ABSTRACT

A CHARACTERIZATION OF HYPER-ARID NITRATE SOILS IN THE BAQUEDANO VALLEY OF THE ATACAMA DESERT, NORTHERN CHILE

by Joel S. Prellwitz

This study examines the physical, geochemical, and isotopic characteristics of hyper-arid nitrate soils within the Baquedano Valley of the Atacama Desert in northern Chile. Ages of ~4.7-1.5 Ma are determined for these soils based on cosmogenic 10Be results from surface boulders at one locality. The natural flux rate of atmospheric nitrate is derived from the age and soil nitrate concentration at this site. Ages of other neighboring soils are determined by this flux rate and respective nitrate inventories. Soil morphological factors (i.e. bulk density and percent salt) support this age model, however, the ages proposed are likely minimum ages as nitrate accumulation rates in soils decrease with age. Soil carbonate δ13C and δ18O values indicate a CAM-plant dominated paleo-environment with changing moisture sources over time. Soil sulfate δ34S values largely reflect eolian evaporate sulfate, and trend negatively with depth, indicating fractionation via dissolution/precipitations reactions during down-profile migration of sulfate .

A CHARACTERIZATION OF HYPER-ARID NITRATE SOILS IN THE BAQUEDANO VALLEY OF THE ATACAMA DESERT, NORTHERN CHILE

A Thesis

Submitted to the

Faculty of Miami University

in partial fulfillment of

the requirements for the degree of

Master of Science

Department of Geology

by

Joel Scott Prellwitz

Miami University

Oxford, OH

2007

Advisor:______Jason Rech

Reader:______Hailiang Dong

Committee Member:______Elisabeth Widom

TABLE OF CONTENTS

1: INTRODUCTION………………………………………..……...... ………………..……1

2: THE ATACAMA DESERT……………………………..………...... …….………………3 2.1 Location and Climate……………………………………...... ……….……….……3 2.2 Antiquity of the Atacama Desert…………………………………...... …….……..3 2.3 Atacama Central Valley Soils…………………………………...... ……………4 2.4 Field Site Description…………………………………………………………………5

3: METHODS...... 5 3.1 Sample Collection………………………………………….…………...... ………..5 3.2 Analytical Methods……………………………………………………...... ………6

4: RESULTS……………………………………………...………………………...... ………7 4.1 Cosmogenic Nuclide Exposure Age Dates…………………………...... …………7 4.2 Soil Morphology……………………………………………………...... …………7 4.3 Mineralogy……………………………………………………………...... ……….8 4.4 Geochemistry………………………………………………………...... ………….8 4.4.1 Oficina Ercilla……………………………………………...... ………….8 4.4.2 Valenzuela………………………………………………...... …………...9 4.4.3 Rencoret N.W. …………………………………………...... ………….10 4.4.4 Summary………………………………………………...... …………...10 4.5 Isotope Systems…………………………………………………...... …………...11 4.5.1 δ13C and δ18O of Carbonate…………………………...... ……………..11 4.5.2 δ34S of Sulfates…………………………….…………...... …………….12

5: DISCUSSION………………………………....…………………………...... …………...12 5.1 Soil Characterization………………………………………………...... …………12 5.1.1 General Geochemical Trends……………………………...... …………12 5.1.2 Site-specific Geochemical Trends………………………...... …………13 5.2 Age Assessments…………………………………………………...... ………….14 5.2.1 Relative Age Assessments………………………………...... …………14 5.2.2 Empirical Age Assessments……………………………...... ………….15 5.3 Natural Flux Rates…………………………………………………...... ………...15 5.4 Isotopes……………………………………………………………...... …………16 5.4.1 δ13C and δ18O……………………………………………...... …………16 5.4.2 δ34S………………………………………………………...... …………17

6: SOIL DEVELOPMENT IN THE HYPER-ARID ATACAMA DESERT…...... …...... 18

7: CONCLUSIONS…………………………………...... ……………………...... ……...19

REFERENCES……………...…………………………………………………...... ………..20

ii

LIST OF TABLES

Table 1: Cosmogenic exposure age data...... 24

Table 2: Soluble soil concentrations and bulk density...... 25

Table 3: Mineralogy of bulk soil and soil residuum after dissolution of soluble salts...... 27

Table 4: Geochemistry of soluble soil salts...... 29

Table 5: Major anion mass per unit volume (kg / 0.5 m³)...... 31

Table 6: Isotopic values...... 32

iii

LIST OF FIGURES

Figure 1: Location of Baquedano nitrate district, including site localities and locations of nitrate mining operations...... 34

Figure 2: Landsat thematic mapper satellite image of the Baquedano nitrate district, including site locations, locations of modern mining operations, and major faults in the vicinity...... 35

Figure 3: Photographs of site localities in the Baquedano nitrate district. a) Low bedrock hill with alluvial fan that has been mined by nitrate operations at Oficina Ercilla, b) Upper soil horizons at Oficina Ercilla (1-2 cm thick Avyz, 9-17 cm thick Byz, and 4-7 cm thick Bz, see horizon descriptions in text) at Oficina Ercilla and Rencoret NW. (white, powdery Bz horizon is absent at Valenzuela), c) Portion of upper alluvial fan outcrop from early 20th century nitrate mining operations that was sampled at Oficina Ercilla, d) Fluvial terrace outcrop with numerous vertical soil fractures (~15-30 cm thick) exposed along small drainage at Valenzuela, e) surface of fluvial terrace at Valenzuela, inset photo shows nitrate test pit that was used for describing and sampling soil profile, f) Alluvial fan outcrop exposed by early 20th century mining operations at Rencoret NW...... 36

Figure 4: Photographs of cosmogenic nuclide field sampling. a) Typical granitic boulder size and shape sampled at OE 2. b) Samples were cut from the top 1-5 cm of the boulders with a portable generator-powered diamond-tipped circular saw. Background shows well- developed patterned ground on the soil surface. c) An andesitic boulder sampled at Valenzuela. d) Image of a boulder after sampling...... 37

- - Figure 5: Cosmogenic exposure age (Myr) vs. total NO3 (kg) for each profile. NO3 quantity for each site is normalized to a 3.5 m deep profile...... 38

Figure 6: Soil profiles for site localities. Abundant secondary minerals are listed beneath each horizon...... 39

Figure 7: Bulk density (g/cm³) with depth (cm) for each soil profile. Left side of plot shows range of typical soil bulk density...... 40

Figure 8: Percent salt with depth (cm) for each soil profile. Closed symbols are bulk soil measurements and open symbols are vertical fracture fill...... 41

Figure 9: Concentrations of major anions and cations with depth (cm) for soil profile at the upper fan surface of Oficina Ercilla (OE 1)...... 42

Figure 10: Concentrations of major anions and cations with depth (cm) for soil profile at the lower fan surface of Oficina Ercilla (OE 2)...... 43

iv

Figure 11: Concentrations of major anions and cations with depth (cm) for soil profile at Valenzuela...... 44

Figure 12: Concentrations of major anions and cations with depth (cm) for soil profile at Rencoret NW...... 45

Figure 13: δ13C values of soil carbonate with depth for each soil locality and for all soils grouped together. No data were obtained for OE 2 or at depth in Valenzuela due to lack of carbonate...... 46

Figure 14: δ18O values of soil carbonate with depth for each soil locality and for all soils grouped together...... 47

Figure 15: δ13C values plotted against δ18O values for all soil carbonate samples...... 48

Figure 16: δ34S values of soil sulfate with depth for each site. Trend lines and associated R² values are displayed...... 49

v

ACKNOWLEDGEMENTS

I would first like to thank Jason Rech, my primary graduate advisor for his unending patience and guidance. Jason has been a remarkable mentor, teacher, and friend to me over the course of my studies at Miami University, and this study would not have been possible without his encouragement.

I would like to thank the many colleagues for their laboratory assistance on this project, including John Morton, Elisabeth Widom, John Rakovan, Olaf Borkiewicz, and Tomasz Marchlewski at Miami University; Harry Rowe at the University of Kentucky; Lewis Owen, Jason Dortch, and Byron Adams at the University of Cincinnati; Marc Caffee and Susan Ma at the Purdue University PRIME Lab; Greg Michalski at Purdue University; David Dettman and Christopher Eastoe at the University of Arizona; and Dave Parker at the University of California, Berkeley.

For advice, guidance, and encouragement I thank mentors Jay Quade, Elisabeth Widom, Darin Snyder, and Jonathan Levy; and my friends and colleagues William Wilcox, Justin Pierson, and Emily Winer.

Finally, I would like to thank my wife Holly, and my parents for their unrelenting belief in me and constant source of motivation.

This project was funded largely by J. Rech and in part by NSF grant # EAR0609571.

vi

1. INTRODUCTION

Nitrate-rich soils in the Atacama Desert of northern Chile have intrigued geologists and geomorphologists for over 250 years. Scientific enquiry into the origin of these soils began around 1832 when Darwin visited the area during his voyage on the Beagle and made some of the first hypotheses on the origin of the deposits. Darwin believed the nitrate was the result of deposition along the margin of an inland extension of the Pacific Ocean (Darwin, 1871). From the early 19th century through early 20th century there was much interest in the Chilean nitrate deposits because nitrate was an essential ingredient for explosives and fertilizers. During this period nitrate ores were mined extensively, and many geologists speculated on the origin of the nitrate. Early theories for the origin of the nitrate in the Atacama included: 1) decay of vegetation in marine coastal marshes or inland saline lakes (Forbes, 1861; Noellner, 1867; Darwin, 1871; Flagg, 1874; Müntz, and Marcano, 1885; Sieveking, 1887); 2) nitrification of ammonia from bird guano from cutoff seaways, saline lakes, or eolian coastal guano deposits (J.C. Hillinger, 1860; Ochsenius, 1888, 1903; Gautier, 1894; Penrose, 1910; Brüggen, 1928, 1938); 3) bacterial decay of Cenozoic or older flora and fauna during a less arid climate (Plagemann, 1898; Newton, 1896); 4) soil bacterial fixation of atmospheric (Gale, 1912, 1917; Brüggen, 1938); 5) chemical weathering of feldspathic igneous rocks with atmospheric nitric acid formed by either chemical reactions between atmospheric nitrogen and ozone or electrical discharge in dense fogs (Pissis, 1878; Sundt, 1921; Wetzel, 1928; Knoche, 1939); 6) accumulation of nitrogen compounds from Andean volcanic gases (de Kalb, 1916; Feistas, 1922, 1923; Steinman, 1925). At the turn of the century the Haber-Bosch process, which is the synthesis of ammonia by reacting with atmospheric nitrogen, was developed in Germany (Smith, 2002). By the 1930s synthetic production of nitrate surpassed production from Chilean nitrate mines, effectively ending interest in the Atacama nitrate ores (Ericksen, 1983). The nitrate mining industry became economically unviable and interest in the area subsided until the second half of the 20th century. During the 1960s and ‘70s a collaborative research program between the USGS and the Instituto de Investigaciones Geológicas de Chile, under the direction of George Ericksen, began scientific investigation into the nature and origin of the Chilean nitrate deposits. Their work included descriptions of nitrate soils and their distribution, as well as chemical and mineralogical characterization of the soil that would become the basis for comparison in future studies. This research group documented the general location of commercial (>5%) and low-grade nitrate deposits, and identified minerals within the deposits including sulfates, iodates, chromates, borates, and minor amounts of perchlorates (Ericksen, 1981). The generalized commercial-grade nitrate soil profile included a surficial horizon of unconsolidated sand, silt, and rock fragments with high concentrations of sulfate minerals, overlying several well-cemented horizons with variable amounts of nitrate, chloride, and sulfate salts. Ericksen’s research group also made the first identification of natural perchlorate in these soils. Ericksen’s hypotheses regarding the origin of the nitrate deposits included 1) chemical reactions between magmatic material and hydrothermal groundwater and subsequent transport, or the eolian transport of volcanic weathering products (Ericksen, 1961; Ericksen, 1981); 2) groundwater and eolian deposition of

1 microbial nitrification products from dry saline lake beds (salars) (Ericksen, 1983); 3) sea spray aerosols from the Pacific Ocean (Ericksen, 1981); 4) redistribution of Andean- derived salts within groundwater via capillary action and eolian processes deposition (Ericksen, 1983); and 5) deposition of products derived from atmospheric chemical reactions (Ericksen, 1981). In 1983 Ericksen stated that “no single theory can explain the areal or vertical distribution of the richest nitrate deposits and that the origin is neither simple nor obvious. Determination of the age of the deposits via flux rates of atmospheric nitrate deposition is therefore difficult and unreliable” (Ericksen 1983: p. 372). Clearly a new approach was necessary to better constrain the source inputs and age of the soil. Over the last few decades several researchers have analyzed the isotopic composition (δ15N, ∆17O & δ18O, ∆33S & δ34S, 87Sr/86Sr) of soluble salts in Atacama soils (Böhlke et al., 1997; Bao et al., 2004; Rech et al., 2003; Michalski et al., 2004). Böhlke et al. (1997) analyzed δ15N and δ18O in nitrate and δ34S in sulfate from high-grade nitrate soil samples collected by Ericksen. They found δ15N values of ~0‰, consistent with either deposition of atmospheric nitrogen or microbial nitrogen fixation, but the range of 18 δ Onitrate values (+31 to +50‰) were significantly higher than possible from nitrification with meteoric water and more closely resemble that of atmospheric nitrate (Böhlke et al., 1997). Michalski et al. (2004) identified ∆17O mass-independent fractionation anomalies from the same samples that Böhlke analyzed and found that soil nitrate had average ∆17O values of +17.6‰. These results clearly indicated that the majority of Atacama soil nitrate is associated with tropospheric electrochemical or photochemical reactions, as ∆17O values for terrestrial sources are 0 ‰ (Michalski et al., 2004). Sulfur isotopic analyses (∆33S & δ34S) on both surficial and deep soil samples have identified marine aerosols and local eolian dust derived mainly from deflated to be the main sources of sulfate in the Atacama soils (Rech et al., 2003; Bao et al., 2004). Stable landscapes with the least input of marine aerosols and local dust appear to correspond with the distribution of high-grade nitrate deposits, reinforcing atmospheric deposition as the primary source of nitrate (Rech et al., 2003). Atmospheric nitrate deposition occurs globally but accumulation is restricted to hyper-arid environments where leaching and nitrogen uptake by organisms is limited. Hence, nitrate can accumulate over time, presenting a unique opportunity to determine natural flux rates of this and other important nutrients. Ewing et al. (2006) identified the deposition rates of atmospheric inputs into hyper-arid soils in the Atacama through dust traps and soil mass balance calculations. They found that the accumulation of atmospheric inputs (such as salts) outweigh the loss of material over the course of soil development. Ewing’s work determined an unusually high volumetric expansion (~120%) in one Central Atacama soil, a total atmospheric salt accumulation of 830 kg/m2 in a 2.1 Ma soil, and empirically derived soil-based deposition rates for nitrate (0.0081 g/m2•yr), sulfate (0.1345 g/m2•yr), and chloride (0.0425 g/m2•yr) (Ewing et al., 2006). Over the last two centuries geoscientists have made significant advancements towards understanding the genesis of the unique soil deposits found within the Atacama Desert. The geochemical composition of some soils have been identified and the sources of the saline materials have been fairly well constrained. However, the age range for soils in the Atacama and rates of soil genesis are still unknown, as well as the variability in soil morphology and chemistry. Previous work has focused on sampling the upper soil

