A last glacial and deglacial pollen record from the northern South China Sea: New insight into coastal-shelf paleoenvironment

Quaternary Science Reviews 157 (2017) 114e128

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A last glacial and deglacial pollen record from the northern South China Sea: New insight into coastal-shelf paleoenvironment Shaohua Yu a, *, Zhuo Zheng b, **, Fang Chen a, Xia Jing a, Peter Kershaw c, Patrick Moss d, Xuechao Peng a, Xin Zhang a, Chixin Chen a, Yang Zhou a, Kangyou Huang b, Huayang Gan a a

Key Laboratory of Marine Mineral Resource, Ministry of Land and Resources, Guangzhou Marine Geological Survey, Guangzhou, 510760, China School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou, 510275, China School of Geography and Environmental Science, Monash University, Melbourne, VIC, 3800, Australia d School of Geography, Planning and Architecture, The University of Queensland, Brisbane, Queensland 4072, Australia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 August 2016 Received in revised form 13 December 2016 Accepted 15 December 2016

This study presents a marine palynological record of the Asian summer monsoon and sea level change in the Last Glacial Maximum (LGM) and the deglacial period in the northern South China Sea (SCS). A fossil core STD 235 (855 cm in length) and 273 surface sediment samples from the northern SCS were pollen analysed to reconstruct the paleoenvironment of the continental shelf during the last glacial period. Results from fossil pollen show that the main pollen source region fundamentally changed from the LGM to the deglacial period as sea level rapidly rose. The modern marine surface samples show that pollen concentrations in the estuary of the Pearl River are extremely high, and modern pollen assemblages are in good agreement with the regional vegetation. However, wind transport becomes more important in the deeper ocean as the percentages of Pinus, a taxon with very high pollen production and dispersal capacity, is highest in these sediments, which otherwise have very low pollen concentrations. The concentration of total pollen between surface and fossil pollen samples is compared in order to determine the possible vegetation sources areas for the marine core. Pollen concentration as high as >100 grains/g at the LGM suggested that the paleo-shoreline was located within 80 km of the core. Consequently, pollen would mostly have derived from the exposed continental shelf in the northern SCS. By contrast, pollen concentrations were very low due to a much greater transport distance (318 km at present, core STD 235) under higher sea levels, and windblown pollen played a more important role because of the limitation of riverine input into the deep ocean during this highstand period. Such alternation of pollen flux and source distance should be repeated during all glacial-interglacial cycles, reflecting closely sea level and climate dynamics. According to fossil pollen assemblages from Core STD 235, we conclude that wetland and/or grassland communities with sparse subtropical trees dominated most of the exposed shelf during the LGM rather than forest that characterizes the region today. The existence of a predominantly open landscape on the exposed continental shelf suggests lower precipitation during the LGM, which in turn indicates a weaker Asian summer monsoon. This finding is supported by other records from the Okinawa Trough and the East China Sea, suggesting that a weaker summer monsoon was a key characteristic of the LGM in East Asia. © 2016 Elsevier Ltd. All rights reserved.

Keywords: South China Sea Coastal-shelf Vegetation Sea-level change Asian summer monsoon

1. Introduction

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Yu), [email protected]. cn (Z. Zheng). http://dx.doi.org/10.1016/j.quascirev.2016.12.012 0277-3791/© 2016 Elsevier Ltd. All rights reserved.

Vegetation distribution in south China is controlled by variations in the Asia monsoon, which is sensitive to climate variability over a range of temporal scales (An, 2000; Seddon et al., 2016). Most past vegetation records for the south China region are based

