A Complete Isotope (δ¹⁵N, δ¹⁸O, Δ¹⁷O) Investigation of Atmospherically Deposited Nitrate in Glacial‐Hydrologic Systems Across the Third Pole Region

Abstract

The Himalayas and Tibetan Plateau, identified as the Third Pole (TP), is a unique region because of its insertion into global environmental and climatic changes. Deposition of atmospheric nitrate in this region is one of the most important sources of reactive nitrogen to glacial-hydrologic system and ecosystems. The isotopic composition of atmospherically deposited nitrate preserved in ice bodies plays a central role in delineating environmental and climatic changes, present, and past. Here, we provide an overview of the complete isotopic compositions (δ 15 N, δ 18 O, and Δ 17 O) of nitrate in aerosol, snow, ice, and water samples (n = 46) collected across the Southern, Southeastern, Central, and Northern TP to constrain complex nitrogen cycles of the atmosphere, cryosphere, hydrosphere, and, potentially, the biosphere. A large variability of snow nitrate isotopic compositions is observed across the TP at different spatial scales (from a single glacier to the entire plateau), likely due to the complex landscape and relevant physical and chemical processes across the TP. Large nitrate Δ 17 O values are observed in water samples collected from the Mt. Everest region, highlighting the considerable fraction (up to 45%) of atmospheric nitrate to the nitrate load in this Himalayan hydrologic system. Our work reveals the complex chemical, depositional, and postdepositional processes over the TP that are greater than previously thought and identifies further comprehensive investigations, which entail using nitrate isotopic compositions as an identifier for nitrate source apportionment in various ecosystems and understanding past atmospheric and climatic conditions in this region. The Himalayas and Tibetan Plateau hosts the largest number of glaciers (>36,000) on the planet outside of polar regions with thousands of lakes and is referred to as the Third Pole (TP) (Yao et al., 2012). This massive water reservoir within this pristine region sustains the water supply and food security for billions of people in ©2020. American Geophysical Union. All Rights Reserved.

Isotopic Constraints on Sources and Transformations of Nitrate in the Mount Everest Proglacial Water

“…64−67 In our study, we observed an insignificant correlation (p > 0.05) between δ 18 O NOd 3 and δ 15 N NOd 3 in the glacial lake and glacier-fed river samples (Figure 7), suggesting a limited denitrification process occurring within the proglacial water of the RGB. This finding is consistent with previous studies that have indicated negligible denitrification in the same glacierized basin 13 and in other glacier-fed aquatic environments of the Tibetan Plateau. 44 Furthermore, it is important to note that denitrification primarily occurs in anoxic or near-anoxic environments where dissolved oxygen levels are below 2 mg/ L, as most denitrifying heterotrophic microorganisms are anaerobic.…”

“…Everest proglacial water (Figure 6). 13 Conversion of NO 3 − −N in the Water of RGB. The conversions of NO 3 − in water mainly include nitrification, denitrification, and assimilation.…”

“…1,29,32 Previous studies on NO 3 − isotopic characteristics in highmountain-glacierized regions mainly focused on the formation mechanisms and source of NO 3 − in the aerosol. 13,33 Isotopic signatures of NO 3 − archived in snow and ice were utilized to indicate atmospheric composition and microorganism activities in the glacial environment. 34 For example, studies demonstrated that NO 3 − in the aerosol of Mt.…”

Isotopic Evidence for Microbial Nitrogen Cycling in a Glacier Interior of High-Mountain Asia

“…Amoroso et al also reported low Δ 17 O values of NO 3 – in the surface snow (0–24 cm depth) of the European High Arctic. Similar decreases in δ 18 O or Δ 17 O of NO 3 – in glacial downstream (i.e., glacial thaw, runoff, and surface snow) have also been reported in the French Alps, the Colorado Front Range, Tibetan Plateau glaciers, , and UG1 . These studies tend to concentrate on supraglacial and subglacial nitrification; therefore, this study adds a potential additional source of NO 3 – produced in englacial areas for the microbial NO 3 – in glacial runoff.…”

“…In UG1, the accumulation of 50−100 cm a −1 of snow 33 and dust impurities is greater than that in polar regions. 34 Regarding actinic flux, the postdepositional effect on isotopic compositions may be significant under conditions of high UV radiation, such as those found in the Tibetan Plateau, 56 even if snow accumulation and impurities are high. However, actinic flux in the UG1 region is 10 times lower than in the Tibetan Plateau, 57 indicating that this is not the case.…”