The Changes Of Depth To Water Table And Ecological Evolution Of Dajiuhu Peatland On Shennongjia Mountain Over The Past 13000 Years

The global peatlands include 500-700 Gt carbon.The distribution and evolution of global peat can be affected by the climatic changes significantly.How the deposition and ecological evolution of peatlands respond to the climatic changes,which is the key scientific question of‘global changes and regional responses'.The frequent hydrological events including floods and droughts caused by regional precipitation under the global changes have a significant impact on the ecological and carbon balance of peatlands even global carbon cycle.The abundant signals of palaeo-environment and palaeo-ecology can be extracted from peat deposits.Recognizing responses of ecological evolution to the palaeo-hydrological changes of peatlands is of prime importance.Dajiuhu is a rare alpine peatland of central China and has developed since the last deglaciation.Here,we explore the hydrological changes of Dajiuhu Peatland and the correlation of peatland hydrology and regional precipitation over the past 13000 years,and unltimatly focus on the ecological responses to the hydrological changes based on the AMS¹⁴C,modern ecological survey and peat deposits analysis.The main conclusions are as follows:1.The chronology and sedimentology of ZK5 Core:The deposition environmental evolution was reconstructed based on several geophysical and geochemical proxies.The grain size frequency curves displays as a unimodal model.The peak values are around 10?m,which indicates a stable deposit environment.The trace elements?except Cu?Pb?Zn?Cr?mainly derive from the mineral and have a negative correlation with the Total organic carbon?TOC,R<-0.9?.The evolution of Dajiuhu basin agrees well with the East Asian summer monsoon variations under the control of boreal summer insolation.2.The modern ecological survey and biological identification:The modern ecological survey showed that six wetland community types from wet to dry conditions responded to the six vegetation types including hygrophyte peat bogs,hygrophyte herbaceous marshes,degraded semi-hygrophyte marshes,hygrophyte-mesophyte meadow,moderate-xeric meadow,and xeric meadow according to the soil moisture?from wet to dry conditions?and vegetation distribution in study area.The phytolith analysis from surface soil showed that abundant phytoliths are found in topsoil samples from the Dajiuhu Peatland including cuneiform bulliform cells,parallelepipedal bulliform cells,saddles,bilobates,crosses,rondels,trapeziform poylobates,cylindrical polylobates,elongates,hair cells and some phytoliths from gymnosperm and broad-leaved trees.Differences in six phytolith assemblages from 43 surface soil samples in this study exist along the DWT gradients and are agreement with the characteristic differences in the six wetland community types.3.The reconstruction of phytolith-DWT transfer function based on modern process:The correlation of eight environmental factors including TOC,pH,magnetic susceptibility???,total nitrogen,total phosphrous,surface soil temperature,volume average particle size and Depth to water table?DWT?and phytolith assemblages is analyzed and the relative conclusions are as follows:Redundancy analysis?RDA?showed eight enivironmental variables captured 51.6%of the variance.Among them,DWT is the dominant environmental factor affecting phytolith assemnblages and captured 25%of variance,which is higher than other environmental variables.Weighted averaging partial least-squares regression?WA-PLS?was employed to reconstruct the phytolith-DWT transfer function and model performance is assessed using bootstrap cross-validation.The highly correspondence of the predicted DWT results based on phytolith-environment calibration data with observed data reflects that phytolith-DWT transfer function according to WA-PLS can be used to quantify the DWT changes of ombrotrophic?rain-fed?subalpine peatland.4.The reconstruction of palaeoclimate and its environmental significance:Quantitative DWT reconstructions of ZK5 are performed based on phytolith-DWT transfer function using modern phytolith data from the Dajiuhu Peatland topsoils and July DWT