Characteristics and sources of dissolved organic matter in a glacier in the northern Tibetan Plateau: differences between different snow categories

Abstract

Dissolved organic matter (DOM) in mountain glaciers is an important source of carbon for downstream aquatic systems, and its impact is expected to increase due to the increased melting rate of glaciers. We present a comprehensive study of Laohugou glacier no. 12 (LHG) at the northern edge of the Tibetan Plateau to characterize the DOM composition and sources by analyzing surface fresh snow, granular ice samples, and snow pit samples which covered a whole year cycle of 2014/15. Excitation–emission matrix fluorescence spectroscopy analysis of the DOM with parallel factor analysis (EEM-PARAFAC) identified four components, including a microbially humic-like component (C1), two protein-like components (C2 and C3) and a terrestrial humic-like component (C4). The use of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) showed that DOM from all these samples was dominated by CHO and CHON molecular formulas, mainly corresponding to lipids and aliphatic/proteins compounds, reflecting the presence of significant amounts of microbially derived and/or deposited biogenic DOM. The molecular compositions of DOM showed more CHON compounds in granular ice than in fresh snow, likely suggesting newly formed DOM from microbes during snowmelting.

Measurement report: Molecular composition, optical properties, and radiative effects of water-soluble organic carbon in snowpack samples from northern Xinjiang, China

“…The U samples had the highest number of assigned compounds among four groups of sites in both ESI+ and ESI− with averages of 1113 ± 203 and 871 ± 287, respectively (Tables 1 and 2), whereas the number of as-signed species from S samples was lowest (mean: 727 ± 146 and 438 ± 84 for ESI+ and ESI− modes, respectively), reflecting the high molecular complexity of U samples. The numbers of assigned formulas in this study are comparable to the assignments reported for urban aerosol samples (∼ 800-1800) (Lin et al, 2012;Wang et al, 2017a) and WSOC of LHG glacier from the TP region (∼ 700-1900) (Feng et al, 2016(Feng et al, , 2018, but they are lower than those of WSOC from the Antarctic (∼ 1400-2600) and Greenland ice sheets (∼ 1200-4400) (Antony et al, 2014;Bhatia et al, 2010). The mass spectrum plots constructed from individual samples showing integrated composition of U, R, S, and site 120 samples along with the corresponding number contributions from different formula categories are shown in Fig.…”

“…BC is also shown for reference (Bond and Bergstrom, 2006 et al, 2007a, b;Hood et al, 2015), glaciers in the European Alps (∼ 100-300 ng g −1 ) (Legrand et al, 2013;Singer et al, 2012), and the remote TP region (∼ 150-700 ng g −1 ) (Yan et al, 2016). However, our reported WSOC mass concentrations are in the same range as those in the fresh snow samples collected from Laohugou (LHG) glacier, northern TP (2000-2610 ng g −1 ) (Feng et al, 2018). It has been reported that glaciers and ice sheets from polar or alpine regions store a large amount of WSOC and discharge it to their downstream terrestrial ecosystems (Hood et al, 2009(Hood et al, , 2015Singer et al, 2012).…”

Reply on RC1

“…The U samples had the highest number of assigned compounds among four groups of sites in both ESI+ and ESI-with averages of 1113±203 and 871±287, respectively (Tables 1 and 2), whereas the number of assigned species from S samples were lowest (mean: 727±146 and 438±84 for ESI+ and ESI-modes, respectively), reflecting high molecular complexity of U samples. The numbers of assigned formulas in this study are comparable with the assignments reported for urban aerosol samples (~800-1800) (Lin et al, 2012;Wang et al, 2017a) and WSOC of LHG glacier from the TP region (~700-1900) (Feng et al, 2016;Feng et al, 2018), but they are lower In ESI+ mode, CHO+ and CHON+ were the main components in all samples, accounting for 35% to 61% and 20% to 28% of total formulas, respectively (Table 1).…”

Reply on RC2

“…than those found in the snow and ice from polar regions (~40 -500 ng g -1 ) (Fellman et al, 2015;Hagler et al, 2007a,b;Hood et al, 2015), glaciers on the European Alps (~100 -300 ng g -1 ) (Legrand et al, 2013;Singer et al, 2012), and the remote TP region (~150 -700 ng g -1 ) (Yan et al, 2016). However, our reported WSOC mass concentrations are in the same range as those in the fresh snow samples collected from Laohugou (LHG) glacier, northern TP (2000 -2610 ng g -1 ) (Feng et al, 2018). It follows that in addition to the snow and glaciers from polar or alpine regions, the seasonal snow in Northern Xinjiang is also an important organic carbon source for the covered ecosystems.…”

“…The U samples showed the lowest CHO+ abundance (mean: 42%±5%) while the sample from site 120 had the highest value (59%). The fractions of CHON+ species in our samples were significantly lower than those of the aged firn/ice samples from the TP (Feng et al, 2016(Feng et al, ,2018(Feng et al, ,2020Spencer et al, 2014) in which WSOC showed high bioavailability; but were comparable with the WSOC samples from fresh snow (Feng et al, 2018(Feng et al, ,2020. These results indicate that WSOC from the snowpack in northern https://doi.org/10.5194/acp-2020-1260 Preprint.…”