shift towards a herbdominated one (forest-steppe, steppe or steppe-desert) with Artemisia and Chenopodiaceae as the most frequent herbs while in some regions Ephedra and Nitraria play also an important role in the vegetation composition (Sun and Wang, 2005). There, these herb taxa dominate the pollen spectra with abundances of more than 80% (Sun and Wang,

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2005), while the percentages of these taxa make up only 10% at Badaogou. For paleofloristic comparison, the Plio–Pleistocene sedimentary sequence of the Yushe Basin (Shanxi Province, Eastern China), (Shi et al., 1993; Liu et al., 2002) shows a similar later switch from forest to open vegetation dominated by Artemisia and Chenopodiaceae at around 2.3 to 2.2 Ma (Shi et al., 1993). Slightly earlier, the paleofloras in North and Central Japan changed in between 3.0 and 2.5 Ma. Around Osaka, 17 plant taxa were extinct at the Gauss–Matuyama magnetic reversal (~ 2.588 Ma), including Liquidambar, Nyssa, Sequoia, and Tsuga (Momohara, 1994). Despite the common acceptance of the switch to a cold- and drought-adapted flora in China during the Pliocene, this appears not to apply for NE China as several well-dated paleofloras throughout Jilin and Heilongjiang provinces show a certain delay in the vegetation shift (Sun and Wang, 2005). Songliao (Xia and Wang, 1987; Zhao et al., 1994) and the paleomagnetically dated flora of Qianan (Jia et al., 1989) indicate the switch to a more arid climate after 2.4 Ma. Additionally, the paleomagnetic and thermoluminescence-dated Fulareji record a mixed temperate forest, e.g., Betula, Castanea, Ulmus, Alnus, Juglans and elements known from warmer climate (Carya, Liquidambar, Podocarpus), which did not evolve into a herb-dominated vegetation until approx. 2 Ma (Liu et al., 1990). These data support our vegetation reconstruction for Badaogou during the latest Pliocene–earliest Pleistocene, when this region of China was obviously at least partly forested.

5.1.3. Freshwater environment The diatom flora from Badaogou consists of both extant and extinct species. The main element in all samples is Stephanodiscus minutulus (in all samples abundance N 80%), which is a planktonic, freshwater, cosmopolitan, alkalobiotic (pH N7) and eutraphentic (N-autotrophic) recorded from the Pliocene to recent (Khursevich and Kociolek, 2012). The second most common taxon is Pliocaenicus changbaiensis (abundance up to 18%). This species was planktonic, extinct and so far endemic to the Changbai Mountains. The planktonic, freshwater genus Pliocaenicus is known from the Late Miocene until recent (Khursevich and Stachura-Suchoples, 2008); the age of P. changbaiensis (StachuraSuchoples and Jahn, 2009) is revised in this paper. The species diversity inside this genus was highest during the Pliocene, while it is a relict genus today, recorded only in the Asian arctic and alpine zone (two species) (Khursevich and Stachura-Suchoples, 2008). Both species prefer a neutral pH (6–7) environment. Important factors for the distribution of the modern Pliocaenicus costatus could be wind induced turbulence and the length of the open water period (b 3 months) as suggested for its growth during the last ~100 years (Flower et al., 1998). Furthermore, there is a record of two more species of Plioceanicus from China, both from the Pliocene (?) of Jilin Province (Wang, 1999). Other four species are published from Early Pliocene–Pliocene deposits of Japan (see review in Khursevich and Stachura-Suchoples, 2008; Tanaka and Saito-Kato, 2011). The best documented fossil record of Pliocaenicus is published from Lake El'gygytgyn (Chuckotka, Russia) (Snyder et al., 2013). In the sediment core spanning the last 1.2 Ma, Pliocaenicus seczkine occurred in some of the warm interglacial intervals (mean temperature warmest month N 10 °C). Additionally, the most dominant diatom species from our study, S. minutulus, was recorded in Lake El'gygytgyn too, where S. cf. minutulus co-occurred with P. seczkine in the warm MIS-31 (1057–1113 ka). In none of the cold intervals Pliocaenicus was observed in a significant number, while Stephanodiscus occurred in the cold interval between 22 and 30 ka (Snyder et al., 2013).The dominance of planktonic diatoms in all samples (96 to 98%) indicates most likely a deep-water basin. Non-planktonic diatoms are rare and commonly observed as broken valves, which suggests they were transported from shallow-water (photic zone) to deeper parts of the basin. But a conclusive estimate of the water depth is not possible, since the transition of the photic to aphotic zone in lakes is dependent

