High-resolution palynological record for vegetation and environment change during MIS 2 in the southern South China Sea

Marine Micropaleontology 151 (2019) 101769

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Research paper

High-resolution palynological record for vegetation and environment change during MIS 2 in the southern South China Sea

T

Sazal Kumara,b,c, Chuanxiu Luoa,b, , Rong Xianga,b, Jianguo Liua,b, Chixin Chend, Xiaodan Fange ⁎

a

Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China c University of Chinese Academy of Sciences, Beijing 100049, China d Guangzhou Marine Geological Survey, Guangzhou 510075, China e School of Geographical Sciences, Guangzhou University, Guangzhou 510006, China b

ARTICLE INFO

ABSTRACT

Keywords: Last glacial maximum Continental shelf Sundaland Winter monsoon East Asian monsoon

Vegetation evolution and corresponding climate changes during Marine Isotope Stage (MIS) 2 on the exposed shelves in the southern South China Sea (SCS) region are still controversial. Palynomorph data, including percentage, concentration, and influx were used to reconstruct the history of vegetation and environment changes during MIS 2 and MIS 1. It recommends three significant changes occurred during these periods at 21.51 cal kyr B.P., 17.82 cal kyr B.P., and 12.36 cal kyr B.P. The findings indicate that during MIS 2 and the start of MIS 1, the northeastern part of exposed shelves were covered with tropical rainforests. Ferns were present in ground flora and sub-canopy of tree cover in the rainforests. In the coastal areas, mangrove forests survived up to the start of MIS 1 and then disappeared due to the rapid rise in sea level. Grassland was present to a limited extent and probably in the beach areas only. Winter monsoon was strong, although the temperature was lower than at present. From 11.70 cal kyr B.P., ferns were the dominating vegetation while herbs disappeared completely. Trees of lower and upper montane rainforests were present to some extent. The climate was warm, sea level rose, the continental shelf was submerged in the ocean, and the summer monsoon became stronger than in previous times. This study supports the previous works from the southern SCS region and suggests that a combination of pollens with other proxies give a better understanding of vegetation and climatic dynamics in the past.

1. Introduction In the western Pacific, the South China Sea (SCS) is still an attractive region for paleoenvironmental and paleoceanographic researches. Since the sediments deposited on the sea floor in the southern SCS over a long period are affected by the East Asian Monsoons (EAMs) and the evolution of the Western Pacific Warm Pool (WPWP), they also provide an essential insight of the global climate change. Marine Isotope Stage (MIS) 2 (from 24 to 11 cal kyr B.P.) is the coldest part of the last glaciation, is also the early stage of Last Glacial Maximum (LGM) (24–18 cal kyr B.P.) (Martinson et al., 1987). The transition from MIS 2 to MIS 1 (11 cal kyr B.P.-present) occurred at about 14.5 cal kyr B.P. (Railsback et al., 2015). The climate in these periods (MIS 2 and MIS 1) in the SCS was dynamic, as in most places in the world (Dai and Weng, 2015). Reconstructions of vegetation and environment during the period could pursue a geological basis to predict future climate change and boundary conditions for global paleoclimate modeling (Wang et al., 2012).

Recent studies have shown that the tropical ocean (like the SCS) plays a crucial role in global climate change (Wang et al., 2001a). To date, the changes in the environment and the variations in EAMs in this region since MIS 2 have been recognized in several paleoclimatic proxies (Liu et al., 2011; Wang et al., 2014; Wei et al., 2007; Yu et al., 2000). Deep-sea pollen records in the SCS have been studied to depict the vegetation evolution on land and on emerged continental shelves during a glaciation (Sun et al., 2000; Sun and Luo, 2001; Wang et al., 2009). Between 25 and 15 cal kyr B.P., a GIS-based vegetation map of the world showed that China had relatively little tree cover, whereas the vegetation was dominated by grassland in the low landscape, and trees were sparsely distributed in the uplands across southern China (Ray and Adams, 2001). On the contrary, from a palynological study, it is inferred that the exposed shelves (e.g., Sunda Shelf) in this period were covered by lowland rainforest (Li and Sun, 1999; Sun et al., 2002). Wang et al. (1999) studied sediments from the seabed in the SCS targeting the East Asian monsoon history.

⁎ Corresponding author at: Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. E-mail address: [email protected] (C. Luo).

https://doi.org/10.1016/j.marmicro.2019.101769 Received 6 March 2019; Received in revised form 17 July 2019; Accepted 1 August 2019 Available online 02 August 2019 0377-8398/ © 2019 Elsevier B.V. All rights reserved.

