Pedogenic destruction of ferrimagnetics in Alaskan loess deposits

Abstract

The positive correlation between magnetic susceptibility and pedogenic degree or weathering intensity, observed in the loess-palaeosol sequences from China and central Europe, has been widely used by Quaternary scientists for palaeoclimatic studies. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and lower values in loess, is not replicated in some loess deposits. Alaskan and Siberian loess deposits display a completely opposite susceptibility behaviour: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Alaskan samples in this paper suggests that there are notable differences in magnetic properties between Alaskan loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Alaskan palaeosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. Magnetic susceptibility has variable behaviour corresponding to different temperature–moisture environments. Ferrimagnetic minerals are produced (positive correlation) under pedogenic conditions resulting from low precipitation and high evaporation but will be destroyed (negative correlation) under high moisture (waterlogged) pedogenic conditions. If pedogenic development occurs under conditions which oscillate between ferrimagnet formation and destruction then it may be difficult to find a correlation between them. Therefore, great care should be taken when using susceptibility values for palaeoclimatic reconstruction.