Pedogenic destruction of ferrimagnetics in Alaskan loess deposits

The positive correlation between magnetic susceptibility and pedogenic degr ee 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 sus ceptibility in soil units. However, this pattern of high magnetic susceptib ility in palaeosols, and lower values in loess, is not replicated in some l oess deposits. Alaskan and Siberian loess deposits display a completely opp osite susceptibility behaviour: high values in loess and low values in pala eosols. This inverse relationship has been explained by the idea that magne tic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to a verage wind velocity. Our magnetic study of Alaskan samples in this paper s uggests that there are notable differences in magnetic properties between A laskan loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult t o 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 conte nt by post-depositional processes. Magnetic susceptibility has variable beh aviour corresponding to different temperature-moisture environments. Ferrim agnetic minerals are produced (positive correlation) under pedogenic condit ions resulting from low precipitation and high evaporation but will be dest royed (negative correlation) under high moisture (waterlogged) pedogenic co nditions. If pedogenic development occurs under conditions which oscillate between ferrimagnet formation and destruction then it may be difficult to f ind a correlation between them. Therefore, great care should be taken when using susceptibility values for palaeoclimatic reconstruction.