Origin of terrestrial gypsum dunes Implications for Martian gypsum-rich dunes of Olympia Undae
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The Estancia, White Sands, Guadalupe and Cuatrocinegas Dune Fields are among the largest known aeolian gypsum sand-dune accumulations on Earth and occupy closed-drainage basins within the Rio Grande Rift. High sedimentation rates of lacustrine gypsum occur in topographic depressions within the closed basins. The gypsum accumulations result from long-term, complex, interaction between tectonism, climate, and a hydrologic cycle that involves geochemical recycling of sulfur from older sedimentary rocks flanking and underlying the basins. Gypsum precipitation in lacustrine environments is strongly controlled by local groundwater/bedrock interaction. The ranges of 34S values (per mil, vs. VCDT) for gypsum sand in the White Sands Dune Field (12.1 to 13.9), Guadalupe Dune Field (10.2 to 12.5) and Cuatrocinegas Dune Field (14.6 to 15.9) indicate that the main sources of dissolved sulfate are evaporite strata of Lower Permian, Middle Permian and Cretaceous ages, respectively. A spatial increase of 34S values across the White Sands Dune Field, in the direction of prevailing winds, matches well with a stratigraphically upward increase of 34S values recorded in Lake Otero (an ancient lake in the basin) strata. This finding supports previous suggestions that White Sands Dune Field evolved as a result of the step-wise deflation of previously stored sediments in Lake Otero.The modern gypsum dune fields are primarily wet eolian systems in which dune accumulation is controlled by a near-surface groundwater table, which promotes early cementation of the dune accumulations. Early cementation in the interdune surfaces and, to a lesser degree, on the dune surfaces reduces the amount of sand available for transport and slows rates of dune migration. Based on an average migration rate of 2 m/year, the time needed for a dune at White Sands to migrate the downwind distance of the dune field is approximately 6500 years, which matches well with other estimates of the initiation of the dune field at approximately 7000 years ago and indicates that White Sands likely evolved as a wet aeolian system.As on Earth, gypsum-rich dune fields apparently are rare on Mars. Gypsum has been identified only within the Olympia Undae Dune Field which encircles a portion of the Martian north polar residual ice cap. Analogous to terrestrial gypsum dunes, the gypsum within the Olympia Undae gypsum-rich dunes might have originated from transport and deposition via aeolian processes. In this model, gypsum-rich source sediment could have been formed by confined groundwater or surface water activity and later transported by the wind. Any subsequent gypsum precipitation in the source area could have been initiated by episodic melting of the Martian polar ice cap. It is likely that the gypsum was deposited in the Olympia Undae dunes after the main deflation events that led to formation of the siliciclastic components of the dune field. 2009 Elsevier B.V.
