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marchant

HR: 11:15h
AN: H52B-04
TI: High-resolution nitrate and sulfate stable isotope profiles for soils in the Antarctic Dry Valleys
AU: * Bao, H
EM: bao@lsu.edu
AF: Louisiana State University, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803
AU: Marchant, D R
EM: marchant@bu.edu
AF: Boston University, Dept. of Earth Sciences 685 Commonwealth Ave, Boston, MA 02215
AB: The Antarctic Dry Valleys (ADV), being hyperarid and frigid, provide an ideal surface for the accumulation of atmospheric salts (aerosols and residues of snow sublimation). At any site, salt composition and flux, and degree of post-depositional ion migration can be attributed to the duration of exposure (age of the surface), distance to the open ocean, direction of prevailing wind, snow cover, brightness of the surface (albedo), elevation, and if the deposit is a glacial till, the nature of the till (basal or sublimation till). We expect to encounter, therefore, a variety of profiles in the distribution of salts and their stable isotope compositions in Antarctic soils. This exclusive salt repository offers us two unique opportunities. First, the polar atmospheric process associated with the origin of salts can be studied in soils by examining the abundant atmospheric salt accumulated over the time, an alternative to the use of expensive atmospheric sampling techniques. Second, ion migration has never been quantified for cold and hyper-arid desert soils, due partly to the difficulty of obtaining realistic parameters through laboratory experiments. Chemical and stable isotope profiles from a variety of soils in the ADV place critical constrains to any model that combines the deposition and migration of atmospheric salts with the vapor flux and with the formation of subsurface ice cement in this unique surface environment. We have analyzed more than 20 soil profiles with the highest vertical sampling resolution physically attainable in dry, non-cohesive soils of the ADV. Nitrate and sulfate are two of the major salt components in soils of the ADV. We predict that there should be only one nitrate source, i.e., all secondary, due to the lack of local primary source. Our results show that the $\Delta$$^{17}$O values of nitrate in the ADV are quite uniform ($\sim$ +31.0 $\pm$ 1.0$\permil$) both in space and in depth, supporting our prediction. However, accompanied $\delta$$^{15}$N values are not uniform, ranging from $\sim$ -13.0 to $\sim$ -25.0$\permil$, and are significantly higher in sites that are at low elevations and close to the ocean. Some local anomalies in both $\Delta$$^{17}$O and $\delta$$^{15}$N for nitrate are observed in specific soil horizons. We suspect that there might be an unidentified nitrate process, either from the local atmospheric contribution or from post-depositional alteration (biological?). On the basis of observed variations in earlier $\Delta$$^{17}$O data, sulfate appears to have two sources; sea-salt sulfate and secondary sulfate. This hypothesis can be independently tested by accompanied $\delta$$^{34}$S values for sulfate, because sea-salt sulfate has a fixed $\delta$$^{34}$S value, whereas secondary sulfate should have a lower $\delta$$^{34}$S value. Our $\delta$$^{34}$S results are consistent with the two-end-member sulfate source scenario. Furthermore, we found that the sulfate $\delta$$^{34}$S values are decreasing with depth consistently in most of the soil profiles, except for those disturbed by ancient glacial movement or those near the base of sublimation tills. These high-resolution geochemical and multiple stable-isotope data for salts in the ADV provide a solid ground to test the theories on the origin and migration of atmospheric salts in the ADV and to quantify landscape stability and the long-term affects of soil re-working by cryoturbation in hyperarid and cold deserts.
DE: 1827 Glaciology (1863)
DE: 1866 Soil moisture
DE: 1040 Isotopic composition/chemistry
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0365 Troposphere--composition and chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting