Centennial Records of PAHs and Black Carbon in Altay Mountain Peatlands, Xinjiang, China


 Black carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) are often used to indicate anthropogenic impacts on natural environmental changes during the past century. In this study, a 30 cm peat core was collected from the Jiadengyu (JDY) peatland in Altay Mountain and dated by the 137Cs and 210Pb methods. The total organic carbon, BC and PAHs contents in JDY peat core were 17.09-47.16%, 1.14-67.14 mg g-1 and 260.58-950.98 ng·g-1, respectively. The δ13CBC ranged from -31.5‰ to -27.43‰, with an average of -30.52‰. Scanning electron microscope (SEM) showed that the BC particles in the peat were lumpy or irregular in shape and retained the structure of plant fiber. The PAHs ratios, δ13CBC and the SEM result indicated the dominant biomass combustion source of BC in the peatland. The BC content increased from 1950 to 1980 and decreased after 1980. The change of BC and δ13CBC is different from the national BC emission pattern, probably reflecting the impact of local agricultural exploration and thus crop burning increase.


Introduction
Black carbon (BC) is produced by incomplete combustion of fossil fuels or biomass in environment and plays an important role in the carbon biogeochemical process of ecosystem (Hammes et al., 2007;Meehl et al., 2008;Ramanathan and Carmichael, 2008). Global BC was emitted by 50-270 Tg C a − 1 as residues of vegetation res (Kuhlbusch, 1995), and 4.4 Tg C a − 1 from fossil fuel consumption with a liner increase trend (Bond et al., 2007). In recent years, the increased industrial and agricultural activities have signi cantly changed the global carbon cycle through the emission of greenhouse gases and BC particles (Hu et al., 2020;Kuhlbusch, 1995). For example, BC contents in atmosphere were high up to 2 µg C. m − 3 in Beijing and Shanghai (China), which was nearly 2.5 times of that in Gosan (Korea) (Chen et al., 2013).
Identifying sources of BC is helpful in better understanding the proportion of the in uence of regional climate change and human activities on BC uxes (Lehndorff et al., 2015;Sun et al., 2017). The formation and morphological structure of BC is related to the type of fuel, the temperature and duration of combustion (Preston and Schmidt, 2006). Previous methods for identifying soil BC sources included potassium dichromic oxidation, 375°C thermal oxidation, Thermal-light transmission carbon analyzer (TOT) and Thermoscopic carbon analyzer (TOT/RT), ratio between BC and total organic carbon (TOC) analysis, stable carbon isotope analysis (δ 13 (Sage and Wedin, 1999). In addition, PAHs were usually used to identify burning sources and then indicate pyrolysis carbon sources of BC due to their coemission from burning process (Pontevedra-Pombal et al., 2012). The diagnostic ratios of PAHs are useful for indicating the sources of PAHs, mainly by the ratios of Flt/(Flt + Pyr), BaA/(BaA + Chr) and Ant/(Ant + Phe) (Gao et al., 2018;Yunker et al., 2002).
BC is capable of absorbing and transporting polycyclic aromatic hydrocarbons (PAHs), and thus forms a potential pollution source (Lohmann and Lammel, 2004). Recently, BC and PAHs uxes in sediment archives (e.g. lake sediment, peat bog) were widely used for reconstructing the regional environmental pollution history and evaluating the degree of natural and anthropogenic contributions (Gao et al., 2014b;Ruppel et al., 2015;Shen et al., 2020). Altay Mountain is located in the mid-high latitudes in the Alpine region, where the peat resources are abundant due to the cold and wet climate. This region is sensitive to climate change and it is gradually affected by human activities, especially with the increasing tourism. However, the research on anthropogenic impact on the wetland environment is quite limited.
In this study, a 30 cm peat pro le from the Jiadengyu (JDY) in Altay Mountain was dated by the 137 Cs and 210 Pb methods, and multi-proxies including BC, 16 priorities of PAHs and δ 13 C BC were measured. The main objectives are: 1) to characterize the history of BC and PAHs uxes in the JDY peat core; 2) to primarily reveal the main sources of BC uxes in the Altay mountain. In August 2019, a large block was dug up in the Jiadengyu peatland (marked as JDY) and then sectioned in-site at 1-cm intervals. All samples were stored at polyethylene bags and then brought back to the laboratory for cryopreservation.

