Abstract
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.
Highlights
Sulfate minerals show a wide spatial distribution around the north polar ice cap on Mars
While While we recognize that thethat sulfate remain withunusual this unusual we recognize the origin sulfatewill origin will speculative without in situ measurements for the Martian circumpolar dunes, we propose remain speculative without in situ measurements for the Martian circumpolar dunes, that some valid assessments on the sulfate be made using we propose that some valid assessments on theorigin sulfatecan origin can be made geochemical proxies for the sulfur cycle in terrestrial environments
We assumed that the majority of sulfide derived from the Ferrar Dolerite and Granitic Basement Complex bedrock, the main rock types in this portion of the McMurdo Dry Valley, which comprised the majority of the detrital grains
Summary
Sulfate minerals show a wide spatial distribution around the north polar ice cap on Mars. Various geochemical models have been proposed for the origin of gypsum including atmospheric deposition of volcanic aerosols [4], sulfide weathering within the dunes by infiltrating melt waters [5], and erosion of gypsum-rich sand layers underlying the ice cap [2,3]. None of these models have strong support from terrestrial analog studies. An additional difficulty is that previous geochemical models for Mars [2,4,5,10] are mainly
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