ELSEVIER

Marine Geology 156 (1999) 227–244

A pollen record of the last 37 ka in deep sea core 17940 from the northern slope of the South China Sea Xiangjun Sun a,b,Ł , Xun Li a a

Institute of Botany, Chinese Academy of Sciences, Xiangshan, Beijing, 100093, China b Laboratory of Marine Geology, Tongji University, Shanghai 200093, China Received 17 April 1998; accepted 8 July 1998

Abstract This paper presents the palynological record of deep sea core 17940 from the South China Sea (20º070 N, 117º230 E, water depth 1727 m). This core is about 13.30 m long and covers the last 37 ka. A total of 103 samples were counted at 10 cm intervals with a time resolution of ca. 360 yr. Three pollen zones are recognized. Zone P1 (14 C age ca. 37.0–15.0 ka B.P., 13.06–8.70 m) is characterized by the expansion of montane conifers (Tsuga, Picea and Abies) and colonization by temperate grassland, mainly Artemisia, on the exposed northern continental shelf, as well as by frequent natural fires indicated by high values of charcoal concentrations. This implies that the climate of this period was cool and dry. Frequent alternations of montane conifers and grassland dominated by Artemisia indicate that oscillations of comparatively cool and humid conditions with temperate and dry conditions occurred. According to the oxygen isotope data, the period corresponds to stage 3 and the Last Glacial Maximum. Climatic warming (P2-a, 8.70–7.23 m, 15.0–11.3 ka B.P.), associated with a rapid rise in sea level from ca. 14 ka B.P. is inferred by expansion of tropical–subtropical broad-leaved forest with a replacement of grassland by mangroves (mainly Rhizophora and Sonneratia). This time interval may be assigned to the Bølling–Allerød warming stage. Subsequently, climatic cooling reflected by expansion of both montane conifers and upper montane rain forests (mainly Podocarpus and Dacrydium) during the period of ca. 11.3–10.0 ka B.P. (P2-b, 7.23–6.6 m) may be correlated with the Younger Dryas cold spell. The similarity of pollen assemblages during the Holocene and in the surface sediment from the northern South China Sea implies that the vegetation and climate during the last 10,000 years was close to those of the present. During the last 1400 years, human activities are reflected in the sharp increase of Dicranopteris fern spores.  1999 Elsevier Science B.V. All rights reserved. Keywords: palynology; environmental change; Last Glaciation; Holocene; South China Sea

1. Introduction Among various environmental proxies, pollen and spores, together with charcoal, are unique in providing information on terrestrial vegetation in marine Ł Corresponding

author. Fax: C86 10 625 90 833; E-mail: [email protected]

records. Of particular interest is the pollen record in marginal seas which directly bridge continental and marine palaeoenvironmental studies on the basis of high-resolution stratigraphy. In the China Sea, however, marine palynology has been restricted to shallow water environments with neither continuous sediment records nor precise stratigraphy. The only report on deep-sea pollen is from two short sediment

0025-3227/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 9 8 ) 0 0 1 8 1 - 9

228

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

cores in the Nansha Islands area, SE South China Sea (Shen et al., 1991). Due to the absence of dating and to the large sample interval, these records are of very limited use in palaeoenvironmental reconstruction. The German–Chinese joint cruise in the South China Sea (SCS), Sonne-95, has for the first time provided high-quality samples of surface sediments and sediment cores from the deep-water parts of the SCS (Sarnthein et al., 1994). The results of palynological analysis of over 40 surface sediment samples have yielded information on pollen distribution, source areas, and transport in the SCS (Sun et al., 1999). The present paper is a brief report on variations of pollen assemblages in Core 17940, a sediment core from the northern slope of the SCS. Being the first late Quaternary pollen data from a deep-water sediment sequence in the China Sea, it has revealed significant new insight into vegetation and climate changes over the last 37 ka. This paper gives only a general outline of the results, and more detailed reports and environmental interpretations will be published later.

2. Material and methods Core 17940 (20º070 N, 117º230 E) was taken from the northeastern part of the SCS, 400 km southeast of Hongkong at a water depth of 1727 m. From the 13.30 m core length, 103 samples of 10 ml each were obtained every 10 cm. All samples were prepared for pollen analysis in the Laboratory of the Institute of Palynology and Quaternary Sciences, Go¨ttingen University, in the way briefly described in Sun et al. (1999). The AMS 14 C dates from this core are carried out at Kiel University (L. Wang et al., 1999). The 12.72 m of the core has been dated at 35.5 ka B.P. The age model of Core 17940 based on oxygen isotope data and the AMS 14 C dates (L. Wang et al., 1999) is presented in Table 1.

3. Environmental setting The northeastern slope of the SCS is located near 20ºN, west of the Bashi Strait. The slope is covered by hemipelagic sediments, with the exception of the Dongsha Islands in its uppermost part. Due to the

Table 1 Age model of core 17940, northern South China Sea (L. Wang et al., 1999) and correspondence with the pollen zones Stage

14 C

Holocene Termination I Last Glacial Maximum Oxygen Isotope Stage 3

present–10,000 0–660 P3 10,000–15,000 660–870 P2 15,000–25,300 870–1050 P1-b, P1-c 25,300–37,000 1050–1306 P1-a

age (a B.P.)

