Saturday, 15 July 2006

Nitrate Concentrations in Atacama Desert soils and Their Implications for the Antiquity of the Atacama Desert.

Joel Prellwitz1, Jason Rech1, Gregory Michalski2, Brenda Buck3, Michael S. Howell4, and Amy Brock3. (1) Miami Univ, 114 Shideler Hall, Oxford, OH 45056, (2) Purdue Univ, 550 Stadium Mall Dr., West Lafayette, IN 47907, (3) Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 454010, Las Vegas, NV 89154-4010, (4) Univ of Nevada, Las Vegas, Dept of Geoscience, 4505 Maryland Pkwy Box 4010, Las Vegas, NV 89154

The Atacama Desert is situated between the Andes Mountains and Pacific Ocean in northern Chile. This region is extremely hyper-arid (<5mm/yr) and hosts thick soils comprised of a variety of saline minerals, including nitrates, sulfates, chlorides, carbonates, and even minor amounts of perchlorates and iodates (Ericksen, 1981). The origin and age of these soils, and especially the nitrate, has been discussed and debated since their discovery in the early 19th Century (see Ericksen, 1981 for review).Today, however, the origin of the saline minerals in these soils is fairly well understood. Isotopic studies over the last few years (Böhlke et al., 1997; Rech et al., 2003; Michalski et al., 2004) have identified that the majority of sulfate and likely chloride minerals enter soils as eolian dust derived from local evaporite salars, or in a few locations, from sea spray particulates from the Pacific Ocean (Rech et al., 2003). Saline eolian particles are then dissolved by occasional rainfall events and re-precipitated at depth in the soil profile. Nitrate and most likely perchlorate and iodate, however, have a different origin. These oxyanions form as a result of photochemical reactions in the upper atmosphere and are deposited all over the Earth's surface. But, they only accumulate to significant concentrations in extremely hyperarid and stable landscape surfaces, such as the Atacama and Dry Valleys of Antarctica. Mass independent fraction anomalies (Δ17O) of oxygen in the Atacama nitrate provide clear evidence of an atmospheric origin (Michalski et al., 2004). The age of Atacama Desert soils, however, is still debated. Age estimates for the onset of hyperaridity range from 25 Ma (Dunai et al., 2005) to 14 Ma (Alpers & Brimhall, 1988) to 3 Ma (Hartley & Chong, 2003). We examined the chemical composition of a soil profile in the Ercilla Valley just north of the Baquedano nitrate district in the Atacama. This locality was chosen because initial results identified large concentrations of nitrate and other soluble salts in the soil profile, and Δ17O analyses averaged +19.06, indicating an atmospheric origin of nitrate and little/no modification by moisture dependant microbial processes. Soil in the upper 1m of the profile at this locality is >50% salt and has an average bulk density of 2.05 g/m3. Average concentrations (weight percent) of anions in three bulk soil samples are 14.8% SO4, 7.76%NO3, 5.27%Cl, 0.28%PO4, 0.03%F, and 0.017% ClO4. Soils with nitrate concentrations >7.0 wt% were considered ‘ores' by nitrate miners and were the focus of mining operations. We used the average nitrate concentration of this soil (7.76 wt.%), which we consider typical for soils with high nitrate concentrations in the Atacama, along with the average bulk density (2.05g/cm3) and estimated deposition rate of pre-industrial atmospheric nitrate of 0.016g/m2 yr-1 to calculate an age of 9.9 million years for this soil. This calculation assumes no additions of eolian nitrate from the reworking of other soils (which would cause the age to be too old), but also does not account for any nitrate leached below 1m-depth in the soil profile (which would cause the 9.9 My age to be a minimum estimate). When combined with evidence of nitrate paleosols covered by volcanic ash deposits of 8.3 and 9.4 million years in age, this evidence suggests that hyperaridity initiated in the Atacama 10-15 million years ago, and the region has not experienced wetter climatic conditions since this time that were significant enough nitrate from these soils. Continued work on these soils will test these initial results.

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