Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene

Significance

Signatures of sulfur isotopic anomalies (a proxy used in tracking the atmospheric oxygen/sulfur cycles in the past) preserved in the Himalayas (“Asian water towers”) reveal significant changes in the regional atmospheric sulfur cycle and glacial hydrological system during the second industrial revolution. The record extends our atmospheric sulfur isotopic anomaly observation to a unique region and different time and transitional period. Distinct from most existing aerosol measurements made in the twenty-first century, the 200-y record mimics the Archean (4–2.5 billion years ago) barite record and may provide a broader view of the mechanistic origin of sulfur isotopic anomalies in the modern atmosphere and another tool to deepen insights into the Earth’s sulfur cycle during the evolution of early life.

Abstract

Increased anthropogenic-induced aerosol concentrations over the Himalayas and Tibetan Plateau have affected regional climate, accelerated snow/glacier melting, and influenced water supply and quality in Asia. Although sulfate is a predominant chemical component in aerosols and the hydrosphere, the contributions from different sources remain contentious. Here, we report multiple sulfur isotope composition of sedimentary sulfates from a remote freshwater alpine lake near Mount Everest to reconstruct a two-century record of the atmospheric sulfur cycle. The sulfur isotopic anomaly is utilized as a probe for sulfur source apportionment and chemical transformation history. The nineteenth-century record displays a distinct sulfur isotopic signature compared with the twentieth-century record when sulfate concentrations increased. Along with other elemental measurements, the isotopic proxy suggests that the increased trend of sulfate is mainly attributed to enhancements of dust-associated sulfate aerosols and climate-induced weathering/erosion, which overprinted sulfur isotopic anomalies originating from other sources (e.g., sulfates produced in the stratosphere by photolytic oxidation processes and/or emitted from combustion) as observed in most modern tropospheric aerosols. The changes in sulfur cycling reported in this study have implications for better quantification of radiative forcing and snow/glacier melting at this climatically sensitive region and potentially other temperate glacial hydrological systems. Additionally, the unique Δ³³S–δ³⁴S pattern in the nineteenth century, a period with extensive global biomass burning, is similar to the Paleoarchean (3.6–3.2 Ga) barite record, potentially providing a deeper insight into sulfur photochemical/thermal reactions and possible volcanic influences on the Earth’s earliest sulfur cycle.

The Himalayas and Tibetan Plateau (HTP), the largest and highest plateau on the Earth, is climatically unique and important due to its location, topography, and teleconnection with other parts of the world (1). It hosts the largest number of glaciers outside the polar regions and thousands of lakes, and is commonly referred to as the “Third Pole.” The ice and lake sediment cores from this midlatitude region provide valuable paleoclimatic and paleoatmospheric records that cannot be obtained from polar regions (2–5). The HTP is also known as the “Asian water tower” because snow and glacier melting in this region sustains water availability for major rivers in Asia and sustenance for >1.4 billion people (6). A persistent increase in aerosol loading over this region has been altering the atmospheric/glacial chemical composition, snow/glacier melting rate, and glacial river water quality (3, 6, 7). Sulfate is one of the major components of aerosols (especially in Asia), but the relative contributions of varying sources (e.g., combustion, mineral dust) and its mixing state in aerosols remain uncertain because measurements of source-specific tracers in sulfates are absent. This fragmentary understanding limits our ability to accurately quantify the aerosol budget and evaluate its influences on climate and hydrological systems.

