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Atmospheric Sulfur Isotopic Anomalies Recorded at Mt. Everest Across the Anthropocene

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Atmospheric Sulfur Isotopic Anomalies Recorded at Mt. Everest Across the Anthropocene Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene Mang Lina,b,1,2, Shichang Kangc,d,e, Robina Shaheena, Chaoliu Lid,f, Shih-Chieh Hsub,3, and Mark H. Thiemensa,1 aDepartment of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093; bResearch Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan; cState Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, 730000 Lanzhou, China; dCenter for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, 100101 Beijing, China; eUniversity of Chinese Academy of Sciences, 100049 Beijing, China; and fKey Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, 100101 Beijing, China Edited by Barbara J. Finlayson-Pitts, University of California, Irvine, CA, and approved May 17, 2018 (received for review February 2, 2018) Increased anthropogenic-induced aerosol concentrations over the the aerosol budget and evaluate its influences on climate and Himalayas and Tibetan Plateau have affected regional climate, hydrological systems. accelerated snow/glacier melting, and influenced water supply The sulfur isotopic anomaly (or mass-independent fraction- and quality in Asia. Although sulfate is a predominant chemical ation [MIF]) is quantified by nonzero Δ33S and Δ36S values, component in aerosols and the hydrosphere, the contributions where Δ33S = δ33S − 1,000 × [(1 + δ34S/1,000)0.515 − 1] and from different sources remain contentious. Here, we report Δ36S = δ36S − 1,000 × [(1 + δ34S/1,000)1.9 − 1] (Materials and multiple sulfur isotope composition of sedimentary sulfates from a Methods). In the modern atmosphere, sulfate emitted from remote freshwater alpine lake near Mount Everest to reconstruct a combustion and secondarily produced from photolytic oxidation two-century record of the atmospheric sulfur cycle. The sulfur of stratospheric SO2 are the only two known types of isotopically isotopic anomaly is utilized as a probe for sulfur source appor- anomalous sulfates (8–12). Other sulfates (e.g., terrigenous sulfate tionment and chemical transformation history. The nineteenth- in mineral dust, secondary sulfate produced in the troposphere via century record displays a distinct sulfur isotopic signature compared 33 SO2 oxidation) are all isotopically normal (Δ S ∼ 0‰)(8–13). with the twentieth-century record when sulfate concentrations This unique isotopic fingerprinting has been utilized in recon- increased. Along with other elemental measurements, the isotopic structing changes in sources and chemical formation pathways of proxy suggests that the increased trend of sulfate is mainly sulfur in the past atmosphere using ice/snow samples obtained attributed to enhancements of dust-associated sulfate aerosols and climate-induced weathering/erosion, which overprinted sulfur from polar regions (11, 14), but the difficulty of drilling ice cores at Himalayas in terms of the harsh environment at high altitudes isotopic anomalies originating from other sources (e.g., sulfates > produced in the stratosphere by photolytic oxidation processes ( 6,500 m above sea level) (2, 5) hampers such studies in this and/or emitted from combustion) as observed in most modern region. Here we provide multiple sulfur isotopic analysis span- Materials and tropospheric aerosols. The changes in sulfur cycling reported in ning over 200 y in a Himalayan lake sediment core ( this study have implications for better quantification of radiative forcing and snow/glacier melting at this climatically sensitive Significance region and potentially other temperate glacial hydrological sys- 33 34 tems. Additionally, the unique Δ S–δ S pattern in the nine- Signatures of sulfur isotopic anomalies (a proxy used in track- teenth century, a period with extensive global biomass burning, ing the atmospheric oxygen/sulfur cycles in the past) preserved is similar to the Paleoarchean (3.6–3.2 Ga) barite record, potentially in the Himalayas (“Asian water towers”) reveal significant providing a deeper insight into sulfur photochemical/thermal re- changes in the regional atmospheric sulfur cycle and glacial actions and possible volcanic influences on the Earth’s earliest hydrological system during the second industrial revolution. sulfur cycle. The record extends our atmospheric sulfur isotopic anomaly observation to a unique region and different time and transi- Himalayas | mass-independent fractionation | aerosol | glacier | Archean tional period. Distinct from most existing aerosol measure- ments made in the twenty-first century, the 200-y record he Himalayas and Tibetan Plateau (HTP), the largest and mimics the Archean (4–2.5 billion years ago) barite record and Thighest plateau on the Earth, is climatically unique and im- may provide a broader view of the mechanistic origin of sulfur portant due to its location, topography, and teleconnection with isotopic anomalies in the modern atmosphere and another tool ’ other parts of the world (1). It hosts the largest number of gla- to deepen insights into the Earth s sulfur cycle during the ciers outside the polar regions and thousands of lakes, and is evolution of early life. commonly referred to as the “Third Pole.” The ice and lake Author contributions: M.L., R.S., and M.H.T. designed the sulfur isotopic study; M.L. and sediment cores from this midlatitude region provide valuable S.K. collected aerosol, glacial snow, and river samples; S.K. and C.L. collected sediment paleoclimatic and paleoatmospheric records that cannot be samples; S.K., C.L., and S.