Concentration , sources and light absorption 1 characteristics of dissolved organic carbon on a typical 2 glacier , the northern Tibetan Plateau

Light-absorbing dissolved organic carbon (DOC) constitutes a major part of the organic 19 carbon in glacierized regions. It has important influences on the carbon cycle and radiative forcing of 20 glaciers. However, currently, few data are available in the glacierized regions of the Tibetan Plateau 21 (TP). In this study, DOC characteristics of a typical glacier (Laohugou glacier No. 12 (LHG glacier)) in 22 the northern TP were investigated. Generally, DOC concentrations on LHG glacier were comparable to 23 those in other regions around the world. DOC concentrations in snowpits and surface snow were 332 ± 24 132 μg L -1 and 229 ± 104 μg L -1 , respectively, which were slightly higher than those of the Greenland 25 ice sheet. DOC concentration of surface ice (superimposed ice) was 426 ± 270 μg L -1 , comparable to 26 that of the Antarctic ice sheet. The average discharge-weighted DOC of proglacial streamwater was 27 238 ± 96 μg L -1 , which is lower than that of Mendenhall glacier, Alaska. The annual DOC flux released 28 2 from this glacier was estimated to be 6,949 kg C yr -1 , of which 46.2 % of DOC was bioavailable and 29 could be decomposed into CO2 within one month of its release. The mass absorption cross section 30 (MAC) of DOC at 365 nm was 1.4 ± 0.4 m 2 g -1 in snow and 1.3 ± 0.7 m 2 g -1 in ice, similar to the values 31 of dust transported from adjacent deserts. Based on this finding and the significant relationship between 32 DOC and Ca 2+ , the main source of DOC might be desert mineral dust. Meanwhile, autotrophic or 33 heterotrophic biological activities and autochthonous carbon could also contribute to glacier DOC. The 34 radiative forcing of snowpit DOC was considered to be 0.43 W m -2 , implying the necessity of 35 accounting DOC of snow for accelerating melt of glaciers on the TP. 36


