三沙永乐龙洞洞底沉积速率变化及其影响因素
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摘要
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22.9～123.9μm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~(210)Pb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。
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 ~(210)Pb 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.
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 ~(210)Pb 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.
引文
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[2] Steadman D W,Franz R,Morgan G S,et al.Exceptionally well preserved late Quaternary plant and vertebrate fossils from a blue hole on Abaco,The Bahamas[J].Proceedings of the National Academy of Sciences of the United States of America,2007,104(50):19897-19902.
[3] Gregory B R B,Reinhardt E G,Gifford J A.The influence of morphology on sinkhole sedimentation at little salt spring,Florida[J].Journal of Coastal Research,2017,33(2):359-371.
[4] Land L A,Paull C K,Hobson B.Genesis of a submarine sinkhole without subaerial exposure:straits of Florida[J].Geology,1995,23(10):949-951.
[5] Lane P,Donnelly J P,Woodruff J D,et al.A decadally-resolved paleohurricane record archived in the late Holocene sediments of a Florida sinkhole[J].Marine Geology,2011,287(1/4):14-30.
[6] Ortiz M,Winfield I,Varela C.First records of peracarid crustaceans from the Cayo Matias Ocean Blue Hole,SW Cuba,with the description of two new species[J].Zootaxa,2012,3505:53-66.
[7] Gischler E,Shinn E A,Oschmann W,et al.A 1500-year holocene caribbean climate archive from the blue hole,lighthouse Reef,Belize[J].Journal of Coastal Research,2008,24(6):1495-1505.
[8] Gischler E,Anselmetti F S,Shinn E A.Seismic stratigraphy of the Blue Hole (Lighthouse Reef,Belize),a late Holocene climate and storm archive[J].Marine Geology,2013,344:155-162.
[9] Van Hengstum P J,Scott D B,Gr?cke D R,et al.Sea level controls sedimentation and environments in coastal caves and sinkholes[J].Marine Geology,2011,286(1/4):35-50.
[10] 庞仁松,潘少明,王安东.长江口泥质区18#柱样的现代沉积速率及其环境指示意义[J].海洋通报,2011,30(3):294-301.Pang Rensong,Pan Shaoming,Wang Andong.Modern sedimentation rate and its implications for environmental evolutions of the 18#core in the Changjiang Estuary in China[J].Marine Science Bulletin,2011,30(3):294-301.
[11] 盖广生.最深的海洋蓝洞——三沙永乐龙洞[J].海洋世界,2016(11):72-77.Gai Guangsheng.The deepest ocean:Blue Hole[J].Ocean World,2016(11):72-77.
[12] 徐国强,吕炳全,王红罡.新生代南海北部碳酸盐岩台地的淹没事件研究[J].同济大学学报,2002,30(1):35-40.Xu Guoqiang,Lv Bingquan,Wang Honggang.Drown event research:insights from Cenozoic carbonate platforms in northern South China Sea[J].Journal of Tongji University,2002,30(1):35-40.
[13] 张明书.西沙生物礁碳酸盐沉积地质学研究[M].北京:科学出版社,1989.Zhang Mingshu.Sedimentary Geology of Reef Carbonate Deposits in Xisha Island[M].Beijing:Beijing Science Press,1989.
[14] Ma Yubo,Wu Shiguo,Lv Fuliang,et al.Seismic characteristics and development of the Xisha carbonate platforms,northern margin of the South China Sea[J].Journal of Asian Earth Sciences,2011,40(3):770-783.
[15] Wu Shiguo,Yang Zhen,Wang Dawei,et al.Architecture,development and geological control of the Xisha carbonate platforms,northwestern South China Sea[J].Marine Geology,2014,350:71-83.
[16] 刘健,韩春瑞,吴建政,等.西沙更新世礁灰岩大气淡水成岩的地球化学证据[J].沉积学报,1998,16(4):71-77.Liu Jian,Han Chunrui,Wu Jianzheng,et al.Geochemical evidence for the meteoric diagenesis in pleistocene reef limestones of Xisha Islands[J].Acta Sedimentologica Sinica,1998,16(4):71-77.
[17] 叶锦昭.西沙群岛环境水文特征[J].中山大学学报:自然科学版,1996,35(S1):19-25.Ye Jinzhao.Environmental hydrologic features of the Xisha Archipelago[J].Acta Scientiarum Naturalium Universitatis Sunyatseni,1996,35(S1):19-25.
[18] 刘焱雄,杜军,吴志露,等.西沙永乐龙洞探秘前期调查工作综述[J].海洋科学进展,2017,35(3):305-316.Liu Yanxiong,Du Jun,Wu Zhilu,et al.Preliminary investigation on the Yongle Blue Hole in Xisha Islands[J].Advances in Marine Science,2017,35(3):305-316.
[19] Xue Bin,Yao Shuchun.Recent sedimentation rates in lakes in lower Yangtze River basin[J].Quaternary International,2011,244(2):248-253.
[20] Ramsey C B.Bayesian analysis of radiocarbon dates[J].Radiocarbon,2009,51(1):337-360.
[21] Shepard F P.Nomenclature based on sand-silt-clay ratios[J].Journal of Sedimentary Petrology,1954,24(3):151-158.
[22] 赵焕庭,王丽荣,袁家义.南海诸岛珊瑚礁可持续发展[J].热带地理,2016,36(1):55-65.Zhao Huanting,Wang Lirong,Yuan Jiayi.Sustainable development of the coral reefs in the South China Sea islands[J].Tropical Geography,2016,36(1):55-65.
[23] 赵焕庭,王丽荣.南海诸岛珊瑚礁人工岛建造研究[J].热带地理,2017,37(5):681-693.Zhao Huanting,Wang Lirong.Construction of artificial islands on coral reef in the South China Sea islands[J].Tropical Geography,2017,37(5):681-693.
[24] Zhao Meixia,Yu Kefu,Shi Qi,et al.The coral communities of Yongle atoll:status,threats and conservation significance for coral reefs in South China Sea[J].Marine and Freshwater Research,2016,67(12):1888-1896.
[25] Yang Hongqiang,Yu Kefu,Zhao Meixia,et al.Impact on the coral reefs at Yongle Atoll,Xisha Islands,South China Sea from a strong typhoon direct sweep:Wutip,September 2013[J].Journal of Asian Earth Sciences,2015,114:457-466.
[26] 朱晓金,张庆红.南海台风LEO1999的加强机制研究[J].北京大学学报:自然科学版,2007,43(3):325-329.Zhu Xiaojin,Zhang Qinghong.A numerical study of the intensification mechanism of typhoon LEO over South China Sea (1999)[J].Acta Scientiarum Naturalium Universitatis Pekinensis,2007,43(3):325-329.