The sulfur isotopic anomaly (or mass-independent fractionation [MIF]) is quantified by nonzero Δ³³S and Δ³⁶S values, where Δ³³S = δ³³S − 1,000 × [(1 + δ³⁴S/1,000)^0.515 − 1] and Δ³⁶S = δ³⁶S − 1,000 × [(1 + δ³⁴S/1,000)^1.9 − 1] (Materials and Methods). In the modern atmosphere, sulfate emitted from combustion and secondarily produced from photolytic oxidation of stratospheric SO₂ are the only two known types of isotopically anomalous sulfates (8–12). Other sulfates (e.g., terrigenous sulfate in mineral dust, secondary sulfate produced in the troposphere via SO₂ oxidation) are all isotopically normal (Δ³³S ∼ 0‰) (8–13). This unique isotopic fingerprinting has been utilized in reconstructing changes in sources and chemical formation pathways of sulfur in the past atmosphere using ice/snow samples obtained from polar regions (11, 14), but the difficulty of drilling ice cores at Himalayas in terms of the harsh environment at high altitudes (>6,500 m above sea level) (2, 5) hampers such studies in this region. Here we provide multiple sulfur isotopic analysis spanning over 200 y in a Himalayan lake sediment core (Materials and Methods) to gain insight into the change of sulfur source in this climatically sensitive and important region. As a part of the world’s highest freshwater lake system, this remote lake is significantly different from many saline lakes on the HTP (15). The low background sulfate concentration and low biological activity (i.e., minimal postdepositional processes) (16) renders this alpine lake an ideal site to reconstruct regional atmospheric sulfur cycle and potentially to record climate-induced changes in weathering/erosion within the glacial hydrological system.

Atmospheric Sulfur Isotopic Compositions in the Central HTP

We first report Δ³³S (and Δ³⁶S) values in sulfate aerosols [in total suspend particles (TSP)] collected at the central HTP (Materials and Methods and SI Appendix, Fig. S1) to gain an overview of the atmospheric sulfur isotopic anomalies in this arid region (Fig. 1). Because mineral dust or local soil accounts for ∼3/4 of TSP at our sampling site in today’s environment (17) and sulfate derived from this terrigenous source is isotopically normal (Δ³³S = 0‰), the observed Δ³³S values (ranging from 0.05‰ to 0.12‰) are less than most sulfate aerosols collected at midlatitudes (8–10). A first-order estimation based on isotopic mass balance (SI Appendix) yields a Δ³³S value of nondust sulfate to be 0.36 ± 0.12‰. This estimation is at the higher end of the Δ³³S range of currently measured tropospheric sulfate aerosols (maximum: 0.53‰) (8–10), likely because the HTP is frequently affected by the downward transport of stratospheric air (18, 19), which contains stratsopheric sulfates with positive Δ³³S values (8–10). Such a large anomaly cannot be explained by small variations of Δ³³S (±0.1‰) led by slight differences in mass-dependent fractionation (MDF) exponents (³³θ) of varying SO₂ oxidation pathways in the troposphere (13). The scattered Δ³³S–Δ³⁶S relationship observed in aerosols collected from Beijing and California (La Jolla, Bakersfield, and White Mountain) (Fig. 1) indicates that at least two types of isotopically anomalous sulfates (produced in the stratosphere by photolytic oxidation processes and emitted from combustion) may affect Δ³⁶S/Δ³³S slopes in differing ways. If this is the case, the relatively well-defined Δ³⁶S/Δ³³S slope (−1.80 ± 0.85, n = 6) observed in this study (Fig. 1), close to that of stratospheric sulfates estimated from polar records of stratospheric volcano eruptions (−1.93 ± 0.62, n = 13) (14), may imply that the central HTP is predominately affected by a single sulfur isotopic anomaly source (likely stratospheric sulfates) in the twenty-first century.

Fig. 1.

The Δ³³S and Δ³⁶S in sulfate aerosols collected at the HTP; δ³⁴S values (ranging from 3.0‰ to 6.7‰) are reported in SI Appendix. The Δ³³S and Δ³⁶S values of tropospheric sulfate aerosols collected at California (8) and Beijing (9), primary sulfate aerosols emitted from biomass and fossil fuel combustion experiments (12), elemental sulfur produced from OCS photolysis experiments (47) are also shown in this figure.

