Stable carbon and oxygen isotopic study of carbonate along a loess/paleosol section in Lantian County, Shaanxi Province, on the southeastern edge of the Chinese Loess Plateau

Studies of carbonate carbon isotope of loess/paleosol (delta C-13(carb)) in the Chinese Loess Plateau (CLP) have shown delta C-13(carb) less negative in loess and more negative in paleosol, which is opposite to that of bulk organic matter. Although some mechanisms have been proposed to explain this inconsistency, few studies have been conducted to investigate how carbonate migration could affect the reliability of utilizing delta C-13(carb) as an effective indicator. Here, a loess/paleosol profile with a nodule horizon intercalated in the loess layer, located on the southeastern edge of the CLP, was investigated to understand the influence of carbonate eluviation and reprecipitation on delta C-13(carb) along the section. The mean grain size and magnetic susceptibility generally conform to the field observed loess/paleosol stratigraphy. However, carbonate content shows distinct differences in the two sides of the nodule horizon, clearly indicating eluviation along the section. The variation of carbon and oxygen isotopic compositions of soil carbonate (delta C-13(SC) and delta O-18(SC)) and nodule carbonate (delta C-13(NC) and delta O-18(NC)) along the profile does not present a clearly meaningful picture. Generally, delta C-13(SC) and delta O-18(SC) have a similar change trend along the profile and are positively correlated, but there is no apparent relationship between delta C-13(NC) and delta O-18(NC). More importantly, delta O-18(NC) values fall in the range of delta O-18(SC), whereas delta C-13(NC) values are much more positive than delta C-13(SC). Detailed analyses of the data indicate migration of carbonate along the profile, which is an important factor that determines that loess/paleosol delta C-13(carb) could not be employed as a high-resolution paleovegetational and paleoenvironmental indicator in the CLP, at least on or below the glacial/interglacial scales.