Pollen–spore distribution in the surface sediments of the western Bohai Sea, China

Pollen–spore distribution in the surface sediments of the western Bohai Sea, China

Quaternary International xxx (2015) 1e11 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locat...

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Quaternary International xxx (2015) 1e11

Contents lists available at ScienceDirect

Quaternary International journal homepage: www.elsevier.com/locate/quaint

Pollenespore distribution in the surface sediments of the western Bohai Sea, China Shixiong Yang a, b, c, Jie Li c, *, Kam-biu Liu d, Rihui Li c, Zhenhe Wen c, Siyuan Ye a, b, c, Sangheon Yi e, f, Xiaohui Chen c a

Key Laboratory of Coastal Wetland Biogeosciences, China Geologic Survey, Qingdao 266071, Shandong, China Function Laboratory for Marine Geology, National Oceanography Laboratory, Qingdao 266071, China Qingdao Institute of Marine Geology, The Ministry of Land and Resources, Qingdao 266071, Shandong, China d Department of Oceanography and Coastal Sciences, School of the Coast and Environment, Louisiana State University, Baton Rouge, LA 70803, USA e Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea f Korea University of Science and Technology (UST), Daejeon 305-350, South Korea b c

a r t i c l e i n f o

a b s t r a c t

Article history: Available online xxx

Eighty-six seafloor surface sediment samples from the western Bohai Sea were analyzed palynologically to understand the distribution pattern and transport paths of pollen and spores. The results reveal that the pollen assemblages are dominated by arboreal pollen, which accounts for an average of 52% and mainly includes Pinus, deciduous Quercus, Carpinus, Betula, Nitraria, Castanea, and Ulmus. The pollen percentages of herbaceous taxa reach an average of 25%, mostly represented by Chenopodiaceae, Artemisia, Gramineae, Liliaceae, Polygonum, Typha, and Cyperaceae. The fern spores are mostly Selaginella, Osmunda, Triletes, and Monoletes. The distribution characteristic of pollen and spores implies that the pollen assemblages correspond well with the watershed vegetation. Variation in the pollen assemblages in different parts of the marine area could reflect differences in local vegetation, especially the vegetation along the inflowing rivers. The spatial distribution of pollen assemblages further suggests that the discharge from the Yellow River and Luanhe River has a great contribution to the pollen inputs into the western Bohai Sea. Pollen concentrations are lower in the nearshore sea area (water depth < 20 m) due to the combined effects of dynamic sedimentary environment and inflowing rivers. By contrast, pollen concentrations show higher values in the deeper waters of fine sediments near estuaries of Yellow River and Luanhe River and the sea area north of 39.5 N, reaching a maximum concentration of 7000 grains/g. Based on the distribution characteristics of pollen and spores, the PCA analysis results of the distribution of the dominant pollen taxa confirm that pollen grains and spores deposition into the western Bohai Sea was primarily affected by the hydrodynamic condition or water sorted effect (including river flows and ocean currents), and secondarily through aeolian transportation. Non-arboreal pollen were mainly carried by fluvial input and deposited in the nearshore shallow water area. Arboreal pollen dominated by Pinus were mainly transported by winds and ocean currents, and fern spores mainly by river flows and ocean currents. The latter two types were mostly concentrated in relatively deeper water area. © 2015 Elsevier Ltd and INQUA. All rights reserved.

Keywords: Bohai Sea Surface sediments Pollen and spores Pollen transport Distribution

1. Introduction Pollen grains in marine sediments are derived from terrestrial vegetation. Therefore, they can provide not only vegetation information of a pericontinental area, but also direct evidence for marineeterrestrial environmental change. However, pollen sources

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

in marine sediments are very complex, and accurate interpretation of the palynological information relies on a sound understanding of the distribution patterns and transport mechanisms of pollen grains and spores in marine sediments (Stanley, 1966; Heusser, 1988; Kershaw, 1994; van der Kaars, 2001; Dupont and Wyputta, 2003). Theoretically, pollen source areas can be quantitatively traced based on pollen assemblage changes, but multiple factors can complicate reconstruction attempts, such as pollen production, transmission mode, transport distance, and depositional environments (Xu et al., 2012). Hence, an adequate knowledge of the

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dispersal mechanisms of pollen from land to ocean and pollen provenance is a prerequisite for fossil pollen data interpretation. An understanding of the modern processes of pollen dispersal is vital for Quaternary palaeovegetation, palaeoclimatic and palaeoenvironmental reconstruction. Due to limited knowledge about pollenespore sedimentary process, past changes in the research area were mostly reconstructed through palynological study on the core sediments (e.g. Meng and Wang, 1987; Chen et al., 2012). Therefore studies on the modern processes of marine pollenespore dispersal are necessary, especially in the Bohai Sea, where only a few studies on marine pollenespore distribution and dispersal are available, and only with a low spatial resolution (Cheng, 1978; Jin, 1984; Xu and Sun, 1988; Wang, 1993). In this paper, we present the results of pollenespore analysis of the surface sediments from the western part of the Bohai Sea, aiming to determine the distribution patterns of pollen grains and spores as well as their transport mechanisms and provenance. 2. Study area The Bohai Sea, a shallow, low-gradient and semi-enclosed continental shelf area off the coast of northeastern China, is connected with the Yellow Sea through the narrow Bohai Strait (Fig. 1). Covering an area of about 78,000 km2, it can be geographically divided into five parts: the Liaodong Bay in the northeast, the Bohai Bay in the west, the Laizhou Bay in the south, the Bohai Basin in the center, and the Bohai Strait in the east (Qin et al., 1990). The water depth is mostly less than 30 m throughout the Bohai Sea with an average depth of ~18 m and maximum depth of 84 m toward the

