Zhuo Zheng

Cong Chen

Kangyou Huang
email: hkangy@mail.sysu.edu.cn

Xiao Zhang

Peter Kershaw

Jun Cheng

Jie Li

Yuanfu Yue

Qiuchi Wan

Yaze Zhang

Yongjie Tang

Mengyuan Wang

Xiayun Xiao

Rachid Cheddadi

Holocene warming and evergreen/deciduous forest replacement across eastern China

Keywords: Evergreen broadleaved forest, Deglaciation, Holocene, Biome change, Transient simulation, Climate threshold

Combined variations of insolation and ice sheet led to major global climatic and ecosystem changes between the Last Glacial Maximum (LGM) and the Holocene. However, little is known about effects of these changes on the alternating dominance of deciduous and evergreen vegetation in East Asia, particularly in the regions now dominated by evergreen broadleaved forest (EBLF) that forms a vast ecotonal zone between tropical and temperate belts. In this study, we have used two representative records in center of the EBLF distribution, associated with broader regional pollen data from eastern China, to reconstruct spatiotemporal biome changes over the last 22 ka. The results showed a dominance of deciduous broadleaved forest at the LGM in most mountainous regions of the subtropical zone, and the EBLF began to migrate and expand into the low latitude subtropical areas during the last deglaciation, particularly at ca.11.3 ka after the Younger Dryas. However, major biome replacement from deciduous to evergreen forest across eastern China did not occur until 8.1 ka, indicating an earlier forest replacement at sites in low latitude regions as compared to sites located further north or at higher mountains. Our evidence agrees well with quantitative climate reconstruction and simulations at local and regional scales, showing that forest transformed when thermal conditions reached relevant thresholds. Both simulated and reconstructed paleotemperatures are consistent with the early-tomiddle Holocene being a key period when climatic thresholds (particularly winter and/or annual temperatures) gradually approached then exceeded bioclimatic constraints on EBLF recolonization. By contrast, the precipitation in most part of mountainous subtropical southeast China may not be a key constraint for evergreen/deciduous forest transformation since the last deglaciation. During the late Holocene, the reconstructed biome has shifted unstably, probably due to interactive effects of several regional-scale factors, including seasonal climate biases and major fluctuations in effective moisture associated with weakening of summer monsoon before the enhanced human interventions of the last 3000 years.

Introduction

In East Asia, evergreen broadleaved forest (EBLF), also known as laurophyllous forest, is currently the most widespread vegetation type in the subtropical zone, extending from ~22° N, near the Tropic of Cancer, to the Yangtze River basin at 31-33° N in China and 33-35° N in southern Japan [START_REF] Wu | Vegetation of China[END_REF][START_REF] Fang | Atlas of Woody Plants in China: Distribution and Climate[END_REF]. During the Last Glacial Maximum (LGM), the mean surface temperature of mainland China and adjacent areas is estimated to have been 2-8°C colder than today [START_REF] Jiang | Last glacial maximum climate over China from PMIP simulations[END_REF][START_REF] Tian | Revisiting last glacial maximum climate over China and East Asian monsoon using PMIP3 simulations[END_REF]. Simulated and reconstructed biome distribution patterns during the LGM indicate a large-scale southward shift of temperate deciduous forests (DBLF) in regions where EBLF dominates today, followed by a gradual recolonization by continental evergreen biomes during postglacial global warming [START_REF] Yu | Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum[END_REF]Kageyama et al., 2012;[START_REF] Ni | Biome distribution over the last 22,000yr in China[END_REF]. However, little is known about the high-resolution temporal changes in forest transformation from deciduous to evergreen vegetation and the threshold climatic conditions required for forest replacement across the Asia since the last deglaciation. Important questions remain regarding the timing of the onset, dynamics, and causes of this large biome transformation over the subtropical belt in East Asia [START_REF] Zhao | Vegetation response to Holocene climate change in monsoon-influenced region of China[END_REF][START_REF] Li | Large-scale vegetation history in China and its response to climate change since the Last Glacial Maximum[END_REF].

As yet, there are few well-dated, high-resolution fossil records dating back to the LGM from subtropical regions [START_REF] Zheng | Review and data integration of pollenbased quantitative paleoclimate reconstruction studies in China and adjacent areas[END_REF], which severely hinders reconstruction of the nature of spatial vegetation shifts and their relationships with past climate change. Moreover, the modeled cold-tolerant DBLF biome in southern China is poorly represented as a vegetation type in the LGM fossil pollen record, casting doubt on the accuracy of reconstructed vegetation, both spatially and temporally [START_REF] Yu | Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum[END_REF][START_REF] Harrison | Palaeovegetation diversity of temperate plants in East Asia[END_REF]. Therefore, we here have used two representative pollen records, together with other regional pollen and biomarker data, to reconstruct past biome changes, focusing on the EBLF's migration history in subtropical Asia. The two high-resolution records are located in a sensitive montane forest zone in central subtropical regions, where current climatic conditions are near the temperature thresholds for shifts between evergreen and deciduous forest. As temperatures constrain plant photosynthetic efficiency physiologically [START_REF] Bai | Photosynthetic response to seasonal temperature changes in evergreen and deciduous broad-leaved trees in montane forests of Ailao Mountain and Mao'er Mountain[END_REF][START_REF] Song | Evergreen-deciduous broad-leaved forest ecotone in eastern China: Retrospect and new perspectives[END_REF], and thus are key controls of biome change between EBLF and DBLF, we quantitatively reconstructed paleotemperatures using pollen and biomarker data and linked them to local and regional climate simulations by a transient model. The results indicate dramatic biome and climate changes during the last 22 ka in subtropical Asia, and past temperature thresholds for forest transformation under deglacial warming from the LGM to the Holocene. The study has important implications for the projection of future forest migration in Asia under global warming.

Materials and Methods

Regional setting and materials

Vegetation in the subtropical zone of China is composed mainly of EBLF that can be divided into two main components: humid forest in southeastern China and semi-humid mountainous forest in southwestern China, the southeast margin of the Tibetan Plateau [START_REF] Wu | Vegetation of China[END_REF]. The eastern subtropical humid EBLF region generally includes three subzones: a northern subtropical evergreen and deciduous broadleaved forest zone (STa), middle subtropical zone (STb) and southern subtropical zone (STc). The northern boundary of the subtropical zone is near a geographical line joining Qinling mountain ranges and Huaihe River, and the northern subzone (STa) is dominated by deciduous oak forest mixed with evergreen broadleaved trees. The STa subzone has also been assigned to an independent vegetation ecotone (evergreen-deciduous broadleaved forest) of the warm-temperate zone, dominated by deciduous trees [START_REF] Fang | Vegetation-climate relationship and its application in the division of vegetation zone in China[END_REF]. However, the large lowland river plains of this subzone are cultivated with rice and aquatic plants. The middle subzone (STb), restricted to the area between the Yangtze River and Nanling mountain ranges, is characteristically covered by EBLF dominated by evergreen genera such as Cyclobalanopsis, Castanopsis, Lithocarpus, Schima, Machilus, Phoebe, Altingia, Elaeocarpus and Eurya. The southern subzone (STc) hosts monsoonal humid evergreen forest characterized by genera from families including Fagaceae, Lauraceae, Theaceae, Myrtaceae, Meliaceae, Moraceae, Myrsinaceae, Rubiaceae and Hamamelidaceae. In the west, there is a semi-humid subzone (STw), typically comprising alpine EBLF characterized by montane sclerophilous Quercus (Sect. Heterobalanus), subalpine Cyclobalanopsis and pine forests [START_REF] Wu | Vegetation of China[END_REF]. The distribution of EBLF in the subtropical East Asia is strongly influenced by monsoon climate circulation. The mean annual temperature of the EBLF distribution area is usually between 16 and 22℃, with a relatively warm winter generally higher than 0℃ [START_REF] Song | Perspective of the vegetation zonation of forest region in Eastern China[END_REF]. Annual precipitation generally exceeds 1100 mm [START_REF] Fang | Climate and vegetation in China V. Effect of climatic factors on the upper limit of distribution of evergreen broadleaf forest[END_REF], and the relative humidity is about 75-80% on average. We reconstructed past vegetation changes during the transition from the LGM, at around 22 ka, to the Holocene. For this, we used two high-resolution pollen records, based on cores from the sites of Gutian (GT) and Shuizhuyang (SZY) in the middle subtropical EBLF subzone (STb), together with other pollen data from sites across the zone (Fig. 1a, Table 1). The GT site is in the hinterland of the Yuecheng Mountains in the west Nanling Ranges, southern China (110.366°E, 26.084°N;1677 m asl). This is in the southern part of the central subtropical zone, which is strongly influenced by the monsoon climate system. The local mean annual temperature is about 14-15°C, and the coldest mean temperature (January) is only 1-2°C at an altitude of about 1000 m. The annual precipitation, of 1200-1600 mm, is concentrated between the end of March and July. Lithologically, the GT core comprises a thin sand layer interbedded with gravel at the base, grey silty clay at 140-194 cm depth, dark clay with a high plant debris content at 22-140 cm, and brown root-rich gyttja in the upper 0-22 cm layer. The SZY core was collected from a mountain peat bog in Jiufeng Mountains, southeastern China (119.034°E, 26.777°N; 1007 m asl) [START_REF] Yue | A continuous record of vegetation and climate change over the past 50,000 years in the Fujian Province of eastern subtropical China[END_REF]. The site is in the mid-subtropical zone, with an annual mean temperature and precipitation of about 15°C and 1400-1800 mm, respectively.

