Nano-Magnetite Aggregates In Red Soil On Low Magnetic Bedrock, Their Changes During Source-Sink Transfer, And Implications For Paleoclimate Studies

Soil and lake sediments are important paleoclimate archives often forming a source-sink setting. To better understand magnetic properties in such settings, we studied red soil on low-magnetic bedrock and subrecent sediments of Caohai Lake (CL) in Heqing Basin, China. Red soil is the only important source material for the CL sediments, it is highly magnetic with susceptibilities (chi) of similar to 10(-5) m(3)/kg. The red soil is dominated by pedogenic nano-magnetite (similar to 10-15 nm) arranged in aggregates of similar to 100 nm, with particle interaction that causes a wide effective grain size distribution in the superparamagnetic (SP) range tailing into stable single-domain behavior. Transmission electron microscopy and broadband frequency chi(f) suggest partial disintegration of the aggregates and increased alteration of the nanoparticles to hematite during transfer of red soil material to CL. This shifts the domain state behavior to smaller effective magnetic grain sizes, resulting in lower chi(fd)% and chi values, and a characteristic change of chi(f). The SP-stable single-domain distribution of the aggregates in red soil could be climate dependent, and the ratio of saturation remanence to chi is a potential bedrock-specific paleoclimate proxy reflecting it. Magnetic properties of the CL sediments are controlled by an assemblage of nanoparticle aggregates and larger-sized bedrock-derived magnetite. The results challenge the validity of the previous paleoclimate interpretation from the 168-m-long Core-HQ (900-30 ka) in Heqing Basin. Disintegration of aggregates could lead to SP behavior with low chi(fd)% without extinction of individual magnetite nanoparticles, and the chi(fd)%-based assumption of SP magnetite dissolution may be wrong.