Periodicities of palaeoclimatic variations recorded by loess-paleosol sequences in China

摘要

Paleoclimatic periodicity recorded by chinese loess-paleosal sequence has been investigated for a number of years. However, conclusions from previous investigations are still controversial, and interpretation of cycle evolution is quite equivocal. In this study, two typical loess-paleosal sequences (148 and 191m in thickness resepectively) in the central Chinese Loess Plateau are sampled (3872 samples total) and measured for grain size distribution and magnetic susceptability in order to reconstruct the paleoclimatic changes over the past three million years. On the basis of a new sensitive proxy indicator of paleoclimatic and a newly developed independent time scale (not orbitally-tuned), two time series of Asian dust storm variations, which are highly related to the paleoclimatic system changes, are obtained. Wavelet and spectrum analysis indicate that there are approximately 400,200,100,66,57,41,31,27, and 22 kyr cycles in these typical loess-paleosol records. Some orbital driven cycles are weak and are not well presented in the new time series while some non-orbital cycles are found. Since the eccentricity frequencies of the solar irradiance of approximately 400 and 100 kyr are preserved in these paleoclimatic sequences, the lack of relativiley short time orbital cycles of 41-kyr-obliquity and 22-kyr-possession cycles in part of the two time series may be explained by the relatively low-time resolution of the loess-paleosol deposits. Through an astronomical estimate, the obliquity and precession cycles should leave stronger footprints on records of paleoclimatic variations at the middle latitudes of the northern hemisphere. The presence of non-orbital cycles may be explained by unstable dust deposition process and pedogenic process in the paleosol units, which could be misrepresent or obliterate the imprint of the solar irradiance frequency. This conclusion may indicate that one should be cautious when investigating specific paleoclimatic changes (e.g., at sub-orbital time scales) recorded in loess deposits, especially in paleosal units. (C) 2004 Elsevier Ltd. All rights reserved.