海洋地质与环境重点实验室知识库

通过对南海南部150个表层沉积物样品及位于巽他陆坡西部的CG2孔106个层位和东部CB19孔86个层位的积物样品进行孢粉组合、孢粉浓度、百分含量等综合研究，对南海南部孢粉来源和传播方式进行了讨论，并将CG2孔和CB19孔孢粉数据与其它古气候记录对比，重建了南海南部4万年以来植被演化历史及水文气候特征并探讨了其影响因素。具体研究内容如下：

1. 南海南部表层沉积物孢粉分布特征研究

对南海南部150个表层沉积物样品的研究发现，沉积物样品中的孢粉呈现以蕨类孢子为主，木本花粉次之，草本花粉最少的特征：1）木本花粉浓度和百分含量分布特征显示出高值区分布在距离海岸线及主要岛屿较近的位置，离海岸线越远，数值越低。其中，松属花粉可能来自南海北部周边山地，而热带高山雨林花粉的源区为其南部的婆罗洲（以东北部山区为主），由地表径流输运而来。低山雨林花粉在研究区的分布普遍偏少，这与其花粉颗粒小而致密有关，只能借助水流传播，且传播距离较短。热带低地雨林花粉分布特征表明花粉源区位于婆罗洲及纳土纳群岛等南海南部主要岛屿，传播的主要载体是地表径流及河流。红树植物花粉主要来源是纳土纳群岛及婆罗洲的海岸带，且由表层洋流来运输。2）草本植物花粉在南海的分布普遍不高，且主要来源地为纳土纳群岛及婆罗洲，并以河流运输为主。草本植物中的禾本科花粉主要分布在南沙海槽及巽他陆坡附近，可能与婆罗洲与纳土纳群岛上广泛分布的农田有关。3）蕨类孢子浓度高值区分布在巴拉望岛西海岸，婆罗洲北部海岸的南沙海槽，这一区域孢子浓度大都大于800粒/g。巽他陆架及陆坡区浓度也相对较高，一般大于400粒/g。这种离岸越远数值越低的分布特征表明，蕨类孢子的主要来源是巴拉望岛，婆罗洲，南沙群岛也有部分贡献，传播途径为河流及洋流。蕨类孢子的孢粉含量分布呈现出与浓度相反的方式，即离岸越远数值越高。首先表明在南沙群岛上的植被类型中，蕨类植物占优势；其次，因蕨类孢子体积大密度小，容易随洋流进行远距离输运。芒萁属孢子的浓度和百分含量分布特征相同，呈现距离婆罗洲北部岸线越远，数值越低的趋势，表明相比于南沙群岛婆罗洲上的森林植被破坏比较严重。4）通过对研究区内，东北向和西南向的两个表层孢粉剖面研究发现，孢粉更容易沉积在具有较高周围地形的南沙海海槽中，使得孢粉向更远距离的传输变得困难，说明海底地形对孢粉的空间分布具有强烈的影响。

2. CG2孔重建的LGM以来南海西南部植被演化及水文气候记录

CG2孔106个层位的沉积物样品进行孢粉分析，并基于8个层位的浮游有孔虫¹⁴C测年建立的年代框架，重建了南海西南部LGM来南海西南部植被演化。CG2孔的降水数据（蕨类孢子百分含量、海水剩余氧同位素和粘土矿物）可以很好的反映研究区内的水文气候变化。与邻近婆罗洲石笋数据代表的热带对流强度的变化趋势一致。研究结果表明，南海南部水文气候在H1时期，发生重大转变，蕨类植物孢子含量明显降低，草本花粉含量增多，δ¹⁸O_seawater值变重、蒙脱石/（伊利石+绿泥石）比值减小，均表明区内降水大幅减少。在进入全新世阶段后，区内降水稳步增加。根据现代松属植被的分布及松属花粉传输方式，将CG2孔松属花粉的浓度作为东亚冬季风强度替代指标。CG2孔松属花粉浓度与东亚冬季风呈现显著的正相关。在末次冰消期期间，当AMOC关闭时，强盛的EAWM甚至可以跨赤道影响ASM。将CG2孔的水文气候数据与南北半球夏季风强度数据对比发现，在南北半球间的降雨在末次冰消期时存在“翘翘板”式的变化。在AMOC关闭，北半球变冷使得南北半球间温度梯度加大，EAWM增强，且与ASM产生跨赤道的联系，并推动ITCZ的向南移动，导致研究区内较为干旱的条件。现代ENSO的变化对南海南部的降水变化有显著影响，经对比发现，LGM以来，赤道太平洋ENSO-like状态也会影响南海南部的降水。尤其是末次冰消期时，El Niño-like状态可能是CG2水文资料逐渐响应AMOC陡峭变化的主要原因。植被的破坏能够减少陆地的蒸发量，从而降低研究区内岛屿上的对流降水。在末次冰消期时，海平面“扮演”植被破坏者的角色，影响植被分布的同时也减少了陆地降水。植被在冰消期海平面上升时期的破坏也可能是H1事件降水减少的一个因素。