northern Bohai Strait (Chen and Zhao, 1985; Chen et al., 2013). The huge amounts of terrigenous sediments in the Bohai Sea are mostly carried by the coastal rivers, including the Yellow River, the Luanhe River, the Haihe River, and the Liaohe River (Fig. 1). The surface sediments in Bohai Sea are mainly soft clay mud, fine silt mud, coarse silt, and fine sand (Fig. 2). The Bohai Sea circulation is composed of the predominant extension of the Yellow Sea Warm Current (YSWC), the Liaonan Coastal Current (LNCC), and the southern Bohai Sea Coastal Current (BSCC) (Guan, 1994; Fang et al., 2000). In winter, the YSWC is driven by the incursion of a branch of the Kuroshio subsurface water onto the shelf, extends northward, and can reach the Bohai Sea. The current moves westward along the central part of the sea and splits into two branches: one moving toward the northeast to form a clockwise gyre (Liaoxi Coastal Current (LXCC)) and another moving southward and then turning eastward along the southern coast to form a counterclockwise gyre (Lubei Coastal Current (LBCC); Fig. 1) (Guan, 1994; Su, 1998). In summer, the YSWC disappears in the Bohai Sea, and the eddies produced by the Bohai Sea itself are stronger than in winter and the central eddy is missing, while the eddy in the Laizhou Bay is more pronounced, and a coastal current along the southern and western coastlines of the Bohai Sea is established (Su and Weng, 1994). The coastal current flows southward in winter and northward in summer, reflecting the prevailing wind direction (Guan, 1994; Fang et al., 2000). The hydrological conditions of Bohai Sea are not only affected by the open sea, but are also controlled by the configuration of the coastline, coastal river runoff variations, submarine topography, and atmospheric circulation (Department of Marine Geology, Institute of

Fig. 1. Current circulation in the Bohai Sea in the summer (arrows indicate circulation direction) (Zhu et al., 2007), surface sediment sample, locations in the western Bohai, Sea and the location of the selected transect (thick black line).

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Fig. 2. Formation environment and sediment distribution of the seafloor sediments in the western Bohai Sea (Li et al., 2005).

Oceanology, Academia Sinica, 1985). Moreover, the waves, ocean currents, and tidal currents also play an important role in the hydrodynamics of the Bohai Sea (Wei et al., 2004). The climate around the Bohai Sea is mainly regulated by the East Asian monsoon, with cold sunny-dry winters and warm wet summers. The mean annual temperature is about 9 e12.5  C, and the

mean annual precipitation is around 500e900 mm (Qin et al., 1990). Due to the great influence of the adjacent ocean, Liaodong peninsula and Shandong peninsula have sufficient rainfall, but less rainfall occurs in the mountainous regions of northern Hebei (Han et al., 2011). In such climate conditions, vegetation in the land areas surrounding the Bohai Sea is dominated by warm temperate deciduous broadleaved forests and shrub grasslands (Wang, 1993). However, most of the natural vegetation has been removed and remnants of it can only be observed in the mountain areas because of the human disturbance (Fig. 3) (Meng and Wang, 1987). In the coastal land areas around the Bohai Sea, the deciduous broadleaved forests are mainly dominated by Quercus, including Q. liaotungensis, Q. dentate, Q. acutissima, and Q. variabilis. Co-dominant plants are Pinus, such as P. densiflora growing in the coastal humid zone, and P. tabuliformis developing in the relatively dry North China plain. In the plain area, apart from P. tabuliformis and the small conifer Platycladus orientalis, there are some deciduous broadleaved trees, such as Ailanthus altissima, Koelreuteria paniculata, and Morus alba. The other broadleaved trees, such as Betula ermanii, Populus tremula, Acer spp., Tilia amurensis, and Carpinus turczaninowii, mainly grow in the hills and lowlands. Herbs, especially Chenopodiaceae and Artemisia, primarily grow in the coastal salt marshes. Other herbs include Gramineae and Asteraceae. Common shrubs mainly include Ziziphus jujube, Vitex negundo, and Rhamnus davurica (Wang, 1993). 3. Materials and methods 3.1. Sampling

Fig. 3. Vegetation map (1. temperate grassland; 2. deciduous oak forest in the northern warm temperate zone; 3. Pinus densiflora and deciduous oak forest in the southern warm temperate zone; 4. cultivated vegetation in the northern warm temperate zone; 5. cultivated vegetation in the southern temperate zone) (Meng and Wang, 1987).

One hundred surface sediment samples from the western part of the Bohai Sea (west of 120 E) were obtained in 2012 by means of a grab sampler (Ekman's dredge). To ensure that the modern samples were representative, only the mud-like portion at the top of the samples to a depth of 1 cm was returned to the laboratory for pollen

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analysis, each of which was 50e100 g. In this study, 86 surface sediment samples in the western Bohai Sea were palynologically analyzed (Fig. 1, Table 1).

