Biogeochemical processes and response to climate change recorded in the isotopes of lacustrine organic matter, southeastern Qinghai-Tibetan Plateau, China

Palaeogeography, Palaeoclimatology, Palaeoecology 453 (2016) 93–100

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Biogeochemical processes and response to climate change recorded in the isotopes of lacustrine organic matter, southeastern Qinghai-Tibetan Plateau, China Weiwei Sun a, Enlou Zhang a,⁎, Richard T. Jones b, Enfeng Liu a, Ji Shen a a b

State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, PR China Geography, College of Life and Environmental Sciences, University of Exeter, Amory Building, Rennes Drive, Exeter EX4 4RJ, UK

a r t i c l e

i n f o

Article history: Received 27 September 2015 Received in revised form 2 March 2016 Accepted 5 April 2016 Available online 09 April 2016 Keywords: Climate change Biogeochemical cycling Stable isotope Qinghai–Tibetan Plateau Muge Co

a b s t r a c t Large amount of labile organic matter sequestered in Qinghai–Tibetan Plateau (QTP), the highest plateau in the world. However, large uncertainties remained in the response of biogeochemical cycling to climate change due to the lack of spatial and temporal resolution of climatic and ecosystem records in the plateau. Here, we present the stable carbon (δ13C) and nitrogen (δ15N) isotope records from Muge Co, a lake in the southeastern QTP, in order to investigate the response of biogeochemical processes to climate change. In this record, changes in δ13C and δ15N values were more likely to reflect the rate of organic matter decomposition in the catchment and within the lake, while early diagenesis, changes in organic matter sources and aquatic primary productivity may only play an insignificant role. The trend of the greenhouse gases efflux index (GGEI) based on the δ13C and δ15N records is similar to that of other archives which record the organic matter mineralization, and that of climate change records from the eastern QTP and the adjacent regions, with higher GGEI values indicating a higher degree of decomposition of organic matter is associated with a warmer and wetter climate, and vice versa. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Qinghai–Tibetan Plateau (QTP), with a total area of more than 2.5 million km2 and an average altitude above 4000 m above sea level, is one of the most sensitive and fragile regions to global climate change (Rockstrom et al., 2009). Due to the prevailing low temperatures and low turnover rates, large amount of labile organic matter stocks in the high-cold alpine regions, including more than 33.5 Pg of carbon stored in the meadow and steppe soils of the plateau which accounts for about 2.5% of the global soil carbon pool (Dörfer et al. 2013; Wang et al., 2002). At present, the margin of the QTP is primarily controlled by the warm–humid Asian monsoon system in the summer and by the cold–dry Northern Hemispheric middle latitude westerlies in the winter, however, the interior of the QTP is less influenced by the Asian monsoon system and dominated by continental climate due to the block effect of the huge topographic landform (Yao et al., 2012). The mean temperature has increased by 0.2 °C per decade on the QTP during the past five decades and is predicted to rise an additional 2.6–5.2 °C by the end of this century, similar to other high elevation and high latitude areas and much greater than the world as a whole (Chen et al., 2013; Liu and Chen, 2000). Elevated temperatures would largely affect the biogeochemical processes in these plateau ecosystems, such as ⁎ Corresponding author. E-mail address: [email protected] (E. Zhang).

http://dx.doi.org/10.1016/j.palaeo.2016.04.013 0031-0182/© 2016 Elsevier B.V. All rights reserved.

decomposition of soil organic matter and fluxes of greenhouse gases, assuming no changes in other components of climate such as rainfall (Chen et al., 2014; Davidson and Janssens, 2006; Fang et al., 2005). Great efforts have been made to investigate the response of greenhouse gas fluxes to climate warming from different ecosystem on the QTP, based on seasonal field observations or controlled warming experiment (Chen et al., 2010a; Du et al., 2008; Hu et al., 2010; Li et al., 2015; Qin et al., 2015; Saito et al., 2009; Yang et al., 2014; Zhao et al., 2006; Zheng et al., 2012), and the QTP will become an increasing source of CO2, CH4 and N2O (Chen et al., 2013). Large uncertainties remained, however, due to the lack of spatial and temporal resolution of climatic and ecosystem parameters describing the heterogeneous landscape and vegetation communities of the plateau. Gaining a deeper understanding of the long-term biogeochemical processes on the QTP in response to climate change is therefore essential. The sediments of high-elevation alpine lakes are well-suited to studies of environmental changes because of the sensitivity of these lakes to climate change and the fact that they are relatively undisturbed by human activity (Catalan et al., 2013). Lacustrine organic matter serves as a useful archive of lake internal processes and external terrestrial influx entering the lake (Leng and Marshall, 2004). The variations in carbon and nitrogen cycling in lakes and their catchments associated with climate change can be reflected by stable carbon (δ13C) and nitrogen isotope (δ15N) data obtained from lake sediments (Meyers, 1997; Talbot, 2001; Wolfe et al., 1999). In the present study, we present the

