Abstract
At a number of sandy soil sites in Mid Jutland, Denmark, with iron
content of 1-2%, very red spots (Munsell colour: dusky red 10R 3/4) of
a few square meters are found. These spots are most likely due to
burning events. After the fire ashes raised pH. This dispersed silt and
clay size soil particles which were then transported with seepage water
down into lower soil horizons. These particles contain hematite and
maghemite due to influence of the fire. However, a long-standing
unresolved question is how hematite and maghemite can also be present
along with goethite and ferrihydrite, in the same geographical region,
and in extended areas with high iron content (8-40 %) in the topsoil.
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 in top of the groundwater zone in deeper Miocene
deposits. From there Fe2+ is brought to the surface by the groundwater,
and in wells oxidized by meeting the atmosphere and precipitated as two
line ferrihydrite. This is later transformed into goethite. However,
along with these two minerals hematite and maghemite are present in the
topsoil around the well area. Forest fires would be a likely
explanation to the hematite and maghemite. But a body of evidence 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. Microbial
activity might have influenced the mineral transformations.
| Original language | English |
|---|---|
| Journal | Geophysical Research Abstracts |
| Volume | 14 |
| Issue | EGU2012-3400 |
| Pages (from-to) | 3400 |
| Number of pages | 1 |
| ISSN | 1607-7962 |
| Publication status | Published - 1 Apr 2012 |
| Event | EGU GEneral Assembly 2012 - Vienna, Austria Duration: 22 Apr 2012 → 27 Apr 2012 |
Conference
| Conference | EGU GEneral Assembly 2012 |
|---|---|
| Country/Territory | Austria |
| City | Vienna |
| Period | 22/04/2012 → 27/04/2012 |