摘要
本文通过对榕江下游汕头市牛田洋南北两岸的2个钻孔进行^(210)Pb测年,建立了榕江河口冲积平原近代沉积物年代序列。并结合这2个钻孔沉积物粒度和微古生物指标,探讨了近150年来榕江冲积平原的沉积特征及其对灾害性气候的响应。沉积物粒度分析结果显示,榕江冲积平原沉积物以粘土质粉砂为主,沉积物分选性差,牛田洋南岸沉积物(Core 1)颗粒稍细于北岸(Core 2)。由于南岸受潮流影响较北岸强烈,因此南岸粘土质粉砂层含有孔虫(Ammonia spp.和Elphidium spp.);而北岸则仅在钻孔底部一个样品中发现有孔虫(Ammonia sp.)。上述粘土质粉砂沉积为正常气候条件下的沉积物,间隔其间的是颗粒较下覆和上伏层相对粗的砂层。这些砂层在牛田洋南岸以细砂为主,分选性很差,峰态狭窄,^(210)Pb_(exc)活性低,不含有孔虫,钻孔顶部和底部的细砂层中见大量贝壳碎屑;牛田洋北岸的砂层以中、粗砂为主,分选性很差,峰态多为宽,^(210)Pb_(exc)活性低,不含有孔虫。这些砂层呈现出显著的陆源沉积特征,很可能是在洪水期间形成的洪水沉积层。因为组成这些砂层的粗颗粒物质只有在洪水期间经异常强大的径流输送才能到达牛田洋。研究结果表明,根据这些洪水沉积层重建的榕江冲积平原地区历史洪水事件与实际观测的强台风事件在发生年份上较为吻合,如1922年、1969年和2006年等。由于榕江流域的洪水事件多由台风导致,因而这些洪水沉积层可以用来指示历史强台风事件。为估计该区历史强台风的发生周期,我们对时间序列相对长的Core 1的粒度参数进行频谱分析。分析结果显示该地区的强台风事件有较为显著的50年和120年周期,从而预测接下来的20年仍为强台风频发期。基于以上分析,本文认为榕江冲积平原沉积对台风洪水响应敏感,是重建该地区长时间尺度灾害性台风气候的重要载体之一;建议将该区的洪水沉积与滨海地区的风暴潮沉积相结合,二者互相补充以取得更高分辨率的古台风记录,分析其时空分布特征,为潮汕地区防灾抗灾决策提供科学参考。最后,本文还初步探讨了榕江冲积平原沉积对人类活动的响应,研究结果显示,近150多年来牛田洋沉积速率逐渐增大,大致可以分为3个阶段,即从19世纪末至20世纪初的0.33cm/a,增加到20世纪30~80年代期间的1.36cm/a,以及20世纪末80年代后的3.22cm/a。本研究认为造成该地区沉积速率快速增加的一个重要原因是当地围海造田活动的不断加强。
Chaoshan plain in Southern China experiences seasonal typhoons that commonly result in considerable loss of both human life and infrastructure. This is particularly the case for the low-lying Rongjiang River deha(〈3m above PMSL) in western Chaoshan plain. From 1900 to 2010,there have been a total of 91 typhoons traced within 100km radius from Niutianyang,in the lower reach of the Rongjiang River. Among them,large typhoons, such as Chanchu in2006,Viola in 1969 and an unnamed typhoon in 1922, have caused considerable destruction and basin-wide flooding in the Chaoshan plain. As flooding events in this area are mainly induced by large typhoons,it is important to understand the spatial and temporal distribution of large typhoons as such evidence is key to reducing future losses. However, monitoring data collected only since 1950 are not adequate for investigating the recurrence interval of large typhoons,which are usually one in 50 year events or even one in 100 year events. Sedimentary records have the potential to span millennia and have proved to be very useful archives elsewhere, such as the Yangtze delta and the Yellow River delta in China. This study was carried out to examine the sedimentary features of the Rongjiang River flood plain, and to explore its applicability as an indicator for palaeo typhoon hazards and as an indicator for recently enhanced human activities. Two sediment cores were collected from Niutianyang at the west of Rongjiang River estuary in September 2011 using a gouge auger. Core 1, 1.9m in length, was collected from the Hudong Village (23~21.376'N, 116~31.817'E) ,south of Niutianyang; and Core 2,2.2m in length, was collected from the Tiangang Village (23~22. 537'N, 116~36. 096'E), north of Niutianyang. For both cores ,the chronology was developed using the Zl^Pb dating methods via the CRS( Constant Rate of Supply)model, suggesting a timeframe of ca. 150 years. To examine the sedimentary features, the two cores were then analyzed for grain size composition and faunal composition (foraminifera) at a 1 -5em sampling interval. The results of sedimentary analyses show that deposits from both sites are dominated with clayey silt, interrupted by sandy layers of 1 - 20em in thickness. The sedimentary features of south Niutianyang ( Core 1 ) are different from that of the north(Core 2) ,as tide influence is stronger on the south than on the north. Core 1 has a mean grain size value of 7.2~b,poorly sorted,with a near-symmetrical skewness and a mesokurtic kurtosis. Slightly etched foraminifera(Ammonia spp. and Elphidium spp. )were found in the clayey silt deposits. Core 2 has a mean grain size value of 6.8b, slightly coarser than sediments from the south. Sediments are poorly sorted,with a near- symmetrical skewness and a leptokurtic kurtosis, foraminifera(Ammonia sp. )were only found in one sample from bottom of the core. For both cores, intercalated among the normal clayey silt deposits are several sandy layers. In Core 1, these sandy layers are dominated by very poorly sorted fine sand that is fine skewed, and exhibits leptokurtic kurtosis. The sand layers also exhibit low 21~Pboxc activity (141.0Bq/kg, lower than the core mean value of 155.2Bq/kg) and large amounts of bivalve debris were found in fine sand layers from bottom and top of the core. In Core 2, sandy layers were dominated with very poorly sorted medium and coarse sand that is fine or strongly fine skewed,and exhibits platykurtic or very leptokurtie kurtosis. The sand layers here exhibit low 21~Pbex~ activity (118.6Bq/kg,lower than the core mean value of 123. 1Bq/kg)and no foraminifera or shell debris were found in these layers. Different sedimentary features for the sandy layers are most likely due to stronger tide influence on the south than on the north. In these sandy layers,the high proportion of sand and the absence of foraminifera indicate their terrigenous nature. We imply that the terrigenous sediments for these sandy layers were delivered into Niutianyang by strong river runoff during large typhoons, and formed the flood deposits. In return, these flood deposits can be used to indicate big flooding events which mainly induced by large or intense typhoons in this area. Thus, we conclude that flood deposits from the Rongjiang River flood plain can be used to reconstruct palaeo large typhoon events. In order to estimate the recurrence interval of palaeo typhoon events,spectral analysis was applied to particle size parameters of Core 1 ,suggesting four main periodicities as 30 and 80 years by kutosis values,and 50 and 120 years by mean size,sorting and skewness values. By comparing the result of spectral analysis with the chronologically constrained sedimentary record as well as historical records of typhoons,we suggest that the recurrence intervals of 50 and 120 years are more obvious,and we suggest the next 20 years a period with intense large typhoons and probable basin- wide flooding events. Furthermore, this study also suggests that recently enhanced human activity is the main driving mechanism for the significantly increased sedimentary rate since late 1980s of 3.22cm/a,with comparison to 1.36cm/a during 1930 - 1980 and 0.33cm/a in early 20th century.
出处
《第四纪研究》
CAS
CSCD
北大核心
2013年第6期1171-1182,共12页
Quaternary Sciences
基金
新加坡国家研究基金会(the National Research Foundation
Singapore)颁发给Adam D.Switzer博士的NRF研究项目(编号:NRF-RF2010-04)
厦门大学近海海洋环境科学国家重点实验室授予余凤玲博士的青年访问学者野外经费(批准号:2010-2012)共同资助
关键词
洪水沉积
台风
粒度分析
2010Pb测年
牛田洋
榕江三角洲
潮汕平原
typhoons, flood deposits, particle size analysis, 210Pb dating, Niutianyang, Rongjiang River,Chaoshan plain,Southern China