Rainfall-dependent transformations of iron oxides in a tropical saprolite transect of Hainan Island, South China: Spectral and magnetic measurements

Pliocene Vertisols from the Turkana Basin of northwest Kenya (~4°N latitude) have been examined using isothermal remanent magnetization (IRM) experiments, susceptibility measurements, and diffuse reflectance spectroscopy. The complete vertical profile of each paleosol is almost intact, measuring >2 m in thickness and being strongly magnetic (IRM at 1.0T ranges from 4 to 25 × 10−3 Am2/kg). Downprofile changes to the proxy indicators suggest uninterrupted pedodevelopment and a lack of stratigraphic inversions caused by argilli‐pedoturbation. Magnetic minerals consist of hematite to a lesser degree, and pedogenic ferrimagnets derived from moisture cycles that were controlled by monsoonal rainfall. Basal pedogenic zones of the paleosols are magnetically intense, and preserve the greatest slickenside development indicating pronounced seasonal wetting and drying. These observations indicate a deeper dry season water table as compared to poorly drained temperate/tropical Vertisols, which suffer reductive dissolution of Fe oxides in basal pedogenic horizons associated with weak magnetic intensities. Magnetic susceptibilities and the well‐represented presence of fine pedogenic ferrimagnets suggest that Pliocene rainfall was greater than the local modern rate of 200 mm/year and may have been within the range of 500–1,000 mm/year. The study highlights the need to expand the database on Vertisols Fe oxides and magnetism, especially considering the anomalously dry and bimodal rainfall seasonality of East Africa.

Section: Methodsmentioning

confidence: 99%

Constraints on Fe‐Oxide Formation in Monsoonal Vertisols of Pliocene Kenya Using Rock Magnetism and Spectroscopy

Lepre

2019

“…Diffuse reflectance spectra (DRS) were obtained spanning the range from 400 to 700 nm at 1 nm intervals, using a Perkin Elmer Lambda 950 spectrophotometer. According to the method of Long et al (), the redness% was used to estimate the Hm content (Hm%) using a linear regression model, as follows: Hm% = 0.05 × redness% − 1.15. Free iron oxides (Fed) were estimated by the citrate/bicarbonate/dithionite (CBD) method, and the Gt content was estimated as Gt%=1.59 × (Fed − Hm%/1.43) (Torrent et al, ).…”

Section: Laboratory Methodsmentioning

confidence: 99%

Variations in the Iron Mineralogy of a Loess Section in Tajikistan During the Mid‐Pleistocene and Late Pleistocene: Implications for the Climatic Evolution in Central Asia

Jia

Wang

et al. 2018

The loess sequences in Tajikistan are an important archive of information about the development of climate and atmospheric circulation in central Asia during the Pleistocene. Here we present the results of an iron mineralogical study of a loess sequence in Tajikistan to reconstruct paleoclimate evolution during the mid‐Pleistocene and late Pleistocene. The record indicates that interglacial intervals were relatively humid and glacials were dry. We propose a shift in the character of the interglacial climate of the region to more humid after beginning of MIS 9; however, temperature was relatively stable. The location and intensity of Westerlies have a close link with Asian Summer Monsoon (ASM) circulations. From the beginning of MIS 9, there was a southward movement of the Westerlies circulation, which corresponded to a retreat of the Indian Summer Monsoon (ISM). A similar relationship also existed between the East Asian Summer Monsoon (EASM) circulation and the Westerlies, which is evidenced by the formation of the most weakly developed soil unit in the EASM‐dominated regions during MIS 9, in contrast with the formation of the most strongly developed soil unit in the Westerlies‐dominated regions. As conditions in the areas of loess deposition became more humid, the sedimentary basins which are the dust source areas became progressively more arid. The aridification of the source areas may be the result of increased Northern Hemisphere ice volume and accelerated high mountains and/or plateaus uplift in the surrounding regions.

“…As one of the most abundant iron hydroxides in soils, goethite plays potentially important roles in recording information of past climatic and environmental changes (Balsam et al, ; Cornell & Schwertmann, ; Hyland et al, ; Ji et al, ; Long et al, ). Higher excess moisture (precipitation exceeding evapotranspiration), lower pH, and higher organic carbon content in soils favor production of goethite (Kämpf & Schwertmann, ).…”

Section: Introductionmentioning

confidence: 99%

Goethite Concentration Variations in the Red Clay Sequence on the Chinese Loess Plateau

Zhang

Nie

2017

The Goethite is a common magnetic mineral in soils and is important for understanding past climatic and environmental change. Although goethite concentration records are available for the Quaternary loess‐paleosol sequences from the Chinese Loess Plateau (CLP), no goethite concentration record is available for the underlying Red Clay sequence. Here, we generate a record of goethite concentration from the Chaona section on the CLP and investigate the relationship between goethite and ferrimagnetic mineral concentration and climate parameters on the Red Clay sequence deposited from 6 to 2.7 Ma. We found that increasing goethite concentration generally corresponds to decreasing magnetic susceptibility for the Red Clay sequence, with the exception of an anomalous section from 4.3 to 3.7 Ma where both records show increasing trends. Precipitation seasonality could explain this opposite variation pattern between magnetic susceptibility and goethite concentration in the Red Clay sequence. From 3.6 to 2.7 Ma, goethite concentration has a decreasing trend while magnetic susceptibility increased, which we interpret as more seasonally distributed precipitation associated with intensification of the East Asian summer monsoon (EASM). Similar pattern was observed from 0.65 to 0 Ma from the Lingtai section. Taken together, the goethite record reveals that the EASM experienced two stages of rapid intensification with apparent seasonally distributed precipitation since 6 Ma.