The CO₂ in the air-filled spaces (pores) between solid particles in soils consists of a mixture of CO₂ from the atmosphere and CO₂ produced by organisms (microbes, plants and animals) living in the soil. A simple way to envision this is to imagine a sand dune with no plants growing on it. The pores are filled with atmospheric air and thus the CO₂ concentration equals the concentration of CO₂ in the atmosphere. Now imagine that plants grow on the sand dune. Their roots respire and microbes decompose their dead tissues, both of which release CO₂ into the pore spaces. This soil-respired CO₂ mixes with the atmospheric CO₂ in the pore spaces. The resulting mixture is termed soil CO₂. The concentrations and carbon isotope compositions of soil CO₂ are controlled by the relative contributions and the carbon isotopic compositions of the endmembers (soil-respired and atmospheric CO₂, see Figure a for a graphic presentation).

Figure a. A graphical representation of the mixing between soil-respired CO₂ (δ¹³C_r) and atmospheric CO₂ (δ¹³C_a) resulting in soil CO₂ (δ¹³C_s). The relative contribution of CO₂ from soil respiration and from atmospheric air control the value of δ¹³C_s. Therefore, if the contribution from soil respiration (S(z)) and the respired and atmospheric δ¹³C values can be quantified, then atmospheric CO₂ can be determined. The largest source of error using the paleosol carbonate proxy is in estimating values of S(z). The star denotes that the effect of diffusion of CO₂ through the soil pore spaces must also be considered.

Figure displays that the stable carbon isotope composition of soil CO₂ (δ¹³C_s) can move back and forth between the atmospheric and respired endmembers. The value of δ¹³C_s will be more similar to δ¹³C_r if the CO₂ from soil respiration (S(z)) is much greater than the atmospheric CO₂ concentration. All else being equal, higher atmospheric CO₂ results in larger δ¹³C_s values (closer to δ¹³C_a values, i.e., δ¹³C_s moves to the right on the figure).

Calcium carbonate nodules formed in soils record the value of δ¹³C_s. Calcium carbonate nodules in paleosols (Figure b) can therefore be used to estimate past atmospheric CO₂. Other required parameters include δ¹³C_r (determined from organic matter in fossil soils), temperature (required to calculate δ¹³C_s from measured δ¹³C values of the calcium carbonate nodules, determined using temperature proxies) and δ¹³C_a (determined as described above in General background and some commonalities of paleo-CO₂ proxies). Values of S(z) are more uncertain and represent the largest source of error for this proxy, but they can be approximated from mean annual rainfall (for which there are other proxies), the depth of calcium carbonates below the paleosurface and the morphology of the fossil soil (based on comparison with modern soils).

The advantages of the paleosol proxy are that it does not lose sensitivity at high CO₂ as much as some of the other proxies do, it can be (and has been) applied to much of the past 400 million years, and it does not rely on biological species effects that might change with the evolution of organisms. The primary disadvantage is the large uncertainty related to rather poor empirical calibrations of S(z). In addition to calcium carbonate, the iron oxide mineral goethite can be used to estimate paleo-CO₂, although soils precipitating goethite tend to be richer in respired CO₂, so that δ¹³C values of goethite are less sensitive to changes in atmospheric CO₂. However, an advantage of the goethite proxy is that S(z) is related to the concentration of carbon in the goethite, which is measurable. For more information see Breecker (2013).

Figure b. Photograph of a 3-million-year-old paleosol (fossil soil) developed in sedimentary rocks outcropping near Teruel, Spain. The top of the paleosol is marked by the tape measure, which shows approximately where the land surface was at the time this paleosol formed. This paleosol has since been buried by younger sediments and then exhumed and exposed by erosion. The color and structure of the paleosol change gradually with depth. There are abundant nodules of calcium carbonate at about 30 cm giving a ‘rubbly’ appearance. Picture credit: Dan Breecker.

Paleosol proxy flow chart

References cited