-C. H. proposed elemental and lead isotopic measurements; M.L. obtained from polar regions (2–5). The HTP is also known as the performed isotopic and elemental measurements; M.L. analyzed data; M.L., S.K., and M.H.T. interpreted results; and M.L., R.S., and M.H.T. wrote the paper. “Asian water tower” because snow and glacier melting in this The authors declare no conflict of interest. region sustains water availability for major rivers in Asia and sustenance for >1.4 billion people (6). A persistent increase in This article is a PNAS Direct Submission. aerosol loading over this region has been altering the atmo- Published under the PNAS license. 1To whom correspondence may be addressed. Email: [email protected] or spheric/glacial chemical composition, snow/glacier melting rate, [email protected]. and glacial river water quality (3, 6, 7). Sulfate is one of the major 2Present address: School of Materials and Chemical Technology, Tokyo Institute of components of aerosols (especially in Asia), but the relative Technology, 266-8502 Yokohama, Japan. contributions of varying sources (e.g., combustion, mineral dust) 3Deceased October 10, 2014. and its mixing state in aerosols remain uncertain because mea- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. surements of source-specific tracers in sulfates are absent. This 1073/pnas.1801935115/-/DCSupplemental. fragmentary understanding limits our ability to accurately quantify Published online June 18, 2018. 6964–6969 | PNAS | July 3, 2018 | vol. 115 | no. 27 www.pnas.org/cgi/doi/10.1073/pnas.1801935115 Downloaded by guest on September 24, 2021 Methods) to gain insight into the change of sulfur source in this study (Fig. 1), close to that of stratospheric sulfates estimated climatically sensitive and important region. As a part of the from polar records of stratospheric volcano eruptions (−1.93 ± world’s highest freshwater lake system, this remote lake is signif- 0.62, n = 13) (14), may imply that the central HTP is pre- icantly different from many saline lakes on the HTP (15). The low dominately affected by a single sulfur isotopic anomaly source background sulfate concentration and low biological activity (i.e., (likely stratospheric sulfates) in the twenty-first century. minimal postdepositional processes) (16) renders this alpine lake an ideal site to reconstruct regional atmospheric sulfur cycle and Two-Century Record of Atmospheric Sulfur Isotopic potentially to record climate-induced changes in weathering/ero- Anomalies sion within the glacial hydrological system. The sulfur isotopic analysis of post-1930 lake sediments (n = 8) display zero Δ33S values within analytical error (±0.01‰) (Fig. Atmospheric Sulfur Isotopic Compositions in the Central HTP 2A and Materials and Methods), likely a result of a larger con- We first report Δ33S (and Δ36S) values in sulfate aerosols [in tribution of mineral dust to TSP in the Himalayas than the total suspend particles (TSP)] collected at the central HTP central HTP as noted by previous aerosol and ice core studies (4, Δ33 (Materials and Methods and SI Appendix, Fig. S1) to gain an 20) and in part supported by near-zero S values in glacial ‰ ‰ overview of the atmospheric sulfur isotopic anomalies in this arid snow and river sulfates (0.03 and 0.02 , respectively) col- SI Appendix region (Fig. 1). Because mineral dust or local soil accounts for lected at the Mt. Everest in this study ( ). The iso- ∼3/4 of TSP at our sampling site in today’s environment (17) and topically normal sulfates may also originate from the weathering i sulfate derived from this terrigenous source is isotopically nor- process of parent rocks, which is supported by ( ) the low en- Δ33 = ‰ Δ33 richment factors of major, trace, and rare earth elements, and mal ( S 0 ), the observed S values (ranging from 206 207 ‰ ‰ (ii) the observed stable lead isotopic composition ( Pb/ Pb 0.05 to 0.12 ) are less than most sulfate aerosols collected 208 207 – and Pb/ Pb) that is nearly identical to soils and river sedi- at midlatitudes (8 10). A first-order estimation based on isotopic SI Appendix mass balance (SI Appendix) yields a Δ33S value of nondust sulfate ments over the HTP (21, 22) ( , Figs. S2 and S3). In contrast, most sulfate samples (six of eight) deposited in the to be 0.36 ± 0.12‰. This estimation is at the higher end of the pre-1930 period display small but distinguishable nonzero Δ33S Δ33S range of currently measured tropospheric sulfate aerosols values (>±0.02‰, at the 2σ level) (Fig.
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