Browsing by Subject "Pedogenesis"

Pedogenic ferromanganese nodules and concretions are prevalent redoximorphic features in tropical and sub-tropical soils. The nodules are typically highly enriched in Fe and Mn that are present as oxides, hydroxides and oxyhydroxides. The formation of nodules happens via precipitation and translocation of metals as the soil redox state undergoes cyclical changes between reductive and oxidizing settings. As the nodule elemental distribution and structure is primarily and expression of the prevailing soil redox conditions, Fe-Mn nodules have the potential to be a useful tool of paleoclimatological analysis. The Chinese Loess Plateau (CLP) is a terrestrial archive for study of changes in the monsoon climate system. During Late Miocene, the intensification of the Asian Monsoon system caused an increase in warmth and humidity in inland Eastern Asia during a global trend of increased aridity and decreasing temperatures. Fe-Mn nodules from three different soil horizons, formed 8.07, 7.7 and 3.7 Ma ago in Lantian, southern CLP, were studied to compare nodules from varying sedimentary settings formed under different moisture regimes. Using electron microscopy methods, the structure and elemental distribution of nodules were described to compare their redoximorphic features. Large Fe-Mn nodules from floodplain sediments (8.07 Ma) show a well-developed structure, high metal enrichment and signs of variations in rate of formation and dominant redox states. The soil redox conditions are likely primarily controlled by the river flooding. Nodules from two eolian deposits (7.7 Ma and 3.7 Ma) were, on average smaller and showed less metal enrichment, less elemental differentiation and less variance in the dominant redox conditions. Only small, poorly developed nodules were found from older eolian sediments whereas younger soil horizon contained larger nodules with evidence of higher hydromorphism. While potential for using the nodules from eolian sediments to assess changes in precipitation exists, the lack of paleoclimatological information in smaller nodules, the small sample count, limitations of the methods and variance in depositional settings increase the uncertainty of the interpretation.