(excluding spores) were counted for each sample. Pollen percentages were calculated based on the terrestrial pollen and spore sum. 3.3. PCA analysis

3.2. Laboratory analysis 10e20 g of dry sediments was taken for pollen analysis in the lab. The samples were prepared for palynological analysis using hydrochloric (10%) and hydrofluoric acids (40%) to remove the carbonates and silicates, respectively, followed by KOH treatment to remove the organic matter. The pollen and spores were concentrated using a standard heavy liquid separation method (Berglund and Ralska-Jasiewiczowa, 1986; Nakagawa et al., 1998). To calculate pollen concentrations, exotic Lycopodium spore tablets were added with a known concentration (27,637 ± 563 spores) before the first step in sample treatment (Zhang et al., 2002). Pollen identification and counting were carried out with a Nikon Eclipse Ni binocular at 400 magnification, and at 1000 magnification when necessary. The modern pollen reference collection in the Palynology Lab of Sun Yat-sen University, the atlas of the Pollen Flora of China (Wang et al., 1995), and the Angiosperm Pollen Flora of Tropical and Subtropical China (IBSCIBAS, 1982) were consulted to aid with pollen identification. A minimum of 200 pollen grains

In order to further understand the pollen dispersal pattern and provenance in the western Bohai Sea, Principal-components analysis (PCA) was performed on the pollen surface sample dataset. PCA is an ordination procedure that calculates the distances between samples in a way that approximates their Euclidean distance in a multidimensional space (Lamb, 1984). Square-root-transformed pollen proportions in these ordinations increase the influence of rare pollen types and downweighs the more abundant ones (Minckley and Whitlock, 2000). In the surface pollen assemblages, as each sample contains many pollen taxa, PCA can highlight the main taxa in the pollen assemblages, making the analysis results more intuitive and effective (Wang et al., 1996). Canoco 4.5 software (http://www.canoco.com) was used to undertake the PCA based on the pollen percentage data of 86 samples in the study area. Twelve pollen types were selected for the analysis based on the environmental information reflected by the pollen assemblages. The 12 selected taxa, including Pinus, Betula, Carpinus, deciduous Quercus, Ulmus, Chenopodiaceae, Artemisia, Gramineae,

Table 1 General information on the marine surface pollen samples of Bohai Gulf. Station no.

Longitude (E)

Latitude (N)

Depth (m)

Station no.

Longitude (E)

Latitude (N)

Depth (m)

TJB1 TJB2 TJB3 TJB4 TJB5 TJB6 TJB7 TJB8 TJB9 TJB10 TJB11 TJB12 TJB13 TJB14 TJB15 TJB16 TJB17 TJB18 TJB19 TJB20 TJB21 TJB22 TJB23 TJB24 TJB25 TJB26 TJB27 TJB28 TJB29 TJB30 TJB31 TJB32 TJB33 TJB34 TJB35 TJB36 TJB37 TJB38 TJB39 TJB40 TJB42 TJB43 TJB44

117 530 52.06600 117 540 11.13400 117 530 49.12300 117 530 50.69700 117 530 53.44300 117 540 46.70800 118 80 53.47500 118 80 54.35700 118 80 52.38200 118 80 47.83500 118 240 24.07900 118 230 56.98500 118 230 56.28300 118 230 54.66400 118 230 58.12800 118 230 55.06500 118 380 57.45500 118 380 57.07100 118 380 54.64500 118 380 54.82500 118 380 56.01200 118 380 55.96500 118 530 56.78600 118 530 59.90600 118 530 57.92100 118 530 56.89300 119 80 49.200 119 80 5.32800 119 90 3.600 119 90 0.15300 119 80 59.3800 119 90 1.2700 119 80 57.5200 119 80 58.97900 119 90 1.81700 119 80 59.95100 119 90 0.57400 119 80 57.63500 119 80 58.32400 119 90 0.18600 119 230 58.800 119 240 0.69700 119 240 0.0200

39 10 51.29300 38 540 1.90200 38 460 58.17900 38 390 26.82300 38 310 57.74600 38 240 59.42500 38 160 55.57900 38 310 57.34400 38 460 57.04700 39 00 24.74500 38 540 27.7100 38 460 58.96400 38 390 28.29700 38 310 57.22700 38 240 25.44800 38 160 56.66800 38 160 53.86900 38 240 25.00500 38 310 58.24500 38 390 28.06100 38 460 56.21100 38 540 24.5900 39 10 55.12400 38 460 56.7800 38 310 55.90100 38 160 52.82700 37 230 58.5600 37 290 34.12100 37 390 14.400 37 540 13.67800 38 10 48.14200 38 90 22.64600 38 160 53.88700 38 240 26.86700 38 310 57.48400 38 390 27.31300 38 460 55.86300 38 540 26.78800 39 10 56.31500 39 90 22.04200 39 160 48.11700 39 90 21.07800 39 10 53.90800

