末次冰消期以来福建福宁湾钻孔的粒度特征及其环境指示意义

Grain Size Characteristics of Boreholes in Funing Bay,China and Their Environmental Indications since the Last Deglaciation Period

海岸沉积物是记录海-陆-气相互作用下区域古环境变化的重要载体。因短历时古气候事件发生时间短、各物质载体和代用指标都存在一定不足,对华南海岸带沉积物研究多探讨其与海平面波动等长时间尺度上的关系,限制了对粒度信息中可能记录的短期气候事件的深入研究,不利于解译古气候与古人类历史。本研究选取福建霞浦福宁湾FN1钻孔0~8 m部分,以光释光年代标尺为基础,使用粒度作为主要环境代用指标,并通过端元分析、主成分和相关性分析方法,阐明福建东北沿海地区沉积物粒度特征;再结合TOC/TN含量变化,探讨其环境指示意义。结果表明:(1)钻孔0~8 m沉积物以粉砂为主,不同沉积相中各粒级百分比含量波动较大,根据粒径变化趋势和各粒度参数变化曲线分为U1、U2、U3三层。(2)根据BasEMMA方法选取3个端元进行分析。其中,EM1代表低能沉积条件,以粉砂为主,是受冬季风影响的敏感组分,可指示冬季风强度变化;EM2代表中能沉积条件,出现粒级、TOC/TN及沉积速率的突变。受东亚冬季风影响,陆源物质含量增加;EM3代表高能沉积条件,区域水动力增强,沉积物粗颗粒含量增加;高能与低能条件的频繁变化使沉积过程变得复杂,形成海陆过渡相。(3)3.9 ka B.P.~3.7 ka B.P.、1.4 ka B.P.~1.2 ka B.P.以及距今约500 a以来,沉积物粒度与TOC/TN均出现明显变化。与前人对东亚季风与ENSO变化的研究对比发现,环境代用指标的突变与华南沿海在该时段发生的风暴潮频率密切相关,可能对该地区人类生活居住产生严重威胁,进而影响区域文化演替,这表明沉积物粒度特征和TOC/TN含量变化特征等具有指示区域人类活动和环境演变的潜力。本研究可以为古环境重建和人类文化研究提供支持。

As the key zone of sea-land-air interaction,the coastal zone of South China has developed coastal sediments with a scale of thousands of years in evolution history,which is an important carrier to accurately record the regional paleoenvironment changes under the sea-land-air interaction.There were lots of scientific discussion about coastal sediments of South China in association with sea level fluctuation on a long-term scale,but hardly involving with short-term paleoclimatic events which may be recorded by granularity information in the coastal sediments on a particle-size scale.Because short-duration paleoclimate events occurred in a short time and their occurrences were difficult to be accurately determined,and available material carriers and proxy indicators in sediments,all of which prevented scientists from comprehensively interpreting and understanding paleoclimate and paleoanthropology.Sediment grain size is an effective index for paleoenvironment reconstruction.The granularity end-member analysis(EMA)method can isolate sensitive components from complex granularity data and is widely used in paleoclimate research.In this study,the 0-8 m segment of samples at FN1 borehole in Funing Bay,Xiapu,Fujian was selected for test.Based on photoluminescence dating scale,and sediment granularity as main environmental proxy index,it elucidates the characteristics of sediment grain size by means of end-member analysis,principal component analysis and correlation analysis;then by combined with the change of TOC/TN content,it discussed the significance of environmental indication.It found that(1)The sediments from 0-8 m in the borehole were dominated by silt,with widely fluctuated percentage content of each grain size in different sedimentary phases.And it can be divided into three sections(U1,U2 and U3)according to the grain size trend and the curve of grain size parameters.(2)Three end-members were selected for analysis by BasEMMA method.Among them,EM1 represented low-energy deposition conditions,dominated by fine silt,which was a sensitive to East Asia winter monsoon(EAWM),a indicative of intensity of EAWM.EM2 represented medium-energy deposition conditions,suggesting possible mutations in grain size,TOC/TN and deposition rate.Due to East Asian monsoon,it brought out an increase in terrigenous geomaterial.EM3 represented high-energy deposition conditions,manifested as enhanced regional hydrodynamics and increased sediment coarse-grained content.Frequent shifts in high-energy and low-energy conditions complicated sedimentary processes and formed sea-land transition phase.(3)From 3.9 ka B.P.to 3.7 ka B.P.,from 1.4 ka B.P.to 1.2 ka B.P.and about 500 years ago,the grain size and TOC/TN of sediments changed significantly.By comparison with previous studies on East Asian monsoon and ENSO changes,it revealed that abrupt changes in environmental proxies was closely related to the frequency of storm surges occurring along the coast of South China in the same period,which might pose a serious threat to habitation in the region,and consequently affected the regional cultural succession,suggesting that characteristics such as sediment grain size characteristics and changes in the content of TOC/TN have the potential to be indicative of regional human activities and environmental evolution.This study can support paleoenvironmental reconstruction and human cultural studies for the coastal zone of South China.