Magnetic Enhancement and Iron Oxides in the Upper Luochuan Loess-Paleosol Sequence, Chinese Loess Plateau

摘要

Variations in the low-field magnetic susceptibility of the wind-blown Chinese Loess Plateau (CLP) loess-paleosol sequences reflect changes in the global paleoclimate on different time scales. Magnetic enhancement in paleosols has been ascribed to the neoformation of fine-grained maghemite; however, little is known about the pathway through which this mineral was formed in the CLP paleosols, its relationships with the other pedogenic Fe oxides (viz. hematite and goethite), and the pedoclimatic significance of such relationships. In this work, we characterized various magnetic, chemical, and mineralogical properties of the loess-paleosol units at depths from about 23 to 55 m in the Upper Luochuan section, central CLP. The concentration of pedogenic hematite (delta Hm) and the frequency-dependent magnetic susceptibility (chi(FD)), which is used as a proxy for the concentration of fine-grained pedogenic maghemite, were found to be linearly correlated (R2 = 0.825, P < 0.001). This supports the idea that these two minerals were formed concomitantly during pedogenesis, which is consistent with the results of previous in vitro experiments showing that the ferrihydrite to maghemite to hematite transformation takes place under aerobic conditions. By contrast, the concentration of pedogenic goethite (delta Gt) was only weakly correlated with either chi(FD) or delta Hm, which suggests that goethite formed through an alternative pathway. The paleosols above 40 m (S4, S5, corresponding to marine isotope stages 9 and 11, respectively) exhibit a higher degree of weathering and higher delta Hm/(delta Hm + delta Gt) ratio than those below such a depth (S6-S8). This was ascribed to differences in paleoclimatic conditions, which are moister and warmer in the former paleosols than in the latter, rather than to differences in pedogenesis duration.