HIRM variations in the Chinese red-clay sequence: Insights into pedogenesis in the dust source area

Although extensive rock magnetic studies have been performed on the Chinese loess–paleosol sediments, few comparable studies have been applied to the underlying red-clay sediments. Because of this paucity of data, our understanding of the mineral magnetic properties of the red-clay sequence, and the paleoenvironmental information encoded in them, is limited. Here we report “hard” isothermal remanent magnetization (HIRM) variations from ∼8.1 to ∼1.2 Ma in the lower part of the loess–paleosol and the underlying red-clay sequences of the Chaona section on the Chinese Loess Plateau. The results indicate that there is a negative correlation between HIRM and frequency dependence of magnetic susceptibility χfd (χfd = χlf − χhf, where χlf and χhf are magnetic susceptibility measured at 470 Hz and 4700 Hz, respectively) from ∼3.4 to ∼2.6 Ma and a positive correlation between these two proxies from ∼8.1 to ∼3.4 Ma and ∼2.6 to ∼1.2 Ma. We argue that the HIRM variations from ∼8.1 to ∼3.4 Ma and from ∼2.6 to ∼1.2 Ma reflect hematite availability in the dust source region, determined by the size of the area and climatic conditions of the dust source region. The negative correlation between HIRM and χfd between ∼3.4 and ∼2.6 Ma suggests that magnetic enhancement during this time interval may be related to the alteration of hematite/goethite to magnetite/maghemite. We also find that HIRM data between 8.1 and 3.4 Ma show high coherence with the 100-kyr eccentricity signals, suggesting eccentricity forcing of concentration variations of hematite in the dust source area. Higher resolution samples are needed to determine if concentration variations of hematite in the dust source area are coherent with the orbital obliquity and precession forcing signals.