2 horizons (e.g., Rech et al., 2003; Bao et al., 2004), however, little detailed work has been done on complete soil profiles. There are currently no detailed soil descriptions for - nitrate ore (>7 % NO3 ) soils. The soils of the Atacama provide a unique geochemical and isotopic record over the course of soil development in a hyper-arid climate regime. In this study, I characterize the soil chemistry and mineralogy with depth in ~3 m-deep soil profiles from three sites slightly north of the Baquedano nitrate district of the Atacama Desert. I construct an age model based on cosmogenic 10Be results and nitrate concentrations in soil. Carbon and oxygen (δ13C and δ18O) isotopes of soil carbonate are analyzed to identify environmental and climatic factors through the period of soil development, as well as sulfur isotopes (δ34S) to infer the origin of sulfate minerals and determine processes influencing the stable isotopic composition of sulfate minerals.

2. THE ATACAMA DESERT

2.1 Location and Climate The core of the Atacama, where soils with the highest accumulation of nitrate salts reside, lies between ~69.5º W and 70º W longitude and extends from ~19.5º S to 26º S latitude (Ericksen, 1981). Situated between the Coastal Cordillera (~1,000 m to 2,000 m in elevation) to the west and the Andean Cordillera (~2,500 m to >6,000 m in elevation) in the east, the central desert is an in-filled series of alluvial valleys and closed basins filled with Oligocene to Pliocene aged sediments (Sillitoe et al., 1968; Ericksen, 1981). The Central Valley consists of the Pampa del Tamarugal and a southern longitudinal basin, which are separated by the transverse Baquedano valley (Ericksen, 1981). This study focuses on a small, longitudinal transect within the Baquedano valley (Figure 1). The combination of the Andean rainshadow effect and the cold, upwelling waters of the Humboldt Current along the coast limits precipitation from reaching the Central Valley of the Atacama. Pacific precipitation and fog infiltration (locally-termed camanchaca) is limited by the Coastal Cordillera and the temperature inversion at this latitude (Trewartha, 1981). The Coastal Cordillera is typically less than 2000 m in elevation, but this is sufficient to block clouds from entering the Central Valley to the east (Ericksen, 1981). Annual precipitation of <5 mm/yr is typical in the core of the Atacama, and larger rain events of a few centimeters occur only a handful of times every century (Ericksen, 1983; McKay et al., 2003). The scarcity of water causes vascular plant life to be nonexistent, with soils supporting only scattered cyanobacteria colonies that grow on the underside of translucent minerals such as quartz and (Drees et al., 1996, Warren-Rhodes et al., 2006, Dong et al., 2006). Landscape surfaces show little evidence of channeling or runoff from precipitation (Ericksen, 1981; Rech et al., 2003). Limited precipitation either soaks into the unconsolidated topsoil and quickly evaporates, or the evidence of runoff and channeling has been buried by the eolian materials that constitute the topsoil.

2.2 Antiquity of the Atacama Desert There has been considerable debate regarding the age of soils and geomorphic surfaces, and the onset of hyper-aridity, within the core of the Atacama. Most studies suggest that hyper-arid conditions initiated sometime between the Middle Miocene to

3 Pliocene, however, estimates range from late Triassic to Pliocene (Alpers and Brimhall, 1988; Nishiizumi, 1988; Hartley and Chong, 2002; Hartley et al., 2005; Dunai et al., 2005; Clarke, 2006; Rech et al., 2006). Hartley (2005) and Clarke (2006) argue that the presence of Triassic and Jurassic evaporites within the Atacama indicate a Mesozoic age for the formation of the Atacama. Dunai et al. (2005) used cosmogenic 21Ne on erosion- sensitive sediment surfaces to determine that there has been little to no erosion since ~25 Ma, suggesting a late Oligocene onset for hyperaridity. Alpers and Brimhall (1988) used K-Ar age dating to identify the cessation of supergene mineralization (thus the transition to hyper-aridity) at ~14 Ma. Rech et al. (2006) inferred a transition from arid to hyper- arid climate along the eastern Atacama between 19-12 Ma from Miocene paleosols overlain by ignimbrites dated to 9.4 and 8.3 Ma. Hartley and Chong (2002) used sedimentological data to infer a semiarid environment persisting until the late Pliocene, followed by a transition to hyper-aridity. This young age was supported by cosmogenic surface exposure ages of 3-4 Ma by Nishiizumi et al. (2005).

2.3 Atacama Central Valley Soils Soils in the Central Valley of the Atacama Desert are different than most soils on Earth in that they are constantly accumulating material such as salts and silicate dusts, and not losing material through chemical weathering (Ewing et al., 2006). Salts and dust are introduced to the soil as eolian dust sourced from adjacent dry lakes and soils, and from the deposition of atmospheric particulates (Ericksen, 1983; Michalski et al., 2004). Secondary downward salt migration from dissolution/precipitation reactions occur along with occasional rainfall events over the course of soil development (Garrett, 1983; Ewing et al., 2006). As such, these soils have become extremely well cemented by saline materials and volumetric/thermal salt expansion has caused the sediment to crack, allowing the introduction of more eolian dust and surface clasts. This is visible as vertical fractures in the soil profile, and as patterned ground on the landscape surface. The nitrate-rich, alluvial soil deposits that are the focus of this study are common in the Central Valley. Noncommercial (1-5 %) deposits are thought to be fairly widespread throughout the central Atacama, although their spatial distribution is not well known. Commercial-grade (>5 %) nitrate soils occur less frequently (Ericksen, 1981; Ericksen, 1983; Searl and Rankin, 1993). Soil nitrate ores (>7 %) were mined in the late 1800s and early 1900s, and the current understanding of the distribution of such deposits comes from these mining localities and records. Most deposits occur within alluvial fans, as well as on hilltops and the upper slopes of hills in the Central Valley, typically at elevations less than 2000 m (Ericksen, 1981). Nitrate soils generally consist of a series of characteristic physical and chemical horizons described by Ericksen (1981). The upper 10-30 cm of powdery, poorly cemented sand, silts, and gravels are termed chusca. This layer commonly has a thin (1-5 cm thick) white saline horizon of sulfate saline materials such as gypsum, anhydrite, or thenardite. Below the chusca is 0.5 to 2 m of costra, which is a transitional layer of poorly to moderately cemented material. The 1-3 m layer of caliche below the costra is firmly cemented and contains the ore-grade nitrate deposits (caliche-blanco) that have been mined. Below this horizon is the conjelo horizon, which consists of up to 2 m of saline-cemented regolith, and the coba, which is unconsolidated regolith.

4 2.4 Field Site Description The study area consists of three field sites located along a ~15 km north-south transect within an alluvial valley just north of Baquedano and south of Oficina Pedro de Valdivia, one of the few nitrate mines still in operation (Figures 1 & 2). This transect is located in an area of the central desert where the average elevation is ~1400 m. Each field site has deep soil exposures as a result of nitrate mining or nitrate exploration pits (Figure 3). The southernmost site, Oficina Ercilla (23.12º S, 69.86º W), is an alluvial fan deposit (Figure 3). This fan had been mined in the 1910s, allowing access to approximately 3.5 m of exposed soil. The second site, Valenzuela (23.06º S, 69.85º W), is an extensive fluvial terrace with a north to south paleo-flow direction (Figure 3). At these first two sites, a number of soil test pits were excavated as the mining industry searched for high-grade nitrate deposits. These pits range from ~1 - 3.5 m in depth, and one of these was sampled at both the lower fan surface of Oficina Ercilla and at Valenzuela. The surface at Oficina Ercilla is sparsely covered with granitic boulders up to 0.9 m in diameter, and the surface at Valenzuela is littered with ventifacted andesite boulders up to nearly 1 m in diameter. The northernmost site, Rencoret NW (22.98º S, 69.82º W), is the location of another abandoned nitrate operation and also is an alluvial fan deposit (Figure 3). The exposed soil profile sampled at this locality is approximately 1.85 m deep.

3. METHODS

3.1 Sample Collection Soil profiles were described and sampled at three localities situated ~5 to10 km apart along a longitudinal transect to the north of the Baquedano nitrate district. This region was selected to examine soil development in the hyper-arid core of the Atacama because the area is largely undisturbed by nitrate mining, yet there are a few nitrate mines with exposures to allow for description and sampling of soils. Also, initial field observations by J. Rech in 2003 identified well-developed patterned ground and large vertical salt fractures indicative of a dry, old, and stable landscape surface. Chemical analysis of samples collected on this initial survey identified high concentrations of soil nitrate (>5 %) with positive ∆17O values, supporting field observations. A field team consisting of the author, J. Rech, and three soil scientists (B. Buck, M. Howell, and A. Brock) from the University of Nevada returned in October of 2005 to sample and describe soils along this transect. At Oficina Ercilla, samples were collected both from a ~3.18 m exposed nitrate mining outcrop (OE 1) and from within a nitrate test pit, ~3.35 m deep (OE 2). Valenzuela soil samples were also collected from within a ~3.05 m deep test pit. Rencoret NW samples were collected solely from a ~1.85 m exposed profile. At all sites, samples were collected from each horizon within the overlying unconsolidated soil (~30 cm depth), both within vertical fractures and in the surrounding peds. Below 30 cm, heavily cemented soil samples were cut from the profile with a portable generator- powered diamond-tipped circular saw at sampling intervals of ~25-50 cm. Care was taken to sample both within vertical fractures and in the undisturbed soil. Surficial soil horizons were not sampled at OE 2, as they are believed to be similar to that of the upper fan surface. Vertical fractures also were not sampled. Soils were described initially in the field according to the guidelines of Soil Taxonomy by B. Buck. Samples were collected

5 for cosmogenic exposure age dating from large granitic boulders at OE 2 and from andesite boulders at Valenzuela. Samples with the least evidence of erosion were chosen for sampling and boulder orientation was accounted for.

3.2 Analytical Methods Approximately 20-40 g of each soil sample was crushed and homogenized using a SPEX 8000 Mixer/Mill for geochemical analysis. Percent salt was determined by weighing ~200 mg of sample into 50 mL centrifuge tubes and filling the tubes with Millipore water. Samples were then placed in an ultrasonic bath for one hour, centrifuged for 15 minutes, and decanted. This process was repeated three times. Afterwards samples were placed in a drying oven overnight and the mass difference yielded the percent salt. Every tenth sample was run in triplicate to assess precision. The average standard deviation of triplicates is 2.36 %. The same process was used for determining the chemistry of soluble salts in soil samples, except that less sample (~50 mg) was required. The decanted solution was placed into 1.5 mL micro-centrifuge tubes and analyzed on a Dionex DX-500 HPLC/ion chromatograph. For anions, an IonPac AS14 analytical column with a 3.5 mM carbonate/1.0mM sodium bicarbonate eluent was used. For cations, an IonPac CS16 analytical column with a 25 mM methane sulfonic acid eluent was used. Measurements were by suppressed conductivity. Standards were run after every seventh sample and yield an average standard deviation of 2.22 ppm. Perchlorate analysis was performed at the University of California by IC-ESI-MS. Crushed and homogenized soil samples (100 mg) were extracted with de-ionized water to 100 mL and were diluted as necessary. Bulk mineralogy was determined on a Scintag XGEN-4000 X-ray diffraction unit for the crushed and homogenized samples and for residuum after salt dissolution. For each profile, samples were analyzed from every soil horizon at roughly a 1 meter interval. Samples were analyzed from 4-60º 2θ at 2º per minute by non-continuous step scanning. Bulk density was determined on salt-cemented soil clods by measuring their mass on a Harvard Trip 1400 series balance and by covering the clods in paraffin to determine their volume by water displacement (Blake and Hartage, 1986). Each sample was analyzed in triplicate, except for eepit-5 and eepit-7, which were too unconsolidated to perform repeat analyses. Granite boulders at OE 2 and andesite boulders at Valenzuela were sampled for cosmogenic nuclide exposure (Figure 4). Samples were processed at the University of Cincinnati. Average sample thickness before preparation was ~3.25 cm. Approximately 500 g of each sample were crushed and sieved to the 250-500 µm fraction. Quartz grains were isolated through a series of acid leaches (HF, HNO3, and HClO4) and LST heavy liquid separation. 10Be was isolated and purified using anion and cation exchange columns. The processed samples were sent to the PRIME Lab at the Purdue University to determine 10Be concentrations via accelerator mass spectrometry (AMS). Percent CaCO3 and C and O isotopic analyses were performed on the fine fraction (< 500 µm) after soluble salts had been removed. Soil samples were dissolved in 150 mL Millipore water overnight, decanted, and the fine fraction was sieved into beakers and allowed to settle overnight. Residual water was decanted and the remaining fines were oven-dried overnight. Samples were then repeatedly rinsed, decanted, and oven-dried. Percent CaCO3 was determined coulometrically at the University of Kentucky on a UIC

6 Inc. coulometer. Carbon and oxygen isotopic values of soil carbonate were analyzed at the University of Arizona. Samples were heated at 150ºC for three hours in vacuo and processed using an automated sample preparation device (Kiel III) attached directly to a Finnigan MAT 252 mass spectrometer. Values were normalized to NBS-19 based on internal lab standards and precision of repeated standards is ± 0.1 ‰ for δ18O and ± 0.06 ‰ for δ13C (1σ). Sulfur isotopic values were determined for sulfate minerals. Approximately 400 mg of homogenized sample was placed in a 50 mL centrifuge tube with 2 N HCl. The tubes were bathed twice in an ultrasonic bath for one hour, centrifuged, and decanted into new 50 mL centrifuge tubes. 300-400 mg of BaCl was added and allowed to react overnight. The solution was centrifuged, decanted, and the BaSO4 precipitate was rinsed twice and dried. Samples were then sent to the University of Arizona and δ34S values were measured on SO2 gas in a ThermoQuest Finnigan Delta PlusXL continuous-flow

gas-ratio mass spectrometer. Solid BaSO4 samples were combusted at 1030 ºC with O2 and V2O5 using a Costech elemental analyzer coupled to the mass spectrometer. Standardization is based on international standards OSG-1 and NBS123, and several other sulfide and sulfate materials that have been compared between laboratories. Precision is ± 0.15‰, based on repeated internal standards.