Core depth Pollen zone (cm)

prevalence of the monsoon climate, the surface water is characterized by northeastern currents in winter and southwestern currents in summer. East of the Pearl River, the third largest river in the SCS area, there are only small rivers emptying into this part of the SCS. Hanjiang River, the largest river in this area, yields only one tenth of the runoff, as much as that of the Pearl River (Fig. 1). The study area is close to the southern coast of mainland China (situated north of the study site) with Taiwan Island to the northeast. South of the Tropic of Cancer, the south China coast area is relatively flat, with hills less than 150 m a.s.l. and some isolated mountains, covered by tropical seasonal rain forest or rain forest. The annual average temperature ranges from 20º–22ºC to 25º–26ºC, the annual precipitation exceeds 1500–2000 mm. North of the Tropic of Cancer, between 24º and 27ºN the area is occupied by the Nanling Mountains; these range from 500 to 1200 m in altitude, with the highest peak at 1902 m. The vegetation mainly consists of evergreen broadleaved forest (e.g. Castanopsis, Theaceae, Lauraceae), while above 1200 m Podocarpus, Cyclobalanopsis, Betula and other taxa appear (Wu, 1980). The reader is referred to our paper on surface sediment pollen of the SCS (Sun et al., 1999) for further discussion on vegetation in the surrounding areas. In central and southern Taiwan there are mountain ranges rising above 3500 m a.s.l., descending southward to below 100 m a.s.l. along the coast. Tropical rain forest is developed under a tropical climate with high temperature and humidity. Tropical rain forest is diverse with well represented genera including Myristica and Pterospermum. On the lowlands, as the mountains are ascended, it is replaced by evergreen broad-leaved forest (Castanopsis, Lithocarpus, Magnolia, etc.). The montane conifers Abies, Picea

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

229

Fig. 1. Map of the northern South China Sea showing the main mountain ridges and rivers on the adjacent continent and islands.

and Juniperus appear between 3000 and 3600 m a.s.l. (Wu, 1980).

4. Results The hemipelagic sediments throughout the length of Core 17940 are rich in pollen and spores. A total of 161 pollen types was identified. According to their ecology, several groups of taxa can be distinguished. These include tropical and subtropical broad-leaved taxa (mainly Castanopsis, Quercus, Ilex, Altingia, Elaeocarpus, Palmae, Sapindaceae, Araliaceae, Ges-

neriaceae, etc.), temperate broad-leaved taxa (mainly Betula, Carpinus, Alnus, Juglans, etc.), montane conifers (Picea, Abies, Tsuga) and upper montane rain forest taxa (Podocarpus, Dacrydium, Phyllocladus, Dacrycarpus, etc.). 4.1. The pollen diagram Percentages of pollen and spores were calculated based on the total pollen sum of terrestrial arboreal and herbaceous plants. Aside from spores, at least 200 pollen grains were counted from each sample, and the results are presented as a pollen diagram (Fig. 2).

230

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

On the basis of downcore variations in different ecological groups, the pollen diagram is divided into three zones (P1, P2, P3) and eight subzones. 4.1.1. Zone P1 (13.06–8.7 m, ca. 37.0–15.0 ka B.P.) This zone is characterized by strong fluctuations in percentages of most pollen taxa with alternating predominance of tree and herb taxa. Large amounts of Anthoceros spores are found only in this zone. Three subzones are recognized as P1-a, P1-b, P1-c in ascending order. 4.1.1.1 Subzone P1-a (13.06–10.5 m, ca.37.0–25.3 ka B.P.) Three cycles of alternating predominance of tree and herb taxa are observed. Apart from highly fluctuating frequencies of pine pollen, the tree-dominated pollen assemblages are marked by high representations of both montane conifers (18% of Picea C Abies C Tsuga) and upper montane rain forest taxa (23.5% of Podocarpus C Dacrydium C Phyllocladus C Dacrycarpus), along with fairly high values for Anthoceros (25%) and fern spores (20%); the herb-dominated (up to 75%) pollen assemblages are distinguished by strikingly high percentages of Artemisia (up to 46.7%). Cyperaceae and Poaceae are also well represented. Pollen of tropical and subtropical broad-leaved taxa, which occur in moderately high values, fluctuate together in phase with the herb. Charcoal concentrations in this subzone are the highest for the whole profile and also fluctuate in phase with the herb taxa. This subzone corresponds to Oxygen Isotope Stage 3 (L. Wang et al., 1999). 4.1.1.2 Subzone P1-b (10.5–9.8 m, ca. 25.3–21.0 ka B.P.) The pollen frequency of montane conifers reaches its maximum in the core (Picea 15.3%, Tsuga 4.3%, Abies 5.7%), and its range of downcore fluctuations is much less than in the previous subzone. Pollen percentages of both herb and tropical– subtropical broad-leaved taxa decline sharply to minimum values in this subzone, as does the charcoal concentration. 4.1.1.3 Subzone P1-c (9.8–8.7 m, ca. 21.0–15.0 ka B.P.) Pollen percentages of herb taxa, Artemisia in particular, rapidly increase again, although they are very variable. Three rapid cycles can be recognized

