Pinpointing the mechanism of magnetic enhancement in modern soils using
high-resolution magnetic field imaging
Abstract
In well-buffered modern soils, higher annual rainfall is associated with
enhanced soil ferrimagnetic mineral content, especially of ultrafine
particles that result in distinctive observable rock magnetic
properties. Hence, paleosol magnetism has been widely used as a
paleoprecipitation proxy. Identifying the dominant mechanism(s) of
magnetic enhancement in a given sample is critical for reliable
inference of paleoprecipitation. Here we use high-resolution magnetic
field and electron microscopy to identify the grain-scale setting and
formation pathway of magnetic enhancement in two modern soils developed
in higher (~580 mm/y) and lower (~190
mm/y) precipitation settings from the Qilianshan Range, China. We find
both soils contain 1-30 µm aeolian Fe-oxide grains with
indistinguishable rock magnetic properties while the
higher-precipitation soil contains an additional population of ultrafine
(<150 nm), magnetically distinct magnetite grains. We show
that the in situ precipitation of these ultrafine particles, likely
during wet-dry cycling, is the only significant magnetic enhancement
mechanism in this soil. These results demonstrate the potential for
quantum diamond microscope (QDM) magnetic microscopy to extract magnetic
information from distinct, even intimately mixed, grain populations.
This information can be used to evaluate the contribution of distinct
enhancement mechanisms to the total magnetization.