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Calculating Atmospheric pCO2 from Paleosols:
Case study in the Palouse Loess
Kyle Gosnell, Dr. Alex Lechler
Pacific Lutheran University, Tacoma, WA
Discussion
Acknowledgements
References
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in modern soils from central New Mexico, USA. GSA Bulletin 121: 630–640.
Breecker, D.O. Retallack, G.J. (2014) Refining the pedogenic carbonate atmospheric CO2 proxy and application to Miocene CO2.
Paleogeography, Paleoclimatology, Paleoecology. 406: 1-8.
Cerling TE (1984) The stable isotopic composition of modern soil carbonate and its relationship to climate. Earth and Planetary Science
Letters 71: 229–240.
Cerling, T.E., (1999) Stable carbon isotopes in paleosol carbonates. Special Publication for the International Association of
Sedimentologists. 27: 43-60.
Davis, Owen, University of Arizona GEOS 462. “Desert Varnish”. [Online] http://www.geo.arizona.edu/palynology/
geos462/14rockvarnish.html (accessed August 3, 2015).
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carbonate. Earth and Planetary Science Letters 237: 56-68.
Ekart, D.D., Cerling, T.E., Montañez, I.P., Tabor, N.J., 1999. 400 million year carbon isotope record of pedogenic carbonate: implications for
paleoatmospheric carbon dioxide. American Journal of Science 299, 805–827.
Huang, C., Retallack, G.J., Wang, C., Huang, Q., (2013) Paleoatmospheric pCO2 fluctuations across the Cretaceous-Tertiary boundary
recorded from paleosol carbonates in NE China, Paleogeography, Paleoclimatology, Paleoecology 385: 95-105.
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isotopic ratio during the penultimate deglaciation. Quaternary Science Reviews 29: 1983-1992.
McDonald, E.V., Sweeney, M.R., Busacca, A.J., (2012) Glacial outbust floods and loess sedimentation documented during Oxygen Isotope
State 4 on the Columbia Plateau, Washington State, Quaternary Science Review, 45: 18-30.
Montañez, I., (2013) Modern soil system constraints on reconstructing deep-time atmospheric CO2. Geochimica et Cosmochimica Acta 101:
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Stevenson BA, Kelly EF, McDonald EV, and Busacca AJ (2005) The stable carbon isotope composition of soil organic carbon and
pedogenic carbonates along a bioclimatic gradient in the Palouse region, Washington State, USA. Geoderma 124: 37–47.
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• Paleoenvironmental reconstruction helps improve accuracy for future climate
predictions and numerical simulations.
• In Palouse region of eastern WA, periodic megafloods during last glacial period
produced thick loess (wind-blown silt) sequences (Figure 1)
 Loess carbonate carbon and oxygen isotopes record paleoclimate
• Dug terraced trench (Figure 3) to remove modern
(weathered) layer and vegetation.
• Bulk analysis every 10 cm
• Selected areas for specific analysis (Figure 4).
• Paleosol layers:
–Superior cementation
–Lighter hue
–Often contain plant roots (Figure 4)
• δ13
C and δ18
O analysed on Kiel III device.
–Reacted phosphoric acid with samples
–CO2 analyzed via mass spectrometer
• Selected samples analyzed for clumped isotope ratio (T(∆47)).
–Record of temperature of formation
–Accurate estimates of paleotemperatures
–Allow correlation to CO2 [atm] concentrations.
• Selected samples dated using 14
C analysis.
–Correlation with ice core/marine records.
• Mentor, Dr. Alex Lechler
• M.J. Murdock Charitable Trust
• Pacific Lutheran University Natural Sciences Department
• Fellow researchers, Justin Johnsen and Isabellah von Trapp
Future Work
• Compare marine sediment records and ice core records to Palouse loess.
• Explore aridity effects on S(z).
• 14
C/OSL, magnetic correlation
• Combine MAT with clumped isotope temperatures.
Equation 2: Calculation for δ13
C of soil based on MAT. δ13
Ccc (δ13
C value measured by
Kiel III), T (temperature °C) (Romanek, et al., 1992).
Equation 1: CO2 atmospheric concentration formula that was used to calculate pCO2 levels.
The variables δ13
Cs (δ13
C value of soil(‰))[Equation 2] δ13
Cr (δ13
C value of soil organic
matter(‰)) [Stevenson, et al., 2005] and δ13
Ca (δ13
C value of atmosphere(‰))[Equation 4]. S
is CO2 contributed by soil respiration, z represents depth (cm) (Cerling, 1999, Ekart, 1999).
Equation 3 (Above Left): Calculation of MAT (°C) δ18
OC (δ18
O value measured by Kiel III).
Equation 4 (Above Right): Calculation of δ13
Ca values (Arens, 2000).