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on terrestrial peat bogs or lakes from the southern mountains of this region, with results suggesting that forests still dominated during glacial periods, and temperature rather than precipitation played the key role during glacial periods (Lee and Liew, 2010; Liew et al., 2013; Xiao et al., 2007; Yue et al., 2012). However, palynological records from maar lake core TY1 in the coastal area of south China (Zheng and Lei, 1999) and deep ocean sediments from core ODP 1144 in the northern SCS (Sun et al., 2003) have demonstrated that herbaceous taxa dramatically increased during glacial periods, suggesting that precipitation changes may have also been important in vegetation representation during these periods. Additionally a record from Taiwan Island indicates that high non-arboreal pollen (NAP) values in MIS 2 might represent a forest-steppe environment existing under precipitation levels as low as half those of today (Liew et al., 2006). Nevertheless, recently, a core MD05-2906 taken from the northern SCS indicates that the exposed continental shelf was still dominated by subtropical and tropical forest at this time (Dai and Weng, 2015; Dai et al., 2015) suggesting that a southern subtropical climate prevailed under high precipitation in the South China during the LGM. This disagreement has focused on the source of pollen (especially herbs) to marine sediments in the northern SCS (Sun et al., 2003; Dai et al., 2015) as well as to terrestrial cores from maar lakes in the coastal area of south China (Wang et al., 2012; Zheng and Lei, 1999), and whether the high abundance of herbs in south China during glacial periods is sufficient to explain the vegetation representation as well as the strength of precipitation change. Therefore, new palynological records in the subtropical-tropical area of south China are needed to further investigate the relative role of temperature and precipitation in vegetation distribution and whether climate alterations are sensitive to variations in the Asian summer monsoon. The SCS is in a subtropical to tropical monsoon area, which is situated close to the Eurasian continent and the Pacific Ocean, with one of the largest marginal seas (~3.6 million km2) and widest continental shelf areas in the world. It connects with the Pacific Ocean through the Luzon Strait and the East China Sea through the Taiwan Strait, and the whole of Taiwan Strait was exposed during the LGM when sea level was around 130e140 m lower than present (Barrows and Juggins, 2005; Wang et al., 1995; Yokoyama et al., 2000). As the most recent ice-sheet maximum period (Clark et al., 2009; Mix et al., 2001), the LGM is well recorded in deep ocean sediments in the SCS by a range of proxies including foraminifera, geochemical elements and biomarkers (Huang and Tian, 2012; Shintani et al., 2011; Steinke et al., 2010; Tian et al., 2010). However marine pollen in this area has been poorly represented due to confusion about pollen source areas especially with the herbs, making interpretation of marine pollen records problematic. Here we present a novel, empirical approach to assess pollen source area for the northern SCS, through analysis of 273 surface seafloor samples, which are then linked to the last glacial to deglacial pollen record from deep ocean core STD 235 (8.55 m in length). This paper provides insight into the pollen transport mechanisms and their terrigenous source areas that greatly assists in reconstructing the paleovegetation of the exposed continental shelf during the LGM. In addition, it provides insight into the strength of the Asian summer monsoon and the impact that sealevel variation and deglacial warming had on East Asian coastal environments. 2. Regional setting The study area is located in the northeast SCS, which is dominated by subtropical and tropical climates, with mean annual temperatures between 18 and 24
C and high annual precipitation (>1000 mm). Precipitation is controlled by shifts in the inter-

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tropical convergence zone (ITCZ) and summer monsoon winds (Turney et al., 2004; Wang et al., 2004). In summer, southwesterly winds gather large amounts of moisture and blow towards the Eurasian continent as the ITCZ moves northward. In winter, dry and cold air moves from the interior of the Eurasian continent to the SCS as the ITCZ moves southward. The general surface circulation of the SCS changes seasonally with the monsoon winds and is also heavily impacted by the Kuroshio Current intrusion (Zhang et al., 2006) (Fig. 1). In winter, the Loop Current and SCS Branch of the Kuroshio Current are distributed along the continental slope from the southwest of Taiwan to the west of the Dongsha Islands, which are warmed by the Kuroshio Current. The SCS Warm Current is a northeastward flow separated from the SCS Branch of the Kuroshio Current. The Guangdong Coastal Current flows northwest and is largely controlled by the winter monsoon. In summer, the SCS Branch of the Kuroshio Current also shifts southeast and the Guangdong Coastal Current is reversed, while the SCS Warm Current becomes the prevailing northeasterly surface current forced by summer monsoon winds. Besides, there exists a deep water current (2000e2500 m) that turns southwest along the continental margin off the southeast China coast called the SCS Contour Current (Qu et al., 2009). The topography of much of south China is characterized by widespread hills (mainly between 200 and 2000 m) except for the Yushan Mountains in Taiwan, which reach an elevation of 3952 m and the peak of Fujian coastal mountain that have a height of 2158 m. The submarine topography of the northern SCS consists of a broad continental shelf, that transitions to a continental slope and then into a deep ocean (Wang et al., 1995). There are several large rivers in southeast China, including the Pearl River in Guangdong Province and the Han and Min Rivers in Fujian Province. The Pearl River is the largest and flows into northeastern SCS with a substantial amount of terrestrial sediment. It delivers 3.5*1011 m3/yr of freshwater and 85 *106 tons/yr of sediment load into the SCS (Bai et al., 2015). The catchment of these river systems is located in an area directly influenced by the East Asian Monsoon. Nearly 80% of the discharge occurs during the wet season from AprileSeptember and only 20% during the dry season of OctobereMarch (Yin et al., 2004). The vegetation in southeast China is dominated by subtropical evergreen broadleaf forest in the hills, with tropical semi-evergreen broadleaf forest along the coast, as well as tropical evergreen broadleaf forest on Hainan and Luzon islands (Olson et al., 2001; Zheng et al., 2014). The native forests in this area, especially along the coastal plain, have been mostly destroyed by anthropogenic activities and are now dominated by tropical and subtropical grasslands. Extensive areas of plantation forests (Pinus massoniana, Cunninghamia lanceolata and Eucalyptus) also occur within the catchment and were planted in the 1960s and 1970s. Tropical evergreen broadleaf forests are rare and are now only found in the lowlands of southern Hainan Island and southern Taiwan at elevations below 600 m, and consist mainly of taxa such as Dipterocarpaceae, Pterospermum, Heritiera, Sterulia, Aglaia, Moraceae and Sapindaceae (Whitmore, 1989). Natural subtropical evergreen broadleaved forests are scarce and mostly located on the low hills (elevation 600e1500 m) of Jiangxi and Fujian provinces and consist mainly of Castanopsis, Cyclobalanopsis, Laurance, Ilex, Hamamelidiaceae and Magnoliaceae (Wu, 1980). The vegetation of this region tends to be controlled by latitude, although mountains above 1500 m support temperate deciduous broad-leaved forest with, for example, Carpinus, Betula, Alnus, Acer, Liquidambar and Tsuga. In addition, the Yushan Mountains, the highest range in the region (elevation peak of 3925 m), supports cold-temperate coniferous (Abies and Picea) forest.

