See more from this Session: Symposium--Mineral-Organic Interactions Across Time and Space: I & II
Monday, October 17, 2011: 1:15 PM
Henry Gonzalez Convention Center, Room 212B, Concourse Level
Spots of a few square metres in size, as red as Munsell colour dusky red 10R ¾ are found in a number of low iron containing sandy soils in Denmark. These soils were exposed to fire, and after ashes raised pH, dispersed soil particles were transported with seepage water down into lower soil horizons. These particles contain hematite and maghemite due influence of the fire. However, a long-standing unresolved question is how hematite and maghemite can be present along with goethite and ferrihydrite in extended areas with high iron content (8-40 %) in the topsoil, in the same geographical region. Hematite and particularly maghemite would normally not be expected to form under the temperate humid Danish climate, but be interpreted as the result of high temperature as found in tropical regions or as seen in soils exposed to fire. The high iron content most likely has its origin in pyrite dissolution at the top of the groundwater zone in deeper Miocene deposits. Fe2+is then brought to the surface in wells by the groundwater and oxidized. It precipitates as two line ferrihydrite, and is later transformed into goethite, (hematite and Maghemite?). Forest fires would be a likely explanation to the hematite and maghemite. There is, however, a body of evidence that argues against these sites having been exposed to fire. 1) The pH in the topsoil is 3.6 – 4.8 and thus not raised by ashes. 2) No charcoal is present. 3) There is no indication of fire outside the high iron content areas. 4)Goethite is present along with hematite and maghemite in microparticles, and the mineralogical zonation produced in a forest fire is not seen. The natural sites contain a uniform mixture of goethite/ferrihydrite, hematite and maghemite down to 20 cm depth. An experimental forest fire left charcoal and ashes at the topsoil, produced high pH, mineral zonation and decreased organic matter content, all of which is in contrast to the natural sites. In the freshly precipitated iron materials iron oxidizers as Gallionella sp. were found, but also iron reducing Geobachter sp.were present. Could microbial activity explain the mineral transformations?