Zonal vegetation types at the GT and SZY sites are evergreen broadleaved and evergreen/deciduous mixed forest, respectively. Each site is in a summit wetland of a mountainous area, with a local landscape mainly covered by shrubs and grasses of various families, including Ericaceae, Theaceae, Saxifragaceae, Clethraceae, Rosaceae, Symplocaceae, Poaceae, Cyperaceae, and Asteraceae. [START_REF] Wu | Vegetation of China[END_REF]. The dots are locations of the examined pollen records from eastern China (108 sites in total), and those in red (1-17) were used for EBLF data integrations (Table 1). (b) The potential distribution of EBLF today as simulated by the Species Distribution Model (SDM). The most important bioclimatic thresholds are indicated using quartile values (25%, 50% and 75% in green boxes). (c) Percentiles for the five key climatic variables characterizing the EBLF distribution in southern China, based on 500 random grids of digital georeferenced data: annual precipitation (Pann) and mean temperatures of the coldest month (Tjan), winter (Twin), summer (Tsum), and year (Tann). The chronology for the GT core was based on 10 AMS 14 C dates (Fig. 2, Table 2). For the SZY core, new 14 C dates were obtained and additional pollen analyses carried out to improve the temporal resolution, especially for the late glacial and deglacial intervals. To incorporate age uncertainties associated with the resolution of pollen sampling, Bayesian estimation was applied to age model outputs using the Bchron package in R statistical software (Parmell et al., 2008). This provided probability distribution over the mean age of transitions between successive sampling levels. All 14 C dates were calibrated to calendar years using IntCal13 [START_REF] Reimer | Intcal13 and Maeinre13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]. Median values were used for interpolation at different sample depths, except for the transition from the last glacial to the Holocene in the SZY core, where 90% quantiles were assumed between the two dating points. 153 Following a preliminary examination of a broader fossil pollen dataset [START_REF] Cao | A late Quaternary pollen dataset from eastern continental Asia for vegetation and climate reconstructions: Set up and evaluation[END_REF][START_REF] Penny | A 40,000 year palynological record from north-east Thailand: Implications for biogeography and palaeo-environmental reconstruction[END_REF][START_REF] Zheng | Review and data integration of pollenbased quantitative paleoclimate reconstruction studies in China and adjacent areas[END_REF], we selected records from sites in relatively closed basins (upland lakes or marshes) within the subtropical zone with a reliable age model for comprehensive regional analysis. Pollen records from of alluvial, deltaic and river-fed lacustrine sedimentary materials were excluded due to the uncertainty of pollen sources transported by rivers. Thus, only 14 well-dated pollen records (Table 1) were used for regional data integration. Of the selected records, pollen analysis from GT, SZY, JXA, NT and DJH cores were conducted by our laboratory [START_REF] Yue | A continuous record of vegetation and climate change over the past 50,000 years in the Fujian Province of eastern subtropical China[END_REF][START_REF] Li | Vegetation changes during the past 40,000 years in Central China from a long fossil record[END_REF][START_REF] Han | Holocene climate records from a mountain wetland in Luoxiao ranges[END_REF].

Methods

We analyzed pollen in samples from several cores collected from the subtropical zone after preparation following standard techniques [START_REF] Faegri | Textbook of Pollen Analysis[END_REF] with minor amendments. Each sample consisted of 2-3 g of sediment and after chemical treatment approximately 500 pollen grains were counted and classified in most samples. The pollen total used for percentage calculations was based on the total number of pollen grains counted, excluding aquatic pollen and spores, and percentages of pteridophyte and fungal spores were based on total counts of palynomorphs. Pollen taxa were grouped by ecological affinity and distribution of parent plants. Pollen groups classified by plant ecological affinity were used to summarize structural-functional changes in the vegetation, with no pollen taxon assigned to more than one group. We summed relative values for the following groups: cold-tolerant conifers, temperate deciduous broadleaved trees, subtropical evergreen broadleaved trees, eurythermic and subtropical warm conifers, forbs, and wetland plants. For comparative study of regional variation trend, the percentages of arboreal pollen group of evergreen broadleaved trees (EBT) in the selected lacustrine pollen records were standardized by rescaling to values from 0 to 1, and the tropical records (from three sites) and subtropical records (from 14 sites) were separately combined. The EBT pollen data from temperate zone were not used for synthetic analysis because the dominant component is always deciduous taxa rather than evergreen.

To illustrate spatiotemporal vegetation changes in the study area, we used biomes that are geographically and climatically distributed in broad physiognomic zones. Pollen-based biome reconstruction has been successfully performed by applying the biomization technique to various plant functional type (PFT) schemes [START_REF] Yu | Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum[END_REF][START_REF] Chen | Quantifying modern biomes based on surface pollen data in China[END_REF]. A recently described scheme for converting pollen taxa to biomes, with some modifications to the PFTs to suit the particular subtropical situations in China [START_REF] Ni | Biome distribution over the last 22,000yr in China[END_REF] and adjacent islands [START_REF] Lee | Late Quaternary vegetation and climate changes inferred from a pollen record of Dongyuan Lake in southern Taiwan[END_REF][START_REF] Gotanda | Biome classification from Japanese pollen data: Application to modern-day and Late Quaternary samples[END_REF] was applied to the GT, SZY and other records. Essentially, we applied a biome PFT taxon matrix previously developed for East Asia [START_REF] Ni | Biome distribution over the last 22,000yr in China[END_REF], apart from the exclusion of some PFTs from highlatitude Arctic and boreal zones, such as Arctic shrub, boreal summergreen, and conifers PFTs. The biome taxa matrix was then simplified to focus on the subtropical region that includes locations of the fossil pollen sites. In total, 136 pollen taxa were assigned to 18 PFTs (Table 3). The defined composition of each biome in terms of PFTs is given in Table 4, and 9 biomes from the study area was used. In accordance with previous modifications of the subtropical zone biome, ts2 (a PFT characterized by Alnus and Fagus) as proposed by [START_REF] Yu | Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum[END_REF] was assigned exclusively to the DBLF biome. The alpine shrub-land biome (ALSH) [START_REF] Lee | Late Quaternary vegetation and climate changes inferred from a pollen record of Dongyuan Lake in southern Taiwan[END_REF], characterized by subtropical mountain vegetation, was also included. The results of biome reconstruction for modern pollen spectra indicate that most samples are correctly assigned. The improved result of biome reconstruction has good correspondence with the present-day vegetation distribution across eastern China (Fig. 3), and some exceptions from the northern subzones of subtropical regions show a dominance of DBLF and CLMX biomes caused by temperature gradients in mountainous regions (samples below 800 m were deleted due to strong human disturbance). We used the difference in biome scores between EBLF and DBLF (E-T) to evaluate the biome shifts. We applied the transfer function method to reconstruct the paleoclimate quantitatively from the SZY pollen record, using the Locally-Weighted Weighted-Averaging (LWWA) model [START_REF] Juggins | Exploratory Data Analysis and Data Display[END_REF][START_REF] Juggins | Quantitative Environmental Reconstructions from Biological Data[END_REF]. Modern pollen data were extracted from the recently updated Eastern Asia Pollen Dataset (EAPD) [START_REF] Zheng | East Asian pollen database: modern pollen distribution and its quantitative relationship with vegetation and climate[END_REF][START_REF] Zheng | Modern pollen data from the East Asian Pollen Database (EAPD): pollen, vegetation and climate relationship[END_REF], with the addition of some recently completed samples. An area in mainland China between 20-40 N latitude and 105-120 E longitude around the research site (SZY) was selected (Fig. 4). We screened the modern samples and excluded those that had clearly been disturbed by human activities, leaving 683 surface samples for use in the calculations. We use the branched glycerol dialkyl glycerol tetraether (brGDGT) as an independent method to quantify past temperature changes [START_REF] Wang | Branched GDGT-based paleotemperature reconstruction of the last 30, 000 years in humid monsoon region of Southeast China[END_REF]. Samples for biomarker analysis were taken from the same core (SZY) of pollen analysis, which was measured using high-performance liquid chromatography/atmospheric pressure chemical ionization-mass spectrometry. GDGTs were detected by using the selected ion monitoring (SIM) mode, following the methods described by [START_REF] Weijers | Environmental controls on bacterial tetraether membrane lipid distribution in soils[END_REF]. The quantification of GDGTs was achieved by integration of the peak area of [START_REF] Liu | Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming[END_REF]2012). We selected an area covering 22°-27° N to 105°-120° E to represent subtropical southern China to compare results of the simulations with corresponding proxies. The simulated local climate variables of the studied sites were extracted directly from the corresponding spatial grids simulated by TRACE21 model [START_REF] Liu | Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming[END_REF]. Six climatic variables are analyzed:

annual mean precipitation and mean temperature, mean winter (December-February) and summer (June-August) temperatures, and indices of temperature and precipitation seasonality (corresponding to WorldClim bioclimatic variables 4 and 15) [START_REF] Hijmans | Very high resolution interpolated climate surfaces for global land areas[END_REF]. These variables were chosen because they provided the maximal explanatory power of potential predictors, although some colinearity was still present, especially for winter temperature and temperature seasonality.

Results and discussion

Pollen records and biome reconstructions

We regard results from the two high-resolution pollen records as representative of vegetation changes in the middle subtropical zone during the last glacial-interglacial transition. Both records indicate a substantial change in distribution of the subtropical ecosystem from the LGM at 22 ka BP to the Holocene. The pollen at the LGM at sites of the two records was dominated by cold-tolerant, temperate broadleaved forest taxa (Fagus, Carpinus, Betula, Corylus, Juglans and deciduous Quercus), mixed with cold-tolerant alpine conifer taxa (Tsuga and Abies), indicating that temperatures were much lower than today (Figs. 5 and6). During the Bölling/Alleröd (B/A) warm period (15.5-13 ka BP), there was an early increase in pollen of the evergreen forest tree genus Cyclobalanopsis.

However, the taxa Castanopsis/Lithocarpus and Altingia increased significantly and reached dominant percentages until the middle Holocene at both sites. Among the non-arboreal pollen (NAP) taxa,

Poaceae was the most abundant, and tended to increase, reaching maximum percentages in the late Holocene. Cyperaceae, a wetland plant, began to increase at ca.12 ka and peaked in the mid-Holocene, positively correlating with pollen taxa of evergreen broadleaved trees. We used pollen groups classified by plant ecological affinity to synthesize the taxonomic pollen data (Fig. 7), and reconstructed biomes using the biomization technique [START_REF] Yu | Palaeovegetation of China: a pollen data-based synthesis for the mid-Holocene and last glacial maximum[END_REF] based on its successful application with modern pollen samples (Fig. 3). Arboreal taxa of the two pollen records indicated the dominance of temperate deciduous broadleaved trees (DBT), corresponding to the DBLF biome, during the glacial and early Holocene periods. Such temperate deciduous forest vegetation indicates glacial climatic conditions different from those of today in the subtropical zone. During the Bölling/Alleröd (B/A) warm period (16-13 ka), there was a moderate increase in pollen group of evergreen broadleaved trees (EBT). The primary increase in evergreen broadleaved tree (EBT) during the B/A was more pronounced at SZY than GT, reflecting the difference in sites' elevation (1007 and 1677 m asl, respectively). The biomization result shows that the evergreen forest biome (EBLF) during the last glacial and early deglacial periods did not exceed those of temperate deciduous broadleaved forest (DBLF). The primary increase in broadleaf evergreen forest components since the last deglaciation was interrupted during the cold Younger Dryas (YD). Such an abrupt change was also marked by increases in forbs and sedges, reflecting drier condition on land and locally shallowing lake waters. After the YD, EBT forest group slowly increased again from about 11.3 ka, and biome change from DBLF to EBLF occurred at 8.7 ka in GT and 8.1 ka at SZY (Fig. 7), reflecting a time difference between latitudinal locations. In particular, the reconstructed biome changes during the mid-Holocene showed that the decrease in DBLF was more significant than the increase in EBLF. There was also only a marginal dominance of EBLF or a mixed EBLF and DBLF biome at sites characterized by relatively high elevation and low air temperature, such as GT (1677 m asl), during the middle to late Holocene. This elevational effect suggests that deglacial migration of vegetation occurred not only along a north-south latitudinal transect but also along an elevational gradient. Over the last 4,000 years, EBLF decreased relative to DBLF, indicating that the forest type was more consistent with the evergreen/deciduous mixed forest since then. In addition, the abundance of non-arboreal taxa (forbs) increased progressively in both records, suggesting a drying trend that probably had interactive effects with anthropogenic influences, at least within the last 2 ka [START_REF] Ma | Holocene fire and forest histories in relation to climate change and agriculture development in southeastern China[END_REF]. 3).