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Fig. 1. The location of core STD 235 and surface sediment samples (black dots). Monsoon winds and current systems in the northeastern South China Sea are modified from (Liu et al., 2016). Numbers for winter (black) and summer (red) surface currents: 1, Loop Current; 2, SCS Branch of Kuroshio; 3, NW Luzon Cyclonic Gyre; 4, NW Luzon Cyclonic Eddy; 5, NW Luzon Coastal Current; 6, SCS Warm Current; 7, Guangdong Coastal Current; 8, Kuroshio Current; 9, Luzon Deep Current; 10, Deep Cyclonic Current; 11, SCS Contour Current. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3. Material and methods The STD 235 marine core (20
200 3700 N, 118
2101600 E, water depth 2630 m, core length 8.55 m) was obtained by the Guangzhou Marine Geological Survey, China Geological Survey. Lithological analysis showed that the lower part (855e550) consists of alternating layers of silt and sandy silt, while the upper part (550-0 cm) is composed mainly of homogeneous, gray clayey silt. Eighty-six samples were taken at 10 cm intervals from the core for palynological, total organic carbon and carbonate content analyses. In order to analyze the transport mechanism and pollen deposition on the seafloor and to clarify the relationship between relative and absolute pollen content, 273 surface samples were also collected for analysis. These samples were taken from coastal areas of the southeast China, the Pearl River mouth, the Taiwan Strait, the SCS continental shelf and the SCS deep ocean basin. All samples were collected, between 2011 and 2013, using sediment boxes. All samples were prepared in the paleontological laboratory at the Guangzhou Marine Geological Survey (Davis, 1984; Tweddle and Edwards, 2010). Hydrochloric and hydrofluoric acids were used initially to remove carbonates and silicates respectively from the samples. Thereafter, a heavy liquid (HBr, KI, Zn) with a specific gravity of 2.0 (d ¼ 2) was used to separate pollen from the remaining inorganic fraction. Finally, the residues were filtered through a 10 mm nylon sieve by ultrasonic treatment. Each sample was counted for more than 100 pollen grains under a Zeiss light microscope (400*magnification). Absolute pollen concentration was calculated using the volumetric method. In order to calculate

pollen concentration of each sample, counts were made on total pollen slides. Diagrams of percentage and concentration of fossil pollen were made using TILIA software. CONISS was adopted to assist pollen zonation based on pollen assemblages (separated into tree, shrub, herb and fern groups). In order to estimate the modern marine pollen transport distance, the distance of each surface pollen sample from shore was calculated by Near analysis in the ArcGIS software. The chronology of the core is based on 6 AMS 14C dates (Table 1) and samples (bulk organic) were dated at the Guangzhou Institute of Geochemistry, Chinese Academy of Science and Beijing University. These dates were calibrated by d13C fractionation correction, then calibrated based on the IntCal 04 tree ring curve (Calib version 5.0.1) (Stuiver and Reimer, 1993).

4. Results 4.1. Chronology An age model was established from the 6 radiocarbon dates by assuming constant sedimentation rates between two contiguous control points and extrapolation beyond the oldest control point (Fig. 2). The linear interpolation suggests that the core represents a continuous sequence, from 25383 to 8069 cal yr BP and, therefore this record covers the LGM and the deglacial period. The sedimentation rate was highest during the LGM reaching 0.16 cm/yr and then fell to 0.009 cm/yr during the Last Glacial to Interglacial transition, with the lowest rates (0.029 cm/yr) during the early

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Table 1 Conventional and calibrated AMS 14C dates for the core STD 235. Tested samples

Depth (cm)

Sample core

d13C (‰)

AMS

STD235-2 STD235-3 STD235-5 STD235-7 STD235-9 STD235-10

50 cm 100 cm 300 cm 500 cm 700 cm 855 cm

GZ4995 GZ4996 GZ4997 GZ4998 GZ4999 GZ5000

23.15 22.19 21.31 21.42 21.99 22.92

8330 ± 30 9595 ± 30 14330 ± 45 16170 ± 60 17455 ± 50 25350 ± 90

Holocene (Fig. 2). The total organic carbon and carbonate contents are compared with the sedimentation rate and the highest organic carbon values correspond to the peak sediment accumulation rate, from 19.4 to 20.7 ka. Organic carbon then decreases dramatically to 19085 cal yr BP from 470 cm, while carbonate content increases, except between 210 and 190 cm (14502-13887 cal yr BP) where there is a sharp dip.