on many factors, such as water turbidity and color, shading effects, climate regime and lake size (Cohen, 2003). Thus, our preliminary results of the fossil diatom flora from Badaogou indicate a freshwater system with deep waters. The basin was relatively nutrient rich with a suggested pH of N7. 5.2. Paleoclimatic implications 5.2.1. Paleoclimate of Badaogou in comparison to recent The paleovegetation of Badaogou records a mainly deciduous forest, where tree taxa still distributed in this region are mixed with others known from more southern regions of China today. In particular, the presence of Cedrus, Liquidambar, Sassafras accompanied by Podocarpus, Tsuga, Nyssa and Zelkova limit the CI borders. The conifers Tsuga, Cedrus and Podocarpus are distributed mainly in higher elevations in southern China, e.g., Tibet/Xizang, Yunnan, Taiwan, Sichuan, etc. (Brach and Song, 2006; Flora of China, 'eFloras, 2008; Huang et al., 2008; Fang et al., 2011). Yet, Cedrus never grows in areas with coldest month mean temperatures below − 0.3 (Utescher and Mosbrugger, 1990– 2014; Fang et al., 2011). Similar climatic estimates derive from Sassafras, Liquidambar, Nyssa and Zelkova, which are distributed today in warmer and more humid climates of central and southern China (Brach and Song, 2006; Flora of China, 'eFloras, 2008; Fang et al., 2011), e.g., Liquidambar tolerates a coldest month mean of −1.0 °C and Sassafras of −3.3 °C (Fang et al., 2011). Thus, indicated by the recent global distribution of these plants, winter monthly temperatures did not significantly drop below 0 °C at the Plio–Pleistocene boundary in the region of Badaogou. Additionally, the presence of Sassafras points towards a mean annual temperature higher than 9.3 °C (Fang et al., 2011), which is supported by the recent distribution of Liquidambar in climates with a mean annual temperature higher than 11.5 °C and Cedrus N 11.6 °C. The cold season was opposed by a warm season of more than ~19 °C indicated by e.g., Nyssa, Cedrus, Carya and Sassafras (Utescher and Mosbrugger, 1990–2014; Huang et al., 2008; Fang et al., 2011). Precipitation reconstructions are mainly based on the presence of Patrinia accompanied by the also temperature significant Cedrus, Podacarpus, Sassafras and Liquidambar. Unfortunately, less exact conclusions can be drawn. Today, Sassafras requires at least a mean annual precipitation of 843 mm, Podocarpus of 652 mm or Liquidambar of 619 mm, although many other elements may suggest an even more humid climate above 1000 mm of rainfall per year (Utescher and Mosbrugger, 1990–2014; Greenwood, 2005; Fang et al., 2011). Today, Liquidambar is common in areas, where the wettest month of the year ranges between 109 and 340 mm precipitation while Sassafras can be found in areas with a highest monthly rainfall between 71 and 295 mm (Utescher and Mosbrugger, 1990–2014). A relatively drier month is suggested by e.g., Carya requiring a rainfall of at least 8 mm each month, Podocarpus more than 13 mm, Sassafras more than 17 mm (Greenwood, 2005; Fang et al., 2011). Therefore we may conclude for a seasonal rainfall, but there is no clear indication of a long drought season. All these overall climatic estimates (Fig. 4) differ significantly from the recent climate in NE China, temperature in particular. Along the Changbai Mountain, a mean annual temperature of only 2.8 °C is recorded at the foot (Changbai Mountain Weather Station, Chinese Academy of Sciences, 740 m) and about − 7.3 °C on the top (Tianchi Weather Station, 2,623.5 m; Wu et al., 2002). Mean annual temperatures in e.g. Shenyang 8.4 °C or Changchun 5.7 °C, Linjiang 5.3 °C (China Meteorological Administration, 2015.) are higher, but still several degrees lower than suggested for the paleoclimate of Badaogou. These recent values can be explained by the long and severe winters in NE China. Temperatures in December and January stay below a monthly mean of less than − 10 °C (China Meteorological Administration, 2015.), but also the previous and proceeding months (November to March) commonly have frosty and dry conditions due to the winter