Introduction
Ice sheets and mountain glaciers cover 11% of the land surface of the Earth and store approximately 6 Pg (1 Pg=10 15 g) of organic carbon, the majority of which (77%) is in the form of dissolved organic carbon (DOC) (Hood et al., 2015).The annual global DOC release through glacial runoff is approximately 1.04±0.18Tg C (1 Tg=10 12 g) (Hood et al., 2015).Therefore, glaciers not only play important role in the hydrological cycle by contributing to sea-level rise (Rignot et al., 2003;Jacob et al., 2012), but also potentially influence the global carbon cycle (Anesio and Laybourn-Parry, 2012;Hood et al., 2015) in the context of accelerated glacial ice loss rates.In addition, a large portion of glacierreleased DOC has proven to be highly bioavailable, influencing the balance of downstream ecosystems (Hood et al., 2009;Singer et al., 2012).
Although DOC storage in ice sheets is much larger than that of mountain glaciers, the annual mountain glacier-derived DOC dominates the global DOC release (Hood et al., 2015).Currently, many studies on the concentration, age, composition, storage and release of DOC have been conducted around the world (Stubbins et al., 2012;Bhatia et al., 2013;Lawson et al., 2014 ;Hood et al., 2015).The sources of glacier-derived DOC were found to be diverse (Bhatia et al., 2010;Stubbins et al., 2012;Singer et al., 2012), with large variations in concentrations and ages (Singer et al., 2012;Hood et al., 2015).For example, a study on the Greenland ice sheet showed that the concentration of exported DOC exhibited slight temporal variations during the melting period, with subtly higher values in early May than late May and July (Bhatia et al., 2013).Additionally, concentration of total organic carbon in snow across the East Antarctic ice sheets exhibited remarkable spatial variations due to the marine source of organic carbon (Antony et al., 2011).Studies on both radiocarbon isotopic compositions and biodegradable DOC (BDOC) have proposed that ancient organic carbon from glaciers was much easier for microbes to utilize in glacier-fed rivers and oceans, implying that large amounts of these DOC will return to the atmosphere quickly as CO2 and participate in the global carbon cycle, thereby producing a positive feedback in the global warming process (Hood et al., 2009;Singer et al., 2012).In addition to black carbon (BC), another DOC fraction known as water-soluble brown carbon (WS-BrC) has also been considered a warming component in the climate system (Andreae and Gelencsé r, 2006;Chen and Bond, 2010).This type of DOC exhibits strong light-absorbing properties in the ultraviolet wavelengths (Andreae and Gelencsé r, 2006;Chen and Bond, 2010).The relative radiative forcing caused by water-soluble organic carbon (WSOC, the same as DOC) relative to BC in aerosols was estimated to account for 2-10% and The Cryosphere Discuss., doi:10.5194/tc-2016Discuss., doi:10.5194/tc- -53, 2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.
approximately 1% in a typical pollution area of North China (Kirillova et al., 2013) and a remote island in the Indian Ocean (Bosch et al., 2014), respectively.Unfortunately, so far, few direct evaluations have been conducted in the glacierized region around the world, including the Tibetan Plateau (TP), where DOC accounts for a large part of the carbonaceous matter (Legrand et al., 2013;May et al., 2013) and potentially contributes to radiative forcing in the glacierized region.
The TP has the largest number of glaciers at moderate elevations.Most of the glaciers on the TP are experiencing intensive retreat because of increases in temperature (Kang et al., 2010;Yao et al., 2012;Kang et al., 2015) and anthropogenic carbonaceous particle deposition (Xu et al., 2009;Qu et al., 2014;Kaspari et al., 2014).However, to date, no study has quantitatively evaluated the light absorption characteristics of DOC in the glacierized region on the TP, despite some investigations of concentrations and sources (Spencer et al., 2014;Yan et al., 2015).The primary results of these studies have showed that DOC concentrations in snowpits in the northeastern TP were higher than those in the southern TP (Yan et al., 2015).In addition, a large fraction of the ancient DOC in the glacier in the southern TP has high bioavailability characteristics (Spencer et al., 2014).However, knowledge of DOC in TP glaciers remains lacking due to the large area and diverse environments of the TP and the relatively limited samples and studies.Therefore, numerous snow, ice and proglacial streamwater samples (n=310) (Table S1, Fig. 1) were collected from a typical glacier in the northeastern TP (Laohugou glacier No. 12 (LHG glacier)) based on the preliminary research of snowpit samples (Yan et al., 2015).The concentrations of DOC and major ions (Ca 2+ , Mg 2+ , Na + , K + , NH + 4 , Cl − , NO - 3 and SO 2− 4 ) and DOC light absorbance were measured to comprehensively investigate the sources, light absorption characteristics and carbon dynamics in this glacieried region to provide a basis for the study of DOC across the TP and other regions in the future.
The LHG glacier features typical continental and arid climate characteristics (Li et al., 2012;Zhang The Cryosphere Discuss., doi:10.et al., 2012b).Precipitation occurs mainly from May to September, accounting for over 70% of the total annual precipitation (Zhang et al., 2012b).The monthly mean air temperatures in the ablation zone of the glacier range from -18.4°C in December to 3.4°C in July (Li et al., 2012).Like other glaciers on the TP, LHG glacier has been experiencing significant thinning and shrinkage at an accelerated rate since the mid-1990s (Du et al., 2008;Zhang et al., 2012b).

Sample collection
Two snowpits were dug in 2014 and 2015 in the accumulation zone of LHG glacier.In total, 15 and 23 snow samples were collected in 2014 and 2015, respectively, at a vertical resolution of 5 cm for each snowpit.Moreover, 71 snow/ice samples were collected along the eastern tributary at an approximate elevation interval of 50 or 100 m from the terminus to the accumulation zone, and 201 proglacial streamwater samples were collected at the gauge station during the melting period (Fig. 1, Table S1).The concentrations of glacier DOC have been observed to be very low and prone to contamination, causing an overestimation of DOC concentration (Legrand et al., 2013).Therefore, during the whole sampling procedure, samples were collected according to the "Clean hands-Dirty Hands" principle to prevent any contamination (Fitzgerald, 1999).Meanwhile, at least one blank was made for every sampling process to confirm that the contamination was low (Table S2).Moreover, another batch of samples were also collected for BC concentration measurement following the protocol discussed in detail in our earlier work (Qu et al., 2014) and these results will be presented in another article.
All the collected samples were stored in 125 mL pre-cleaned polycarbonate bottles, kept frozen and in the dark in the field, during transportation and in the laboratory until analysis.In addition, two sand samples from the desert and four dust samples from Dunhuang (39º 53'-41º 35'N, 92º 13'-93º 30'E)a potential source region of the dust deposited on LHG glacierwere collected to compare the light absorption characteristic of dust-sourced DOC to those of the snowpit and ice samples.