values.Three relatively shallow DWT stages?wet?occurring at 13000-11500cal yr BP(DWT_mean=7.28cm),9600-7500 cal yr BP(DWT_mean=10.37cm),and 3000 cal yr BP-present(DWT_mean=7.11cm)could be recognized during the past 13000 cal yr BP,compared with two extended deep DWT stages?dry?occurring at 11500-9600 cal yr BP(DWT_mean=19.14cm)and 7500-3000 cal yr BP(DWT_mean=25.73cm).The comparison between DWT variations of Dajiuhu and precipitation of the Middle Yangtaze Valley since the last deglaciation showed that the five DWT intervals are agreement with the regional monsoon palaeo-precipitation records.The climatic monitoring results also indicate that the shallow DWT(DWT_mean=2.1cm)occurs in summer and deep DWT(DWT_mean=6.0cm)occurs in winter,DWT agrees well with the rainfall(P_mean=5.6mm in summer,P_mean=0.9mm in winter)in Dajiuhu Peatland during 2015-2017.The both core records and monitoring results show that Dajiuhu Peatland is a typical ombrotrophic peatland and the DWT changes are controlled by regional rainfall.Quantitative temperature reconstructions of ZK5 are performed based on phytolith-temperature transfer function using modern phytolith data from the China topsoils and annual tempreture values.The temperature of Dajiuhu region agrees well with the boreal climatic changes during the past 18000 years,undergoing the last deglaciation?18000-10000 cal yr BP?,Holocene Optimum?10000-3000 cal yr BP?and Holocene Katathermal?3000 cal yr BP-present?.The assemblages of hydrology and temperature showed warm-dry B/A period and Mid-Holocene and cold-wet YD interval,early and late Holocene.On millennial timescales,the Warm-dry and cold-wet climatic patterns are performed in the middle Yangtze Valley over the past 13000 years.5.The relationship between?past/present?ecosystem and hydrology of Dajiuhu Peatland:The modern survey and peat core records indicate that the ecological changes including carbon cycle,microorganism activities and peatland vegetation respond to the hydrological variations sensitively.During deep DWT?dry?periods,the decreased CH₄emission(mean daily emission is 5.3 nmol.m^-2.s^-11 in winter),and increased CO₂emission(mean daily emission is 0.7?molCO₂.m^-2.s^-1in winter),damped peat?the lowest accumulation rate is 0.02 mm/yr?and carbon accumulation rates?6 gC/yr/m²?in middle Holocene induced weak carbon sequestration and carbon sink function;Increased aerobic bacterial activities?Hopnoid flux was more than 1 ug/g/yr in middle Holocene?,occurrences of more xerotolerant species of testate amoeba?e.g.Trigonopyxis arcule,Euglypha rotunda type,Assulina muscorum,Habrotrocha angusticollis?and diatoms?e.g.D.contenta,L.mutica,P.borealis,H.amphioxys?agree well with the dry conditions;The vegetation responses include decreased Pooideae percentages and plant biomass of hygrophytes,and increased plant biomass of mesophytes.On the contrary,during shallow DWT?wet?periods,increased CH₄emission(mean daily emission is 30.8 nmol.m^-2.s^-11 in summer)and decreased CO₂emission(mean daily emission is-3.6?molCO₂.m^-2.s^-11 in summer),enhanced peat?>0.07 mm/yr during early and late Holocene?and carbon accumulation rates?>10gC/yr/m² during early and late Holocene?induced enhanced carbon sequestration and carbon sink function;decreased aerobic bacterial activities?Hopnoid flux is about 0.2?g/g/yr during early and late Holocene?,rare xerotolerant species of testate amoeba and diatoms responded to the wet conditions.The vegetation responses include increased Pooideae percentages and plant biomass of hygrophytes,and decreased plant biomass of mesophytes.In total,this study quantifies the DWT changes of Dajiuhu Peatland over the past13000 years based on the phytolith-DWT transfer function using modern phytolith data from the Dajiuhu Peatland topsoils and July DWT values.The palaeo-DWT of Dajiuhu Peatland agrees well with the palaeo-precipitation of the Middle Yangtze Valley.This finding raises a new hydrological record of the Middle Yangtze Valley.Accroding to the modern survey and peat core records,the ecological changes of peatland respond to the DWT variations.Our results provide a reference to deeply explore the influences of peatland DWT on the ecological changes and global carbon cycle.