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Moreover, from the study of the coccolithophores (e.g., Florisphaera profunda) and through empirical orthogonal functions Su et al. (2013) found that the evolution of the East Asian Summer Monsoon (EASM) was due to the precession (~20 kyr). Furthermore, sea surface temperature (SST), sediment accumulation rates and source of sediments in the southern SCS (from core NS07-25) have been studied based on trace and major metal analysis (Liu et al., 2011). The SST in the SCS was lower and about 1 °C cooler as estimated by foraminiferal faunal transfer techniques (Chen et al., 2005) than as estimated by the CLIMAP Project (1981). In southern China, there was no evidence of temperature fluctuations, but moisture changes occurred at the last glacial (Wang et al., 1990). Although, controversy remains about the vegetation and climate change during MIS 2 in the SCS. This dispute may be due to the difference in proxies and lack of high-resolution data to elucidate the vegetation evolution in the lowland and exposed landmasses. In this study, we chose a drilling core on the slope of Sundaland in the southern SCS for pollen analysis. Our study investigated the regional vegetation evolution in the continent and shelves, and their response to climate change during MIS 2. We further investigated the dynamics in EAM based on palynofloral composition and compared it with other paleoclimatic proxies.

Strait, and with the Indian Ocean through the Strait of Malacca and Strait of Sunda. With a complex bathymetry, the water depth in the SCS varies significantly, with a maximum depth of approximately 5000 m and an average depth of 1200 m (Shaw and Chao, 1994). The southern part of the SCS is in the middle transition zone of the Western PacificEast Indian Ocean Warm Pool (collectively referred to as the WPWP); it is the place of origin of the East Asian summer monsoon and the intersection of the East Asian monsoon, the South Asian monsoon, and the Australian monsoon. Controlled by the equatorial low-pressure zone, high temperature (average annual temperature of 28–30 °C) and high humidity characterize the southern SCS. The annual precipitation is > 2800 mm, and rainfall is abundant. Tropical rainforests and seasonal forests are distributed on the southern islands around the SCS, such as Sumatra, Kalimantan, Indochina, the Malay Peninsula and the Philippine Islands (Florin, 1963) (Fig. 2). The islands around the Sunda shelf, such as Sumatra, Java, and Borneo are mainly covered with tropical rainforest vegetation (Wang et al., 2009). The vegetation changes with the change in altitude. For example, tropical lowland rainforests, which are dominated by Dipterocarpaceae, are distributed below 1000 m above sea level. Fagaceae, Lauraceae, Hamamelidaceae, and other lower montane rainforest species are distributed from approximately 1000 to 2000 m; alpine rainforest and shrubbery are distributed in the mountainous areas above 2400 m, and they are mainly Vaccinium, Myrsine, and Rhododendron. Tropical montane coniferous trees, such as Podocarpus, Dacrycarpus, Phyllostachys, and Phyllocladus, are distributed in mountainous areas above 1800 m. On the surface of Mt. Gunung at Kerinci, Sumatra, there is a large amount (about 40%) of Dacrycarpus at 1900–2100 m above sea level (Newsome, 1988). Mangroves such as Rhizophoraceae, Sonneratia, Avicennia are found along the coasts and river mouths of Sumatra, Kalimantan, Indochina, the Malay Peninsula, and the Philippine Islands (Kitayama, 1992).

2. Study site and environmental setting The core NS07-25 (6°39.945′N, 113°32.936′E, water depth 2006 m) is located in the southern SCS (Fig. 1). The SCS is one of the largest marginal seas in the western Pacific. It connects with the Pacific Ocean through the Luzon Strait, with the East China Sea through the Taiwan

3. Materials and method 3.1. Coring and sampling The length of gravity piston core NS07-25 is 556 cm and was obtained from the northwest slope of Nansha Trough by the R/V Shiyan 3 in May 2007 by the South China Sea Institute of Oceanology, Chinese Academy of Sciences (Fig. 1). Through comparison of oxygen isotope analysis of benthic foraminifera and AMS14C planktonic foraminifera chronostratigraphic tests, a high-resolution chronostratigraphic sequence of nearly 40 cal kyr B.P. was developed from which about 26 cal kyr B.P. was analyzed in more details using palynomorph content (Table 1, Xiang et al., 2009). For the current study, the core sediments were sampled at 4 cm intervals (from 0 to 60 cm core depth) and 1 cm intervals (from 60 to 200 cm core depth) in 2014 and 2015. A total of 148 samples were then palynologically analyzed covering a resolution of 178 years per sample Fig. 3. 3.2. Laboratory analysis Dried samples of 4–10 g were processed for pollen and spore analysis. The samples contained small amounts of clay and slightly larger amounts of sand. A hydrofluoric (HF) acid treatment was used to remove siliceous materials from the samples. After that, a heavy liquid (HBr, KI, Zn) with a specific gravity of 2.0 (d = 2) was used to separate the spores and pollen by flotation. Finally, the pollen was concentrated through a 7 mm nylon sieve with ultrasonic treatment. Pollen and spores were counted under 400× magnification, and the identification was performed using a 1000× oil immersion lens, mostly to the family level. The pollen reference collection in the laboratory of Quaternary Paleoenvironment and several palynomorph identification keys (Wang et al., 1995) were used for identification. More than 200 pollen grains were counted for each sediment sample, except for a few