Quantitative climate reconstruction and comparison with simulation result

We applied a quantitative method to reconstruct the paleoclimate from the SZY pollen record. The mean annual temperature (Tann), which may strongly influence regional forest distributions, passed the significance test for the reconstruction (Fig. 4). We selected annual mean temperature as the representative parameter to explore the vegetation-climate relationship. The result shows that the estimated Tann from the SZY fossil pollen record was lowest (about 8°C) during the LGM, and it increased to 15-17°C during the mid-Holocene, with an amplitude in variation of up to ~8°C between the glacial and postglacial periods (Fig. 8). The reconstructed temperatures below 12°C

during the glacial period from 22 ka to 15 ka are consistent with conditions suitable for the deciduous forest biome, but far from the threshold temperatures required by evergreen forest. This quantitative estimate falls within the framework of previously simulated temperature using climate models,

showing that in subtropical areas annual climatic conditions for deciduous forest expansion during the LGM could include annual temperatures as low as 11°C [START_REF] Ju | Simulation of the Last Glacial Maximum climate over East Asia with a regional climate model nested in a general circulation model[END_REF][START_REF] Tian | Revisiting last glacial maximum climate over China and East Asian monsoon using PMIP3 simulations[END_REF].

According to the reconstruction there was a temperature rise at 15 ka, corresponding with the B/A warming. However, after the YD the rate of warming in the SZY locality slowed, with fluctuations, and only increased rapidly to near the EBLF threshold temperature (>15°C) after the 8.2 ka event.

The perfect match of quantitative climate data with biome changes revealed that the temperature threshold leading to EBLF as the dominant forest was about 15-17℃ in the mountains of the middle subtropical zone. It should be noted that both the SZY and GT sites are on summits of subtropical mountains, which may receive pollen carried by upward wind from nearby lower forest belts. To validate the pollen-based temperature proxy, we performed a parallel reconstruction of air temperature from the same core (SZY) using brGDGT [START_REF] Wang | Branched GDGT-based paleotemperature reconstruction of the last 30, 000 years in humid monsoon region of Southeast China[END_REF], as an independent climate reconstruction method. The brGDGT-based Tann indicated a rapid increase 13°C to >15°C at ~8.1 ka, which is largely consistent with the pollen-based climate reconstruction. Similar degrees of variation and thresholds apply to the two temperature proxies, particularly for the key timing of the YD event and the substantial forest transformation starting at ~8.1 ka, despite some differences between absolute values of the LGM derived by the two methods. Overall, there is strong agreement between forest transitions and quantitative climate changes derived from pollen and other biomarkers in the same sediment material (SZY), both long-term (orbital scale) and short-term (rapid shifts, e.g., in the YD event), indicating sensitive response to climate change (Fig. 3). Since about 8.1 ka, tThe reconstructed annual temperature derived from both pollen and branched glycerol dialkyl glycerol tetraethers (brGDGT, see below) remained consistently high in the mid-to late Holocene (15-18℃), since about 8.1 ka, with only a clear cooling event at about 3.5 ka.

We used transient simulation, generated with TRACE21 [START_REF] Liu | Transient simulation of last deglaciation with a new mechanism for Bølling-Allerød warming[END_REF], to assess the seasonal and annual climatic changes in the study area during the last 21 ka. The modeled climate data corresponding to the geographical location of the SZY study site were extracted, and systematically corrected according to the present-day local climate observations. The results show that modelled mean annual and winter temperatures, characterized by progressive increase towards the late Holocene, were crucial for changes in forest components (Fig. 8). The simulated mean annual temperature (Tann) threshold for local EBLF as the dominant forest was higher than 15°C, in good agreement with the pollen-and brGDGT-based climate reconstructions. In terms of seasonal climate, a threshold simulated winter temperature of about 4°C for EBLF expansion in subtropical mountainous areas was obtained. The simulation indicates that precipitation may not be a key factor for the focal forest transformations, because it remained similar during the covered period, with annual rainfall consistently exceeding 1000 mm since the LGM (Fig. 8e). In summary, the modelproxy comparison revealed good consistency for the climate thresholds for local forest transformation from DBLF to EBLF, except for a few differences in the late Holocene.

Evergreen/deciduous forest alternation since the last glacial maximum across eastern China

Three distinct evolutionary stages of vegetation change over the subtropical zone could be identified (Fig. 8). During the LGM (stage I), the dominant biome was DBLF, with negative score values of biome difference between EBLF and DBLF (E-D) ranging from -15 to -5. During stage II (from 15 to 8.1 ka), this score gradually increased from -10 to +10, most strongly between 11.3 and 8.1 ka. This indicates a transition period characterized by an increase in the evergreen biome to an extent equal to or greater than that of the deciduous biome. However, the score only rose from -10 to 0 in the SZY and GT records, due to their locations in mountainous areas at altitudes higher than 1000 m. During stage III (after 8.1 ka), substantial evergreen forest recolonization occurred, with E-D scores rising up to +5 to +20. Integration of pollen records from the subtropical region consistently shows that the two abrupt global cooling events (i.e., the YD and 8.2 ka event) had major impacts on the regional vegetation and Asian climate system [START_REF] Wang | A highresolution absolute-dated late Pleistocene monsoon record from Hulu Cave[END_REF][START_REF] Cheng | Timing and structure of the Younger Dryas event and its underlying climate dynamics[END_REF]. Our data from across the subtropical region indicate a period of maximum EBLF dominance between 8 and 5 ka, corresponding to the Holocene Thermal Maximum (HTM). The late Holocene records from the study sites show a decline of EBLF or an unstable fluctuation with complex changes in the local ecosystems, such as a decline in wetlands, colonization by terrestrial forbs instead of forest trees, and expansion of secondary forest with abundant pioneer ferns (Figs. 5 and6).

Previously compiled vegetation proxy data suggest that the modern forest distribution in southeastern China does not differ greatly from that of the LGM, and that forest cover has changed little (mostly between 5%) during the last 22 ka (Tian et al., 2016). However, our pollen data and biome reconstruction, which distinguish between deciduous and evergreen forests, suggest a fundamental change in forest type occurred during the last glacial cycle. In this study, we compiled data on pollen groups composed of representative evergreen broadleaved trees from multiple records across the subtropical zone (Fig. 9). The results indicate that an initial increase in EBT during the B/A warming coincided with a peak period in the tropical zone, and hence an early northward expansion of evergreen forests. However, since the YD event, the decline of EBT in the tropics indicates an opposite trend to that of the subtropical zone. Such contrasting trends in EBT between the tropical and subtropical zones since the end of YD can be interpreted as a northward biome migration. The timing of EBLF biome transformation at four locations from 22° N to 27° N latitude shows a successive northward shift, with dates at 11.2, 10.2, 8.7, and 8.1 ka (Fig. 9c), strongly suggesting a timetransgressive pattern. That is, vegetation response to Holocene warming apparently occurred earlier in the southern subzone than in the middle and northern subzones. The results also indicate that the latitudinal limit of the evergreen forest was south of site DJH, located at the northern subtropical boundary (31.5° N latitude and 1751 m asl). Site DJH was not dominated by EBLF during the mid-Holocene, and it is still not dominated by EBLF today, which is not surprising as the current annual temperature (Tann) is about 7-8°C at an elevation of 1700 m, far below the climatic range for EBLF dominance. The findings suggest that the EBLF's northward migration during the HTM only reached the mountainous regions south of 31° N. In order to reconstruct the regional shifts of EBLF across eastern China and better understand its spatiotemporal changes, we selected a key pollen taxon Castanopsis/Lithocarpus which is dominant in evergreen broad-leaved forest for data integration of different climate zones. The taxon of Quercus was not used because of the great uncertainty in the identification of pollen taxa at the genus level between evergreen-type and deciduous-type. A total of 108 records from the pollen database in eastern China [START_REF] Cao | A late Quaternary pollen dataset from eastern continental Asia for vegetation and climate reconstructions: Set up and evaluation[END_REF][START_REF] Zheng | Review and data integration of pollenbased quantitative paleoclimate reconstruction studies in China and adjacent areas[END_REF] have been examined. The data integration results of different climate zones in eastern China (longitude and latitude ranges from 100 to 125°E and from 18 to 40°N) show the increases in Castanopsis/Lithocarpus (equivalent to EBLF) since the last deglaciation represent a significant time-transgressive forest response from south to north. The proportion of evergreen forest elements decreased obviously in the northern subtropical subzone, and almost disappeared northward in the temperate zone (Fig. 10). It is clear that the EBLF distribution was restricted to the south of northern subtropical boundary even during the HTM. Besides, the EBLF expansion along altitudinal gradients in mountain ranges of the middle subtropical zone (e.g. GT record at 1677m asl.) also lagged behind those in the southern subtropical lowland regions and/or coastal plains. The large-scale biome shifts across eight degrees of latitude, from 22 to 30°N, and the high temporal precision of the fossil record, enabled us to estimate the forest migration rate. Our results indicate that a fundamental biome upheaval began as early as 11.2 ka, initially in the southern subtropical subzone (south of ~22°N). General recolonization of evergreen forests in the middle subtropical mountains (around 24-28°N) then accelerated during the mid-Holocene (8-6 ka), until the modern northern subtropical boundary was reached at around 6-5 ka. According to the relationship between migration distance and time, the average migration rate was about 155km/ka northward during the transition period from early to middle Holocene.

Regional climate changes and thresholds leading to EBLF expansion

The general patterns of deglacial warming observed in the climate simulations may help to improve understanding of the forcing and dynamic mechanisms involved in spatial migration history of EBLF.

For each individual locality, the EBLF variation in many records shows a rapid change in forest replacement within a relatively narrow time span, suggesting that climatic thresholds may play a key role in forest transformation. Among the bioclimates required for EBLF growth, temperatures are the most important determinants for its boundaries in China [START_REF] Fang | Climate and vegetation in China V. Effect of climatic factors on the upper limit of distribution of evergreen broadleaf forest[END_REF][START_REF] Hu | Climate boundaries in vertical forest vegetation zonation of subtropical east Chinese mountains-a case of the southwest mountainous areas in Zhejiang. Resour[END_REF].

