基于叶蜡正构烷烃重建的南海及周边地区植被类型

VEGETATION CHANGES RECORDED BY LEAF-WAX n-ALKANES AROUND THE SOUTH CHINA SEA

陆地植被类型的变迁是对地质历史时期古环境变化的响应，而植物表层产生的叶蜡则记录了这些变化。叶蜡中长链正构烷烃的参数指标CPI、∑Odd（C25-33）、ACL和为判识沉积物的来源、陆源物质供应量、植被类型变化等提供了重要参考。本文利用现代遥感技术提供的植被类型、植被覆盖度等信息，初步调查了南海周边植被类型的覆盖情况，结果表明C3植被覆盖率具有压倒性优势，而C4植被大多分布在中国云南省附近以及柬埔寨境内；南海北部表层沉积物的δ13C31记录与遥感数据得到的珠江流域平均δ13C31值较为一致，据此我们认为南海北部沉积物的记录能够反映周边河流流域的植被变迁。本文还将南海北部MDl2—3433站位作为实例，通过分析长链正构烷烃的几个参数，认为末次盛冰期早期南海海平面降低，气候相对变干，C4植被相对增多；而后C4植被丰度逐渐降低，在新仙女木事件期间植被类型存在明显波动变化，在较为暖湿的中晚全新世时期C3植被占据主导优势。

Changes of terrestrial vegetation which are recorded by leaf wax from higher plants are in response to paleoclimate changes. Several indexes of n-alkanes derived from leaf wax, including CPI, Z Odd（C25-33）, ACL and δ13Cn-alka^o are used to indicate changes of sediment sources, the terrestrial input and vegetation types. We collect the remote sensing dataset of the Land Cover Type and the Vegetation Continuous Fraction from LP DAAC, and preliminarily investigate and calculate the CJC4 plant coverage around the South China Sea （SCS）. Our results show that the C3 plants hold the dominant position, especially in South China, Taiwan Island, Kalimantan Island, Sumatra, Peninsular Malaysia and the North Vietnam, while C4 plants are mostly distributed in Yunnan Province and Cambodia. We also calculate average δ13C31 of Pearl River basin （30.6‰） and Taiwan Island （-31.7‰） from remote sensing productions. We extract n-alkanes of 19 surface samples on the northern South China Sea to research the relationship between results of the remote sensing dataset and surface sediment records on the northern SCS. δ13C31 of these surface samples ranges between -28.2‰ and -30.7‰ and is more consistent with the average δ13C31 of Pearl River basin, thus we consider that vegetation changes of Pearl River basin are recorded by n-alkanes in sediment of the northern SCS. Besides, we also show our research on samples at 0-3.5m depth （Ca.25Cal.ka B.P.） of core MD12-3433 （19°16.88＇N, 116°14.52＇E; 2125m water depth） from the northern SCS as an example to further illustrate the implications of n-alkanes indexes. The carbon preference index （CPI） values of the MD12-3433 core range from 1.6 to 4.3, with the lowest value at Late-Mid Holocene and the peak value at the Last Glacial Maximum （LGM）. During high sea level periods, a relative lower terrestrial input is counteracted by increasing accumulation of marine-source particles, resulting in reduced CPI. Thus we deem that the source of the n-alkanes was mostly from terrestrial higher plants. Z Odd （C25_33） of the MD12-3433 core drops as much as ca.580ng/g from ca.780ng/g during the LGM to ca.20Ong/g during the Mid-Late Holocene. It implies that sea-level changes act as a decisive factor in Z Odd （C25-33） in the SCS. The average chain length （ACL） of MD12-3433 core varies from 28.5 to 29.5 with an average value of ca.29.4 during the LGM, an average value of ca.28.6 during the Mid-Late Holocene. We suggest that, during the early LGM, C4 plants expanded under a relative dry climate. Afterwards, wet and warm climate enabled C3 plants to keep a high and stable level in the Middle and Late Holocene.