Geochemical and Isotopic Characterization of the Luochuan Loess-Paleosol Sequence of China and its Paleoclimatic Implications

Sylvain Gallet Géosciences Rennes, UPR 4661-CNRS, Université de Rennes 1, 35042 Rennes Cedex, France

Bor-ming Jahn Géosciences Rennes, UPR 4661-CNRS, Université de Rennes 1, 35042 Rennes Cedex, France:

The Luochan loess-paleosol sequence of China is probably the best known and studied loess deposit in the world. It offers a rare and precious opportunity to study Quaternary paleoclimatic change from the continental environmental record. The purpose of our study is to examine the chemical and isotopic variation between different loess layers and between loess and paleosol. The variation between different loess layers has important implication to the problem of source heterogeneity and possible change of major wind patterns since 850 ka. The variation between loess and paleosol pairs could allow us to evaluate elemental mobilities during pedogenesis, hence distinguishing mobile (pedogenesis intensity indicators) from refractory elements (protoliths indicators).

Our new results clearly indicate that Ba, Rb, Sr, U, Ce are strongly mobile and their abundances severely depleted in paleosol relative to parental loess. Their behavior is strongly controlled by the breakdown of primary minerals like feldspars (Ba), carbonate (Sr) or by alteration in a highly oxidizing environment (Ce and U). The pattern of elemental variations through the upper section (L1 to S7) mimics the magnetic susceptibility signal both in position and in intensity, hence reinforcing the opinion of pedogenetic origin for the enhanced magnetic susceptibility.

In terms of refractory elements, all loess samples show highly uniform REE patterns (except Ce), characterized by the UCC (upper continental crust)-like ratios of (La/Yb)N × 10 and Eu/Eu* × 0.66. REE patterns of paleosols are similar to those of loess, showing the same LREE/HREE fractionation with (La/Yb)N × 10 and Eu/Eu* × 0.66, but they further show distinct and variable negative Ce anomalies.

Nd isotopic compositions are very homogeneous in both loess and paleosols (eNd(0) = -9 to -11) and no distinction between loess and paleosol ca be made. TDM model ages are uniform (1.7 - 1.9 Ga) and similar to those of major river particulates, other loess deposits in China and most recent clastic sediments. Apparently, loess as well as fine-grained clastic sediments have sampled protoliths that have been extremely well mixed through multiple stages of recycling. 87Sr/86Sr ratios show a small range (0.715 to 0.720), but paleosols have slightly higher 87Sr/86Sr ratios (mean = 0.7171) than loess (mean = 0.7156), which is related to leaching of detrital carbonate with marine Sr isotopic composition during pedogenesis. Overall, the Nd-Sr isotopic homogeneity within the whole loess-paleosol sequence strongly suggests a single and globally uniform source region during the entire period of deposition since about 800 ka ago.

Several conclusions may be reached from the present study: (1) No geochemical distinction can be made between loess layers S1 to S9. The sources and dust storm trajectories must have been essentially the same since the last 800 Ka. (2) Some elements (Ca, Ba, Rb, Sr, U, Ce) are strongly fractionated between loess and paleosols as a result of pedogenesis. The Ce mobility in soils is clearly demonstrated but the process responsible for this depletion is still poorly understood. (3) Systematic variation of elemental abundances and ratios between loess and paleosol can be used as chemical indicators for paleoclimatic changes. These chemical indicators serve as a function similar to that of oxygen isotopes in deep sea sediments (pelagic foraminiferas), 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 tool for the estimation of the average composition of the upper continental crust.