4.2. Seafloor surface pollen assemblages All palynomorphs were grouped into three major types, e.g. terrestrial arboreal pollen, terrestrial herbaceous pollen, and terrestrial Pteridophyta spores, fresh-water algae and marinederived taxa. Among the pollen, 65 taxa have been identified including 49 tree and 16 herb types. A total of 27 terrestrial Pteridophyta spore and fresh-water algae taxa have also been identified. Based on the modern pollen database of eastern Asia, the identified taxa are common components of the continental regions

14

C age (yr B.P.)

d13C fractionation correction

14

8360 ± 30 9640 ± 30 14390 ± 45 16228 ± 60 17504 ± 50 25383 ± 90

9425 ± 25 11119 ± 39 17271 ± 207 19408 ± 67 20632 ± 140 25383 ± 90

C calibrated age (a BP, 1s)

of south China (Zheng et al., 2014). In order to distinguish pollen source areas for marine sediments, all pollen taxa were assigned into groups according to their ecological features and distribution (i.e. from coastal lowland to montane habitat). The key groups include mangroves; lowland and sub-montane forest; lower montane evergreen forest; mid-montane mixed forest; sub-alpine coniferous forest taxa; plantation taxa and herbs (Table 2) (van der Kaars, 2001; Zheng and Lei, 1999). The most abundant mangrove pollen taxon is Rhizophora, which is only found in the samples distributed near the coastline. Arboreal taxa such as Altingia, Ardisia, Capparis, Eurya, Helicia, Homalium, Hypericum, Maesa, Magnolia, Myrsine, Olea, Pittosporum, Sapindaceae, Syzygium, Thymelaeaceae, Vitex and Wikstroemia are grouped into the lowland and sub-montane forest because they most often occur at lower elevation areas. These taxa generally show low percentages (2%e5%) in the study area and are found only in the surface samples located less than 150 km from the shoreline. The plantation group is composed of Pinus and Mimosaceae; Pinus is

Fig. 2. Core STD 235: lithological description, age-depth model, sedimentation rate, total organic matter and CaCO3 contents.

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Table 2 Identified pollen taxa in surface and core pollen samples. Nothing in the superscript of the taxon means that only surface pollen samples have presented this pollen taxon. One star (*) means that both of the surface and core pollen samples have presented this taxon. Two stars (**) means that only core pollen samples have presented this taxon.

one of the most abundant pollen taxa (40%e80%) in the marine samples and, although native, its high values may be related to expanded plantation areas. Lower montane evergreen forest yields 11%e35% of the total sum and mainly includes Quercus, Castanopsis, Podocarpus, Dacrydium, Hamamelis, Ilex, Keteleeria, Sambucus, Taxodiaceae and Tsuga. The most abundant pollen types in this group are the main elements of broadleaved and tropical coniferous forests - Quercus (30%e3%) and Castanopsis (10%e3%). Mid-montane

mixed forest is composed of temperate taxa including Alnus, Betula, Carpinus, Carya, Castanea, Corylus, Fraxinus, Juglans, Liquidambar, Pterocarya, Rosaceae, Salix and Ulmus, which have percentages of less than 10%. Sub-alpine coniferous forest, composed of Abies and Picea, is a group reflecting long distance transportation because it occurs in mountains far from the coast at elevations over 3000 m. The herb group (values of 15e30%) consists mainly of Poaceae and Artemisia, with other common taxa including

S. Yu et al. / Quaternary Science Reviews 157 (2017) 114e128 Fig. 3. Pollen percentage diagram of the STD 235 core (all taxon percentages show 5 times exaggeration in grey). SACF ¼ subalpine coniferous forest taxa; MMF ¼ mid-montane mixed forest; LMF ¼ lower montane evergreen forest; LSMF ¼ lowland and sub-montane forest.

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Amaranthaceae, Caryophyllaceae, Chenopodiaceae, Compositae, Cyperaceae, Humulus, Polygonum and Thalictrum. The pteridophyte group consists of trilete and monolete spores (mostly Dicranopteris and Polypodiaceae). 4.3. Fossil pollen record of STD 235 Ninety-four taxa have been identified in the marine core samples, with 49 arboreal types, 13 herbs and 32 fern taxa. Some marine-borne microfossils such as dinoflagellates and freshwater algae (i.e. Concentricystes) are also recognized. The arboreal pollen has been grouped based on their occurrence in different ecological communities linked to surface pollen results. Arboreal pollen taxa were particularly abundant during the deglacial period, with the most frequent taxon being Pinus (between 17 and 90%). Subalpine coniferous forest taxa (SACF), including Abies (7.69%e1.18%) and Picea (1.40%e0.05%) were the next most important group. Mid-montane mixed forest taxa (MMF) were composed mostly of Alnus (2.15%e0.9%), Juglans (1.59%e 0.07%), Liquidambar (1.58%e0.8%), Pterocarya (5.68%e0.9%) and Tsuga (4%e1%). The lower montane evergreen forest taxa (LMF) group was composed of Podocarpus ranging from 9% to 2%, Dacrydium (4.88%e0.51%), Taxodiaceae (6%e2%), evergreen Quercus (19.11%e1.2%) and Castanea-Castanopsis (12.36%e1.9%). The lowland and submontane forest taxa (LSMF) were rare and consisted mostly of Eurya (0.94%e0.01%), Syzygium (2.43%e0.94%). There was also only one type of tropical mangrove (Carallia) (1.15%e0.04%) identified. Among the herbs, Artemisia was most abundant taxon in the