Concentration measurements of DOC and major ions
DOC concentrations were determined using a TOC-5000A analyzer (Shimadzu Corp, Kyoto, Japan) after the collected samples were filtered through a PTFE membrane filter with 0.45 μm pore size (Macherey-Nagel) (Yan et al., 2015).The detection limit of the analyzer was 15 μg L -1 , and the average DOC concentration of the blanks was 31.9±7.4μg L -1 , demonstrating that contamination can be ignored The Cryosphere Discuss., doi:10.5194/tc-2016Discuss., doi:10.5194/tc- -53, 2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.

Light absorption measurements
The light absorption spectra of DOC was measured using an ultraviolet-visible absorption spectrophotometer (SpectraMax M5, USA), scanning wavelengths from 200-800 nm at a precision of 5 nm.The mass absorption cross section (MAC) was calculated based on the Lambert-Beer Law (Bosch et al., 2014;Kirillova et al., 2014a;Kirillova et al., 2014b): where I0 and I are the light intensities of the transmitted light and incident light, respectively, A is the absorbance derived directly from the spectrophotometer, C is the concentration of DOC, and L is the absorbing path length (1 cm).
In order to investigate the wavelength dependence of DOC light absorption characteristics, the Absorption Ångström Exponent (AAE) was fitted by the following equation (Kirillova et al., 2014a;Kirillova et al., 2014b): AAE values were fitted from the wavelength of 330 to 400 nm, within this wavelength range, light absorption by other inorganic compounds can be avoided (Cheng et al., 2011).
The radiative forcing caused by BC has been widely studied.Therefore, in this study, using a simplistic model (the following algorithm) the amount of solar radiation absorbed by DOC compared to BC was estimated: previous study, we used MAC550=7.5±1.2 m 2 g -1 (Bond and Bergstrom, 2006), and AAE for BC was set as 1, while hABL was set to 1000 m, which has little influence on the integration from the wavelengths of 300-2500 nm (Kirillova et al., 2013;Bosch et al., 2014;Kirillova et al., 2014a;Kirillova et al., 2014b).

In situ DOC bioavailability experiment
The bioavailability experiment was conducted from August 17th to 31st, 2015, at the glacier terminus during fieldwork.In brief, surface ice samples were collected in pre-burned (550°C, 6 h) aluminum basins and melted in the field.The melted samples were filtered through pre-burned glass fiber filters (GF/F 0.7 μm) into 12 pre-cleaned 125-mL polycarbonate bottles and wrapped with three layers of aluminum foil to avoid solar irradiation.Two samples were refrigerated immediately after filtering to obtain initial DOC concentrations; the others were placed outside at the terminus of the glacier, and 2 samples were refrigerated every 3 days.The BDOC was calculated based on the discrepancies between initial and treated samples.

Snowpits
LHG glacier is surrounded by arid and semi-arid regions and frequently influenced by strong dust storms (Dong et al., 2014b) (Fig. 1).Therefore, heavy mineral dust deposition contributes to high DOC concentrations in LHG glacier.Average DOC concentration of the snowpit samples was 332.4±132.3μg L -1 (Fig. 2), with values ranging from 124.4 μg L -1 to 581.0 μg L -1 (Fig. S1).The highest values appeared in the dirty layers (Fig. S1), similar to the pattern observed in the Greenland summit snowpit (Hagler et al., 2007) and glaciers in the southern TP (Xu et al., 2013), indicating DOC concentrations were mainly influenced by dust deposition in this region.Spatially, our results were higher than those of Tanggula glacier (TGL) in the middle TP and Rongbuk glacier on Mount Everest (EV) in the southern TP (Fig. 1) (Table 1) (Yan et al., 2015), which was similar to the distributions of mercury and particle on the TP