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monsoon. Yet, these low temperatures are missing in the paleoclimate estimates and contradict the presence of more southern elements in our vegetation (MAT 11.5–15.7 °C; CMT −0.3–9.6 °C). Rain mainly falls during the summer months (June to August), when the East Asian monsoon brings up to 90 to 160 mm rainfall per month (China Meteorological Administration, 2015.). Along the slope of the Changbai Mountain, annual precipitation increases with altitude, with a range from 600 to 900 mm at the lowlands to up to 1340 mm on the top (Wu et al., 2002). These seasonality values are within the Coexistence Interval, although annual records are far below. However, the paleorelief is yet not well understood for the surroundings of Badaogou, but certain slopes and regions of higher altitude are indicated by the presence of taxa as Tsuga, Cedrus, Picea and Podocarpus (Brach and Song, 2006; Flora of China, 'eFloras, 2008). Therefore, the plants growing at higher and/or lower altitudes could be mixed into the assemblages biasing the paleoclimatic estimates slightly by representing different elevations. Changes along the sections are indicated by Pliocaenicus changbaiensis. Its increased abundance at the top of the lithological column (X27-2-11, X-2-13, X27-4-5 and X27-4-6) may indicate more favorable pH conditions and/or other nutrient conditions. Other controlling factors for its abundance could also be climatically-driven. 5.2.2. Plio–Pleistocene climate in Eastern Asia Globally, the Late Pliocene represents the beginning of the glaciation cycles (Zachos et al., 2001), when a gradual but paced onset of the permanent Northern Hemisphere glaciations from 3.6–2.4 Ma (Mudelsee and Raymo, 2005) is linked with a significant strong drop of temperature in between ~2.6 and 2.4 Ma (Raymo et al., 1989, Shackleton et al., 1995). Furthermore, this increased and permanent ice mass around the North Pole caused an intensification of the high pressure system over Siberia during the winter strengthening the dry and cold monsoon winds (Ding et al., 1995; Chan and Li, 2004). For China, this further means a drop in mean annual precipitation as clearly indicated by modeling results (Prell and Kutzbach, 1992; Liu and Dong, 2013 and references therein) increasing the cold and dry climate in NE China today. Such cold winters are clearly not (yet) indicated by the herein studied 2.5 Ma old paleoflora of Badaogou (Fig. 4). As the ongoing volcanism had already formed a mountain chain (Geological Bureau of Jilin, 1988; Wei et al., 2007), favorable microclimates inside valleys may have enabled the paleoflora to persist there for longer times than in other regions. Yet, water temperatures in the Sea of Japan were a few degrees lower than today (Amano, 1994; Cronin et al., 1994; Ogasawara, 1995) since the warm water Tsushima Current was poorly established before the opening of the Tsushima Strait (Kitamura et al., 2001). A severe cooling in the Japanese Sea took place later than ~ 2.7 Ma, characterized by high fluctuations in deep and surface water temperatures due to glaciation events (Cronin et al., 1994). Studies on the fossil macrofloras in Central Japan suggest an onset of a temperature drop already around 3 Ma, causing the extinction of many subtropical and warm temperate plants until ~ 2.588 Ma (Momohara, 1994). The first climate change in Northern Asia predates these records to 3.31–3.28 Ma, when drier conditions started to favor the expansion of Poaceae and Artemisia around the Siberian crater Lake El'gygytgyn (Andreev et al., 2013). Later, warm and wet phases alternated with dry and cold ones until 2.6 Ma (MIS stage 103), when temperature finally dropped and a very dry steppe vegetation was permanently established in this region in Siberia (Andreev et al., 2013; BrighamGrette et al., 2013). Nevertheless, the paleoclimate in Far East Russia (Primory'e Peninsula) showed significantly warmer mean annual temperatures and winter temperatures even during the Early Pleistocene (Utescher et al., 2015). In Central China, a continuous record for the past 3.5 Ma indicates the position of the margin of the Mu Us desert (Jingbian, Shaanxi Province; Ding et al., 2005), where a significant expansion occurred at