core (13.77%e41%), and other common herbaceous taxa included Compositae (8.69%e0.94%), Poaceae (7.40%e0.86%), Chenopodiaceae (4.76%e0.86%), Polygonum (2.04%e0.82%) and Corallodiscus (4.63%e0.82%). Fern spores were mostly trilete spores (Dicranopteris) and monolete fern spores. Other Peridophyta included Cibotium (2%e0.1%), Cyathea (3.8%e0.31%), Davalliaceae (1.92%0.13%) and Pteris (9.33%e1.65%). There were also algae found in some samples, and the most frequent of them were Dinoflagellates (1.82%e0.04%) and Concentricystes (2.38e0.35%). With the aid of CONISS, the whole sequence was vertically divided into three zones, with zone 1 further divided into subzones 1a and 1b (Fig. 4). Summary concentrations of selected pollen taxa are shown in Fig. 5. Major characteristics of each zone are as follows.

4.3.1. Zone 1 (855-450 cm, 25383 -18872 cal yr BP) This is the oldest stage of the core containing 47 samples and covering the LGM. It is characterized by high percentages of herbs, especially Artemisia (41%e10%), Poaceae (7%e1%), Compositae (8.7%e1.1%) and Chenopodiaceae (4.7%e0.9%). Arboreal pollen includes coniferous trees dominated by Pinus and Podocarpus. Quercus is the most common taxon from the lower montane evergreen forest group while Castanopsis is the most common taxon from the evergreen broad-leaved group. The temperate component consists of Pterocarya and Liquidambar, while pollen from the lowland and sub-montane forest group was rarely observed. The dominant ferns are Dicranopteris and Polypodiaceae. Pollen concentrations are generally high in this zone (310 grains/g).

Fig. 4. Pollen concentration (grains/g) diagram of STD 235 core (SACF ¼ subalpine coniferous forest taxa; MMF ¼ mid-montane mixed forest; LMF ¼ lower montane evergreen forest; LSMF ¼ lowland and sub-montane forest).

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Fig. 5. Surface total pollen and spores distribution in three sections.

4.3.2. Subzone 1a (855-690 cm, 25383 -20570 cal yr BP) The most significant characteristic of this subzone is that herbaceous pollen have high abundances, especially Artemisia (~23.6%), as well as Compositae and Poaceae. Arboreal pollen is dominated by Pinus (44%) while lower montane evergreen trees, particularly Quercus and Castanopsis are also important in this subzone. Spores are dominated by Dicranopteris (6.17%) and Polypodiaceae (16.83%). In addition, fresh water algae (i.e. Concentricystes) frequently occur in this zone comparing little marine microfossil Dinoflagellates. The concentration of total pollen is the highest in the record with an average of 152 grains/g. 4.3.3. Subzone 1b (690-450 cm, 20570-18872 cal yr BP) Herb abundances are even higher in this subzone, largely due to Artemisia values increasing to ~37%. There is also a slight increase in arboreal pollen, with Pinus still being the most important tree taxa, but with key increases in the mid-montane mixed forest taxa Alnus, Betula, Salix, Corylus and Juglans. In addition, lower montane subtropical evergreen trees show a slight increase, mainly due to an increase in Quercus. Ferns values are markedly lower and Concentricystes still occurs in this zone, while Dinoflagellates are absent from this subzone. The pollen concentration is between 98 and 103 grains/g. 4.3.4. Zone 2 (450-290 cm, 18872-16963 cal yr BP) This zone displays a dramatic increase in arboreal pollen, specifically Pinus. In contrast, herbs have decreased sharply, with a marked decline in Artemisia (<10%). Quercus is the most important lower montane broadleaved tree taxon, while there is a slight decrease in the lowland sub-montane tropical components and

mid-montane mixed broad-leaved forest taxa, except for Pterocarya, Liquidambar and Myrica. Ferns have decreased slightly and Dinoflagellates are present in the record again. Pollen concentrations decrease markedly in this zone from 103 grains/g to 62 grains/g. 4.3.5. Zone 3 (290-10 cm, 16963-8069 cal yr BP) Arboreal pollen is most prominent in this zone, with Pinus still being by far the most important taxon while mid-montane mixed broad-leaved forest taxa have declined. Quercus is still the most important broadleaved forest taxa, although it has slightly declined. Herb abundances are markedly lower, with key taxa such as Artemisia, Poaceae, Chenopodiaceae and Compositae recording values of less than 1%. There is a dramatic increase in spores, an obvious decrease in Concentricystes and a slight increase in Dinoflagellates in this zone. Pollen concentration has dropped from 62 grains/g to 33 grains/g in this zone. 5. Discussion 5.1. Pollen transport mechanism based on marine surface pollen data In order to understand the pollen transportation and deposition mechanism for the SCS, the total pollen and spore concentration data is presented on a regional map (Fig. 5). Pollen concentrations generally higher than 500 grains/g are only located in the sites near the Pearl River estuary, within the paleo-Pearl River Delta (Pang et al., 2007). These pollen quantities are more than fifty times higher than the other samples located on the continental shelf and in the deep ocean. Three sections have been identified based on

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Fig. 6. Relationship between concentration and distance offshore for total pollen, Pinus and Artemisia in surface samples and core STD 235.

