The isotope composition of sulfate (SO42-) is an important geochemical tracer used to study elemental cycling globally. In polar desert environments such as Antarctica, sulfate in ice, snow, and surficial deposits has primarily been considered to derive from atmospheric deposition. However, recent studies suggest inputs from multiple sources may be important and possibly controlled by weathering, changing climate and seawater interactions in coastal areas. To better constrain the sources of SO42- in the McMurdo Dry Valleys (MDV), the largest of the ice-free regions in Antarctica, we investigated the delta S-34 and delta O-18 of SO42- in various depositional environments (lakes, ponds and valley bottoms) including archived Dry Valley Drilling Project sediment cores and modern surface materials. We found that, in general, the delta S-34 and delta(18)O of SO42- in the studied sediments, bedrock, and water samples exhibited wide ranges in values, consistent with multiple SO42- sources and post-depositional alterations in thaw zones of valley bottoms. For instance, the delta S-34 and delta O-18 of SO42- in Antarctic snow/ice and atmospheric aerosols generally had lower (+2 to +17 parts per thousand) and higher (-2 to +12 parts per thousand) values, respectively, compared to the higher delta S-34 (+3 to +71 parts per thousand) and lower delta O-18 (-25 to +1 parts per thousand) values in the MDV surficial sediments, bedrock, and lake deposits. This implies less SO42- contributions from atmospheric deposition that may also include local inputs from oxidation of H2S emitted from Antarctic lacustrine settings. Conversely, the distinctive delta S-34 ranging from +10 to +17 parts per thousand suggested varied inputs of SO42- from seawater (+21 parts per thousand) and weathering (oxidation) of bedrock- and lake-derived sulfide with low and high delta S-34 values (-2 to +3 parts per thousand and +5 to +14 parts per thousand, respectively). The highest SO42- concentrations (up to 3.5 wt% S) were measured in Lake Vanda sediments and were consistent with the presence of marine fossils and expected seawater-derived SO42- (median delta S-34 +22 parts per thousand) from past marine transgressions. Additionally, in many of the studied MDV lacustrine settings the delta S-34 and delta O-18 of SO42- showed significant increases (up to +71 and +7 parts per thousand, respectively) accompanied by distinctive high delta S-34 of sulfide/H2S (0 to +18 parts per thousand), suggesting microbial sulfate reduction in closed (ice-covered) lake systems. Furthermore, the negative delta O-18 values of SO42- (-20 to 0 parts per thousand) were considerably lower compared to seawater-SO42- (+9 parts per thousand) suggesting microbially- and/or abiotically driven oxygen isotope exchange between SO42-/SO32- and water, likely occurring on short and longer time scales. Overall, our results imply that without significant seawater inputs through marine transgressions to the MDV, the total SO42- deposition would be relatively small in lakes and valley bottoms due to low bedrock sulfide concentrations (<0.01 to 0.5 wt% S) and minor atmospheric deposition. Coupled isotope analyses and comparisons of the MDV materials used in this and previous studies enabled greater insight into assessment of variable atmospheric and geological sulfate sources and investigation of further isotope fingerprinting by microbial processes and marine inputs.

Carbon (C) and nitrogen (N) isotopes have been widely used as biosignatures to study the origin of organic materials. However, bulk S13C and S15N analyses have been underutilized in hydrothermal settings, making relevant comparisons to early Earth environments and other planetary materials difficult. Therefore, in this study we examined if bulk isotope analysis can be successfully used to detect a microbial signature in hydrothermal environments similar to where life might have begun on Earth. We analyzed concentrations and isotope compositions of C and N within acidic hydrothermal sediments in Iceland and the United States. The measured bulk S13C of the hot spring and mudpot sediments was higher in barren Icelandic sites (-25.6 to-14.5 %o) compared to lower values in more forested sites of the United States (-26.5 to-21.0 %o). These corresponded to lower C concentration in Iceland (0.13 to 0.55 wt%) and higher C concentrations in the United States (0.04 to 4.78 wt%). The distinctive negative ranges of bulk S13C were indicative of life processes occurring in-situ (microbial activity) and in the surrounding area (production of plant biomass). The higher S13C of the Icelandic sediments were consistent with smaller C isotope fractionations associated with thermophile (microbial) activity, wherein more negative bulk S13C values corresponded to increasing input of allochthonous plant matter in the United States. Conversely, the bulk S15N results (-18.4 to +3.7 %o) were less useful in differentiating biosignatures because of overlapping S15N values between various N sources such as allochthonous plant matter, volcanic/hydrothermal gases, and microbial processes.

