Holocene Climate Change Revealed By?- Cellulose ?¹⁸O Data Of An Alpine Peat Core From The Altai Mountains,Northern Xinjiang

In the past,the ice-core and stalagmite?¹⁸O records have been used to track the precipitation oxygen isotopic composition(?¹⁸O_p)during the Holocene in the arid central Asia?ACA?that mainly controlled by westerlies,but lacking other indicators with reliable chronology and clear hydroclimatic significance.Peat has the advantages of high-resolution and well-dating,and stable oxygen isotopic composition in peat?-cellulose(?¹⁸O_cell)is very important tracer of regional hydroclimate.Theoretically,the stable hydrogen and oxygen isotopic composition of plant?-cellulose are determined only by the source water isotopic composition.If soil water derived from meteoric water,thus the hydrogen and oxygen isotopes in plant?-cellulose can record the signal of precipitation isotopes.It has long been recognized that?-cellulose oxygen isotope in tree-ring is an indicator of paleo-thermometer,especially in middle and high latitudes.However,with the further study of modern process,many scholars have recognized that?¹⁸O_cell is also can be influenced by other climatic factors such as the precipitation amount,relative humidity,moisture source and hydrological inputs.In China,the relevant application is relatively simple,some scholars directly thought that the?¹⁸O_cellell data in the herbaceous peatlands in the Eastern monsoon region can record the changes of surface temperature during the Holocene.However,use of peat?-cellulose oxygen isotope for paleoclimatic and paleohydrological reconstructions in the ACA is very rare,and there is no relevant research regarding the linkage between hydroclimatic processes and?¹⁸O_cell in modern vascular sedge plants,especially for vascular-plant-dominated alpine peatlands.What the hydroclimatic significance of?¹⁸O_cell in alpine peatlands in the ACA?Whether the peat?¹⁸O_cell data in these regions can still reflect variations in temperature?Relevant research needs to be urgently addressed.Therefore,we used the stable oxygen isotope of peat?-cellulose from a continuous peat deposited based on the research of the modern process to reconstruct the paleoclimatic evolution history during the Holocene in a vascular-plant-dominated alpine peatland in the southern Altai Mountains,Northern Xinjiang.The main results are as follows:In order to determine the potential of?-cellulose?¹⁸O(?¹⁸O_cell)data from multiple terrestrial plant species to record the?¹⁸O data of precipitation(?¹⁸O_p),especially in relatively cold and arid environments,we collected samples of terrestrial plant residues from 104 surface soils covering a large environmental gradient?²9°N to 51°N in latitude,⁸1°E to 124°E in longitude,²00 to 5100 m in elevation?in inland China.Despite the diverse vegetation types,wide range of climatic conditions and potentially complex biogeochemical processes operating within the vast study region,the?¹⁸O_cellell data showed a similar spatial pattern to that of?¹⁸O_p.The?¹⁸O_cell values of the terrestrial plant residues have a significant positive correlation with?¹⁸O_p along the entire transect?R²=0.62,n=104,p<0.001?.The results demonstrate that the measured values of?¹⁸O_cell data have the potential to record variations in?¹⁸O_p.Further analysis indicated that the?¹⁸O_cell data from the Tibetan Plateau?TP?were primarily controlled by the differing moisture sources including Indian summer monsoon and westerlies,and were less influenced by other environmental factors.The?¹⁸O_cell data from the northern region of China including the East Asian monsoon-margin region?EAMM?and the Arid central Asia?ACA?were mainly controlled by temperature,although the influence of relative humidity in this region is not negligible.To test whether?¹⁸O_cell can be used as a hydroclimatic indicator in vascular-plant-dominated alpine peatlands,we investigated the hydro-climatology,?¹⁸O-values of?-cellulose extracted from the dominant plant species?Carex pamirensis?and those of potential source waters during two growing seasons from May to September in 2014and 2017 at the Sahara sand peatland?SSP?in the southern Altai Mountains,Northern Xinjiang.We found that the meltwater from snow/ice cover and seasonally-frozen soil played a critical role in controlling the hydrological processes in this alpine peatland.The?