4. RESULTS

4.1 Cosmogenic Nuclide Exposure Age Dates Nine boulders from atop the landscape surface were sampled for in situ cosmogenic 10Be concentrations in quartz grains in order to constrain exposure ages. Samples were collected at two sites: 1) five granitic boulders from the lower fan surface at Oficina Ercilla (OE 2), and 2) four andesitic boulders at Valenzuela (Figure 4 a-d). Average boulder diameters were ~0.85 m and ~0.5 m, for samples at OE 2 and Valenzuela, respectively. Of the nine samples, only the five granitic samples produced enough quartz for analysis. Using the scaling factor employed by Lal (1991) and Stone (2000), samples EEC1-EEC5 yield exposure ages of ~2.9 Ma, 2.8 Ma, 1.9 Ma, 3.2 Ma, and 2.7 Ma, respectively (Table 2; Figure 5). This produces an average minimum age of ~2.7 Ma for the soil profile at OE 2, assuming no prior inheritance of cosmogenic nuclides.

4.2 Soil Morphology The alluvial and fluvial soils along the transect consist of silt, sand, and gravel intermixed with a variety soluble saline minerals. The upper ~15-25 cm at each site consists of a set of unconsolidated soil horizons that overlie an additional 1.6-3.2 m of heavily-cemented soil horizons (Figure 3; Figure 6). The topmost horizon (Avyz, or Avz at Valenzuela) is a thin layer of dust and gravel that is weakly cemented. Below this is a Byz horizon ~9-17 cm thick consisting of prismatic peds and small vertical fractures that have been in-filled by eolian dust. At Oficina Ercilla and Rencoret NW, this horizon is underlain by a thin (~4-7 cm thick) Bz horizon that is white and crystalline in appearance. This layer is absent at Valenzuela. The lower, consolidated horizons are much less consistent (Figure 6). In general, each horizon is massive, consisting of a silt to gravel sized matrix indurated by salts.

7 However, large (up to ~17 cm wide) vertical fractures commonly intrude into these lower horizons, to depths of ~2 m. These horizons include: Bzm and Bzkm (OE 1); Byzm and Bzm (OE 2); Bym, Byzm, and Bzm (Valenzuela); and Byzm, Byzkm, and Bzm (Rencoret NW) (Figure 6). The average soil bulk density (ρbulk) from all sites is 2.29 g/cm³ with a standard deviation of 0.18. (Table 2; Figure 7). At Oficina Ercilla, the exposed soil profile on the upper alluvial fan surface (OE 1) has an average ρbulk of 2.38 g/cm³ and ranges from 1.99–2.82 g/cm³, while the pit residing on the lower fan surface (OE 2) has an average ρbulk of 2.29 g/cm³ with a range of 2.06–2.51 g/cm³, trending towards increased values with depth. Valenzuela has a range of ρbulk values from 1.9–2.63 g/cm³ and an average of 2.19 g/cm³. At Rencoret NW bulk density ranges between 1.79–2.50 g/cm³ with an average of 2.15 g/cm³, the lowest of the three sites. Most typical soils have a range of bulk densities between 1.00 and 2.00 g/cm³, with heavily compacted soils often in excess of 1.6 g/cm³. At these sites, soil void space has been in-filled with secondary salts (~2.1 g/cm³) and eolian siliclastic material (~2.6 g/cm³).

4.3 Mineralogy Soil mineralogy was analyzed on bulk soil samples and on soil residuum after salt dissolution. For each site, five to six samples were analyzed within specific soil horizons and at roughly 1 m intervals. Each profile is quartz and feldspar-rich, with variable amounts of secondary salts (Table 3). The unconsolidated topsoil horizons are a complex mixture of materials but generally contain more sulfate minerals (gypsum, anhydrite, thenardite, and glauberite) than are present in the lower, consolidated horizons. This is especially clear in the Bz horizon at both OE 1 and Rencoret NW, which is a nearly pure, thenardite layer (Table 3; Figure 6). This thin (~4-7 cm) set of horizontal lamina cap the lower, consolidated horizons. Below this boundary, and salts are more abundant, with the exception of Valenzuela, which contains elevated levels of gypsum and anhydrite at multiple depths within the profile, and additionally has bloedite, glauberite, and darapskite at the base of the profile. is present (in greater than trace amounts) only within the lower soil horizons at OE 1 and Rencoret NW (Table 3; Figure 6).

4.4 Geochemistry This section discusses the geochemical characteristics of the soluble soil salts at each site, including trends in percent salt and the concentrations of major anions and cations of dissolved salts with depth.

4.4.1 Oficina Ercilla OE 1 has an average percent salt of 40 % (Table 2; Figure 8). The greatest variation is within the upper, unconsolidated soil horizons, which range from 13 % in the surface Avyz horizon to 99 % at the boundary between unconsolidated and heavily cemented horizons. In the lower horizons, percent salt ranges from 18 % to 54 %, with a general trend towards lower percentages at depth. Fracture samples have variable salt percentages; though tend to range between 25 and 50 %. Sulfate has the highest average weight percent at this locality (7.94 %), followed closely by nitrate (7.67 %), chloride (5.35 %), and carbonate (1.34 %) (Table 4; Figure 9). Sulfate is minimal on the surface in

8 the Avyz, but peaks with a concentration of 71.08 % in the Bz horizon. Concentrations drop and remain low (~3.82 %) in the lower horizons, with the exception of vertical fracture samples at 2.5 m depth, which reach ~23.88 %. Nitrate is noticeably low or absent in the surficial soil horizons, but has higher concentrations at depth (Figure 9). Nitrate concentrations reach as high as 12–16 % at depths of 0.75, 1.53, and 3 m, with lower concentrations (<6 %) elsewhere. Fracture samples contain higher concentrations of nitrate than bulk soil, with the exception of one sample at 2.5 m. Chloride has a consistent trend with nitrate. Carbonate is present at depth, with concentrations increasing to 3.72 % from just below the Bz to ~3 m depth. This site also contains low concentrations of perchlorate (0.02 % on average) and trace amounts of phosphate. Dominant cations include sodium (7.99 %), calcium (1.91 %), and lesser amounts of (Mg+2) and potassium (K+) (Table 4; Figure 9). Calcium follows a similar trend with depth as sulfate, with a maximum weight percent of 10.14 in the Bz horizon. Sodium tends to follow the same trend as nitrate and chloride, however, it reaches a maximum concentration (29.55 %) more shallowly within the Bz, which is consistent with both sulfate and calcium. OE 2 has an average percent salt of 28 % in the lower, consolidated horizons, with a range of 18–44 %, and also trends towards lower percentages with depth (Table 2; -2 - - Figure 8). Dominant anions are SO4 (7.02 wt. %), NO3 (4.48 %), and Cl (4.19 %), respectively (Table 4; Figure 10). Sulfate concentrations range between 3.11–9.16 % between the various Bzm and Byzm horizons. Nitrate reaches a maximum concentration of 11.71 % at 0.825 m depth and decreases below. Chloride concentrations are highest in the shallowest Byzm horizon (9.59 %), decrease with depth, and increase slightly at the base of the profile. Phosphate, perchlorate, and carbonate are present in trace amounts. Dominant cations Na+ and Ca+2 have average weight percent values of 4.62 and 1.43 %, respectively. Calcium concentrations are variable, with depth concentrations not matching any particular anion with consistency. Sodium is consistent with chloride throughout the profile.

4.4.2 Valenzuela The soil pit at Valenzuela has an average percent salt of 26 % (Table 2; Figure 8). Although there is little surficial salt variation observed at any given depth, there is a trend towards higher salt percentages with depth (11 % in the Avz to 45 % in the deepest Bzm -2 - horizon). The dominant anions at Valenzuela are SO4 (9.35 average wt. %), NO3 (2.95 %), and Cl- (2.21 %) (Table 4; Figure 11). Sulfate is minimal in the Avz, but reaches a high concentration of 21.83 % in the Byz. Concentrations decrease in the lower, consolidated soil horizons, with the exception of a small increase near 1.5 m depth and a maximum concentration at the base of the profile (24.33 %). Vertical fracture samples show lower sulfate concentrations than bulk soil (~4.6 % on average). Both nitrate and chloride concentrations are minimal near the surface and reach high concentrations at ~0.93 m depth (10.62 and 6.11 %, respectively). Below this depth, concentrations decrease slightly, fluctuating down-profile. Vertical fracture samples contain elevated concentrations at 0.45 m depth, compared to the bulk soil, but contain nearly no nitrate or chloride salts at other depths. Phosphate and perchlorate are present in trace amounts, and carbonate is present but in low quantities (≤0.66 %). Dominant cations are Na+ (3.31 %) and Ca+2 (2.94 %) (Table 4; Figure 11). Calcium has a similar trend to that of sulfate

9 except at the base of the profile, where calcium remains low. Sodium follows a consistently similar trend to that of nitrate and chloride, except for a peak of 7.96 % at the base of the profile.

4.4.3 Rencoret NW Percent salt at the exposed soil profile of Rencoret NW ranges 9–81 % and averages 30 %, with no trend with depth (Table 2; Figure 8). Only 1.85 m of soil is exposed at this site, so the geochemistry below this depth cannot be compared to the -2 - - -2 other sites. Dominant anions SO4 , NO3 , Cl , and CO3 have average weight percentages of 7.02, 4.48, 4.19, and 1.17, respectively (Table 4; Figure 12). Sulfate concentrations are low in the Avz, increases to 56.45 % in the Bz at the boundary between the unconsolidated and well-cemented soil horizons, and drops to concentrations below 20 % in the lower profile. Vertical fracture samples fluctuate near the values of the average bulk soil. Nitrate is minimal in the first 0.4 m, but increases to 4.46 % by 0.725 m depth. Concentrations then decrease until ~1.2 m before increasing to 4.63 % near the base of the profile. Vertical fracture samples have similar concentrations as average bulk soil. Chloride mimics nitrate at the surface, but not lower in the profile. Concentrations are highest (17.71 %) just below the Bz, then steadily decrease with depth. Carbonate reaches a maximum concentration of 5.71 % at 1.2 m depth. Phosphate and perchlorate are present, but only in trace amounts. Dominant cations are Na+ (8.61 %) and Ca+2 (2.01 %) (Table 4; Figure 12). Calcium reaches a maximum concentration in the Bz horizon (3.38 %), then decreases down-profile, with a minor increase at 0.725 m. Concentrations are significantly higher at 0.56 m from a sample taken from the center of a 17 cm-wide vertical fracture, which extends downward from the surface. Sodium has a trend that mimics chloride throughout the profile.

4.4.4 Summary The average percentage of salt from all three sites is 34.15 % including samples from within fractures. The average salt percentage from OE 1, OE 2, Valenzuela, and Rencoret NW are 40 %, 28 %, 26 %, and 30 %, respectively (Figure 8). Both profiles at Oficina Ercilla display decreasing trends with depth in the lower, consolidated horizons, with variable but elevated levels of salt in the unconsolidated topsoil. Valenzuela has an opposite trend, with the highest percentage of salt deep in the profile. Rencoret NW displays no trend with depth. -2 - - -2 At each site SO4 , NO3 , Cl , and CO3 are the dominant anions, from most - abundant to least abundant, except at OE 1, where NO3 is most dominant. The dominant cations at each site are Na+ and Ca+2. Sulfate is highest at Valenzuela with a total inventory of 785.61 kg for a 1 x 1 x 3.05 m soil profile (Table 5). OE 1 contains the highest total amounts of nitrate (613.31 kg), chloride (416.97 kg), and carbonate (134.7 kg). OE 2 and Rencoret NW have medial amounts of each major anion, although Rencoret NW contains significant amounts of carbonate (84.3 kg). At all sites, + -2 concentrations of Ca and SO4 appear to fluctuate together with depth. Sulfate minerals do not appear to be as abundant below the upper, unconsolidated horizons, with the exception of Valenzuela, which has high concentrations of sodium sulfate minerals (bloedite, glauberite, and darapskite) at the base of the profile (Table 4; Figure 11). In the lower, consolidated horizons at all sites, halite and nitrate minerals are most abundant.

