Iron geochemistry of loess and red clay deposits in the Chinese Loess Plateau and implications for long-term Asian monsoon evolution in the last 7.0 Ma

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Abstract

Recently, some groups of authors have demonstrated that the Tertiary red clay deposits underlying the Pleistocene loess–paleosol sequence in the Chinese Loess Plateau are wind-blown in origin, thus extending the eolian record in the Plateau from 2.6 Ma back to about 7.0 Ma, and providing a good opportunity to reconstruct long-term monsoon changes. As magnetic susceptibility, the widely used paleomonsoon proxy of loess deposits, is problematic in reconstructing the monsoon history recorded in the red clay, the development of other proxies is urgently needed. In this study, we analyzed the ratios of citrate–bicarbonate–dithionite (CBD)-extractable Fe2O3 to total Fe2O3 concentrations in four loess sections along a north–south transect of the Loess Plateau, and in the Lingtai loess–red clay sequence of ∼305 m thick. The Fe2O3 ratio in the loess sections clearly shows higher values in soil units than in loess horizons, and a southward increase in this ratio is also remarkable along the transect. These are in good agreement with the temporal–spatial variations in the East Asia summer monsoon intensity identified by other monsoon proxies and soil development observations. The Fe2O3 ratio thus attests to being a good proxy indicator of the summer monsoon evolution. The Lingtai Fe2O3 ratio record shows high values over three time intervals: ∼4.8–4.1 Ma, ∼3.4–2.6 Ma, and during the interglacial periods of the past 0.8 Ma. The increase in summer monsoon intensity over the three intervals also coincides with the well-developed soil characteristics. It is therefore concluded that the East Asia summer monsoon has experienced a non-linear evolution since the late Miocene. The occurrence of a monsoon prime at about 4.8–4.1 Ma implies that the Tibetan Plateau, one of the most important boundary conditions in maintaining the monsoon circulation, must have been uplifted to a critical height at that time.

Introduction

Long-term monsoon evolution in the geological past can be understood in terms of mutual interactions of the atmosphere, hydrosphere, lithosphere, and biosphere [1], [2], [3]. On the globe, the interhemispheric phenomenon of Asian monsoon circulation represents one of the most dynamic atmospheric systems [4]. Meteorological studies have shown that the present-day Asia summer monsoon system actually consists of two major subsystems, one being the Indian (or southwest) monsoon and the other the East Asia (or southeast) monsoon [5], [6]. The India summer monsoon is driven by the pressure difference between the high atmospheric pressure cell over the southern subtropical Indian Ocean and the low pressure cell over the Asian Plateau (India Low), while the connection of the Pacific High with the India Low causes the southeast summer monsoon.

In recent years, the India summer monsoon evolution during the Cenozoic has been well established by studying various proxies of oceanic sediments [3], [7], [8], [9], [10]. However, reconstruction of the East Asia summer monsoon before the Pleistocene is hindered by the paucity of long geological records. The East Asia summer monsoon changes during the Pleistocene are recorded in the loess–paleosol sequence of the Chinese Loess Plateau [11], [12]. Interpretation of some proxy records, such as pedogenetic micromorphology [13], [14], [15], magnetic susceptibility [16], [17], [18], [19], pollen and snail assemblages [20], [21], and geochemical and isotopic ratios [22], [23], [24], suggests that the intensity of the East Asia summer monsoon during interglacial periods is much stronger than during glacial times.

The Chinese loess–soil sequence, which is dated back to 2.4–2.6 Ma [11], [25], is underlain directly by reddish clay–silt-sized sediments, named the red clay [11]. Recently, field observations and geochemical and sedimentological studies [26], [27], [28], [29] have demonstrated that the red clay is also wind-blown in origin, thus extending eolian deposits in the Loess Plateau down to about 7.0 Ma. This extension provides a good opportunity to reconstruct long-term monsoon history from the loess–red clay sequence by using uniform proxy records. Some authors [29], [30] have proposed the validity of the readily measurable magnetic susceptibility of the red clay to describe monsoon changes, and suggested an overall weaker summer monsoon intensity during the late Miocene and Pliocene than during the Pleistocene. It is true that magnetic susceptibility of the red clay is much lower than the loess–soil deposits, i.e. only about half that of the paleosol horizons of the loess [27]. However, field observations show that a large portion of the red clay is pedogenetically developed better than the paleosols within the Pleistocene loess deposits [27], thus in contrast with the interpretation of the magnetic susceptibility record. This means that new and more reliable proxy indicators should be developed.

It has been recognized that the ratio of citrate–bicarbonate–dithionite (CBD)-extractable iron (or free iron) to total iron concentrations is controlled basically by pedogenetic weathering degrees of the loess–soil sequence, and soil weathering is in turn closely associated with monsoonal precipitation [31], [32]. In this study, we conducted iron analyses of the loess–soil deposits of the last two glacial cycles in four loess sections and the 305-m Lingtai loess–red clay section in the Loess Plateau, with the objective of addressing the East Asia summer monsoon evolution in the past 7.0 Ma.

Section snippets

Setting and stratigraphy

To examine temporal–spatial variations in the iron geochemical indicator, four loess sections along a north–south transect, which are located respectively near Yanchang (latitude 36°35′15″N, longitude 110°04′40″E, elevation 1040 m), Yichuan (latitude 36°06′55″N, longitude 110°13′45″E, elevation 1017 m), Huanglong (latitude 35°36′50″N, longitude 109°46′50″E, elevation 1390 m) and Weinan (latitude 34°20′14″N, longitude 109°29′45″E, elevation 850 m) in the Loess Plateau (Fig. 1), were studied. The

Analyses and results

In the four loess–paleosol sections of the transect, samples were taken at intervals of 5–10 cm, with a total of 1345 samples collected. In the Lingtai loess–red clay sequence, we took a total of 1707 samples spaced at 15–20-cm intervals. Free iron was extracted by the CBD method [34]. The samples for total iron concentration determination were dissolved with a HF–HClO4–HNO3 mixed solution. Both the free and total iron concentrations were measured with a Hitachi 220A spectrophotometer.

Discussion and conclusions

As mentioned in Section 2, the red clay sequence at Lingtai is composed of about 120 reddish soil B horizons and about 110 intervening carbonate nodule horizons, and the overlying loess deposits have about 40 paleosols. Field observations show that the Lingtai paleosols both within the red clay and within the Pleistocene loess, despite their different appearance and morphology, do not show fundamental differences in terms of soil classification. The soils are mostly characterized by a brownish

Acknowledgements

This study is supported by the National Natural Science Foundation of China (49894170, 49525203). Critical comments by Drs. S. Clemens and K. Kohfield on an early version of the paper are greatly acknowledged.[EB]

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