in this subzone with two alternating assemblages: one is characterized by high frequencies of Artemisia pollen (30.1–42.7%), and the others by relatively high values for pollen of montane conifers (8.3– 17.9%) associated with Anthoceros spores. Mangrove pollen begin to appear, though in very low frequencies. Proportions of tropical–subtropical broadleaved taxa and charcoal concentration values moderately increase. P1-b and P1-c are assigned to the Last Glacial Maximum according to the oxygen isotope and 14 C data (L. Wang et al., 1999). 4.1.2. Zone P2 (8.7–6.6 m, ca. 15.0–10 ka B.P.) Pollen frequencies of tropical–subtropical broadleaved taxa and herb are gradually reduced relative to those of montane conifers and upper montane rain forests. Pine progressively increases through this zone. Two subzones are recognized as P2-a and P2-b. 4.1.2.1 Subzone P2-a (8.7–7.23 m, ca. 15.0–11.3 ka B.P.) This subzone is characterized by high values of tropical–subtropical broad-leaved taxa together with significant representation of Artemisia and low values or absence of both montane conifers and upper montane rain forest taxa. In the middle of the subzone (ca. 14.0 ka B.P.) appears a sharp peak in mangroves (29%) composed mainly of Rhizophora and Sonneratia. This peak is accompanied by an abrupt decline in Artemisia. Chronologically, this subzone corresponds to the Bølling and Allerød warm stages. 4.1.2.2 Subzone P2-b (7.23–6.6 m, ca. 11.3–10.0 ka B.P.) Pollen frequencies of both montane conifers and upper montane rain forest taxa rise again, reaching 11.2 and 20.6%, respectively. Those of tropical– subtropical broad-leaved and herbaceous plants decline significantly, Artemisia and Anthoceros almost disappearing. This subzone is correlated with the Younger Dryas event well recognized also in the oxygen isotope curve (L. Wang et al., 1999). 4.1.3. Zone P3 (6.6–0 m, ca. 10.0 ka B.P.–present) This zone is distinguished by an absolute dominance of pine pollen (90%). Diversity and proportions of other taxa are much lower than those during the Last Glaciation. Spore percentages of both Di-

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

Fig. 2. Pollen percentage diagram of deep sea core 17940 from the South China Sea.

pp. 231–236

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

cranopteris and Cyathea gradually increase through the zone. Three subzones are recognized as P3-a, P3-b and P3-c. The main features of this pollen zone, particularly subzones P3-b and P3-c, are quite similar to those of pollen assemblages in surface sediment from the northern SCS. This zone is assigned to the Holocene.

237

In order to aid in the interpretation of the pollen diagram, a short description of some ecological groups and individual taxa is provided.

in cool areas with distinct seasonality where moisture is plentiful during their growing seasons. Being limited by the hottest monthly temperature of 10º– 15ºC (up to 19ºC) and annual precipitation between 600–1000 mm, they dominate the boreal coniferous forests from 57º to 67ºN on the Eurasian continent. Further south, they occur only in mountainous regions, with their southernmost occurrence at 2500– 3000 m elevation in the ranges of Mt. Qingling and Mt. Dabashan in the east of China. In the west, they are generally localized around the eastern margin of the Tibetan Plateau and the mountain areas between 3000–4300 m in both western Sichuan and northern Yunnan, China (Wu, 1980). In the 1980’s, patches of Abies were found on Mt. Baishanzhu (27º450 N, 119º110 E) about 1857 m a.s.l., southern Zhejiang (WFFCCC, 1976) and at 2023 m a.s.l. on Mt. Yuanbaoshan (25º250 N, 109º100 E), Guangxi, China (Fu et al., 1980). As these Abies patches are far south of their main present distribution, they are thought to be relics from the Last Glaciation. Tsuga (Fig. 3c) favors warm, moist and shady environments, usually mixed within broad-leaved forest. In Asia, Tsuga is mostly found in subtropical mountains from the Himalayas (2500–3100 m a.s.l.) to Mt. Qingling, but reaches as far as central Kyushu, Japan (40ºN) in the north, and south of Mt. Qingling (1450 m a.s.l., 26ºN) in the south. It also appears at 2000–3000 m in the Chungyang Ridge of Taiwan (Fu and Jin, 1992; Florin, 1963). Only single grains of these three taxa have been found in the surface sediments of the SCS, probably due to the long distance from their present growth areas and ineffective transport of their pollen grains. However, during the Last Glaciation at the studied area the amount of these pollen grains increased noticeably, implying that the distribution areas of montane conifers expanded both in altitude and latitude. The shortened distance from their source areas was responsible for the increased occurrence of montane conifers in pollen assemblages during the last glacial period.