Secondary sulfate minerals are abundant in Gale crater on Mars and in some cases exhibit distinctive morphological features on the exhumed surfaces of sedimentary formations. These are bright-toned sulfate-rich halos and salt accumulations on the eroded bedding planes, but their origin and aqueous processes are still not well understood. To address this knowledge gap, we investigated similar features of sulfate-rich deposits in the Rio Puerco watershed, New Mexico to better understand their formation processes. During five individual months between 2012 and 2024, the studied halo-like features, bedding planes, and pseudo-layers were enriched in various Ca-Mg-Na sulfate minerals such as gypsum, thenardite, bloedite and hexahydrite. These sulfate-rich accumulations were common at the outcrop scale on the exhumed surfaces of sedimentary rocks but in some years, they were also visible on aerial images. Comparisons between the individual wet and dry months suggest that their formation is directly controlled by episodic (ephemeral) water activity related to the semi-arid climate. Episodic flows and infiltration of meteoric water and repeated cycles of redissolution of the already existing sulfate-rich secondary minerals on and near the surface lead to their reprecipitation and distinctive accumulations in halo-like features, bedding planes, and pseudo-layers. We hypothesize that sulfate-rich halos and bedding planes observed in Gale crater on Mars might have been related to the late-stage fluid flows on or within the exhumed surfaces induced by past meteoric water activity under dry conditions.

Secondary sulfate minerals are common throughout the sedimentary deposits of Mount Sharp, located within Gale crater on Mars. However, the source of sulfate (SO42- ) and past climatic conditions during their formation are not well understood. Therefore, we investigated the S34S, S18O, and S2H of gypsum veins and other Mg- and Ca- sulfates forming as salt crusts and cement within the shallow sediments of the Rio Puerco watershed in central New Mexico. The S34S values of vein gypsum and acid-soluble SO42- (cement) varied over the same range (-33.3 to -12.9 %o and -34.6 to -12.1 %o, respectively), which was similar to the S34S of bedrock sulfide minerals (-37.4 to -5.9 %o). This implies that sulfide oxidation is the main source of SO42- in the Rio Puerco aqueous system. The measured S18O values of SO42- (-8.9 to +3.1 %o) as well as S18O and S2H values of gypsum hydration water (-8.9 to +0.6 %o, and -112 to -82 %o, respectively) overlapped with the isotope composition of local meteoric precipitation, suggesting that sulfide oxidation to SO42- and gypsum formation have occurred under semi-arid climate conditions. The isotope results suggest the top-down infiltration of meteoric water leads to leaching of SO42- , Mg+, and Ca2+ from bedrock sulfide weathering followed by abundant formation of Mg- and Ca-sulfates in surface deposits and gypsum veins with depth. Because of spatial and mineralogical similarities in the secondary Mg- and Ca-sulfate mineral occurrences, we hypothesize that chemical weathering of sulfide minerals could have been the main source of SO42- in the aqueous system of Gale crater.