Annual and winter temperatures from the transient model over the subtropical zone show consistent variation with integrated subtropical biome changes between deciduous and evergreen broadleaved forests. There was a moderate increase in the modeled temperatures, particularly in summer, during the B/A warming. This resulted in a limited increase in EBLF, mainly in the southeastern and lowaltitude parts of the subtropical area near the west Pacific Ocean, such as the Chinese coastal regions and islands (e.g., sites 11 and 12 in Fig. 9c). However, both simulated and proxy climate variables for the mountainous localities in the central subtropical zone (e.g., SZY record) indicate that temperatures remained below the thresholds for evergreen forest transformation during the early deglacial warming (B/A). Thus, dominance of the deciduous forest biome persisted there. Soon after the YD cooling, simulated temperatures for the subtropical area reached regional mean thresholds of ca.16.7°C (Tann) and 8.0 °C (Twin) (Fig. 9d). Such average climate thresholds for EBLF dominance during the Holocene in subtropical China are highly consistent with current optimum climate conditions for EBLF simulated using the Species Distribution model (Hijmans et al., 2016), with median percentiles of 16.7°C and 8.1°C for Tann and Twin, respectively (Fig. 1c). Our results are also consistent with meteorological data delineating the modern EBLF distribution in the subtropical mountains [START_REF] Liu | Variations in the vertical vegetation zonation of subtropical Chinese mountains: The importance of climatic seasonality[END_REF]. It should be noted that in the vertical and northern boundary areas of the subtropical region, the temperature thresholds for EBLF transformation are slightly lower than the regional averages. For in proxy records of the East Asian monsoon region [START_REF] An | Asynchronous Holocene optimum of the East Asian monsoon[END_REF][START_REF] Zhou | Time-transgressive onset of the Holocene optimum in the East Asian monsoon region[END_REF][START_REF] Ren | Characteristics of time-transgressive Holocene Optimum in the East Asian monsoon region[END_REF].

Low winter and/or minimum temperatures compared with summer, linked with the length of the growing season, were likely important constraints on the rate of EBLF migration during this period. A vegetation shift from the early to mid-Holocene triggered by the winter warming effect on vegetation has been corroborated by sensitivity experiments involving rainfall and thermal forcing with a landvegetation model [START_REF] Cheng | Vegetation feedback causes delayed ecosystem response to East Asian summer monsoon rainfall during the Holocene[END_REF]. As well as temperature, mountainous topography in the study area may have also played a role, as a geographic barrier hindering northward biome migration [START_REF] Xu | Topographic and biotic factors determine forest biomass spatial distribution in a subtropical mountain moist forest[END_REF]. Notably, records for site TCK1, at 3898 m asl, indicate that the major forest biome change in the southwestern plateau of China occurred at ~6 ka [START_REF] Xiao | Latest Pleistocene and Holocene vegetation and climate history inferred from an alpine lacustrine record, northwestern Yunnan Province, southwestern China[END_REF], more than 2,000 years later than at a similar latitude in southeastern China. This may have been a consequence of the temperature lapse rate along the altitudinal gradient, so both latitude and altitude may have geographically influenced temperature gradients and constrained the vegetation belts [START_REF] Cheng | Differentiated climate-driven Holocene biome migration in western and eastern China as mediated by topography[END_REF]. overlapping windows for ENSO events exceeding 3 k (in °C) [START_REF] Clement | Suppression of El Nino during the mid-Holocene by changes in the Earth's orbit[END_REF]. (b) Speleothem oxygen isotopic records from the Hulu and Dongge caves [START_REF] Wang | A highresolution absolute-dated late Pleistocene monsoon record from Hulu Cave[END_REF][START_REF] Dykoski | A high-resolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge cave, China[END_REF]. (c) July insolation at 65°N latitude.

(d) and (e) TRACE21 winter (blue) and summer (red) temperatures for the subtropical zone; (f) Alkenone-sea ruface temperature estimates for the northern South China Sea [START_REF] He | Biomarker evidence of relatively stable community structure in the northern South China Sea during the Last Glacial and Holocene[END_REF]. (g) Pollen-based temperature reconstruction (Tann) from the SZY record (the orange line shows the five-point averages). (h) Ice-volume equivalent sea level estimate [START_REF] Lambeck | Sea level and global ice volumes from the Last Glacial Maximum to the Holocene[END_REF]. (i) Stacked data of the biome scores (EBLF-DBLF) for the subtropical zone. (j) Timing and latitude positions for the EBLF as a dominant forest from record sites 4, 7, 8, 10, 11, 12 (Table 1). Effects of changes in land-sea configuration associated with sea-level changes also suggest that internal variability in the climate system linked with global energy fluxes on millennial time scales play important roles in radiative imbalances [START_REF] Baggenstos | Earth's radiative imbalance from the Last Glacial Maximum to the present[END_REF], and hence both temperature and vegetation. In addition, coupled trends between temperatures reconstructed from a middle subtropical record and surface temperatures of the northern South China Sea [START_REF] He | Biomarker evidence of relatively stable community structure in the northern South China Sea during the Last Glacial and Holocene[END_REF] imply a close landsea relationship and the importance of tropical ocean circulation (DiNezio and Tierney, 2013). Our data reveal that the EBLF distribution peaked in a Holocene interval between 8.1 and 5 ka, associated with the maximum forest coverage in China [START_REF] Li | Large-scale vegetation history in China and its response to climate change since the Last Glacial Maximum[END_REF]. Such interval of forest expansion indicates the warmest condition, which is consistent with the HTM, traditionally thought to have occurred during the mid-Holocene in Asia [START_REF] Shi | Mid-Holocene climates and environments in China[END_REF][START_REF] Jiang | Synchronous strengthening of the Indian and East Asian Monsoons in response to global warming since the last deglaciation[END_REF].

Our analysis shows a time-transgressive pattern of forest replacement from south to north, consistent with the assumption that there was an expansion and/or northward retreat of the Holocene Optimum [START_REF] Ren | Characteristics of time-transgressive Holocene Optimum in the East Asian monsoon region[END_REF]. Similarly, monsoon precipitation previously reconstructed from pollen data also shows a time-transgressive pattern of maximum precipitation, with earlier occurrence in the southern area and later in the northern area [START_REF] Zhou | Monthly insolation linked to the time-transgressive nature of the Holocene East Asian monsoon precipitation maximum[END_REF]. They suggested that the monthly insolation changes force regional climate in different parts of the monsoonal China through a shift in the Western Pacific Subtropical High. In northern China, there was also a delayed ecosystem (tree cover) response to changes of monsoon intensity which was likely caused by winter warming and the subsequent feedback effects on soil moisture [START_REF] Cheng | Vegetation feedback causes delayed ecosystem response to East Asian summer monsoon rainfall during the Holocene[END_REF]. In this study, we suggest that temperature change (particularly winter warming) was crucial for the EBLF expansion. Apparently, rainfall in the subtropical mountainous areas of southeast China during the last glacial cycle remained relatively high due to the orographic effects, indicating that precipitation changes may not be a key factor for the EBLF/DBLF alternation. Moreover, simulated variations in precipitation and temperature were not coupled during the early to mid-Holocene.

, implying a gradual weakening of the temperature gradient between high and low latitudes. This may have been related to airflows linked to the Hadley circulation and changes in position of the Intertropical Convergence Zone (ITCZ) [START_REF] Reimi | Constraints on the magnitude of the deglacial migration of the ITCZ in the Central Equatorial Pacific Ocean[END_REF]. Such evidence of time-transgressive The EBLF biome and climate variation trends during the mid-to late Holocene indicated by proxy records showed a pattern of temperature decline or unstable fluctuations, conflicting with modeled indications of sustained warming (Fig. 9a). This contradiction between the reconstructed cooling (or unstable variation) and simulated progressive warming has been attributed to biases in the seasonality of the proxy reconstruction and climate sensitivity of models (Liu et al., 2015). Moreover, we found that the pollen assemblages indicating cooler conditions in the late Holocene were complex and differed from those from the last glacial period, as the evergreen forest components were not simply replaced by the deciduous components. Instead, grassland and secondary plant elements such as Poaceae, Pinus, and Alnus gradually increased. Some studies suggest that there was a decline in effective moisture in southern Asia resulted from the weakened summer monsoon and decreased precipitation since the middle Holocene [START_REF] Li | A continuous simulation of Holocene effective moisture change represented by variability of virtual lake level in East and Central Asia[END_REF]. In the meanwhile, the mean El Niño Southern Oscillation (ENSO) amplitude significantly increased [START_REF] Clement | Suppression of El Nino during the mid-Holocene by changes in the Earth's orbit[END_REF], leading to frequent floods and droughts in China. Further, numerous proxy studies have indicated that a major loss of plant biodiversity, spread of rice cultivation and increasing frequency of fires in southern China occurred 2000-3000 years ago [START_REF] Kaplan | Holocene carbon emissions as a result of anthropogenic land cover change[END_REF][START_REF] Ma | Holocene coastal evolution preceded the expansion of paddy field rice farming[END_REF][START_REF] Xu | Holocene fire history in China: Responses to climate change and human activities[END_REF]. Therefore, we suggest that combinations of factors, such as the weakening Asian summer monsoon, reduced precipitation and effective moisture, increased amplitude of ENSO events and associated changes in the western Pacific subtropical high, as well as fire frequencies associated with anthropogenic disturbances, may have affected the regional climate and thus EBLF/DBLF forest transformation from the mid-to late Holocene.

Conclusion

Although the EBLF components began to increase in low latitude regions during the deglaciation, biome change from deciduous to evergreen forest in the southern subtropical zone did not begin until 11.3 ka, after the Younger Dryas when regional annual and winter temperatures reached the zonal average thresholds of ~16.7C and ~8.0C, respectively. These values are highly consistent with the simulated results for current EBLF distribution. Both modeled and proxy data are consistent with the early Holocene being a key period when climatic thresholds (particularly winter and annual temperatures) approached then exceeded bioclimatic constraints for EBLF expansion. The EBLF biome recolonized the entire subtropical landscape including northern and mountainous areas during 8-5 ka, when regional air temperature (Tann) was higher than 15°C, indicating a warmest climate interval in accordance with the age of HTM commonly defined by multi-proxy records in China. Our integrated data indicate a time transgressive pattern that an earlier forest replacement at sites in low latitude regions as compared to sites located further north or at higher elevation in the subtropical zone of east China. This study highlights the greater importance of postglacial warming than rainfall changes for the EBLF recolonization across southeast China. However, the environment factors causing forest changes in the latter half of the Holocene were more complex. The sensitivity of forest responses to complex seasonal and abrupt climate changes (such as ENSO events), as well as increasing human activities during the late Holocene, may lead to the pollen-based climate reconstruction different from the model results.

Our study provides new quantitative information regarding past biome and climate changes, as well as thresholds for forest transformation, that enhance understanding of long-term vegetation dynamics in the ecologically sensitive region between temperate and tropical bioclimatic zones. The observed amplitude of subtropical ecosystem changes during the postglacial warming may also serve as an analog for projections of forest responses to the increasing greenhouse conditions expected under future climate scenarios.

Fig. 1 .

 1 Fig. 1. Locations of the examined pollen records across East Asia, present-day biome map and simulated EBLF

Fig. 2 .