Two-Century Record of Atmospheric Sulfur Isotopic Anomalies

The sulfur isotopic analysis of post-1930 lake sediments (n = 8) display zero Δ³³S values within analytical error (±0.01‰) (Fig. 2A and Materials and Methods), likely a result of a larger contribution of mineral dust to TSP in the Himalayas than the central HTP as noted by previous aerosol and ice core studies (4, 20) and in part supported by near-zero Δ³³S values in glacial snow and river sulfates (0.03‰ and 0.02‰, respectively) collected at the Mt. Everest in this study (SI Appendix). The isotopically normal sulfates may also originate from the weathering process of parent rocks, which is supported by (i) the low enrichment factors of major, trace, and rare earth elements, and (ii) the observed stable lead isotopic composition (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb) that is nearly identical to soils and river sediments over the HTP (21, 22) (SI Appendix, Figs. S2 and S3).

Fig. 2.

Time series of (A) Δ³³S, (B) δ³⁴S, and (C) sulfate concentration and flux. Note that horizontal bars represent time intervals of combined sediment samples instead of dating errors. The ²¹⁰Pb-dated age was reported by Kang et al. (3). Vertical errors bars represent one SD uncertainties made on the basis of reproducibility of the laboratory standard with varying sample sizes (Methods and Materials). The red shaded area between −0.02‰ and 0.02‰ in A represents the mass-dependent field. Points distinguished from this field are interpreted to be sulfur isotopic anomalous (see Materials and Methods for details).

In contrast, most sulfate samples (six of eight) deposited in the pre-1930 period display small but distinguishable nonzero Δ³³S values (> ±0.02‰, at the 2σ level) (Fig. 2A), which may not be an analytical artifact because multiple laboratory controls throughout the study period show that the measured Δ³³S values are highly reproducible (SI Appendix and SI Appendix, Table S1). Even if the mass-dependent field is defined as ±0.05‰ (at the 5σ level), two samples in the nineteenth century (1818–1853 and 1885–1886) still possess nonzero Δ³³S values. In this study, we attempted to extract acid volatile sulfide [(AVS) reduced forms of sulfur] (SI Appendix), but none of our sediment samples contain AVS. The absence of AVS suggests that microbial sulfur reduction processes are negligible in this oligotrophic lake (15) due to insufficient supply of organic matter (16). Subsequently, the observed nonzero Δ³³S values, particularly in two samples in the nineteenth century (1818–1853 and 1885–1886), could not be explained by the slight differences in MDF exponents in various microbial sulfur reduction/disproportionation processes and mixing of various sulfur reservoirs, as those observed in geological samples of Phanerozoic age (23, 24). This interpretation is further supported by the small δ³⁴S variation in our sedimentary sulfates (Fig. 2B) and zero Δ³³S values (within ±0.01‰) in all post-1930 samples. We therefore suggest that the origin of the relatively large anomalies in the 1818–1853 and 1885–1886 samples, if not all statistically resolvable anomalies, is attributed to MIF signatures in atmospheric sulfates deposited in lake sediments.