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~2.6 Ma. The authors link the spread of arid conditions directly to the retreat of the East Asian Monsoon to the SW and/or a general weakening of the system (Ding et al., 2005). Furthermore, a comparison of different records of the desert shows, that the sedimentary sequences of these eolian deposits show a transition already at ~ 2.8 Ma (Yang and Ding, 2010). The older weathered red clay records change to loess and paleosol deposits which are characterized by larger grain sizes and higher content of dust particles. Both features can be explained by strong seasonal variations of cold-dry and warm-humid fluctuations as well as an intensification of arid conditions in the source area of these wind-derived sediments (Yang and Ding, 2010). Farther in the East of China, a drop in temperature caused a decrease of warmth-requiring elements in the paleovegetation of Hebei Province (Nihewan Basin; Yuan et al., 1996) after 2.5 Ma. This significant and sudden cooling was also observed in the well-studied and well-dated Yushe Basin around the Gauss–Matuyama reversal (~2.588 Ma). Similar to the plant record in Hebei, all warm temperate broad-leaved deciduous elements (Carya, Juglans regia-type, Carpinus, Liquidambar, etc.) started to decrease (Shi et al., 1993; Liu et al., 2002). First, a turn-over in elements indicated colder temperature but a stable humidity (Liu et al., 2002) before a transition towards a predominantly steppe vegetation took place around 2.3 Ma ago. This represents either a possible delay in the vegetation response in comparison to the expansion indicated by the sedimentological records further to the West or a delay in the change towards dry and colder conditions. These paleobotanically based climatic estimates are supported by independently dated records from parts of Jilin and Heilongjiang provinces (Xia and Wang, 1987; Jia et al., 1989; Liu et al., 1990; Zhao et al., 1994). Besides the macro- and microbotanical record, Yao et al (2010) analyzed the carbon and oxygen isotope composition of soil carbonates in the North China Plain. C4 reached up to 40–60% in the regional ecosystems during Late Pliocene and Earliest Pleistocene, but declined to ~25% after ~2.2 Ma. It is commonly assumed, that the onset of the Northern Hemisphere glaciations strengthened the winter monsoon intensity (e.g. Wang, 2006a,b), consequently intensifying dry and cold conditions known in NE China today. Yet, the alternation of the dominance of winter monsoon winds and summer East Asian monsoon winds are controlled by Milankovitch forcing (Wang et al., 2005; Lisiecki and Raymo, 2007). Regular shifts in the paleovegetation were observed in the Yushe Basin, where the decline of thermophilic elements along the expansion of drought-adapted herbs occurred stepwise. Milankovitch cyclicities (40–60 ka alternation in Liu et al., 2002) are assumed, but unfortunately, the resolution did not allow proper statistical analysis. More detailed conclusions on astronomical forcing were drawn in the studies of Lake El'gygytgyn, where vegetation and climate reconstructions point towards phased shifts in accordance with orbital 41,000 ka forcing (Brigham-Grette et al., 2013). This co-occurs with the onset of the dominance of the obliquity cycle at ~ 2.5 Ma (Lisiecki and Raymo, 2007), which could further complicate the analysis of the different monsoon intensities. This astronomically controlled shift between the different monsoon systems could also have an effect on the moderately warmer climatic conditions in NE China around 2.5 Ma. As the thickness of the diatomite is only ~20 m, it could well be fully deposited within a warm obliquity interval as the flora only captures a short time window. Onwards, the flora could also represent a relatively warm and humidity-adapted forest prior to the winter monsoon affecting NE China, which remains a possibility due to the errors in the radiometric dating. In any case, the Changbai volcanic mountain chain should have offered protected habitats where microclimates favored the requirements of temperate to moderately thermophilic taxa thus delaying their decrease there. 6. Conclusion We present a paleoenvironmental and paleoclimatic study of the leaf, pollen and diatom flora of a single diatomite sequence of Badaogou