8.72 9.03 11.96 11.3 9.2 6.3 5.4 11.6 17.9 9.89 20.4 28.1 20.4 17.9 14.46 10.76 12.72 17.84 20.1 21.1 26.6 28.1 18.13 28.6 22.73 16.95 6.8 5 3.2 7.7 12.6 16.1 20.2 23.21 24.44 25.62 27.39 25.6 21.18 16.88 18.2 22.53 24.75

TJB45 TJB46 TJB47 TJB48 TJB49 TJB50 TJB51 TJB52 TJB53 TJB54 TJB55 TJB56 TJB57 TJB59 TJB60 TJB61 TJB62 TJB63 TJB64 TJB65 TJB66 TJB67 TJB68 TJB69 TJB70 TJB71 TJB72 TJB73 TJB74 TJB75 TJB76 TJB77 TJB78 TJB79 TJB80 TJB81 TJB83 TJB84 TJB85 TJB86 TJB87 TJB88 TJB89

119 240 2.50500 119 240 0.9200 119 240 0.62600 119 240 1.22400 119 240 2.58800 119 240 0.6300 119 240 1.92600 119 240 2.66200 119 240 2.55400 119 240 1.85500 119 230 59.8500 119 240 1.46500 119 240 1.2500 119 390 3.49100 119 390 1.4900 119 390 4.47100 119 390 3.78300 119 390 2.54400 119 390 5.58400 119 390 4.64700 119 390 21.600 119 390 3.600 119 390 3.12800 119 390 4.18100 119 540 5.01200 119 540 4.55700 119 540 2.27200 119 540 4.55700 119 540 3.66700 119 540 4.94500 119 540 5.35200 119 540 5.05700 119 540 3.33700 119 540 4.28100 119 540 6.08400 119 540 3.38100 119 540 3.78200 119 540 5.47900 119 540 2.49100 119 540 3.4700 119 540 6.41300 119 540 4.2400 119 540 3.51500

38 540 28.09200 38 460 56.1200 38 390 27.19900 38 310 57.63200 38 240 26.17300 38 160 54.07700 38 90 22.65600 38 10 49.35800 37 540 14.24200 37 460 36.76100 37 390 2.04300 37 310 25.52900 37 230 48.04700 37 160 10.28900 37 310 26.02100 37 460 37.39100 38 10 47.96800 38 160 55.14800 38 310 55.51600 38 460 56.86800 39 20 4.200 39 170 6.7200 39 310 37.76600 39 460 24.81100 39 530 48.96700 39 460 25.2700 39 390 1.33200 39 310 37.17700 39 240 10.65500 39 160 49.20300 39 90 18.49100 39 10 53.14800 38 540 26.29400 38 460 57.49400 38 390 28.81500 38 310 56.89800 38 160 55.18300 38 90 22.10400 38 10 49.47400 37 540 13.59700 37 460 39.10200 37 390 3.47100 37 310 25.22200

26.21 26.82 26.38 25.4 24.1 21.8 19.9 17.2 15.5 13.52 11.34 12.41 9.8 8.07 13.94 16.28 19.8 23.4 27.38 26.2 24.5 22.7 17.3 14.3 17.61 20.51 22.76 24.5 26.2 26.6 24.6 22.6 22.5 24.57 25.9 26.86 23.87 24.71 19.84 18.41 16.8 15.4 12.8

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Typha, Selaginella, Triletes, and Monletes, account for more than 90% of the pollen and spore sum.

4. Results 4.1. Pollen and spore assemblage characteristics in the marine area A total of 71 taxa were identified in the study. The average number of counted pollen grains in each sample was 327. A variety of palynomorphs were encountered, including 41 arboreal pollen taxa, 24 non-arboreal pollen taxa, and six Pteridophyta, Bryophyta, and algae spore taxa. Arboreal pollen types constituted 52% of the total, non-arboreal pollen 25%, fern spores 22%, and algal spores 0.7% (Fig. 4). The gymnosperm pollen mostly contained Pinus, Taxodiaceae, and Tsuga. The angiosperm tree pollen mainly included deciduous Quercus, Betula, Carpinus, Nitraria, Pterocarya, Castanea, Ulmus, Tilia, Corylus, Rutaceae and Moraceae. Nonarboreal pollen grains mainly consisted of Chenopodiaceae, Artemisia, Gramineae, Liliaceae, Polygonum, Typha, and Cyperaceae. Fern spores primarily consisted of Selaginella, Osmunda, Triletes, and Monoletes. Freshwater algae were dominated by Concentricystes and Zygnema, and there are also some Protista such as the marine Hystrichosphaera. The pollen concentration exceeded 1000 grains/g in most samples, with an average value of 3132 grains/g. Only 20 samples contained less than 1000 grains/g, while the highest concentration reached 19,839 grains/g. Higher pollen concentrations lie near the mouths of the Yellow River and the Luanhe River and the sea area north of 39.5 N, and the lower values occur in Bohai Bay and Laizhou Bay (Fig. 5).

Fig. 5. Total pollen concentration diagram for the surface sediments in the Bohai Sea (legend as Fig. 2).

Fig. 4. Pollen percentage diagram for the surface sediments in the Bohai Sea.