4.2.1. Montane conifers Abies (Fig. 3a) and Picea (Fig. 3b) are the dominant components amongst the alpine coniferous vegetation in both the northern boreal areas and the high mountains of the subtropical zone. They are found

4.2.2. Upper montane rain forest Taxa from this vegetation type are mainly Podocarpus, Dacrydium, Dacrycarpus and Phyllocladus. They commonly occur between 1500 and 3000 m a.s.l. in the tropics, with highest daily tem-

4.1.3.1 Subzone P3-a (6.6–4.4 m, ca. 10.0–0.79 ka B.P.) Pollen percentages of pine rise from ca. 70 to 95% through the subzone, although with significant fluctuations. The percentages of tropical–subtropical broad-leaved taxa display a similar increasing trend, but the fluctuations are even greater ranging from 1–2% to 20%. Herbaceous pollen are rare. In comparison with the Younger Dryas, montane conifers and upper montane rain forest taxa show low pollen frequencies. Spores of Cyathea and Dicranopteris are represented continuously, although in very low proportions. 4.1.3.2 Subzone P3-b (4.4–0.55 m, ca. 0.79–0.14 ka B.P.) This subzone shows much less fluctuation in pollen percentage for each taxon. Pine pollen dominate this subzone absolutely, averaging 90%. Upper montane rain forest taxa are well represented, while tropical–subtropical broad-leaved taxa decline in percentage and montane conifers are presented only by single grains. Charcoal concentration values become much lower than during glaciation. 4.1.3.3 Subzone P3-c (0.55–0 m, ca. 0.14 ka B.P.– present) This subzone is very similar to P3-b. The only difference is the striking rise of Dicranopteris spores, reaching 34.3% of the total pollen sum. 4.2. Ecological information

238

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

Fig. 3. Present distributions of the main montane conifer and upper montane rainforest taxa, compiled from Florin (1963). (a) Abies, (b) Picea, (c) Tsuga, (d) Podocarpus, (e) Dacrydium, stippled area: present distribution.

peratures between 15º and 24ºC and annual precipitation above 2000 mm (Kitayama, 1992). Phyllocladus is only found in the southern hemisphere. In China, Podocarpus (Fig. 3d) is widely distributed south of the Changjiang (Yangtze) River, mostly mixed with evergreen broad-leaved forest between 400 and 1600 m a.s.l. For Dacrydium (Fig. 3e), there

is only one species, Dacrydium pierrei, occurring in the mountains from 300 to 1600 m a.s.l. in Hainan Island (DFRPS, 1978). 4.2.3. Pinus Pinus is widespread in the northern temperate zone. In the south of China, Pinus massoniana and

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

Pinus ikedai are also components of subtropical and tropical conifers. Study from pollen assemblages in surface sediments of SCS indicates that, more than 80% of pine pollen are dominated in most of the surface samples in the northern SCS. Its distribution pattern suggests that most of them were transported through the Bashi and Taiwan Straits from relatively large source areas, probably including eastern China. Therefore, pine pollen have less ecological importance in the interpretation of the palaeoenvironment. The high abundance of Pinus in pollen assemblages during the Holocene is close to that from surface samples. It might be explained that the vegetation and pollen transportation at that time are similar to those at present. The sharp decline of pine pollen frequency during the glaciation might be the result either from the retreat of the pine tree northward due to deterioration of the climate or a change of the pollen dispersal mechanism and route as a result of the exposure of the Sunda Shelf and the change with the sea current pattern. 4.2.4. Mangroves Mangroves thrive in silt-rich and brackish-water environments along tropical coasts. Sea surface temperature (SST) and salinity are limiting factors controlling the growth of mangroves. Mangroves are only found in tropical areas where SST achieves 24ºC at least once a year (Van Campo, 1986), and are usually near river estuaries. In China, small patches of mangrove occur along the southern coast with its northern limit in the southern Fujian province (Wu, 1980). The rise of mangrove percentage in the pollen diagram is closely related to sea level rise (Ellison, 1993; Grindrod and Rhodes, 1984). 4.2.5. Artemisia Found as herb and small shrubs, Artemisia is widely distributed in temperate grasslands within the northern hemisphere, especially on sandy soils (Sun et al., 1994). High percentages of Artemisia pollen grains are characteristic of modern surface samples from northwestern China. A pollen-climate response surface for that area (Sun et al., 1996) indicated that 30–50% of Artemisia pollen require an annual precipitation of between 300 and 500 mm and an average July temperature around 15º–24ºC,

239

a climate more temperate and drier than montane conifers require. In the pollen assemblages from the surface sediments of the SCS, only a few pollen grains of Artemisia occur in rare samples adjacent to the northern continent. To our surprise, Artemisia prevails in the pollen assemblages during the Last Glaciation (zone P-1). High percentages of Artemisia ranging from 30.1 to 46.7% occurred frequently and periodically within zone P-1. Where did they come from? One could explain that pollen grains of Artemisia were transported from the nonglaciated inland which was covered by alpine vegetation units with Artemisia. From the percentage isopoll map of Artemisia pollen at 18 ka in China (Fig. 4), high frequencies of Artemisia pollen gathered in two areas far from our studied site: along the northern ranges of Mt. Qingling as components of alpine meadow and within the eastern part of Inner Mongolia grassland, and decreased sharply eastward and southward. In southeastern China (south of 24ºN, east of 110ºE) close to core 17940, only single pollen grains of Artemisia were found and are even absent at 18 ka. Moreover, Artemisia pollen were rare throughout the pollen profile of the Last Glaciation at this site (Zheng and Li, 1992, China Quaternary Pollen Data Base). Thus no sources from the nearby inland could give rise to such high ratio of Artemisia pollen in the studied core during the glacial time. One plausible explanation for such high levels of Artemisia pollen grains is that, during the Last Glaciation, they came from the exposed continental shelf of the SCS which was occupied by grassland, dominated by Artemisia, under a dry and temperate climate. 4.2.6. Anthoceros Numerous spores of Anthoceros, a small liverwort, are recorded during the Last Glaciation in this core. Among them, three species (A. levis, A. dichotomas and A. sp.) are documented in the sediment records for the first time from China. These dwarf plants (about 10 cm tall) are now distributed from the temperate zone to the tropics. In Germany, they prefer to grow in deforested areas, on the edges of forests, or on moist soil (Prof. H.-J. Beug, pers. commun.). Anthoceros levis and A. dichotomas are also reported to occur in the Heilongjiang and Liaoning provinces, northeastern China (Gao and Zhang,

