A CARBON-ISOTOPE RECORD OF THE UPPER CHINESE LOESS SEQUENCE - ESTIMATES OF PLANT TYPES DURING STADIALS AND INTERSTADIALS

The upper three couplets of paleosol-loess in the Luochuan section of northern China were analyzed for carbon isotopes. These units date back to about 205 ka and have been correlated with the marine oxygen isotope stages 16. Calcite is present as nodules, concretions and interstitial cement; analyses were performed on bulk samples of slightly cemented soil and loess material. An ashing study indicates the presence of organic matter in the samples, and it is possible that analyzed gases included a contribution from this source. The measured deltaC-13 values and the estimates of CO, content in the atmosphere over this time, derived from studies of the Vostok ice core and from the DELTAdeltaC-13 values of planktic-benthic foraminifera, are used following the method of Cerling (1984, 1991) to calculate the proportions of C3 vs C4 plants which contributed CO, to the formation of calcite in the paleosols and the loess. C3 plants were dominant at all times (paleosols approximately 70-80%; loess approximately 55-60% C3), and both loess and paleosols show a decrease toward the present. These relationships suggest a progressive drying of the environments, due either to climate change or to changes in the local drainage network.