Rapid semi-quantitative identification of greigite in lacustrine
sediments using SIRM/χ and χARM: Insights from the Cuo E core, Tibetan
Plateau, China
Abstract
The Cuo E (CE) core contains the entire Quaternary stratigraphy of the
Cuo E Lake (31°24′ - 31°.32′ N, 91°28′ - 91°33′E, Tibetan Plateau,
China). Here, we study the magnetic properties of 1748 samples from the
CE core, combined with scanning electron microscopy and X-ray energy
dispersive spectrometer analysis on selected samples. The main magnetic
minerals appear to be magnetite and greigite. To further quantify the
presence of greigite in CE core, 84 samples were rock magnetically
analyzed in more detail including: susceptibility (χ) vs. temperature,
coercivity component analysis of acquisition curves of the isothermal
remanent magnetization (IRM), and Principal component analysis (PCA) of
the first-order reversal curve (FORC) diagrams.
The greigite content and saturation IRM over susceptibility (SIRM/χ)
appear to be exponentially related. SIRM/χ can be used as greigite
concentration indicator. We also propose a rapid way to identify
greigite in the CE core: When the SIRM/χ value increases and
anhysteretic remanent susceptibility (χARM) increases only slowly, the
sample contains greigite. Samples with SIRM/χ < 15kAm-1 also
obey this relation. This method has a lower limit of detection than the
traditionally used SIRM/χ parameter for greigite detection. It is also
faster than FORC analysis and thus particularly suitable for analysis of
large sample collections. Twenty greigite-bearing layers were identified
in the CE core using this method. The location of the Jaramillo subchron
in the CE core is controversial in the currently available
magnetostratigraphy, possibly related to the presence of greigite in
this core interval.