Isotopic Approach to Soil Carbonate Dynamics and Implications for Paleoclimatic Interpretations

Abstract

The radiocarbon content and stable isotope composition of soil carbonate are best described by a dynamic system in which isotopic reequilibration occurs as a result of recurrent dissolution and reprecipitation. Depth of water penetration into the soil profile, as well as soil age, determines the degree of carbonate isotope reequilibration. We measured δ13C, δ18O and radiocarbon content of gravel rinds and fine (<2 mm) carbonate in soils of 3 .different ages (1000, 3800, and 6300 14 C yr B.P.) to assess the degree to which they record and preserve a climatic signal. In soils developing in deposits independently dated at 3800 and 6300 radiocarbon yr B.P., carbonate radiocarbon content above 40 cm depth suggests continual dissolution and reprecipitation, presumably due to frequent wetting events. Between 40 and 90 cm depth, fine carbonate is dissolved and precipitated as rinds that are not redissolved subsequently. Below 90 cm depth in these soils, radiocarbon content indicates that inherited, fine carbonate undergoes little dissolution and reprecipitation. In the 3800- and 6300-yr-old soils, δ13C in rind and fine carbonate follows a decreasing trend with depth, apparently in equilibrium with modern soil gas, as predicted by a diffusive model for soil CO2. δ18O also decreases with depth due to greater evaporative enrichment above 50 cm depth. In contrast, carbonate isotopes in a 1000-yr-old deposit do not reflect modern conditions even in surficial horizons; this soil has not undergone significant pedogenesis. There appears to be a lag of at least 1000 but less than 3800 yr before carbonate inherited with parent material is modified by ambient climatic conditions. Although small amounts of carbonate are inherited with the parent material, the rate of pedogenic carbonate accumulation indicates that Ca is derived primarily from eolian and rainfall sources. A model describing carbonate input and radiocarbon decay suggests that fine carbonate below 90 cm is mostly detrital (inherited) and that carbonate rinds have been forming pedogenically at a constant rate since alluvial fans were deposited.

Pedogenic carbonates: Forms and formation processes

“…Moreover, the difference of δ 13 C values of the remaining GC to that of PC (Fig. 10) leads to a less negative δ 13 C of PC (Pendall et al, 1994;Quast et al, 2006) and consequently to doubtful paleoecological interpretations.…”

“…Considering the first assumption, mixing of old PC as well as "dead" (i.e. not applicable for radiocarbon dating) limestone particles with newly formed PC overestimates the absolute ages of soils, landscape or geomorphological surfaces (Pendall et al, 1994;Pustovoytov and Terhorst, 2004). For instance, if only 1% dead limestone particle remain in the structure of a given PC specimen (i.e.…”

“…The new isotopic signals of a PC feature will reflect the altered and not the original environmental conditions (Pendall et al, 1994). Considering the first assumption, mixing of old PC as well as "dead" (i.e.…”

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

“…Pendall et al (1994) estimated the recrystallization rate indirectly by comparing radiocarbon ages of these carbonates with ages determined independently from isotopic methods. Very low recrystallization rates impede assessing the process and rule out applying stable carbon isotope natural abundance.…”

Influence of Elevation on Carbonate Contents in Stratified Soils, Northern Great Basin and Adjacent Mountains, U.S.A

“…Because any intense leaching event would have caused the dissolution of older calcic horizons higher in the profiles [28], this implies a trend of decreasing leaching intensities with time, assumed to be glacial and post-glacial in time, respectively [27,29]. If so, at least the paleoclimatic framework of fossil CEZ at depths of several meters would come into question.…”