摘要
第三纪的红粘土和第四纪黄土同为风成沉积，二者是否有相同的物质来源？长尺度风尘的加积与上地壳演化之间有怎样的联系？二者均是非常重要的科学问题。由于Sr，Nd同位素可以有效示踪沉积岩的物质来源，我们近年研究了8Ma以来黄土高原泾川剖面红粘土-黄土序列〈20μm的细粒硅酸盐组分的Sr，Nd同位素变化。结果表明：1）黄土高原的黄土与北太平洋中部的硅酸盐组分的Nd同位素组成非常接近，二者应在一定程度上具有相似的物质来源，即来自蒙古国南部及中国境内的戈壁、沙漠地区，所不同的是前者由近地面风搬运，而后者为远距离输送的风尘；2）Sr同位素在8．00～2．58Ma期间呈现略为增加的趋势，应当与红粘土的来源物质经受了较长时间的化学风化有关，而Sr同位素自2．58Ma以来的显著降低趋势与第四纪时期黄土-古土壤的物质来源和第三纪红粘土不同有关，新构造运动使更多相对年轻的造山带褶皱成山，第四纪冰期来临使更多高海拔的年轻造山带遭受剥蚀而成为黄土的来源物质，这是导致黄土、红粘土物源不同的重要原因。
The thickest and oldest eolian deposits occur on the Chinese Loess Plateau, and both the chronology and climatology of the eolian deposits have been well-studied. However, there are still many basic questions that remain unclear, for instance, although both the Quaternary loess-soil successions and the Late Tertiary Red Clay are eolian origin, we still do not know if they have the same provenance or what relationship exists between the accumulation of the eolian deposits and the upper crustal evolution.
Because Nd and Sr isotopes have long been used as powerful tools in tracing sediments provenance, in this study a record of changes in Nd and Sr isotopic composition of the eolian deposits from the central Loess Plateau has been determined for the past 8Ma. The isotopic records of the silicate fraction of the Quaternary and Tertiary eolian deposits allow interpreting the interplay between the Sr isotopic variations in the eolian deposits and the Late Cenozoic tectonic and climatic changes.
The results indicate： （1） the St, Nd isotopic compositions of the Chinese eolian deposits are comparable with that of the silicate fraction of the deep sea deposits in the central North Pacific, implying they have the similar provenance, transported from the gobi desert in southern Mongolia and China, the terrestrial eolian deposits are transported by near-surface winds, while the eolian fraction in the central North Pacific is distal deposits transported long-distance mainly by westerlies; （2） the slightly increasing trend of ^87 Sr/^36 Sr ratios from ca. 8.00Ma to 2.58Ma ago at Jingchuan is mostly related to the prolonged chemical weathering; and （3） the remarkable decreasing trend of ^87 Sr/^86 Sr ratios after 2.58Ma ago towards the end of the Quaternary is attributed to the additions of relatively younger crust materials, in response to the climatic cooling and the Late Cenozoic uplift induced glacial grinding in the high orogenic belts in central Asia. In this context, the Quaternary glacial grinding and neotectonics have modified dust sources significantly, and the Quaternary loess forming processes are preferentially sampling relatively younger and high relief crust materials than that of the Tertiary Red Clay.
出处
《第四纪研究》
CAS
CSCD
北大核心
2007年第2期187-192,共6页
Quaternary Sciences