The Mount Meager Volcanic Complex (Q̓welq̓welústen) is an active glacier-capped volcanic massif in the Garibaldi Volcanic Belt (British Columbia) and the only known glaciovolcanic cave system in North America steadily releasing sulfur-rich gases. In September 2022, leveraging specialized cave explorer expertise, the fumarole-carved ice cave at the Job Glacier on Mt. Meager was surveyed. Direct measurements of fumarolic gas concentrations were taken at the source, with H2S >200 ppm, SO2 >100 ppm, CO2 ∼5,200 ppm, and CO ∼230 ppm. Snowpack and fumarole-associated sediments were characterized for microbial diversity, functional potential, and biogeochemistry including measurements of nutrients, major ions, dissolved organic and inorganic carbon concentrations as well as the stable isotope compositions of carbon, sulfur, hydrogen and oxygen. Green algae (Chlorophyta) dominated the snowpack, consistent with other Pacific Northwest glaciers. Representatives of Firmicutes were the most abundant bacterial sequences detected in our samples, contrasting with other glacier and snowpack samples which harbor abundant Sphingobacteria, Betaproteobacteria, and Alphaproteobacteria. Sediments and water collected inside the cave were mostly high in SO42- (5.3–185.2 mg/L) and acidic (pH = 3.6–6.0), while most other major anions and cations were below detection of the method used. Snow at the cave entrance had more SO42- (0.08 mg/L) and lower pH (5.9) than snow collected at a distance (SO42- undetectable, pH 7.6), suggesting influence by fumarole exhalations. Negative δ13C values of organic matter (−29.0‰ to −26.1‰, respectively) in sediments suggest in-situ microbial carbon transformations, findings that are supported by the presence of genes encoding complete heterotrophic and autotrophic carbon transformation pathways. The δ34S value of H2S was ∼0‰, suggesting a deep magmatic origin; however, both sulfur-oxidizing and sulfate-reducing microbial phyla were present in the sediment samples as were genes encoding both dissimilatory sulfur-oxidizing and sulfate-reducing pathways. Metagenomic data suggest diverse chemosynthetic lifestyles in the cave microbial community. This study provides insight on the microbiomes associated with a sulfidic glaciovolcanic system and identifies unique analog features for icy celestial bodies like Saturn’s moon Enceladus, where cryovolcanic activity may carry biomarkers from the subsurface and deposit them on surface ice.

The cycling of sulfur (S) to the upper crust and surface via thermal springs at convergent margins has not been explored outside areas with active arc volcanism, even though subduction plays a key role in the Earth's longterm S cycle. To address this knowledge gap, we analyzed stable sulfur and oxygen isotope compositions ((SS)-S-34 and (SO)-O-18 values) of dissolved sulfate (SO42- ) in 55 thermal springs from five distinct settings in the Andean orogen. These regions are the Peruvian flat slab and backarc, transition between these two, Argentinian backarc, and Chilean forearc. Although the flat-slab settings had lower SO42- concentrations (<2000 mg/L) compared to the steep-slab settings (<12,700 mg/L), there was no significant relationship between isotope composition of SO42- and slab geometry. The (SS)-S-34 and (SO)-O-18 values of SO42- varied widely across the studied areas (+0.2 to +23.5 %o and -3.3 to +16.0 %o, respectively) and reflected the isotope compositions of local bedrock endmembers from dissolution of marine evaporites (+5 to +25 %o and + 10 to +20 %o, respectively) and oxidation of magmatic and/or hydrothermal S and ore sulfide minerals with variable S34S (0 to +16 %o). The (SO)-O-18 and (SH)-H-2 values of thermal spring water (-18.5 to -3.3 %o and - 141.1 to -23.7 %o, respectively) were consistent with meteoric precipitation, and in most cases decreased with increasing altitude following precipitation in the Andes. Generally, our isotope results do not support the direct transfer of slab-derived S/SO42- to thermal springs in the investigated settings. Rather, the (SS)-S-34 and (SO)-O-18 of SO42- in the thermal springs are a sensitive indicator of local water-rock interactions that remobilize bedrock S originating from a complex orogenic cycle reflecting tectonic uplift, erosion, weathering, and exhumation history across the duration of Andean Mountain building.

Abstract Understanding past and present aqueous activity on Mars is critical to constraining martian aqueous geochemistry and habitability, and to searching for life on Mars. Assemblages of minerals observed at or near the martian surface include phyllosilicates, sulfates, iron oxides/hydroxides, and chlorides, all of which are indicative of a complex history of aqueous activity and alteration in the martian past. Furthermore, features observed on parts of the martian surface suggest present-day activity of subsurface brines and at least transient liquid water. Terrestrial analogs for younger and colder (Hesperian–Amazonian) martian geologic and climatic conditions are available in the McMurdo Dry Valleys (MDV) of Antarctica and provide opportunities for improved understanding of more recent aqueous activity on Mars. Here, we study the VXE-6 intermittent brine pond site from Wright Valley in the MDV region and use coordinated spectroscopy, X-ray diffraction, and elemental analyses to characterize the mineralogy and chemistry of surface sediments that have evolved in response to aqueous activity at this site. We find that brine pond activity results in mineral assemblages akin to aqueous alteration products associated with younger sites on Mars. In particular, surficial chlorides, a transition layer of poorly crystalline aluminosilicates and iron oxides/hydroxides, and a deeper gypsum-rich interval within the upper 10 cm of sediment are closely related at this Antarctic brine pond site. Activity of the Antarctic brine pond and associated mineral formation presents a process analog for chemical alteration on the martian surface during episodes of transient liquid water activity during the late Hesperian and/or more recently. Our results provide a relevant example of how aqueous activity in a cold and dry Mars-like climate may explain the co-occurrence of chlorides, clays, iron oxides/hydroxides, and sulfates observed on Mars.

