SHORT-TERM VARIATIONS IN PALEOCLIMATE AND VEGETATION AS REVEALED BY STABLE ISOTOPE COMPOSITIONS OF PEDOGENIC CARBONATE NODULES AND SOIL ORGANIC MATTER IN SELECTED CHINESE LOESS PALEOSOLS

Thick successions of interbedded loess and paleosols from the Chinese Loess Plateau provide a continuous terrestrial record of climate variability for the past 2.6 Ma. The d¹³C and d¹⁸O compositions of pedogenic carbonate nodules associated with the paleosols have been used as a paleoenvironmental proxy, however, pedogenic carbonate nodules can be overprinted leading to artificial homogenization of their bulk isotopic composition. To evaluate the isotopic complexity of pedogenic nodules from selected paleosols (S1, S5, S15-S19) of the Jiaodao section, we carried out a high-resolution (micrometer) analysis of various micromorphological features (matrix, peloids, pore coatings, in-fillings) within nodules, and compared these results to homogenized bulk carbonate samples from the same nodules. The d¹³C values of most micromorphological components vary between -3.6 and -7.0 ‰, whereas d¹⁸O values vary between -8.4 and -10.4 ‰ PDB. Some pore coatings and in-fillings, however, have d¹³C values as negative as -8.5 ‰ PDB. Intranodule variability in d¹³C and d¹⁸O values is <1 ‰ PDB in small nodules (<50 mm), and up to 2‰ or more in larger nodules. Intranodule isotope variability is similar to internodule variability within each soil and between paleosols of different ages. Bulk nodule samples do not display any relationship between d¹³C and d¹⁸O isotope values, but the values of micromorphological components covary. Bulk isotope compositions (d¹³C=-5.1 to -6.5 ‰, d¹⁸O=-8.9 to -10.0 ‰ PDB) do record long-term conditions at the site, but they mask smaller-scale temporal environmental variations. Bulk values suggest that the nodules formed when the percentage of C4 vegetation at the site was elevated and the precipitation levels were relatively high. Measured soil organic matter d¹³C values from the same paleosols indicate elevated C4 vegetation at the site during peak paleosol formation periods. The isotopic values of microsamples confirm that the bulk of nodule formation occurred under warm and wet soil forming conditions, however, the isotopic composition of younger pore coatings and in-fillings suggests that these formed during the transition to colder and less wet conditions when C3 vegetation prevailed at the site.