CLIMATIC AND ELEMENTAL THRESHOLDS OF THE FORMATION OF SOIL Fe-Mn NODULES IN THE WARM AND HUMID REGION OF CHINA
Fe-Mn nodules in the warm and humid region of China(19°～36° N,98°～122° E)are formed in the alternating dry and wet soil environment,controlled by monsoon climate with cold and dry conditions in winter and warm and humid conditions in summer.It has long been theoretically proposed that the appearance of soil Fe-Mnnodules corresponds to warm and humid environment.However,a quantitative understanding of the climatic threshold for the formation of soil Fe-Mn nodules based on empirical data is still lacking on a large scale.Aside with the climatic threshold,the existing quantitative understandings are limited by the fact that data on formation of soil Fe-Mn nodules are typically only available for a single soil profile.Here,using a comprehensive compilation of the mean annual temperature(MAT),mean annual precipitation(MAP),chemical index of alteration(CIA)and Fe-Mn element contents of 154 soil profiles with 912 samples across over the warm and humid region of China,located in warm temperate,subtropical and tropical regions,we investigate the relationship between climate and the formation of soil Fe-Mn nodules during silicate weathering.In the case of these soil profiles,43 soil profiles exclusively provide Fe-Mn nodules,65 profiles contain Fe-Mn nodules,and the nodule has not been observed in the other 46 profiles through field investigation in the region.Comparative and statistical analyses of soil profiles exclusively provided Fe-Mn nodules and spatial variations in meteorological factors reveal that the climatic threshold for the formation of soil Fe-Mn nodules is MAT＞15 ℃ and MAP＞800 mm in the warm and humid region of China.The threshold is demonstrated by the relationship of soils with and without Fe-Mn nodules and contour maps of meteorological variables in the region.Furthermore,correlation of soil silicate weathering intensity and Fe-Mn content,with and without Fe-Mn nodules,suggests that soil TFe2O3＞5％and MnO＞0.1％are the elemental threshold of the formation of some soil Fe-Mn nodules in the study region.The region with soil Fe-Mn nodules is also marked by an extreme chemical weathering stage(CIA values exceeding 80),which is observed in regions adorned with supply-limited(i.e.,weathering is limited mainly by the supply of fresh material)weathering regimes.Although several soil profiles without Fe-Mn nodules from Sichuan Basin meet the climatic threshold and soil TFe2O3 content,the content of soil MnO is less than 0.1％and consequently limits the formation of soil Fe-Mn nodules in the basin.Additionally,the pedogenesis also poses a significant effect on the formation of some soil Fe-Mn nodules,as the relatively-weak chemical weathering corresponds to these soils without Fe-Mn nodules in the study region.A significant conclusion from the foregoing is that a large area of exposed soil Fe-Mn nodules that developed in high temperature climate points to the warm and humid condition with MAT＞15℃ and MAP＞800 mm.The conclusion plays a critical role in our in-depth understanding of the formation soil Fe-Mn nodules and its underlying climate significance.Moreover,the climatic threshold provides an important baseline for climate reconstruction based paleosol for the intervals of hothouse throughout Earth's history.
soil Fe-Mn noduleclimatic thresholdchemical alteration indexwarm and humid region
万方数据
维普
