THE ROLE OF MAGHEMITE IN THE ENHANCEMENT OF MAGNETIC SIGNAL IN THE CHINESE LOESS-PALEOSOL SEQUENCE - AN EXTENSIVE ROCK MAGNETIC STUDY COMBINED WITH CITRATE-BICARBONATE-DITHIONITE TREATMENT
Ww. Sun et al., THE ROLE OF MAGHEMITE IN THE ENHANCEMENT OF MAGNETIC SIGNAL IN THE CHINESE LOESS-PALEOSOL SEQUENCE - AN EXTENSIVE ROCK MAGNETIC STUDY COMBINED WITH CITRATE-BICARBONATE-DITHIONITE TREATMENT, Earth and planetary science letters, 133(3-4), 1995, pp. 493-505
We have conducted a rock magnetic study of selected Chinese loess samp
les before and after citrate-bicarbonate-dithionite (CBD) treatment. O
ur study is more extensive than previous ones and reveals new details
of the magnetic carriers which are related to paleoclimate change. Thr
ee pairs of paleosol S1 (the top paleosol layer in the Lishi Formation
) and loess L2 (underlying S1) from three localities (Xining, Xifeng,
and Jixian) were investigated by using a variety of magnetic methods.
After the paleosols were treated with CBD, which removes all maghemite
grains and the finest grains of magnetite, magnetic susceptibility de
creased from 77 X 10(-8) to 20 X 10(-8) m(3)/kg in Xining, from 165 x
10(-8) to 30 X 10(-8) m(3)/kg in Xifeng, and from 180 X 10(-8) to 25 X
10(-8) m(3)/kg in Jixian, anhysteretic remanent magnetizations (ARM)
were reduced to less than one tenth of the original values in the thre
e sites, and saturation magnetization (J(s)) decreased by about 60% in
Xining and 70% in Xifeng and Jixian. These reduced values in the pale
osols after CBD treatment are similar to those of the corresponding un
derlying loess samples, which, however, did not show significant decre
ase after CBD treatment. Room-temperature frequency dependence of susc
eptibility, high-temperature dependence of susceptibility, and low-tem
perature SIRM demagnetization of the paleosol and loess samples before
and after CBD treatment confirm that in addition to superparamagnetic
(SP) particles (magnetite and/or maghemite), the CBD removable magnet
ic carriers also include maghemite of grain sizes larger than SP.