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stratigraphic variations in the elemental and stable isotopic signatures of C and N in the sediments of Muge Co, southeastern QTP, in order to investigate the response of the plateau's biogeochemical cycles to climate change in the monsoonal area.

2. Study site Muge Co (30°08′ N, 101°50′ E, Fig. 1a) is located at a NW–SE oriented mountain valley, 3780 m above sea level in the Gongga Mountains, which are part of the larger Hengduan Range in the southwestern China. During the field investigation in 2011, hydrochemical parameters were measured from a single sample. The open freshwater lake (specific conductivity of 121 μS/cm) has a surface area of about 3.0 km2, a catchment area of 75 km2 and a maximum water depth of 31.4 m. The lake has a pH of 8.4 and is classified as oligotrophic water body (total phosphorus concentration is 10 μg/L and total nitrogen concentration is 230 μg/L). Muge Co is fed by seasonal snow melt derived from the higher altitude areas of the lake catchment, and by precipitation. There is one outlet on the northeast side of the lake, which flows into the Yala River. The regional bedrock geology is mainly composed of granite. Muge Co is near the local tree line, the catchment vegetation is dominated by subalpine meadow on the western banks, with forest across the upper areas of the catchment rich in Pinus, Picea, Abies, Cyclobalanopsis and Taxoidiaceae, Betula, Rhododendron, Salix, Ribes and Sorbus are also common in the catchment, along with a range of herbaceous taxa including Artemisia. The study area is mainly influenced by the Asian monsoon system (Wang et al., 2003). During summer it transports warm and moist air masses to the QTP leading to a comparatively warm and humid climate. In winter, dry and cold continental air masses prevail in the study area, driven by the anticyclone over Siberia and Mongolia. Mean annual temperature is 7.2 °C and mean July temperature is 15.6 °C, the mean annual precipitation is ~830 mm, and most (77%) of the annual rainfall occurs during the rainy season between May and September (measured at the nearest meteorological station Kangding Station, 30°1′48″N, 101°34′48″E, at 2615 m above sea level). Based on the observed temperature gradient of 0.5 °C decrease per 100 m increase in altitude, the mean annual and July temperatures at Muge Co are about 1.4 °C and 9.8 °C, respectively (Sun et al., 2015).

3. Material and methods 3.1. Coring, sampling and sediment dating A 383 cm long sediment core (MG1) from the deepest part (about 30-m depth) of Muge Co was collected in 2011 using a Kullenberg Uwitech Coring Platform System (Fig. 1b). In addition, 20 samples of common terrestrial plant species around Muge Co were also collected. The core sections were split, photographed, and described in the laboratory, and continuously subsampled at 1-cm intervals for laboratory analysis, except the interval between 331 and 323 cm, which may be disturbed by sediment slipped down before the section was drilled (Sun et al., 2015). The core chronology is based on 137Cs dating of the surface sediments at 0.5-cm intervals and 8 accelerator mass spectrometry (AMS) 14C radiocarbon dating, including 7 bulk sediment samples and one sample of concentrated pollen (Sun et al., 2015). 137Cs dating was conducted using the EG and G Ortec well-type, coaxial, low background, intrinsic germanium detectors (HPGe GWL-120-15) at the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. The bulk sediment samples were dated by Beta Analytic Inc., Miami, USA, and the pollen sample was dated at the Oxford Radiocarbon Accelerator Unit. All of the 8 AMS 14C dates obtained were calibrated to calendar years before present (0 BP = 1950 AD) using the IntCal13 calibration dataset (Reimer et al., 2013). The relationship between age and depth is interpolated by Bayesian model utilizing the Bacon program with default settings for lake sediments (Blaauw and Andres Christen, 2011; R Development Core Team, 2013) (Fig. 2). The basal age of the core is ~12 cal ka BP and the average sedimentation rate is about 33 cm ka−1. 3.2. Analytical methods Samples of modern terrestrial plants were washed with distilled water and freeze-dried. Samples of the sediment were obtained every 2-cm for total organic carbon (TOC), total nitrogen (TN) content, and stable isotopic analysis of the bulk organic matter. Sample aliquots were treated with 5% HCl to remove possible trace amounts of carbonates and oven-dried at 40 °C. The dried samples were then crushed in an agate mortar to homogenize them and reduce the grain size. The TOC and TN contents were determined using an elemental analyzer