Fig. 1. Locations of core NS07-25 (Yellow dot) in the southern South China Sea. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2

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Fig. 2. Vegetation type map of Southeast Asia (Poliakova et al., 2014).

samples in which the pollen quantity was insufficient to reach the standard for statistics. In those cases, > 1000 Lycopodium spores were counted to validate the procedure.

The pollen or spore concentrations per unit volume of sediment, R, were calculated with this equation:

R = (27,637/ Lycopodium spore counted per slide)

3.3. Palynomorph percentages, concentration, and influx calculation

× (Pollen or Spore count per slide/Volume of each sample)

The total pollen numbers and the percentages of core sediment samples were calculated. The pollen percentage was based on the pollen sum, including herbs, trees, and aquatic algae, whereas spore percentage was based on the total number of fern spores. To calculate the pollen concentration, a Lycopodium spore tablet with a known number of 27,637 ± 563 spores was added before the experiment as a trace.

(1)

The number of Lycopodium spores counted in a sample served as a reference to normalize the concentration values (Zhang et al., 2002a, 2002b). This Lycopodium allowed us to compare the values between sediment samples directly. Palynomorph influx was calculated with the following equation:

3

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zones (Grimm, 1987).

Table 1 Radiometric ages of core NS07-25 in the southern South China Sea (modified from Xiang et al., 2009). 14

Depth (cm)

Materials

Conventional (kyr B.P.)

4–5 10–11 29–32 57–60

G. sacc P. obli + N. dut P. obli + N. dut G. sacc + N. dut + G. mena P. obli + N. dut + G. mena + G. sacc Most planktonic foraminifera

5165 ± 34 10,282 ± 29 12,890 ± 38 14,153 ± 46

5480 ± 40 11,200 ± 40 14,520 ± 200 16,320 ± 120

15,680 ± 49

18,620 ± 70

22,175 ± 195

26,310 ± 210

98–102 197–202

C age

4. Results

Calendar age (cal kyr B.P.)a

4.1. Palynological results A total of 91 genera of spores and pollen from 148 samples in the core NS07-25 from the Nansha Trough were identified, where there are 9 genera of fern spores, 61 genera of tree pollen, 20 genera of herb pollen, and Concentricytes. According to the ecological distribution of all species of pollen, pollen types (except for Pinus) can be divided into the following ecological groups:

G. sacc = Globigerinoides sacculifer; G. menna = Globorotalia menardii; N. dut = Neogloboquadrina dutertrei; P. obli = Pulleniatina obliquiloculata. a All ages were converted to calendar years with the CALPAL 2007 Hulu Software (Joris and Weninger, 1998).

(1) Spores: fern spores are plentiful, containing large amounts of Microlepia, Cyathea (tree ferns), Pteris, Polypodiaceae, Lygodium, and others. (2) Herbs: There are Chenopodiaceae, Poaceae, Araceae, and Commelinaceae. (3) Upper Montane Rainforests: The main types are Podocarpus and Dacrydium, reflecting relatively cold and wet habitats. (4) Temperate Forests: The primary type is Tsuga. (5) Lower Montane Tropical Rainforests: This group includes Castanopsis (Castanopsis/ Lithocarpus/ Castanea), Dipterocarpaceae, and other types. (6) Tropical Lowland Rainforests: The main types are Euphorbiaceae, Magnoliaceae, Hamamelidaceae, Palmae, Proteaceae, and Araceae. (7) Mangroves: This group includes Rhizophoraceae (mangrove) and Sonneratia. (8) Algae: Concentricytes. 4.2. Pollen assemblages according to pollen zones

Fig. 3. Age-depth curve of core NS07-25 (After Xiang et al., 2009).

