Testing the magnetic proxy χ_FD/HIRM for quantifying paleoprecipitation in modern soil profiles from Shaanxi Province, China

Soils are natural archives of paleoclimates, especially for paleoprecipitation. Environmental magnetic parameters of soils are of great interest for paleoenvironmental and paleoclimatic investigations. Generally, magnetite, maghemite, hematite, and goethite are the main minerals controlling the magnetic properties of soils. The application of magnetic methods in large areas is, however, limited due to the potential effects of parent material. Here, magnetic measurements were conducted on modern soil profiles along a North-South transect in Shaanxi Province, China, to test a new magnetic proxy, defined as the ratio between the absolute frequency dependent susceptibility and the hard isothermal remanence magnetization (χ_FD/HIRM) for paleoprecipitation estimation. More specifically, χ_FD and HIRM were used as semi-quantitative proxies for the ferrimagnets (magnetite and maghemite) and hematite concentrations, respectively. The proxy χ_FD/HIRM, estimated from the scatter plots of χ_FD and HIRM ranges between 4.5 and 97.6×10^-5mA^-1 and systematically increases with the mean annual precipitation (MAP) from 300 to 1000mm (χ_FD/HIRM=0.124×MAP-31.5, R²=0.92) with an error of ±70mm for the paleoprecipitation estimation, but is less related to mean annual temperature (MAT) (R²=0.29 for a MAT range of 8-15°C). For MAP<1000mm, both pedogenic ferrimagnet and hematite production increases with increasing MAP, which suggests that these minerals are likely to be genetically related. Nevertheless, the concentration of ferrimagnets increased more markedly than that of hematite. For a soil profile located in an area where MAP>1000mm, the χ_FD/HIRM value was only 17.8×10^-5mA^-1 as the likely result of reductive dissolution or alteration, the effect being more marked for ferrimagnets than for hematite. In summary, this new paleoclimatic transfer function has potential to predict MAP for Holocene soils with MAP<1000mm, providing that there is no reductive dissolution or alteration of the iron oxides and the effect of other influential factors (e.g. temperature) is taken into account.