涠洲岛近1500年以来孢粉炭屑记录的环境变化

过去2000年来是人类高速发展的时期，环境变化已经从自然驱动主导转变为受人类和自然共同影响。涠洲岛位于南海西北部的北部湾海域中部，受南亚热带季风性气候和频繁的台风的影响，湿地沉积发育丰富，是研究亚热带海岛气候环境变化以及揭示人类活动的理想区域。本研究通过对涠洲岛湿地钻孔（WZD）长度160 cm的剖面开展高分辨率孢粉、炭屑和粒度分析等多指标综合分析，在AMS ¹⁴C测年的基础上，同时，结合当地历史文献资料，讨论了涠洲岛近1500年的植被演化与气候变化历史，并进一步揭示了人类活动影响下的涠洲岛环境变化过程。研究结果显示，涠洲岛环境变化过程分为4个阶段：1）1471~669 cal.a B.P.（479~1281 cal.A.D.），剖面孢粉最低，优势植物基本以耐旱品种为主，表明此时期高温干旱。松花粉含量高，指示涠洲岛周围地区可能发育大量针叶林，形成以耐旱灌木、草本为主的稀树草原景观。在高温干旱的气候背景下，发生自然火灾的可能性大，此阶段的炭屑多为地方性火灾产生，涠洲岛基本保持着原始生态面貌。2）669~372 cal.a B.P.（1281~1578 cal.A.D.），孢粉浓度增加，表明木本植物类型变丰富，气候温暖湿润。该时期涠洲岛上植被茂盛，变为从稀树草原到北亚热带季节性雨林的过渡型。温暖湿润的气候以及可供用于烧火的木材增多、有利于人类迁入并定居。3）372~93 cal.a B.P.（1578~1857 cal.A.D.），喜凉的杜鹃花粉继续增加，耐寒冷的漆树科常有出现，表征此时气温进一步下降。十字花科、莴苣属等常见伴人植物花粉的出现以及剖面炭屑浓度和禾本科花粉迅速上升，说明人类在涠洲岛开始修建人工池塘，并开展水稻种植及其他农业活动。人类活动对岛上原始环境产生影响，但原始植被的防御力使得涠洲岛仍然部分保留了原始特征。4）93 cal.a B.P.~现在（1857 cal.A.D.），木本植物花粉先减少后急剧增加，热带性植物木麻黄属增多指示此期间气候转暖。此时期大致对应现代暖期。仙人掌已经成为该岛优势植物，仙人掌、樟、楝、木麻黄、金合欢组合植被成为涠洲岛特有的植被景观。此外，D＜100 μm、D≥100 μm这两种粒径的炭屑和禾本科花粉含量急剧升至整个剖面的最高峰，中值粒径迅速细化，记录了强烈的人类活动。涠洲岛近1500年以来的孢粉炭屑记录表明，在中世纪暖期期间（900~700 cal.a B.P.），涠洲岛高温干旱，海平面有所上升。在小冰期期间（500~350 cal.a B.P.），气候温暖湿润并随着海平面下降。涠洲岛气候环境变化主要受太阳辐射调控，同时受ENSO活动等因子影响。

Environmental changes recorded by pollen and charcoal in Weizhou Island since last 1500 years

The past 2000 years was a period of human's rapid development, environmental changes have changed from nature-driven dominance to the common influence of human and nature. Weizhou Island(20°54'~21°10'N, 109°00'~109°15'E) is located in the middle of the Beibu Gulf in the northwest of the South China Sea. Due to the influence of the southern subtropical monsoon climate and frequent typhoons, the wetland deposition is well developed. It is therefore an ideal area to study the climate and environment change of subtropical islands and reveal human activities. In this study, a 160-cm wetland core, named WZD(21°03'26″N, 109°07'26″E), has been collected from Weizhou Island. Multi-proxies analysis mainly including AMS 14C dating, high-resolution pollen, charcoal and particle size analysis in combination with local historical literature, provide powerful evidence for exploring paleo-environmental variability over the past ca.1500 years, as well as the history of vegetation evolution and climate change in Weizhou Island, and further revealing the environmental change process of Weizhou Island under the influence of human activities. The main conclusions are as follows: (1) 1471~669 cal.a B.P.(479~1281 cal. A.D.), it was characterized by the lowest pollen concentration, but the dominant plants were mainly drought tolerant varieties, which indicated a high temperature and drought at this period. High content of Pinus pollen suggested a large number of coniferous forests might be developed around Weizhou Island. It probably indicated a savanna landscape dominated by drought tolerant shrubs and herbs. Under the climate background of high temperature and drought, there was a high possibility of natural fire. We thus inferred that charcoal was mostly produced by local fires at this stage, and Weizhou Island basically maintained its original ecological appearance. (2) 669~372 cal.a B.P.(1281~1578 cal. A.D.), the pollen concentration and the arboreal pollen type both increased, which indicated the vegetation on Weizhou Island was lush. It became a transitional type from savanna to north subtropical seasonal rain forest, and suggested a warm and humid climate during this period. The warm and humid environment made good preparations for human landing on the island in the later stage. The warm and humid climate and the increase of wood available for fire were conducive to human migration and settlement. (3) 372~93 cal.a B.P.(1578~1857 cal. A.D.), this period was featured by the continuous increase of the cool-tolerant Rhododendron and the often appearance of the cold-resistant Anacardiaceae, which indicated a cold climate during this time. The common appearance of human-associated pollen such as Brassicaceae and Lactuca, and the rapid increase of the concentration of charcoal and the content of Poaceae, which inferred that human beings had carried out rice planting and other agricultural planting in Weizhou Island and built artificial ponds or lakes. Human beings had a certain impact on the original environment of the island, but the defensive power of the original vegetation made Weizhou Island still retain some of its original characteristics. (4) 93 cal.a B.P.~now(since 1857 cal. A.D.), it roughly corresponded to the modern warm period, and the arboreal pollen decreased first and then increased sharply, and the increase of tropical plant Casuarina, which indicated that the climate became warmer during this period. During this period, Cactus has become the dominant plant on the island, and the combined vegetation of Cactus, Cinnamomum camphora, Melia azedarach, Casuarina equisetifolia and Acacia confusa became the unique vegetation landscape of Weizhou Island. In addition, the contents of charcoal and Poaceae rose sharply to the peak of the whole profile, and the median particle size was rapidly refined, suggesting strong human activities. The climate and environmental changes in Weizhou Island in recent 1500 years recorded by pollen and charcoal reflected that during the warm period of the Middle Ages(900~700 cal.a B.P.), Weizhou Island was hot and dry, and the sea level rose. During the Little Ice Age(500~350 cal.a B.P.), the climate was warm and humid and decreased with the sea level. The climate and environment change in Weizhou Island is mainly controlled by solar radiation and affected by ENSO activities.