 2 Fig. 2. Age-depth model of the GT (a) and SZY (b) cores.

Fig. 3 .

 3 Fig. 3. Predicted biomes for modern pollen surface samples from latitudes of 20N to 40N in eastern China. Some

Fig. 4 .

 4 Fig. 4. Modern pollen samples (left; gray circles) and a significance test for the pollen-based climate

[

  M+H] + ion traces of branched GDGTs. Replicate HPLC/APCI-MS analysis of more than 10 samples showed the reproducibility of MBT'(Methylation index of Branched Tetraethers), which was used for calculating the annual mean temperature with the equations given by[START_REF] Weijers | Environmental controls on bacterial tetraether membrane lipid distribution in soils[END_REF] and[START_REF] Peterse | Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils[END_REF]. For climate simulation, we use the TRACE21 model at the T31 spatial resolution (3.75°x3.75°), which is the first state-of-the-art transient simulation of global climate change over the last 21 ka

Fig. 5 .

 5 Fig. 5. Detailed pollen diagram of selected taxa from the GT core. Percentages were calculated from the sum of all

Fig. 6 .

 6 Fig. 6. Detailed pollen diagram of selected taxa from the SZY core. Percentages were calculated from the sum of all

Fig. 7 .

 7 Fig. 7. Pollen groups and reconstructed biomes for the last 22 ka based on the GT (a) and SZY (b) cores from the

Fig. 8 .

 8 Fig. 8. Reconstructed and modeled temperatures compared with biome changes in the SZY record. (a) brGDGT-

Fig. 9 .

 9 Fig. 9. Relationship between biome and climate changes in the subtropical zone. (a) Winter (blue), summer (red) and

Fig. 10 .

 10 Fig. 10. Synthetic changes of the dominant pollen type of Castanopsis/Lithocarpus, a representative taxon in

  example, records for mountainous parts such as the SZY site (altitude, 1007 m) indicate that the Tann and Twin thresholds are 15 and 4 °C, respectively, and these areas cover roughly 10-30% of the modern EBLF distribution. During the transition period of the early Holocene, the warming reached the thresholds for EBLF dominance earlier in the south than in the north. Such time difference in temperature rises to the threshold along latitude and vertical gradients since the LGM are reflected in delayed biome changes northward. During the early Holocene, temperatures in subtropical Asia gradually increased to levels near the EBLF thresholds. However, temperatures in a large area to the northern boundary of the zone may have still been below the thresholds, resulting in a north-south temperature gradient, and thus there was an earlier forest response at sites along low latitudes as compared to sites located further north. Similar time-transgressive patterns have already been observed

Fig. 11 .

 11 Fig. 11. Comparison of proxy records, simulations and paleoclimate forcings. (a) Mean amplitude of 500-year

  (k) Standardized values of pollen group of evergreen broadleaved taxa (EBT) in the subtropical zone; (l) July insolation at 65°N latitude.(m) Proportion of biome coverages in China at 1000-yr resolution[START_REF] Li | Large-scale vegetation history in China and its response to climate change since the Last Glacial Maximum[END_REF]. (n) Frequency variation of the HTM interval indicated by pollen data in China[START_REF] Jiang | Synchronous strengthening of the Indian and East Asian Monsoons in response to global warming since the last deglaciation[END_REF].Our solid evidence of the subtropical biome shift during the last glacial-interglacial transition enables better understanding of the sensitivity and potential mechanism of vegetation response to global climate change. The observed onset of primary increases in evergreen arboreal elements coincides with the B/A warming during the early deglaciation. The initial gradual increase in EBLF biome coincided with the main phase of deglaciation occurred from ∼16.5 ka to ∼8.2 ka BP at an average rate of sea-level rise of 12 m•ka -1 inferred by[START_REF] Lambeck | Sea level and global ice volumes from the Last Glacial Maximum to the Holocene[END_REF] since about 16 ka (Fig.11).

Table 1 .

 1 Locations of 14 subtropical (1-14) and 3 tropical pollen records (15-17) for the regional EBLF integration study, selected from 108 examined sites across East Asia.

	No.	Core (site name)	Location	Elevation (m)	Type of archive* BP) and age range (ka	Reference
	1	DJH (Dajiuhu)	110.001°E, 31.484°N	1751	P (0-42)	Li et al., 2013
	2	LTY (Lantianyan)	119.310°E, 28.430°N	902	L (1-8)	Ma et al., 2016
	3	JLC (Fanjingshan)	108.775°E, 27.847°N	2200	P (0-10)	Gao et al., 2019
	4	NT (Nantun)	104.340°E, 26.810°N	2203	L (1-22)	This paper

Table 2 .

 2 AMS 14 C dates and calibrated ages for the GT and SZY cores Beta Analytic Inc.; Poz, Poznan Radiocarbon Laboratory; XA, Xi'an Institute of Earth and Environment, Chinese Academy of Sciences; GZ, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

		(a)						(b)
	0	5	10	15	20	0	5	10	15	20	25	30	35	40
			(cal ka BP)						(cal ka BP)
	154
	Core	Depth (cm)	Laboratory* Number		Dated material			Radiocarbon age (a BP)			Calibrated age (cal. a BP)
	GT	25	Beta-440773		Bulk sediment			2030 ± 30				1907-2090
		45	Poz-84956		Pollen			3970 ± 40				4299-4522
		64	XA 6643		Bulk sediment			4233 ± 28				4662-4850
		74	Poz-84957		Pollen			5740 ± 70				6384-6695
		87	Beta-326017		Bulk sediment			7600 ± 40				8390-8534
		114	XA 6639		Bulk sediment			8738 ± 43				9568-9891
		125	Poz-84958		Pollen			9390 ± 70				10422-10984
		137	Beta-326015		Bulk sediment			11210 ± 60				13025-13237
		175.5 Poz-84960		Pollen			16170 ± 140				19148-19861
		192	XA 6638		Bulk sediment			17395 ± 66				20813-21181
	SZY	30	Beta-440771		Pollen			890 ± 30				738-903
		70	Beta-324767		Bulk organic matter		3170 ± 30				3347-3447
		105	GZ 4920		Bulk organic matter		3555 ± 35				3729-3953
		134.5 Poz-84961		Pollen			5020 ± 40				5661-5888
		165	GZ 4922		Bulk organic matter		6230 ± 35				7023-7245
		215	GZ 4921		Bulk organic matter		8690 ± 45				9553-9814
		245	Beta-324765		Bulk organic matter		9240 ± 40				10240-10440
		265	GZ 4860		Bulk sediment			11000 ± 60				12747-13016
		277.5 Beta-469574		pollen			24700 ± 100				28470-28980
		284.5 Beta-469575		pollen			22400 ± 90				26371-27076
		290	Beta-324768		Wood fragment			8430 ± 40				9410-9529
		300	Poz-84966		Pollen			17700 ± 120				21075-21742
		315	GZ 4923		Bulk sediment			24770 ± 110				28620-29004
		325.5 Poz-84968		Pollen			29320 ± 610				31791-34435
		333	Beta-324766		Wood fragment			21880 ± 100				25883-26352
		345.5 Poz-84969		Pollen			37900 ± 1100			40117-43875

Table 3 .

 3 Assignment of pollen taxa from southern China to plant functional types (PFTs) for biomization.

	Code	PFT	Pollen taxa
	bec	Boreal evergreen conifer	Abies, Picea, Pinus (Haploxylon)
	ctc	Cool-temperate conifer	Abies, Tsuga
	ec	Eurythermic or heliophilous conifer	Cupressaceae, Pinus

ts Temperate summergreen Acer, Araliaceae, Betula, Buxus, Carpinus, Cornus, Corylus, Euphorbia, Fraxinus, Galium, Quercus (deciduous), Rhamnaceae, Rhus, Rosaceae, Rubiaceae, Rutaceae, Salix, Sambucus, Spiraea, Tilia, Viburnum ts1 Intermediate-temperate summergreen Alnus, Caprifoliaceae, Carpinus, Carya, Celtis, Fagus, Hamamelidaceae, Hedera, Juglans, Liquidambar, Mimosaceae, Myrica, Nyssa, Pterocarya, Thymelaeaceae, Ulmus-Zelkova, Urticaceae, Vitis ts2 Warm-temperate summergreen Ebenaceae, Elaeagnaceae, Engelhardtia, Platycarya, Pterocarya, Rhus wtc Subtropical evergreen conifer Cephalotaxaceae, Dacrydium, Keteleeria, Podocarpus, Taxodiaceae wte Subtropical broadleaved evergreen Altingia, Anacardiaceae, Antidesma, Aporosa, Araliaceae, Berberidaceae, Breynia, Buxus Camellia, Castanopsis-Lithocarpus, Celastraceae, Claoxylon, Cucurbitaceae, Cyclobalanopsis, Elaeocarpus, Engelhardtia, Eriobotrya, Euphorbiaceae, Eurya, Fabaceae, Gesneriaceae, Hamamelidaceae, Lauraceae, Mallotus, Malvaceae, Melastomataceae, Meliaceae, Mimosaceae, Moraceae, Myrtaceae, Oleaceae, Papillonaceae, Phylanthus, Polygala, Proteaceae, Pterospermum, Reevesia, Rhamnaceae, Rubiaceae, Rutaceae, Sapindaceae, Solanaceae, Symplocos, Ternstroemia, Tetracentron, Theaceae sse Southern subtropical evergreen Apocynaceae, Castanopsis/Lithocarpus, Celastraceae, Cucurbitaceae, Euphorbiaceae, Exoecaria, Fabaceae, Flacourtia, Hamameliaceae, Loranthaceae, Lythraceae, Mallotus, Malvaceae, Meliaceae, Nyssa, Papilionaceae, Sapindaceae, Symplocos, Tilia, Vitaceae te Tropical evergreen Anacardiaceae, Antidesma, Apocynaceae, Aporosa, Araceae, Araliaceae, Berberidaceae, Camelia, Claoxylon, Eurya, Ficus, Lauraceae, Macaranga, Melastomataceae, Mimosaceae, Moraceae, Myrtaceae, Olacaceae, Oleaceae, Phylanthus, Proteaceae, Pterospermum, Caryophyllaceae, Crassulaceae, Fabaceae, Gentiana, Geraniaceae, Gesneriaceae, Impatiens, Labiatae, Onagraceae, Papaveraceae, Papilionaceae, Pedicularis, Plantago, Polygala, Polygonum, Potentilla, Primulaceae, Ranunculaceae, Rosaceae, Sanguisorba, Thalictrum,

Table 4 .