Fig. 1. (a) Location of the study site and paleoclimatic sites referenced in Fig. 6. Triangle indicates the location of Muge Co, and the circles indicate the location of Dongge Cave (Dykoski et al., 2005), Hongyuan peatland (Yu et al., 2006), and Qinghai Lake (Ji et al., 2005). (b) Bathymetric chart of Muge Co with coring site indicated by the triangle.

W. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 453 (2016) 93–100

a

8 4

b

0

0.8 0.6 0.4

TN(%)

TOC (%)

12

95

0.2

c

0

16 12 -16 8

d

-20 -24

(Flash EA 1112). In this study, TOC/TN ratios are expressed as molar ratios. Stable isotopes were measured using a Thermo Delta Plus mass spectrometer coupled with a Flash EA 1112 elemental analyzer. The results are presented as per mil deviation relative to conventional standards, i.e. the Vienna PeeDee Belemnite (VPDB) for carbon isotopic ratios (δ13C), and atmospheric N2 for δ15N values. Replicate analyses indicated that the analytical precision was better than 0.1‰ for the δ13C values and 0.2‰ for the δ15N values.

δ15N(‰,AIR)

6 Fig. 2. Age–depth model based on calibrated AMS 14C dates for core MG1 (Sun et al., 2015).

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e

4

δ13C(‰,VPDB)

C/N ratio

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2 0 -2 0

2

4

6

8

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12

Age (calkaBP) Fig. 3. Stratigraphic variation of various proxy records from the core MG1. (a) TOC content; (b) TN content; (c) TOC/TN ratio; (d) δ13C and (e) δ15N.

4. Results The δ13C values of all modern terrestrial plant samples range from − 30.8 to − 24.1‰, with an average of − 26.5‰. The δ15N values of the samples range from − 5.6 to 2.1‰, with a mean of − 0.9‰. The TOC/TN ratios are high, with a maximum of 84.1 (Table 1). The results of elemental and stable isotopic analyses of the sediments are plotted in Fig. 3. The TOC contents range from 0.5 to 9.2% by weight, with a mean of 5.5% (Fig. 3a). TN values range from 0.04–0.73% by weight, with a mean of 0.48%, exhibiting a similar trend over time as the TOC record (Fig. 3b). The calculated TOC/TN ratios are rather constant, ranging from 8.8 to 16.3 with a mean of 12.8 (Fig. 3c). The δ13C values exhibit large variations, from − 27.4 to − 18.4‰ with a mean of − 24.6‰ (Fig. 3d). During the interval from 12 to 9.7 cal ka BP, the δ13C values exhibit high variability, fluctuating around −21.0‰. The δ13C values were generally more negative in the first half of Holocene (9.7–5 cal ka BP) than in the late Holocene (5 cal ka BP to the present). In contrast, the δ15N values (range from − 1.2 to 4.5 ‰, with a mean of 2.1‰) have a distinct interval with large negative excursions that occurs from 12 to

9.7 cal ka BP, and the values are higher from 9.7 cal ka BP onwards (Fig. 3e). 5. Discussion 5.1. Sources of organic matter The origin of organic matter preserved in lake sediment archives can be deduced through the analysis of elemental ratios and stable isotopic signals (Meyers, 1997; Meyers, 2003; Meyers and Ishiwatari, 1993). Previous studies have shown that TOC/TN ratios can be used to detect the contribution of terrestrial and aquatic plants to bulk sediment organic matter (Meyers, 1997; Meyers, 2003). Phytoplankton typically records TOC/TN ratios between 4 and 10 due to the high protein/low carbohydrate content, while submerged and floating aquatic macrophytes generally have TOC/TN ratios between 10 and 20. In contrast, TOC/TN ratios of vascular land plants often exceed 20 due to the high carbohydrate content (Meyers, 1997; Meyers and Ishiwatari, 1993;