¹⁸O-values of swale water?in 2014?and soil water?in 2017?responded more strongly to the variations of inflow meltwater rather than to summer precipitation?r=0.72,n=15,p=0.002?.IsoSource modelling demonstrated that the mean 76-24%split between meltwater and summer precipitation inputs determined the overall isotopic composition of peatland water.These findings suggest that inflow meltwater is the dominant source of water for the alpine peatland,while summer precipitation is less important.The LMWL,meltwater line and peatland water lines are all closely aligned,suggesting minimal evaporative isotopic enrichment from those external source waters due to the high relative humidity?73⁸0%?in the studied fen.However,the slopes of the stem water line?4.53?and leaf water line?3.57?were generally lower than that of the LMWL,indicating that the internal plant waters?especially leaf water?have experienced isotopic enrichment during transpiration.Accordingly,the?¹⁸O variations of swale/soil water?especially for deeper soil water?can directly reflect that of precipitation?¹⁸O_p.Interestingly,the variations of?¹⁸O_cell?of shoots,stems and leaves?fall within a relatively narrow range?<±3‰?,which is mainly attributed to a nearly constant monthly net ¹⁸O enrichment factor(11.13±1.07‰for?_Modelled)due to the very limited variation of relative humidity during daytime?08:00–20:00?on the monthly timescale,together with a smoothing effect.Furthermore,empirical correlation and Roden–Lin–Ehleringer?RLE?mechanistic modelling both showed the co-variation of external source water and cellulose synthesis,demonstrating that the cellulose oxygen isotope composition of C.pamirensis can faithfully reflect variations in soil/swale water.Therefore,we conclude that winter half-year?October-April?precipitation?¹⁸O_p plays the predominant role in determining the?¹⁸O_cell values of C.pamirensis in the study region.Analysis of data?1980–2003?from six stations?Wulumuqi,Barabinsk,Salekhard,Pechora,Perm and Kirov?of the Global Network of Isotopes in Precipitation?GNIP?in the adjacent Altai Mountains,demonstrated that the winter precipitation?¹⁸O_p has a significantly positive correlation with winter temperature?R²=0.51,n=263,p<0.001?rather than winter precipitation?R²=0.02,n=263,p>0.05?.For the months?October-April?of winter half-year of the past² decades,our observational precipitation isotopes data of 2014,2017-2018,and the previously reported precipitation isotopes data of 2000-2002 and 2015-2016,showed that mean air temperature explains 69%of the variance in?¹⁸O_p during winter(p<0.001;slope=0.4‰?^-1),indicating a strong temperature effect on the winter precipitation isotopes in the Altai area.Furthermore,we investigated the linear correlation between coldest month?¹⁸O_p and corresponding temperature on inter-annual timescales,using data from the Wulumuqi?1987–2003?stations of the GNIP in the south of the Altai area.The results showed that there is a significantly positive correlation between coldest month?¹⁸O_p and corresponding temperature during the period of 1996-2003?R²=0.90,n=5,p=0.01?,suggesting winter temperature is the predominant factor controlling?¹⁸O_p variations on interannual to decadal timescales in these regions.In addition to the temperature effect,moisture source is also a significant control on?¹⁸O_p during the cold season at our study area in some abnormal years.For example,we found that above-average winter?¹⁸O_p in the Wulumuqi is associated with enhanced westerly flow over the North Atlantic region at January of 1988,whereas atmospheric blocking in the North Atlantic and Scandinavian regions resulted in a more northerly moisture source and below-average?¹⁸O_p at January of 1991.The changing moisture sources driven by atmospheric pressure anomalies probably contributed to a lesser extent to the?¹⁸O_p during the cold season in some years,but the winter temperature is the predominant factor controlling winter precipitation?¹⁸O_p at the long-term in Altai Mountains.Therefore,we concluded that the?¹⁸O_cell values of C.pamirensis is a paleoclimatic proxy of winter temperature in these regions.We present a 45-year-resolution alpine peat?