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Project Progress

Sulfates appear to be a significant part of rocks and regolith exposed at or near the surface of Mars. Important occurrences of sulfate minerals have been detected at latitudes extending from the polar region to lower latitudes in Meridiani Planum and Vallis Marineris. We are pursuing, therefore, a latitudinal study of Salt Basins developed along the Rio Grande Rift as a terrestrial analog to sulfate deposition in the past on Mars. Complex physical and chemical factors control the initial precipitation of sulfate deposits and early diagenesis at regional and local scales. The following processes are being addressed: 1) formation and transformation of sulfate-rich deposits in lakes and playas from closed-drainage basins, 2) erosion of evaporitic crusts and eolian grain transportation at sites of gypsum dune formation, 3) early diagenesis within gypsum dunes under arid and semi-arid climate condition, and 4) local and regional influences of volcanism on sulfur cycles. Geochemical (major and trace elements) and stable isotopic (³⁴S/³²S, ¹⁸O/¹⁶O, ²H/¹H) compositions of water and sediment samples are combined with geomorphic analysis of satellite images to gain quantitative and qualitative insights into processes influencing production and preservation of gypsum in saline basins along the Rio Grande Rift system.

Geochemical analyses are coupled with field observations at the Estancia Basin (EB), White Sands National Monument (WSNM), regions of Guadalupe and Cuatro Ciénegas. Collection of materials includes sampling of 1) sulfate minerals in vertical profiles of evaporite crusts and detrital chemical sediments from small playa lakes (EB) and from Lake Lucero (WSNM), and 2) gypsum sand and silt from dunes to address age, morphology, sediment source, sedimentary structures, and degree of cementation for gypsum dunes. Based on sample collection in January and May 2007, a few general conclusions can be stated:

1)the sulfur source for gypsum dunes and paleo-lake sediments is largely controlled by dissolution of Permian evaporates by meteoric waters;

2)there is no direct evidence for input of magmatic-derived sulfur to the small saline playas in the study areas under study but some gypsum deposits (pillar shape) in WSNM appear to be associated with fracture lineaments that may focus upward flow of saline brines to the surface environment;

3)lakebeds from EB contain more organic carbon (mean 0.64 %) than WSNM (mean 0.18 %); burial of organic matter is inferred to sustain higher rates of sulfate reduction in EB compared to WSNM;

4)the formation of gypsum deposits appears to be governed largely by evaporation and subsequent oversaturation of water in dissolved salts; present-day groundwater systems at all locations show chemical equilibrium with gypsum deposits;

5)early diagenesis of dunes is mainly controlled by upward movement of groundwater and further loss of water due to evaporation.

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