Climatic Response Of Pedogenic Iron Oxides In Subtropical And Tropical Regions

Climatic response of continental chemical weathering is a basic theoretical question of earth science. It is a hot issue of the research on Critical Zone. Exploring the climate response of continental chemical weathering in regional scale can help strengthen the weathering theory and evaluate the environmental effect of continental chemical weathering accurately. Iron oxides are highly concentrated in the surface of the earth as the.main final product of continental chemical weathering. The iron oxides can be divided into a coloring group and a magnetic group according to their physical properties. The content and ratio of iron oxides have been widely employed in soil taxonomy and paleorainfall reconstruction. However, the climatic implication of iron oxides under extreme climate remains ambiguous. Exploring the climatic response style of iron oxides in subtropics and tropics with different combinations of temperature and rainfall can help understand the concentration and distrubtion mechanism of iron oxides systematically. More importantly, it lay the foundation for the paleoclimate reconstruction accurately in future.With the purpose of exploring the climate response style of pedogenic iron oxides in extremely warm regions, we chosed eight saprolitic climosequences with The content and ratio of pedogenic iron oxides is determined by magnetic and spectral method. The climatic response of total amout of iron oxides and earch gourp of iron oxides has also been systematically discussed. The main results of this study are listed as below:1. Hematite and goethite are the main coloring agents in subtroipical and tropical soils. Hematite has much stronger dying power than goethite in natural samples. Both the color index of redness and the peak position of of first derivative spectrum can be employed to determine the content of hematite in tropical and subtropical soils effectively. The response sensitivity of redness to hematite is dependent on the content of quartz in soil matrix.2. There is a rainfall inflection point around1400-1500mm controlling the relationship between ferrimagnets, hematite and rainfall in the soils derived from basalt and granite under tropical humid climate in Hainan Island. The ferrimagnets and hematite has positive correlation with rainfall below the point while they have negative correlation above the point.3. The formation of magnetic and coloring iron oxides is sensitive to the change of temperature in subtropical regions of Yunnan without sufficient heat. In addition, they show strong dependence on the rainfall in low rainfall (<1500mm) regions.4. The formation of magnetic and coloring iron oxides is senstive to the change of rainfall in North Africa without sufficient water. In addition, the index of Redness/Fed can act as a substitute for Hm/(Hm+Gt) to trace the relationship between rainfall and temperature in aerobic soils derived from different parent materials acoss arid and semi-arid regions5. The concentration of total amount of iron oxides is dependent on the parent material and chemical weathering intensity. The increasing of both temperature and rainfall can promote the chemical weathering and the concentration of iron oxides.The combinative index of RTIp can help understand the comprehensive effects of temperature and rainfall on the concentration process of iron oxides. The accumulation of iron oxides is more sensitive to the change of RTIp at a low level than at a high level.6. The distribution of coloring iron oxides including hematite and goethite is independent on chemical weathering and dependent on the relative supply of heat and water. High temperature and low rainfall favors the formation of hematite whereas low temperature and high rainfall favor the formation of goethite on a large spatical scale across subtropical and tropical regions. The combinative index of RTIq can be helpful to estimate the comprehensive effects of temperatrue and rainfall on the distribution of coloring iron oxide. The index of Hm/(Hm+Gt) is more sensitive to the change of RTIq at a high level than at a low level.7. The content of hematite and ferrimagnets has commonly positive correlation in saprolitic soils derived from granite and basalt across subtropical and tropical regions with different combination of temperature and rainfall. The widespread correlation verified the ageing path of hematite in laboratory experiments.8. Based on the specific climosequences and profiles with high ferrimagnets or high Fe-bearing primary minerals, it has been found that the magnetic susceptibility shows negative correlation with the frequency magnetic susceptibility in the early stage and late stage of chemical weathering. The former is attributed to the destruction of multi-domain magnetic particles derived from high magnetic parent material while the latter is attributed to the grainsize growth of magnetic particles with highly concentrated pedogenic ferrimagnets. A conceptual model has been established for the evolution of the grainsize distribution of ferrimagnets with increasing chemical weathering and pedogenic ferrimagnets.9. Based on the systematic research on the climosequences in subtropical and tropical regions, we propose that the formation process of pedogenic iron oxides in surfical environment involves in three processes including the concentration process of total amount of iron oxides, distribution process between hematite and geothite, and coupling process between hematite and maghemite. A2-D model considering rainfall and temperature is proposed to help understand the climate response of pedogenic iron oxides. It is suggested the rainfall inflection point controlling opposite patterns between rainfall and ferrimagnets across tropical and subtropical regions should increase with the temperature..