The Evolution Of Holocene Sedimentary Environment In Southern Flank Of The Yangtze River Delta And Its Driving Mechanisms

Deltas are the products of land-sea interaction and the hotspots of human economic activities.Understanding the evolution of deltas and their response mechanisms to human activities and climate change will help to forecast their evolutionary trends,thereby providing scientific support for the sustainable development of deltas.The southern flank of the Yangtze River Delta(YRD),where the most economically developed regions Shanghai and Jiangsu province in China are located,has been rarely studied regarding its environmental evolution process and mechanisms due to a lack of high-resolution chronological studies.In this paper,five Holocene drill cores from the southern flank of the Yangtze River Delta—Jiandong(JD),Haiqiao(HQ),Jiqiao(JQ),Niudu(ND),and Waizhong(YZ)—are investigated using a combination of methods,including optically stimulated luminescence(OSL)dating and radiocarbon dating(14C),grain size analysis,environmental magnetic and geochemical analysis of bulk and size-specific samples,and diffuse reflectance spectroscopy(DRS).A high-resolution chronological framework for the Holocene sediments in the southern flank of the YRD is reconstructed,and the magnetic and geochemical indicators for provenance tracing explored.Based on theses analyses,environmental evolution process and driving mechanisms in the study area are reconstructed.The main conclusions are as follows:1.In the southern flank of the YRD,a 25-meter-thick Holocene deposit is overlingthe Late Pleistocene stiff clay layer.The Holocene deposit can be divided into three major sedimentary facies from bottom to top:tidal flat(progradational)facies(F1),estuarine-shallow marine facies(F2),and tidal flat(aggradational)facies(F3).According to the sedimentation rates revealed by dating results,the the southern flank of the YRD shows three stages of depostional rate,i.e.,fast(9-6 ka),slow(6-2.5 ka),and fast(2.5-0 ka).From 6 ka to 2.5 ka,there was a relatively long hiatus.Before 2.5 ka,the paleo-water depth increased from west to east,which has a depth of 10-17 meters below the modern surface.2.Based on dating results,Jiandong village,10 km east of the chenier ridges formed in 7-3 ka BP,a geomorphological marker on the YRD,was emerged around 1.5 ka.The land formation of the academically disputed Zhoupu-Xiasha line occurred around 1.1 ka,that of Chuansha-Nanhui line around 0.8 ka,and Bailonggang-Machang line around 0.3 ka.According to historical changes of the paleocoastline,the coastal progradational rate of the southern flank since 1.5 ka has exhibited significant fluctuations(2.7-80.2 m/a),with slower rates during 0.8-0.44 ka and 0.28-0.07 ka,and faster ones during 1.1-0.8 ka and 0.44-0.28 ka.3.Magnetic characteristics of the sediments in the southern flank are dominated by ferrimagnetic minerals.Grain size significantly affects some magnetic parameters,with higher values of χARM,HIRM,S-100,and S-300 found in finer particle fractions.In some cores,the χARM/SIRM and SIRM/χ values are positively correlated with coarse grain size.The lower part of tidal flat facies deposited in cores HQ and JQ are affected by reductive diagenesis,resulting in the presence of a large amount of greigite.The top part of tidal flat facies of all cores is affected by pedogenesis,causing an increasing content of hematite.Overall,magnetic characteristics of cores JD,ND,and YZ are less affected by diagenesis effects and can be used for provenance analysis.Principal Component Analysis(PCA)was performed using size-specific magnetic parameters x,SIRM,χARM,HIRM,S-100,S-300,and χARM/SIRM.The results show that samples before 0.9 ka were dominately influenced by the Han River,while samples after 0.9 ka were more influenced by the Jialing River.4.Geochemical characteristics of cores JD,ND,and YZ exhibit significant vertical changes.Through PCA of the bulk elemental composition and their correlation analysis with particle size,it is found that Ca,Na,Sr,and Zr elements are mainly enriched in coarser grains,while the remaining elements increase with finer grain size,indicating that these elements are significantly affected by particle size.The vertical elemental changes in the cores ND and YZ show that sediments in the lower part of F2 had higher concentration of sulfur,while the corresponding Mn/Fe values were lower,reflecting the enrichment of S and loss of Mn elements under redox conditions.However,diagenesis does not significantly affect redox-sensitive elements U,V,Mo,and Cd.Neglecting the influence from the Yellow River,a qualitative provenance analysis was conducted using bulk-sample Ca/Al-Na/Al-Ti/Al ternary diagrams and size-specific Na,Ca,Mg,and Ti elemental characteristics.The results are consistent with that obtained by magnetic analysis.5.By integrating size-specific magnetic and geochemical characteristics for quantitative provenance analysis,the result shows that in sediment fraction smaller than 63 μm,the Han River is the largest contributor,with a percentage of about 60%.In sediment frations larger than63 μm,the contribution ratios of the Jialing River and the Han River are similar,both around 50%.The Han River played a dominant role before 0.9 ka(with a percentage of about 60%),and the contribution of the Jialing River increased from 34%to 40%during 0.90-0.44 ka.The latter two rivers showed a further increas of contribution during 0.44-0 ka,with the Jialing River increasing from 40%to 46%,and the Jinsha River from 6%to 26%.6.The evolution process and provenance changes of the southern flank are influenced by the combined effects of climate change,human activities,and the shift of depocenter.In the early Holocene(9-6 ka),the humid climate in the Yangtze River catchment and the rapid rise of sea level together led to rapid sedimentation.During the mid-Holocene(6-2.5 ka),the climate in the catchment became drier,leading to a relatively lower sedimentation rate.In the late Holocene(2.5-0 ka),human activities increased the sedimentation rate.Since 2.0 ka,due to different development times of various tributaries by human soceity,the provenance has shown a trend of declining contribution of the Han River while increasing contribution of the Jialing River and the Jinsha River over time.The shift of depocenter is also a major influencing factor.During the 0.8-0.44 ka period,the sedimentation rate of the northern flank of the YRD was 4-6 times than that of the southern flank,with the depocenter shifting to the northern flank.Since 0.44 ka,the fastest direction of coastline advancement has changed from west-east to northwest-southeast,which is related to the southward shift of the Southern Channel in the YRD.