-cellulose oxygen isotopic(?¹⁸O_cell)record from a 742 cm-long peat cores?ATM10-C7?at the SSP in the Altai Mountains.The chronology of the record has been well-constrained by 22 AMS¹⁴C dates of the peat?-cellulose samples.Based on above detailed modern-process study results,the indicative significance of the peat?¹⁸O_cell record has been carefully determined as winter temperature indicator.The record reveals an overall trend of increasing winter temperature for the past¹1 ka,with a colder early?11⁸ ka BP?and middle?6⁴ ka BP?Holocene but warmer late Holocene?4⁰ ka BP?.Concurrently,the oxygen isotopes of Altai peat?-cellulose also clearly recorded several significant climatic abrupt events such as 8.2 ka BP,4.2 ka BP and LIA.The winter temperature is mainly controlled by winter solar radiation,ice-sheet and greenhouse gases?GHG?forcings.Specifically,the lowest winter temperature in the early Holocene is mainly affected by lower winter solar radiation and survived high-latitude continental ice-sheet,while the lower winter temperature in the mid-Holocene is related to lower atmospheric greenhouse gas content form ca.8⁶ ka BP.The rapid warming trend in winter during the late Holocene may be related to the increase of winter solar radiation and superimposed upon the increasing the atmospheric greenhouse gas content.Finally,we combined the high-resolution Holocene winter temperature and summer temperature records based on measurements of peat?-cellulose?¹⁸O_cell and?¹³C_cell from the well-dated alpine peat core?ATM10-C7?at the SSP in the Altai Mountains.The results reveal long-term warming trends of annual mean temperature at the regional scale since the past¹1 ka,with a colder early and middle Holocene but warmer late Holocene.There are higher peat accumulation rates at the two lower temperature periods of 11⁸ ka BP and 6⁴ ka BP and showed obviously two stages of carbon sink.In contrast,there are lower peat accumulation rates at the two higher temperature periods of 8⁶ ka BP and 4⁰ ka BP correspond to two stages of carbon source.Interestingly,the changed in peat accumulation rate at the Altai Mountains during the Holocene was consistent with the growth rate of stalagmite at the Ural Mountains,and both the maximum values appear⁵ ka BP,indicating that the good correspondence between the carbon budget records in the middle and high latitude regions and Altai temperature records may be of universal significance.Comparison of the more results based on broader space ranges,we reasonably surmised the internal and external feedback mechanisms of the Earth's climate system during the Holocene.The temperature changes before⁵ ka BP can largely be explained by insolation;however,greenhouse gas?GHG?forcing was the dominant factor after⁵ ka BP as caused by increasing human activity and its positive feedback effect.Thus,we proposed that the onset of the climatological Anthropocene occurred at⁵ ka BP of middle Holocene rather than the early diagnosis of industrial revolution period.Overall,it is helpful to re-understand the real palaeoclimatic significance of oxygen isotope records of relevant meteoric precipitation(e.g stalagmite?¹⁸O)in Xinjiang region from our alpine peat?¹⁸O_cell records in the Altai Mountains.More importantly,the long-term warming trends during the Holocene revealed by our winter and summer temperature records from the Altai Mountains provide a reliable temperature evolution background,which is also very important for further reconciling the‘westerlies'and‘monsoon'models regarding Holocene moisture evolution in these regions.Against the background of long-term warming during the Holocene,the increased meltwater supply from snow and ice in high-altitude mountain areas fed the low-altitude lakes,which finally resulted in higher lake levels,expanded oasis areas,and enhanced local moisture recycling.In the future,with the likely acceleration of climatic warming,the further ablation of ice and snow in the high-altitude mountain regions will present a major and growing threat to the sustainable socioeconomic development of Xinjiang.Consequently,it is important that both the government and the public seriously consider the implications of an ice-free environment in these regions.