Table 1 Elemental ratios and isotopic compositions of modern plant samples collected in the vicinity of Muge Co. Sample

C/N ratio

δ13C (‰)

δ15N (‰)

Sample

C/N ratio

δ13C (‰)

δ15N (‰)

Salix oreinoma Sabina squamata Primula poisoni Rhododendron orthocladum Abies fabri Rhododendron tapetiforme Rhododendron phaeochrysum Cotoneaster mirophyllus Potentilla peduncularis Tsuga yunnanensis

13.5 84.1 49.2 40.9 71.0 30.0 40.4 44.0 25.5 27.8

−24.4 −24.7 −26.0 −26.4 −25.6 −26.2 −25.0 −29.5 −26.5 −27.2

1.7 −1.5 −3.2 −4.1 −2.6 −0.1 −0.1 −1.9 1.2 −0.4

Sabina squamata Larix potaninii Sabina squamata Rhododendron primulaeflorum Abies georgei Potentilla glabra Potentilla chinensis Allium cyathophorum Caryopteris frichosphaera Calliergonella cuspidata

22.0 41.8 38.2 26.6 22.9 27.9 18.3 17.4 70.8 27.8

−26.8 −24.7 −27.0 −28.0 −24.1 −26.7 −27.8 −27.3 −30.8 −25.0

0.2 −5.6 −1.8 −1.4 −0.2 0.4 1.1 2.1 −1.2 −0.6

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Talbot and Lærdal, 2000). The TOC/TN ratios of nonvascular aquatic plants may also be influenced by nitrogen limitation in the lake water (Hecky et al., 1993; Olsen et al., 2013; Talbot and Lærdal, 2000), or preferentially degraded proteinaceous components during early diagenesis (Meyers, 1997), resulting high values than the source. In the Muge Co sediment core, TN and TOC are positively correlated (Fig. 4a), suggesting that the sedimentary nitrogen is generally in the organic form (Carsten and Stephen, 2001; Nara et al., 2014). The sedimentary TOC/TN ratios should therefore provide a good insight into the origin of the organic matter fraction in the sediment record. The TOC/TN ratios of the MG1 sediments range from 8.8 to 16.3, with a mean of 12.8, distinctly different from that of the terrestrial plants in the catchment. Stable carbon isotope provides an additional tool to distinguish organic matter source. The δ13C values of organic matter are principally determined by the dynamics of carbon assimilation during photosynthesis and by the isotopic composition of carbon source (Hayes, 1993). Terrestrial plants are divided into C3, C4 and Crassulacean Acid Metabolism (CAM) types, according to their photosynthetic pathway. C3 plants, which include most trees and shrubs, typically have a carbon isotope composition ranging from −35 to −20‰. The C4 pathway, characteristic of many savanna grasses and sedges, has a carbon isotope composition ranging from −16 to −10‰ (Deines, 1980). Except in extremely dry conditions, the CAM group has low biomass productivity compared to C3 and C4 plants, and can use either the C3 or C4 pathway; their carbon isotope values are typically in the range of − 20 to − 10‰ (Lüttge, 2004). For aquatic organisms, δ13C values range between − 42 and −26‰ for emergent plants and phytoplankton because their photosynthetic process uses atmosphere CO2. In normal circumstances, δ13C values of dissolved bicarbonate are 7–10‰ higher than those of atmospheric CO2 dissolved in water. Thus, submerged and floating macrophytes, which may absorb CO2 released from bicarbonate in water, have relatively high δ13C values between − 30‰ and − 12‰ (Leng and Marshall, 2004). On the eastern slope of Mount Gongga in the southeastern QTP, C4 plants only occur below 2100 m and the abundance of C4 plants within the community gradually declines with increasing altitude (Li et al., 2009). Although C4 plants are found at altitudes above 4000 m and a maximal elevation of 4520 m in the QTP (Wang et al., 2004), the δ13C value of surface soil organic matter is lighter than −22‰ above 2100 m (Chen et al., 2010b). This suggests that the biomass contribution of C4 plants to the community is very low or zero at high altitude, consistent with the fact that most of the plants sampled in the Muge Co catchment are C3 types. Combining TOC/TN ratios and δ13C records for MG1 with the same parameters for the modern vegetation in the catchment (Fig. 4b) indicates a mixed contribution of endogenous and exogenous material, but with a preponderance of autochthonous material.

