STABLE ISOTOPE RECORD IN PALEOSOL CARBONATES FROM THE CHINESE LOESS PLATEAU: IMPLICATIONS FOR LATE NEOGENE PALAEOCLIMATE AND PALAEOVEGETATION

Stable carbon and oxygen isotope analyses were conducted on pedogenic carbonate concretions collected from a continuous, late Neogene terrestrial sequence located in Lantian, Chinese Loess Plateau. The sequence consists of 300 m of fluvial deposits (Bahe Formation) with a basal age of 11 Ma overlain by ca. 50 m thick sequence of aeolian red clay (Lantian Formation) deposited between ca. 7 – 3 Ma.

Pedogenic carbonates from the fluvial part of the sequence show only slight variation in d¹³C and d¹⁸O values suggesting that no dramatic change in local climate or vegetation took place during ca. 11 – 7 Ma. The carbon isotope compositions imply a pure C₃ vegetation. Most data are, however, in the upper C₃ plant range suggesting water stress conditions.

A shift in both carbon and oxygen stable isotope curves occur at the boundary of the two formations where sedimentation regime changes from fluvial to mainly aeolian. The d¹³C record shows more depleted values in the Lantian Fm, whereas the formation boundary marks a general rise in d¹⁸O values. Moreover, the d¹³C and d¹⁸O values scatter more in the Lantian Fm compared to those measured from the Bahe Fm. As the shift in d¹³C and d¹⁸O values at the formation boundary is concurrent we hypothesize that the reason for the changes in both values are most likely interrelated. The changes most probably reflect increased summer precipitation related to the onset or intensification of the Asian monsoon system.

Sedimentological and fossil vertebrate analyses suggest that stable environments with relatively arid conditions prevailed during the time when the Bahe Fm was laid down. Slightly below the formation boundary there is a faunal turnover implying a marked change into more humid and closed habitats. The paleoenvironmental patterns deduced from carbon and oxygen isotopes are in good agreement with the paleoenvironments inferred from sedimentological and paleontological studies from the Lantian area.