240

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

Fig. 4. Pollen percentage isopoll map of Artemisia at 18 ka B.P. in China with data from the China Quaternary Pollen Data Base.

1981). Although there has been no systematic survey of Anthoceros in China, their modern distribution in Guangdong province is in hilly areas below 1000 m a.s.l., on the edges of forests, near springs and on moist soils (Prof. Wu Pengchen, pers. commun.). The low productivity of Anthoceros spores, together with their large size, thick wall and the dwarf nature of the plants, makes them difficult to transport. Accordingly, the frequent occurrence of their spores in the deposits of the last glacial suggests that Anthoceros grew on the exposed continental shelf at the lowered sea level, and that its spores were transported to the SCS by either a strong winter monsoon or by the water flow across the ground surface. With the rise of air temperature and sea level after glaciation, the continental shelves of the SCS were submerged and thus ended the ephemeral history of Anthoceros. 4.2.7. Dicranopteris This is a tropical and subtropical fern. Dicranopteris occurs only sparsely in open areas of primary evergreen broad-leaved forests. In the hilly grasslands widely scattered in southern China, Dicranopteris is the main component together with Poaceae. It is typically 30–80 cm tall, forming 50– 80% of the total cover. Such hilly grassland generally results from the destruction of forests with human in-

terference (GIB, 1976). Thus, the high representation of Dicranopteris spores in surface samples from the northern SCS is probably correlated with human activities. 4.2.8. Charcoal Charcoal is a product of plants after incomplete burning. Prior to the impact of human burning, it can be regarded as an indicator of the strength and frequency of natural fires which are closely correlated with aridity (Chen, 1986). The much higher charcoal concentration during the Last Glaciation than during the Holocene is probably caused by drier condition during glaciation.

5. Discussion On the basis of pollen data and ecological information on major taxa and vegetation types a vegetation and climate history for the last 37 ka can be reconstructed. 5.1. Last Glaciation (zone P1, ca. 37.0–15.0 ka B.P.) The pollen analysis of Core 17940 for the first time provides some insight into the vegetation covering the exposed continental shelf of the northern

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

SCS during the last glacial period. Judging from the pollen data, the exposed shelf was probably occupied by grassland, with montane conifers growing on nearby mountains. Therefore, the northern part of the SCS experienced a remarkable decrease both in temperature and humidity during the last glacial. Despite the fluctuations, the amount of charcoal was much higher than during the Holocene, suggesting that fires occurred frequently due to a drier climate. As is well known, controversy has surrounded estimates of glacial temperatures derived from marine and terrestrial records in low-latitude areas (Anderson and Webb, 1994). The CLIMAP data show only little, if any, change in SSTs in the low-to-middle latitude areas of the Western Pacific (CLIMAP Project Members, 1976), whereas terrestrial records from New Guinea, Java and Sumatra suggest a significant cooling (6º–8ºC) during the LGM (Webster and Streten, 1978; Stuijts et al., 1988). Core 17940 provides new evidence for considerable glacial cooling and drying in the low-latitude SCS where the micropalaeontological and isotopic analyses have also revealed a remarkable cooling in winter SST at the LGM (L. Wang and P. Wang, 1990; P. Wang et al., 1995). This SST cooling in the northern SCS has been confirmed by recent alkenone palaeotemperature analyses showing a LGM=Holocene SST contrast of 4º–4.5ºC (Huang et al., 1997; P. Wang, 1998). Moreover, the glacial exposure of vast shelf areas and the decrease of SSTs would have greatly reduced evaporation from the sea and hence strengthen aridity in China (P. Wang et al., 1997). The pollen records from Core 17940 strongly support these conclusions. However, glacial aridity has been found only on the northern shelf of the SCS, not on its southern shelf, as shown by our studies on Core 17964 (Li, 1997). Of particular interest are the sub-orbital frequency variations in vegetation and climate recorded here. As can be seen on Fig. 2, the pollen diagram displays an alternating predominance of two groups of pollen taxa during the glaciation (below 8.7 m), suggesting frequent changes in the character of the vegetation covering the exposed continental shelf. The first group consists of montane conifers, upper mountain rain forest taxa and Anthoceros, indicating a relatively cool and humid climate. The second group comprises herb, tropical and subtropical

