Release of iron from chlorite weathering and links to magnetic enhancement in Chinese loess deposits

Abstract

Chlorite weathering is potentially useful as an indicator of in situ weathering related to the strength of the East Asian summer monsoon in Chinese loess deposits. The chlorite ratio (the integral area ratio of chlorite diffraction peaks at 0.47nm and 0.7nm) is a new proxy for evaluating the release of free iron from chlorite weathering. This ratio was measured using X-ray diffraction (XRD) on the <2μm fraction of samples from four loess sections spanning the last 130ka across the eastern Chinese Loess Plateau. The variations in the chlorite ratio in the four loess sections indicate that the release of free iron from chlorite weathering had taken place in all four last Interglacial S1 soils and also, with the exception of the northernmost Huanxian section, in the middle Holocene S0 soils. In contrast, the illite/chlorite (I/C) ratio measurements point to a reduction in chlorite content only in the southernmost Weinan section. This comparison suggests that chlorite weathering was initially characterized by Fe2+ leaching rather than by conversion of chlorite to another mineral phase during the incipient weathering stage. The southward increase in the chlorite ratio along the four sections is entirely consistent with the weathering gradient resulting from the present-day summer monsoon climate pattern. Thus, the chlorite ratio provides a more quantitative and sensitive tool for evaluating chlorite weathering. The results also suggest that chlorite weathering may be responsible for supplying free Fe for the formation of fine-grained secondary ferrimagnets in most, if not all, of the eastern Loess Plateau region.