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Fig. 7. Surface pollen distribution of Pinus, Artemisia and fern spores.

their location and pollen concentration values (Fig. 5). Section A is situated close to the Pearl River estuary with extremely high concentration. Section B is located near the Taiwan Strait and continental slope with water depth less than 1000 m. Section C is found in the deeper ocean, and the water depth is more than 1000 m. The nearest distance offshore has been measured using the Near analysis function from ArcGIS to determine the relationship between pollen concentration and distance from the coast (Fig. 6). The key finding is that pollen concentration values gradually decrease as the distance increases from the coast, with consistently low values recorded when the distance to shore is greater than 150 km (i.e. in section C). The key influence on pollen representation in the SCS appears to be related to the Pearl River, with this catchment providing the main transport mechanism of pollen to the SCS and

resulting in much higher concentration values in section A (~550 grains/g) compared to section B (<50 grains/g) and section C. Such characteristics have been noted elsewhere, in northern California (Heusser, 1988), northeastern Australia (Moss et al., 2005) and the Gulf of Lions (Beaudouin et al., 2007) based on other surface pollen datasets, which suggests that fluvial pollen transport is a key mechanism for pollen deposition in these marine settings. As noted above, the concentration of all of pollen taxa (including Pinus etc.) and spores is extremely high in the Pearl River mouth while pollen concentrations in the marine areas far from the coastline and in the deep-sea regions are very low (Fig. 6). This can dramatically impact pollen representation in the marine sediments and is particularly demonstrated by the high relative values (percentage) of Pinus in the deep-sea region (Fig. 7 A and a). In these

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Fig. 8. Percentages of selected pollen taxa and taxon groups plotted by age.

areas, which are more than 150 km from the fluvial pollen source, wind transportation mechanisms become far more important and Pinus, which has strong aeolian transport capabilities, becomes overrepresented in the low pollen concentration sediments of the deep-sea region (Luo et al., 2014; Zheng et al., 2013). In addition, surface ocean currents can also impact pollen representation by interrupting and/or altering river flow into the SCS and may favor spore representation in the deep ocean sediments due to their lighter weight and greater buoyancy (Dai et al., 2014; van der Kaars and De Deckker, 2003; van der Kaars, 2001).

5.2. Changes of key pollen types through the LGM to deglacial period 5.2.1. Pinus as an index of sea level change High values of Pinus are usually found in modern surface marine sediments in the northern SCS (Dai et al., 2014; Luo et al., 2014; Sun et al., 1999). The reason for such high percentages and concentrations of Pinus in the surface sediments from the Pearl River estuary is that they are derived from plantations within the broader catchment. However, in the fossil record, Pinus pollen increases, markedly before forestation practices, from the glacial to deglacial period. A similar trend is observed but with much lower concentrations, in the Quaternary deep ocean sediments during deglacial periods (Sun and Li, 1999; Sun et al., 2003; Zheng et al., 2013, 2011).

However, the modern distribution of Pinus at the genus level extends from the subtropical to temperate zones and has a large amplitude of temperature thresholds (Zheng et al., 2008). Therefore, it is difficult to suggest that climate variation alone is responsible for the high Pinus representation in deglacial deep ocean sediments. Our findings suggest that alterations in sea-level during deglacial periods play a key role in Pinus representation in deep ocean sediments. That is, as sea-level rises, the distance to the coast increases and this increases the relative amount of Pinus pollen in the marine sediments due to the strong capacity for pine pollen, with its buoyant air bladders, to be transported by aeolian mechanisms relative to other pollen taxa (Fig. 7). Similar results have been found in the Gulf of Lion (Western Mediterranean Sea) (Beaudouin et al., 2007) and the Okinawa Trough (Zheng et al., 2013, 2011). During glacials, a drop in sea-level reduces the distance to shore, and fluvially derived pollen from more coastal vegetation is better able to reach the deep-sea. In contrast during interglacials, relative wind input increases and aeolian- derived pollen grains, particularly Pinus, become dominant in pelagic sediments.

5.2.2. Herbaceous pollen as an indicator of strengthening terrestrial input The pollen spectra reveal that there were highest values of herbs, especially Artemisia, in the LGM (pollen zones 1). Surface

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Fig. 9. Comparison of the present pollen proxies with the sea surface temperature in the South China Sea (Wei et al., 2007), the relative sea level change (Lisiecki and Raymo, 2005), and regional monsoon index of cave stalagmites (Wang et al., 2001).