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(Changbai Mountains, Jilin Province, NE China) in a new light of radiometric ages. Five Ar–Ar samples below and above the fossil-bearing strata point towards a rapid deposition of the sequence between 2.6 and 2.5 Ma. The paleovegetation outlines vegetation of deciduous forest mixed with conifers; main components are woody angiosperms such as Acer, Ulmus, Quercus, Tilia and Zelkova along with less abundant Alnus, Juglans, Betula and the more warmth requiring Sassafras and Nyssa. High pollen abundances of Tsuga, Picea, Abies, Podocarpus, and Fagus indicate mountainous regions close to the depositional area of Badaogou. The total amount of herbs is below 10% in all palynological samples, which clearly points towards a mainly forested habitat. This contrasts the expansion of steppe-dominated vegetation in other regions of China. The diatom flora is strongly dominated by the planktonic taxa Stephanodiscus minutulus (in all samples abundance N 80%) and Pliocaenicus changbaiensis (abundance up to 18%) pointing towards a relatively nutrient rich freshwater lake system as depositional environment and a pH N7. The paleovegetation of Badaogou shows significant differences compared to the modern vegetation of the Changbai Mountains. Mean annual temperature was significantly higher than today (Coexistence Interval of 11.5–15.7 °C) due to far less severe winters (Coexistence Interval − 0.3–9.6 °C) while annual rainfall exceeded current values of 800 mm (Coexistence Interval 843–1577 mm). This contrasts the assumption of a winter monsoon induced temperature and humidity drop following the onset of the Northern Hemisphere glaciation in NE China and shows the complexity of large-scale paleoclimate reconstructions. Acknowledgments This study was funded by the Sino-German Center for Science Promotion (project GZ654) and supported by the FAPESP fellowship 2014/05582-0. The field work would not have been possible without the help of the students of Shenyang Paleontological College and Jilin University in Changchun as well as Zhang Yi (PMOL Shenyang) and Prof. Sun Ge (Shenyang Normal University). Additionally, we kindly thank Regine Jahn (BGBM, Berlin-Dahlem) and B.M. Suh for joining our field work team 2013 in the last minute. For field work assistance and laboratory work, we are grateful to Marit Kamenz (SMNS). Further, we thank Torsten Utescher (University of Bonn) for providing background information for the paleoclimatic reconstruction and Angela A. Bruch (Senckenberg Research Institute) for field work during 2006 and sharing these samples for the herein summarized study. Additionally, we give thanks to Sarah Sherlock (Open University) for discussion about the crucial basalt dating. This study is a contribution to the NECLIME network. References Ai, Y., Zheng, T., He, Y., Dong, D., 2003. A complex 660 km discontinuity beneath northeast China. Earth Planet. Sci. Lett. 212, 63–71. Amano, K., 1994. An attempt to estimate the surface temperature of the Japan Sea in the Early Pleistocene by using a molluscan assemblage. Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 369–378. Andreev, A.A., Tarasov, P.E., Wennrich, V., Raschke, E., Herzschuh, U., Nowaczyk, N.R., Brigham-Grette, J., Melles, M., 2013. Late Pliocene and Early Pleistocene vegetation history of northeastern Russian Arctic inferred from the Lake El'gygytgyn pollen record. Clim. Past 10, 1017–1039. Brach, A.R., Song, H., 2006. eFloras: new directions for online floras exemplified by the Flora of China Project. Taxon 55, 188–192. Brigham-Grette, J., Melles, M., Minyuk, P., Andreev, A., Tarasov, P., DeConto, R., Koenig, S., Nowaczyk, N., Wennrich, V., Rosén, P., Haltia-Hovi, E., Cook, T., Gebhardt, C., MeyerJacob, C., Snyder, J., Herzschuh, U., 2013. Pliocene warmth, extreme polar amplification, and stepped Pleistocene cooling recorded in NE Russia. Science 340, 1421–1427. Chan, J., Li, C.Y., 2004. The East Asian winter monsoon. In: Chang, C.P. (Ed.), East Asian monsoon. World Scientific, Singapore, pp. 54–106. Chen, X., Liu, L., 2012. Geochemistry study of Cenozoic Wangtian'e Volcano in Northeast China. Geophys. Res. Abstr. 14 (EGU2012-7917-1).

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