5.2. Environmental implications of the stable isotope records At sites with low allochthonous organic matter input, several factors may control δ13C and δ15N in lacustrine environments: (1) early diagenesis; (2) changes in nutrient utilization by aquatic primary producer; (3) the isotopic signature of the source (Finlay and Kendall, 2008; Meyers, 1997). These potential effects, which may weaken the strength of the isotopic signal as an environmental indicator, need to be discussed, identified and eliminated. Variations in post-burial degradation may affect the isotope signals of organic matter in sediments. Post-depositional preferential degradation of organic compounds such as proteins, can result in an increase in the TOC/TN ratio of the remaining organic fraction relative to that in the upper water column along with a depletion in 13C and 15N relative to that in the upper water column (Herczeg et al., 2001; Prahl et al., 1997). In MG1, the sedimentation rate is relatively low and uniform, and thus the magnitude of diagenetic effects on the organic matter is likely to be similar throughout the core (Nara et al., 2014). If early diagenesis was responsible for the isotopic values measured in this study, the δ13C and δ15N values should show a down-core decrease over time. Such a decrease in carbon/nitrogen isotopes is not observed in MG1, although the observed increase in TOC/TN ratio is consistent with this hypothesis. The effects of selective diagenesis on the isotopic record are typically less than 2‰ (Meyers, 1997); which would equate to an insignificant diagenetic effect on δ13C and δ15N values in MG1 sediments. The isotopic fractionation of aquatic plant may be closely related to growth rate in a closed system (Fogel and Cifuentes, 1993). In general, primary producers preferentially assimilate lighter isotope with an increase in primary productivity. During photosynthesis, the preferential uptake of 13C-depleted dissolved inorganic carbon (DIC) by primary producers results in a 13C-enriched DIC reservoir in the lake. In highly productive and alkaline lakes, CO2 dissolved in the water column can drop below a level that may limit the rate of photosynthesis (Hodell and Schelske, 1998b). Thus δ13C values of primary producers can be seen to increase with productivity, resulting in the incorporation of 13 C-enriched organic matter into the sediment record. Comparable to carbon assimilation, nitrogen isotope fractionation between dissolved inorganic nitrogen (DIN) and primary producers is seen to decrease with increasing productivity (Brenner et al., 1999; Hodell and Schelske, 1998a; Hodell and Schelske, 1998b; Talbot, 2001). The δ15N values of sedimentary organic matter should increase in response to the degree of increase in DIN utilization (Brunelle et al., 2007). However, the positive relationship between δ13C and δ15N values is absent in the Muge Co record. Furthermore, kinetic isotope effects may not be significant in oligotrophic lakes where DIN is limited, although limnological

a

0.8

-16

b

sediment samples land plants

δ C(‰, VPDB)

0.4

-20

-24

13

TN(%)

0.6

0.2

-28

2

R =0.88, p<0.001 -32

0 0

2

4

6

TOC(%)