241

broad-leaved taxa together with a high concentration of charcoal, implying drier and=or more temperate conditions. During the cooler stages, the distribution area of montane conifers shifted substantially southward and, together with upper-montane rain forest, migrated from higher to lower altitudes. As a result, the Mt. Nanling area was occupied by montane conifers. When the climate became temperate and drier, the widely exposed continental shelf to the north of the SCS was quickly covered by herb vegetation dominated by Artemisia, while the frequent occurrence of Cyperaceae and Poaceae pollen probably derived from swamps or wetlands scattered on the continental shelf. Taking into account the monsoon climate of this region, the alternation in vegetation is probably related to changes in the East Asian monsoon circulation. The more cold and humid, as indicated by montane conifer-dominated pollen assemblages, must correspond to a strengthened winter monsoon. There are at least four such quasi-cyclic alternations during the later part of δ18 O Stage 3 (zone P1-a, Fig. 2), and three in the LGM (zone P1-b, P1-c, Fig. 2), recorded in our pollen diagram. The period from ca. 24–21 ka B.P. in contrast, may, however, have been relatively stable. Since the pollen analysis was based on a 10 cm sampling interval, the temporal resolution of the pollen diagram during the glacial is about 360 yr and, hence, not adequate to reveal cycles at the millennial scale. Nevertheless, the quasi-cycles recorded in Core 17940 very much resemble recurring cold episodes in the North Atlantic (Heinrich, 1988; Dansgaard et al., 1993). Evidence of climate episodes correlative with Heinrich events has been found in Chinese loess sequences and interpreted as changes in the strength of the East Asian winter monsoon (Poter and An, 1995). The quasi-cyclic predominance of the second pollen group in Core 17940 may provide, for the first time, a record of glacial sub-orbital variations in the East Asia monsoon that may correspond to Dansgaard– Oeschger and=or Heinrich events. Similar recurrent events at the millennial scale have been discovered in the south part of the SCS in Core 17964 (6º100 N, 112º130 E) where the pollen diagram shows alternating predominance of upper montane rain forest and of lowland rain forest with mangrove (Li, 1997). The different expressions in the north and south of the

242

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

SCS of monsoon variations, and their possible correlation with North Atlantic events, will be discussed elsewhere. In view of the importance of climate variability, further studies with higher temporal resolution pollen analyses are required. 5.2. Termination I (zone P2, ca. 15.0–10.0 ka B.P.) This is a period during which pronounced oscillations occurred in both vegetation and climate. From 15.0 to 11.3 ka B.P. (subzone P2-a, Fig. 2), tropical–subtropical broad-leaved taxa increased remarkably in pollen assemblages, indicating that with the rise of temperature, the distribution area of montane conifers became smaller, giving way to tropical–subtropical forests. The δ18 O curve also implies a climatic warming occurred during this period. Therefore it might be comparable with the Bølling–Allerød warm stage. During subzone P2-a, a mangrove pollen spike around 14.0 ka in age is sandwiched between two short sections with high pollen contents of herbaceous and tropical– subtropical broad-leaved taxa (Fig. 2) indicative of dry and=or warm climate. The mangrove spike might represent a short event of sea level rise when grassland was replaced by mangroves during the initial part of the marine transgression. From 11.3 to 10.0 ka B.P., the percentages of tropical–subtropical broad-leaved taxa are suddenly decreased, due to the re-expansion of taxa of montane conifers and upper montane rain forest taxa, implying a substantial drop of temperature but an increase in moisture. Data from both palynology and oxygen isotopes indicate that this event is closely correlated with the Younger Dryas event. 5.3. Holocene (zone P3, ca. 10.0 ka B.P.–present) At the transition to the Holocene, frequencies of herbaceous pollen suddenly decreased, and these taxa are almost absent by the middle of the Holocene. The post-glacial submergence of the continental shelf and the increased temperature and humidity are probably responsible for this phenomenon. The simultaneous decrease of montane conifers also implies that these cold-adapted conifers retreated northward and westward, gradually being limited to the area they presently occupy. After 1400

a B.P., the conspicuous increase of Dicranopteris spores in the pollen assemblage provides evidence for the destruction of forests as a result of human interference. For the Holocene section of the pollen diagram of Core 17940, the most striking feature is the predominance of pine, making up 80% or more of the pollen sum and presenting a sharp contrast to the last glacial (Fig. 2). The glacial section of the core is distinguished by a higher diversity in the pollen assemblages, with the percentages of tropical–subtropical and temperate taxa surpassing those during the Holocene; this apparently contradicts climate cooling during the glacial. This glacial= Holocene contrast, however, is believed to be a consequence of taphonomic rather than ecological changes. The pollen assemblages of the Holocene (zone P3, Fig. 2) are very close to that in the surface sample from the same site. As shown by a sediment trap experiment (Jennerjahn et al., 1992), the surface currents driven by the NE winter monsoon bring a large amount of fine-grainal terrigenous sediments in suspension into the SCS through the Bashi Strait and maybe through the Taiwan Strait. Those are the major sources of modern deep-water sediments in the northeastern part of the SCS. The floating-prone pollen grains of pine and fern spores are effectively transported by the currents, explaining the predominance of pine in the modern, as well as the Holocene, pollen assemblages of the core. The pollen transport mechanism was different during the Last Glaciation. Because the glacial sedimentation rate was much lower than that of the Holocene in Core 17940, we infer that sediment supply from the northeastern source via the above described transport mechanism was obviously diminished or obstructed during glacial times. The grain size analyses show that the terrigenous sediments during the glacial were predominantly by aeolian dust (L. Wang et al., 1999). Therefore, pollen grains together with mineral particles might have been brought by the strengthened winter monsoon from the north to the sea. Meanwhile, river runoff was probably another mechanism responsible for pollen dispersal. Since the river discharge was much closer to the studied area during the lower sea-level stand of the glacial, a component of the pollen grains in the glacial section of the core probably was transported into the