Palynomorph influx = R × Sedimentation rate

(2)

Sedimentation rate = Depth/Age

(3)

Based on the CONISS results, the whole sequence was vertically divided into four pollen zones (Fig. 4). The important characteristics of each zone are as follows: Zone A (137–200 cm; 21.51–26.39 cal kyr B.P.) is characterized by the highest abundance of ferns such as Polypodiaceae (average: 73.64%), and Microlepia (average: 20.85%), followed by tree pollens including Pinus (average: 24.94%), Castanopsis (average: 16.90%), and Rhizophoraceae (average: 18.53%). The percentage of other taxa are relatively low (Fig. 4).

Palynomorph percentage, concentration, and influx diagram were constructed using TILIA software (version 2.0.41). Constrained Incremental Sum of Squares (CONISS) was adopted to make pollen

Fig. 4. Pollen and spore percentages in core NS07-25 in the southern SCS (The pollen percentage was based on the pollen sum including herbs, trees, and aquatic algae; spore percentage was based on the total fern spores). 4

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Fig. 5. Pollen and spore concentration (grains/cm3) of core NS07-25 in the southern SCS (red shade indicates 5 times exaggeration). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Pollen and spore influx (grains cm−2 kyr−1) of core NS07-25 in the southern SCS.

Total pollen concentration ranges between 8 and 58 grains/cm3 and that of spores between 24 and 210 grains/cm3 (Fig. 5). Total pollen influx ranges from 106 to 749 grains cm−2 kyr−1, among which Pinus influx is 11–257 grains cm−2 kyr−1. Total spore influx is between 309 and 2714 grains cm−2 kyr−1 (average: 1203 grains cm−2 kyr−1) among which Microlepia and Polypodiaceae are dominant taxa. Among the trees, mangroves (belonging to Rhizophoraceae) show the highest pollen influx (267 grains cm−2 kyr−1), followed by Pinus (257 grains cm−2 kyr−1) and lower montane rainforest especially Castanopsis (185 grains cm−2 kyr−1). Herb pollen influx shows a fluctuation (Fig. 6). Zone B (85–136 cm; 17.82–21.44 cal kyr B.P.) is also dominated by ferns. The average fern percentage is 73.59%, among which Polypodiaceae is 76.86% and Microlepia is 15.82%. Average fern percentage is relatively lower than that of zone A (average: 78.06%). Among the tree pollens Pinus (average: 31.47%), Rhizophoraceae (average: 11.79%), and Castanopsis (average: 19.28%) are dominant (Fig. 4). Tree pollens are higher than in zone A; however, herb

percentage decreases more than in zone A. Total pollen concentration ranges between 6 and 75 grains/cm3 (average: 33 grains/cm3) which is relatively higher than in zone A. Additionally, spores show an increase in concentration which ranges between 10 and 262 grains/cm3 (average: 105 grains/cm3) (Fig. 5). Total pollen influx ranges between 78 and 1317 grains cm−2 kyr−1 (average: 459 grains cm−2 kyr−1), which is almost three times higher than in zone A. On the contrary, mangrove pollen influx (the influx rates of Rhizophoraceae and Sonneratia) is almost the same as in zone A. The herb influx rate is lower than in zone A; however influx rates of Poaceae are higher than in zone A. Total spore influx ranges between 129 and 4722 grains cm−2 kyr−1 with an average value of 1530 grains cm−2 kyr−1. Fern spores (except Polypodiaceae) and herb pollens reduced while tree pollens increased in this period (Fig. 6). Zone C (17–84 cm; 12.36–17.76 cal kyr B.P.) is also dominated by higher fern and tree percentage. However, the average fern percentage (71.82%) is lower than in previous zones. Among tree pollens, Pinus 5

