Magnetism of a red soil core derived from basalt, northern Hainan Island, China: Volcanic ash versus pedogenesis

Similar to loess-paleosol sequences in northwestern China, terrestrial sedimentary sequences (red soils) in southern China also provide sensitive Quaternary records of subtropical/tropical paleoclimate and paleoenvironment. Compared with red clay sequences originated from eolian dust, red soils derived from bedrock have received little attention. In this study, a long core of red soil derived from weathered basalt in northern Hainan Island, China, was systematically investigated by using detailed magnetic measurements and rare earth element analyses. The results show that an extremely strong magnetic zone with a maximum magnetic susceptibility (>10x10(-5)m(3)kg(-1)) is interbedded in the middle of the core profile. This layer contains a significant amount of superparamagnetic magnetite/maghemite particles that primarily originated from volcanic ash, with secondary contributions from pedogenesis. The former has an average grain size of similar to 19nm with a normal distribution of volume, and the latter has a much wider grain size distribution. The presence of volcanic ash within the red soil indicates that these Quaternary basalts were not formed by continuous volcanic eruptions. Moreover, the magnetic enhancement patterns differ between the upper and lower zones. The upper zone is more magnetically enhanced and experienced higher precipitation and temperature than the lower zone. Discrimination of superparamagnetic particles originating from pedogenic processes and volcanic ash thus provides a sound theoretical base for accurate interpretation of magnetism in red soils in this region.