At mercury (Hg)-contaminated sites, streambank erosion can act as a main mobilizer of Hg into nearby waterbodies. Once deposited into the waters, mercury from these soils can be transformed to MeHg by microorganisms. It is therefore important to understand the solid-phase speciation of Hg in streambanks as differences in Hg speciation will have implications for Hg transport and bioavailability. In this study, we characterized Hg solid phases in Hg-contaminated soils (100-1100 mg per kg Hg) collected from the incised bank of the East Fork Poplar Creek (EFPC) in Oak Ridge, TN (USA). The analysis of the soil samples by scanning electron microscopy-energy dispersive spectroscopy indicated numerous microenvironments where Hg and sulfur (S) are co-located. According to bulk soil analyses by extended X-ray absorption fine structure spectroscopy (EXAFS), the near-neighbor Hg molecular coordination in the soils closely resembled freshly precipitated Hg sulfide (metacinnabar, HgS); however, EXAFS fits indicated the Hg in the HgS structure was undercoordinated with respect to crystalline metacinnabar. This undercoordination of Hg-S observed by spectroscopy is consistent with transmission electron microspy images showing the presence of nanocrystallites with structural defects (twinning, stacking faults, dislocations) in individual HgS-bearing particles. Although the soils were collected from exposed parts of the stream bank (i.e., open to the atmosphere), the presence of reduced forms of S and sulfate-reducing microbes suggests that biogenic sulfides promote the formation of HgS nanoparticles in these soils. Altogether, these data demonstrate the predominance of nanoparticulate HgS with crystal lattice defects in the bank soils of an industrially impacted stream. Efforts to predict the mobilization and bioavailability of Hg associated with nano-HgS forms should consider the impact of nanocrystalline lattice defects on particle surface reactivity, including Hg dissolution rates and bioavailability on Hg fate and transformations.

Sulfate (SO42- ) has been found in elevated concentrations in modern hydrothermal settings on Earth and linked to coupled ferric iron (Fe3+)- and molecular oxygen (O2)-driven oxidations of hydrothermal H2S and secondary sulfur (S) deposits enriched in elemental S and sulfide minerals. However, the relative proportions of SO42- from these two oxidation mechanisms in surface acidic hydrothermal systems are unclear. To address this uncertainty, we determined the aqueous SO4 2-contributions via Fe3+- and O2-driven oxidation pathways in acidic hot springs and mudpots of Iceland (N & PRIME;amafjall, KrATIN SMALL LETTER Y WITH ACUTEsuvik) and the United States (Valles Caldera, Lassen, Yellowstone). Approximately 40 water and 11 fumarole sediment samples were collected for oxygen isotope analyses (& delta;18O). The oxidation processes appear to be accompanied by large variation of the & delta;18O of SO42- (-8.8 to +5.5 %o) similar to the & delta;18O of hot spring and mudpot water (-15.5 to +6.3 %o), which is greatly controlled by inflow of meteoric water (e.g., runoff, groundwater) and surface evaporation. In the studied sites, the Fe3+-driven oxidation of hydrothermal elemental S and sulfide minerals appears to be an important source of SO4 2-, contributing -55 to 100% of SO42- when compared to contributions from O2-driven oxidation. Additionally, the distinctive high & delta;18O values of SO42- (up to +13.7 %o) accompanied direct oxidation of H2S and/or hydrothermal S minerals by atmospheric O2 in dry (fumarolic) environments. This, in turn, is in agreement with previously determined O isotope fractionations (& epsilon;sulfate-atm.oxygen - 9.8 %o) for low-temperature sulfide oxidation in the presence of O2. Our new quantitative measures of oxidation pathways signify the importance of Fe3+-driven oxidation of hydrothermal S to SO42- in the surface volcanic settings, which have significant implications for understanding widespread occurrences of SO42- found on the Fe-rich volcanic terrains of Mars.