pollen analysis of northeastern SCS has suggested that nonarboreal pollen grains have greatest representation close to the Pearl River estuary. Herbs values, particularly Artemisia, sharply decrease the further away from the Pearl River estuary (Fig. 7B and b), suggesting that the increased representation of Artemisia during LGM was a result of a closer fluvial source to the site. A closer fluvial source is suggested also by the higher organic content and lower calcium carbonate values found in the record during the LGM indicating that the sediments have a great terrigenous input during this period. Organic carbon in ocean sediments is mostly derived from terrestrial materials, and variation of sedimentary TOC content depends on the input of terrestrial organic matter (Mei et al., 2010; Zhisheng et al., 2011). CaCO3 is mostly sourced from the authogenic marine materials, like shells and foraminifer (Wang et al., 1999). The schematic diagram of pollen concentration, TOC, CaCO3 as well as the aquatic algae Concentricystes from core STD235 clearly indicates higher terrestrial input from 25 to 18 ka BP (Fig. 3). Conversely, the absence of terrestrial algae and aquatic pollen, as well as lower pollen concentrations and TOC during the deglacial period suggests that terrestrial river input had declined and wind derived pollen had become more important. 5.3. Pollen provenance analysis High percentages of Artemisia have been documented in marine cores from the East China Sea and the Okinawa Trough (Kawahata et al., 2011; Xu et al., 2013; Zheng et al., 2013, 2011) during glacial periods. Our fossil pollen results also records relatively high Artemisia values (more than 30%) during the LGM period, in contrast to our modern surface samples (Figs. 6 and 7) and terrestrial pollen data from terrestrial locations in south China (Zheng et al., 2007). This phenomenon has been recorded in the other records from the northeastern SCS (Dai and Weng, 2015; Sun and Li, 1999; Sun et al., 2003; Chang et al., 2013) as well, but the source of Artemisia is disputed based on the discrepancy between the surface pollen analysis and fossil data from the marine records. It has been suggested that the dominance of Artemisia during the LGM was due to this taxon growing on the exposed northern continental shelf (Sun et al., 2003; Chang et al., 2013). Another view suggests that the Asian winter monsoon transported Artemisia from the eastern continental shelf (Dai and Weng, 2015; Dai et al., 2015). Using the comparison between the modern pollen surface samples and the

fossil pollen data from the STD 235 marine core we can gain a better insight into the possible cause of higher Artemisia representation during the LGM. The results of this comparison show that pollen concentration was as high as >100 grains/g during the LGM, and suggests that the paleo-shoreline was located less than 80 km from the core (Fig. 6). Therefore, based on such a high concentration during the LGM, the herbs cannot have been transported from further away, for example on the eastern continental shelf of China. The source of herbs, especially Artemisia in the LGM, was most likely derived from the paleo-Pearl River or other paleo-rivers like the Han or Min at present within the eastern Guangdong and Fujian Provinces, but which flowed across the exposed Taiwan Strait and northeastern continental shelf during the LGM. From seismic surveys, there is a paleoPearl River Delta in the estuary of the modern Pearl River and paleo-delta in the Taiwan Strait, as well as a series of submarine and buried channels (Horng and Huh., 2011; Ma et al., 2015; Pang et al., 2007; Peng and Yao, 1993). These paleo-channels along with ocean currents would have been favourable for the transportation of terrestrial sediments to the deep ocean when the continental shelf was extensively exposed during the LGM (Wang et al., 1995; Tija, 1996; Woodroffe and Horton, 2005). As the distance from the inflowing river to the deep ocean had been shortened, there had been a corresponding increase in herb representation. In contrast, pollen representation during higher sea level periods was characterized by long-distance (aeolian) transport from more distant source areas (Fig 7 A and a). This scenario indicates that changes in sea-level have played a key role in pollen representation and terrestrial sediment input for the SCS during the late Quaternary period. 5.4. Terrestrial vegetation changes during the 25 -8 ka period 5.4.1. The last glacial maximum (25 ka-18 ka) It has been mentioned that there is a key debate about whether the SCS continental shelf during the LGM was dominated by grassland or subtropical/tropical forest (Dai and Weng, 2015; Dai et al., 2015; Chang et al., 2013; Sun and Li, 1999; Sun et al., 2003). Resolving this debate is important for understanding regional climates, with grassland reflecting a cooler and dryer LGM (Sun et al., 2003) and forest representing climate conditions not too different from the present day (Dai and Weng, 2015; Dai et al., 2015). The