Surface snow and ice
Average DOC concentration in LHG glacier surface snow was significantly lower than that in surface ice because more impurities were presented in the latter (Fig. 2).Like those of the snowpits, DOC concentrations in the glacier surface ice (Fig. 2) were higher than those in glacier of the southern TP (Nyainqentanglha glacier) (Spencer et al., 2014) (Table 1) mainly due to heavy dust load of LHG glacier.
No significant relationship was found between DOC concentration and elevation for either the surface snow or ice (Fig. S2), suggesting no "altitude effect" of DOC in this glacier.Therefore, the distributions of DOC concentrations in the glacier surface snow and ice were influenced by complicated factors, such as the terrain, surface moraine and atmosphere circulation.Furthermore, DOC concentrations of snow and ice of this glacier were within the range of previously reported values for glacieried regions outside the TP (Table 1).
BDOC reached 43.2% if the experiment duration was extended to 28 days, according to the equation derived from the 15-day experiment (Fig. 3).Despite different incubation conditions, this finding agreed well with the reports of BDOC from a glacier in the southern TP (28-day dark incubation at 20 °C, 46-69% BDOC) (Spencer et al., 2014) and European Alpine glaciers (50-day dark incubation at 4 °C, 59±20% BDOC) (Singer et al., 2012).Therefore, the previous results obtained in the laboratory are close to the reality and can be used to estimate the bioavailability of glacier-derived DOC.

Sources of snowpit DOC
The sources of glacier DOC are diverse and include microbial activities (viruses, bacteria and algae) (Anesio et al., 2009), terrestrial inputs (DOC deposition from vascular plants and dust) (Singer et al., 2012) and anthropogenic sources (fossil fuel and biomass combustion) (Stubbins et al., 2012).In this study, major ions were adopted as indicators to investigate the potential sources of snowpit DOC, because The Cryosphere Discuss., doi:10.5194/tc-2016-53,2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.
the sources of major ions in snowpit samples from Tibetan glaciers have been investigated in detail (Kang et al., 2002;Kang et al., 2008).It was found that DOC and Ca 2+ (a typical indicator of mineral dust) were significantly related (R 2 =0.84,Fig. S3), suggesting that the major source of DOC was mineral dust, which is consistent with the previous DOC source investigations of snowpits on this glacier (Yan et al., 2015).
In addition, the combined study of geochemistry and backward trajectories for LHG glacier showed that the dust particles on the glacier were mainly derived from the deserts to the west and north of the study area (Dong et al., 2014a).

AAE
AAE is generally used to characterize the spectral dependence of the light absorption of DOC, which is important input data for radiative forcing calculations.The fitted AAE330-400 values ranged from 1.2 to 15.2 (5.0±5.9) for snow samples and from 0.3 to 8.4 (3.4±2.7) for ice samples (Fig. S5).The relatively low AAE330-400 values of ice indicated that the DOC experienced strong photo-bleaching due to longduration exposure to solar irradiation.Previous studies have found that the AAE of brown carbon (BrC) in aged aerosols (Zhao et al., 2015) and secondary organic aerosols (SOAs) (Lambe et al., 2013) were much lower compared to that of the primary values.Therefore, the large divergence in AAE values might suggest different chemical compositions of DOC due to multiple possibilities, such as different sources and photo-bleaching processes.In general, AAE330-400 had a negative relationship with MAC365, especially in the ice samples (Fig. S5), suggesting that stronger absorbing DOC might contribute to lower AAE values, which was also found in other aerosol studies (Chen and Bond, 2010;Bosch et al., 2014;Kirillova et al., 2014b).

