CARBON AND OXYGEN ISOTOPE COMPOSITIONS OF CARBONATE CONCRETIONS IN LOESS PART 2:CARBON ISOTOPE AND PALEO-ARIDITY
黄土中钙结核的碳氧同位素研究(二)碳同位素及其古环境意义

Carbonate is a common component in soils and Quaternary sediments. The stable isotopic compositions of secondary carbonates in Quaternary deposits, mainly the δ18O and δ13C values, were used as the important paleoenvironment data carriers,from which the information of the paleoclimatic and paleoecological conditions were extracted. Loess is one kind of the Quaternary sediments rich in carbonates. During the paleosol formation original carbonates became soluble bicarbonates in the local environment with high CO2 partial pressure. These soluble bicarbonates reprecipitated in water lost and/or the decreasing of CO2 content. Isotopic exchange took place during this dissolution and reprecipitation cycle.The carbonate concretions related to paleosols in Lishi Loess (Formation) of the Luochuan loess section were systematically collected. The measurement of carbon and oxygen isotopic compositions was carried out in the laboratory. Statistical analysis on the obtained δO and δC values was optimistic about the possibility of carrying out quantitative paleoclimatic and paleoecological reconstruction. In the first part of this paper the authors discussed the variation of δO values in different paleosols and an attempt was made to correlate the δO values to the paleotemperature when paleosols were formed. What is reported here is the variation of the carbon isotopic compositions of paleosol carbonates in Lishi Loess (Formation) of Luochuan and the paleoclimatic and paleoecological significance of the carbon isotopic composition.δ13C values range from -2.78 to -7.19‰, with the maximum value in less developed paleosol S2-1 and the minimum value in best developed paleosol S5-2. It changes in a opposite direction to that of δ18O. For δ18O, the minimum value is measured in less developed paleosol S2-1 and the maximum value in best developed paleosol S5-2. The variation magnitude between different paleosols (about 4.4‰) is bigger than that of δO(1.5‰).The carbon isotopic composition of pedogenic carbonate is controlled by the isotopic composition of soil CO2 in soil gases, which is usually well correlated with the proportion of C3 and C4 photosynthesis in the local ecosystem. Using the δC values of different paleosols the proportion of C3 and C4 biomass was estimated for each paleosol by geochemical calculation. The results imply that for most paleosols there should be almost same proportion of the C3 and C4 biomass and only for those well developed paleosols with higher C3 biomass up to 66% (S5-2) and for less developed paleosol with lower C, biomass (35%, S2-1) indicating the local ecosystem in Luochuan area is dominated by the grass vegetation during most of paleosol formation except for the well developed paleosol S5 which was developed under the vegetation with some trees and shrubs.C4 plants, which are adapted to high light and water stressed conditions, have δ13C values between about -9 and -19‰ quite different from that of C3 plants (between about -22 and -