The long-term variations in sulfate concentrations recorded in the Himalayan lake sediment (Fig. 2C) match the inventory of global anthropogenic sulfur emissions (5, 25). The most striking finding of this study is that nonzero Δ³³S values are not observed after the second industrial revolution, when sulfate concentrations started to rise (>0.5 μg mg⁻¹) (Fig. 2 A and C). This observation is in marked contrast to nonzero Δ³³S values in most existing aerosol measurements made in the twenty-first century (8–10). Simultaneous increasing trends for Hg, U, Mo, Sb, and Tl in the same sediment core and atmospheric CO₂ mixing ratios (Fig. 3) also indicate that the increased sulfates are likely anthropogenic, but the absence of sulfur isotopic anomaly provides strong observational evidence that sulfate emitted from combustion (or produced from photolytic oxidation of stratospheric SO₂) is a highly unlikely source because they possess nonzero Δ³³S values as observed at two sides of the Pacific Ocean (8–10), the South Pole (11), and controlled combustion experiments in a chamber (12). As discussed, the Himalayas are strongly affected by dust aerosols (4, 20), and the increased sulfate is therefore likely attributed to two types of dust-associated sulfate aerosols (Δ³³S ∼ 0‰): (i) secondary sulfate produced from transition metal ion catalysis or O₃ oxidation of anthropogenic emitted SO₂ on the surface of mineral dust (26, 27), and/or (ii) terrigenous sulfate in anthropogenic emission of mineral particles from South Asia (SI Appendix, Fig. S1) (28). Another important source of isotopically normal sulfate that diluted/overprinted the MIF signature is strong weathering/erosion in this Himalayan glacial hydrological system in the twenty-first century as revealed by various weathering indices and Hf in the same sediment core (Fig. 3 and SI Appendix, Fig. S4), which is linked to pronounced warming trends, enhanced precipitation, and glacier melting (29–31). Our interpretation is independently supported by results from the positive matrix factorization (PMF) model (a receptor model widely used for source apportionment; https://www.epa.gov/air-research/positive-matrix-factorization-model-environmental-data-analyses) based on major, trace, and rare earth element measurements in the same sediment core (SI Appendix): the relative background contribution decreased from 65 ± 20% in the pre-1930 period to 10 ± 11% in the post-1930 period; relatively contributions of anthropogenic influences and weathering/erosion (14 ± 9% and 20 ± 14%, respectively) in the pre-1930 period enhanced to 45 ± 14% and 45 ± 10%, respectively, in the post-1930 period (SI Appendix, Figs. S5 and S6).

Fig. 3.

Time series of (A) U and Hg, and (B) Mo, Sb, Tl, and Hf. The ²¹⁰Pb-estimated age and Hg concentrations were obtained from Kang et al. (3). These elements are selected out of 49 measured elements because their concentration ratios of the post-1930 to pre-1930 periods are greater than 2. (C) Time series of atmospheric CO₂ record (Scripps CO₂ Program: scrippsco2.ucsd.edu/) based on ice core data and direct observation from Mauna Loa and the South Pole.

Possible Origins of Negative Δ³³S Values

Three samples in the nineteenth century possess negative Δ³³S values (< −0.02‰) and the largest anomaly (Δ³³S = −0.10 ± 0.01‰) is found in sulfates deposited from ∼1818 to ∼1853 (Fig. 2A). The negative values are surprising in that they contrast with positive Δ³³S values in most sulfate aerosols (101 of 118) collected at midlatitudes (including the HTP) in the twenty-first century (Fig. 4). Large positive Δ³³S values found in sulfates produced from laboratory SO₂ photochemistry experiments (in the presence of O₂) (32) indicate that the observed positive Δ³³S values in most tropospheric sulfate aerosols are likely attributed to the frequent downward transport of stratospheric sulfates at midlatitudes, which have been noted by previous studies (8–10). However, negative Δ³³S values measured in this study and recently observed in urban Beijing during a highly polluted season with active industrial/residential coal combustion and minimal stratospheric influences (−0.21 ± 0.19‰; minimum: −0.66‰) (10) are not expected, indicating that an additional MIF process may be required.

Fig. 4.

Stable sulfur isotopic compositions in sulfates extracted from the Himalayan sediment core, modern aerosols, and Archean barites. Modern aerosol data (blue) is obtained from this study and refs. 8–10. The Archean barite data (red) is obtained from refs. 50–58. Note that the Δ³³S values for sediment samples are one order of magnitude smaller than aerosol and barite data because the signature of sulfur isotopic anomaly is greatly diluted by isotopically normal sulfates as discussed in the main text.

