Response Of Vegetation Dynamics To Climate Change Since The Younger Dryas In The Longgang Region,northeastern China

Lakes and mires are widely distributed across northeastern China.Those lacustrine sediments and peats are well-placed as excellent archives for paleoclimate and paleovegetation reconstruction since the Late-Quaternary.During the past decades,numerous paleo-records have been recovered from lakes and peatlands in NE China,which have significantly improve our understanding of the interactions between vegetation dynamics and climate change.Because of the wide distributions of maar lakes in the Longgang Volcanic Fields,it has received increasing attention from global scientists.However,though a series of high resolution multiple proxy-based reconstructions have been carried out in the Longgang Region,its regional paleoclimate history,especially the paleo-precipitation variation,is still controversial.Younger Dryas?YD?is a key time period not only because it connects the last deglaciation and the Holocene but also means a significant shift from the cold/dry glacial time to the warm/humid post-glacial periods.A better understanding of the vegetation history and climate change since the Younger Dryas in the Longgang Reion is critical for the recognition of the East Asia monsoon evolution characteristics and forcing mechanism,and will therefore aid in predicating the magnitude and rapidity of ongoing climate change.In this thesis,two peat cores were recovered from an ombrotrophic bog in the Gushantun peatlands in the Longgang Reion,northeastern China.Employing absolutely AMS radiocarbon dating and referring to modern processes investigations,it presents a multiple proxy based?including pollen,charcoal,stable carbon and nitrogen isotopes,biomarker and compound-specific stable carbon isotope,as well as metal elements?paleovegetation,paleo-fire and paleoclimate reconstruction spanning last 13 ka?10³ years?.Compared with other lacustrine and peat records nearby,responses of past vegetation dynamics to climate change in the study region were investigated and evaluated.Spectrum,wavelet and complete ensemble empirical mode decomposition?CEEMD?were employed to find out the potential forcing mechanism for the climate change and vegetation variations.Pollen analysis results indicated that the regional landscape changes from temperate boreal forest,deciduous broadleaved forest to broadleaf and conifer mixed forest in the Longgang Region.During the YD,there was a significant expansion of boreal forest in the study region,as the Betula,Picea,and Larix are more tolerant to the cold and dry climate.The openness of the boreal forest is high,which inhabited several drought-tolerant herbaceous species,such as Artemisia and Poaceae.Following the increase of regional temperature and intensified monsoon activity in the early and mid-Holocene,the boreal forest was gradually replaced by temperate deciduous broadleaf forest which is dominated by Quercus,Juglans,Tilia,Corylus,Carpinus,and Ulmus.The booming of deciduous trees introduced a decline in the openness of the forest and resulted in peatland local vegetation/communities are dominated by Cyperaceae and some Sphagnum species.During the late-Holocene,continuous insolation diming resulted in a decrease in regional temperature.Climate deterioration favored the expansion of Pinus in the study region,while the previous deciduous broadleaf forest was replaced by a deciduous and conifer mixed forest about 5 ka BP which lasts until now in the Changbai Mountains.Younger Dryas in the Longgang Region is characterized by cold and dry regional climate conditions.During which,aeolian dust flux carried by the East Asian winter monsoon is relative high.A depletion of ?¹³C during the YD was observed in the Gushantun record,however,it was not due to climate shifts.Instead,it is because the lacustrine Algae utilize dissolved CO₂,which is usually in isotopic equilibrium with atmospheric CO₂.As a consequence,the carbon isotopic composition of Algae-derived organic matter is typically less enriched than that from the vascular plants.Since the early Holocene,East Asian summer monsoon was significantly intensified over the winter monsoon,causing a decreasing in dust flux deposition in the Gushantun peatland.Mid-Holocene is a period with intensified regional precipitation and high water-table levels in the peatland.However,during the late-Holocene,the Ti/Al recorded an intensifying of winter monsoon and aeolian dust flux increased again,as a sequence.Several abrupt climate anomaly events are recognized by paleoclimate proxies in the Gushantun peat cores.Almost most of them are comparable to stacked global cold events and the ice rafted debris?IRD?events observed in the marine sediments in the North Atlantic,implying the teleconnection of the monsoonal climate to other global circulation and climate systems.An early-Holocene volcanic eruption tephra layer?600-610 cm?was recognized from polarized optical microscope survey,geochemical analyses,and stratigraphic and chorological comparisons.However,maybe due to the sampling peatland is located in the upwind direction of the volcano,besides receiving volcanic clasts,peatland pollen assemblages didn't show any obvious evidences for the transition and/or succession of the regional vegetation.Millennial,centennial and even decadal periodic variations of paleocliamte and paleovegetation were found out via spectral analyses,at the frequency of ³000a,²000a,¹000a,⁸00a,⁵00a,and ²10a.Most of those periods are comparable with the cyclic variations of solar activities that reconstructed from cosmic isotopes preserved in ice cores(¹⁰Be)and/or tree rings(¹⁴C).Moreover,CEEMD results showed that the Quercus pollen percentages varied in-phase with past solar activities reconstructed from tree ring ?¹⁴C at the frequency of 500⁶00a.¹000a,and ²300a,providing a visible way showing the modulation role of the sun in governing the regional climate and vegetation dynamics.Inspired by this,this thesis further proposed a conceptual model illustrating how the sun regulates the climate and vegetation variations in the study region under different solar insolation scenarios.