澳大利亚悉尼 Long Reef Beach 中新世古土壤岩石磁学特征及环境意义

Early Miocene paleosols, dated back to 17 Ma BP, are prevalently distributed in the Sydney area, of which laterite strata was previously thought to form in tropic environments with high temperature and humidity. However, the Australia plate did not yet drift to current position during the early Miocene when Sydney paleolatitude was 45°S-50°S, thus belonging to the temperate zone. So, what were the paleoenvironments like during the laterite development remains controversial. In order to better understand and investigate the paleoclimate of Sydney during the early Miocene, a typical paleosol profile with laterite, located at the Long Reef Beach (LRB) town (northeast of Sydney), was taken for analysis, which was divided into three parts: Holocene part, Miocene Ultisol part and Miocene Laterite part. Environmental magnetism, an efficient approach to acquire environment information recorded by magnetic minerals, has been successfully used in study of loess-paleosols. Herein, this approach is first introduced to study the Miocene paleosol, i.e. LRB. Rock magnetic measurements on all samples (n=55) include low field magnetic susceptibility (χ), saturation isothermal remanent (SIRM), saturation magnetization (M_s), anhysteretic remanent magnetization (ARM), magnetic hysteresis loops, and thermomagnetic analysis (i.e. M -T & κ -T curves). Magnetic assemble in the Holocene part, which is compatible with counterparts in the Chinese loess plateau (CLP), is dominated by magnetite in the multi-domain (MD). For the Miocene Ultisol part, a trend of magnetic transformation from magnetite into maghemite against depth was observed, which coexisted with appearance of hematite. It is characteristic of higher magnetic concentration than developed paleosol in CLP and a large grain size distribution ranging from single domain (SD) to MD, and the most important is psuedo-single domain (PSD). The part of Miocene Laterite is much higher magnetic concentration and characteristic of predominant hematite with slight concentration of maghemite and goethite. The grain size of hematite is dominated by SD, at the same time showing a bit of PSD and MD. LRB Miocene paleosol is speculated to have formed in a context of exceeding rain precipitation and high temperature, where intense chemical weathering on the Earth surface occurred. The gradual alternation of magnetite into maghemite and hematite seen in the part of Miocene Ultisol can be attributed to low temperature oxidation due to eluviation caused by abundant rainfall. Maghemite, formed in the part of Miocene Ultisol, leached downwards and was transformed into stable hematite due to high temperature and recrystallization in the part of Miocene Laterite. The conversion process of magnetic minerals with increasing chemical weathering can be summarized as magnetite-magnetite core wrapped by maghemite shell-maghemite-hematite. Maghemite thermal stability was observed in a part of samples, which might be due to its coarser grain size or Fe³⁺ replacement by Al³⁺.