MAC365
MAC365 for DOC is another input data point for the radiative forcing calculation.The light absorption ability at 365 nm is selected to avoid interferences of non-organic compounds (such as nitrate) and to be consistent with previous investigations (Hecobian et al., 2010;Cheng et al., 2011).The MAC365 of DOC was 1.4±0.4m 2 g -1 in snow and 1.3±0.7 m 2 g -1 in glacier ice (Fig. S5).The MAC values for DOC from different sources varied widely.Normally, the MAC365 of DOC derived from biomass combustion was as high as 5 m 2 g -1 (Kirchstetter, 2004).Correspondingly, the value for SOAs was as low as 0.001-0.088m 2 g -1 (Lambe et al., 2013) and humic like substances (HULIS) in Arctic snow was 2.6±1.1 at 250 nm (Voisin et al., 2012)  considered that the snowpit DOC should be SOAs with low MAC365 values, however, the high MAC365 value of the snowpit DOC indicated that these DOC may not be entirely derived from SOAs.Therefore, it was proposed that mineral dust-sourced DOC caused high MAC365 values in the snowpit samples.The light absorption characteristics of DOC from both snowpit and ice showed similar patterns to those of WSOC in dust from the adjacent deserts, further indicating that LHG glacier DOC was transported via mineral dust and shared similar light absorption characteristics (Fig. 4).Moreover, the difference in light absorption characteristics (especially for wavelengths larger than 400 nm) between snow/ice samples and aerosols in Beijing, China, also indicated different sources (Fig. 4) (Table 2).Light absorbance was significantly correlated with DOC concentrations in both snow and ice samples (Fig. S4), suggesting that DOC was one of the absorption factors.Nevertheless, MAC365 values of surface ice (0-3 cm) were lower than those of subsurface layers (3-5 cm), despite its higher DOC concentrations (Fig. 5), reflecting stronger DOC photo-bleaching in the surface ice due to the direct exposure to solar irradiation.

Radiative forcing of DOC relative to BC
The radiative forcing contributed by WSOC relative to BC in aerosols has been proposed to be as high as 2-10 % (Kirillova et al., 2013;Kirillova et al., 2014a).Furthermore, it was estimated that BrC accounted for a higher ratio of 20 % of the direct radiative forcing of aerosols at the top of the atmosphere because BC concentrations decrease faster than BrC in the high-altitude atmosphere (Liu et al., 2014).
Because the studied glacier is located at high elevations near the top of the troposphere and features relatively high DOC/BC ratios in the snowpit samples (Fig. S6), the radiative forcing caused by DOC relative to BC should also be high.
Our results showed that the relative radiative forcing caused by DOC relative to BC ranged from 2.1 % to 30.4 % (9.5±8.4 %) for snowpit samples and from 0.01% to 0.5 % (0.1±0.1 %) for surface ice samples (Fig. S5), mainly because of the higher DOC/BC ratio (0.65) in the snowpit samples than in the ice samples (0.012) (Fig. S6).The value in ice was much lower due to the enrichment of BC in surface glacier ice during the intensive ablation period (Xu et al., 2009).Because snowpit samples can be approximately considered to be fresh snow; thus, radiative forcing caused by DOC is a non-ignorable contributor in addition to BC in reducing the albedo of a glacier when the glacier is covered by fresh snow.

DOC export during the melt season
The two-year average discharge-weighted DOC concentration was 237.5±95.6 μg L -1 during the The Cryosphere Discuss., doi:10.5194/tc-2016-53,2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.
melting period, comparable with the proglacial streamwater of Mount Nyainqentanglha glacier in the southern TP (Spencer et al., 2014).Seasonally, high DOC concentrations appeared during the low discharge periods (May to July and September to October) (Fig. 6), suggesting that DOC concentrations were slightly enriched to some extent.However, there were no clear diurnal variations in DOC concentrations with the discharge, indicating that the discharge from different parts of the glacier was well mixed at the glacier terminus (Fig. S7).
The seasonal variations in DOC flux were similar to those of the discharge (Fig. 6), indicating that discharge (rather than DOC concentrations) played a dominant role in the DOC mass flux.Hence, the majority of the glacier DOC export occurred during the summer melting season.Over the whole melting season, the annual flux of DOC from LHG glacier was 340.7 kg km -2 yr -1 , with peak DOC fluxes from mid-late July to late August (70% of annual flux).Combined with the value of BDOC determined above, at least 3,001.5 kg C yr -1 was ready to be decomposed and returned to the atmosphere as CO2 within 28 days of its release, producing a positive feedback in the global warming process.