Combustion is a highly likely candidate (10, 11) because negative Δ³³S values (minimum: −0.23‰) have been observed in the primary sulfates emitted from combustion in controlled chamber experiments of biomass and diesel fuels (12) (Fig. 1). The fundamental chemical physics remain inadequately described, but recombination of elemental sulfur shown in a recent theoretical study (33) is the most likely mechanism based upon the isoelectronic reactions of sulfur species that mimic the well-known ozone formation reaction where the first chemically produced MIF anomalies were observed as a result of symmetry effects (34, 35). A number of radical driven reactions (e.g., SH + H → S + H₂, SH + SH → S + H₂S, S + SH → S₂ + H) in combustion plumes (especially biomass burning) (36, 37) potentially produce elemental sulfur allotropes (e.g., S and S₂) at these high molecule number densities. These reactions, based upon gas phase recombination reaction theory and symmetry effects (33), are likely mass independent. Laboratory experiments have shown that symmetry reactions in sulfur-bearing species produce sulfur MIF isotopic compositions (38). Further laboratory investigation and field-based measurements of both Δ³³S and Δ³⁶S along with combustion tracers in the future are crucial to examine the extent to which recombination reactions (33) in combustion processes affect the signature of sulfur isotopic anomalies in emitted sulfates on experimental and observational basis, respectively.

Global lake sedimentary charcoal records, including the one retrieved from the southeast HTP (SI Appendix, Fig. S7), have shown that the nineteenth century is a period with extensive biomass burning around the globe, especially in the northern hemisphere extratropics (39, 40). Although black carbon records in the central HTP showed that biomass burning activities after the late twentieth century may be stronger than the nineteenth century (41), impacts of large-scale fires in the nineteenth century (likely attribute to land-clearing processes as a result of rapid population growth) (40) on sulfur partitioning (in isotopes and species) are probably different from small-scale biomass burning after the late twentieth century (as a consequence of fire suppression management policy, decreased vegetation density, and forest fragmentation) (39, 40). In large and persistent biomass-burning plumes, high molecule number densities of S and S₂ occur, and therefore isotopically anomalous sulfates may be directly emitted from combustion plumes (12) via the mechanism discussed earlier, and responsible for the negative Δ³³S values observed in our sediment core. The biomass-burning end member of Δ³³S was not unambiguously identified in the early combustion experiments (12), and further investigation is required. Here we provide a first-order estimation. Given that anthropogenic sulfur emissions before 1850 are dominated by grassland and forest fires (25), we assume that the PMF-model–estimated anthropogenic influence in the 1818–1853 period (15 ± 5%) (SI Appendix, Fig. S6), when the most negative Δ³³S value (−0.10‰) is observed, is exclusively attributed to biomass burning. Based on isotopic mass balance (SI Appendix), a biomass-burning end member of Δ³³S is determined to be −0.67 ± 0.20‰. This first-order estimation independently agrees well with the most negative Δ³³S value (−0.66‰) observed in Beijing during heavy pollution events (10).

Another possible influence of biomass burning in the production of sulfur isotopic anomalies is the stratospheric photochemistry of carbonyl sulfide (OCS). With biomass burning as one of its major sources, OCS is the most abundant sulfur-bearing molecule in the terrestrial atmosphere and stratospheric photochemical loss is one of its major sinks as a consequence of its slow oxidation rates in the troposphere (42). OCS is photolyzed in the high stratosphere (>25 km above sea level), oxidized to SO₂, and serves as an embedded SO₂ stratospheric source at those high altitudes. Shortly after SO₂ production it is photolyzed and oxidized to sulfates, which acquire large sulfur MIF signatures as observed at the South Pole after prolonged wildfires due to extremely dry weather ensuing the 1997–1998 super El Niño–Southern Oscillation (ENSO) (11). Although Δ³³S values in the South Pole ENSO record are positive (11), we do not rule out the possibility that this pathway may provide a consistent source of isotopically anomalous sulfates and contribute in part to the observed negative Δ³³S values because the Himalayas are likely a global hotspot for stratospheric intrusions (18, 19) and Δ³³S values in the stratosphere are spatially and temporally heterogeneous (14, 43). It has been observed that volcanic plumes produced varying MIF signatures (in sign and magnitude) over time as the plume photochemically evolves isotopically during long-range transport of sulfur compounds (14). The detailed chemistry and transport mechanism between the stratosphere and troposphere is particularly complex for the HTP (44–47), especially in the vicinity of Mt. Everest, and requires further modeling in the future. To improve and constrain such models, field-based measurements of quadrupole stable sulfur isotopes and biomass-burning tracers at varying locations including emission sources and background receptors become important. We also note that laboratory experiments of OCS photolysis (48) show slightly negative Δ³³S values (minimum: −0.23‰) in the produced elemental sulfur (Fig. 1), which may be oxidized to sulfate in today’s oxygen-rich atmosphere. The negative Δ³³S values in the OCS photodissociation may be too small to contribute to our observed anomalies, but its role should not be neglected and further laboratory and field investigations are required.

