Origins of hard carbonate nodules in arable Chernozems in the Central Russian Upland

Abstract

The genesis of hard carbonate nodules in the lowermost horizons (180–200 cm) of arable soils in the southern part of the forest‐steppe region of the Central Russian Upland was associated with a change in soil water regime. The conversion of forest to arable lands was studied in three agro‐chronosequences located on flat interfluves and consisted of undisturbed soils under deciduous forests and arable soils with different durations of agricultural use. Due to arable agricultural activity, the upper soil horizons become drier in the summer during the growing season, whereas the lowermost parts get wetter in the spring and autumn after harvests. As a result, two types of hard carbonate nodules, which differed in morphology, origin and age, formed in the arable soils. The first type of hard nodules had a dense cryptocrystalline fabric in thin sections and colloform morphology viewed under an electron microscope, consisting of calcite with Si, Al and Fe peaks in EDS spectra, and had a 14C‐age from 16,410 ± 200 to 13,570 ± 150 years BP. Their formation occurred due to an ascending of “old” carbonate matter in colloidal suspensions through capillary pores from parent rocks in the periods of strong heating of the soil surface; these nodules had an evaporative origin. The second type of hard nodules consisted of crystalline pure calcite and had a 14C‐age of < 4,500 years BP. They had a hydromorphic genesis and developed in periods of water stagnation in the deep horizons and can be considered to be markers of a seasonal hydromorphism of arable soils in the studied area. Highlights The conversion of forest to arable lands resulted in a change of soil water regime followed by the formation of two types of hard nodules The uplifting of calcite colloidal solutions/suspensions from parent material enriched newly formed hard nodules with “old” 14C The seasonal stagnant water in the lowermost soil horizons caused hard nodule recrystallization and rejuvenation by “young” 14C from organic acids and atmospheric CO2.

Stable and radioactive carbon isotopes as indicators of restructuring the pedogenic carbonates in Chernozems during their transition from arable to abandoned state (Cis‐Azov steppe, Russia)

“…We believe the observed transformation of both radioactive and stable carbon isotopes in the carbonates of the R-14 is related to the vegetation growing there. The carbonates raised and precipitated by root water uptake have relatively ancient 14 C age (Khokhlova et al, 2021), which explains the anomalously high 14 C-dates obtained for this soil for all horizons.…”

“…We believe the observed transformation of both radioactive and stable carbon isotopes in the carbonates of the R‐14 is related to the vegetation growing there. The carbonates raised and precipitated by root water uptake have relatively ancient 14 C age (Khokhlova et al., 2021), which explains the anomalously high 14 C‐dates obtained for this soil for all horizons. Since the carbonate suspensions move upward predominantly along the interpedal and intercracks mass (where the nonsegregated carbonates were sampled), the most negative or lowest values of δ 13 С were observed in this mass.…”

“…The radiocarbon age of carbonates in the Bk1 horizon of the R‐P is higher than in the R‐20, R‐30, and R‐86 soils, especially if we take into account the highest location of this horizon in the R‐P. Previously, we showed the radiocarbon age of carbonates increases when they move upward in colloidal suspensions through capillary pores without exchange with C of soil solutions (Khokhlova et al., 2021), which also occurs in the R‐P. More negative δ 13 С values of soft concretions compared with nonsegregated carbonates were detected in the two sampled horizons Bk and BCk.…”

The Evolution of the Chernozems in the Central Part of Russia and their Complex Analysis in Modern Conditions

“…The horizon of intense illumination of CaCO 3 lying below is characterized by a maximum gradient of variation in its content with depth and a relatively high coefficient of variation of this indicator. The lowest carbonated strata Post, 2006a, 2006b;Khokhlova et al, 2020) is characterized by a high content of carbonates and a minimum value of variation of the considered indicator, i.e. with a conservative state of the trait (Figure 1).…”

Preservation of molecular fossils in carbonate concretions in cretaceous shales in the songliao basin, northeast China

“…② The local equilibrium model suggests that at a shallow burial site, when microbial action (e.g., sulfate reduction) against an organic matter that acts as a nucleus is sufficiently fast, a specific microenvironment is created and maintained. Here, the concentrations of carbonates and other products are higher than in surrounding sediments, leading to carbonate oversaturation (Coleman, 1993;Khokhlova et al, 2021). ③ According to the mixed fluid model, in some areas, atmospheric precipitate or deep fluids get mixed with pore water, inducing the formation or maintaining the growth of carbonate concretions (Abedini and Calagri, 2017).…”