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4.2. Percentage and concentration distribution of pollen and spores 4.2.1. Spatial distribution of arboreal pollen taxa Arboreal pollen occurs at relatively high percentages in the research area, and the average percentage is more than 40% (Fig. 6A, a). The arboreal pollen concentrations range from 97 grains/g to 11,537 grains/g, and both arboreal pollen concentrations and percentages occur at lower values in the open Bohai Bay (excluding nearshore samples of Tuhai River) and higher values in the nearshore sea area of Yellow River and Luanhe River and the area north of 39.5 N. The predominant taxa of arboreal pollen were Pinus, deciduous Quercus, and Betula. Pinus: The concentrations and percentages of Pinus generally show relatively high values in the western Bohai Sea (Fig. 6B, b). The pollen percentage is mostly more than 12% and the highest value was 68%. The pollen concentrations also present relatively high values of 81e10,692 grains/g, comparable to the arboreal pollen. The distribution pattern of Pinus is consistent with that of the total arboreal pollen. Betula: Although the pollen percentages are generally no more than 6% and the maximum concentration only reach 338 grains/g (Fig. 6C, c), it occurs commonly in all samples. Pollen percentages decrease with increasing offshore distance and the lower values are concentrated in the central part of the study area. The higher pollen concentrations are clustered in the nearshore sea area of Yellow River and Luanhe River and the area north of 39.5 N. The distributions of Betula concentration and percentage are not consistent in the study area. Deciduous Quercus: The pollen percentage is very low and generally no more than 4.5%. The maximum value of concentration is only around 243 grains/g (Fig. 6D, d). The distribution patterns of deciduous Quercus are similar to those of Betula. 4.2.2. Spatial distribution of herbaceous pollen taxa Herbaceous pollen contents are generally lower than the arboreal plants, with the pollen percentages ranging from 1% to 60%, and pollen concentrations from 45 grains/g to 3173 grains/g (Fig. 6E, e). Herbaceous palynomorphs show higher percentages in the nearshore shallow waters and decrease seaward. However, higher pollen concentrations are concentrated in the nearshore sea area of Haihe River, Yellow River, Luanhe River and the area north of 39.5 N. Chenopodiaceae: Chenopodiaceae is one of the dominant herb taxa, with relatively high concentrations and percentages (Fig. 6F, f). The higher percentages occur in the Bohai Bay and the nearshore of Laizhou Bay, with the highest value up to 36%. Whereas, the higher pollen concentrations mainly occur in the nearshore sea area of Haihe River, Yellow River, and Luanhe River (with the highest value up to 2547 grains/g), and the concentration values decrease with increasing distance from the coast. Artemisia: The higher percentages of Artemisia mainly appear in Laizhou Bay, and the highest value can reach up to 36%. However, the higher pollen concentrations mainly show in the nearshore sea area of Yellow River, Laizhou Bay, and Luanhe River and the area north of 38.5 N, with the maximum concentration of 1410 grains/g (Fig. 6G, g). Gramineae: The higher percentages concentrate in the nearshore sea area of Yellow River, Liaozhou Bay, and the nearshore around Bohai Bay, with the highest value at about 10%. However, the higher pollen concentrations mainly occur in the nearshore sea area of Yellow River, the north of 39 N, with the maximum to 441 grains/g (Fig. 6H, h). Typha: The pollen percentage is relatively low and the maximum value only reaches about 7%. The pollen percentage shows a relatively high value in Bohai Bay. However, pollen

Fig. 6. Pollen concentration and percentage distribution diagrams for the surface sediment in the western Bohai Sea (legend as Fig. 2).

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Fig. 6. (continued).

concentration values show a somewhat different distribution pattern. The high concentrations of Typha mainly lie in the nearshore sea area off the mouth of the Yellow River, the central part of western Bohai Sea, and off the mouth of the Haihe River, with the peak value of 293 grains/g (Fig. 6I, i). 4.2.3. Spatial distribution of fern spores Fern spores have higher percentages in the central part of the sea area (Fig. 6J, j), with the highest value up to 75%. The predominant taxa of the fern spores are Selaginella, Osmunda, Monletes, and Triletes. The distribution centre of fern spores lies in the sea area north of 39 N, in both percentages and concentrations. The maximum concentration can reach up to 6844 grains/g, second only to the arboreal pollen. 4.2.4. Variation in pollenespore distribution with offshore distance A transect originating from the nearshore toward the central Bohai Sea was set to determine the pollenespore dispersal patterns in the research area (Figs. 1 and 7). Herbaceous pollen, especially local pollen types such as Chenopodiaceae, Artemisia, and Typha, gradually decreased with increasing distance from the shore. However, arboreal pollen dominated by Pinus increased seaward. Furthermore, the fern spores show similar dispersal patterns to the arboreal pollen. Distribution variations of the relative abundance among arboreal pollen, non-arboreal pollen, and spores in the western area of Bohai Sea could reflect their transportation characteristics to some degree.

Fig. 6. (continued).