The Meridiani Planum region on Mars has extensive sulfate-rich sedimentary deposits (similar to 20 wt% SO42-) that are hypothesized to have formed from regional groundwater upwelling that led to the precipitation of secondary Fe-, Mg-, Ca-sulfate minerals and cementation of basaltic sediments. However, the primary source of sulfur (S) for these abundant secondary sulfate minerals is unclear. Therefore, in this study the contributions of volcanic S via surface water and groundwater were investigated in the terrestrial basaltic analogs of Hawaii and Iceland to determine the importance of active volcanism and climate on S cycling as well as the resulting timescale of aqueous activity in the Meridiani Planum region. SO42- fluxes (contributions) were calculated in metric tons/yr using historical data from online repositories and normalized to the catchment area to determine the SO42- load in metric tons/yr/km(2). Our results show that the SO42 load is greatly affected by climate, typically ranging from similar to 7.3 to 170 metric tons/yr/km(2) under wetter conditions and similar to 2.6 to 43 metric tons/yr/km(2) under dry conditions. Active S degassing and accompanying S-rich mineralization from current hydrothermal activity greatly increased the SO42- loads (similar to 2.8 to 170 metric tons/yr/km(2)) compared to non-active catchments (2.6 to 13 metric tons/yr/km(2)). Younger basaltic bedrock with greater permeability and groundwater-rock interactions was also found to be important, resulting in higher SO42- loads (similar to 26 to 170 metric tons/yr/km(2)) compared to older, less permeable catchments (similar to 2.6 to 12 metric tons/yr/km(2)). Based on these terrestrial SO42- loads in Hawaii and Iceland, we calculated a range of possible loads and timescales of SO42 transport in Meridiani Planum under variable environmental conditions. Results show that the smallest SO42- loads and longest timescales would occur in Meridiani under dry, non-volcanically active conditions, typically requiring similar to 16 to 65 Ma of an active aqueous system, as in the older catchments of Hawaii and Iceland. Conversely, the largest SO42- fluxes and shortest timescales would occur under wet, volcanically active conditions, requiring similar to 1.0 to 6.9 Ma, as in the younger catchments of Hawaii and Iceland. Our results suggest that moderately wet conditions with some active hydrothermal S input would be needed to transport and deposit the equivalent mass of SO42- currently present in the sulfate-rich deposits of Meridiani Planum.

