PEDOGENIC IRON OXIDE MINERALOGIC VARIATION FROM A CLIMATIC AND GEOGRAPHIC TRANSECT OF THE LOESS PLATEAU, CENTRAL CHINA

The Loess Plateau of north central China (CLP) contains a 2.5 ma record of paleoclimate change. The modern climate is dominated by alternation of the warm, wet summer and cold, dry and windy winter monsoons. Former glacial climates on the CLP were likely dominated by winter-monsoon-like conditions and enhanced loess deposition, whereas interglacials were dominated by summer-monsoon-like conditions conducive to rapid weathering and soil formation.

The purpose of this work is to compare the iron oxide/oxyhydroxide mineralogy contained within 2 loess-paleosol couplets collected from 2 sites on the CLP. Wiggle matching of sedimentary magnetic susceptibility variations with the marine SPECMAP d18O record has established that the S1/ L2 (paleosol and parent loess respectively) couplet represents the Last Interglacial soil and preceding OIS-6 glacial loess. The S5/L6 paleosol-loess pair corresponds to the MIS 13-15/16 interglacial and glacial stages respectively.

The study sites represent a transect from the drier northern Plateau margin to the wetter southern margin. The northern Shimao site is characterized by coarse silts and oxidized sands derived from proximal desert sources. The southern Wei'nan site is distal to the dust sources and is characterized by thin, clayey, highly weathered loess and paleosols containing abundant redoxymorphic concentrations.

This project compares iron oxide mineral occurrence in loess and paleosols on an intra-site and regional basis. Oxide mineral variation within loess-paleosol couplets includes differences in grain size, mineralogy and origin. Silt to clay-size oxides are transported, whereas submicron oxides are formed in situ, often as coatings. Oxide mineral phases differ at the nanoscale in structure and reactivity compared to their macroscopic equivalents. The goal of this work is to establish correlations between the nanoscale iron oxides and the soil environmental conditions of their formation.

Bulk pedogenic iron oxides have been isolated and will be examined via analytical TEM for crystal chemistry and structure. This and surface scaling patterns will be used to help distinguish discrete populations of minerals. Finally, oxygen isotopes of iron oxides will be measured to directly compare regional hydrologic conditions with global proxies of climate change.