Abstract

This petrographic and stable (δ13C and δ18O) and radiogenic (14C) isotope study of pedogenic carbonates from late Quaternary soils in the Rio Grande Rift region of New Mexico documents that carbonate formation in semiarid to arid soils (Aridisols) occurs principally by overprinting and can result in a complex petrographic and isotopic record. With increasing duration of exposure (<7 to 900 ky) of the four studied profiles, pedogenic carbonates exhibit: (1) an increase in the degree of intraprofile δ13C and δ18O heterogeneity, (2) increased development of down-profile δ13C and δ18O trends, (3) an increase in the degree of intermorphologic and intramorphologic stable-isotope variation, commonly in samples distributed laterally within narrow depth intervals, and (4) development of covariation between δ13C and δ18O values. Radiocarbon ages of the carbonates permit assessment of the degree of isotope heterogeneity in contemporaneous carbonates and recognition of temporal isotopic trends.

The range of exposure times recorded by the studied soils are analogous to those associated with the formation of fossil soils. This makes the studied soils ideal for evaluating the origin of stable-isotope heterogeneity in paleosol carbonates and the implications of those heterogeneities for the use of stable isotopes as paleoenvironmental proxies. Specific implications of this study include the following: (1) Pedogenic carbonates that form by overprinting are a cumulative record of the soil conditions present during their accumulation; thus, isotope analysis of overprinted carbonates can result in "time averaging" of changing climatic, paleoenvironmental, and paleoecologic conditions. (2) Carbonates may record only a fraction of the duration of development of their host profile. (3) There is a high level of uncertainty associated with using the sequence of morphologic development of carbonate as a function of time or maturity in paleosols. (4) An exponential decrease in carbonate δ13C values with depth in a paleosol does not imply that the carbonates are contemporaneous or accumulated under invariant soil conditions. (5) Shifts in soil and paleoclimatic conditions may be recorded by the δ13C and δ18O values of carbonates distributed within narrow depth intervals of a given soil in addition to those distributed down-profile, or even from within individual carbonate samples.

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