10 + - -2 Trends are generally consistent between Na , Cl , and NO3 , with concentrations reaching their peaks between ~0.5-1.25 m depth and fluctuating to lesser amounts -2 towards the base of the profiles. CO3 is present in large amounts at OE 1 and Rencoret -2 NW, but occurs only in trace amounts at OE 2 and Valenzuela. At both sites, CO3 has low concentrations near the surface and trends towards higher concentrations at depth.

4.5 Isotope Systems Carbon and oxygen isotopic values were determined on fine grained (< 500 µm) soil residuum after dissolution of soluble salts. Therefore, any small detrital limestone fragments would also be included in this analysis. Visual inspection of all lithic clasts failed to identify any limestone. However, hydrothermal calcite was identified in a few samples from Oficina Ercilla. The δ13C and δ18O values for one sample of hydrothermal calcite were determined to be -8.34 ‰ and -19.62 ‰, respectively. Therefore, any potential inclusion of hydrothermal calcite in our bulk soil carbonate samples would have been easily identifiable by significantly more negative δ18O values than adjacent soil carbonate. Sulfur (δ34S) isotopic values were determined for bulk soil sulfates. The isotopic signatures and trends with depth from each site are discussed in the following sections.

4.5.1 δ13C and δ 18O of Carbonate Soil carbonate is present at depth at OE 1 (134.70 kg) and Rencoret NW (84.3 kg) but occurs in only trace amounts at OE 2 (1.61 kg) and Valenzuela (4.09 kg) (Table 5). As such, no data were obtained for OE 2 or the lower, consolidated horizons at Valenzuela. The data is consistent between sites for δ13C in the lower consolidated horizons (-3.6 ‰, on average), with a range of -3.41 to -3.92 ‰ (except for sample, rnw5, that is slightly lower at -4.92 ‰) (Table 6; Figure 13). Carbonate from the unconsolidated, surficial horizons have more positive values and greater scatter than carbonate from lower in the soil profile. At Rencoret NW, one bulk soil sample (rnw t.s.p.-1) and one small vertical fracture sample have δ13C values of -1.81 and -0.03 ‰, respectively. Two bulk surface soil samples at OE 1 (ee t.s.p.-1 and ee t.s.p.-3) have δ13C values of +1.13 ‰ and +1.25 ‰, respectively. Finally, the topsoil at Valenzuela include two bulk soil samples (v t.s.p.-1 and v t.s.p.-3), and one small fracture sample (v t.s.p.-2). Here, soil δ13C values are +3.21, +2.97, and +2.61 ‰, respectively (Figure 13). Soil carbonate δ18O values do not share the same inter-site consistency and trends observed in the δ13C values. Instead, there is a site-specific consistency in the lower, consolidated horizons (Figure 14). Samples at OE 1 display a scatter of <3 ‰ (+0.42 ‰, on average) with a range of -1.06 to +1.82 ‰ (Table 6; Figure 14). Values at Rencoret NW are more consistent, ranging from -5.83 to -5.36 ‰ (-4.96 ‰, on average). The relationships between δ18O values in the topsoil samples and the deeper samples are more complex than those observed in the δ13C trends. Topsoil δ18O values fall between the average values in the deeper soil at OE 1 and Rencoret NW, ranging from -4.09 to -3.02 ‰ at Rencoret NW, -3.29 to -3.05 ‰ at OE 1, and -2.43 to -0.94 ‰ at Valenzuela (Figure 14). Soil carbonate δ13C and δ18O values within the lower consolidated soils at each site are extremely consistent, and vary noticeably from that of the topsoil. However, the trends are not the same between δ13C and δ18O. Topsoil samples display more positive

11 δ13C values than that of the deeper soils (Figure 15). The δ18O values of the topsoil interestingly lie between the more negative values in the deeper soil at Rencoret NW and the more positive values in the deeper soil at OE 1.

4.5.2 δ34S of Sulfates At all sites, δ34S values are more positive near the top of the soil profiles and trend towards more negative values near the base of the profiles. The range in values is fairly consistent at sites OE 1, OE 2, and Rencoret NW (+3.8 to +8.0 ‰) (Table 6; Figure 16). However, δ34S values from Valenzuela are more positive (+6.9 to 11.8 ‰). At each site vertical fracture samples have similar values as bulk soil. The R² values for the positive trend in δ34S values with depth are 0.5596, 0.7241, 0.831, and 0.4998 for OE 1, OE 2, Valenzuela, and Rencoret NW, respectively (Figure 16).

5. DISCUSSION

5.1 Soil Characterization 5.1.1 General Geochemical Trends The observed soils correspond only slightly to the generalized nitrate soil profile (Ericksen, 1981). The unconsolidated, surficial horizons correspond to Ericksen’s chusca; consisting of up to 25 cm of powdery sand, silt, and rock fragments intermixed with high amounts of sulfate minerals, and with a lower boundary horizon (Bz) of friable thenardite. Below these horizons are extremely well-cemented soil horizons that contain variable amounts of more soluble salts such as nitratine and halite, while sulfate mineral concentrations generally decrease with depth. At each site, nitrate, chloride, and sodium ions tend to fluctuate together down profile (Figures 7–10). Concentrations are generally higher just below the boundary between the unconsolidated topsoil and lower, well- cemented horizons, and then decrease with depth. Nitratine and halite do not appear to occur in veins or as textural boundary accumulations as has been observed in previous studies (Ericksen, 1981, Ewing et al., 2006). While sand, silt, and clay content do vary in these alluvial soils, this does not appear to have significantly hindered the downward migration of soluble saline minerals. The downward migration of saline material appears to have been hindered by the extreme cementation of the lower horizons. This has caused a thin sulfate accumulation layer (Bz) that is visible at Oficina Ercilla and Rencoret NW. Small, infrequent rainfall -2 events have likely allowed for dissolution and re-precipitation of SO4 minerals of variable hydration, including gypsum, and specifically anhydrite and thenardite in the Bz horizon (Ewing et al., 2006). The presence of anhydrite is a testament to the extreme aridity of the region and is likely the product of the dehydration of gypsum over time. Rare, naturally occurring perchlorate is present at each site, though in trace amounts, with the exception of OE 1 (Table 4). It is uncertain what the exact source mechanism is for the creation of naturally occurring perchlorate. An atmospheric reaction between chlorine gas and ozone, and subsequent deposition has been suggested as the method of creating natural perchlorate (Ericksen, 1981; Erickson, 2004; Orris, G. J., 2005). Others have suggested that formation of natural perchlorate under strongly oxidizing conditions is possible through evaporation and production of ozone during electrical storms (Longmire, 2005). Positive ∆17O anomalies, indicating an atmospheric

12 source in a handful of perchlorate samples from the Atacama Desert seem to support these theories (Bao et al., 2004; Erickson, 2004). Regardless, perchlorate has consistently been detected at a number of sites within the Central Desert (Ericksen, 1981; Erickson, 2004). Though carbonate is a common soil mineral in arid settings, it is absent or present only in low concentrations in most Atacama Desert soils (Quade et al., 2007). Carbonate +2 formation is dependent upon Ca concentrations, pH, and CO2 partial pressures (PCO2) (Butler, 1982; Cerling and Quade, 1993; Amundson, 2004). Though calcium content is plentiful, soil pH values are relatively neutral (Ewing et al., 2006), and soil PCO2 levels are likely near atmospheric, due to low organic carbon content and the lack of life (Ewing et al., 2006). Carbonate formation therefore is currently unfavorable. The presence of carbonate lower in the profiles at both Oficina Ercilla and Rencoret NW (Table 4) is likely relict from less arid, past climatic conditions.

5.1.2 Site-specific Geochemical Trends The heavily saline-cemented soils in our study area have significantly higher bulk densities (2.29 g/cm³, on average) than regular soils (typically 1-2 g/cm³), which is the result of a mixture of the high percent salt content and igneous lithics (Timm et al., 2005). OE 1 has the highest average bulk density (2.38 g/cm³) and highest percent salt (40 %) of all the profiles (Table 2). Also, at this site nitrate, chloride, and carbonate are more abundant than at all of the other profiles. The high bulk density is reflective of the extreme salt-cementation of the lower soil horizons as well as andesite as the dominant intermixed lithic. Percent salt in the upper, unconsolidated soil horizons is extremely variable (13 % in the Avyz, to 99 % in the Bz) but trend towards lower amounts in the lower, consolidated horizons (Figure 8). The observed geochemical trends and mineralogy at OE 1 indicate an abundance of sulfate minerals such as gypsum (CaSO4·2H2O), anhydrite (CaSO4), and thenardite (Na2SO4) in the topsoil, transitioning to an abundance of halite (NaCl), calcite (CaCO3), and nitratine (NaNO3) in the lower soil horizons (Figure 9). At Oficina Ercilla’s lower fan surface (OE 2) the unconsolidated topsoil was not sampled as the profile is close enough to the upper fan surface that the material overlying the well cemented horizons should be similar. Here, the average percent salt (28 %) is significantly lower than at OE 1 (Table 2). Sulfate concentrations range between 3.11– 9.16 wt. % and are consistent with the lower levels observed at OE 1. However, the total nitrate inventory at OE 2 (368.28 kg) is only slightly more than half that observed at OE 1 (613.31 kg), and carbonate is only present in very low amounts (1.61 kg) (Table 5). Sodium trends are consistent with chloride throughout the profile, indicating the presence of halite (Table 4). This site has a slightly lower bulk density (2.29 g/cm³) as well, though this is still higher than at Valenzuela and Rencoret NW, likely due to the presence of granite as the dominant intermixed lithic. The average percent salt (26 %) at the soil pit at Valenzuela is the lowest of the three sites (Table 2). The variation in the unconsolidated topsoil observed at Oficina Ercilla is not present at this site, and there is an opposite trend toward higher percent salt with depth (11 % in the Avz, to 45 % in the deepest Bzm horizon). Here, the total inventories of major anions are significantly lower than the other sites with the exception -2 of SO4 (785.61 kg), which is the highest of all sites (Table 5). Also, the diagnostic Bz

13 horizon is absent at this site. The upper, unconsolidated horizons instead transition into a moderately well-cemented Bym horizon in which sulfate concentrations reach a high concentration of 21.83 wt. % (Table 4; Figure 11). Sulfate concentrations then decrease with depth, with the exception of a small increase near 1.5 m depth (12.78 %), and a maximum concentration of 24.33 % at the base of the profile. Also in the lower horizons, sodium follows a consistently similar trend to that of nitrate and chloride, except for an increase to a maximum concentration of 7.96 wt. % at the base of the profile (Table 4; Figure 11). This is consistent with the aforementioned basal peak in sulfate, and another in magnesium which indicates the presence of bloedite (Na2Mg(SO4)2 · 4H2O) and glauberite (Na2Ca(SO4)2). These minerals at the base of the Valenzuela soil pit are indicative of dissolved sulfate that has mixed with Na+ (and Mg+2) from throughout the profile, which then re-precipitated lower in the profile. Rencoret NW has the lowest bulk density (2.15 g/cm³) and the second highest percent salt (30 %) of all the sites (Table 2). The diagnostic Bz horizon is present -2 (81 % salt; 56.33 wt. % SO4 ), however, there is no other noticeable trend between percent salt and depth, as at other sites (Figure 8). Here, total inventories of major anions are the lowest of all sites, with the interesting exception of carbonate (84.30 kg). It is likely that this is relict pedogenic carbonate, the formation of which may have been strongly influenced by the parent material at Rencoret NW. At ~1 m depth (Byzkm) this profile contains significant amounts of large, well-rounded cobbles (1-13 cm in diameter). Carbonate may have formed on the underside of these clasts (stage I and II carbonate) during a less arid climate in the past when formational conditions would have been more optimal. This material could have then been dispersed slightly through the lower profile from subsequent dissolution/precipitation reactions.

5.2 Age Assessments 5.2.1 Relative Age Assessments Many of the differences in the nitrate soil profiles discussed above are likely the result of differences in age of the soils, as well as possible influence of site formation processes. Several factors suggest that the upper fan surface at Oficina Ercilla is the oldest soil profile from our transect. Large inventories of soil anions, especially those with slower deposition rates, are typically reflective of older, more mature soils. OE 1 has the highest percent salt (40 %), bulk density (2.38 g/cm³), nitrate content (613.31 kg), chloride content (416.97 kg), and carbonate content (134.7 kg) indicating that it is the oldest and most mature soil site (Table 2; Table 5). Meanwhile, OE 2 has significantly lower anion inventories, as well as a lower bulk density (2.29 g/cm3) and percent salt (28 %), indicating the lower fan surface is younger and less developed than the upper surface. The soil profile at Valenzuela is less well cemented and saline minerals are much more variable with depth than at the other sites. The absence of the Bz horizon, as well as the low percent salt (26 %), medial bulk density (2.19 g/cm3), and lower nitrate, chloride, and carbonate inventories indicate that this profile is less well developed than both profiles at Oficina Ercilla, and likely much younger. Rencoret NW has a lower total inventory in each major anion except for carbonate (84.3 kg), indicating that it is also a younger soil (Table 5). This profile has a higher average percent salt (34 %) but the lowest average bulk density (2.15 g/cm3) of all the sites (Table 2). The alluvial parent material is grain-

14 supported by abundant well-rounded cobbles, rather than matrix supported like the other sites. This has likely affected the degree of salt cementation at this site.