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

sea area by rivers draining the surrounding land and islands, giving rise to a higher diversity in the pollen assemblages.

6. Conclusions Hemipelagic sediments from the northern continental slope of the SCS, as shown by our analysis of core 17940, are rich in pollen and spores, providing valuable information on vegetation and climate changes on the neighboring land. The glacial=interglacial contrast in the pollen diagram is remarkable. Both Oxygen Isotope Stage 3 and the LGM are distinguished by high percentages of montane conifers (Tsuga, Picea and Abies), herb, liverwort (Anthoceros) indicating lowered temperature and humidity. The pollen diagram also displays sub-orbital variations in vegetation and climate, suggesting alternating predominance of relatively cool=humid and dry=temperate climate cycles at the millennial scale during the glacial period. The Last Deglaciation (Termination I) experienced significant changes in vegetation and climate. The early stage (15.0–11.3 ka B.P.), corresponding to the Bølling–Allerød, is characterized by an increase of tropical–subtropical broad-leaved taxa with a spike of mangrove pollen around 14.0 ka B.P. The late stage (11.3–10.0 ka B.P.), correlated with the Younger Dryas, is marked by enhanced percentages of montane conifers and upper montane rain forest taxa. Pollen records of the Holocene are quite similar to those of the surface sediments from the northern SCS, implying that the vegetation and climate during the last 10 kyr was similar to that at present. An abrupt rise of Dicranopteris spores, a fern growing mainly on deforested areas, indicates active human interference with the vegetation over the last 1400 yr.

Acknowledgements This study resulted from the joint project ‘Monitor Monsoon’ of the Kiel University, Germany, and the Tongji University, China. The laboratory work

243

and pollen counts were undertaken in the Institute ¨ ttingen of Palynology and Quaternary Research, GO University, and financially supported by the German Ministry of Education, Science and Technology. The National Natural Science Foundation of China (Grant No. 49732060) and a key grant of the Chinese Academy of Sciences supported those research activities performed in China.We sincerely acknowledge Prof. M. Sarnthein and Dr. Wang Luejing for providing research samples and dating the studied core. Prof. H.-J. Beug is thanked for organizing the laboratory work. Prof. Wang Pinxian is thanked for discussions and advice with this paper. We also wish to thank Prof. Peter Kershaw for assistance with the English. Mrs. Hongsernant is sincerely thanked for arranging the senior author’s visit to Germany.

References Anderson, D.M., Webb, R.S., 1994. Ice-age tropics revisited. Science 377, 23–34. Chen, Y., 1986. Early Holocene vegetation dynamics of lake Barrine Basin, N.E. Queesland, Australia. Ph.D. thesis, Aust. Natl. Univ., Canberra. CLIMAP Project Members, 1976. The surface of the ice-age earth. Science 191, 1131–1137. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjornsdottir, A.E., Jouzel, J., Bond, G., 1993. Evidence for general instability of past climate from a 250-kyr ice core record. Nature 364, 218–220. Delectis Florae Reipublicae Popularis Sinicae Agendae Academiae Sinicae Edita (DFRPS), 1978. Flora reipublicae popularis sinicae. Tomus 7, Gymnospermae (in Chinese). Ellison, J.C., 1993. Mangrove retreat with rising sea level, Bermuda. Estuarine Coastal Shelf Sci. 37, 75–88. Florin, R., 1963. The distribution of conifer and taxad genera in time and space. Acta Horti Bergiani 20 (4), 121–312. Fu, L.K., Jin, J.M., 1992. China Plant Red Data Book — Rare and Endangered Plants, Vol. 1. Science Press, Beijing, New York. Fu, L.K., Lu, Y.J., Mo, S.L., 1980. The genus Abies discovered for the first time in Guangxi and Hunan. Acta Phytotaxon. Sin. 18 (2), 205–208 (in Chinese with English title). Gao, Q., Zhang, Q., 1981. Bryophytes of Northeast China. Science Press, Beijing (in Chinese). Guangdong Institute of Botany (GIB), 1976. Vegetation of Guangdong. Science Press, Beijing (in Chinese with English title). Grindrod, J., Rhodes, E.G., 1984. Holocene sea-level history of a tropical estuary: Missionary Bay, North Queensland. In: Thom, B.G. (Ed.), Coastal Geomorphology in Australia. Academic Press, Sydney, pp. 151–178.

