Carbon Stable Isotope Composition of Modern Calcareous Soil Profiles in California: Implications for CO2 Reconstructions from Calcareous Paleosols

Abstract

Fourteen soil profiles from California were collected in order to measure the d 13 C of coexisting soil calcite and organic matter. Thirteen of the profiles contained a measurable amount of calcite ranging from 0.04 to 54.6 wt %. Soil calcite d 13 C PDB (d 13 C value vs. the calcite standard Peedee Belemnite) values range from À14.4 to 1.3ø, whereas organic matter d 13 C PDB values range from À24.0 to À27.7ø. The hydrology of these profiles is divided into two broad groups: (1) soils characterized by gravity-driven, piston-type vertical flow through the profile and (2) soils affected by groundwater within the profile at depths where calcite is present. The difference between soil calcite and organic matter d 13 C PDB values, D 13 C cc-om , is smaller in profiles affected by groundwater saturation as well as most Vertisols and may be a product of waterlogging. The larger D 13 C cc-om values in soils with gravity-driven flow are consistent with open-system mixing of tropospheric CO 2 and CO 2 derived from in situ oxidation of soil organic matter with mean soil PCO 2 values potentially in excess of ;20,000 ppmV at the time of calcite crystallization. There is a correlation between estimates of soil PCO 2 and a value termed ''E PPT-U '' (kJm 2 /yr) among the soil profiles characterized by gravity-driven flow. E PPT-U is the energy flux through the soil during periods of soil moisture utilization, and it is the product of water mass and temperature in the profile during the growing season. Thus, soils with high water-holding capacity/storage and/or low/high growing season temperature may form soil calcite in the presence of high soil PCO 2 , and vice versa. The results of this research have important implications for reconstructions of paleoclimate from stable carbon isotopes of calcareous paleosol profiles.

Regional-scale variability in the spread of grasslands in the late Miocene

“…Even if one assumes instead that the highest δ 13 C org value of -23.2‰ was the most representative, the minimum ∆ 13 C value would be +18.7, which is still substantially out of equilibrium. The degree of disequilibrium is consistent with groundwater affected or diagenetically altered carbonates (Tabor et al, 2013 plants in the absence of other proxies that can rule out pedogenic or diagenetic alteration.…”

Latest Permian paleosols from Wapadsberg Pass, South Africa: Implications for Changhsingian climate

“…However, studies of modern soils indicate that partial pressures of soil CO 2 (C S ) as well as soil-respired CO 2 (Sz) at the time of calcite crystallization can range from <1000 to >10,000 ppmV (Breecker et al, 2009;Mintz et al, 2011;Montañez, 2012;Tabor et al, 2013). For the kinds of paleosols that occur in the Karoo Basin (Calcisols), corresponding modern soils intimate soil-respired (Sz) values of <1000 ppmV (Breecker et al, 2009) to 2000 ppmV (Montañez, 2012).…”

New constraints on using paleosols to reconstruct atmospheric pCO2

“…1), as the soil Δ 13 C increases, the calculated S(z) decreases. If the Δ 13 C is less than ~14‰ (depending on temperature), then the calculated S(z) value will be negative, implying precipitation of carbonates from a one-component soil CO 2 system (Tabor et al, 2012). Figure 6 and supplementary Table 2 (see footnote 1) include data from fi ve soil series in South Dakota, two in Minnesota, and one in New Mexico that clearly do not fall on the S(z)-MAP relationship defi ned based upon the literature data.…”

“…Modern soils with Δ 13 C values less than 14‰ can occur in a system in which the fractionation caused by diffusion is small or absent. Low Δ 13 C values have been observed to occur in areas with groundwater saturation (Mintz et al, 2011), or with soils that do not exhibit gravitydriven hydrologic fl ow, which lowers diffusivity and decreases the diffusive enrichment factor (Tabor et al, 2012). Water-saturated soils may undergo advection as well as diffusion.…”