4.2.5. Results of principal-components analysis The PCA results (Table 2) show that the eigenvalue of axis 1 is dominant, with a score of 0.54, while the eigenvalue of the second axis is 0.11. The analysis results on the pollen types (Fig. 8a) show that on axis 1 Selaginella has the lowest score and Artemisia has the

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highest value, whereas on axis 2 Chenopodiaceae shows the lowest score and Pinus shows the highest. Although both environmental factors of these two axes dominate the pollen distribution of the western Bohai Sea, the first axis plays the major role. 5. Discussion 5.1. Relationship between modern pollen distribution in surface sediments and source vegetation The present-day vegetation pattern in the hinterland around Bohai Sea affects the distribution characteristics of pollen taxa in the marine surface sediments (Sun et al., 2003a; Luo et al., 2013, 2014). The dominant and common arboreal pollen taxa such as deciduous Quercus and Pinus show higher percentages and concentrations in the sea area of Laizhou Bay, the sea north of 39.5 N, and the estuary of Luanhe River. The land northeast of Bohai Sea (the Liaodong Peninsula) belongs to the southern temperate zone, where it mainly supports a conifer and broad-leaved mixed forest dominated by Pinus densiflora and Quercus acutissima (Wang et al., 1987). The mountainous and hilly areas in the Hebei and Liaoning provinces belong to the northern warm temperate zone, where it mainly supports a deciduous oak forest dominated by Pinus tabuliformis, Quercus liaotungensis, and Quercus dentate (Wang, 1993). The Shandong Peninsula belongs to the southern warm temperate zone, where it mainly supports a pineeoak forest dominated by Pinus densiflora and Quercus spp. (Meng and Wang, 1987). The ubiquitous distribution of these plants on the adjacent terrain explain why Pinus and deciduous Quercus are the most common pollen taxa occurring in the western Bohai Sea. As Quercus pollen cannot be transported further than Pinus, percentages of Quercus are higher in the nearshore shallow waters than offshore. The land southwest of the Bohai Sea is the cultivated region of the Yellow River plain, where it is mainly covered by shrub and herbs (Wang et al., 1987). This results in the higher percentages of nonarboreal pollen in the nearshore shallow waters surrounding the Bohai Bay. Furthermore, lying directly to the west of the Bohai Sea is the vast north China plain. Here the coastal areas are covered by saline herbaceous plants, especially Chenopodiaceae and Artemisia, accompanied by Gramineae and Cyperaceae (Wang et al., 1987; Wang, 1993). Thus, the herbaceous pollen dominated by Chenopodiaceae are mainly distributed in the nearshore of the research area, and their pollen percentages decreased with increasing distance from shore. The low fern plants, with the exception of Cyathea spinulosa, mainly grow on the floor of humid forests. Previous palynological studies have documented that fern spores are abundant in surface samples from the deciduous broad-leaved forest of northern China (Sun et al., 2003b; Xu et al., 2005a). It has also been reported that fern spores are mainly transported by flowing water and deposited in marine sediments, while few spore grains are transported by wind (Sun et al., 2003a; Dai et al., 2012). In the present study, higher concentrations of fern spores are found in the relatively deeper waters near the estuaries of the Yellow and the Luanhe Rivers and in the central open area of Bohai Sea, suggesting that the fern spores mainly originate from the watershed forests of the Luanhe River and the Yellow River. However, fern spores show lower concentrations near the estuary of Haihe River, which can be explained by the vast cultivated area in the North China plain which contains very little forest cover. In addition, palynological studies on the modern samples (including aerial, surface and alluvial pollen grains and spores) in the Luanhe River basin and the alluvium in the North China Plain (based on the deposits of the Yellow River) also indicated that arboreal pollen grains were dominated by Pinus, and herbaceous pollen grains were dominated by Artemisia and Chenopodiaceae (Xu et al., 2001, 2005b), which are similar to the

characteristics of pollen assemblages in the surface sediment from the western Bohai Sea. Moreover, the distribution patterns of pollen assemblages in different reaches seaward could reflect the variations of vegetation composition along the Luanhe River and the Yellow River, and these river flows played essential roles in pollen deposition of the Western Bohai Sea. Therefore, pollen assemblages of the surface sediment in the research area generally correspond well with the vegetation distribution surrounding the western Bohai Sea. Variation of the pollen assemblages in different parts of the marine area could reflect differences in local and regional vegetation, especially the vegetation along the inflowing rivers. 5.2. Effects of depositional environment on pollen distribution In the western Bohai Sea, higher pollen concentration of the surface sediments mainly occurs in the nearshore sea area of the Yellow River, Luanhe River and the area north of 39.5 N, where the water depths are generally greater than 20 m. This distribution pattern suggests that the Yellow River and Luanhe River are the dominant pollen discharges into the western Bohai Sea. With an annual sediment load of 12  1011 t and 6  109 t, respectively, being discharged to the sea (Wang, 1993), the Yellow River and the Luanhe River are a major source of sediment and pollen for the western Bohai Sea (Xu et al., 2005b), forming a large depositional center in front of the submerged delta. In these areas, sediments are dominated by clayey silt or silty clay and low energy (Li et al., 2005), which are favorable for pollen deposition and preservation, hence the higher pollen concentrations (Figs. 5 and 6a, b). In addition, the sea area north of 39.5 N, affected by the summer circulation and the coastal currents of Bohai Sea, receive sediments from the coastal rivers such as the Liuguhe River, Gouhe River and Dashihe River and form a sandesilteclay sedimentation after transportation and redeposition, where the silt and clay sediments are deposited in the boundary transgressive environment (Li et al., 2005; Tong et al., 2012), favouring pollen preservation. However, in coastal areas of shallower water (e.g. the west part of Bohai Bay), the complex and strong hydrodynamics caused by the waves, tides, coastal currents, and ocean circulation are more conducive to pollen resuspension than to pollen deposition. Moreover, most inflowing rivers have less river runoff and sedimentation flux into this area, resulting in lower pollen concentrations even in the fine sediments (Figs. 5 and 6a, b). Therefore, not only the terrigenous pollen input but also the hydrodynamic and sedimentary environment of the western Bohai Sea has great impact on the pollen assemblages of surface sediments, and regulates the pollen distribution patterns to a great extent. 5.3. General transportation pattern for most pollen and spore taxa Airborne pollen dominates in arid and sub-arid regions where few rivers discharge into the sea (Dupont and Agwu, 1991; Dupont and Wyputta, 2003; Hooghiemstra et al., 2006; Dupont et al., 2007). On the other hand, waterborne pollen grains are much more dominant in Southeast Asia and equatorial America, where precipitation is abundant (Muller, 1959; van der Kaars, 2001; van der Kaars and Deckker, 2003; Moss et al., 2005). Pollen and spores in marine sediments were carried from land through multiple paths into the sea area. Knowledge of pollen provenance and dispersal mechanisms are critical for better explanation of pollen spectra (Sun et al., 2003a). Numerous rivers discharge into the western Bohai Sea area, including Luanhe River, Xunyunhe River and Haihe River in the north, and Yellow River, Tuhaihe River and Majiahe River in the south. Previous study on clay mineral assemblages of the surface sediments in the Bohai Bay revealed that the Haihe River, Yellow

