y = 0.49x + 1.19R² = 0.92
0
1
2
3
4
5
6
0 2 4 6 8 10
Dep
th W
eigh
ted
LOI (
%)
Depth Weighted RR (unitless)
y = 3E-06x + 1.93R² = 0.86
0
1
2
3
4
5
6
7
8
9
1.00E+04 1.00E+05 1.00E+06 1.00E+07
Dep
th W
eigh
ted
RR (u
nitle
ss)
log(Approximate Age (yr))
y = -0.73ln(x) + 15.80R² = 0.89
4
5
6
7
8
9
10
1.00E+04 1.00E+05 1.00E+06 1.00E+07
Dep
th W
eigh
ted
pH K
Cl(
unitl
ess)
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. 1Amanda Kerr, 2Christopher Shepard, 2Craig Rasmussen
1Department of Marine and Environmental Sciences, Northeastern University, Boston, MA, USA2Department of Soil, Water and Environmental Science, The University of Arizona, Tucson, AZ, USA
Introduction• Soil chronosequences quantify changes
in soil chemical and physical propertiesover time.
• Alluvial fans preserve traces of theclimatic conditions under which theywere deposited.
Research Objectives• How do fundamental soil properties
change with time across differently agedfan surfaces?
• Does pedogenic iron concentrationincrease with time, and thus function as asuitable proxy for soil age and degree ofsoil 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 changesin soil properties with time.
Methods
AcknowledgmentsThe 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 depthweighted LOI; indicative of the adsorption oforganic matter onto iron-oxide surfaces over time.
Depth weighted average redness rating increasedlinearly 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
Dep
th (c
m)
RR (unitless)
Q0-1Q1-1Q2-1Q3-1
Redness Rating of each profile increased withdepth but decreased at carbonate-rich layers.Generally, RR increased alongside surface agewith exception of unrepresentative Q1 samplecomposed of mostly rock fragments.
Depth weighted average pH KCl decreased linearlywith age, indicative of increased chemicalweathering on older surfaces.
• Generally, pH decreased with surface age, while loss on ignition and redness rating increased with surface age.
Formulas & ReferencesRednessRating =
10− MunsellHue ×MunsellChromaMunsellValue
+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).
%𝐿𝑂𝐼 = 105°Csoilweight − 550°Csoilweight
105°Csoilweight − tinweight ×100
Site CharacteristicsPinaleño Mountains near Safford, AZ, approximately 120 km northeast of Tucson, AZ
Q0
1800 KYR Q1
600 KYR Q2
130 KYR Q340 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
Dep
th (c
m)
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 Q0 fan surface (left) next tocarbonate-rich horizon from Q3 fan surface (right).Illustrates observed change in soil properties overtime.