3. CB19孔重建的四万年以来南海东南部植被演化

CB19孔86个层位的沉积物样品进行孢粉分析，并基于4个层位的浮游有孔虫¹⁴C测年建立的年代框架，重建了南海东南部4万年来南海东南部植被演化的历史。研究结果表明，在MIS 3阶段，婆罗洲北部水热条件相对适宜，婆罗洲上植被的垂直分布结构维持在相对稳定的水平，热带高山雨林、低地雨林、热带低山雨林和红树林均有分布，且覆盖面积保持相对稳定。MIS 2期时，虽然降水保持稳定，但气温大幅降低，热带高山雨林植被的扩张，压缩了热带低山雨林和低地雨林的生存空间。虽然海平面降低，但较短的陆架不足以为热带低山雨林和低地雨林提供足够扩展空间，同时陡峭的陆架破折地区，也严重影响了红树植物的生长。CB19孔孢粉数据中重复出现较高含量的高山雨林花粉与北半球高纬度的Heinrich事件相对应。通过与其位于热带地区的温度记录进行对比，发现热带高山林成分（以罗汉松属为主）的相对含量，能够用来作为热带地区千年尺度气候变化的有效指标，并揭示了赤道附近南海周边岛屿上的植被对北半球高纬度地区千年尺度的快速冷事件的响应模式。CB19花粉序列中，两个松属花粉的浓度峰值出现在25 ka和31 ka，与Heinrich事件有很好的对应关系，分别对应H2和H3，同时符合黄土记录的东亚冬季风强度变化，进一步验证了来自南海南部的松属花粉浓度可以作为东亚冬季风强度的指标。

综上，通过对表层沉积物和柱状样孢粉的综合研究结果显示，表层沉积物孢粉样品中，除松属来源较广外，其他孢粉主要来自南海南部以婆罗洲为主的岛屿，为CG2孔和CB19孔孢粉重建区域古气候、古环境演化提供孢粉来源的基础。两个柱样站位的松属花粉的浓度，是良好的冬季风强度指标，其高值与北半球高纬冷事件相对应。婆罗洲的植被以高山雨林向山下扩张的模式响应千年尺度的气候变冷。以蕨类含量为指标的降水数据显示，南海南部降水H1时期大幅减少，与南半球的降水增加相对应，表明ITCZ的向南移动，但其对北半球高纬冷事件的平缓响应，表现出El Niño-like状态对研究降水的调节作用。

Through the study of the 150 samples of surface sediments in the southern South China Sea by analyzing pollen assemblage, pollen concentration and pollen percentage, it can be inferred the pollen source and the transport mechanisms. The vegetation evolution history, hydrological and climatic characteristics of the southern South China Sea were reconstructed by analyzing pollen data of sediment core CG2 and core CB19 and comparing with other paleo-climatic records.

1. Pollen and spores distribution of surface sediments in the southern South China Sea

In the study of 150 surface sediment samples in the southern South China Sea, it is found that the pollen and spores in the sediment samples was dominated by pteridophyte spores, followed by trees pollen. The characteristics of pollen and spores distribution were as follows: 1) the distribution characteristics of trees pollen concentration and percentage showed that the high value area was located near the coastline, and the farther away from the coastline, the lower the value. Among them, pine pollen may come from the mountains areas around the northern South China Sea, while the source area of the tropical high mountain rainforest pollen is the southern Borneo (mainly in the northeastern mountainous area), which is transported by surface runoff. The distribution of low mountain rainforest pollen is generally less in the study area, which is related to the small and dense pollen grains, which can only be transported by means of water flow. The pollen distribution characteristics of the tropical lowland rainforest indicate that the pollen source area is located in the southern islands of the South China Sea, such as Borneo and Natuna Islands, and the main carriers are surface runoff and rivers. The main source of mangrove pollen is the coastal zone of the Natuna Islands and Borneo, and is transported by surface ocean currents. 2) Herb pollen are scarce in surface sediments of the Southern South China Sea, and mainly derived from Natuna and Borneo. Poaceae pollen mostly distributed in the Nansha Trough and Sunda slope, and probably related to the widespread distribution of farmland in Borneo and the Natuna Islands. 3) The high concentration of fern spores is located in the Nansha trough on the west coast of Palawan and the northern coast of Borneo, where the concentration of spores is greater than 800 grains/g. The Sunda shelf and continental slope concentrations are also relatively high, typically greater than 400 grains/ g. This distribution pattern shows that the main source of fern spores is Palawan, Borneo and the spores carry by river and ocean current. The distribution of percentage of fern spores showed the opposite pattern to it concentration. This pattern reflects the dominance of ferns in the Nansha Islands vegetation and the spores can be long distance transport by ocean current. The concentration and percentage of Dicranopteris spore have the same pattern of distribution and display a decreasing trend from the North of Borneo to Nansha Islands which reveal the deforestation in the Borneo. 4) Based on the study of two pollen profiles in the study area, it is found that pollen and spore are more likely to be deposited in the Nansha trough with higher surrounding terrain. The higher surrounding terrain makes it difficult to transfer pollen, which indicate the seafloor topography has a strong influence on the spatial distribution of pollen.

