Jarosite formation in deep Antarctic ice provides a window into acidic, water-limited weathering on Mars

The presence of jarosite, a ferric-potassium hydroxide sulfate mineral [KFe³⁺₃(SO₄)₂(OH)₆], in Martian surficial sediments has sparked considerable debate regarding its formation mechanisms. While jarosite is relatively rare on Earth, its formation is typically associated with low-temperature acidic oxidative weathering of iron-bearing minerals in water-limited settings. The discovery of widespread jarosite at Meridiani Planum by the Opportunity rover in 2004 confirmed earlier predictions but raised questions about the specific geological context. The mineral's presence has been interpreted as evidence for past liquid water on Mars, but the limited water availability necessary for jarosite formation also implies a geologically short period of water activity. A paradox arises from the observation that jarosite occurs within mafic (basaltic) volcanic material, whose interaction with acidic solutions would have a neutralizing effect, seemingly incompatible with jarosite formation. This paradox is resolved by proposing that the interaction occurs in isolated environments, minimizing water-rock interaction and maintaining low pH during diagenesis. Several hypotheses posit that Martian jarosite formed through interactions between acidic fluids and weathered sediments in transient lacustrine-evaporative basins (similar to Earth playas) or volcanic settings like fumaroles. An alternative, yet untested, model proposed jarosite formation through weathering of mafic dust or fine-grained ash trapped within massive ice deposits. This 'ice-weathering' model suggests that the ice interior promotes acidic weathering via cryo-concentration of sulfur-rich volcanic aerosols, leading to jarosite precipitation. Previous studies have identified jarosite in Antarctic rock varnishes, weathering rinds on meteorites, and soils, but not in englacial environments. This research aims to address this gap by investigating the possibility of englacial jarosite formation in deep Antarctic ice and its implications for understanding Martian geology.

Burns (1987) initially hypothesized the presence of ferric sulfates, including jarosite, on Mars, a prediction later confirmed by the discovery of widespread jarosite at Meridiani Planum by Klingelhöfer et al. (2004). Subsequent studies confirmed the widespread presence of jarosite across Martian regions (Farrand et al., 2009; Weitz et al., 2015; Rampe et al., 2017). Madden et al. (2004) highlighted the significance of jarosite as an indicator of water-limited chemical weathering on Mars, emphasizing the critical role of limited water in its formation and preservation. Experimental evidence (Papike et al., 2006; Tosca et al., 2008) showed that exceeding a certain water-to-rock ratio leads to jarosite transformation into goethite, supporting the interpretation of Martian jarosite as evidence for limited and short-lived liquid water activity. The debate also involved the lithology of the protolith, with studies highlighting mafic volcanic material (McLennan et al., 2005; McCollom & Hynek, 2005) and exploring the role of acid-basalt interaction and its neutralization effect (Zolotov & Shock, 2005; Zolotov & Mironenko, 2007). Niles and Michalski (2009) proposed the ice-weathering model, suggesting that jarosite formation could occur within massive ice deposits, a hypothesis supported by Michalski and Niles (2012) and Niles et al. (2017). However, prior to this study, direct evidence for englacial jarosite formation remained elusive, with previous Antarctic findings limited to surface environments (Giorgetti & Baroni, 2007; Hallis, 2013; Terada et al., 2001; Simas et al., 2006).

This study utilized the Talos Dome ice core (TALDICE) in East Antarctica, a 1620 m long core providing a climate record extending to ~153,000 years before present. The research focused on identifying jarosite within dust samples from the deeper sections of the core (>1000 m). The identification of jarosite in microgram-sized samples presented significant analytical challenges, necessitating a multi-technique approach for robust results. Several techniques were employed to confirm the presence of jarosite: 1. **Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDX):** This technique was used to observe the morphology and elemental composition of mineral dust particles. The identification of hexagonal platelets, a habit compatible with jarosite's trigonal system, was a key observation. 2. **X-ray absorption spectroscopy (XAS):** XAS, performed at the Diamond Light Source, was used to determine the depth profile of jarosite and analyze the Fe oxidation state. The progressive increase in Fe K-edge absorption energy with depth indicated increasing Fe oxidation, consistent with weathering processes. A convex feature between 7135 and 7142 eV, characteristic of jarosite, further supported its presence and quantification. 3. **Scanning Transmission Electron Microscopy (STEM) and EDX:** STEM-EDX provided high-resolution imaging and diffraction analysis to confirm the presence of jarosite crystals with the expected diffraction spacings and stoichiometry. 4. **Dust concentration and grain size analysis:** A Coulter counter was used to determine dust concentration and grain size distribution. A novel dust grain size index was developed to distinguish between upper and lower TALDICE dust based on granulometric features. The study also involved analyzing the physico-chemical conditions in deep ice, considering ice metamorphism and re-crystallization, the accumulation of impurities at ice grain boundaries, and the potential formation of acidic brines. The study critically evaluated alternative explanations for jarosite presence, such as contamination from the bedrock, ruling them out based on the ice stratigraphy and other available evidence.