5.2.2 Empirical Age Assessments Determining the age of soils in the Atacama Desert has been a constant challenge to scientists. In the absence of widespread and readily dateable material (e.g. volcanic ash or fossil remains), researchers must turn to other methods. Cosmogenic nuclide analysis has emerged as an alternate means of dating landscapes. This method is useful in a hyper- arid environment such as the Atacama, where weathering losses are negligible (as evidenced by the distribution of salts with depth), and landscape surfaces have remained remarkably stable. Researchers do not currently agree on a preeminent scaling factor to account for spallation reactions during the accumulation of cosmogenic nuclides in terrestrial materials. As such, this study will focus on the scaling factor from Lal (1991)/Stone (2000), as it has been used in other previous studies in the Atacama. However, to facilitate comparison, the results from scaling factors of Desilets et al., 2006, and Liften et al., 2005) are also presented (Table 1). The granitic samples from the surface of OE 2 yield average ages of 2.71 ± 0.56 Ma, 2.91 ± 0.72 Ma or 3.23 ± 1.02 Ma, using the scaling factors of Lal (1991)/Stone (2000), Lifton et al., 2005, and Desilets et al., 2006, - respectively (Table 1). The soil profile OE 2 contains a total NO3 inventory of 384.77 kg - 2 over a thickness of 3.35 m (Table 5). A natural NO3 flux rate of 0.041 g/m •yr is - obtained via the age of OE 2 and total NO3 accumulation at that site. If a constant flux - rate across the desert is assumed, this rate and the total NO3 inventories at each site yield ages of 4.76, 1.68, and 1.47, for OE 1, Valenzuela, and Rencoret NW, respectively - (Figure 5). A linear relationship between age and total NO3 accumulation is assumed for the purpose of this study in order to designate the ages of the other sites. However, these are likely minimum ages since nitrate accumulation rates in soils likely decrease with age as each soil becomes progressively more cemented with soluble salts. The calculated ages correspond well to the aforementioned relative age assumptions (see Relative age assessments, Section 5.2.1). These young ages are also supported by Nishiizumi et al. (1998) in which cosmogenic surface exposure ages of 3-4 Ma were derived for gravels and cobbles on an alluvial fan in the Central Atacama Their study also reports low erosion rates of >0.1 m/Myr, further evidencing the applicability of incorporating cosmogenic dating methods.

5.3 Natural Flux Rates Soils forming within a hyper-arid environment undergo little, if any silicate weathering and reduction of nutrients or major elements. Instead they tend to accumulate atmospheric particulates and dusts, resulting in volumetric expansion (Ewing et al., 2006). Soils within the study area have high concentrations of saline minerals, not only of nitrate, but also sulfates, chlorides, and perchlorates. Quantification of the age, bulk density, and the total inventory of the dominant ions throughout the thickness of a soil profile allows for a determination of the natural flux rates of the various ions. The oldest site (OE 1) has a minimum age of 4.76 Ma, a bulk density of 2.38 g/cm³, 3.18 m profile depth, and large ion inventories of 570.36 kg, 416.97 kg, 1.89 kg, 620.92 kg, and 141.66 -2 - - + +2 kg for SO4 , Cl , ClO4 , Na , and Ca , respectively (Table 2; Table 5). This data yields

15 2 -2 2 - 2 natural flux rates of 0.03769 g/m •yr (SO4 ), 0.02756 g/m •yr (Cl ), 0.0001 g/m •yr - 2 + 2 +2 (ClO4 ), 0.04103 g/m •yr (Na ), and 0.0094 g/m •yr (Ca ).

5.4 Isotopes 5.4.1 δ13C and δ18O Carbon and oxygen isotopes (δ13C and δ18O) are commonly used as a proxy for paleoclimate within soils and paleosols. The carbon isotope system is largely controlled by CO2 in soil gases, affected by atmospheric CO2 and the proportion of C3, CAM, or C4 plants living in the soil (Cerling and Quade, 1993). However, the Baquedano Valley lies within the absolute desert, where biological activity is absent or negligible today. Thus soil carbonate should show a large influence from atmospheric CO2. Quade et al. (2007) determined δ13C values of soil carbonates across the Atacama that are the highest of any desert in the world ranging from -8.2 to +7.9 ‰ (VPDB) from the wettest sites to the driest. Our observed δ13C values (-4.92 to +3.21 ‰) fall within this range, and are consistent at depth (Table 6). The oxygen isotope system is directly related to the isotopic composition of meteoric water, the amount of evaporation, and temperature during precipitation (Cerling and Quade, 1993). Possible sources of meteoric water include the camanchaca, or coastal fog (though it is mostly blocked from the Central Valley of the Atacama by the Coastal Cordillera) and infrequent, seasonal Andean precipitation. The extreme aridity of the Atacama would cause a large amount of evaporative enrichment in the δ18O values of soil carbonate as well. The boundary between the topsoil and deeper soil separates carbonate derived from different source processes. The deeper soil is likely relict pedogenic carbonate formed during a slightly less arid climate, while the carbonate in the surficial soils is likely eolian detrital carbonate and widespread in origin. From the δ13C values alone, one may argue that it is the same carbonate throughout the profiles, with diffusion processes causing the topsoil to have more positive δ13C values (Figure 13). However, the δ18O values do not support this hypothesis, as values of the topsoil are more negative than those of the deeper soil (Figure 14). If all carbonate was pedogenic, one would expect to see more positive values of δ18O due to evaporative enrichment of soil water near the surface. Therefore, the carbonate analyzed in the topsoil must be from a different source, and likely multiple sources, from the range of values between sites (Table 6; Figure 15). Non-soil carbonate data from surficial samples in Quade et al., (2007) show a similar range of δ13C and δ18O values, further supporting this conclusion. The deeper soil horizons at OE 1 and Rencoret NW each have very consistent δ13C values, averaging ~-3.6 ‰ (Table 6; Figure 15). This remarkable homogeneity indicates that it is relict pedogenic carbonate that formed in situ. The δ13C composition of soil carbonate is largely affected by that of atmospheric CO2, the variety of plant life, and from soil respiration rates (Cerling and Quade, 1993). Today there is no plant life in the Central Desert and soil respiration rates are very low (0.3-0.4 mm/m2•hr), but this may not have been the case in the past, when soil carbonate was forming. In the absence of plant life in the soil, one would expect to see carbonate that formed in equilibrium with atmospheric CO2, which would yield values of ~+4.1 ‰ (Quade et al., 2007). The more negative observed δ13C values indicate that the carbonate formed in slightly less arid conditions, with at least some vascular plant life present. Carbonate forming in the presence of plants undergoes a phase transformation fractionation of ~+10 to 12 ‰ from

16 soil CO2 to soil carbonate. Diffusion processes between atmospheric CO2 also occurs, causing an enrichment of ~+4.4 ‰ (Cerling, 1984). Differences in metabolism between 13 plant species (i.e. C3, CAM, or C4) creates varying δ C values in the soil CO2 which is then used to create soil carbonate. CAM plants (such as cacti) use a mixture of C3 and C4 metabolic pathways and produce soil CO2 of ~-19 ‰ (Clark and Fritz, 1997). After phase transformation and diffusion processes, the δ13C soil carbonate values resulting from a CAM-dominated landscape would be in the range of -5 to -2 ‰. Our observed data fall within this range of values (Table 6; Figure 15). As previously discussed, the difference in soil chemistry and degree of development between OE 1 and Rencoret NW suggest, a difference in age between the sites. OE 1 has large inventories of nitrate, sulfate, chloride, and carbonate, while Rencoret NW has much lower inventories of each, indicating that it is a younger and less mature soil (Table 5; Figure 5). The ~5 ‰ difference in δ18O values of carbonate within the deep soils at OE 1 and Rencoret NW is yet another verification that each profile developed at different times. It is unclear what is causing the observed shift in values. The δ18O composition of soil carbonate is largely dependent on the amount of evaporative enrichment that occurs within the soil water that the carbonate is forming from, and the δ18O composition of local meteoric water (Quade and Cerling, 1993). One possible explanation is that each site received moisture from different sources during each respective period of carbonate formation. Values at Rencoret NW average -5.62 ‰, which reflects values for Andean summer precipitation for a mean elevation of ~1400 m (Quade et al., 2007). However, values at OE 1 average -0.22 ‰, which reflects values for precipitation derived from coastal fog. It is possible that the dominant moisture source at the time soil at OE 1 was forming was from the coastal region, and that the dominant moisture source had transitioned to Andean precipitation by the time the soil at Rencoret NW began to form. A second possibility is that climate was slightly more arid while carbonate at OE 1 was forming, leading to more evaporative enrichment of δ18O in the resultant soil carbonate. The latter hypothesis would be ideal, as it allows for a constant Andean moisture source during the formation of soil at both sites. This hypothesis is also supported by the observed δ34S values which indicate only a minimal marine influence (discussed in the following section). Furthermore, the δ18O values from Quade et al. (2007) suggest that extreme soil dewatering by evaporation occurs at most sites in the Atacama, prior to carbonate formation. However, the ~5 ‰ difference in δ18O values is a very large range to be explained by evaporative enrichment alone. It may be explained by one or a contribution of both hypotheses but at present the answer to this phenomenon remains unclear.

5.4.2 δ34S Previous studies have identified the source of sulfate minerals in Atacama Desert soils as a combination of particulates derived from marine sea spray, atmospheric chemical reactions, and Andean-derived salar salts and volcanic weathering products via well constrained δ34S signatures (Rech et al., 2003, Bao et al., 2004). The relative impact of any given input is largely dependent on location and proximity to specific sources (Rech et al., 2003; Bao et al., 2004). The study area is located in an area of the Central desert where the average elevation is ~1400 m and the Coastal Cordillera is sufficiently high enough to block marine aerosols from having a major influence. At Oficina Ercilla

17 and Rencoret NW the δ34S values are fairly consistent, with values ranging from +3.79 to +8.01 ‰ (Table 6; Figure 16). Values at Valenzuela are slightly more positive with values ranging from +6.97 to +11.84 ‰ and all sites share similar trends toward lower δ34S values at depth. The range of observed values (approximately +4 to +12 ‰) is consistent with that of sulfate derived from Andean groundwater, with a minor influence from marine aerosols, which may account for the more positive values at Valenzuela (Rech et al., 2003). The longitudinal similarity of these sites is such that significant variation in source material is unexpected, and the data largely supports this conclusion. The similarity in values and trends between the differently aged sites indicates that a changing sulfate sources over time cannot explain the negative shift in δ34S values with depth. This conclusion is supported by Ewing et al., 2005. In this study, the ∆17O values of soil sulfates were analyzed from within both arid and hyper-arid Atacama soils and results produced positive, unchanging values with depth, indicating a constant tropospheric source of sulfate. Mass-dependent fractionation from dissolution/precipitation reactions of sulfate minerals down profile may explain the observed range in values. This hypothesis depends on the assumption that the leaching frequency and intensity of sulfate from the topsoil is not greater than the rate of sulfate input (Bao et al., 2004). In this hyper-arid climate, rainfall is sufficiently limited and infrequent that during rare precipitation events fractionation should occur. Regardless, the range of sulfate values at any given depth begs the question as to how different sulfate minerals fractionate. There are various sulfate minerals observed across the transect, including: gypsum (CaSO4•2H2O), anhydrite (CaSO4), thenardite (Na2SO4), glauberite (Na2Ca(SO4)2), bloedite (Na2Mg(SO4)2•4H2O), and kieserite (MgSO4•H2O). For example, at OE 1 there is a range of δ34S values from 5.1 – 7.9 ‰ within the topsoil for samples that have large variations in sulfate mineralogy (Table 6; Figure 16). This variation is likely the result of the dominant sulfate minerals present in the various topsoil horizons. Bloedite is the dominant sulfate mineral in the Avyz and has a δ34S value of 7.2 ‰. Gypsum and anhydrite dominate the Byz and have a δ34S value of 7.9 ‰. A pure thenardite sample from the Bz yielded a δ34S value of 5.1 ‰. At depth, gypsum and anhydrite are the only observed sulfate minerals, and δ34S values decrease to 3.8 ‰ by 2.65 m (Figure 16). It is unclear how different sulfate minerals fractionate in such an environment. A more complete study should be undertaken to assess the importance of δ34S fractionation between different soil sulfate minerals.

6. SOIL DEVELOPMENT IN THE HYPER-ARID ATACAMA DESERT

Soil development in extremely arid regions differs from typical soil development in much of the world. Most soils experience silicate weathering and a loss of nutrients and major elements, which results in volumetric collapse over time. In arid regions however, there is a significant accumulation of both chemical weathering products and atmospheric inputs. Thus arid soils display volumetric expansion over the course of soil development (Ewing et al., 2006). The soil geochemistry in arid soils reflects retention of many soluble minerals that are normally not present, or occur in lower quantities in more humid soils. Nitrate and sulfate minerals in particular are well preserved in the Central Valley.

18 Soils in the Baquedano region appear to have formed as typical alluvial fan and fluvial terrace deposits during a slightly less arid past climatic regime. The carbon isotope data indicates soil carbonate precipitation within a CAM plant-dominated paleo- environment. While CAM-dominant landscapes are not often observed today, they are present in marginal areas of the absolute desert, and likely were more prevalent in the past, before the complete emergence of C4 plants. This study suggests a CAM plant- dominant landscape between at least 4.76 and 1.47 Mya. As the climate transitioned to hyper-aridity, saline minerals from tropospheric and eolian depositional processes began to accumulate within the soil. Eolian sources include salts from salars (dried playas), Andean weathering products, and marine aerosols. Playas receive saline minerals through the capillary action of groundwater, and evaporation causes precipitation of a salt crust. These salts are then blown across the desert by prevailing and local winds (Rech et al., 2003). Today, marine aerosols are largely blocked from this region of the Central Valley by the Coastal Cordillera (Rech et al., 2003). However, they may have been a dominant source in the past. Regardless, previous studies of mass-independent ∆17O anomalies in nitrates, sulfates, and perchlorates have indicated that the accumulation of these saline minerals in arid soils is dominantly attributed to atmospheric photochemical reactions and subsequent deposition (Michalski et al., 2004; Bao et al., 2004). Periodic, but infrequent rainfall has caused dissolution, downward migration, and re-precipitation of - - - -2 -2 soluble salts (NO3 , Cl , ClO4 , and to a lesser extent, SO4 and CO3 ) into the lower soil horizons, slowly filling soil void space until the soil has become extremely well- cemented. The lower, consolidated horizons now act as a somewhat impermeable layer, upon which further downward migration of soluble salts is hindered. Sulfate minerals have a fairly high deposition rate and thus currently dominate the unconsolidated topsoil. The boundary between the lower, consolidated horizons and topsoil often contains a noticeable, thin Bz horizon of nearly pure thenardite (Ericksen, 1981), although this horizon is noticeably absent at Valenzuela. The limited downward redistribution of sulfate minerals appears to coincide with mass-dependent fractionation of lighter sulfate isotopes through dissolution/precipitation reactions, which has caused a trend towards more negative δ34S values at depth (Figure 16).