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combination of surface pollen and fossil pollen data here demonstrates that pollen representation is directly impacted by transport mechanisms, with distance from shore and paleo-river channels playing major roles. Based on our results, particularly the dominance of herbaceous taxa and consistent representation of arboreal taxon pollen, it is suggested that the exposed SCS continental shelf was dominated by grasslands and wetlands with a sparse tree cover rather than representing either pure grassland or tropical/subtropical forest. In relation to the question of whether conditions were dryer during the LGM, the presence of high herbs on the exposed SCS continental shelf would support a decline in precipitation at this time. The dominance of Artemisia in the record suggests precipitation less than 500 mm, around half that received in the area today, as estimated from its present day distribution (Zheng et al., 2008). The presence of both Artemisia and Poaceae in the northern SCS (Dai and Weng, 2015; Dai et al., 2015; Sun et al., 2003), and the Toushe basin in central Taiwan (Liew et al., 2006) also support dryer conditions. The expansion of grassland especially in the northeastern continental shelf of SCS may be explained, at least partially, by the influence of Taiwan Island as a rain shadow during the LGM. Such records of lower precipitation suggest a much weaker summer monsoon in the south China during the LGM. Similarly, evidence for decreased precipitation through increased herb representation is also seen in records from the Okinawa Through, East China Sea (Xu et al., 2013, 2010; Zheng et al., 2013, 2011). This suggests that the weakening summer monsoon was synchronous across the region and potentially linked to a significant sea-level fall during LGM. However forests may have been maintained in eastern Japan and Philippines due to orographic rainfall on their narrow continental shelf (Bian et al., 2011; Igarashi et al., 2011). 5.4.2. Deglacial period (18e8 ka) There is an obvious difference in vegetation and environments in the deglacial period, with a dramatic decrease in herbs and a sharp decline in pollen concentrations due to the greater distance to shore that developed during this period. This is in contrast with other regions of East Asia, such as the islands of Philippines or Taiwan, which showed increases in mangrove and tropical forest pollen taxa (Bian et al., 2011), both of which are rare in our record during the deglacial. Instead the STD 235 record has high percentages of Pinus as well as the significant representation of lower mountain evergreen forest including Quercus, Castanea-Castanopsis, Hamamelis, Eurya, Helicia and Syzygium (Fig. 8). These pollen types are only now recorded in the natural vegetation of hills and mountains in the Jiangxi and Fujian Provinces of South China (Yue et al., 2015; Zheng et al., 2014). Therefore, we suggest that there has been a change in the pollen source area during the deglacial towards the hills and mountains mainly in the south China with the pollen being transported from southeast China to the study area. The significant decrease in herbs combined with the high percentage of subtropical trees (Fig. 8) points towards a strengthening of the Asia summer monsoon, a feature also recorded in the regional monsoon index of cave stalagmites (Wang et al., 2001), sea level change (Lisiecki and Raymo, 2005) and the sea surface temperature in the South China Sea (Wei et al., 2007) (Fig. 9). In addition, the lower montane evergreen trees in the deglacial period expanded over the hills or mountains of South China, reaching modern elevations of 600e1500 m during this period. 6. Conclusions The pollen record from core STD 235 in north SCS has revealed

paleoenvironmental changes from the LGM to the deglacial period, with variation in pollen source areas caused by sea level changes. The most abundant forest taxon has been Pinus, especially in the deglacial. Of secondary importance were taxa from lower montane evergreen forest, mainly Quercus, Castanea-Castanopsis, as well as lowland and sub-montane forest taxa like Hamamelis, Eurya, Helicia and Syzygium. Mid-montane mixed forest was moderately represented, mainly by Alnus, Juglans, Liquidambar and Pterocarya. Among herbs, Poaceae, Compositae, Polygonum and Chenopodiaceae and especially Artemisia, had significant representation, particularly during the LGM. In order to promulgate pollen sources of marine sediment and their transport mechanism, a total of 273 surface pollen samples from the Pearl River estuary, continental shelf, slope and deep-sea areas were studied. Results demonstrate that the Pearl River has been the most important means of transporting pollen from the continent to the SCS. The marine sediments near the river mouth contain extremely high pollen concentrations in comparison with the other areas on the continental shelf and deep ocean basin. The sharp decrease in pollen concentrations from the Pearl River Estuary suggests that distance from shore is crucial for pollen representation in marine sediments. Windblown pollen was more important in the deep ocean basin samples as the highest percentage of Pinus and influence of ocean currents that interrupted and/or totally stopped fluvial input. The comparison of pollen concentration between surface and fossil record made it possible to determine the changes in pollen flux and source. Results indicate that pollen concentrations were as high as >100 grains/g during the LGM suggesting that the shoreline was within 80 km. Therefore, the pollen source area was largely from the exposed continental shelf (120 m relative to present) and pollen was transported by the paleo-rivers across the north continental shelf and Taiwan Strait to the study site. Vegetation during the LGM on the exposed continental shelf of north SCS was predominantly grassland and/or wetland with sparse subtropical trees. The widespread grass communities on the coastal-shelf of the SCS have reflected the lower precipitation during the LGM. Similar changes were also recorded on the continental shelf of the East China Sea, suggesting a synchronous decline in the effectiveness of the Asian summer monsoon. All evidence in the deglacial period points towards a strengthening of the Asia summer monsoon. The expansion of pollen source was from the hills or mountains of south China in the deglacial period, company with lower montane evergreen trees reaching the modern elevation range of 600e1500 m during the deglacial, indicating an increase in temperature during the deglacial period. Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant No.41506049, 41472143, 41230101) and the project of China Geological Survey (SN: 1212011220115) as well as the Key Laboratory of Marine Mineral Resources of Ministry of Land and Resources in Guangzhou Marine Geological Survey (No. KLMMR-2014-A-02). References An, Z., 2000. The history and variability of the East Asian paleomonsoon climate. Quat. Sci. Rev. 19, 171e187. http://dx.doi.org/10.1016/S0277-3791(99)00060-8. Bai, Y., Huang, T.H., He, X., Wang, S.L., Hsin, Y.C., Wu, C.R., Zhai, W., Lui, H.K., Chen, C.T.A., 2015. Intrusion of the Pearl River plume into the main channel of the Taiwan Strait in summer. J. Sea Res. 95, 1e15. http://dx.doi.org/10.1016/ j.seares.2014.10.003. Barrows, T.T., Juggins, S., 2005. Sea-surface temperatures around the Australian margin and indian ocean during the last glacial maximum. Quat. Sci. Rev. 24, 1017e1047. http://dx.doi.org/10.1016/j.quascirev.2004.07.020.

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