Holocene Sedimentary Environmental Evolution Recorded By TZK1 Core Sediment In Taiwan Strait

The riverine sediments re-transported by ocean current of the Taiwan Strait,which influenced by East Asian monsoon and anticyclone zone of subtropical belt,provides an ideal material to study sedimentary environment evolution.Compared with other Chinesesea area,the study of the paleoenvironmental evolution of the Taiwan Strait is relatively rare,and the latest research is focused on the mud area on the west bank of the Taiwan Strait,which only reflects the evolution situation of China Coastal Current.The seasonal variation of the oceanic currents is obvious in Taiwan Strait,and most of the areas of the Taiwan Strait are strongly influcened by the Taiwan Warm Current.To better understand the Holocene evolution of the sedimentary environment in the Taiwan Strait,this study based on AMS14C age,grain size,elements and magnetic susceptibility of a selected TZK1(24 °59 '58.18 "N,119°59' 19.26,E)core from north of the Taiwan Strait,thevertical variation and those environmental significances will be analyzed,and then the Holocene evolution of sedimentary environment in the Taiwan Strait will be revealed.The results as follow:(1)The results of grain size analysis show that the content of sand varied from 0.13%to 92.12%and the average is 42.29%.the content of silt is between 6.58%and 80.85%and 45.20%on average,the content of clay varied from 1.05%to 35.27%with an average of 12.51%.The sediment types of TZK1 core sediments are mainly sandy silt and silty sand.The standard deviation(ai),skewness(Ski),and kurtosis(Kg)reflect bad sorting,positive skewness and flat kurtosis,respectively.The 10-224 ?m component is more sensitive to the Holocene sedimentary environment,while the 224-1124 ?m component is more sensitive to pre-Holocene.The grain size cumulative frequency distribution curves exhibit two-sections,indicating two main components.The variation of the proportion,median grain size,and mean grain size of sensitive component of the two main components,reveal that the hydrodynamic condition in north of the Taiwan Strait experienced fluctuate-stable-increasing enhance stages from early to late Holocene.(2)The major and trace elements of TZK1 core sediments are controlled by three factors.SiO2,Al2O3,Fe2O3,K2O,MgO,Na2O,TiO2,Zn,Rb,Co,Ni,V,Zr,Hf and Th are dominanted by terrigenous detris,while CaO,MnO and Sr are controlled by marine biogenic matter,and P2O5,Pb,Ba and Cu may come from multiple sources.Element ratios(Ti/Na,Al/K,Na/K,Rb/Zr,Rb/Sr)and CIA reveal that the chemical weathering intensity of provenance experienced a process of frequent fluctuation-steady-and gradually weakening since the Holocene.(3)Before the Holocene,the magnetic susceptibility of TZK1 core sediment is the highest and fluctuates frequently,and in the early Holocene,that of TZK1 core sediment is low and fluctuates frequently.The susceptibility of sediments in the middle Holoceneincreased and is relatively stable.Subsequently,the magnetic susceptibility fluctuates more frequently but the magnitude is insignificant than that of the early Holocene.The whole process,fluctuation with high value-steady with low value-rapid increasing,reflects the contribution of paramagnetic minerals to sediment susceptibility at different stages.(4)Combining with the AMS14C age and the Holocene sea level changes in the study area,TZK1 core can be divided into three stages(since the Holocene).Stage 1(about 10-6ka BP):With the rapid warming of the climate,Taiwan strait's sea level keep rising continuously in this stage.The major hydrodynamic condition was tide-dominated,the sedimentary environment changed along with the rise of sea level.At this stage the sediment provenance has experienced strong chemical weathering,and all the indexes have been characterized by frequent fluctuations during this period.Stage 2(about 6-2.8ka BP):In this stage climate was stable,the current system formed and was stable in the Taiwan-strait.The hydrodynamic condition is stronger than the previous stage,the sediment particle size is slightly thicker,the sediment source at this stage had some change and was gradually steady,and the chemical weathering in the provenance area was stable at this stage.The 6ka BP,5.5ka BP and 2.8ka BP cold events generally found in the Holocene were reflected in the core.Stage 3(about 2.8ka BP up to now):Following with climate turned cool in late Holocene,sedimentary environment in Taiwan Strait also changed slightly.Hydrodynamic conditions reached the strongest in the whole Holocene,at the same time,the accumulation rates decreased rapidly.The strong hydrodynamic condition tended to continue to transport the sediments rather than deposition,and sediment thickness also confirmed this.Chemical weathering of provenance area turned weakened at this stage,may be related to the monsoon weakened.