Kaolinite dating from Acrisol and Ferralsol: A new key to understanding the landscape evolution in NW Amazonia (Brazil)

Abstract

Ferralsols and Acrisols are major types of soils of the Amazon basin observed on various landform units comprising plateau surfaces, incised hills at their edges and slopes. The present study focuses on an Acrisol developed on plateaus surfaces from northwest Amazonia and a Ferralsol from the convex hills of the incised plateau edges. Local geomorphologic models of weathering covers suggest that Ferralsols are younger than plateau Acrisols but the absolute chronology of their formation is still lacking. This type of information is however critical to understand the evolution of northwest Amazon Basin landscapes and to identify its link with major climatic and geomorphologic events. In this paper, kaolinite-rich samples from soils and saprolites belonging to a transect in the São Gabriel da Cachoeira region (Amazon state, Brasil) are investigated. Based on their crystal-chemical characters, several types of kaolinites are identified. Using a previously developed methodology based on electron paramagnetic resonance (EPR) spectroscopy, crystallization ages are proposed for these different kaolinites. Saprolite kaolinites are dated from 6 to 3.6 Ma in the Acrisol profile and display significantly more recent ages (< 1 Ma) in the Ferralsol saprolite. Kaolinite from solum (soils horizons above the C horizon) display ages ranging from 2.5 to 1 Ma for both the Acrisol and Ferralsol. Three distinct weathering stages are thus unraveled by kaolinite dating. The Acrisol saprolite displays the older weathering stage preserved in the investigated soil sequence. It is followed by a single weathering stage leading to the formation of both soils. These two stages can be correlated to the formation of two paleosurfaces recognized at the scale of the South America subcontinent as the Miocene Vehlas and the Quaternary Paraguaçu surfaces, indicating that the Curicuriari profiles evolved during periods favorable to tropical weathering surfaces development in Amazonia. The last weathering stage corresponds to the saprolite formation in the Ferralsol profile, which is still developing under the present Amazonian climate. This still active, late 2 weathering stage is tentatively related to the more significant drainage and relief dissection occurring on the plateau edges.

Dating kaolinite from the Neogene Içá Formation and overlying laterites, central Amazonia, Brazil: Constraints for a stratigraphic correlation

“…In addition, the ages of the Içá Formation are significantly younger than those of kaolinite samples from the Alter do Chao sediment at 22.2 -65.3 Ma (Balan et al, 2005), as seen in Table 2. They are close to the ages of the oxisols and intermediate horizons from the Manaus region (5.7-19.2 Ma, including the transition horizons; Balan et al, 2005) and some samples from Sao Gabriel laterites (0.21-1.68 Ma for the Ferralsol and 1.03-5.98 Ma for the Acrisol; Mathian et al, 2020). This suggests that common paleoclimatic episodes are revealed in the secondary mineral record at the scale of the Amazon Basin.…”

Tropical landscape evolution and its relationship with paleosurfaces, Ferralsols and ferruginous duricrusts in the Alto Paranaíba region (Central Plateau, Minas Gerais, Brazil)

“…The mineralogy of the clay fraction of all soil profiles is dominated by kaolinite, gibbsite, hematite, goethite, anatase/rutile, and accessory minerals (i.e., zircon in the soils developed in the transient landform) (Figs. 2,3,4 and 5), similar to that observed in others Ferralsols developed over different parent materials, climates, and landscape positions in the intertropical zone (e.g., Curi and Franzmeier, 1984;Vidal-Torrado et al, 1999;Fritsch et al, 2005;Muggler et al, 2007;Mathian et al, 2020). The variation of the basal spacing and broadening of the (001) reflection of kaolinite is not related to the interstratification with 2:1 clay mineral, considering that the Asymmetry Index (AI) of this plan was low (Tab.…”

“…Highly and long-term weathered soils recognised as either ferralitic or lateritic soils may be classified as Oxisols (Soil Survey Staff, 2014), Ferralsols (IUSS Working Group WRB, 2015, or Latossolos in the Brazilian Soil Classification System (Santos et al, 2018). Many of them have been developed under distinct weathering periods for sometimes for millions of years over different elevations, landforms and lithologies in humid and semi-humid tropical climate conditions (e.g., Chauvel and Pedro, 1978a;Bittom and Novikoff, 1991;Tardy and Roquin, 2000;Mathian et al, 2020). The weathering conditions that prevailed during their formation are recorded in their mineralogy, which may be an archive to decipher the major paleoclimates when appropriately constrained their formation by dating methods of secondary minerals (e.g., Balan et al, 2005;Fritsch et al, 2011;Mathian et al, 2019;Allard et al, 2020).…”

“…The general trend of progressive weathering under tropical climate conditions is towards the residual accumulation of iron and aluminum oxyhydroxides: the ferralitization (e.g., Muller, 1972;Chauvel and Pedro, 1978b;Herbillon, 1980). However, varying proportions of gibbsite, kaolinite, iron oxides/oxyhydroxides, and frequently minor accessory minerals have been observed in highly weathered soils in the intertropical zone (Tardy et al, 1991;Curi and Franzmeier, 1984;Furian et al, 2002;Muggler et al, 2007;Mathian et al, 2020). It is well known that the amounts and characteristics of kaolinite and gibbsite are governed by the extent to which desilication occurs and weathering conditions prevailing at the present time of their formation (Muller, 1982;Tardy et al, 1991;Fritsch et al, 2005).…”

“…Other techniques for evaluation of minerals in clay fraction have been used (e.g., Mössbauer spectroscopy, thermogravimetry and differential thermal analysis, isotopic composition, electron paramagnetic resonance, infrared spectroscopy, and transmission electron microscopy) and insights from the mineralogy of highly weathered soils have emerged (e.g., Girard et al, 2000;Mathian et al, 2020;Allard et al, 2020). Among them, the upwards replacement of an old population of well-ordered kaolinites with large particle size by a more recent population of disordered kaolinites with smaller particle size has been verified in Ferralsols from Amazon Basin (Fritsch et al, 2002;Balan et al, 2005;Balan et al, 2007;Mathian et al, 2020) and over an ancient landscape in Minas Gerais State, Brazil (Muggler et al, 2007). Therefore, the direction of weathering and the conditions that prevailed during its formation cannot be adequately revealed by the composition of the clay fraction itself assessed via XRD patterns.…”