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(average: 53.08%), Rhizophoraceae (average: 4.82%), and Castanopsis (average: 9.09%) are prevalent. The average percentage of Polypodiaceae and Microlepia are 69.98% and 17.91% respectively, which indicates an increase in Microlepia and a decrease in Polypodiaceae percentage than in previous zones. Percentages of herb pollen are still lower, but the Poaceae appeared with the highest average value of 3.57%. Percentage of Concentricytes is the highest (average: 4.88%) among the other zones (Fig. 4). Total pollen concentration ranges between 2 and 49 grains/cm3 (average: 15 grains/cm3) and that of spores are between 13 and 169 grains/cm3 (average: 47 grains/cm3) which are relatively higher than in zone B, but lower than in zone A (Fig. 5). Total pollen influx ranges between 35 and 889 grains cm−2 kyr−1 (average: 249 grains cm−2 kyr−1) which is lower than both in zone A and zone B. Total spore influx ranges between 81 and 3049 grains cm−2 kyr−1 (average: 814 grains cm−2 kyr−1). In this zone, all other taxa show lower values than in previous zones (Fig. 6). Zone D (0–13 cm; 2.62–11.70 cal kyr B.P.) is the youngest zone, which is also characterized by higher fern and tree percentage. Average fern percentage is about 88.15% among which Polypodiaceae and Microlepia are 52.40% and 40.25% respectively. In this zone, the percentage of Polypodiaceae is the lowest while Microlepia. shows the highest value. Among tree pollens, Pinus is still the dominant genus (average: 30%), followed by Tsuga (average: 14%), Dacrydium (average: 10.67%), Castanopsis (average: 10%), and Rhizophoraceae (average: 4%). Herbs and Concentricytes are absent in this zone (Fig. 4). Total pollen concentration ranges between 7 and 35 grains/cm3 (average: 14 grains/cm3), which is the lowest among all the zones. Besides, the total spore influx range is 12–329 grains/cm3, with an average value of 129 grains/cm3 (Fig. 5). Total pollen influx ranges between 8 and 39 grains cm−2 kyr−1 (average: 17 grains cm−2 kyr−1), which is also the lowest value among all the zones. All the herb taxa and most of the tree taxa disappear, unlike Pinus and taxa of lower montane tropical rainforests such as Podocarpus, Castanopsis, Castanea, and Rhizophoraceae (Fig. 6). Whereas fern spores are still present in a reasonable amount.

ferns in the sub-canopy of high tree cover (Pérez-Paredes et al., 2014). Moreover, tree ferns require high rainfall and minimum temperature to grow (Sosa et al., 2016). Therefore, ferns were present in ground flora and sub-canopy of tree cover on the exposed Sundaland. Among the tree pollens, Pinus and Castanopsis are dominant in this zone. They were also deposited in the deep-sea basin, just like sediment particles and fluvial transport. This pattern of vegetation indicates regional vegetation prevailed on the shelf. Herb pollens such as Poaceae, Chenopodiaceae cannot be transported from so far, indicating a local expansion of grassland community (Yang et al., 2016). The presence of mangrove pollens (Rhizophoraceae and Sonneratia) imply that mangroves survived along the coastline during the glacial periods. Ferns also dominate zone B (85–136 cm; 17.82–21.44 cal kyr B.P.), but lower in percentage than in zone A. Total pollen influx is also lower than in the previous zone. However, Pinus pollen percentage increased with a decrease in herb pollen percentage. Lower montane tropical tree pollen percentage increased more than in zone A. However, ferns showed a gradual decrease in abundance indicating changes in vegetation and climate. In addition, along the coastline mangroves were present but in less amount than in zone A. On the contrary, based on palynology from one core in Malay Peninsula and six cores from Southern Borneo, Morley and Flenley (1987) proposed a trans-equatorial savannah corridor during the LGM. The corridor stretched from Thailand, through Malay Peninsula, central Sundaland, to the Lesser Sunda Islands (Bird et al., 2005; Earl of Cranbrook, 2010; Meijaard, 2003; Morley and Flenley, 1987; Wurster et al., 2010). However, the Species Distribution Model (SDM) of Dipterocarp species supported by palynological evidence, dynamic vegetation models, extant mammal and termite communities, vascular plant fatty acid isotopic compositions, and stable carbon isotopic compositions of cave Guano profiles demonstrated that during the LGM a rainforest was present in the central part of exposed Sundaland instead of grassland (Raes et al., 2014). Besides, deep-sea core studies on pollen also indicate the presence of rainforest in the northern Sundaland (Sun et al., 2002; Wang et al., 2009). Additionally, Dipterocarp species grow well in the rainforest, with a few exceptional species found in forests of Indochina (Smitinand and Santisuk, 1981). Interestingly, in this study we have also found a reasonable amount of Dipterocarpaceae during 26.39 to 17.82 cal kyr B.P. (Fig. 4). Therefore, during the LGM, exposed shelf (the northern part) was covered with tropical rainforests and ferns prevailed in ground flora and sub-canopy of tree cover as it was in zone A. In zone C (17–84 cm; 12.36–17.76 cal kyr B.P.), fern spores showed the lowest percentage while Pinus showed the highest percentage among all the zones. Poaceae also showed the highest percentage (average: 3.57%) among all other zones. Pollen of tropical lowland rainforest taxa showed lower values in percentage (Fig. 4). Nevertheless, total pollen and spore influx show a fluctuation. Whereas, Pinus showed a higher percentage and influx. Since Pinus is primarily transported by winds from a long distance and higher abundance of Pinus pollen during the Holocene in the marine sediment explains that vegetation and pollen transportation are similar to those at present (Sun and Li, 1999). In the southern SCS, Pinus pollens originate from Indochina and the Malay Peninsula and are transported by southwestern wind and southwestern surface current (Luo et al., 2016). Besides, during interglacial periods, airborne pollens (e.g., Pinus) become dominant in the marine sediments due to increase in relative wind input (Yu et al., 2017). Therefore, during 12.36–17.76 cal kyr B.P. Pinus pollens were transported by wind from Indochina and the Malay Peninsula, but it did not grow on the Sundaland itself. The highest percentage of fern spores is the main feature of zone D (0–13 cm; 2.62–11.70 cal kyr B.P.) indicating the increase in humidity and temperature. Fern spores may be transported from the Borneo region by water, as they are suitable for water transportation over a long distance (Dai and Weng, 2011; van der Kaars, 2001; Luo et al., 2019). In addition, fern abundance is often used to denote the humidity of the

