Sulfur, Chlorine, and Bromine Variations in the Soil Profile at Gusev Crater, Mars

Sulfur, chlorine and bromine abundances in Gusev soils over depths up to ≈10 cm, from MER APXS sols 14-1512 data, are considered to exhibit: (1) nearly constant S/Cl ratios (2) element fractionation possibly facilitated by brine activity either by upward-wicking [1], or downward/lateral migration [2]. We assessed an alternative mechanism similar to terrestrial volatilization of surficial bromide observed at Sabkhas [3] and the polar troposphere at the polar sunrise [4]. In addition, we evaluated whether S/Cl ratios may be more variable than previously thought. Bromine concentrations are higher in the subsurface at Gusev and the increase relative to surface concentrations is roughly in the order Br>S>Cl, consistent with higher Br/Cl, lower S/Br, and higher S/Cl ratios in the subsurface. Furthermore, nearly all subsurface variability in Br can be modeled by Mg and S variations alone. Preferential [5] downward migration of Br with thin films of concentrated brine formed by the eutectic melting of surface frost [2] could explain the trends we observed. Preferential Br substitution into Mg-salts such as bischofite over Na-salts [5] may have also occurred. Alternatively or in conjunction, our observations are consistent with the loss of surficial Br to the atmosphere, driven either by UV photolysis [3] or by chemical oxidants [6]. Where present, reactive Br is known to eliminate O3 [7], which in turn could affect the distribution of perchlorates [8], current models of H2O2 production [9], and atmospheric ozone [10] variation. We are currently engaged in a laboratory program involving experimental evaporation of UV-exposed bromide-bearing salt solutions, the results of which should help to better discriminate among models. A vertically variable S/Cl ratio suggests that bulk soil, which includes material at decimeter depth, may bear a S/Cl mass ratio in the 4 - 5 range with variable correlation between the two elements. This contrasts with the smaller 1.8 [11] - 3.6 ratio and strong correlation in surface soil. Such variability at depth would be consistent with differential vertical mobility of the two elements. Tentatively, the trends appear to be driven primarily by S increase at depth. Neither Br nor Cl correlates strongly with Na in soil samples, unlike the enrichment of halides in rock alteration rinds at Meridiani [12]. Preliminary evaluation of Meridiani soils geochemistry suggests generally consistent, but noticeably subdued chemical trends compared to Gusev. This could be due to a minor contribution from the Meridiani sedimentary rocks where Br varies by more than an order of magnitude. References [1] Wang A. et al.(2006) doi:10.1029/2005JE002513 [2] Yen A. et al.(2005) doi:10.1038/nature03637 [3] Wood W. W. & Sanford W. E.(2007) doi:10.1029/2007GL029922 [4] Zhao T. et al.(2008) doi:10.1029/2008JD010631 [5] Marion G. et al.(2009) doi:10.1016/j.icarus.2008.12.004 [6] Finlayson-Pitts B. J.(2010) doi:10.1073/pnas.1003038107 [7] Finlayson-Pitts B.(2010) doi:10.1021/ac901478p [8] Marion G. et al.(2010) doi:10.1016/j.icarus.2009.12.003 [9] Hurowitz J. A. et al.(2007) doi:10.1016/j.epsl.2006.12.004 [10] Lefevre F. et al.(2008) doi:10.1038/nature07116 [11] Kounaves S. et al.(2010) doi:10.1029/2010GL042613 [12] Knoll A. H. et al.(2008) doi:10.1029/2007JE002949