三沙永乐龙洞洞底沉积速率变化及其影响因素

三沙永乐龙洞位于西沙永乐环礁，是目前世界范围内已探明的最深蓝洞，其内部沉积动力环境相对单一，是研究沉积速率变化的天然实验室。龙洞洞底约24 cm以浅沉积物粒度分析结果显示，洞底浅埋沉积物以砂质组分为主，平均粒径多介于22.9~123.9 μm之间，在表层、4 cm、9.5 cm以及21 cm深度分别出现了沉积物变粗、砂含量增多的现象；利用210Pb的CRS定年模式计算得到底层沉积物沉积时间为1896年，平均沉积速率为0.19 cm/a，在4 cm和9.5 cm深度沉积速率急剧增加。柱状样顶部沉积速率显著增加，与近年来西沙地区人类活动加剧吻合。沉积物粒度变粗以及沉积速率骤然加快现象，与西沙地区台风活动频繁相关，通过对比历史台风记录，验证了沉积物在1999-2001年以及2010-2011年所记录的6次台风事件:199902、199915、200110、201002、201005以及201118。推测龙洞洞底沉积物沉积速率主要受台风活动影响，近期有人类活动影响的痕迹。

Sedimentation rate and its impact factors revealed through a pushcore at the bottom of the Sansha Yongle Blue Hole

Sansha Yongle Blue Hole (SYBH), known as the deepest blue hole worldwide, is located in the Xisha Yongle Atoll. Due to its comparatively quiescent internal environment, SYBH has been set as a natural laboratory for understanding how different factors impact the sedimentation rate changes. The grain sizes analyses of a 24 cm long pushcore collected at the bottom of SYBH reveals that the sediments there are primarily consisted of sand with mean grain sizes varying from 22.9 to 123.9 μm. Meanwhile significant grain size coarsening is observed 0 cm, 4 cm, 9.5 cm and 21 cm, in the pushcore. We adopte CRS model of 210Pb and related analysis to calculate the age of the sediments, and find the oldest sediment was preserved in 1896, and the mean sedimentation rate is 0.19 cm/a. Sharp increases of the sedimentation rate is observed at 4 cm and 9.5 cm. We hypothesize that such increased sedimentation rate near the top pushcore, is a result of anthropogenic influences that happened in the Xisha district within the past few years. Analysis of historical typhoon records near our study area indicates six of typhoon events presumably contribute to the grain size variation in the SYBH during 1999-2001 and 2010-2011, i.e., 199902, 199915, 200110, 201002 and 201118. Thus, we can conclude that the sedimentation rate of sediment at the bottom of SYBH is mainly affected by typhoon events and there is also anthropogenic influence recently.