5. Discussion 5.1. Source of palynomorphs All of the recovered pollen and spores must have been transported from land by water and wind to the deep-sea basin. Considering seabed topography and other environmental factors on the land, Luo et al. (2016) revealed the modern pollen sources in the Southern SCS. In greater water depths (> 200 m) the pollen and spores originate from the Northern Kalimantan Islands. During the LGM sea level was approximately −123 m lower than the present day (Hanebuth et al., 2009) and a large land mass called Sundaland was exposed near our study site which delivered vast amounts of terrigenous material as well as spores and pollen by the giant fluvial river system “Paleo-Sunda River” (Hanebuth et al., 2011). During the period, 16.5 to 13.9 cal kyr B.P. pollen and spores were transferred from the Northern Kalimantan Islands and from the Nansha Islands around 7.0 cal kyr B.P. (Luo et al., 2016; Wang et al., 2009). 5.2. Pollen and vegetation Ferns and trees dominate pollen assemblages in zone A (137–200 cm; 21.51–26.39 cal kyr B.P.). Ferns grow well in humid places, especially in the shade and wet forest environment or forest environment (Nondorf et al., 2003). Their spores are larger in size and lighter in density than those of tree pollens and hence are transported mainly by water (van der Kaars, 2001) a long distance from its source (Dai and Weng, 2011). In addition, the presence of tree fern (Cyathea) in the pollen assemblages confirm that vegetation was prevailed by 6

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Fig. 7. Comparisons between pollen influxes (grains cm−2 kyr−1) of Pinus, total tree, herb, the percentage of fern based on total palynomorph (%) and the oxygen isotope curve (δ18O) from the NS07-25 core (Xiang et al., 2009), SCS. Dashed line indicates the onset of climate warming.

environment, with more fern spores indicating a more humid climate (Farjon and Filer, 2013). Moreover, the climate warming led to the increase of rainfall, so the number of fern spores brought by rivers also increased, which indicated that the climate showed a warm and wet trend during this period. Combined with the results of Luo et al. (2019), it is known that the total influx of pollens dropped significantly. Additionally, in this zone, Pinus pollen percentage reduced about two times than the previous zone (Fig. 4). This fall in pollen influx and Pinus abundance is an abrupt change in palynomorph assemblage. Almost all tree pollens disappeared, but temperate forest taxa (Tsuga) dramatically showed the highest value. However, lower and upper montane tropical rainforest taxa were present to some extent (Fig. 4). These phenomena prevailed may be due to the submersion of exposed shelves because of the increase in sea level which in turn changed the pollen source area. The rise in sea level during 12.75 and 4.20 cal kyr B.P. was 60 m below modern sea level and 5 m above modern sea level respectively (Hanebuth et al., 2011). Herb pollen percentage became zero indicating an increase in the distance between coastline and study site.

