西峰剖面午城黄土古风化强度变化与 早更新世季风环境演化*

游离铁(FeD)/全铁(FeT)比值被土壤学界用来反映土壤化学风化强度,并已在风尘堆积的古环境研究中得到较好应用。然而,迄今对第四纪黄土古风化强度的研究多集中于中更新世以来的序列,只有少数剖面的分析包含午城黄土。文章基于西峰剖面661个样品的FeD和FeT分析,建立了该剖面午城黄土的古风化强度序列,并与剖面原有1.2Ma以来的序列进行对比。结果表明,早更新世古土壤的风化强度与后期相比总体较低,而黄土层的风化强度总体偏高,反映了较小的冰期-间冰期反差; 古风化强度的波动总体上与深海氧同位素有较好的一致性,体现了全球冰量变化对我国北方季风环境的控制。2.6~1.6Ma期间,古土壤的平均风化强度在整个第四纪最低,指示了较弱的夏季风环境效应。从约1.6Ma开始到1.2Ma,古土壤和黄土层的风化强度均增加,并开始有较显著的约100ka周期。这些特征在深海氧同位素记录中没有表现,而与热带地区一些环境特征相吻合,从而指示了低纬过程对我国北方早更新世季风环境的显著影响。

VARIATIONS OF PALEO-WEATHERING INTENSITY OF THE WUCHENG LOESS FORMATION AT XIFENG AND THE IMPLICATIONS ON ENVIRONMENTAL CHANGES

FeD/FeT(free iron/total iron) ratio can be used as a sensitive indicator of the weathering intensity in the Quaternary loess-soil sequences and in the Late Miocene-Pliocene Red Earth Formation.Up to date,high-resolution paleo-weathering record of the Early Pleistocene Wucheng Loess Formation has been only generated in the Lingtai loess-soil section while those for the Xifeng and Changwu sections cover only the past 1.2Ma.This study aims at generating a new paleo-weathering record based on the Wucheng Loess Formation in the Xifeng loess-section.Through analyzing 661 samples from the Wucheng Loess Formation,a paleo-weathering timeseries is obtained that covers the interval from 2.6Ma to 1.2Ma.Then we examine the monsoonal environment changes at extra-obital and orbital scales and attempt to explore the links of paleomonsoon evolution with global climate.In comparison with the portion of the last 1.2Ma,Early Pleistocene weathering is weaker in soils while stronger in loess,indicating rather weak glacial-interglacial contrasts.The Early Pleistocene changes of weathering intensity at orbital frequencies are dominated by the 40-ka obliquity band.This suggests a dominant control of global ice-volume on the monsoon climate in Northern China,probably through modulating the dust accumulation rates and the strength of the winter monsoon.FeD/FeT progressively increases since 1.6Ma in both soil and loess units,suggesting increased summer monsoon intensities.These trends are inconsistent with the marine oxygen isotope record,but match well a series of environmental changes in tropical and sub-tropical regions,suggesting an origin of low-latitude forcing.An early onset of the 100-ka cycles is also observed at 1.6Ma,which is unlikely to be linked to global ice volume.Rather,it is attributable to the modulation of precession by eccentricity and the consequent impacts on the summer monsoon.