Paleoenvironmental Reconstruction Of The Mid-late Holocene: Pollen Record From The Yellow River Flooding Sedimentary Sequence In Kaifeng City,China

Flood events and their climate backgrounds have attracted attention in quaternary environmental change and global climate change research.The Yellow River,which is praised as“the mother river”of China,is invaluable in Chinese⁵000-year civilization development and evolution.Many cities that located in its middle and lower reaches were the political,economic and cultural centers for a long time.However,as the saying goes,“every coin has two sides”.In past thousands of years,the Yellow River was beneficial to Chinese civilization,but simultaneously made fatal damage to cities nearby.Naturally,its frequent flood events and the carried sediments have received considerable attention in past decades.Many previous studies focused on the floods and sediments in upper reaches and branches of the Yellow River,but fewer in the lower reaches,where the river channel migrated many times.Kaifeng,which is located in the lower reaches of the Yellow River,was seriously damaged by the floods for many times.However,few research on its flooded sediments has been carried out before.Also,Kaifeng as a representative of central plains,its paleoclimate and paleovegetation has been less studied.Located in the middle reaches of the Yellow River,Chinese Loess Plateau?CLP?is focused on its Holocene vegetation change by scholars who are devoted to environmental evolution.There are large discrepancies accounting for the existence of extensive forests in CLP during mid-Holocene.Kaifeng has been imposed on the flood sediments from the Yellow River for many times.As thus,it provides us an excellent sedimentary sequence for studying flood events and well-preserved information about paleoenvironment in middle and lower reaches of the Yellow River during the Holocene.Hence,two cores(named as ZK_jm and ZK_sz core respectively)from Kaifeng City were drilled and subsampled.On the basis of detailed pollen analysis,along with color identification,AMS¹⁴C and optically stimulated luminescence?OSL?dates,grain size and historical documents,pollen-based sedimentary cycles in the study area were identified,and flood events and their dating frameworks were established.Modern analogue technique?MAT?and BIOME methods were utilized to reconstruct the paleoclimate and paleovegetation in Kaifeng City,respectively.Pollen data from upper reaches of the Yellow River were excluded and the rest data were input to BIOME for reverting the macro-vegetation change in CLP during mid-late Holocene.Our study is concluded as follows.?1?The divided limit lines of pollen-based cycles are generally consistent with those of the comprehensive cycles based on granularity,black carbon and geochemical elements.Thus,pollen is considered as a feasible indicator for dividing the sedimentary cycles of flood sediments in Kaifeng City.Moreover,the number of pollen-based cycles?15?is more than that based on granularity?12?,black carbon?12?and geochemical elements?12?.It proves that pollen is more feasible than the other three indicators?i.e.,granularity,black carbon and geochemical elements?when used for dividing cycles of human-disturbed sediments.?2?From nine flood events that recorded in ZK_jm core,seven flood events can be aged.Their ages and depths of buried sediments are as follows:⁴000 a BP?14.7-20.3 m?,225 BC?13.2-14.7 m?,1387 AD?11.8-13.2 m?,1399 AD?10.0-11.8 m?,1461 AD?7.8-10.0 m?,1642 AD?4.0-7.8 m?and 1841 AD?0.8-4.0m?,respectively.Simultaneously,sediments in ZK_sz core from the same flood events as ZK_jm core,were buried at 11.9-20.6 m,10.2-11.9 m,8.8-10.2 m,6.9-8.8 m,5.9-6.9 m,2.8-5.9 m and 0-2.8 m,respectively.Additionally,sediments at 20.3-22.1 m in ZK_jm core and 20.6-23.0 m in ZK_sz core were both lacustrine deposition from mid-Holocene.?3?The pollen-based reconstruction of paleoclimate and paleovegetation in Kaifeng City shows that the climate maintained warmest and wettest during mid-Holocene.The maximum precipitation was 800mm,and surroundings had warm-temperate deciduous broad-leaved forest with evergreen trees grown.The climate change fluctuated obviously from the Spring and Autumn?abbreviated as SA?period to the Warring States?abbreviated as WS?period.High temperature and abundant precipitation with the mean annual of⁶00 mm occurred in SA period when climate maintained warm/wet for forest vegetation.After that,temperature and precipitation decreased in WS period when climate maintained temperate-cool and less wet for forest and grassland vegetation.Then,slight increase of temperature and decrease of precipitation occurred in Tang-Song periods and the vegetation landscape was dominated by grassland or forest-grassland.The pollen record indicates four stages of climate changes from Ming to Qing dynasties,of which three stages occurred in Ming Dynasty.Climate in these three stages was characterized by warm/wet?early Ming?,temperate-cool/dry?mid-Ming?and temperate-cool/dryer?late Ming?,with precipitation of⁶50mm,⁴80mm and³90 mm,respectively.Another stage occurred in late Qing Dynasty,which characterized by cold/wet climate.The vegetation landscapes in above four stages were dominated by savanna?early Ming?,grassland?mid-Ming?,steppe and xeric shrubland?late Ming?,and forest-grassland?late Qing?.?4?Paleoclimate that reconstructed based on pollen from flood sediments in Kaifeng can help to identify widely-received four events about climate changes?i.e.,Holocene Thermal Maximum,4200 a BP,Medieval Warm Period and Little Ice Age?.Moreover,the paleoclimate in various stages of Kaifeng are well consistent with that among other biogeographic regions in mid-low latitudes of north hemisphere.Therefore,it is feasible and believable that using flood-hiatus pollen to reconstruct local paleoclimate in floodplains in lower reaches of the Yellow River,being absent of high-resolution indicators?e.g.,lake and stalagmite?.?5?Paleovegetation changes that based on the alluvial pollen from the CLP indicate that the changes experienced several stages.During mid-Holocene,the CLP was overall dominated by grassland.In particular,its southern and southeastern regions with wet climate were dominated by meadow-or forest-grassland as well as the forest vegetation in valley-and bedrock-mountains.In late-Holocene,it was firstly dominated by grassland and secondly by forest-grassland,while forest and pine-dominated mixture of coniferous and broad-leaved forests were distributed in valley-and bedrock-mountains.From Qin-Han to Song-Yuan dynasties,the vegetation was characterized by grassland and a small portion of forest-grassland and steppe.In Ming and Qing dynasties,vegetation was featured as grassland and coniferous-dominated forest in valleys and hills in Qing Dynasty.?6?Wind erosion,rain-sweeping and leaching made it difficult to get access to well-preserved and high-resolution indicators in the CLP for reconstructing the paleoclimate and paleovegetation.In current,the pollen sediments are unable to exactly revert the vegetation in CLP,but pollen from flood sediments in Kaifeng City can offset this defect.Compared with existing studies that about reverted vegetation changes of CLP,our study is more qualified to revert the macro-vegetation changes of CLP.We further conclude that it is advisable for pollen in flood sediments from lower reaches of rivers to revert the vegetation changes in middle and upper reaches.