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Ferrimagnetic (FM) and antiferromagnetic (AFM) iron oxide particles are considered pedogenic and climatic indicators due to their enrichment with comparable increasing in rainfall and temperature. However, the opposite changes in rainfall and temperature result in rapid change of relative humidity (RH), which could lead to their competition and transformation. We examined two soil sequences which have undergone contrary climate development on the eastern edge of the Tibetan Plateau. Dry and warm climate with low RH favors the coordinative enrichment of AFM hematite and FM particles, while wet and cool climate with high RH mainly produces goethite but leads to competition between low content AFM hematite and FM particles. The outcome well interprets the changing relationship between color and magnetism in soils and sediments, and suggests that temperature is as important as precipitation in paleoclimate reconstruction based on iron oxides, especially during strong dry‐wet cycles and climate pattern shifts. Plain Language Summary: Iron oxides are commonly enriched on the surface of Earth as weathering products driven by comparable increases of rainfall and temperature. Color and magnetism of soils dominated by antiferromagnetic (AFM) and ferrimagnetic (FM) iron oxides may present sensitive pedogenic and climatic indicators. However, contrary changes in rainfall and temperature often lead to remarkably changing relative humidity (RH) which could promote their competition and transformation into iron hydroxides. The uplift of the Tibetan Plateau has led to different orographic elevation and differential climate development on the eastern edge on the Yunnan Plateau and the Guizhou Plateau. We found that the present dry and warm climate on the Yunnan Plateau with low RH favors the dehydration of amorphous iron oxides to form AFM hematite and FM particles concomitantly, while the present wet and cool climate on the Guizhou Plateau with high RH mainly produces goethite and leads to a competitive development of AFM hematite at low concentration and FM particles. These conditions explain the variable relationship of magnetism and color in soils and sediments. It is also suggested that temperature is as important as precipitation when reconstructing paleoclimate based on iron oxides, especially during strong dry‐wet cycles and climate shifts. Key Points: Dry and warm climate favors the dehydration of amorphous iron oxides to form antiferromagnetic hematite and ferrimagnetic particlesWet and cool climate mainly produces goethite and leads to the competition between antiferromagnetic hematite and ferrimagnetic particlesTemperature is as important as precipitation when reconstructing paleoclimate with strong dry‐wet cycles and climate pattern shifts [ABSTRACT FROM AUTHOR]

Magnetic particles associated with iron (Fe) oxides are widespread on the surface of Earth and Mars and serve as reasonable climatic indicators. Ferrimagnetic maghemite (Mgh) and antiferromagnetic hematite (Hm), which dominate magnetism and redness, often coexist or compete with each other in soils and sediments. The formation efficiency of Mgh relative to Hm could be modulated by geochemical background besides climate, especially by phosphate (P), which has a high affinity for the surfaces of precursor iron oxides in natural systems. We investigated two Ferralsol sequences around a P mining field with similar climates and contrasting P/Fe ratios. High P/Fe ratios retard iron oxide crystallization, grain growth, and transformation into Hm, thereby promoting more effective accumulation of ferrimagnetic Mgh as an intermediate product. The lack of ligand‐protected effects well interprets asynchronous changes in magnetism and redness in soils and sediments across large spatiotemporal scales, especially in highly weathered soils. Plain Language Summary: Iron oxides are critical carriers of magnetism and dyeing agents of soils and sediments. The concentration of pedogenic maghemite (Mgh) and hematite (Hm), which dominate magnetism and redness, is considered to be controlled by the primary iron input and climate conditions. However, phosphate exhibits a high affinity for the surfaces of precursor iron oxides, especially in highly weathered soils. We investigated two Ferralsol sequences around a P mining field with contrasting P/Fe ratios but similar climates. High P/Fe ratios are observed to impede iron oxide crystallization, grain growth, and transformation into Hm, thereby promoting more effective accumulation of ferrimagnetic Mgh as an intermediate product. The presence and absence of ligand‐protected effects help explain asynchronous changes in magnetism and color in nature systems across large spatiotemporal scales. Key Points: Iron oxide crystallinity increases monotonically as soil P/Fe decreasesFerrimagnets accompanying the formation of hematite are enriched with accelerations under high P/Fe but with even rates under low P/FeFerrimagnets grow and transform into hematite more rapidly without the proper level of P ligand protection [ABSTRACT FROM AUTHOR]