7. CONCLUSIONS

Deep nitrate soils in the Baquedano nitrate district consist of alluvial and fluvial deposits that have become extremely well-cemented by saline minerals over the course of - - -2 soil development. Specifically, NO3 , Cl , and SO4 minerals, deposited from both atmospheric photochemical and eolian processes have migrated down-profile, infilling soil void spaces through dissolution/precipitation reactions. This has resulted in a thick (1.6-3.2 m) set of massive, salt-indurated soil horizons which are overlain by 15-25 cm of unconsolidated, sulfate-rich topsoil horizons. The boundary between the topsoil and lower, consolidated horizons is marked at Oficina Ercilla and Rencoret NW by a thin (4-7 cm thick) Bz horizon of white, crystalline thenardite. The deeper soil horizons largely consist of nitratine and halite, intermixed with variable concentrations of sulfates, carbonates (OE 1 and Rencoret NW), and perchlorates (OE 1). The extreme cementation of the lower soil horizons is evidenced by the large saline geochemical inventories, high average bulk density, and increased salt percentage of each profile.

19 Unlike soils in more humid climate regimes, soils in a hyper-arid environment such as the Atacama have retained and accumulated deposited materials over time, due to the low to negligible weathering losses of deposited soil nutrients. This retention of soluble saline minerals allows for a unique opportunity to determine the natural flux rate of such minerals onto the soil surface. The natural flux rates of major soil anions and cations can thus be determined for hyper-arid soils by dividing the total ion inventory across the depth of a soil profile by the age of the soil. Cosmogenic nuclide analysis of boulders from the landscape surface at OE 2 yield a minimum age of 2.71 Ma. This age, - - combined with the total NO3 inventory in this profile yields a natural NO3 flux rate of - - -2 - 0.04053 g/m²•yr. OE 1 has the highest total inventories of NO3 , Cl , CO3 , and ClO4 , as well as the highest average percent salt and bulk density of the four soil profiles. These characteristics indicate that is the oldest and most developed soil profile. This conclusion is empirically evidenced by a minimum age of 4.76 Ma determined from the calculated - - natural NO3 flux rate and total NO3 inventory at this site. The nearly homogeneous δ13C values of soil carbonate within the lower, consolidated horizons are indicative of pedogenic carbonate which precipitated under a CAM plant-dominated paleo-environment. Carbon isotopic values within the topsoil are indicative of eolian carbonate dust from various sources. Soil carbonate δ18O values range -5.83 to +1.82 ‰ from Rencoret NW to OE 1, indicative of either 1) drier conditions resulting in extreme evaporative enrichment of δ18O in soil carbonate precipitated at OE 1, 2) changing moisture sources through time, or 3) a combination of both processes. The δ18O values at Rencoret NW correspond to that of Andean-derived precipitation at an elevation of ~1400 m. Although δ18O values at OE 1 are more positive than those at Rencoret NW, it is unlikely that a ~5 ‰ increase is possible simply from more arid conditions (at OE 1) and evaporative enrichment alone. Similarly it is unlikely that moisture sources have changed significantly in the interim between soil formation at both sites to account for the difference in values. I infer a combination of these factors to account for the range of δ18O values. However, the contribution of each remains unclear. The sources of soil sulfate were determined via analysis of the δ34S values of soil sulfate minerals at each site. Observed values are consistent with δ34S signatures of eolian evaporite sulfate derived from Andean groundwater, with a minor influence from marine aerosols. At each site, there is a negative trend in δ34S values with depth. This trend suggests fractionation via dissolution/precipitation reactions of the various sulfate minerals as they migrate through the soil profiles. However, more detailed study is required to verify this hypothesis.

REFERENCES

Alpers, C.N., Brimhall, G.H., (1988) Middle Miocene climatic change in the Atacama desert, northern Chile: evidence from supergene mineralization at La Escondida. Geol. Soc. Am. Bull., v. 100, 1640–1656.

Amundson, R. (2004) Soil Formation. In: Treatise on Geochemistry, H.D. Holland and K.K. Turekian (Eds in Chief). Elsevier Press, Amsterdam, Chap. 5.01, pp. 1-35.

Bao, H., Jenkins, K. A., Khachaturyan, M., and G. C. Díaz (2004) Different sulfate sources and their post-depositional migration in Atacama soils. Earth and Planetary Science Letters, v. 224, 577-587.

Blake, G.R., and Hartage, K.H. (1986) Methods of Soil Analysis: Part 1 – Physical and Mineralogical Methods. SSSA Book Series 5, 371-373.

20 Böhlke, J. K., Ericksen, G. E., and Revesz, K. (1997) Stable isotope evidence for an atmospheric origin of desert nitrate deposits in northern Chile and southern California, U.S.A. Chemical Geology, v. 136, 135-152.

Brimhall, G.H., Chadwick, O.A., Lewis, C.J., Compston, W., Williams, I.S., Danti, K.J., Dietrich, W.E., Power, M.W., Hendricks, D., Bratt, J. (1992) Deformational mass transport and invasive processes in soil evolution. Science, v. 255, 695–702.

Brüggen, J. (1928) La geología de los yacimientos del salitre de Chile y las teorías que tartan de explicar su orgien. Sociedad Nacional de Minería, Boletín Minero, v. 40, 394-406.

Brüggen, J. (1938) El salitre. Sociedad Nacional de Minería, Boletín Minero, v. 50, 737-754.

Burke W. H., Denison R. E., Heatherington E. A., Koepnick R. B., Nelson N. F., and Otto J. B. (1982) Variation of seawater 87Sr/86Sr throughout Phanerozoic time. Geology, v. 10, 516–519.

Butler, J.N. (1982) Carbon Dioxide Equilibria and Their Applications. Addison-Wesley, Reading, MA.

Castleman A. W. Jr., Munkelwitz H. R., and Manowitz B. (1974) Isotopic studies of the sulfur component of the stratospheric aerosol layer. Tellus, v. 26, 222–234.

Cerling, T. E. (1984) The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth Planet Science Letter, v. 71, 229-240.

Cerling, T. E. and Quade, J. (1993) Stable Carbon and Oxygen Isotopes in Soil Carbonates. In Continental Indicators of Climate, Proceedings of Chapman Conference, Jackson Hole, Wyoming, American Geophysical Union Monograph (eds. P. Swart, J. A. McKenzie, K. C. Lohman ), v. 78, pp. 217-231.

Chong G. (1988) The Cenozoic saline deposits of the Chilean Andes between 18° and 27° South. In The southern Central Andes (eds. H. Bahlburg, C. Breitkreuz, and P. Giese), Springer-Verlag, pp. 135–151.

Clark, I. and Fritz, P. (1997) Environmental Isotopes in Hydrogeology. Lewis Publishers, 2nd edition, 382.

Darwin C. (1871) The Journal of a Voyage in H. M. S. Beagle. D. Appleton and Co., London.

De Kalb, C. (1916) Origin of nitrate. Mining and Scientific Press, v. 112, 663-664.

Desilets, D., Zreda, M., Prabu, T. (2006). Extended scaling factors for in situ cosmogenic nuclides: New measurements at low latitude. Earth Planet. Sci. Lett., v. 246, no.3-4, 265-276.

Dong, H., Rech, J.A., Jiang, H., Sun, H., Buck, B.J., (2006) Endolithc cyanobacteria in soil sulfates from hyperarid environments on Earth. Geological Society of America annual meeting, paper no. 216-3.

Dunai, T.J., Lopez, G.A.G., Juez-Larre, J., (2005) Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms. Geology, v. 33, 321–324.

Ericksen, G.E., (1981) Geology and origin of the Chilean nitrate deposits. Geological Society Professional Paper 1188. United States Government Printing Office, Washington.

Ericksen, G. E. (1983) The Chilean Nitrate Deposits. American Scientist, v. 71, 366 - 374.

Ericksen, G. E. and M. E. Mrose (1970) Mineralogical studies of the nitrate deposits of Chile. II. Darapskite, Na3((NO3)(SO4) H2O. The American Mineralogist, v. 55, 1500-1517.

Ericksen, G. E. (1961) Rhyolite tuff, a source of the salts of northern Chile. Short papers in the geologic and hydrologic sciences.

Erickson, B. E. (2004) Tracing the origin of perchlorate. American Chemical Society Nov. 1, 388-399.

Ewing, S. A. et al., (2006) A threshold in soil formation at Earth’s arid–hyperarid transition. Geochimica et Cosmochimica Acta, v. 70, 5293–5322.

Fiesta, B. (1922) El origin del salitre. Caliche 4, 97-106; (1923) Real Academia Ciencias Madrid Revista, v. 22, 489-523.

Flagg, J. W. (1874) Nitrate of soda, its locality, mode of occurrence and methods of extraction. American Chemist, v. 17, 403-408.

Forbes, D. (1861) On the geology of Bolivia and southern Peru. Geological Society of London Quarterly Journal, v. 17, 7-62.

Gallardo L., Olivares G., Langner J. and Aarhus B. (2002) Coastal lows and sulfur air pollution in central Chile. Atmos. Environ., v. 36, 3829–3841.

Gale, H.S. (1912) Nitrate deposits. U.S. Geological Survey Bulletin, v. 523, 1-36.

21

Gale, H.S. (1917) Origin of nitrates in cliffs and ledges. Mining and Scientific Press, v. 115, 676-678.

Garrett, D. E. (1983) Chemistry and Origin of the Chilean Nitrate Deposits. Sixth Internation Symposium on Salt, v. 1: 285-302.

Gautier, A. (1884) Sur un gesement de phosphates de chaux et d’alumine contenant des espèces rares ou nouvelles et sur la genèse des phosphates et niters naturels. Annales des Mines sér., v. 9, 5, 5-53.

Hartley, A.J., Chong, G., (2002) Late Pliocene age for the Atacama Desert: implications for the desertification of western South America. Geology, v 31, 4346.

Hartley, A.J., Chong, G., Houston, J., Mather, A.E., (2005) 150 million years of climatic stability: evidence from the Atacama Desert, northern Chile. J. Geol. Soc., v. 162, 421–424.

Knoche, W. (1930) Zur Entstehung des Chile-Saltpeters. Forschungen und Fortschritte, v. 6, 196-197.

Konnert, J. A., H. T. E. Jr., et al. (1994) Mineralogical studies of the nitrate deposits of Chile: VII. Two new saline minerals with the composition K6(Na, K)4Na6Mg10(XO4)12(IO4)12 12H2O: Fuenzalidaite (X=s) and (X=Se). American Mineralogist, v. 79, 1003-1008.

Lal, D. (1991). Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth Planet. Sci. Lett., v. 104, 421– 439.

Lifton, N. A., Bieber, J. W., Clem, J. M., Duldig, M. L., Evenson, P., Humble, J. E., Pyle, R. (2005). Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications. Earth Planet. Sci. Lett., v. 239, no. 3-4, 140-161.

McKay, C.P., Friedmann, I., Go´mez-Silva, B., Ca´ceres-Villanueva, L., Anderson, D.T., Landheim, R., (2003) Temperature and moisture conditions for life in the extreme arid region of the Atacama Desert: four years of observation including the El Niño of 1997-1998. Astrobiology, v. 3, 393–406.

Michalski, G., Böhlke, J. K., and Thiemens, M. (2004) Long term atmospheric depostion as the source of nitrate and other salts in the Atacama Desert, Chile: New evidence from mass-independent oxygen isotopic compositions. Geochimica et Cosmochimica Acta, v. 68(20), 4023-4038.

Mueller, G. (1968) Genetic Histories of Nitrate Deposits from Antartica and Chile. Nature, v. 219, 1131-1134.

Müntz, C.A., and Marcano, V. (1885) Sur la formation des terres nitrées dans les régions tropicales. Académie des Sciences Comptes Rendus, v. 101, 65-68.

Newton, W. (1896) The origin of nitrate in Chili [sic]. Geological Magazine, new series, decade 4, v. 3, 339-342.

Nishiizumi, K., Caffee, M.W., Finkel, R.C., Brimhall, G., Mote, G. (2005). Remnants of a fossil alluvial fan landscape of Miocene age in the Atacama Desert of northern chile using cosmogenic nuclide exposure age dating. Earth Planet. Sci. Lett., v. 237, 499–507.

Noellner, J. F. (1867) Uber die Entstehung der Salpeter- und Boraxlager in Peru. Journal für praktishe Chemie, v. 102, 459-464.

Ochsenius, C. (1887-1888) La formaciíon del nitrato sódico de sales de agues madres. Socidad Nacional de Mineria Boletin,, v. 4: no. 94, 752-753, no. 95, 757-760, and no. 96, 765-767, (1887); v. 5, no. 97, 776-778, no. 98, 781-792, and no. 99, 789-790, (1888).