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Fig. 7. Pollen percentage diagram of the common taxa for the surface samples from the westeeast transect in the western Bohai Sea.

Fig. 8. PCA analysis. a: Species scores Component 1 vs. Component 2; b: Sample scores Component 1 vs. Component 2.

Table 2 Eigenvalue and Explained variation of the main 5 axis of PCA. Code

Axis 1

Axis 2

Axis 3

Axis 4

Axis 5

Eigenvalues Explained variation (cumulative)

0.5403 54.03

0.1075 64.79

0.069 71.69

0.0537 77.06

0.0418 81.24

River, and Luanhe River are the main carriers of clay minerals into this region (Han et al., 2011). The organic carbon content of the sediment in Bohai Sea was also attributed to the input of the terrigenous organic carbon (Chen et al., 2011). Therefore, coastal rivers play a key role in sediment transportation into the nearshore areas of the Bohai Sea. Xu et al. (2005b) further point out that the pollen assemblage characteristics of surface sediments surrounding the watershed of Luanhe River show a good consistency with the surface pollen in the Bohai Bay, implying rivers are the main carriers for pollen from land into the nearshore areas of Bohai Sea. The behaviors of pollen grains and spores in the sea area are more complex, generating the distribution disparity in the research area comparing to pollen assemblages in alluvium from the Luanhe River and the Yellow River. The pollen concentrations for the herbs, Betula, and Quercus decreases with increasing distance from the shore in this study. However, the fern spore percentages increase with increasing distance offshore in the western Bohai Sea. Previous studies reported that the fern spores were mainly transported into the sea by river flows (van der Kaars, 2001). A survey on the airborne pollen and pollen in the surface water from the western

Pacific Ocean further revealed that pollen contents were higher in the surface water samples, and inferred that fern spores could be easily suspended in sea water over long distance due to its lighter density (Dai and Weng, 2011). This can explain our observation that fern spore contents increase with increasing distance offshore in the study region. According to palynological studies on the characteristic behavior of Pinus, the floating property on the water of Pinus will not be lost until both air-sacs are filled with water (Matsushita and Sanukida, 1986; Bohne et al., 2005). This can explain the increasing tendency of Pinus seaward in the research area, because air in the bissacci permits Pinus grains to float on the surface water and reach a relatively long distance from their original sites. Therefore, river and ocean flows play a dominant role for pollen transportation into the western Bohai Sea. The Bohai Sea is a semi-enclosed sea of China, flanked by open terrain and climatically controlled by the Asian monsoon. This physical setting is favorable for pollen transportation from land to sea through wind and water carrier. The low herbs produce pollen grains and fall in situ or migrate shorter distances in contrast to arboreal pollen. Percentages of herbaceous pollen generally show

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S. Yang et al. / Quaternary International xxx (2015) 1e11

