The quality of the loess magnetic susceptibility (MS) record as a clim
ate proxy has been compared to the ice core and deep-sea records, yet
the source and causes of the MS signal remain poorly understood. Four
explanations are currently in circulation. The dominant hypothesis, th
at MS variations largely result from post-depositional weathering and/
or formation of magnetic phases by pedogenic processes, has led to the
use of MS data as a palaeoprecipitation proxy. We present a different
view based on analyses of samples of monthly dust falls over 4 years
in North China. These have MS values comparable to those of Pleistocen
e loess and palaeosols but, unlike the latter, their variations are se
asonal and so cannot be explained by weathering differences. Our exper
iments confirm that MS enhancement in the palaeosols is caused princip
ally by fine-grain magnetic minerals, but, in contrast with previous p
roposals, the major source of the ultrafine magnetic minerals appears
to be decomposed plant litter. Copyright (C) 1997 Elsevier Science Ltd
.