Owen, J., Nishiizumi, K., Sharp, W., Sutter, B., Ewing, S., Amundson, R., (2003) Investigations into the Numerical Ages of post- Miocene Fluvial Landforms in the Atacama Desert, Chile. Eos Trans. AGU 84, Fall Meet. Suppl., Abstract T31C-0857.

Penrose, R. A. F., Jr. (1910) The nitrate deposits of Chile. Journal of Geology, v. 18, no. 1, 1-32.

Pissis, A. (1878) Report upon the Desert of Atacama, its geology and mineral products. In: Nitrate and guano deposits in the Desert of Atacama London, Taylor and Francis, 1-30.

Plagemann, A. (1898) Sobre la formación geológica del salitre bajo el punto de vista de la fermentación química. Sociedad Nacional de Minería, Boletín, v. 10, 12-15.

Quade, J. et al., (2007). Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile. Geochimica et Cosmochimica Acta xxx: xxx-xxx.

Ravelo, A.C., Andreasen, D.H., Lyle, M., Lyle, A.O., Wara, M.W., (2004) Regional climate shifts caused by gradual global cooling in the Pliocene epoch. Nature, v. 429, 263–267.

22 Rech, J. A., Quade, J., and Hart, W. S. (2003) Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile. Geochimica et Cosmochinica Acta, v. 67(4), 575-586.

Rech, J.A., Currie, B.S., Michalski, G., Cowan, A.M. (2006) Neogene climate change and uplift in the Atacama Desert, Chile. Geology, v. 34,761-764.

Rees C. E., Jenkins W. J., and Monster J. (1978) The sulphur isotopic composition of ocean water sulphate. Geochim. Cosmochim. Acta, v. 42, 377–381.

Rieder, R. et al., (1997) The Chemical Composition of Martian Soil and Rocks Returned by the Mokile Alpha Proton X-ray Spectrometer: Preliminary Results from the X-ray Mode. Science, v. 278 (5344), 1771-1774.

Searl, A. and S. Rankin (1993) A preliminary petrographic study of the Chilean nitrates. Geology Magazine, v. 130, 319-333.

Schemenauer R. S. and Cereceda P. (1992) The quality of fog water collected for domestic and agricultural use in Chile. J. Appl. Meteorol., v. 31, 275–290.

Sieveking, J.P., Jr., and Miller, B.L. (1887) Chilean nitre-beds. In: Nitrate and guano deposits in the Desert of Atacama, London, Taylor and Francis, pp. 38-43.

Sillitoe, R.H., Mortimer, C., Clark, A.H., (1968) A chronology of landform evolution and supergene mineral alteration, southern Atacama Desert, Chile. Transactions of the Institution of Mining and Metallurgy, v. 77, 166–169.

Smith, B. E. (2002) STRUCTURE: Nitrogenase Reveals Its Inner Secrets. Science, v. 297, no. 5587, 1654 – 1655.

Steinman, G. (1925) Über den Chemismus in der Wüste. Naturhistorischer Cerein der preussischen Rheinlande Sitzungsberichte 1924A, 8-12.

Stone, J.O. (2000). Air pressure and cosmogenic isotope production. Journal of Geophysical Research, v. 105, no. 23, 753-759.

Sundt, L. (1921) Las teorías sobre la formación de los depósitos de nitrato de sodio. Caliche, v. 3, no. 3, 97-100.

Timm, L. C., Reichardt, K., Bacchi, O. O. S., Oliveira, J. C. M., Pires, L. F., Roveratti, R. (2005) Soil bulk density evaluation by conventional and nuclear methods. Australian Journal of Soil Research,. v. 43, no. 1, 97-103.

Trewartha, G. T. (1981) The Earth’s problem climates. University of Wisconsin Press 2nd edition, 371.

Warren-Rhodes, K., Rhodes, K.L., Pointing, S.B., Ewing, S., Lacap, D.C., Go´mez-Silva, B., Amundson, R., Friedmann, E.I., McKay, C.P., (2006) The influence of water availability on cyanobacterial community ecology and the dry limit to photosynthetic life in the Atacama Desert, Chile. Microbial Ecol.

Wetzel, W. (1928) Die salzbildungen der chilenischen Wüste. Chemie der Erde, v. 3, 375-436.

23 Table 1. Cosmogenic exposure age data

Scaling schemes for spallation Desilets et al., 2006 Lifton et al., 2005 Lal (1991)/Stone (2000)c Sample Quartz Be carrier # of 9Be # of 10Be 10Be / 9Be Exposure External Exposure External Exposure External namea weight weightb atoms atoms age uncertainty age uncertainty age uncertainty (g) (g) (yr) (yr) (yr) (yr) (yr) (yr)

EEC1 15.673 0.416 3.50255E+19 1.841E+07 8.24E-12 3530421 1101101 3171249 742924 2943140 563018 EEC2 15.0258 0.3809 3.20702E+19 1.810E+07 8.48E-12 3384817 1027201 3052540 706243 2839808 542620 EEC3 17.838 0.411 3.46045E+19 1.430E+07 7.37E-12 2115765 696707 1963189 580802 1861227 514743 EEC4 13.5618 0.376 3.16577E+19 1.917E+07 8.21E-12 3935111 1395085 3493248 909589 3219636 677643 EEC5 17.2437 0.3725 3.1363E+19 1.765E+07 9.70E-12 3189560 917655 2891093 641594 2698033 498062

( a ) All samples collected from landscape surface surrounding Site OE 2. Site altitude is 1345 m a.s.l. and lat./long. is 23.116 S / 69.862 W (in decimal degrees) ( b ) LBC Be carrier concentration is 1.26 mg/g ( c ) Scaling scheme used in age model Table 2. Soluble soil concentrations and bulk density

Depth Sample Soil ρbulk ρbulk Salt Notes (cm) Horizon (g/cm³) std. dev. %

Oficina Ercilla (OE 1)

-1.0 ee tsp-1 Avyz nda nd 13 -5.5 ee tsp-2b Byz nd nd 33 Vertical fracture sample. -5.5 ee tsp-3 Byz nd nd 71 -13.0 ee tsp-4 Bz nd nd 99 -34.0 ee1-1 Bzm 2.18 0.17 51 -57.5 ee1-4 Bzm 2.48 0.08 41 -75.0 ee2-2 Bzm nd nd 53 -80.0 ee2-1 Bzm nd nd 55 Horizontal laminae sample. -89.0 ee1-5 Bzm 2.40 0.06 37 -132.0 ee1-2b Bzm 2.37 0.02 37 Vertical fracture sample. -132.0 ee1-3 Bzm 2.25 0.05 38 -146.0 ee1-12 Bzm 2.41 0.02 33 -153.0 ee3-1 Bzkm nd nd 47 Vertically bedded fracture - left -153.0 ee3-2 Bzkm nd nd 35 Vertically bedded fracture - middle -153.0 ee3-3 Bzkm nd nd 44 Vertically bedded fracture - right -155.0 ee2-3 Bzkm nd nd 29 -183.0 ee1-6 Bzkm 2.64 0.16 31 -232.0 ee1-10 Bzkm 2.46 0.05 34 -235.0 ee1-7 Bzkm 2.32 0.04 19 -252.0 ee4-1b Bzkm nd nd 46 Vertical fracture sample. -252.0 ee4-2b Bzkm nd nd 51 Vertical fracture sample. -265.0 ee1-11 Bzkm 2.24 0.07 19 -293.0 ee4-3b Bzm nd nd 50 Vertical fracture sample. -293.0 ee4-4b Bzm nd nd 24 Vertical fracture sample. -300.0 ee1-8 Bzm 2.40 0.04 41 -300.0 ee1-9 Bzm 2.46 0.12 31 n/a ee4-5 nd nd nd 23 Sample displaced from outcrop.

Oficina Ercilla Pit (OE 2)

-32.5 ee pit-7 Byzm 2.19c 0.16 42 -82.5 ee pit-6 Bzm1 2.13 ndd 44 -132.5 ee pit-5 Bzm2 2.20 ndd 23 -182.5 ee pit-4 Bzm2 nd nd 24 -232.5 ee pit-3 Byzm 2.31c 0.21 26 -282.5 ee pit-2 Byzm nd nd 18 -332.5 ee pit-1 Bzm 2.44c 0.01 20

Valenzuela

-4.0 v tsp-1 Avz nd nd 11 -16.5 v tsp-2b Byz nd nd 12 Vertical fracture sample. -16.5 v tsp-3 Byz nd nd 29 Table 2. Soluble soil concentrations and bulk density (cont.)

Depth Sample Soil ρbulk ρbulk Salt Notes (cm) Horizon (g/cm³) std. dev. %

Valenzuela

-28.5 v pit-8 Bym nd nd 30 -45.0 v pit-7b Byzm nd nd 26 Vertical fracture sample. -59.5 v pit-6 Byzm 1.98c 0.08 25 -93.0 v pit-9 Bzm 2.09 0.10 37 -103.0 v pit-5b Bzm nd nd 13 Vertical fracture sample. -152.5 v pit-4 Byzm 2.32c 0.27 38 -204.0 v pit-3 Bzm 2.30 0.05 24 -252.5 v pit-2 Bym 2.25 0.07 25 -303.0 v pit-1 Byzm 2.21 0.02 46

Rencoret N.W.

-1.0 rnw tsp-1 Avyz nd nd 18 -9.0 rnw tsp-2b Byz nd nd 18 Vertical fracture sample. -9.0 rnw tsp-3 Byz nd nd 18 -22.5 rnw tsp-4 Bz nd nd 81 -36.0 rnw-1 Byzm 2.18 0.05 63 -56.0 rnw-2b Byzm 2.07c 0.38 44 Vertical fracture sample. -56.0 rnw-3b Byzm 2.17c 0.19 36 Vertical fracture sample. -72.5 rnw-4 Byzm nd nd 28 -105.0 rnw-5 Byzkm nd nd 24 -122.5 rnw-6 Byzkm nd nd 9 -172.5 rnw-7 Bzm 2.18 0.10 36

( a ) No Data ( b ) Sampled from within vertical fractures ( c ) Limited sample available for bulk density analysis ( d ) Bulk density determined from single sample Table 3. Mineralogy of bulk soil and soil residuum after dissolution of soluble salts

Depth (~cm) Sample Horizon Major secondary minerals Minor secondary minerals Residual parent material Notes

Oficina Ercilla (OE 1)

-1.0 eetsp-1 Avyz Bloedite Quartz, Anorthite Residuum -5.5 eetsp-3 Byz Anhydrite, Gypsum Quartz, Albite Ped -13.0 eetsp-4 Bz Thenardite -57.5 ee1-4 Bzm Calcite Quartz, Albite Residuum -89.0 ee1-5 Bzm Halite, Nitratine Quartz, Albite -146.0 ee1-12 Bzkm Calcite Quartz, Albite Residuum 232.0 ee1-10 Bzkm Halite, Nitratine, Calcite Quartz, Albite -300.0 ee1-8 Bzkm Calcite Quartz, Albite -300.0 ee1-9 Bzm Nitratine Halite Quartz

Oficina Ercilla Pit (OE 2)

-32.5 ee pit-7 Byzm Halite Darapskite Quartz -82.5 ee pit-6 Bzm1 Anhydrite Gypsum Quartz, Albite Redisuum -132.5 ee pit-5 Bzm2 Halite Quartz, Albite -232.5 ee pit-3 Byzm Gypsum Quartz, Albite Redisuum -232.5 ee pit-3 Byzm Halite Nitratine, Kieserite Quartz, Albite -332.5 ee pit-1 Bzm Halite Quartz, Albite

Valenzuela

-16.5 v tsp-2 Byz Gypsum, Anhydrite Quartz, Albite Fracture -16.5 v tsp-3 Byz Anhydrite Quartz, Albite Ped -28.5 v pit-8 Bym Anhydrite, Gypsum, Quartz Residuum -59.5 v pit-6 Byzm Anhydrite, Halite Nitratine, Bytownite Quartz, Albite -93.0 v pit-9 Bzm Halite, Nitratine, Anhydrite Quartz, Albite -103.0 v pit-5 Bzm Thenardite, Anhydrite Darapskite Quartz, Albite Residuum, Fracture -204.0 v pit-3 Bzm Halite, Nitratine Anorthite, Quartz -252.5 v pit-2 Bym Gypsum Quartz Residuum -303.0 v pit-1 Bzm Halite, Glauberite, Bloedite Darapskite Quartz Table 3. Mineralogy of bulk soil and soil residuum after dissolution of soluble salts (cont.)

Depth (-cm) Sample Horizon Mineralogy (Major peaks) Mineralogy (Minor Peaks) Notes

Rencoret N.W.

