Aridity-driven decoupling of δ13C between pedogenic carbonate and soil organic matter

Abstract

Pedogenic carbonate is an invaluable archive for reconstructing continental paleoclimate and paleoecology. The δ13C of pedogenic carbonate (δ13Cc) has been widely used to document the rise and expansion of C4 plants over the Cenozoic. This application requires a fundamental presumption that in soil pores, soil-respired CO2 dominates over atmospheric CO2 during the formation of pedogenic carbonates. However, the decoupling between δ13Cc and δ13C of soil organic matter (δ13CSOM) have been observed, particularly in arid regions, suggesting that this presumption is not always valid. To evaluate the influence of atmospheric CO2 on soil δ13Cc, here we performed systematic δ13C analyses of paleosols across the Chinese Loess Plateau, with the sample ages spanning three intervals: the Holocene, the Late Pleistocene, and the mid-Pliocene warm period. Our paired δ13Cc and δ13CSOM data reveal broadly divergent trending patterns. Using a two-component CO2-mixing model, we show substantial incorporations of atmospheric CO2 (up to 60%) into soil pore space during carbonate precipitation. This result readily explains the enrichment of δ13Cc and its divergence from δ13CSOM. As a consequence, δ13C of pedogenic carbonates formed under semiarid and/or arid conditions are largely driven by regional aridity through its control on soil CO2 composition, and thus cannot be used to evaluate the relative abundance of C3 versus C4 plants. Nonetheless, these carbonates can be applied for atmospheric CO2 reconstructions, even for periods with low CO2 levels.

Overestimate of C4 Plant Abundance Caused by Soil Degradation‐Induced Carbon Isotope Fractionation

“…Because SOM directly inherits the δ 13 C signal of aboveground biomass, the δ 13 C SOM is considered a more robust proxy for regional ecosystems (i.e., C 3 vs. C 4 vegetation) than other indirect proxies such as the δ 13 C of pedogenic carbonate and tooth enamel (Da et al., 2020). When interpreting paleosol‐δ 13 C SOM records, past applications rarely considered 13 C enrichment caused by SOM decomposition.…”

Natural and Human-Induced Factors on the Accumulation and Migration of Pedogenic Carbonate in Soil: A Review

“…There is a large body of literature demonstrating the success of the isotope technique in estimating PC for various objectives, such as paleoenvironmental reconstruction [84,85], neo-formed PC detection [62,86], land-use change [55,56], and labeling experiments [87]. The isotope signature of different types of carbonate varies in a soil horizon, reflecting the parent material, dissolution-reprecipitation cycle, and climatic factors [85,88,89]. Various isotope signatures including δ 13 C [62], 14 C [29], δ 18 O [42,62], clumped isotopes [40], and U [23] have been used to study PC formation and accumulation.…”

“…Nevertheless, estimating the PC pool using isotope techniques usually comes with a degree of uncertainty, arising from the decoupling between CO 2 production and carbonate precipitation spatially and temporally [53]. For instance, an increase in aridity leads to a decrease in the respiration rate of roots, which in turn results in the dominance of atmospheric CO 2 in soil pores [52,88]. In such a case, intrusion of atmospheric CO 2 (with a discreet isotopic signature) into soil pores is predictable, modifying the isotopic signature of soil CO 2 and soil carbonate (shifting to more positive values), particularly in topsoil [27,52,88].…”

Regional Patterns in Miocene‐Pliocene Aridity Across the Chinese Loess Plateau Revealed by High Resolution Records of Paleosol Carbonate and Occluded Organic Matter

“…To address these issues, we present new paired carbon isotope records from paleosol carbonate and occluded organic matter preserved within pedogenic nodules from the Red Clay Formation at the Jiaxian site (Figure 1), which is located at the northern edge of the Loess Plateau. Late Miocene‐Pliocene shifts to higher δ 13 C carb values on the Loess Plateau have commonly been interpreted as tracking increases in C 4 vegetation (An et al., 2005; Ding & Yang, 2000; Passey et al., 2009), but aridity driven changes in soil respiration rates have more recently been invoked to explain fluctuations in δ 13 C carb values (Caves Rugenstein & Chamberlain, 2018; Da et al., 2020). By evaluating the same sample organic matter and carbonate records together, we can interpret what environmental factor is primarily responsible for shifts in carbon isotopes at Jiaxian.…”

“…These proxies include such as magnetic susceptibility and carbonate content (e.g., Sun et al, 2010), abundance and stable isotope geochemistry of pedogenic carbonate, which refers to authigenic carbonate minerals that accumulate in dryland soils (e.g., Ding & Yang, 2000), pollen assemblages (Ma et al, 2005), and phytolith assemblages (e.g., H. Wang et al, 2019). Broad differences in north-south environmental patterns between the Holocene and Pliocene have been suggested from paleosol and tooth enamel records (Da et al, 2020;Passey et al, 2009;Yang et al, 2015). However, creating a single, cohesive picture of how regional moisture patterns evolved throughout the Late Miocene and Pliocene remains challenging based on existing records.…”