Two different illites in samples from S0 to L10 loess-paleosol sequence at Heimugou section in Luochuan are identified by decomposition of X-ray diffraction (XRD) curve, transmission electronic microscopy (TEM) and energy dis- persive X-ray detector (EDX) analyses. Type I illite displays relatively poor crystallinity, wide and blunt XRD peaks, the highest content of which in <2 μm clay minerals is up to 55%. Type II illite shows good crystallinity, narrow and sharp XRD peaks. The contents of these two types of illites show mutually exclusive relationship in the studied section. The EDX analyses indicate that in contrast to the illite with good crystallinity, the illite with poor crystallinity is enriched in K+, Si4+ and depleted inAl3+.
Carbon isotope ratios (δ 13C) of 89 C4 plant samples were determined from the loess area in North China. δ 13C values vary between –10.5‰ and –14.6‰ with a mean of –12.6‰. Along a pre-cipitation gradient from the semi-moist area to the semiarid area, then to the arid area, the δ 13C val-ues of C4 plants show a slight decreasing trend. The δ 13C values of C4 plants in the dry season are found lower than those in the wet season. These trends are opposite to those observed for C3 spe-cies.
WANG Guoan1,2,3, HAN Jiamao 2, ZHOU Liping3, XIONG Xiaogang4, TAN Ming2, WU Zhenhai5 & PENG Jun6 1. Department of Plant Nutrition, China Agricultural University, Beijing 100094, China
A weathered deposit in South China is widespread on the coastal areas of Fujian and Guangdong provinces,China.This deposit consists of slightly cemented,medium-to fine-grained sands,and is characterized by its colors of red,brown red,light reddish brown or dark yellowish orange,and is usually called "Old Red Sand".The uncertainty in its formation age has been a major obstacle to the study of this type of deposit.In this paper,optically stimulated luminescence(OSL) techniques were used to date the "Old Red Sand" sediments from Jinjiang,Fujian Province,China.The effect of the geochemical behavior of uranium and thorium in sediment during chemical weathering on estimation of annual dose was investigated.The results show that the change in annual dose due to weathering poses a major problem for the optical dating of such weathered sediments.The optical dating of these weathered deposits will produce erroneous ages if average annual dose during burial cannot be correctly estimated.For the profiles studied,the OSL dates obtained on samples from the upper part do not represent the burial age of the samples.It is highly likely that they are underestimated due mainly to the accumulated radioactive elements as a result of chemical weathering.It is concluded that changes in annual dose due to chemical weathering must be considered when dating similar sediments in South China.With a detailed analysis of the OSL dating results,the chronology of the marine terraces in this area was suggested.The lowest terrace was formed at ～3.5 ka and the second terrace was dated to ～74 ka.The age of the highest terrace may not be established accurately,but is inferred to be older than the apparent OSL date of～77 ka and so is the Paleolithic artifacts from it.