increasing trends along the Luanhe River and the Yellow River toward the Bohai Sea, because of the widespread herbs in the vast North China Plains and the coastal areas (Xu et al., 2001, 2005b). Herbaceous pollen, due to the poor dispersal efficiency, are mostly carried by river flows and deposited in the nearshore sea area (Fig. 6b). However, most fern spores derived from the forests can be carried by river flows into the sea area. Xu et al. (2005b) also noted that the content of spores in the alluvium of the Luanhe River increased toward the lower reaches. Due to the buoyancy of fern spores, they can be transported by the surface sea currents for a certain distance, generating higher abundances with increasing distance from the shore. Thus, the fern spores were carried and deposited in the research area by the combination of river flows and surface ocean currents. As the arboreal pollen grains are generally more adapted for wind transport, direct supply into the sea area by air currents must be considered. Studies on atmospheric pollen transport over the Yellow Sea showed that the content of arboreal pollen amounted to 74%, with 23% herbaceous pollen and 3% spores (Xu, 1994). Research on monitoring fluvial pollen transportation indicated that the proportions of conifer pollen (including Pinus) were 6% in fluvial samples, 32% in airborne samples and 3% in local vegetation (Brown et al., 2007). In particular, the pollen of Pinus, with its high pollen yield, low density, and saccate structure, can be transported over long distances by wind (Song, 1965; Hooghiemstra et al., 1986). On the other hand, the pollen grains of Pinus can be carried a certain distance by the surface water currents in the waters because of their high buoyancy (Matsushita and Sanukida, 1986; Sun et al., 1999), which can explain the increase of Pinus pollen abundance with increasing distance offshore (Figs. 6a and 7). Accordingly, the combined effect of wind and water (river flows and ocean currents) transportation determines Pinus pollen deposition in the research area. Hence, the distribution patterns of pollen grains and spores in the western Bohai Sea largely depends on water transportation (river flows and ocean currents), while the importance of wind transport for arboreal pollen (dominated by Pinus) cannot be ignored. The PCA results support our interpretation of the pollen distribution data and shed light on the transport mechanisms for different pollen types. On axis 1 Selaginella has the lowest score and Artemisia has the highest value (Fig. 8a). Artemisia pollen mainly occur in the nearshore of the western Bohai Sea, with the highest percentages and concentrations near the river mouths. The fern spores, dominated by Selaginella, increase with increasing offshore distance and water depth, which was mainly transported into the sea by water (Muller, 1959; van der Kaars, 2001; van der Kaars and Deckker, 2003; Moss et al., 2005). Thus, the first axis probably reflects the hydrodynamic condition or water sorted effect for all kinds of pollen types. On axis 2 Chenopodiaceae has the lowest score and Pinus has the highest. Chenopodiaceae pollen grains, with very poor dispersal efficiency in air or water, mainly occur in the coastal sea area. Pinus is adapted to wind transportation and of high buoyancy, and its pollen percentage increases with increasing offshore distance. However, fern spores with the buoyancy, were mostly transported through water flows (Dai and Weng, 2011) and showed relatively lower values on axis 2. Therefore, the second axis reflects the wind transportation ability of pollen and spores. Wind is an important transportation vector for pollen grains into the western Bohai Sea (Faegri and Iversen, 1964; Hooghiemstra et al., 1986; Mudie and Mc Carthy, 1994; Sun and Li, 1997; Rousseau et al., 2008). The PCA results on the pollen samples (Fig. 8b) also show that samples were mainly distributed along Axis 1, indicating that the hydrodynamic condition or water sorted effect play a vital role in pollen and spores deposition in the western Bohai Sea.

6. Conclusions (1) In the western Bohai Sea, the surface pollen assemblages are dominated by arboreal pollen, which accounts for an average of 52% of the pollen and spore sum and mainly includes Pinus, Deciduous Quercus, Carpinus, Betula, Nitraria, Castanea, and Ulmus. The pollen percentages of herbaceous taxa reach an average of 25%, mostly represented by Chenopodiaceae, Artemisia, Gramineae, Liliaceae, Polygonum, Typha and Cyperaceae. The fern spores are mostly represented by Selaginella, Osmunda, Triletes, and Monoletes. The distribution of pollen grains and spores reveals that the pollen assemblages correspond well with the watershed vegetation. Variation in the pollen assemblages in different parts of the marine area could reflect differences in local vegetation, especially the vegetation along the inflowing rivers. The spatial distribution of pollen assemblages in the sea area further suggests that the Yellow River and Luanhe River flows have a great contribution to the pollen input. (2) Pollen concentrations occur at low values in the nearshore areas (e.g. the west side of Bohai Bay) due to the joint effects of the dynamic sedimentary environment and inflowing rivers. Pollen concentrations are higher in the deeper waters off the estuaries of Yellow River and Luanhe River and the sea area north of 39.5 N (the highest concentration reaching up to 7000 grains/g), where the sediments are dominated by silt and clay. (3) According to the pollenespore distribution characteristic and the PCA results on the dominant pollen types, it can be inferred that pollen and spore deposition in the western Bohai Sea were primarily affected by water hydrodynamic conditions or water sorting effects. The dominant herbaceous pollen, Chenopodiaceae, were primarily transported by rivers and deposited in the nearshore shallow water area, while the arboreal pollen, dominated by Pinus, were mainly transported by wind and ocean currents. Therefore, higher percentages of the herbaceous pollen occur in the nearshore shallow waters and higher percentages of the arboreal pollen in the relatively deeper waters. Meanwhile, the fern spores dominated by Selaginella were mainly transported by water flows (including river and ocean flows) and deposited in relatively deeper water areas.

Acknowledgments This study was jointly funded by the National Natural Science Foundation of China (Grant Nos. 40872167, 41240022, 41406069), the Ministry of Land and Resources program: “Special foundation for scientific research on public causes” (Grant No. 201111023), the Marine Safeguard Project (Grant Nos. GZH201200503, GZH200800501), and the China Geological Survey (Grant Nos. 1212010611402, 1212011220113). We sincerely thank two reviewers for their constructive comments on the manuscript.

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