Acidic hydrothermal and fumarolic surface deposits within the Columbia Hills in Gusev crater on Mars were found to have elevated concentrations of Fe-Mg-Ca-sulfate minerals. However, this is inconsistent with analogous terrestrial hydrothermal settings that are usually enriched in elemental sulfur (S). Consequently, this raises questions about the origin and hydrothermal history of the Gusev sediments. To address this discrepancy, we analyzed quantities and S isotope compositions of S-bearing minerals in hydrothermal sediment samples from acidic hot springs, mud pots, and fumaroles with elevated H2S emissions in Iceland and the United States (e.g., Valles Caldera, Lassen, and Yellowstone). Our results indicate that the typical concentrations (e.g., inter-quartile range) of elemental S and sulfide minerals (0.3 to 10.5 wt% S, but as high as -75 wt% S; and 0.1 to 1.7 wt% S, but as high as -10 wt% S) are significantly higher compared to sulfate (0.1 to 1.1 wt% S, but as high as -4.5 wt % S) in the surface hydrothermal deposits. In most cases, the concentrations of elemental S, sulfides, and sulfates in the sediments decreased with increasing hydrological connectivity and in wetter climates. Similar delta S-34 values between sulfate (-0.1 to +1.4 parts per thousand) and elemental S (-0.4 to +1.6 parts per thousand) compared to lower delta S-34 of sulfide (-2.4 to +0.4 parts per thousand) suggest that more sulfate is likely derived from the subsequent oxidation of elemental S than sulfide. Conversely, minor amounts of sulfate are formed via direct oxidation of H2S which had higher delta S-34 values (+1.1 to +5.9 parts per thousand). Our laboratory experiments carried over a wide range of temperatures (25, 65, and 85 ?) and low pH (-2) indicate that elemental S and pyrite undergo subsequent oxidation to sulfate via both ferric iron (Fe3+) and O-2. While the amount of sulfate increased with increasing temperature in the presence of both Fe3+ and O-2, Fe3+ appears to be a more efficient oxidizer than O-2. For example, pyrite oxidation by only Fe3+ resulted in-1.5x more sulfate (-80 to 180 mg/L SO42-) than by only O-2 (-40 to 140 mg/L SO42-). In contrast, considerably less sulfate was formed during the oxidation of elemental S, although in the presence of O-2 -10x more sulfate (-0.1 to 45 mg/L SO42-) was formed than when Fe3+ was present (-0.3 to 7.5 mg/L SO42-). Despite the prevalence of sulfate minerals rather than elemental S and sulfides in the hydrothermal Gusev deposits on Mars, the total S concentrations measured by the Spirit rover (2.9 to 9.3 wt% S) are highly comparable to the total S in hydrothermal sediments formed in colder and moderately wet climates such as coastal Iceland (1.8 to 10.7 wt% S). This contrasts with sediments formed in the high-altitude and drier climate of Valles Caldera (9.9 to 37.6 wt% S), or the wetter climates of Yellowstone (4.1 to 17.3 wt% S) and Lassen (0.5 to 3.5 wt % S). Because water is needed to further oxidize the hydrothermal elemental S and sulfide to sulfate, we infer that the aqueous conditions must have persisted in Gusev crater for a period of time after the main hydrothermal activity ceased. Later, under low water-to-rock conditions with little (or no) H2S emission, complete oxidation of the Gusev hydrothermal deposits likely took place and led to the formation of the sulfate minerals that were identified by the Spirit rover.

Sulfate (SO42-) has been found in elevated concentrations on the surface of Mars and linked to oxidation of volcanically derived sulfur (S) in the past, with the best examples of hydrothermal S-rich deposits in Gusev crater. However, the oxidation mechanisms of volcanic S are unclear due to the lack of abundant molecular oxygen (O-2) in the Martian atmosphere. To address this uncertainty, we investigated the mechanisms of hydrogen sulfide (H2S) oxidation to SO42- in O-2-depleted acidic hydrothermal systems of Iceland and the United States (Valles Caldera, Lassen, and Yellowstone) as geochemical analogs. Approximately 50 water and sediment samples were collected for chemical and stable isotope analyses (delta S-34, delta O-18, delta H-2). At the time of sampling, the measured dissolved O-2 (DO) concentrations were low (0.01 to 1.03 mg/L) because of elevated temperatures (60 to 90 degrees C) and did not account for high concentrations of SO42- (up to 24,461 mg/L) in the acidic hot springs and mud pots. However, high concentrations of leachable iron (Fe) were present in the hydrothermal water and sediments (average of 158 mg/L and 71,302 mg/kg, respectively), implying that in addition to atmospheric O-2, the Fe-driven oxidation of H2S via ferric Fe reduction to ferrous Fe is likely involved. Our laboratory experiments studying H2S oxidation carried out in a wide temperature range (25 degrees C, 50 degrees C, 90 degrees C) and low pH of 2 suggest that under acidic hydrothermal conditions, ferric Fe reduction involves stepwise oxidation of H2S with preferential formation of insoluble elemental S, which is later oxidized to SO42-. In contrast, the Gusev hydrothermal sediments on Mars appear to be enriched in Fe3+-sulfates with minor amounts of Ca-Mg-sulfates and negligible elemental S. This implies that the Gusev sediments must have experienced subsequent oxidation, most likely after the main episode of hydrothermal activity.

