Climatic thresholds for pedogenic iron oxides under aerobic conditions: Processes and their significance in paleoclimate reconstruction. (15th October 2016)

• Record Type: Journal Article
• Authors: Long, Xiaoyong; Ji, Junfeng; Barrón, Vidal; Torrent, José
• Is Part Of: Quaternary science reviews. Volume 150(2016)
• Page Start: 264
• Page End: 277
• Publication Date: 2016-10-15
• Subjects: Climate threshold -- Iron oxides -- Magnetism -- Color -- Paleoclimate reconstruction; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2016.08.031
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Iron oxides are widely distributed across the surface of the Earth as a result of the aerobic weathering of primary Fe-bearing minerals. Pedogenic iron oxides which consist mainly of hematite (Hm), goethite (Gt), maghemite (Mgh), are often concentrated synchronously in aerobic soils under low to moderate rainfall regimes. Magnetic susceptibility (χ) and redness, which respectively reflect the content of Mgh and Hm in soils, are considered reasonable pedogenic and climatic indicators in soil taxonomy and paleorainfall reconstruction. However, under high rainfall regimes, the grain growth of Mgh and transformation to Hm, combined with the prior formation of Gt under conditions of high relative humidity (RH), can result in magnetic reduction and dramatic yellowing of soils and sediments, which explains the existence of rainfall thresholds for Mgh and Hm at a large scale even before the pedogenic environment turns anaerobic. In order to capture the rainfall thresholds for Mgh and Hm occurring under aerobic conditions, we explored a tropical transect across a granitic region where the soil color turned from red to yellow under a wide rainfall range of 900–2200 mm/yr and a corresponding mean annual RH range of 77%–85%. We observed a lower rainfall threshold of ∼1500 mm/yr and a corresponding RH ∼80% for Mgh and Hm along this transect, as well as a higher rainfall threshold of ∼1700 mm/yr and a corresponding RH of ∼81% for Gt and total pedogenic iron oxides (citrate/bicarbonate/dithionite-extractable Fe, Fed ). Cross-referencing with comparable studies in temperate and subtropical regions, we noted that the rainfall or RH thresholds for Fed and Hm or Mgh likewise increase with temperature. Moreover, the different thresholds for total and individual iron oxide phase indicates that a negative correlation between chemical weathering intensity and redness or χ in sediment sequences can occur under the prevalent climate regime just between their thresholds. Finally, we developed an integrated model to interpret the sequential formation of rainfall thresholds for different pedogenic iron oxides in soils and sediments with conditions from aerobic to anaerobic. Graphical abstract: Highlights: Higher rainfall thresholds for Fed and Gt than Hm and χ were observed under aerobic condition Rainfall thresholds for Fed, Mgh or Hm increase with temperature. Negative correlation between chemical weathering intensity, redness or χ can occur between their rainfall thresholds. An integrated model was proposed on the content of different Fe oxides with increasing rainfall.