244

X. Sun, X. Li / Marine Geology 156 (1999) 227–244

Heinrich, H., 1988. Origin and consequences of cyclic ice rafting in the Northeast Atlantic Ocean during the past 130 000 years. Quat. Res. 29, 142–152. Huang, C.-Y., Liew, P.-M., Zhao, M., Chang, T.-C., Kuo, C.-M., Chen, M.-T., Wang, C.-H., Zhang, L.F., 1997. Deep sea and lake records of the Southeast Asian paleomonsoons of the last 25 thousand years. Earth Planet. Sci. Lett. 146, 59–72. Jennerjahn, T.C., Liebezeit, G., Kempe, S., Xu, L.Q., Chen, W.B., Wong, H.K., 1992. Particle flux in the northern South China Sea. In: Jin, X.L. et al. (Eds.), Marine Geology and Geophysics of the South China Sea. China Ocean Press, Beijing, pp. 228–235. Kitayama, K., 1992. An altitudinal transect study of the vegetation on Mount Kinabalu, Borneo. Vegetation 102, 149–171. Li, X., 1997. Palynological record from deep sea core 17964 in the South China Sea. M.Sc. thesis, Inst. Bot., Chinese Acad. Sci. Beijing (in Chinese with English abstract). Poter, S.C., An, Z., 1995. Correlation between climate events in the North Atlantic and China during the last glaciation. Nature 375, 305–308. Sarnthein, M., Pflaumann, U., Wang, P.X., Wong, H.K., 1994. Preliminary report on SONNE-95 Cruise ‘Monitor Monsoon’ to the South China Sea. Geol.-Pala¨ontol. Inst. Mus., ChristianAlbrechts-Univ., Kiel, Ber. Rep. 68, 224 pp. Shen, C.M., Tang, L.Y., Sun, S.X., 1991. A study of Late Quaternary palynological data from the Nansha Sea area in the South China Sea. In: Multidisciplinary Oceanographic Expedition Team of Academia Sinica to Nansha Island (Ed.), Quaternary Biological Groups of Nansha Island and the Neighbouring Waters. Zhongshan University Publishing House, Guangzhou, pp. 239–265. Stuijts, I., Newsome, J.C., Flenley, J.R., 1988. Evidence for late Quaternary vegetational change in the Sumatran and Javan highlands. Rev. Palaeobot. Palynol. 55, 207–216. Sun, X., Du, N., Weng, C., Lin, R., Wei, K., 1994. Paleovegetation and paleoenvironment of Manasi Lake, Xinjing, N.W. China during the last 14 000 years. Quat. Sci. 3, 239–248 (in Chinese with English abstract). Sun, X., Song, C., Wang, F., 1996. Pollen-climate response surface of selected taxa from northern China. Sci. China Ser. D 39 (5), 486–493.

Sun, X., Li, X., Beug, H.-J., 1999. Pollen distribution in hemipelagic surface sediments of the South China Sea and relation to modern vegetation distribution. Mar. Geol. 156, 211–226. Van Campo, E., 1986. Monsoon fluctuations in two 20,000-years B.P. oxygen isotope=pollen records off southwest India. Quat. Res. 26, 376–388. Wang, L., Wang, P., 1990. Late Quaternary paleooceanography of South China Sea: glacial–interglacial contrast in an enclosed Basin. Paleoceanography 5 (1), 77–90. Wang, L., Sarnthein, M., Erlenkeuser, H., Grimalt, J., Grootes, P., Heilig, S., Ivanova, E., Kienast, M., Pflaumann, U., Pelejero, C., 1999. East Asian monsoon climate during the late Pleistocene: High-resolution sediment records from the South China Sea. Mar. Geol. 156, 245–284. Wang, P., 1998. Western Pacific in glacial cycles: Seasonality in marginal seas and variability of Warm Pool. Sci. China Ser. D 41 (1), 35–41. Wang, P., Wang, L., Bian, Y., Jian, Z., 1995. Late Quaternary paleooceanography of the South China Sea: surface circulation and carbonate cycles. Mar. Geol. 127, 145–165. Wang, P., Bradshaw, M., Ganzei, S. et al., 1997. West Pacific marginal seas during the Last Glacial Maximum: Amplification of environmental signal and its impact on monsoon climate. Proc. 30th Int. Geol. Congr. VSP, Utrecht and Tokyo, Vol. 13, 65–86. Webster, P.J., Streten, N.A., 1978. Late Quaternary ice age climates of tropical Australasia: interpretations and reconstructions. Quat. Res. 10, 279–309. Wanli Forest Farm, Chingyuan County, Chekiang (WFFCCC), 1976. Abies beshanzuensis M.H. Wu — a new species of Abies from Chekiang. Acta Phytotaxon. Sin. 14(2), 15-21 (in Chinese with English title). Wu, Z.Y., 1980. Vegetation of China. Science Press, Beijing (in Chinese). Zheng, Z., Li, Q.Y., 1992. Paleoenvironment and paleoclimate after pollen and spore records of the Hanjiang delta since the late Pleistocene. In: Huang, Y.K. (Ed.), Late Cenozoic Geology along the Coast of the South China Sea. Suppl. J. Sunyatsen Univ. 27, 161–172 (in Chinese with English abstract).