Dry bulk density, water content and ash content analyses
The peat samples were put into the aluminum box of xed volume, dried at 105℃ over 12 h, and then weighed to obtain the water content (WC, %) and dry bulk density (DBD, g·cm − 3 ). The dry samples were transferred into mu e furnace at 550 ℃ over 4 h for complete combustion. The ash content (Ash, %) was calculated through the loss on ignition method. These physicochemical measurements and the geochemical analyses below were conducted in the Analysis and Test Center of Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (CAS).

Total organic carbon
About 1 g of dry peat samples were ground to 200 mesh and were digested su ciently by 3 mol/L of HCl to fully remove the carbonate. Then, it was washed to neutral with deionized water, and dried after centrifugation. The dry samples were ground and weighed for TOC analysis by the element analyzer (Italy Euro Vector Company, EA 3000 type).

210 Pb and 137 Cs dating
Approximate 5 g dry samples of each slices were 210 Pb-and 137 Cs-dated by a low-background γ-ray spectrometer with a high pure Ge semiconductor (ORTEC Instruments Ltd., USA) at the State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, CAS. A detailed description of the radiometric dating techniques used for this peat and the equations calculating sedimentation rate (SR) and peat accumulation rate (PAR) has been given elsewhere (Bao et al., 2012).

Black carbon, δ 13 C BC and SEM-ENERGY Spectrum Analysis
About 1 g dry samples were treated for 20 h in 10 ml HCl (1 mol/L) to remove inorganic carbon. Then, the contents were centrifuged and the residue was digested for 20 h with 10 mL acid mixture (3 mol/L HCl + 22 mol/L HF, a volumetric ratio of 1:2). The samples were then centrifuged and the residue were soaked in 10 ml HCl (1 mol/L) for 10 h. The residue is regarded to consist of organic matter, kerogen and BC. The 0.1 mol/L of NaOH (30 ml, 12 h, twice) was used to remove humid acid, and removed kerogen by K 2 Cr 2 O 7 (0.1 mol/L) and H 2 SO 4 (2mol/L) mixed solution (60 h, and keep mixture yellow). All steps were treated in 55℃ bath. The residual carbon as BC and δ 13 C in residual carbon as δ 13 C BC were quanti ed by a continuous-ow isotope ratios mass spectrometer (CF-IRMS) which consists of an EA (Flash, 2000 series) coupled to a Finnigan MAT 253 mass spectrometer. BC reference material (charred wood) produced in the Department of Geography at the University of Zurich (Hammes et al., 2006) was used to verify the BC measurement method. The particle size, morphology and porous structure of BC were analyzed by JSM-IT 500 HR microscope (Hitachi, Tokyo, Japan), which can perfectly combine the large eld optical CCD images and SEM images until the smooth operation of high magni cation observation,

Statistical analysis
Descriptive statistical analyses were conducted to calculate the means, ranges, and standard deviations of the peat parameters. Moreover, linear regression analysis was used to investigate the relationship between BC content in JDY peat core and TOC, PAHs, respectively. These procedures were performed using SPSS 22 software package. Statistical signi cance was determined at the P = 0.05 level.

Physicochemical properties of peat samples
The DBD and Ash decreased while TOC and WC increased from 1-7 cm and from 11-18 cm; DBD and Ash increased while TOC and WC decreased from 7-11 cm and from 18-30 cm (Fig. 2). The DBD value varied from 0.12 to 0.29 g cm − 3 , and the mean value was 0.18 g cm − 3 of the whole peat core. The DBD at 18-30 cm (mean: 0.15 g cm − 3 ) were lower than those at 1-11 cm (mean: 0.21 g cm − 3 ). The Ash ranged between 11.9% and 65.31%, with an average of 37.73% of the whole peat core. In addition, Ash (mean: 27.21%) at 18-30 cm were lower than those at 1-11 cm (mean: 55.45%). The TOC of whole peat core ranged from 17.09-47%, with an average value of 33.84 % and the highest value was 37.72% at 18 cm and the lowest value (17.09%) at 11 cm. The WC ranged between 47.09% and 64.36% and the average was 58.89% of the whole peat core.