Because deposition of isotopically anomalous sulfates of stratospheric volcanic origins occurs within several years after eruptions (11, 14), stratospheric volcanic events (e.g., Tambora in 1815, Cosigüina in 1835) only play a minor role, if any, in contributing to the most negative Δ³³S value observed from ∼1818 to ∼1853.

Comparison with Archean Barite Records and Possible Biogeochemical Implications

The sulfur isotopic anomaly in Archean (∼4 to ∼2.5 Ga) sediments is strong evidence of an anoxic atmosphere and a proxy for understanding the evolution of atmospheric oxygen and early life on primitive Earth (49), but corresponding biogeochemical processes are not fully understood (11). An interesting facet of our data is that the unique Δ³³S–δ³⁴S pattern in the pre-1930 period is similar to published Archean barite (BaSO₄) data (Fig. 4), although the magnitude of Δ³³S is smaller because present-day sediments contain a large amount of isotopically normal sulfates. Archean barites are only observed in a relatively short period (3.5–3.2 Ga) and are characterized by a narrow range of positive δ³⁴S values (from ∼3‰ to ∼11‰) and negative Δ³³S values of −0.75 ± 0.39‰ (n = 224) (50–58), notably different from the Archean pyrite (FeS₂) and sulfide (S²⁻) records (−40‰ ≤ δ³⁴S ≤ 25‰; −4‰ ≤ Δ³³S ≤ 15‰) (32). It has been a mystery why the oxidized form of sulfur can exsit in the anoxic atmosphere and its unique isotopic composition may represent a combined result of atmospheric, oceanic, and microbial processes (56). It is widely accepted that the Paleoarchean (3.6–3.2 Ga) was strongly affected by active volcanism (59) and negative Δ³³S values in barite deposits is likely an isotopic fingerprinting that sulfate is produced from photolytic reactions of SO₂ allowed by an anoxic atmosphere at that time (50–58). However, the deviation of δ³⁴S from the expected photochemical array is debated and explained by various concept models such as microbial sulfate reduction (54), different atmospheric composition (i.e., wavelength-dependent MIF effects) (56), and/or mixing of varying sulfur reservoirs (52).

Our observation of Paleoarchean-barite-like Δ³³S–δ³⁴S pattern in an oxygen-rich atmosphere (Fig. 4) implies that basic reactions responsible for sulfur MIF in combustion as discussed previously may also occur in the Paleoarchean. Although biomass burning is not possible, it is plausible, and cannot be ruled out, that recombination reactions of elemental sulfur (33) may be significant in the active Paleoarchean volcanism because formation of elemental sulfur related to volcanism are commonly observed on both Earth (60) and extraterrestrial bodies such as Io (61). Therefore, the negative ³³S anomalies in Paleoarchean barites produced in the volcano plume could in fact come from a strictly thermal reaction instead of photolytic reactions, which may yield different Δ³³S–δ³⁴S patterns. In addition, OCS is a potentially important chemical component in the Archean atmosphere, for not only its role in photochemically produced sulfur isotopic anomalies (11, 56, 62), but also its greenhouse effect in solving the faint young sun paradox (62). Our long-term sulfur isotopic measurements in a high-altitude nonpolar region across the Anthropocene further support recent interpretations that OCS may be another possible sulfur-bearing molecule for producing isotopically anomalous sulfates in the present-day and possibly Archean atmospheres (11, 56).

