Testing the magnetic proxy x_(FD)/HIRM for quantifying paleoprecipitation in modern soil profiles from Shaanxi Province, China

摘要

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 modem 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 (x_(FD)/HIRM) for paleoprecipitation estimation. More specifically, x_(FD) and HIRM were used as semi-quantitative proxies for the ferrimagnets (magnetite and maghemite) and hematite concentrations, respectively. The proxy x_(FD)/ HIRM, estimated from the scatter plots of x_(FD) and HIRM ranges between 4.5 and 97.6 × 10~(-5) mA~(-1) and systematically increases with the mean annual precipitation (MAP) from 300 to 1000 mm (x_(FD)/HIRM = 0.124 × MAP - 31.5, R~2 = 0.92) with an error of ±70 mm for the paleoprecipitation estimation, but is less related to mean annual temperature (MAT) (R~2 = 0.29 for a MAT range of 8-15 ℃). For MAP ＜ 1000 mm, 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 ＞ 1000 mm, the °C/HIRM value was only 17.8 × 10~(-5) mA~(-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 ＜ 1000 mm, 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.