2. Vegetation evolution and hydrological climate records in the southwestern South China Sea since LGM

Precipitation data (fern spore percentage, δ¹⁸O_seawater, smectite/(illite + chlorite) ratio) inferred form sediment core CG2 well reflect hydroclimate changes in the Southern South China Sea and consistent with the trend of tropical convective intensity derived from Borneo stalagmite. Hydroclimate in study area during Heinrich stadial 1 change significantly, and low fern spore percentage and high herb pollen content accompany with heavier δ¹⁸O_seawater and lower smectite/(illite + chlorite) ratio, indicate precipitation reduce drastically. According to the distribution of modern Pinus vegetation and Pinus pollen transport, the Pinus pollen concentration of core CG2 was used as a indicator for EAWM intensity. The Pinus pollen concentration of core CG2 was positively correlated with the EAWM. During the last deglaciation, when the AMOC was shutdown, the strong EAWM could even affectted the ASM across the equator. Comparing the hydrological and climatic data of the CG2 with the north and south hemisphere summer monsoon intensity, it is found that the rainfall in the north and south hemispheres is in the form of “seesaw” in the last glacial. During the Heinrich stadial 1, the AMOC is closed which make northern hemisphere cool and strengthens the East Asian winter monsoon. Enhanced East Asian winter monsoon can flow cross-equatorial and increase the Australian summer monsoon. This process drives ITCZ move southward and reduces rainfall in the study area. Although the change of modern ENSO has a significant effect on the precipitation change in the southern South China Sea, the precipitation change since the late glacial period in the study area is not consistent with the long-term change of the ENSO state. The destruction of vegetation can reduce the amount of land evaporation, thereby reducing the convection precipitation in the study area. In the last deglacial, sea level "played" the role of deforestation, affecting the distribution of vegetation while also reducing land precipitation. The destruction of vegetation during the rise of the sea level in the deglacial may also be a factor in the reduction of precipitation of H1 events.

3. Vegetation evolution in the southeastern South China Sea since 40,000 years

The pollen sequence of core CB19 showd the evolution of vegetation in the southern the South China Sea during the late last glacial period. The results showd that, in the MIS 3, the hydrothermal conditions in northern Borneo were relatively suitable. The vertical distribution structure of vegetation on Borneo was relatively stable, and it was distributed in high mountain rainforest, tropical lowland rainforest, tropical low mountain rainforest and mangrove forest. During MIS 2, although the precipitation remained stable, the temperature was greatly reduced, and the expansion of the tropical high mountain rainforest vegetation compressed the living space of the tropical low mountain rainforest and lowland rainforest. Although the sea level was reduced, the shorter shelf is not enough to provide sufficient expansion space for the tropical low mountain rainforest and low land rainforest, and the steep slope area also affects the growth of mangrove. Repeated high-level high mountain rainforest pollen in core CB19 corresponds to the Heinrich event. By comparing it with the temperature records in the tropics, the relative content of tropical high mountain rainforest components (mainly Podocarpus) can be used as an effective indicator of millennium-scale climate change in the tropics, and revealed the response patterns of vegetation on the millennium-scale rapid cold event in the high latitudes of the Northern Hemisphere. In the CB19 pollen sequence, two peak concentrations of the Pinus pollen appeared at 25 ka and 31 ka, which corresponded with the Heinrich event 2 and 3, and consistent with the change of EAWM intensity recorded by loess. It is further verified that the Pinus pollen concentration in the southern South China Sea can be used as an indicator of EAWM intensity.