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 minerals. 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