source area. The slope receives palynomorphs from a wide area through the fluvial system, whereas shelf indicates a local source of palynomorphs (Wang et al., 2009). Therefore, we assume that grassland was present in this period to a limited extent and may be in the beach areas only. However, palynological studies from south China (Maar lake core TY1) and northern SCS (sediment core ODP1144) have demonstrated that during the LGM herbs were primary vegetation in these regions (Sun et al., 2003; Zheng and Lei, 1999). Nevertheless, a recent study based on modern marine pollen and fossil pollen from sediment core STD 235 in the northern SCS has speculated that exposed shelves were prevailed by wetland and/or grassland with sparse sub-tropical trees (Yu et al., 2017). In addition, the Sahul shelf, northeastern Australia, Singapore, and Sumba Island in eastern Indonesia were covered by grass and shrub vegetation indicating a dry environment during the LGM (DiNezio and Tierney, 2013; Kershaw et al., 1993; Moss et al., 2017; Partin et al., 2007; van der Kaars et al., 2010). This contrast in vegetation in southern SCS shelves with northern SCS shelves, shelves in Southeast Asia, and northeastern Australia could be due to the following four reasons. Firstly, orographic effects of the Island Mountains, which blocked vapor transportation from the sea (Sun et al., 1999). Secondly, out-of-phase monsoon response to the precessional forcing between the northern (Sunda shelf) and southern (Sahul shelf) hemispheres (Wang, 2009). Thirdly, the local summer position of heavy precipitation band associated with ITCZ and during the LGM, the southern SCS (Sunda shelf) was located in the ITCZ (McGee et al., 2014). Whereas, northern SCS, Southeast Asian shelves, and northeastern Australia were located outside of the ITCZ (McGee et al., 2014). Finally, the core NS07-25 was located in the WPWP during the LGM (Luo et al., 2019). It should be noted that WPWP is the largest source of atmospheric water vapor and one of the highest rainfall places on the earth (Niedermeyer et al., 2014), which might provide sufficient humidity. A large number of tropical taxa and fern plants indicate a hot and humid environment (Luo and Sun, 2013; Sun et al., 2003). In addition, increase in the relative abundance of Pinus pollen indicates a rise in sea

5.3. Vegetation and climate From the above discussions, it can be speculated that, during MIS 2 and the start of MIS 1 that covers the LGM, exposed shelves were covered with tropical rainforests and ferns were present in ground flora and sub-canopy of tree cover in the northeastern part on the exposed Sundaland. The palynomorph source was the exposed shelves. Along the coastline areas mangrove forests survived up to the start of MIS 1 and then disappeared. Palynology of core 17,964 on the southern continental slope also indicates that tropical lowland rainforest and mangroves were present during the LGM (Sun et al., 2002; Wang et al., 2009). Whereas, pollen analysis of core 18,323 on the continental shelf indicated a decreasing trend of lowland rainforest and expansion of herbs during the LGM (Wang et al., 2009). The contrast in pollen assemblages between shelf and slope is due to the difference in the pollen 7

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level (Yu et al., 2017) and an increase in tree pollen influx especially, Pinus pollen influx also indicates enlargement of winter monsoon (Luo and Sun, 2013). In this study, Pinus percentage and influx rose from zone A to zone C covering 26.39–11.70 cal kyr B.P. and then dropped again. It indicates the gradual enlargement of winter monsoon from 26.39–11.70 cal kyr B.P., and then summer monsoon became stronger. Besides, the percentage of Castanea shows a similar trend in increase as Pinus that indicates colder climate during the LGM than at present when compared with current tropical semi-evergreen seasonal rainforest. From 11.70 cal kyr B.P. (i.e., during the Holocene) total pollen influx became very low, herb percentage became zero, but fern spore percentage increased dramatically (Fig. 7). In addition, almost all tree pollens disappeared, but temperate forest taxa (Tsuga) dramatically showed the highest value. This may be inferred as warming of the climate, rising in sea level, and submersion of the continental shelf. The study of foraminifera in the same core also indicated that during the LGM, a stronger EAWM prevailed in the southern SCS (Xiang et al., 2009). Moreover, both the stalagmite δ18O record from Hulu cave (Wang et al., 2001b) and benthic δ18O data from NS07-25 core (Xiang et al., 2009) showed a higher peak during the LGM than Holocene indicating stronger EAWM precipitation over EASM precipitation during the LGM. Another core (17961-2, water depth 1968 m) in the slope of Sundaland revealed that terrigenous markers (n-nonacosan and n-hexacsan-1-ol) were eight times higher in the LGM than in the Holocene (Pelejero et al., 1999). Higher terrigenous markers were brought to the study site by higher precipitation through paleo-river systems on the exposed Sundaland during the LGM (Pelejero et al., 1999). Clay mineral study of core NS07-25 (Liu et al., 2011) and core ODP 1143 (Tamburini et al., 2003) in the southern SCS also indicate stronger EASM, high sea level, humid and warm conditions during the Holocene.