 4 Assignments of plant functional types to biomes.

	203
	Code	Biome	Plant functional type (PFT)*
	ALSM	Alpine shrub-land, meadow/tundra	af, g, h, s
	CLMX	Cold mixed forest	ctc, ec, ts1
	COMX	Cool mixed forest	bec, ctc, ec, h, ts
	DBLF	Temperate deciduous broadleaved forest	ctc, ec, h, ts, ts1, ts2
	EBLF	Subtropical evergreen broadleaved forest	ec,te, tr, ts2, wtc, wte
	STEP	steppe	g, sf
	TDFO	Tropical dry forest/savanna	g, tr
	TRFO	Tropical rainforest	te, sse, suf
	TSFO	Tropical seasonal forest	tr, suf, wtc
	* PFT codes: bec, boreal evergreen conifer; ctc, cool-temperate conifer; ec, eurythermic or heliophilous conifer;
	ts, temperate summergreen; ts1, intermediate-temperate summergreen; ts2, warm-temperate summergreen; wtc,
	subtropical evergreen conifer; wte, subtropical broadleaved evergreen; sse, southern subtropical evergreen; te,

tropical evergreen; tr, tropical raingreen; suf, subtropical shrub/forb; sf, steppe forb/shrub; af, alpine forb/shrub; h, heath; g, grass; and s, sedge.

  The climate at each modern site was interpolated from modern climatic data of 756 meteorological stations in the years of 1981-2010 in China

	Temperate
	forest-steppe
	Temperate deciduous
	broadleaved forest
	Warm-temperate
	mixed forest
	Subtropical evergreen
	broadleaved forest

(http://data.cma.cn). The thin plate spline regression was applied in the interpolation process for each modern pollen site. The robustness of the transfer function model was assessed by calculating the root mean square error of prediction (RMSEP) and coefficient of determination (R2) between measured and predicted values. The R software package

[START_REF] Juggins | Rioja: Analysis of Quaternary Science Data. R package version 0[END_REF]

was used to calculate the pollen-climate model.

Acknowledgments

Funding: This work was financially supported by the National Key R&D Program of China (grant numbers 2022YFF0801500; 2016YFA0600500), the National Natural Science Foundation of China (NSFC) (grant numbers 42072205, 42077414, 41472143, 41630753, 41230101) and the China Postdoctoral Science Foundation (grant number 2021M703659), and is a contribution to the Belmont Forum funded project VULPES (Project ID: ANR-15-MASC-0003).

Data availability

Data of biome and climate reconstructions can be found in the Supplementary data. Modern pollen dataset from EAPD for climate reconstruction is shared at https://doi.org/10.5281/zenodo.6918393.

CRediT authorship contribution statement

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


S U Nin-Ren

Emilie Jarde

Yin Yue-Peng

Marine Liotaud

Wen Zhang

Sedimentary organic matter of channel type reservoirs: Input of lipid biomarker for source assessment

Keywords: words:organic matter, reservoir, source aportionment, lipid biomarkers,

Abstract:

The cascade hydropower development has led to fragmentation of riparian habitats and increased sediment retention in reservoirs, while the impact of channel-type reservoirs on the distribution of organic matter (OM) sources is poorly studied and the mechanism is unclear. In this study, surface sediments from 11 sampling sites in the Pubugou Reservoir on the Dadu River were selected, and the sediment OM was extracted and analyzed by gas chromatography-mass spectrometry(GC-MS), and the OM was quantified and divided into natural, diagenesis, heat source and sewage markers. The results showed that OM was mainly derived from natural input (46.4%), mainly from riverine vegetation (i.e., higher plant sources) and microbial sources in aquatic environments. It is noteworthy that the head of the reservoir is close to the Hanyuan town area, where anthropogenic inputs are significantly higher (59.8%), and further studies on the proportion of effluent, rock-forming and thermal sources to anthropogenic inputs on this basis emphasise the contribution of effluent inputs, in addition to the average OM content of the head of the reservoir (10.7%) > the tail of the reservoir (7.3%) > the middle of the reservoir (6.7%), which may be related to the reservoir's role in trapping OM. Given the high overall primary productivity of reservoirs and the wide variation in the composition of OM sources in reservoirs, more attention should be paid to the discharge of anthropogenic pollution in reservoir areas in channel-type reservoir OM studies.

Introduction

The impoundment effect resulting from the construction of a riverine reservoir often directly affects the spatial and temporal distribution, burial and transport of sediment organic matter (OM) in the watershed. A comprehensive understanding of the dynamics of sediment OM remains challenging due to the morphological structure and throughput flow characteristics of reservoirs [START_REF] Thornton K W | Reservoir limnology: ecological perspectives[END_REF] as well as the complex structure and diversity of sources of sediment OM. The composition of OM in reservoir areas is closely related to its origin and is the joint result of multiple allochtonous and autochtonous primary producers, such as phytoplankton, benthic macroalgae, herbaceous plants, and anthropogenic OM [START_REF] Bianchi T S | Particulate Organic Carbon Cycling and Transformation[END_REF][START_REF] 贾晓斌 | 新安江水库水环境演变的沉积物记录及碳埋藏研究[END_REF][START_REF] 何宜颖 | 惠州白盆珠水库沉积物营养元素时空变化特征及源解析[END_REF][5]. However, a comprehensive origin and dynamics of reservoir sediment OM to further explore the transport transformation of OM in reservoirs is yet to be investigated.

A broad spectrum of characterization methods are available for OM, from basic measurements of dissolved organic carbon, ultra-violet (UV) absorption or molecular weight distribution [START_REF] Wen | Fluorescence excitation-emission matrix regional integration to quantify spectra for dissolved organic matter[END_REF] to advanced techniques such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), homology and heterology correlation multidimensional nuclear magnetic resonance (NMR) and excitation-emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) [START_REF] Nebbioso | Molecular characterization of dissolved organic matter (DOM): a critical review[END_REF],and a variety of tools have been widely used to identify OM origins, including stable carbon isotopes, molecular biomarkers and spectral indices. Of these, the single technique of stable carbon isotopes has inherent limitations in revealing source assignment [START_REF] Feng L K | Tracing dissolved organic matter in inflowing rivers of Nansi Lake as a storage reservoir: Implications for water-quality control[END_REF], spectral indices identify OM based on large molecules of humic acid and fulvic acid in a large number of water samples, while lipid biomarkers target a single group of molecules to track OM sources, which not only distinguish between exogenous, local sources and anthropogenic inputs, but also show the ability to identify subgroups of these three sources and, in some cases and even identify the sources of specific species of organisms (e.g. bacteria, plants, plankton) [START_REF] Parrish C C | Lipid and Phenolic Biomarkers in Marine Ecosystems: Analysis and Applications[END_REF][START_REF] Berge | Fatty Acids from Lipids of Marine Organisms: Molecular Biodiversity, Roles as Biomarkers, Biologically Active Compounds, and Economical Aspects [J][END_REF][START_REF] Li | Biomarkers as a soil organic carbon tracer of sediment: Recent advances and challenges[END_REF], and thus are widely used in OM traceability efforts for their high resolution of mixed sources [START_REF] Jeanneau | Impact of a highly contaminated river on a more important hydrologic system: Changes in organic markers[END_REF][START_REF] Kaal | Downstream changes in molecular composition of DOM along a headwater stream in the Harz mountains (Central Germany) as determined by FTIR, Pyrolysis-GC MS and THM-GC MS [J][END_REF][START_REF] Li Z | Effects of organic matter on polycyclic aromatic hydrocarbons in riverine sediments affected by human activities[END_REF], while few studies have been reported to explore the distribution of organic matter sources in reservoir-type river sediments by this method [START_REF] Kaal | Downstream changes in molecular composition of DOM along a headwater stream in the Harz mountains (Central Germany) as determined by FTIR, Pyrolysis-GC MS and THM-GC MS [J][END_REF][START_REF] Li Z | Effects of organic matter on polycyclic aromatic hydrocarbons in riverine sediments affected by human activities[END_REF][START_REF] Yin | Impact of river dams on phosphorus migration: a case of the Pubugou Reservoir on the Dadu River in China[END_REF].

The development of hydropower in the Dadu River terrace has led to increased fragmentation of riparian habitats and a significant increase in hydraulic retention time (HRT) [START_REF] Yin | Impact of river dams on phosphorus migration: a case of the Pubugou Reservoir on the Dadu River in China[END_REF], increasing sediment trapping [16], so it is important to study the impact of reservoir creation on the source distribution of OM. In this study, we used gas chromatography-mass spectrometry (GC-MS) to quantify molecular markers of OM in the sediments of the Dadu River (a mid-step control reservoir). The lipid markers were classified into natural markers (higher plants, bacteria and microalgae, algae), anthropogenic markers (including petrogenic ones, pyrogenic ones and fecal markers) and non-specific molecules according to their sources, so as to (1) compare anthropogenic and natural inputs; (2) explore the reservoir-type river OM distribution and sources to track the effects of industrialization and agriculture on this small hydrological system. The results of this study can enhance the understanding of OM retention during dam construction on typical rivers and provide a reference for river and dam management.

Sample collection

In this study, 11 representative sampling sites were selected in the watershed based on hydrogeological characteristics and the distribution of pollution sources, and the specific information of the sampling sites is shown in Table 1. sampling time was December 2019, and the sampling sites were spatially divided into three groups: (1) D1-D5 represents the middle of the reservoir; (2) D6-D10 represents the reservoir; (3) D11 represents the end of the reservoir. Surface sediments were collected from 0-5 cm depth using Peterson samplers. To ensure sediment representativeness, triplicate samples were collected at each site, the three samples were mixed to obtain a single 500 g of sediment from each site, and after sampling, the samples were stored in sealed polyethylene plastic bags (stored at 4°C) for for the determination of the particle size and organic matter content. The lipid biomarkers preparation and analyses were performed at the Ladamo group of the CONDATE-eau analytical platform of Rennes University, France (https://osur.univ-rennes1.fr/empreinte-moleculaire).

1.3 Measurement and analysis 1.3.1 Determination of organic matter content and particle size After drying and filtering the sediment samples, OM calcined each sediment sample in a muffle furnace for the amount of combustion loss (550 °C for 2 h). By the analysis of the LS 230 laser diffraction particle analyzer (Microtrac S3500), the sediment particle size is divided into three categories: (1) clay: < 4 μm; (2) silt: 4-63 μm; [START_REF] 贾晓斌 | 新安江水库水环境演变的沉积物记录及碳埋藏研究[END_REF] coarse sand: > 63 μm.

Sample preparation for lipid biomarkers analysis

Samples were collected and frozen for two days and freeze-dried for three days. The freeze-dried samples were sieved and ground to 0-2 mm. freeze-dried samples were extracted with dichloromethane using an accelerated solvent extractor (Dionex™ ASE™ 200) according to the following conditions, modified from Derrien et al [START_REF] Derrien | Extreme Variability of Steroid Profiles in Cow Feces and Pig Slurries at the Regional Scale: Implications for the Use of Steroids to Specify Fecal Pollution Sources in Waters [J][END_REF]: 33 mL cells, 5 min heating at 100°C and 60 bars, 10 min static phase, completed with 80% flush and 20-s purge with nitrogen. Elemental sulfur was removed from the total lipid fraction by reduction on metallic copper. The total lipid extract was then fractionated into aliphatic hydrocarbons, aromatic hydrocarbons and polar compounds on a silica column by successive elution with cyclohexane, cyclohexane/dichloromethane (2/1, v/v) and methanol/dichloromethane (1/1, v/v).