Peat chronology
The 210 Pb and 137 Cs activity decreased with depth in JDY peat pro le (Fig. 3). Ages and sedimentation rates were calculated using the CRS Model by the MATLAB 2012a software. The activities of 210 Pb and 137 Cs decreased with depth downward. The peat record covered about 160 years to reach back to A.D.
1860. The mean SR was 0.47 cm y − 1 and the PAR is 0.06 g·cm − 2 y − 1 .

Temporal variation of BC, δ 13 C BC and PAHs
The average BC content was 21.06 mg g − 1 , with a range of 1.14-67.14 mg g − 1 in the JDY peat core. The range of δ 13 C BC was from − 31.5‰ to -29.42‰ with a mean of -30.52‰. During 1860-1950, the BC and δ 13 C BC consistently showed a relative stable pattern (Fig. 4a, b). From 1950 to 1980, the BC and δ 13 C BC showed an obviously increase trend. After 1980, the BC decreased obviously with time but the δ 13 C BC did not show consistent variation with the BC. The total PAHs in JDY peat core were 260.59-950.98 ng g − 1 , and showed similar variation trend with the BC (Fig. 4a, c). During the nearest 10 years, the PAHs showed a minor increasing trend, which was different from the BC variation.

PAHs composition
The PAHs was mainly composed of low molecular weight compounds (2-3 rings, except Nap, Any), and the content of Any, Flu and Phe content was the highest (Fig. 5)

Comparison of black carbon over the world
The average content of BC in the JDY peat pro le was 21.06 mg g − 1 , which was similar with the average BC content of 24.85 mg g − 1 in Tuqiang peat in Great Hinggan Mountains, China (Cong et al., 2016). Compared to other soils (e.g. forest soil, loess) and sediments (e.g. lake sediment, coastal sediment), contents of BC in the peats were the highest (Table 1) BaA/(BaA + Chr) > 0.35, Flt/(Flt + Pyr) > 0.5 and Ant/(Ant + Phe) < 0.1 indicate that PAHs are mainly from biomass combustion (Gao et al., 2018). In this study, the BaA/(BaA + Chr) ratio, Flt/(Flt + Pyr) ratio and Ant/(Ant + Phe) ratio were 0.45, 0.56 and 0.13 respectively (Fig. 6). Therefore, the main source of PAHs is biomass combustion in JDY pro le, which is consistent with the main source revealed by the δ 13 C BC values.
Previous studies found that the BC particles emitted by diesel vehicle combustion were less than 50 nm and spherical in shape, with an agglomeration and long chain (Accardidey, 2003;Wang et al., 2015). The particle size and morphology of BC particles derived from gasoline vehicles are similar to those of diesel vehicles, but the polymerization is more obvious. BC particles emitted by coal burning are generally porous and not spherical in shape (Zhan et al., 2016). BC particles from biomass combustion releases are lumpy or irregular in shape and retain the structure of plant bers (porous or tubular) (Masiello, 2004). The BC of samples at depths of 18 cm and 28 cm was lumpy or irregular in shape, which retained the structure of plant bers (porous or tubular) (Fig. 7a-d), and indicated the effects of biomass combustion.

Black carbon temporal variation
Compared with the regional background of China's BC emissions (Gao et al., 2014b), this study attempts to analyze the BC emission pattern in the Altay region during the last 150 years. The concentration of BC emissions in China has been increasing, especially after 1980, which is mainly controlled by the increase of industrial emissions (Fig. 8a (Gao et al., 2014a)). However, the changes of BC content and BC ux in JDY show an increasing trend before 1980 but a decreasing trend after 1980 (Fig. 8b). This is because the BC in JDY peat pro le is mainly from biomass combustion, as discussed above. It was reported that the cultivated land area in 1949 from 9.43 km 2 increased to 115.29 km 2 in 1980 in Altay area (Statistics, 2018). As a result, BC content in the JDY peatland signi cantly increased from 1950 to 1980. The decrease in the BC record in the peat is probably a response to the increasing regional environmental protection.

Conclusion
The average accumulation rate of BC in JDY peatland was 0.47g·cm − 2 yr − 1 since the 1860s. The range of δ 13   territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.   The contents of BC (a), δ13CBC (b) ratios and the total PAHs content (c) in the core of JDY.