Geochemical characterization of the Luochuan loess-paleosol sequence, China, and paleoclimatic implications

Chemical (major and trace element) and isotopic (Sr, Nd) analyses of the Luochuan loess-paleosol sequence in China were performed in order to examine the following problems: (1) potential source heterogeneity; (2) element behavior during pedogenesis; (3) paleoclimatic implications; and (4) the use of loess data to determine the average composition of the upper continental crust. New results clearly indicate that Rb, Sr, U and Ce abundances are severely depleted in paleosols relative to their ''parental'' loess. The behavior of these elements is strongly controlled by the breakdown of primary minerals such as carbonate (Sr) or by alteration processes in highly oxidizing environments (Ce and U). The pattern of element variations through the upper section of the Luochuan sequence mimics the magnetic susceptibility signal both in position and in intensity, thus reinforcing the pedogenetic origin of the enhanced magnetic susceptibility in paleosols. All loess samples display highly uniform REE patterns (except for Ce) characterized by the upper continental crust (UCC) ratios: (La/Yb)(N) approximate to 10 and Eu/Eu* approximate to 0.66. The paleosol REE patterns are similar to those of the loess, with the same LREE/HREE fractionation, but they also show distinct and variable negative Ce anomalies. Nd and Sr isotopic compositions are rather uniform in both loess and paleosols and do not vary with stratigraphic position (epsilon Nd = -10.5 to -9.2; Sr-87/Sr-86 = 0.715 to 0.719). The isotopic homogeneity in the Luochuan sequence strongly suggests a uniform source region during the entire period of deposition since similar to 800 ka ago to the present. Several conclusions may be drawn from the present study: (1) No geochemical distinction can be made between loess layers L1 to L7. The sources and the dust storm trajectories must have been essentially the same for the last 800 ka. (2) Some elements (e.g., Ca, Rb, Sr, U, Ce) are strongly fractionated by pedogenesis between loess and paleosols. Ce mobility in soils is clearly demonstrated but the process responsible for this leaching is still poorly understood. (3) Systematic variations of element abundances and ratios between loess and paleosols can be used as chemical indicators for pedogenetic intensity and so for paleoclimatic change. These chemical indicators serve a function similar to that of oxygen isotopes in deep-sea sediments (pelagic foraminifera), or that of magnetic susceptibility in loess sequences. They are different recorders of paleoclimatic change. (4) The striking uniformity of REE patterns and La/Th ratios in the Luochuan loess and paleosols, as well as in loess worldwide, is an excellent starting point for estimating the average composition of the upper continental crust.