Sediment Provenance And Environmental Records On The Inner Shelf Of The East China Sea Since The Last Glaciation Maximum

The continental shelf of the East China Sea,as one of the largest river-dominated continental shelves in the world,has been receiving huge fluvial materials since the Last Glacial Maximum(~ 22.0 kyr BP).Thus,it is an ideal natural laboratory for studying the dynamic processes on the earth's surface,including the source-to-sink process and the interaction between lands and seas.In this thesis,in order to study the sedimentary provenance and the reconstruction of sedimentary environment on the inner shelf of the East China Sea since the Last Glacial Maximum,the sedimentary records,including the chronology,mineralogy,and terrigenous grain size from the sediment cores ECMZ and EC2005,which are located in the mud wedge on the East China Sea inner shelf,and the river surface sediments from the modern Changjiang(Yangtze)River and small rivers in Zhejiang Province,are analyzed.After that,the models for the sedimentary formation and their implication for the regional or global environmental evolution since the Last Glacial Maximum are further studied.As a river-dominated marginal sea,the East China Sea is significantly influenced by the Changjiang(Yangtze River);however,the influence of Changjiang on the sediment dispersal pattern since the Last Glacial Maximum is not well understood.To trace the Changjiang sediment dispersal pattern on the southern East China Sea continental shelf since the Last Glacial Maximum,we present a set of mineral data from the southern inner shelf of the East China Sea.The provenance analyses from heavy and clay minerals demonstrate that the Changjiang has been the predominant sediment source,despite the significant sea-level changes since the Last Glacial Maximum.Based on the evolution of the depositional environment and the diagnostic heavy minerals(e.g.,polysynthetic mica,magnetite,and dolomite),our results indicate that the paleo-Changjiang sediments could have been transported southward as far as approximately 400 km through incised valleys during the Last Glacial Maximum.Then,the study area evolved into an intertidal environment due to the rapid sea-level rise during the last deglaciation,and thus,the sedimentary process was mainly influenced by strong tides during the late stage of the last deglaciation(12.5-9.0 kyr BP).Since 7.5 kyr BP,the Changjiang-derived sediments have been transported southward mainly by coastal currents,which has led to the formation of a mud wedge on the inner shelf of the East China Sea.Our new findings confirm that Changjiang has always been the primary source of sediments on the inner shelf of the East China Sea,and the sediment dispersal pattern has been regulated via incised valleys,tides,and coastal currents at the individual stage since the Last Glacial Maximum.The sea-level curve during the last deglaciation is characterized by two episodes of rapid sea-level rise,termed Meltwater Pulses 1A(14.6-14.3 kyr BP)and 1B(11.45-11.1 kyr BP),as well as an episode of relatively slow sea-level rise during the Younger Dryas stadial period(12.85-11.65 kyr BP).However,the relative sea-level changes during the Younger Dryas and Meltwater Pulses 1B periods are still not well defined.Here,we use precisely dated intertidal sediments derived from 6 sediment cores on the tectonically stable continental shelf of the East China Sea to reconstruct a high-resolution sea-level curve between 14.0 kyr BP and 10.0 kyr BP.Although linear regression analysis suggests that the sea level rose continuously,with an average rate of 10.6 ± 0.6 mm/yr(r =-0.87,p(a)< 0.01,n = 95),the mode of sea-level rise differs significantly before,during,and after the Younger Dryas stadial.A rapid sea-level rise of 30.5 ± 4.9 mm/yr(r =-0.79,p(a)< 0.05,n = 25)occurred during 14.0-12.85 kyr BP.Subsequently,the rate of sea-level rise decreased significantly in response to the Younger Dryas stadial,and it increased smoothly to 12.0 ± 1.4 mm/yr(r =-0.86,p(a)< 0.01,n = 29)from 11.65-10.0 kyr BP,indicating that Meltwater Pulses 1B was absent on the East China Sea continental shelf.Furthermore,based on comparisons of these data with other high-resolution curves of sea level or elevation at different latitudes,we suggest that because of the glacial isostatic adjustment,the sea level heights in high-and low-latitude oceans behave differently.However,due to the same reason,the poleward seawater migration from intertropical broad oceans occurred,generating a relatively consistent global sea-level height during the last deglaciation,especially during the Younger Dryas period,when the rate of sea-level rise was remarkably slowed.The mechanism of the mud depocenter formation on the inner shelf of the East China Sea is unknown.Based on the sedimentary record from several cores around the southern mud depocenter on the inner shelf of the East China Sea,coupled with the modern ocean circulation development,we propose a new model for the formation of mud depocenter.The hydrodynamic barrier,which is well developed among the Zhe-Min coastal current,Taiwan warm current,and Kuroshio intrusion current,can control the development of southern mud depocenter on the inner shelf of the East China Sea,by trapping the majority of Changjiang-dominated suspended material.Subordinately,the formation of mud depocenter is also influenced by the flux and fate of the Changjiang discharged material.Our new findings highlight the oceanic circulations under the context of Asian monsoons for constraining the formation of mud depocenter.Currently,the El Ni?o-Southern Oscillation,as an anomalous ocean-atmosphere condition in the tropical Pacific,can impact the East Asian winter monsoon,and vice versa.However,the evolution of the interrelation between El Ni?o-Southern Oscillation activity and East Asian winter monsoon intensity during the Holocene epoch remains unclear.Here we present a high-resolution sedimentary record from the East China Sea inner shelf to reconstruct the evolutive history of the East Asian winter monsoon since 7.0 kyr BP.Based on the results of sediment provenance analysis and sedimentary process,The grain size records reveal several centennial oscillations of the East Asian winter monsoon during the past 7.0 kyr BP,showing an anti-phase temporal evolution with El Ni?o-Southern Oscillation activity during 5.8-0 kyr BP.These results,which are consistent with modern observations,indicate that the enhanced East Asian winter monsoon could provoke the formation of subsequent intensive El Ni?o-Southern Oscillation activity,whereas the increased El Ni?o-Southern Oscillation activity suppressed the winter monsoon intensity during the mid-late Holocene.Furthermore,based on the 1500-yr period identified from the grain size data,which corresponds to that documented in the Holocene El Ni?o-Southern Oscillation records,we suggest that the Holocene East Asian winter monsoon was a crucial pathway linking the remote polar and equatorial climatic systems.The energy exchanges between high-and low-latitude climatic systems could result in the millennial period through the East Asian winter monsoon.Our study thus highlights the internal feedback in the Earth's climatic systems during the mid-late Holocene epoch.