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Geochemical Journal
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Strontium isotopic and elemental characteristics of calcites in the eolian dust profile of the Chinese Loess Plateau during the past 7 Ma

WENBO RAO, JUN CHEN, JIEDONG YANG, JUNFENG JI, GENGXIN ZHANG, JIANSHEN CHEN
Geochemical Journal, Vol. 42, No. , P. 493-506, 2008

ABSTRACT

Strontium isotopic ratios, calcium and strontium contents of calcites in the eolian dust profile during the last 7 Ma at Lingtai were systematically studied. 87Sr/86Sr ratios of calcites change with three secondary fluctuations in the Quaternary loess-paleosol sequence: slight fluctuations from L33 to S15, a slight increase from L15 to L6, and a dramatic decrease from S5 to L1. They are usually lower in the Quaternary loess than in the interstratified paleosols. 87Sr/86Sr ratios of calcites in the Tertiary red clay are relatively stable, mostly higher than in the Quaternary loess and paleosols. Ca2+ and Sr2+ contents of calcites are relatively high in the Quaternary loess and the Tertiary red clay but low in the Quaternary paleosols. Sr2+ is well correlated with Ca2+, and can thus be an effective tracer for Ca2+. High contents of Ca (Sr) in the Tertiary red clay show that carbonates were poorly leached out of the Tertiary red clay. Like Ca contents, Ca/Sr ratios in the Tertiary red clay are close to those in the Quaternary loess. Elemental ratios and contents have different variations from the Sr isotopic ratios of calcites in the Lingtai profile, implying that isotopic ratios were better proxies for climate change than elemental ratios and contents. Primary carbonate and silicate comprised two endmembers determining the range of the Sr isotopic ratios of calcites, and chemical weathering was an inner driver to induce strontium isotopic variations of calcites in the eolian dust profile. Our isotope data indicate that chemical weathering was strongest in the Tertiary red clay, the second in the Quaternary paleosols, and weakest in the Quaternary loess. Strontium isotopic variations of calcites are in a good agreement with pedogenic characteristics in the Lingtai profile, broadly consistent with a proxy of the summer monsoon—Fe2O3(f)/Fe2O3(t) ratios, and correlate with variations of Fe2+/Fe3+ ratios in the Lingtai profile and the dust flux in ODP885/886 during the late Cenozoic. Strontium isotopic variations of calcites are different from the magnetic susceptibility records in the Lingtai profile, further demonstrating that magnetic susceptibility of the Tertiary red clay could not reliably indicate the Tertiary summer monsoon. As a result, strontium isotopic characteristics of calcites in the Lingtai profile indicate that the Quaternary summer monsoon had a glacial-interglacial alternation and weakened with large-amplitude oscillations since 0.5 Ma B.P., and also reveal that the Tertiary summer monsoon was more stable and overall stronger than the Quaternary, nevertheless, similar climates to those in the Tertiary probably occurred during a few paleosol developments of the Quaternary.

KEYWORDS

Chinese Loess Plateau, the eolian dust profile, strontium isotope of calcite, chemical weathering, the East Asian summer monsoon

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