PEDOGENIC AND LITHOGENIC CONTRIBUTIONS TO THE MAGNETIC-SUSCEPTIBILITY RECORD OF THE CHINESE LOESS PALEOSOL SEQUENCE

We have studied the magnetic properties of 69 loess and palaeosol samples from the loess plateau in China. Our methodology involves the combination of differential dissolution of secondary iron oxides and oxyhydroxides using citrate-bicarbonate-dithionite (CBD) with measurements of magnetic parameters, Because most of these parameters are additive, we can use pre- and post-CBD values to determine the mineral magnetic properties of the CBD-soluble material. We can also calculate the partitioning of iron among various phases. Approximately two-thirds of the total iron in our samples exists as paramagnetic iron in silicate minerals. Antiferromagnetic material (haematite) constitutes almost all of the remainder, However, the magnetic susceptibility record of the loess/palaeosol sequence arises primarily from a small amount of iron in ferrimagnetic phases, This iron consists of a CBD-resistant component which represents multidomain grains inherited from the parent material of the loess, and a CBD-soluble component which represents grains near the superparamagnetic/single domain boundary. The first component appears in nearly uniform amounts throughout the loess column and is the dominant ferrimagnetic phase in the magnetically less-enhanced loess. Accumulation of the second component in the palaeosols gives rise to the magnetic susceptibility enhancement of the loess column. The magnetic properties of the CBD-soluble component indicate that it formed as a result of in situ pedogenesis, which confirms our earlier conclusion that the palaeoclimate record of the loess/palaeosol sequence at the sites that we have sampled is primarily a record of pedogenesis.