-1.0 rnw tsp-1 Avyz Gypsum Quartz, Albite Surfacial -9.0 rnw tsp-3 Byz Bytownite, Gypsum Quartz, Albite Ped -22.5 rnw tsp-4 Bz Thenardite, Glauberite -36.0 rnw-1 Byzm Gypsum Quartz Residuum -72.5 rnw-4 Byzm Halite, Anhydrite Quartz, Albite -72.5 rnw-4a Byzm Halite, Anhydrite Quartz Repeat analysis -105.0 rnw-5 Byzkm Anhydrite, Calcite Quartz Residuum -122.5 rnw-6 Byzkm Calcite Quartz, Albite -122.5 rnw-6b Byzkm Nitratine, Darapskite Albite Repeat analysis -172.5 rnw-7 Bzm Nitratine, Thenardite Calcite Quartz, Albite Table 4. Geochemistry of soluble soil salts

Depth Sample Cations (wt%) Anions (wt%) + +2 + +2 -2 - - - - -2 -2 (cm) Na Mg K Ca SO4 NO3 Cl F ClO4 CO3 PO4

Oficina Ercilla (OE 1)

-1.0 ee tsp-1 0.702 0.077 0.166 0.688 1.480 0.021 0.248 0.017 0.000 1.302 1.480 -5.5 ee tsp-2a 0.563 0.045 0.057 2.904 6.811 0.257 0.269 0.006 0.000 ndb 0.039 -5.5 ee tsp-3 0.484 0.057 0.069 10.139 24.338 0.223 0.189 0.025 0.000 0.139 0.009 -13.0 ee tsp-4 29.555 0.242 0.001 1.092 71.080 0.481 0.097 0.125 0.000 0.001 0.000 -34.0 ee1-1 13.412 0.340 0.913 0.963 8.026 8.604 14.027 0.008 0.005 0.012 0.009 -57.5 ee1-4 10.371 0.021 0.484 0.902 5.485 11.519 6.777 0.017 0.037 1.327 0.004 -75.0 ee2-2 14.248 0.218 1.033 1.181 8.407 13.252 11.746 0.024 0.003 0.208 0.010 -80.0 ee2-1 14.085 0.136 0.675 1.465 10.967 11.281 10.832 0.011 0.003 0.020 0.032 -89.0 ee1-5 5.994 0.030 0.641 0.935 3.222 2.665 6.181 0.023 0.009 1.932 0.009 -132.0 ee1-2a 9.604 0.015 0.198 0.918 3.399 10.963 7.696 0.010 0.064 2.407 0.006 -132.0 ee1-3 9.698 0.038 0.492 0.559 3.185 12.813 6.586 0.015 0.058 0.018 0.008 -146.0 ee1-12 7.290 0.027 0.599 0.774 1.954 8.654 5.795 0.014 0.050 2.156 0.006 -153.0 ee3-1 12.454 0.050 0.425 1.783 5.129 16.189 9.941 0.008 0.003 0.051 0.028 -153.0 ee3-2 7.070 0.028 0.567 0.746 1.122 11.656 3.851 0.015 0.005 1.149 0.005 -153.0 ee3-3 7.120 0.022 0.448 1.282 2.883 9.674 5.025 0.014 0.005 0.869 0.003 -155.0 ee2-3 4.286 0.023 0.443 3.006 7.737 4.019 2.953 0.005 0.001 0.919 0.004 -183.0 ee1-6 7.423 0.024 0.366 1.900 5.401 6.547 6.721 0.007 0.041 1.604 0.004 -232.0 ee1-10 5.431 0.020 0.432 0.705 1.902 6.777 3.460 0.007 0.048 0.495 0.004 -235.0 ee1-7 2.824 0.013 0.267 0.540 0.424 3.464 1.842 0.019 0.015 3.529 0.007 -252.0 ee4-1a 6.073 0.045 0.359 9.647 23.888 4.678 5.396 0.008 0.003 0.416 0.002 -252.0 ee4-2 12.223 0.085 0.315 1.201 6.057 14.728 8.546 0.016 0.005 0.429 0.014 -265.0 ee1-11 3.856 0.027 0.465 0.662 0.641 4.727 2.584 0.010 0.037 2.618 0.008 -293.0 ee4-3a 4.005 0.034 0.274 6.342 15.603 2.011 4.339 0.006 0.005 2.719 0.003 -293.0 ee4-4a 4.962 0.030 0.355 0.812 1.017 7.481 3.085 0.009 0.004 3.722 0.006 -300.0 ee1-8 8.031 0.017 0.326 1.090 1.754 12.430 5.255 0.009 0.040 2.753 0.028 -300.0 ee1-9 5.972 0.019 0.367 0.786 0.536 8.771 3.785 0.010 0.063 3.137 0.009 n/a ee4-5 4.576 0.044 0.395 2.389 7.787 0.866 4.703 0.005 0.004 0.803 0.003

Oficina Ercilla Pit (OE 2)

-32.5 ee pit-7 10.221 0.563 0.605 0.603 8.746 4.905 9.597 0.005 0.000 0.012 0.023 -82.5 ee pit-6 8.626 1.040 0.737 1.895 7.729 11.709 7.128 0.005 0.003 0.005 0.033 -132.5 ee pit-5 3.556 0.549 0.560 0.667 3.114 4.045 3.018 0.004 0.000 0.015 0.060 -182.5 ee pit-4 3.716 0.290 0.333 1.474 4.218 4.285 2.985 0.004 0.005 0.007 0.000 -232.5 ee pit-3 2.875 1.312 0.817 1.692 9.125 4.093 2.675 0.011 0.002 0.007 0.003 -282.5 ee pit-2 2.680 0.425 0.306 1.680 8.178 2.073 0.974 0.017 0.001 0.014 0.003 -332.5 ee pit-1 4.174 0.187 0.316 1.559 3.773 1.054 6.021 0.007 0.001 0.080 0.004

Valenzuela

-4.0 v tsp-1 0.126 0.037 0.116 0.494 0.544 0.000 0.013 0.014 0.000 0.404 0.019 -16.5 v tsp-2a 0.126 0.032 0.098 1.993 4.189 0.000 0.046 0.009 0.000 0.438 0.000 -16.5 v tsp-3 0.324 0.035 0.102 9.171 21.832 0.000 0.077 0.013 0.000 0.665 0.000 -28.5 v pit-8 0.660 0.049 0.132 7.753 18.846 0.066 0.296 0.010 0.000 0.124 0.044 Table 4. Geochemistry of soluble soil salts (cont.)

Depth Sample Cations (wt%) Anions (wt%) + +2 + +2 -2 - - - - -2 -2 (cm) Na Mg K Ca SO4 NO3 Cl F ClO4 CO3 PO4

Valenzuela

-45.0 v pit-7a 5.697 0.057 0.206 1.431 4.127 8.121 3.608 0.024 0.000 0.052 0.000 -59.5 v pit-6 3.238 0.173 0.267 2.577 6.688 4.148 2.352 0.027 0.001 0.033 0.011 -93.0 v pit-9 7.937 0.097 0.174 2.066 5.563 10.617 6.114 0.032 0.000 0.026 0.064 -103.0 v pit-5 0.708 0.304 0.122 1.044 2.791 0.538 0.282 0.033 0.000 0.008 0.414 -152.5 v pit-4 4.682 0.184 0.277 4.679 12.782 2.495 5.019 0.024 0.000 nd 0.087 -204.0 v pit-3 3.956 0.296 0.463 1.374 6.133 4.476 2.041 0.015 0.000 nd 0.283 -252.5 v pit-2 4.502 0.085 0.143 1.986 9.675 0.489 2.747 0.022 0.000 nd 0.000 -303.0 v pit-1 7.957 1.758 0.191 1.456 24.331 1.995 3.669 0.034 0.000 nd 0.000

Rencoret N.W.

-1.0 rnw tsp-1 0.312 0.049 0.140 2.269 5.497 0.000 0.015 0.006 0.000 0.929 0.039 -9.0 rnw tsp-2a 0.792 0.059 0.080 2.747 6.709 0.451 0.330 0.007 0.000 0.348 0.000 -9.0 rnw tsp-3 0.489 0.089 0.090 3.380 8.126 0.206 0.277 0.007 0.000 0.057 0.000 -22.5 rnw tsp-4 22.757 0.107 0.035 1.564 56.389 0.000 0.261 0.070 0.000 0.005 0.004 -36.0 rnw-1 14.969 0.824 0.421 1.756 14.025 2.534 17.709 0.030 0.000 0.017 0.000 -56.0 rnw-2a 5.853 0.628 0.661 5.789 17.466 4.178 6.086 0.010 0.002 0.017 0.000 -56.0 rnw-3a 7.048 0.976 0.691 2.046 8.341 3.696 8.667 0.018 0.001 0.021 0.060 -72.5 rnw-4 3.161 1.077 0.738 2.732 9.561 4.353 2.895 0.018 0.001 0.002 0.000 -105.0 rnw-5 3.592 0.111 0.543 1.933 5.952 0.799 3.243 0.006 0.002 2.372 0.000 -122.5 rnw-6 0.938 0.023 0.270 1.224 1.534 0.827 0.274 0.000 0.000 5.715 0.031 -172.5 rnw-7 5.231 0.026 0.812 0.654 7.625 4.627 0.571 0.018 0.001 3.443 0.000

( a ) Sampled from within vertical fractures ( b ) No Data Table 5. Major anion mass per unit volume (kg / 0.5 m³)

- -2 - -2 - Depth (-cm) NO3 SO4 Cl CO3 ClO4

Oficina Ercilla (OE 1) ( ρ bulk = 2.38 g/cm³ )

0 - 49 22.81 265.93 35.29 4.33 0.01 50 - 99 115.18 83.54 105.72 10.38 0.15 100 - 149 128.64 33.87 79.64 18.17 0.68 150 - 199 114.44 53.00 67.81 10.93 0.13 200 - 249 41.22 5.04 21.92 41.99 0.18 250 - 299 80.03 112.35 57.00 23.57 0.13 300 - 350 110.98 16.63 49.59 25.33 0.60 Totala: 613.31 570.36 416.97 134.70 1.89

Oficina Ercilla Pit (OE 2) ( ρ bulk = 2.29 g/cm³ )

0 - 49 56.16 100.14 109.89 0.14 0.00 50 - 99 134.07 88.50 81.61 0.05 0.04 100 - 149 46.31 35.66 34.56 0.17 0.01 150 - 199 49.06 48.30 34.18 0.08 0.06 200 - 249 46.87 104.48 30.63 0.08 0.03 250 - 299 23.73 93.64 11.15 0.16 0.01 300 - 350 12.07 43.20 68.94 0.92 0.01 Total: 368.28 513.93 370.97 1.61 0.16

Valenzuela (ρ bulk = 2.19 g/cm³ )

0 - 49 17.93 108.49 8.85 3.68 0.00 50 - 99 80.84 67.07 46.35 0.32 0.01 100 - 149 5.89 30.56 3.08 0.09 0.00 150 - 199 27.32 139.96 54.96 ndb 0.00 200 - 249 49.01 67.16 22.35 nd 0.00 250 - 299 5.36 105.95 30.08 nd 0.00 300 - 350 21.85 266.43 40.18 nd 0.00 Total: 208.20 785.61 205.86 4.09 0.01

Rencoret N.W. ( ρ bulk = 2.15 g/cm³ )

0 - 49 6.93 196.92 40.34 2.94 0.00 50 - 99 44.22 127.91 63.83 0.14 0.02 100 - 149 8.82 40.61 19.08 43.87 0.01 150 - 199 50.21 82.73 6.20 37.35 0.01 Total: 110.17 448.17 129.45 84.30 0.04

( a ) Total mass in kg for soil profile (1m x 1m x depth (m)) ( b ) No Data Table 6. Isotopic values

Depth (cm) Sample δ13C (‰ VPDB) δ18O (‰ VPDB) δ34S (‰ CDT)

Oficina Ercilla (OE 1)

-1 ee tsp-1 1.13 -3.29 7.23 -5.5 ee tsp-2a ndb nd 6.79 -5.5 ee tsp-3 1.25 -3.05 7.88 -13 ee tsp-4 nd nd 5.14 -34 ee1-1 nd nd 6.56 -57.5 ee1-4 -3.82 0.29 5.51 -89 ee1-5 -3.74 0.07 4.93 -132 ee1-2a -3.57 -0.28 5.22 -132 ee1-3 nd nd 6.31 -146 ee1-12 -3.52 0.27 5.38 -153 ee3-2 -3.51 1.49 nd -153 ee3-3 -3.48 1.52 nd -155 ee2-3 -3.38 1.18 nd -183 ee1-6 -3.61 0.30 5.90 -232 ee1-10 -3.56 0.70 4.70 -235 ee1-7 -3.86 -1.06 4.00 -252 ee4-1a -3.92 1.62 nd -252 ee4-2a -3.71 1.82 nd -265 ee1-11 -3.65 -0.67 3.79 -293 ee4-3a -3.54 0.47 nd -293 ee4-4a -3.59 0.59 nd -300 ee1-8 -3.45 -0.75 4.42 nd ee4-5 -3.89 -0.12 nd

Oficina Ercilla Pit (OE 2)

-32.5 ee pit-7 nd nd 6.72 -82.5 ee pit-6 nd nd 6.70 -132.5 ee pit-5 nd nd 7.30 -182.5 ee pit-4 nd nd 6.79 -232.5 ee pit-3 nd nd 5.78 -282.5 ee pit-2 nd nd 5.43 -332.5 ee pit-1 nd nd 4.90

Valenzuela

-4 v tsp-1 3.21 -0.94 nd -16.5 v tsp-2a 2.61 -2.35 11.07 -16.5 v tsp-3 2.97 -2.43 11.84 -28.5 v pit-8 nd nd 10.45 -45 v pit-7a nd nd 10.22 -59.5 v pit-6 nd nd 10.15 -93 v pit-9 nd nd 9.71 -103 v pit-5a nd nd 8.84 -152.5 v pit-4 nd nd 10.03 Table 6. Isotopic values (cont.)

Depth (cm) Sample δ13C (‰ VPDB) δ18O (‰ VPDB) δ34S (‰ CDT)

Valenzuela

-204 v pit-3 nd nd 9.15 -252.5 v pit-2 nd nd 8.75 -303 v pit-1 nd nd 6.87

Rencoret N.W.

-1 rnw tsp-1 -1.81 -4.09 7.62 -9 rnw tsp-2a 0.35 -3.02 7.72 -9 rnw tsp-3 nd nd 7.81 -22.5 rnw tsp-4 nd nd 7.24 -36 rnw-1 nd nd 6.67 -56 rnw-2a nd nd 7.75 -56 rnw-3a nd nd 7.43 -72.5 rnw-4 nd nd 8.01 -105 rnw-5 -4.92 -5.83 7.41 -122.5 rnw-6 -3.78 -5.67 6.31 -172.5 rnw-7 -3.78 -5.36 5.72

( a ) Sampled from within vertical fractures ( b ) No Data