Sedimentary Record Of Coastal Hazards In Zhangjiang Estuary Since The Late Holocene

Ongoing global warming is accelerating global sea level rising.Coupled with increasingly frequent extreme events,it is increasingly threatening the coastal areas.The most prominent threat is the extreme sea level change event under extreme weather conditions.China is a country having long coastline.Coastal area is the most densely populated and economically developed regions in China.Besides,coastal area is also an ecologically vulnerable zone under the impact of frequently occurred coastal hazards.Therefore,the study of coastal natural hazards and the exploration of the occurrence mechanism and development period of extreme hazards are helpful to the establishment of coastal hazard information system to guide the formulation of corresponding hazard prevention measures,which has important practical significance for the economic development of human society.Historical documents of coastal hazards,however,are often lacking,because existing measured data cover only a short time span.It is therefore difficult to understand the hazard history and its driving mechanisms.As a consequence,the answers to several key scientific questions,such as the intensity and frequency of regional historical hazards and the driving mechanism of hazards,are still not clear to us.Geological archives are an important record of coastal hazard event at millennial and centennial time scales.Based on those,investigations of geochemical characteristics of coastal sedimentary records,could help us understand the types and characteristics of coastal hazards during the Holocene and improve our knowledge of driving mechanism of coastal hazards.Taking Zhangjiang Estuary in Fujian Province as an example,this study reconstructs the history of coastal paleo-hazard since about 4200 years ago by using biogeochemical indicators,in order to explore the temporal and spatial characteristics and driving mechanism of coastal hazards in study area.Based on the results,this study try to address the following questions(objectives):1)Identification of coastal hazard deposits:identify the high-energy hydrological event layer using grain size data.Source of the sand layer is inferred by combining δ13C,C/N,major/trace elements and diatom assemblages.2)The age:based on AMS 14C dating,this study establishes the age depth model using ’Bchron’ package;3)Periodicity and influencing factors:through spectrum analysis,the periodicity of Late-Holocene hazard sequences in the study area was analyzed.Combined with the history of solar activity intensity reconstructed by predecessors,the response of hazard events to solar activity in the study area is discussed.Through the above work,the main conclusions are listed as following:(1)Core K5 is deposited since the late Holocene,with an age span from 4280 to 530 cal.a BP.The sedimentation rate of Core K5 shows a fast-slow-fast variation.This might be related to the regional sea level change.(2)The sediments in the Zhangjiang Estuary area are mainly composed of silt,which constitutes the normal sedimentary sequence of Core K5.Sediment are poorly sorted,reflecting the characteristics of strong hydrodynamic conditions and complex material sources in the estuary area.The δ13C of Core K5 indicate that they originated from terrestrial and marine sources,reflecting the mixed input of marine and terrestrial materials.(3)37 species of diatoms were identified in the core sediments.10 nearshore and intertidal species are the most dominant species,having a relative abundance greater than 1%.(4)6 event sand layers have been identified based on grain size composition,which represents a period of frequent hazards.They are named S6,S5,S4,S3,S2 and S1.According to the content of major/trace elements,diatom assemblages and the paleoclimate characteristics recorded in the previous study,these layers are presumed to be typhoon deposits.Among those events,S1,S3,S4,and S5 may be storm surge deposits caused by typhoon,S2 and S6 may be flood deposits caused by intensified rainfall caused by typhoon.(5)Sensitive grain size component analyses found that the mean grain size of the 6 event layers showed a coarsering trend,indicating that the intensity of the typhoon hazard in the Zhangjiang Esturay area has been increasing since the Late Holocene.In addition,there were three periods of frequent coastal hazards during 1400～700 cal a BP(3 times/700a),the frequency of which was about twice that during the period of 3900～2500 cal a BP(3 times/1400a).(6)Spectrum analysis of δ13C and sensitive grain size components of Core K5,identified ca.700-year and ca.400～600-year cycle in late-Holocene climate change history in Zhangjiang Estuary.The 400～600-year cycle may be the cycle of frequent strong typhoon period,which may be related to Holocene solar activity at different time scales.