Christian-Albrechts-Universität zu Kiel
MathematischNaturwissenschaftliche Fakultät
Lehrstuhl für Physische Geographie
Landschaftsökologie und Geoinformation
Eolian Contributions to Soil Development in the Vernal Pools
of Enchanted Rock, Texas
Rainer Duttmann, Christian-Albrechts-University Kiel, Germany
James F. Petersen, Texas State University, San Marcos (TX), USA
On the bedrock summit of Enchanted Rock, a 130m high exfoliation
dome in central Texas, some of the granite weathering pits contain
an adequate soil depth to support island-gardens of vegetation.
Although these kinds of natural gardens (”vernal
pools”) have been studied in various places worldwide, little attention has been given to soil genesis
in vernal pools. In granitic regions these soils are
typically attributed to granular disintegration during
pit formation and further weathering of grus.
prevailing wind direction
study site
Source: Google Earth, 01.01.2009
Soil analyses of one of the deeper pits reveal a significant
contribution of eolian deposits to soil formation. Amongst
comparably high portions of wind blown sediments, as fine
sand and silt, Scanning Electron Microscopy (SEM) analyses and Energy Dispersion X-Ray Spectroscopy (EDS) show
an enrichment of gypsum in the deeper horizons of the
studied soil profile.
periodical ponding and drying
eolian sediment
weathering
suspension
ongoing ponding and drying
sediment deposition
saltation
surface creeping
mixing
residuals from granite weathering (e.g. grus)
Since gypsum does not appear on Enchanted Rock but in
the surrounding evaporitic rocks, we assume that its occurrence is indicative of the deposition of wind-blown sediments, as gypsum-rich dust. The originally deposited gypsum has been dissolved and washed out from the top soil
during the wet phases and subsequently enriched above
the impermeable granite surface, where it forms coatings
that cover formerly aggregated soil particles.
EDS quantification results
gypsum coating
(soil depth: 25-35 cm)
Magnification: 470 x
3000
S Ka1 Ca Ka1
weathering
continued weathering
suspension
wetting and drying
pioneer vegetation (lichens, moss, grasses)
weathering
increasing surface roughness
2000
SO4--
Ca++
counts
Ca Kb1
ongoing deposition of eolian sediments
Al Ka
run off,
dissolved mineral
compounds
500
SEM photography:
gypsum coatings
4
keV
6
8
10
EDS results:
element composition of the coatings
Lehrstuhl für Physische Geographie
Landschaftsökologie und
Geoinformation
Geographisches Institut der
Christian-Albrechts-Universität zu Kiel
Ludewig-Meyn-Str. 14
ongoing relocation/leaching of nutrients
increasing surface roughness
S Kb1
2
development of “microdunes”
continued weathering
Si Ka
1500
1000
suspension
ongoing deposition of eolian sediments
run off,
dissolved mineral
compounds
Ca++
initial soil formation
formation of soil
aggregates
relocation/leaching of
nutrients and dissolved organic compounds
2500
Magnification: 1300 x
forming of aggregates
by swell-shrink processes and cementing
Kontakt:
Prof. Dr. Rainer Duttmann
Tel.: ++49 (0)431 880-3426
Fax: ++49 (0)431 880-4658
[email protected]
http://www.lgi.geographie.uni-kiel.de
++
Ca
CaSO4
--
SO4
Ca++
suspension
development of “microdunes”
ongoing relocation/leaching of nutrients
accumulation/precipitation of leached substances