Analytical Methods

Polar fractions were analyzed by capillary gas chromatograph-mass spectrometer (QP2010SE GC-MS, Shimadzu) after derivatization using a mixture of N,O-bis(trimethylsilyl) trifuoroacetamide (BSTFA) and trimethylchlorosilane (TMSC) (99/1, v/v), whereas the aliphatic and aromatic fractions were analyzed without further treatment. The injector used was in splitless mode and maintained at a temperature of 310°C. The chromatographic separation was performed on a SLB-5MS capillary column (length = 60m, diameter = 0.25mm, film thickness = 0.25 µm) under the following temperature conditions: 70°C (held for 1 min) to 130°C at 15°C/min, the 130°C to 300°C (held for 15 min) at 3°C/min. The helium flow was maintained at 1 ml/min. The chromatograph was coupled to the mass spectrometer by a transfer line heated at 280°C. The analyses were performed in full scan mode. The organic compounds were quantified by using an external calibration adding an internal standard in the solutions containing the aliphatic hydrocarbons, aromatic hydrocarbons and polar compounds as per [START_REF] Grossi | Biotransformation pathways of phytol in recent anoxic sediments [J][END_REF]. Grain size, as one of the basic properties of sediments, can reflect their material origin and the relationship between different grain size fractions and the source area of the sediment [START_REF] 徐利强 | 巢湖沉积物粒度特征及其沉积学意义[END_REF]. The surface sediments of the reservoir are dominated by chalk and coarse sand, of which the average contents of clay, chalk and coarse sand are 13.1 ± 5.0%, 47.1 ± 15.6% and 39.7 ± 18.9%, respectively (Table 2). The mean median grain size in the reservoir area is 42.8 ± 25.6 μm, with a decrease in coarse sand content and an increase in the proportion of clay; the median grain size downstream of the reservoir (dam) is 92.9 μm. The mean grain size downstream of the reservoir dam is larger than that in the mid-stream river section before the dam, and similar phenomena have been found (e.g., downstream of the Black River channel (0.82 mm) > mid-stream (0.22 mm)) [START_REF] 王昱 | 筑坝拦截对黑河河道沉积物粒度空间分布的影响[END_REF][START_REF] 穆青 | 青草沙水库浊度与悬浮颗粒物粒径的分布特征[END_REF][START_REF] 屈利红 | 海勃湾水库建设前后下游河道泥沙粒径变化分析[END_REF]. The mean OM content in the reservoir area (10.7 ± 4.8%) was significantly higher than in the downstream (7.3%) and upstream (6.7 ± 1.8%) reaches of the reservoir, indicating that OM was significantly trapped within the reservoir [24,[START_REF] 邬丽姗 | 湖库型水源地有机质污染特征研究--以川南地区 H 水库为例[END_REF].

表 1 采样点经度、纬度和污染类型
The presence of the dam resulted in slower flow rates in the reservoir area, depositing large amounts of fine particles, which in turn adsorbed and preserved more OM, consistent with the findings of Rapin et al [START_REF] Rapin A | Sedimentary phosphorus accumulation and distribution in the continuum of three cascade dams[END_REF] and Xu et al [START_REF] Xu F | Impact of the Three Gorges Dam on the Quality of Riverine Dissolved Organic Matter [J][END_REF]. In addition, the finer particle size of the sediments in the reservoir area creates favorable conditions for longterm physical or chemical accumulation and storage processes of OM [START_REF] Zhao | Preliminary study on the effect of cascade dams on organic matter sources of sediments in the middle Lancang-Mekong River [J][END_REF]. In contrast, under fast flow conditions downstream of the dam, more fine-grained particles are washed away by the water flow and large agglomerates are less protective of OM, which may be a key factor for the lower OM content downstream [START_REF] Zhao | Spatial and seasonal dynamics of organic carbon in physically fractioned sediments associated with dam construction in the middle Lancang-Mekong River [J][END_REF].

OM consists mainly of polar compounds, an average of 51.6% (Figure 2). From upstream to downstream, the proportion of polar OM decreases from 53.7% to 44.4% and the proportion of aliphatic hydrocarbons increases from 33.7% to 42.4%. The proportion of aromatic hydrocarbons remained at around 13.9%. Note: CPI is the carbon preference index; CPIl = nC20 to nC25; CPIh=nC25 to nC35 Fig. 2 Composition of the extractable organic matter

Sources of natural markers

Sediment OM natural source markers can be divided into bacteria and microalgae, higher plants and algae (Figure 3), Among them, the input sources of higher plants were the most prominent, with an average of 70.5±9.1%

(ranging from 51.7%-85.3%); bacterial and microbial inputs were approximately 26.5±10.3% (ranging from 11.0% to 48.1%); OM also contained trace algal inputs of 2.9±1.9% (ranging from 0.1% to 7.2%). This may indicate that higher plant inputs dominate the natural sources of organic matter in the sediment.

Natural molecular markers present in the sediments (Table 2) are mainly terrestrial plants of neophytdiene [START_REF] Meyers P A | Lacustrine organic geochemistry--an overview of indicators of organic matter sources and diagenesis in lake sediments[END_REF] from chlorophyll a and nC24-nC34 n-alanes (odd carbon more than even carbon predominates), indicating that OM is mainly derived from riparic vegetation.The carbon preference index (CPI) numerically represents the degree of preservation of the length of the original biological chain [START_REF] Collister | Compound-specific δ 13C analyses of leaf lipids from plants with differing carbon dioxide metabolisms[END_REF]. When CPI (from nC21 to nC25) >5, it means that high molecular weight n-alkanes (>nC20) are dominant [START_REF] 梁作兵 | 典型岩溶区地下河中溶解态脂类生物标志物来源解析及其变化特征[END_REF], and there are odd and even carbon advantages, which are the characteristics of input from higher plants [START_REF] Bray E E | Distribution of n-paraffins as a clue to recognition of source beds [J][END_REF]. When CPI values are close to 1.0, there is no preference for odd or even carbon, indicating a source of petroleum hydrocarbons [START_REF] Colombo | Sources, vertical fluxes, and accumulation of aliphatic hydrocarbons in coastal sediments of the Rio de la Plata Estuary, Argentina [J][END_REF][START_REF] Jeanneau | Quantitative multimolecular marker approach to investigate the spatial variability of the transfer of pollution from the Fensch River to the Moselle River (France) [J][END_REF]. In the sediments of the study area, CPI ranges from 1.7 to 2.7 (Table 4), emphasizing the mixed input of natural and anthropogenic sources [START_REF]Fossil Fuel Biomarkers, Applications and Spectra[END_REF][START_REF] Leeming R | Using faecal sterols from humans and animals to distinguish faecal pollution in receiving waters[END_REF]. The main land use and cover types in this area are mainly woodland and cultivated land, which may be related to local villagers' annual pruning of fruit tree branches and discarding abandoned branches and vegetable leaves on the shore during crop harvest.

Natural input sources are also characterised by the occurrence of n-alkanols, which have an even over odd carbon number predominance, and these compounds are resistant to degradation after plant decay, so they can be used as biomarkers for the transport of terrestrial higher plants into the water column [START_REF] Eglinton | Leaf Epicuticular Waxes: The waxy outer surfaces of most plants display a wide diversity of fine structure and chemical constituents[END_REF]. The contribution of phytosterols is also evidenced by their presence, such as campesterol and stigmasterol, they are the main sterols in higher plants and they are present in large quantities in the loquat, grape and orange crops that are most grown in the region [START_REF] Phillips K M | Phytosterol composition of nuts and seeds commonly consumed in the United States [J][END_REF], so agricultural activities may be their main source. Brassicasterol may also be markers of aquatic primary production, as this molecule is the main sterol in the early algae family as well as several diatoms (Bacillariophyceae) and methanogens (Methanogaster) [START_REF] Mudge S M | Sterols in the ria formosa lagoon, Portugal [J][END_REF]. The high primary productivity resulting from the enrichment of planktonic nutrients required by agricultural and urban activities suggests a high source of primary OM in the Dadu River.

(Bacteria and microalgae, higher plants, algae) Fig. 3 Source of natural markers

Sources of anthropogenic markers

With the speeding up of urbanization process, many industrial park rise in rural areas. From upstream to reservoir areas, industrial and agricultural areas have increased significantly, and the input of different organic compounds from industrial and domestic sewage, leaching from urban and industrial surfaces, atmospheric deposition and accidental leakage will all lead to the importation of different organic matter from anthropogenic sources. These inputs induce the occurrence of anthropogenic activities signature molecules such as polycyclic aromatic hydrocarbons (PAH) and hopanes (Table 3) [START_REF]Fossil Fuel Biomarkers, Applications and Spectra[END_REF].

Among the anthropogenic source markers, petrogenic markers contributed the most, with an average proportion of 80.8±13.6% (maximum reached 6262.1 ng/g) (Figure 4). These molecules come from non-heat source uses of fossil organics, such as lubricating oils, greases, and road asphalt. Hopanes is the main diagenetic molecular marker in the sediments of the Dadu River. Diagenetic markers in reservoir sediments, with the predominance of hopanes, tricyclic triterpanes, pristane, phytane and n-alkane distributions with a slight predominance of odd-numbered compounds, indicate the presence of OM from petroleum crude oil (crude unprocessed petroleum) or petroleum by-products (e.g. fuels, oils, lubricants or bitumen). Municipal and industrial wastewater, atmospheric fallout, leaching in urbanized areas and accidental oil spills are the causes of their accumulation in river sediments, such as byproducts from industrial activities (industrial wastewater, atmospheric fallout, etc.); oil, asphalt washed down from roads in rain; Exhaust gases from automobiles are hydrocarbons that settle through atmospheric dry and wet settlement.

The sewage marker was greater at 16.0±16.6% in the reservoir than at 11.7±11.5% in the upper reservoir (Figure 4). The main sewage marker compounds present in the samples is coprostanol, which is derived from the degradation of cholesterol by the digestive systems of sheep, cattle, pigs, cats and humans [START_REF] Leeming R | Using faecal sterols from humans and animals to distinguish faecal pollution in receiving waters[END_REF]. The ratio of fecal prostatic coprostanol/cholesterol to fecal coprostanol depends on the animal that produces it, and values above 0.06 are characteristic of human feces [START_REF] Writer | Sewage contamination in the upper Mississippi River as measured by the fecal sterol, coprostanol [J][END_REF]. In the Dadu River sediment, the ratio ranged from 0.1 to 0.7, suggesting that fecal coprostanol was deriving from human origin and domestic sewage. According to the actual site pollution source survey, the coverage of domestic sewage treatment facilities in the area is very low and the vast majority of domestic sewage is discharged on site [START_REF] 罗芳 | 农村环境污染现状调查与污染治理--以四川省汉源县为例[END_REF].