sedimentation rates in the core NS07-25 range from 0.73–17.83 cm kyr−1 during the periods MIS 1 and MIS 2. Between 26 and 18 cal kyr B.P. sedimentation rate was the highest (17.83 cm kyr−1), which is consistent with the higher pollen influx values (Table 1). A nearby core (CG2 core) in the study area also suggests a higher sedimentation rate in the southern SCS due to the intensified monsoon rainfall and southward shift of the ITCZ is responsible for a relatively higher sedimentation rate (Huang et al., 2016). However, the total pollen influx is low at the beginning of MIS 2 and then gradually increases to the highest value at the age of 16.60 cal kyr B.P., and then abruptly falls to a minimum value at the beginning of MIS 1, which explains slow cooling and rapid warming in glacial cycles (Sun et al., 2003). 6. Conclusions Pollen analyses of marine core NS07-25 in the southern South China Sea were performed to elucidate the vegetation and climate change since MIS 2. Palynomorph percentage, concentration, and influx values were used to reconstruct the vegetation and climate changes since MIS 2. Three significant changes in climatic events occurred at about 21.51 cal kyr B.P., 17.82 cal kyr B.P., and 12.36 cal kyr B.P. During MIS 2 and at the very beginning of MIS 1 exposed shelves (the northeastern part) were covered with tropical rainforests. Ferns were present in ground flora and as a sub-canopy in the rainforests on the exposed shelf. Along with the coastline areas, mangrove forests survived during this period; in the beach areas, grasses were present to a minimal extent. Winter monsoon was strong during the MIS 2, but at the beginning of MIS 1, the onset of climate warming is observed, which is an indication of the strengthening of the summer monsoon. The higher pollen influx may be due to the higher sedimentation rate, higher discharge of paleo-river system due to higher precipitation, and evolution of EASM. The high sedimentation rate is due to the emergence of the Sunda shelf and paleo-Sunda River during MIS 2, but the mechanism responsible for the high sedimentation rate is still unknown. The overall vegetation and climate in MIS 2 and MIS 1 are in line with the different studies conducted in the southern SCS region. A combination of different proxies with pollens give a better understanding of vegetation and climatic dynamics.

5.4. The onset of climatic warming The transition from MIS 2 to MIS 1 is marked by Younger Dryas (YD) as illustrated in Fig. 7. From 14.5 cal kyr B.P. herb pollen influx dramatically disappeared whereas, fern percentage began to rise markedly. In addition, Pinus and total tree pollen influx show a decreasing pattern from the YD. This decrease in tree pollen influx is attributed to the onset of climate warming, strengthening of the summer monsoon, and rising in sea level. The record of planktonic foraminifera in the core NS07-25 also shows abrupt warming at around 14.7 cal kyr B.P. (Xiang et al., 2009), which is similar to our findings.

Acknowledgments We are thankful to the editor, Professor Richard Jordan and the anonymous Reviewer for their constructive comments and suggestions. This work was funded by the National Natural Science Foundation of China (grants NSFC 41876062, NSFC 41676047, NSFC 41661144003, and 91228207), the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA11030104), and Innovation Development Fund Projects of Innovation Research Institute on South China Sea Ecological and Environmental Engineering, Chinese Academy of Sciences (no. ISEE2018PY02).

5.5. Higher pollen influx Pollen concentration is the relative amount of pollen or spore deposited in the unit area, whereas, pollen influx reflects the sheer amount of pollen or spore that reaches the deposition site. Nevertheless, pollen influx largely depends on sedimentation rates as well as pollen concentration. In this study, pollen influx is relatively higher due to the higher sedimentation rates, although the relationship between them is not straightforward. Sedimentation rate largely depends on the monsoonal activity. Possibly, the higher sedimentation rate is due to the emergence of the Sunda shelf and paleo-Sunda River (Wang et al., 2014) as well as higher precipitation during MIS 2 (Pelejero et al., 1999). A tremendous amount of terrigenous sediments was delivered by the paleo-Sunda River to the deep-sea basin during the glacial period but shifted to the Northeastern corner in the Holocene (Liu et al., 2011). Although the mechanism responsible for the high sedimentation rate is still unknown. Probably, the exposure of the Sunda shelf had a significant influence on sedimentation rate (Tjallingii et al., 2010). Because, it stopped the exchange of hot saline water between SCS and indo-pacific through the straits of Karimata, Malacca, and Sunda (Steinke et al., 2006). Another factor could be the decrease in relative sea level and an increase in monsoonal rainfall, although the summer monsoon was weak at that time (Huang et al., 2016). The

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