Conclusions and implications
The concentrations and light absorption characteristics of DOC on a typical glacier in the northern TP were reported in this study.The mean DOC concentrations in snowpit sample, fresh snow, surface ice and proglacial streamwater were 332.4±132.3μg L -1 , 229.3±104.4μg L -1 , 425.8±269.9μg L -1 and 237.5±95.6 μg L -1 , respectively.These values were slightly higher or comparable to those of other regions, such as the European Alps and Alaska.DOC in snowpit samples was significantly correlated with Ca 2+ , a typical cation in mineral dust, indicating that mineral dust transported from adjacent arid regions made important contributions to DOC of the studied glacierized region.In addition, the light absorption profile of snowpit DOC was similar to that of dust from potential source deserts, providing further evidence of the influence of mineral dust on snowpit DOC.For the first time, it is estimated that the radiative forcing caused by DOC accounted for 9.5±8.4% and 0.1±0.1 % relative to that of BC, in the snowpit samples and surface ice, respectively.Therefore, in addition to BC, DOC is also an important agent in terms of absorbing solar irradiation in the glacierized region, especially when the glacier is covered by fresh snow, which contains high DOC/BC ratios.It has also been proven that water-insoluble organic carbon has a stronger light absorption ability (Chen and Bond, 2010).Therefore, the total contribution of OC to light absorption in the glacierized region should be higher, which requires further study.Wet deposition is the most effective way of removing carbonaceous matter from the atmosphere (Vignati et al., 2010), and the The Cryosphere Discuss., doi:10.5194/tc-2016Discuss., doi:10.5194/tc- -53, 2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.
removal ratio of OC in remote areas is almost the same as that of BC after long-range transport from source regions (Garrett et al., 2011).Because snowpit samples directly reflect the wet and dry depositions of carbonaceous matter, it is assumed that the contribution of radiative forcing for WSOC relative to BC in the atmosphere in glacierized regions should be close to that of the snowpit samples in this study.
Because proglacial streamwater from different parts of the glacier is well mixed, no clear diurnal variations in DOC concentrations have been found.Combined with discharge and the corresponding DOC concentration, it was calculated that approximately 340.7 kg km -2 yr -1 of DOC was released from LHG glacier.It was also calculated that approximately 43.2% of the DOC could be decomposed within 28 days; thus, 3,001.5 kg C yr -1 would return to the atmosphere as CO2, producing positive feedback in the warming process.Although the flux of DOC from the studied glacier is small, the number of glacial rivers across the entire TP and surrounding areas may be large due to the glacial area of approximately 100,000 km 2 (Yao et al. 2012).These factors need to be comprehensively studied in the future.
Author contributions.S. Kang was the lead scientist of the entire project.F. Yan wrote the first draft of the manuscript with the significant help of C. Li.F. Yan did DOC and ions measurement.Y. Li and Y. Zhang helped with the sample collection.X. Qin and X. Zhang provided the discharge data.M. Sillanpä ä , B. Qu, P. Chen, Z. Hu and X. Li improved the manuscript.S. Kang and C. Li conceived and designed the experiments.

Figure 2 .
Figure 2. Average DOC concentrations of ice, snow and proglacial streamwater of LHG glacier.

Figure 3 .
Figure 3. Exponential deceases in DOC concentrations during the biodegradation experiment.Note: The blue point is calculated using equations derived from the experimental data (black point).Mean values ± standard deviation of duplicate treated samples are presented.

Figure 4 .
Figure 4. Absorption spectra for DOC in snow and ice of LHG glacier and the dust and desert sand from surrounding areas.

Figure 5 .
Figure 5.Comparison of DOC concentrations (A) and MAC365 (B) between surface and subsurface ice.

Figure 6 .
Figure 6.The discharge, DOC concentrations and fluxes exported from LHG glacier.Note: The concentrations

Table 1 .
Comparison of DOC concentrations in snow and ice from the glacier in this study and other regions.The Cryosphere Discuss., doi:10.5194/tc-2016-53,2016 Manuscript under review for journal The Cryosphere Published: 11 April 2016 c Author(s) 2016.CC-BY 3.0 License.

Table 2 .
Mass absorption cross section (MAC) and Absorption Ångström Exponent (AAE330-400) of ice and snow 464 from LHG glacier and aerosols from other regions.