Conclusions

In summary, our measurements of multiple stable sulfur and lead isotopes, major, trace, and rare earth elements in a two-century Himalayan sediment record suggest that the observed changes in sulfur cycling at the second industrial revolution reflects more dust-associated sulfates and climate-induced weathering/erosion affecting the Himalayas in the last century than the nineteenth century.

Dust-associated sulfates have acquired significant attention because of their ability to alter physical/chemical properties of other mineral components in the dust and relevant impacts on climate and ecosystems (26, 63). The Himalayan sulfur isotopic record provides an independent marker of anthropogenic-induced changes in the atmospheric composition and may help to improve our understanding of how historical and future changes in the coupling of sulfur and dust emissions affect radiative forcing, glacier surface albedo, and snow/glacier melting over the HTP (64–66). The discovery of enhancing weathering/erosion in this alpine glacial hydrological system in the past >100 y supports a recent 20-y investigation of water chemistry over the Himalayas (30), provides context for quantifying and projecting how such fragile ecosystems response to climate change, and may also have potentially important implications for other temperate glacial hydrological systems.

Additionally, our study highlights the need for better source apportionment of combustion-associated sulfur isotopic anomalies (such as fuels, OCS photolysis, or potentially underappreciated reactions during combustion) and quantifying their influences on Δ³³S and Δ³⁶S signatures in the present-day atmosphere. Given that chemical reactions responsible for sulfur MIF might have occurred throughout Earth’s history (56), a complete understanding of sulfur MIF in the present-day atmosphere may be an important ingredient for further defining the relative roles of the dynamics, atmospheric chemistry, and microbial metabolisms on the formation and preservation of barites in the Paleoarchean.

Materials and Methods

TSP and glacier snow samples were collected at Nam Co Lake and Mount Everest, respectively (SI Appendix, Fig. S1). The sediment core was drilled at the Gokyo lake system (the world’s highest oligotrophic freshwater lake system, ∼4,800 m above sea level, ∼15 km from the peak of Mount Everest) and dated by Kang et al. (3). The isotope ratios of quadruple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) defined as δ³xS=[(³xS/³²S)_sample/(³xS/³²S)_VCDT − 1]*1,000 (unit: ‰), where x = 3, 4, and 6 and VCDT stands for the Vienna Canyon Diablo Troilite reference material, were determined in the Stable Isotope Laboratory at University of California, San Diego, using the traditional BrF₅ fluorination method (8, 11, 12, 50) and an isotope ratio mass spectrometry (Thermo Finnigan MAT 253). A laboratory Ag₂S standard of approximately the sample sizes comparable to environmental samples (1–6 μmol) was subjected to the same analytical procedure throughout the study period to determine overall uncertainties of measurements (associated with extraction, fluorination, purification and mass-spectrometry measurements) (SI Appendix, Table S1). For large sizes of samples (>2.5 μmol), the errors (one SD) for δ³⁴S, Δ³³S, and Δ³⁶S values were less than 0.4‰, 0.01‰, and 0.1‰, respectively. The Δ³⁶S values in Mt. Everest samples are not reported in this study because of relatively large uncertainties associated with small sample sizes. The slightly greater SDs of δ³⁴S and Δ³³S (1.0‰ and 0.02‰, respectively) for small sizes of samples (<2.5 μmol) would not affect our interpretation and conclusion. Elemental and stable lead isotopic analysis was carried out in the Trace Element Laboratory at Academia Sinica using the microwave digestion method and an inductively coupled plasma mass spectrometry (Perkin-Elmer Elan 6100). Detailed information of sampling sites, sampling, chemical processing, quality assurance/control procedures, and PMF modeling approach can be found in SI Appendix. All stable sulfur isotope data reported in this study is available in SI Appendix, Tables S2 and S3.