1
y = 0.49x + 1.19 R² = 0.92 0 1 2 3 4 5 6 0 2 4 6 8 10 Depth Weighted LOI (%) Depth Weighted RR (unitless) y = 3E-06x + 1.93 R² = 0.86 0 1 2 3 4 5 6 7 8 9 1.00E+04 1.00E+05 1.00E+06 1.00E+07 Depth Weighted RR (unitless) log(Approximate Age (yr)) y = -0.73ln(x) + 15.80 R² = 0.89 4 5 6 7 8 9 10 1.00E+04 1.00E+05 1.00E+06 1.00E+07 Depth Weighted pH KCl (unitless) log(Approximate Age (yr)) Conclusions • Older fan surfaces exhibited greater chemical weathering than younger surfaces. • Decreased pH and increased RR values suggest a wetter, cooler past climate transitioning toward hotter and drier modern conditions. • Soil chronosequences are acceptable models for understanding local influence of past climate change on soil formation and survival. Linking soil chronosequence properties to landscape evolution in southern Arizona. 1 Amanda Kerr, 2 Christopher Shepard, 2 Craig Rasmussen 1 Department of Marine and Environmental Sciences, Northeastern University, Boston, MA, USA 2 Department of Soil, Water and Environmental Science, The University of Arizona, Tucson, AZ, USA Introduction • Soil chronosequences quantify changes in soil chemical and physical properties over time. • Alluvial fans preserve traces of the climatic conditions under which they were deposited. Research Objectives • How do fundamental soil properties change with time across differently aged fan surfaces? • Does pedogenic iron concentration increase with time, and thus function as a suitable proxy for soil age and degree of soil development? Fan surfaces near Safford, AZ. Geologic map of sampling sites. • Collected profiles from four fan surfaces of sequential age. • Samples were air-dried and passed through a 2mm sieve. • Color (Redness Rating), pH, electrical conductivity (EC), and loss on ignition (LOI) were measured to understand changes in soil properties with time. Methods Acknowledgments The authors of this poster kindly thank Dr. Katerina Dontsova, Dr. Kevin Bonine, Dr. Takayuki Kobayashi, Dr. Jim Walworth, Yadi Wang, Rebecca Hingley, Anthony Tornito, the Undergraduate Research Opportunities Consortium at The University of Arizona, and the National Science Foundation EAR grant #1263251 “REU Site: Environmental and Earth Systems Research at Biosphere 2”. Samples from B Horizon of each fan illustrate color gradient from oldest to youngest surface. Results and Discussion Correlation between depth weighted RR and depth weighted LOI; indicative of the adsorption of organic matter onto iron-oxide surfaces over time. Depth weighted average redness rating increased linearly with time from youngest to oldest surfaces, which suggests increased hematite content with age. 0 50 100 150 200 250 0 2 4 6 8 10 Depth (cm) RR (unitless) Q0-1 Q1-1 Q2-1 Q3-1 Redness Rating of each profile increased with depth but decreased at carbonate-rich layers. Generally, RR increased alongside surface age with exception of unrepresentative Q1 sample composed of mostly rock fragments. Depth weighted average pH KCl decreased linearly with age, indicative of increased chemical weathering on older surfaces. • Generally, pH decreased with surface age, while loss on ignition and redness rating increased with surface age. Formulas & References Redness Rating = 10 − Munsell Hue × Munsell Chroma Munsell Value + Gelderman, F.W., Winkelaar, P., Brown, J.E., Meurisse, R.T., 1970, Soil Survey of the Safford Area, Arizona: United States Department of Agriculture Soil Conservation Service and University of Arizona Agricultural Experiment Station, p.52. ++ Western Regional Climate Center, Period of Record Monthly Climate Summary 08/01/1898 to 06/30/1973 for Safford, Arizona, http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?az7388 (accessed August 2016). % = 105°C soil weight − 550°C soil weight 105°C soil weight − tin weight × 100 Site Characteristics Pinaleño Mountains near Safford, AZ, approximately 120 km northeast of Tucson, AZ Q 0 1800 KYR Q 1 600 KYR Q 2 130 KYR Q 3 40 KYR • Elevation 790-1400 m + • N 32° 44’, W 109° 47’ • Average annual max temperature 27°C, min 8°C ++ • Average annual precipitation 22.9cm ++ 0 50 100 150 200 250 3 4 5 6 7 8 9 Depth (cm) pH KCl Q0-1 Q1-1 Q2-1 Q3-1 pH KCl increased with profile depth and across successively younger fan surfaces. Clay-rich horizon from Q 0 fan surface (left) next to carbonate-rich horizon from Q 3 fan surface (right). Illustrates observed change in soil properties over time.