Table 1 (Above): Effect on S(z) values by changing 13
Cr
values and 13
Ca. This shows how little 13
Ca effects S(z) across
the possible ranges of inputs. Figure 7 (Below): Calculated
S(z) using δ13
Ca from ice core estimates (red) and via equation
4 (blue). [Only data from CLY3 shown for clarity]
Approach
Low
(-24.3‰)
Middle
(-25.10‰)
High
(-25.9‰)
Average ∆
S(z)
42.7
(+/- 12.7)
32.8
(+/- 7.2)
-2.97
(+/-0.5)
24.0
(+/-6.5)
Figure 1: Location of
CLY and WA study
sites, including areas of
loess deposition.
(Figure modified from
McDonald et. al, 2012)
Figure 2: Sources of carbon input
for soil carbonates. Atmosphere,
Plant respiration, and soil
organics all combine to effect the
carbonates isotopic value. (δ13
C)
Red Line indicates approximate Bk
soil horizon (Photo modified from
123rf.com)
Figure 3 (Top): Pit at CLY3. (Photo Credit: Dr. Alex
Lecher.) Figure 4 (Bottom): Carbonate casings from
plant roots (Photo from WIN1).
Figure 9 (Left)/Figure10 (Right): Variations in atmospheric CO2 levels while
constraining S(z) levels to those reported by Montanez for aridisols (500-3000),
Standard deviation calculated using low, average and high values of δ13
Cr as reported
by Stevenson, et al., 2005) (Table 1).
METHOD 1: Assume pCO2 calculate S(z)
LGM pCO2 = 185 ppm (Chart 3) results in S(z) values < 1000 ppm
Low S(z) reflects low plant productivity  cold, dry conditions during LGM
• Modern S(z) values (> 1000 ppm) reflect increased Holocene pCO2 [atm] and associated
increase in productivity during Holocene
• Paleosol carbonate sensitive to atmosphere
CO2 concentration (pCO2)
• Use Palouse loess carbonate to investigate
pCO2 of last glacial period
• Calculating pCO2 requires information about
all carbon sources in soil (Figure 2)
(1) δ13
C of atmosphere
(2) δ13
C of Plant respiration
(3) δ13
C soil organics
Figure 5 (Above Left): LGM-Holocene ice core CO2 concentration record (modified from Lourantou et al., 2010).
Figure 6 (Above Right): LGM-Holocene temperature trends derived from carbonate δ18
O and ∆47 values. Samples
binned based on measured 14
C ages. MAT calculated from carbonate δ18
O (Equation 3) (red line). Blue dots =
measured carbonate clumped isotope temperatures (T(∆47)). Note pCO2-MAT correlation. Warm T(∆47) (> 30°C) for
shallow samples in 24 ka Washtucna paleosol likely reflects kinetic fractionation (A. Lechler, pers. comm.)
Methods
Figure 11: Isotopic fractionation
signature of plants based on
method of photosynthesis. Trees
are an example of C3 plants. Hot
weather plants are typically
classified as C4. (Image Credit:
Owen Davis)
The use of stable isotopes to track changes in environmental conditions has been a major focus in
the attempt to forecast potential effects of rising atmospheric CO-2 levels. Stable isotopes rely on soil
carbonates to lock in a mixture of the CO2 isotopic composition the surrounding environment. This
includes plant respiration (δ13
Cr), soil organic matter (δ13
Cs), atmospheric CO-2 (δ13
Ca), temperature
of formation i.e. clumped isotope temperature (T∆47), and CO-2 contributed via soil respiration (S(z)).
A study of the Palouse Loess in Eastern Washington state was conducted in order for a systematic
examination of the variables that effect CO2[atm] estimates. Advances in analytical methods have
removed some variability [δ13
Cr, δ13
Cs-, T∆47) of the equations originally derived from Cerling (1984)
and refined over the years (Ekart, 1999; Cerling, 1999, others). However, δ13
Ca and S(z) remain
variable, the latter has been found to be the most variable. Paleoatmospheric CO-2 values have been
previously estimated and calculated, our findings show little variability between the two values. In the
larger picture they result in pCO2[atm] changes of < 24.0 ppmv. Investigation of S(z) levels agree with
Breecker (2009) in that previous soil respiration estimates have been too high. Carbonate formation
occurs when evaporation exceeds precipitation, vice mean annual conditions, leading to high
estimates of S(z) and T∆47. It was found that when holding pCO2[atm] to levels recorded in ice cores
the S(z) values for the last glacial maximum are within the expected range and are lower than those
of the Holocene which is expected due to cold, dry conditions of the last glacial maximum.Introduction/Motivation
METHOD 2: Assume S(z), calculate pCO2
•Using full empirical S(z) range of 500-3000 ppm (Montañez, 2013) results in
unrealistically high atmospheric pCO2 values (Figure 9, 10)
 Soil carbonate preferentially form during times of low S(z) (Breecker, 2013)
• Similarity of pCO2 in deeper sections of Washtucna paleosol at WA5 and CLY
Exploring Temperature Exploring δ13
Ca
Exploring S(z)
(LGM (>20 kya) and Modern (<14 kya) soil profiles distinguished using 14
C ages)
Given difficulty in determining S(z) (Breecker and Retallack, 2014), a dual approach
was employed to assess the the impact of S(z) uncertainty for pCO2 calculations