Temperature dependence of magnetic susceptibility in an argon environment: implications for pedogenesis of Chinese loess/palaeosols

Abstract

S U M M A R YTemperature dependence of magnetic susceptibility (χ − T ) has been widely used to determine changes in mineralogy of natural samples during heat treatment. We carried out integrated rock magnetic experiments to interpret the χ − T curves of the Chinese loess/palaeosols in argon. We used both raw materials and heated samples. In addition, we also investigated the magnetic properties of magnetic extracts and residues to quantify contributions from each fraction to the bulk magnetic properties. For the heating curves, the susceptibility loss (∼30 per cent) between ∼300-400 • C is caused by the inversion from pedogenic fine-grained maghemite to haematite, suggesting that the susceptibility loss can be used as a new concentration index of the pedogenic fine-grained superparamagnetic (SP) particles in the Chinese loess/palaeosols. Unlike the warming curves, the cooling curves are dominated by newly formed fine-grained magnetites with a dominant size of ∼35 nm. The onset for the new production of these finegrained magnetic particles occurs at ∼400 • C. It is interesting that the room-temperature magnetic susceptibility (χ ph ) of the samples heated after a 700 • C run is independent of the degree of pedogenesis and saturates at approximately 33-35 × 10 −7 m 3 kg −1 , indicating that the susceptibility enhancement is controlled only by the reduction of Fe-bearing aeolian minerals during heating. It appears that the 700 • C thermal treatment in argon could be in some sense an analogue to the pedogenic processes. Thus, we predict that ∼33-35 × 10 −7 m 3 kg −1 is the maximum susceptibility that pedogenesis can generate for the last interglacial palaeosol unit (S1). In practice, χ ph would be useful to quantify the aeolian inputs to the Chinese Loess plateau.

Palaeoclimate evolution across the Cretaceous–Palaeogene boundary in the Nanxiong Basin (SE China) recorded by red strata and its correlation with marine records

“…Maghemite generated during pedogenic 182 processes is generally unstable during heating, as represented by its transformation to 183 haematite at 300-400℃ . In addition to being affected by the 184 magnetic mineral type, -T curves are also affected by magnetic particle size due to 185 that some fine particles could change their domain state during the heating/colling 186 process (Liu et al, 2005).…”

Investigation of the magnetic properties of soils in the Cisna-Wetlina Landscape Park.

“…For the sample from the outside of the anomaly (8.I), the deflection on the SIRM(T) curve around 300°C is less evident, poining to the prevalence of hematite among other magnetic minerals. The presence of hematite in the topsoil indicates the maturity of the soil (Liu et al 2005). The decrease of susceptibility in the range 280-400°C on the κ(T) heating curves can also be related to the transformation of maghemite.…”

Mechanism of variations in environmental magnetic proxies of lake sediments from Nam Co, Tibet during the Holocene

“…The FORC diagrams were calculated using the FORCinel software v1.18 [26]. Temperature-dependent susceptibility curves (-T curves) are effective in characterizing magnetic minerals in samples and alteration of minerals during heating processes [27]. Therefore, -T curves were measured using Kappa Bridge MFK1-FA equipped with CS-3 temperature control system with a maximum temperature of 700°C, and a temperature ramping rate of 2°C /min in argon environment to avoid samples being oxidized when heating.…”