Pyrogenic markers can be obtained from several combustion processes, such as vehicle emissions, wood combustion, coal combustion, and coal tar. Using the ratios calculated from isomers of the same molecular mass to distinguish different heat sources (Figure 5), the ratio of fluorene to fluoranthene and pyrene (Fl/Fl+Py) was used to distinguish between oil combustion and other types of combustion. Fl/Fl+Py is between 0.52 and 0.61, indicating that the heat source is mainly the combustion of grass, wood and coal. In addition, the allocation of combustion sources should also be confirmed by the ratio of other molecules, such as IP/IP+Bghi, BaA/228 ratios between 0.38 and 0.48 and 0.28 and 0.44, all of which further emphasize the combined contribution of grass, wood coal combustion and liquefied petroleum fuels, indicating that nearby rural residents still maintain a more traditional agricultural production lifestyle.

表 4 正构烷烃、芳香分子和类固醇的比率 (pyrogenic,petrogenic, sewage markers) Comparison of natural and anthropogenic inputs Although irregular discharges from towns as well as industrial agriculture were present in the vicinity of the Dadu River at the time of the actual site contamination survey, the presence of molecular markers in the sediment clearly indicates that OM is predominantly a natural input (Fig. 6), with these molecules accounting for 46.4 ± 13.4% (21.5%-67.3%) of the quantitative molecules. The organic matter in the sediment was mainly derived from terrestrial plants and bacterial microalgae, indicating high reservoir productivity, and similar results have been obtained in previous studies [START_REF] Li Z | Effects of organic matter on polycyclic aromatic hydrocarbons in riverine sediments affected by human activities[END_REF][START_REF] Kumar | Discerning natural and anthropogenic organic matter inputs to salt marsh sediments of Ria Formosa lagoon (South Portugal) [J][END_REF][START_REF] Xu W | Organic Matters and Lipid Biomarkers in Surface Sediments from the Northern South China Sea: Origins and Transport [J][END_REF]. Rainfall is one of the main routes of terrestrial input, and the splashing, scouring and transport of surface soils around river dams by rainfall results in runoff carrying large amounts of terrestrial organic matter. With an average annual rainfall of 741.8 mm in the area, the Cascade Gorge Reservoir considers the flood control needs of the Sichuan and Chongqing sections of the river while intercepting and reducing flooding in the basin [START_REF] 贺玉彬 | 瀑布沟水库在岷江"20•8"洪水中的防洪作用分析[END_REF].

The anthropogenic input (31.4%) OM was greater in the reservoir area (D6-D10) than upstream of the reservoir (D1-D5) (15.9%).Significant anthropogenic input (59.8%) was observed at points D6 and D7, probably because D6 and D7 are located in Hanyuan County town, at the end of Hanyuan Lake, where the river channel narrows and is densely populated with multiple sources of discharge such as aquaculture and sewage; at the same time, the flow velocity in the reservoir area slow, coupled with the retention effect of the dam, the OM retention capacity of the reservoir area is enhanced, limiting biodegradation under the conditions of low water velocity [START_REF] Jeanneau | Evolution of the source apportionment of the lipidic fraction from sediments along the Fensch River, France: a multimolecular approach[END_REF]. In a study by Zeng Hongping and others [START_REF] 曾红平 | 滃江长湖水库沉积物营养元素沉积历史重构及源解析[END_REF] on the same reservoir-type river, the Yangtze River, it was also found that most of the organic matter was retained in the reservoir area under the influence of the dam. The proportion of rock-forming molecules remained constant throughout the sampling area, with petroleum crude oil and petroleum by-products (e.g. fuels, lubricants and bitumen) being the main source of rock-forming material in the Dadu River.

The low proportion of anthropogenic input to the Dadu River in reservoir-type rivers compared to natural input may be due to the fact that the waterfall ditch reservoir has both a water supply function and a high demand for water quality, so the anthropogenic input of organic matter to the Dadu River in reservoir-type rivers is better controlled.

(Natural markers, anthropogenic markers) Fig. 6 Comparison of human and anthropogenic markers 3 Conclusion

(1) The average content of OM in this river-type reservoir was (10.7±4.8%) > (7.3%) > (6.7±1.8%). OM was obviously retained in the head of the reservoir, which was related to the deposition of a large number of fine particles in the head of the reservoir.

(2) Natural input was the main contributor of OM content (46.4%±13.4%), mainly from riverine vegetation (33.1%) and from aquatic microorganisms (12.0%). The main organic compounds were nC24-nC34 n-alkanes.

(3) The source of OM at the head of the reservoir (near Hanyuan town) was dominated by anthropogenic input (31.4%). Compared with the source of OM in the reservoir (15.9%), the anthropogenic input significantly increased, and the contribution of sewage input was larger, which was considered to be related to the low coverage rate of domestic sewage treatment facilities in the residential area, and its core mechanism was related to the coupling effect of reservoir retention effect. Therefore, when studying the distribution characteristics of OM sources in channel reservoirs, more attention should be paid to the anthropogenic pollution emission at the head of the reservoir and the influence of cascade dam construction on OM sources.

1

  Materials and methods 1.1 Study Area Dadu River flows through central and western Sichuan Province, with a total length of about 1,062 km and a drainage area of about 77440 km2. It is the largest tributary of Minjiang River, a tributary of the Yangtze River in China. Its main cascade development projects consist of three reservoirs and 22 levels, Among them, Pubugou Reservoir is the 17th hydropower station completed in 2009 and belongs to the control reservoir in the middle reaches of the cascade. In this study, Shufugou Reservoir is taken as the research area, with a reservoir capacity of up to 539 million m3, annual rainfall of 741.8 mm and average temperature of 22℃. Sampling points are selected in the middle, front and end of the reservoir, as shown in Fig 1.

Fig. 1

 1 Fig.1 The map of sample sites

Fig. 4

 4 Fig.4 Source of human activity markers

Table 1

 1 Longitude, latitude and pollution type of sampling sites

	采样点	经纬度	Type of pollution
	D1	102°26′32″ E	29°15′58″ N	Calcium carbonate plant, cement plant
	D2	102°34′25″ E	29°20′9″ N	Relocation of lead and zinc mines
	D3	102°39′40″ E	29°18′27″ N	Lead and zinc mine upstream inundation area
	D4	102°38′8″ E	29°23′20″ N	Agricultural pollution
	D5	102°37′34″ E	29°24′30″ N	Pig farm outfall
	D6	102°41′28″E	29°19′4″N	Fishery aquaculture
	D7	102°43′26″ E	29°20′10″ N	Industrial park outfalls

Table 1

 1 Organic matter composition and particle size parameters of surface sediments

					Coarse sand
	采样点	有机质 OM(%)	Clay(%)	sand(%)	(%)	Median size(μm)
	D1	7.27	13.39	65.46	21.15	27.17
	D2	8.92	14.55	55.63	29.83	41.35
	D3	4.02	7.67	28.59	63.75	86.79
	D4	6.95	11.77	41.46	46.77	57.21
	D5	6.38	11.56	49.49	38.95	47.39
	D6	4.03	6.88	29.92	63.2	84.23
	D7	16.77	14.3	59.03	23.48	33.56
	D8	8.80	20.86	67.15	11.99	20.74
	D9	10.32	20.59	53.93	27.59	26.02
	D10	13.55	15.94	41.11	42.95	49.3
	D11	7.25	6.02	26.79	67.19	92.85

Table 2

 2 Classification of organic compounds determined by GC-MS in organic matter

	Class of biomarkers	Molecules
			n-Alkanes nC25 to nC35 with CPIh1 > 5 except for nC24, nC25 and nC26 from terrestrial origin
	Natural	Higher plants	based on CPIl and CPIh ; n-alcanoic acids from nC20 to nC30 ; w-hydroxyalcanoic acids ; a,w-
			alcanoic diacids n-Alkanols from nC20 to nC30 ; Campesterol ; Stigmasterol
		Bacteria and	n-Alkanes from nC15 to nC19 ; alcanoic acid iso and anteiso C15 and C17 ; Brassicasterol ; Cholesta-
		microalgae	5,22(E)-dien-3b-ol
			n-Alkanes from nC20 to with CPIl > 2 except for nC24, nC25, nC26 from algal origin based on CPIl
		Algae
			and CPIh; néophytadienes ; Fucosterol et Isofucosterol
		Fecal	Coprostanol ; Epicoprostanol ; 24-Ethylcoprostanol ; 24-Ethylepicoprostanol
	Anthropogenic
		contamination
		Petroleum and	n-Alkanes from nC12 to nC14 ; n-Alkanes from nC20 to nC24 with CPIl = 1 ; n-Alkanes from nC25
		petroleum	to nC35 with CPIh = 1 ; Hopanes ; Triterpanes tricycliques, Pristane, Phytane, Bicyclohexane
		product
		Pyrogenic	Polycyclic aromatic hydrocarbones (PAH) from Naphthalene to Benzo(ghi)perylene including
		(combustion)	methyl-PAH
			Cholesterol;Sitosterol;Cholestanol; Epicholestanol; Campestanol; Epicampestanol, 5b-campestanol,
	Ubiquitous (non specific)

Sitostanol; Cholestan-3-one; alcanoic acid from nC10 to nC19 ; alcanoic acid with 12 to 15 carbon atom with a methyl group; alcenoic acid with 16, 18 and 22 carbon atoms; octadecadienoic acid (C18:2), n-Alkanols from nC11 to nC19

Table 3

 3

		Ratio of n-alkanes, aromatic molecules and steroids
			D1	D2	D3	D4	D5	D6	D7	D8	D9	D10	D11
	正 构 烷 烃	CPI	2.6	2.4	1.9	1.7	1.9	1.9	1.7	2.2	2.7	1.8	1.8
			0.6	0.5	0.6	0.6	0.6	0.6	0.6	0.5	0.5	0.6	0.6
	芳 香 分 子	Fl/Fl+Py
			0.5	0.5	0.5	0.4	0.5	0.5	0.5	0.4	0.5	0.5	0.5
		IP/IP+Bghi
			0.4	0.4	0.4	0.4	0.4	0.3	0.4	0.4	0.4	0.3	0.4
		BaA/228
	类固醇	Coprostanoll to
		cholesterol+coprostanol	0.3	0.1	0.1	0.2	0.4	0.11	0.7	0.7	0.4	0.1	0.7
	注: CPI = 0.5⁎((n-C21-25)/(n-C20-24)+(n-C21-25) /(n-C22-26)).

Acknowledgments : This research was supported by the National Natural Science Foundation of China (No. 42007148), open foun dation of State Environmental Protection Key Laboratory of Synergetic Control and Joint Remediation for Soil& Water Pollution (No. GHBK-006), and sponsored jointly by Natural Science Foundation of Sichuan Province (No.2023NSFSC0799).