The key findings of the study include: 1. **Identification of Jarosite in Deep Antarctic Ice:** The researchers successfully identified jarosite in the deep sections (>1000m) of the Talos Dome ice core (TALDICE) using a combination of SEM-EDX, XAS, and STEM-EDX techniques. The jarosite was found adhering to silica-rich particles, suggesting in situ formation. 2. **Englacial Jarosite Formation:** The study proposes a mechanism of englacial formation for the identified jarosite. This is based on the cryo-concentration of sulfuric acid and other impurities at grain boundaries in deep ice, creating localized acidic brines that facilitate the weathering of Fe-bearing dust particles, leading to jarosite precipitation. This increase in jarosite is correlated with the increasing size of ice crystals, an indicator of ice metamorphism, and not with climate-related oscillations. The positive shift of the Fe K-edge absorption energy with depth also supports the progressive oxidation of iron linked to the weathering process. 3. **Significance of Iron Oxidation and Jarosite Abundance:** Analysis of Fe K-edge absorption energies across different depth intervals showed a progressive increase in Fe³⁺ concentration with depth, reaching almost complete oxidation below 1500m. This corroborates the observation of jarosite, which increases significantly below 1033m and remains relatively stable at deeper levels. 4. **Impact on Climate Records:** The findings suggest that chemical weathering processes occurring deep within the ice can alter the climatic signals preserved in dust records, particularly the Fe concentration and speciation, potentially affecting paleoclimatic reconstructions. The correlation between δ¹⁸O and dust concentration is significantly lower below 1400m, indicating a degradation of the climatic signal within the dust record. 5. **Changes in Dust Grain Size:** The grain size analysis revealed an excess of large particles and a lack of fine particles in the deeper parts of TALDICE, suggesting that jarosite precipitation may contribute to chemical aggregation of mineral particles. 6. **Evidence for Localized Acidic Brines:** The presence of jarosite, requiring liquid water for formation, provides strong evidence for the existence of localized acidic brines within the deep ice, potentially at ice grain junctions or intra-grain micro-inclusions. This contradicts the commonly assumed mild acidity in Antarctic meltwater.

The discovery of authigenic jarosite in deep Antarctic ice offers compelling support for the ice-weathering model proposed for jarosite formation on Mars. The similar conditions of cryogenic temperatures, the presence of basaltic dust, and the potential for cryo-concentration of acidic fluids in both deep Antarctic ice and Martian ice deposits strengthen this analogy. The study's findings challenge the prevailing playa-evaporative basin model for Martian jarosite formation, suggesting that ice-mediated weathering may be a more significant process in the formation of extensive jarosite deposits observed on Mars. The findings further imply that the climatic signals in deep ice cores might be affected by englacial chemical reactions, highlighting the importance of accounting for these processes in paleoclimatic studies. The study also suggests the need for further exploration of englacial weathering processes concerning Antarctic meteorites, proposing that englacial weathering might play a more substantial role than previously recognized in shaping the chemical composition of meteorites recovered from Antarctic blue ice fields.

This research presents the first evidence of authigenic jarosite formation in deep Antarctic ice, providing strong support for the ice-weathering model of jarosite formation on Mars. The findings challenge previously held assumptions regarding Martian jarosite formation and emphasize the importance of considering englacial diagenetic processes in both Antarctic and Martian geological contexts. Future research should focus on exploring similar processes in other deep ice cores and improving our understanding of the impact of these reactions on paleoclimatic interpretations and the history of meteorites in Antarctic blue ice fields.

The study focuses on a single ice core (TALDICE) and may not fully represent the diversity of englacial conditions in other Antarctic locations. The analytical techniques used had limitations related to the size of the samples and the complexity of the mineral assemblages. Further studies are needed to confirm the widespread nature of this englacial weathering mechanism and to clarify the precise geochemical conditions required for jarosite formation in these environments.