Soil Development in Absence of Water Examples so far illustrate general trend of chemical weathering over time –Chemical weathering –Element loss Rates of this trend will of course vary with: –1. Bedrock mineralogy –2. Climate What happens as climate (rain) diminishes to zero? Atacama Desert in Chile is an ideal area to test this question. Purpose of this section will be to: –1. Examine soil physical and chemical processes as MAP approaches 0 –2. Examine the role of atmospheric inputs to soil chemistry in absence of weathering

Why is Atacama Desert Dry? Location From Valero-Garcés et al °S 24°S 26°S

Present Climate Dry Site “Wet” Site

Atacama Desert Well Known for Nitrate: why is it there? We will examine cycling of atmospherically derived solutes and their concentration in soils

Soil Morphology: South to North (dry to drier) Parent material = grantic alluvium Age = ??? (hundreds of thousands to > 4 million years)

Soil Profile Excavation Techniques

Soil Morphology: Wettest Site A AB Bw Bk Bkm Bykm By Byk Etc.

Soil Morphology: middle site A Byk Bykm1 Bykm2 Bykm3 Bykm4 Bykm 5

Soil Morphology at Surface of Middle and Dry Sites C By

Formation of Desert Pavement and Gravel Lag

Soil Morphology: Dry Site C By Byk1 Byk2 Bykm1

Surface gypsum layers/prisms gypsum (CaSO 4 2H 2 O)  anhydrite (CaSO 4 ) Polygonal cracking at surface Gypsum prisms determine polygons

Soil Morphology: lower horizons Bykm2 Bykm3 Bykm4 Bzm BCzyk1 BCzyk2 BCzyk3

Where do salts come from and why do they have the pattern observed? Source: coastal fog (marine salts,aerosols) and playa deflation Distribution in soils: related to solubility NaCl =35.7 g/100cc = 6.2 M NaNO 3 =81.5= 9.6M CaSO 4 (gypsum)= 0.241= 0.015M CaCO 3 = = M Soil salts with increasing depth (and increasing rainfall with latitude) should be (deepest, most soluble, first): NaNO 3 >NaCl>gypsum>carbonate This is what we observe…………

Mass Balance View of Soil Chemistry and Physical Changes Volumetric expansion increases with increasing aridity Salt content with depth and latitude consistent with hydrology (but unique for most of Earth)

Measuring Rate and Composition of Atmospheric Inputs

Air Chemistry: 2002 Yields ~ 5  g-h -1 (1.5  g-m -3 ) Little variation among sites Salt chemistry strongly suggests marine aerosols ratioobsSWsea salt aerosol* Br/Cl2x x10 -3 Cl/Na SO 4 /Na NO 3 /Na <1x NH 4 /Na <1x K/Na Ca/Na *Parungo et al Organic carbon = 8-10% (9x10 5  mol/g) Nitrate = 150  mol-g -1 (450  mol-g -1 at Yungay) Ammonium = 1400  mol-g -1 (790  mol-g -1 at Yungay)

Air Chemistry Summary Salt chemistry ratios similar to sea water and marine aerosols Air solids are 8-10% organic C ! Air N is mainly NH4 rather than nitrate

Nitrate in Soils

NITRATE AMMONIUMRain

Soil Biology vs. Rainfall Life (?) in the Atacama Desert Courtesy of F. Rainey, M. Vinson, B. Gatz, J. Battista, and C. McKay.

Figure 2. Cfus/g of soil from 6 Atacama Desert sites and a Sonoran Desert site. Soils were dilution plated on both low nutrient (1/10 PCA) and high nutrient (RM) agar. Counts were determined after 20 days incubation. Table 1: Total viable counts for heterotrophic bacteria determined on 1/10 PCA and RM agar _____________________Sample Sites_________________________ Media S97-3 AT97-3 AT01-03 AT01-16 AT01-19 AT01-23 AT01-22 _____________________________________________________________________ 1/10 PCA 9.6x x10 4 NG* 7.2x x x x10 6 _____________________________________________________________________ RM 5.5x x10 3 NG 1.8x x x x10 6 _____________________________________________________________________ *NG = no growth on agar plates Culturable Bacteria Experiments

Summary of Soil Formation with Decreasing Precipitation Chemical weathering of silicate rocks appears to decline to ~ 0 Soils gradually accumulate exceedingly soluble salts that in most environments are readily lost Salt distribution with latitude, and within a given soil, relate to salt solubility Driest site has: –No plants –Almost no microbiology (virtually sterile soil) –Almost no loss of incoming atmospheric inputs –NaCl cemented soil horizons –Commerical grade nitrate accumulations