8

10

10

100

TOC/TN

Fig. 4. (a) Scatter plot of MG1 sedimentary TOC versus TN. (b) Scatter plots of TOC/TN ratio versus δ13C value for modern plant samples collected in the catchment of Muge Co and core MG1 samples. Land plants are indicated by red circles and sediment samples by green crosses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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studies and laboratory experiments suggest that strong isotopic fractionation could occur during nitrogen assimilation (Fogel and Cifuentes, 1993; François et al., 1996; Wolfe et al., 1999). Primary productivity is therefore assumed to have had only a minor effect on the sedimentary isotope values in MG1. The isotope composition of organic matter is most likely to be influenced by the isotopic signature of the source associated with the biogeochemical processes (Dolenec et al., 2005; Hodell and Schelske, 1998b; Vander Zanden et al., 2005; Teranes and Bernasconi, 2000). The carbon isotopic composition of the DIC is principally a reflection of the δ13CDIC value of inflowing water, the decomposition of organic matter and the respiration of aquatic plants within the lake (Lei et al., 2012; Leng and Marshall, 2004). The most prominent factors determining the δ15N values of DIN are the conversion processes of different DIN species in the catchment and within the lake. Nitrification can occur under oxic conditions, with denitrification occurring under anoxic conditions and ammonia volatilization in high pH (Talbot, 2001). The negative relationship between δ13C and δ15N values in MG1 sediments is a reflection of the decomposition of organic matter and biomass respiration in the catchment and within the lake (Fig. 5, Finlay and Kendall, 2008). The increase in the supply of 13C-depleted CO2 to the lake water, decrease δ13C values in the DIC. When the DIN supply is high relative to biotic demand, nitrogen is lost through fractionating pathways associated with enhanced gaseous exports, and the remaining ecosystem N becomes enriched in 15N (Korontzi et al., 2000; McLauchlan et al., 2013). Therefore, we conclude that the stable isotope records from Muge Co are likely to reflect the intensity of gases efflux over time, such as CO2 and N2O. 5.3. Response of greenhouse gases efflux to the Holocene climate change Several experimental studies have shown that the ability of microbes to decompose organic matter is generally weaker when the soil temperature is below 5 °C and when the soil water-filled porosity is less than 20% (Horváth et al., 2010; Schindlbacher et al., 2004). With increasing temperature and humidity of the soil, the activity of microbes strengthens rapidly and reaches a peak when soil temperature increases to about 20 °C and the soil water-filled porosity is about 70% (Horváth et al., 2010; Schindlbacher et al., 2004). With the increasing decomposition of bulk organic matter in soil, DIN becomes increasingly enriched in respiration/methan oxidation

tio

n

6

denitrification/nitrification ammonia volatilization

δ15N (‰, AIR)

as

sim

ila

4

2

2

(R =0.57 , p<0.001)

0

-2 -28

-26

-24

-22

-20

-18

δ13C (‰,VPDB) Fig. 5. δ15N versus δ13C values of the core MG1. Processes considered relevant for the interpretation of isotope records from Muge Co and their effects on isotopes of aquatic organic matter are given as labeled arrows (modified from Finlay and Kendall, 2008).

97

15 N (Nadelhoffer and Fry, 1988). Warm environments strengthen the activity of nitrobacterias and ammonifiers, accelerating the rate of soil nitrification and denitrification and enriching the remaining substrates in 15N. In addition, microbes prefer to take up the lighter isotope 14N during the process of absorbing inorganic nitrogen, resulting in the enrichment of 15N in the soil (Aranibar et al., 2004). Similarly, the mineralization of organic carbon in lacustrine sediments to CO2 or CH4 is strongly positive related with temperature (Gudasz et al., 2010). High water temperature and a long ice-free season may both have led to more stable summer lake water stratification, resulting in an environment favorable for denitrification (Winder and Schindler, 2004). Nitrogen cycling in soils on the eastern slope of Mount Gongga in the southeastern QTP inferred by nitrogen isotope composition shows that the δ15N value is more negative with increasing elevation (Liu and Wang, 2010). This relationship can also be found in the nitrogen isotope composition characteristics of modern plants from northern China, East Africa, and other tropical and temperate forests (Liu et al., 2010; Liu et al., 2007; Martinelli et al., 1999; Muzuka, 1999). These suggest that the organic matter decomposition rate is likely to be strongly influenced by temperature, since the average warm season temperature in the catchment is about 10 °C, however, significant changes in precipitation and soil moisture content may also impact the related processes. In order to examine the evolution of greenhouse gases efflux since the late deglaciation in the southeastern QTP, the δ13C and δ15N records from MG1 were normalized using a z-transformation, creating a nondimensional greenhouse gases efflux index (GGEI, Fig. 6a), in which large GGEI values represent high greenhouse gases efflux. The index suggests that greenhouse gases efflux over the last 12 ka can be divided in to three periods: From 12 to 9.7 cal ka BP, the efflux of greenhouse gases was low, with a period of high flux between 10.4 and 10.2 cal ka BP. From 9.7 to 5.0 cal ka BP, greenhouse gases efflux is seen to increase significantly after which point it declines gradually through to the present day. The GGEI record for MG1 is in good agreement with Hongyuan peat humification record from the eastern QTP, where high degree of humification is interpreted as a reflection of advanced peat decomposition (Fig. 6b, Yu et al., 2006). This result shows that the greenhouse gases efflux around Muge Co is likely to be strongly influenced by climate change. A previous study from Muge Co suggests that the climate was cold and dry during the periods of Pleistocene–Holocene transition, warm and humid in the first half of the Holocene, and then became gradually cool and dry, although a reversal occurred at the last two millennia (Fig. 6d, Sun et al., 2015). The pattern of climatic evolution inferred from Muge Co sediments is similar to that from the QTP and the adjacent region, such as the speleothem δ18O record from Dongge Cave in southern China (Fig. 6e, Dykoski et al., 2005), and the redness record from Lake Qinghai in the northeastern QTP (Ji et al., 2005). A reconstruction of extratropical Northern Hemisphere (30° to 90° N) temperature shows that the climate was gradually warming from 11.3 to 9.5 cal ka BP, to a temperature plateau extending from 9.5 to 5.5 cal ka BP, and followed by a long-term cooling to the 20th century (Fig. 6f, Marcott et al., 2013). The modeled summer temperature decreases less than 2 °C during the last deglaciation in southwestern China (Ju et al., 2007), the warm season temperature around Muge Co would generally higher than 5 °C during the last 12 ka. Therefore, on the multi-millenial time scale, higher GGEI can be seen to correspond to a warmer and wetter climate, while lower GGEI corresponds to a colder and drier climate. In addition, at Lake Middendorf in Arctic Russia, a warm and moist climate was seen to enhance the decomposition of soil organic matter in the forest in the late Holocene inferred from the δ13C and δ15N records (Wolfe et al., 1999). Unsurprisingly, the period of greatest change in biogeochemical cycling occurred across the transition of the Pleistocene to the Holocene. Permafrost development in the catchment during cold periods, such as the Younger Dryas event, may be responsible for the phenomenon. Controlled warming experiment in the QTP permafrost region shows that the magnitude of soil warming is much slighter than that in other

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Fig. 6. Comparison of various proxy records. (a) GGEI from Muge Co sediments; (b) humification record of Hongyuan peatland (Yu et al., 2006); (c) δ15N record of N2O from the Taylor Dome ice core (Sowers et al., 2003); (d) median grain size from Muge Co (Sun et al., 2015); (e) speleothem δ18O record from Dongge Cave in southern China (Dykoski et al., 2005); (f) extratropical Northern Hemisphere temperature anomaly (Marcott et al., 2013).

warming experiments in the alpine non-permafrost regions (Qin et al., 2015). Degradation of the permafrost, with ameliorating climatic conditions would have resulted in a marked increase in the rate of organic matter decomposition in the permafrost. The most positive nitrogen isotope of atmospheric N2O from the Taylor Dome ice core during Younger dryas event also indicates that continental N2O production probably decreased more significantly compared to oceanic sources, due to weak organic matter mineralization limited by the cold climate (Fig. 6c, Sowers et al., 2003). With about 1.7 million km2 of land on the QTP underlain by permafrost, large volumes of organic matter sequestered in permafrost could be decomposed to increase net sources of greenhouse gases (Qin et al., 2015; Yang et al., 2010).

likely influenced by the rate of organic matter decomposition in the catchment and within in the lake. The GGEI suggests that the maximum effluxes of greenhouse occurred from 9.7 to 5 cal ka BP, after which the rate gradually decreased. The long term trend of the GGEI for Muge Co is comparable to other published Holocene records of organic matter mineralization from the eastern QTP and climate records from China and the Northern Hemisphere. High GGEI values reflecting higher rates of organic matter decomposition are clearly associated with warmer, wetter climate and vice versa. The strong link between biogeochemical cycling in the southeastern QTP and climate change, suggests that this region is likely to become a net source of greenhouse gas.

6. Conclusions

Acknowledgments

A sediment core (MG1) spanning the last 12 ka was retrieved from Muge Co in the southeastern QTP in order to investigate the response of biogeochemical processes to climate change. Temporal variations in TOC/TN ratios and δ13C values suggest that the sedimentary organic matter is mainly derived from autochthonous sources. In this record, changes in δ13C and δ15N values cannot be explained by early diagenesis or simple changes in aquatic primary productivity. Instead, values were

We would like to express our gratitude to the editor, Dr. James Shulmeister and an anonymous reviewer for helpful comments that improved the quality of this manuscript. We also want to thank Dr. Jan Bloemendal for suggested improvement to the English text, and Dr. Q. Jiang and Dr. H. Tang for field assistance. This project was supported by the Strategic Priority Research Program-Climate Change: Carbon Budget and Relevant Issues of the Chinese Academy of Sciences (grant

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