Determining aqueous sulfate sources in terrestrial cold environments can provide an insight into the surface hydrological conditions and sulfur cycle on Mars. In this study, we analyzed sulfur and oxygen isotope compositions of secondary sulfate salts (e.g., gypsum, thenardite) in the surficial sediments and soils of the McMurdo Dry Valleys (MDV), Antarctica to determine contributions of sulfate from bedrock chemical weathering and atmospheric deposition under persistent dry polar conditions. The sulfate showed wider variation of δ34S (+15.8‰ to +32.5‰) compared to smaller ranges of δ18O (−8.9‰ to −4.1‰). In contrast, the δ34S of bedrock sulfide showed significantly lower and consistent values across the studied area (−0.6‰ to +3.3‰). Based on the δ34S trends, sulfide weathering may contribute up to 20–50% of secondary sulfate salts in the MDV. While the remaining 50–80% of sulfate inputs may originate from atmospheric deposition (e.g., sea aerosols, dimethulsulfide oxidation), the subglacial brines derived by relicts of seawater and/or lake/pond water influenced by microbial sulfate reduction could also be important sulfate endmembers particularly in the Antarctic lowland thaw zones. Additional field observations of frost, ponding water, and thin gypsum crusts on the terrestrial gypsum dunes at White Sands supports reactivity of gypsum on the surface of these dunes during cold winter conditions. Combined with our improved geochemical model of the sulfur cycle for cold Antarctic settings, we propose that transient liquid water or frost was available in near-surface environments at the time of gypsum formation in the north polar region on Mars. Ice and/or water interaction with basaltic sand of the basal unit (paleo-erg) would have enhanced leaching of sulfate from both sulfide oxidation and atmospheric deposition and resulted in formation of secondary gypsum salts.

Introduction: Determining aqueous sulfate sources and quantifying their contributions from chemical weathering and atmospheric deposition in terrestrial polar environments is important for understanding surface conditions and the sulfur cycle on Mars. A variety of sulfate minerals have been identified by rovers and orbital spectrometers in numerous locations on Mars [e.g., 1]. However, sources of sulfates in martian regolith and soils are poorly constrained. Most geochemical models are focused on coupled volcanicallyand atmospherically-driven sulfur cycles, but cannot be solely explained by inputs from volcanic emission and atmospheric deposition [2]. Conversely, less emphasis has been placed on water interaction with primary sulfur-bearing minerals in the bedrock. So far, only a few studies have attempted to address whether chemical weathering (sulfide oxidation) could account for some of the sulfates detected on the martian surface [3-5]. One of the difficulties in addressing sulfate sources for Mars is limited research and no clear consensus on the sulfur cycle in terrestrial polar environments that are most analogous to Mars. While sulfide weathering is proposed to be the main sulfate contributor in Arctic glacial/subglacial systems [6], an atmospheric origin has been favored for sulfate accumulations under Antarctic dry polar conditions [7]. The main objective of this study is to critically evaluate chemical weathering and atmospheric processes that contribute to the formation of secondary sulfates in surficial deposits from different settings of the McMurdo Dry Valleys (MDV), Antarctica. The research goal is to quantify inputs of sulfates from chemical weathering and atmospheric deposition in MDV using stable isotope compositions. Study Site: Although the MDV climate is cold and dry, and in many ways analogous to martian settings, liquid water occurs in lakes, ponds and streams with highly saline water during austral summers. Glaciation of Antarctica started ~65 Ma as a result of continental drift that moved Antarctica toward the South Pole. In the MDV, major glacial activity likely ceased by ~3-4 Ma in the valley bottoms, but there is also evidence that persistent cold-desert, ice-free and hyper-arid environmental conditions have existed in this region for up to ~8-11 Myr in the upland valleys. Major rock formations outcropping on the surface are Paleozoic and Mesozoic sedimentary formations and basaltic lavas and dolerite sills. They are underlain by Precambrian basement rocks comprised of magmatic rocks and metamorphosed sedimentary and volcanic rocks. Disseminated grains of pyrite are associated with several of these rock formations [8]. Methods: We performed sulfur sequential extraction and sulfur and oxygen isotope composition analyses on 8 sediment samples collected in 1980 [9] along the South Fork of Wright Valley (Fig. 1), including the main drainage area comprised of Don Juan Pond and VXE-6 Pond [10]. The sediments are from shallow depths (<1 to 20 cm) of then-dry, shallowgroundwater transient salt ponds and surface sediments. Isotopic analyses were performed using a Delta Plus XL mass spectrometer at the University of Tennessee. The analytical precision for both δS and δO was <0.3‰.