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, Fe d ). Cross-referencing with comparable studies in temperate and subtropical regions, we noted that the rainfall or RH thresholds for Fe d 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. [ABSTRACT FROM AUTHOR]

• Lithogenic hematite is enriched on the top of the volcano. • Pedogenic hematite is enriched on leeward slopes, and goethite on windward slopes. • More and finer ferrimagnetic particles are formed on leeward slopes with hematite. • Amorphous iron is more enriched on the shady slopes than on the sunny slopes. Volcanoes are widely distributed on the surface of the Earth and on other planets. Volcanic activities play a key role in the long-term evolution of the planetary atmosphere and geomorphology. Iron oxides on volcanoes often exist as the products of rapid magmatic oxidation driven by earlier lithogenesis or the weathering of primary iron-bearing minerals driven by later pedogenesis. The antiferromagnetic (AFM) and ferrimagnetic (FM) particles associated with the formation and transformation of iron oxides dominate the color and magnetism of soil and can potentially serve as an indicator to trace these processes. In this study, we systematically investigated the subsoils derived from a Pliocene volcano under a tropical monsoon climate. We observed that the crystalline iron oxides are differentiated along the leeward and windward slopes, while amorphous iron oxides are mainly differentiated along the shady and sunny slopes. The AFM hematite is more concentrated on the purplish top and the reddish leeward slopes, while the AFM goethite is more concentrated on the yellowish windward slopes. Coarser FM particles accompanied by lithogenic hematite form via early lithogenesis at the top, while more fine FM particles are concentrated on the leeward slopes with the neoformation of pedogenic hematite. Fewer but finer FM particles form with goethite on the windward slopes. These results suggest that FM particles could coexist with AFM hematite via lithogenesis or pedogenesis and that goethite would be a more reasonable pedogenic indicator in volcanic regions. [ABSTRACT FROM AUTHOR]

• Free iron oxides decrease as amorphous iron oxides increase downslope. • Hematite significantly decreases as goethite keeps little variation downslope. • Ferrimagnetic particles become less and finer downslope. • Ferrimagnetic particles couple with hematite except the finest particles. • Transformation and transportation co-control magnetic particle redistribution. Iron oxides are widespread on the surface of the Earth as a weathering product of iron-bearing primary minerals, which often accompany the enrichment of magnetic particles under aerobic conditions. The magnetic properties determined by the content and ratio of magnetic particles have been widely employed in paleoclimate reconstructions and erosion simulations. However, water redistribution at a slope scale could cause a phase transformation of iron oxides and a corresponding change in magnetic properties. In this study, we investigated a catena with a distinct soil colour change from red to yellow, which commonly occurs in tropical and subtropical hilly and mountainous regions. From upslope to downslope, the free iron oxide content decreases while the amorphous iron oxide content increases. The hematite diminishes significantly while the goethite shows little variation. The ferrimagnetic particles decrease in phase with hematite except for the finest superparamagnetic particles. However, the concentration of finer ferrimagnetic particles relative to coarser particles and the concentration of ferrimagnetic particles relative to antiferromagnetic particles both increase downslope. This increase validates the formation path of ferrimagnetic particles as an intermediate product of hematite but reveals more significant particle growth and transformation into hematite with decreasing water activity in granitic soils with low iron content. In addition, these processes could be enhanced by the preferential migration of the finest ferrimagnetic particles or their transformation from amorphous iron oxides enriched downslope. The slope-scale redistribution of iron oxides and related magnetic particles increases their uncertainty in paleoclimate reconstruction and erosion simulation but brings promising prospects to trace geomorphic evolution and hydrologic change over the long term. [ABSTRACT FROM AUTHOR]