Geology Reference
In-Depth Information
Cell wall material can be a significant part of
the resistant organic matter in soils. Depending
on physico-chemical parameters, enzymatic degra-
dation of polymerous substances from the cell wall
may or may not be possible (Paul & Clark 1996;
Coleman et al. 2004).
In order to check possible artifacts due to high-
vacuum, some representative samples were obser-
ved using XL30 SEM in its LTSEM cryo mode
(Low-temperature SEM). Some fungal strands and
coatings were embedded in an epoxy resin, and
ultrathin sections were performed using a Reichert
Ultracut S (Leica) microtome with a diamond
knife. Ultrathin sections (200-220 nm thick) were
observed using a Phillips CM-200 Transmission
Electron Microscope (TEM) with a voltage of
200 kV. Crystal properties were determined using
microdiffraction.
Materials and methods
Secondary carbonate accumulations have been
sampled at four sites: (i) in the mineral layers
(calcic horizon of a calcisol; IUSS Working Group
WRB 2006) at a quarry near Villiers (Swiss Jura
Mountains, 47804
0
N, 6859
0
E), as well as (ii) at an
outcrop near A¨nsa (Spain, 42821
0
N; 0804
0
E); (iii)
in the narrow entrance of a cave near les Cornettes
de Bises (Swiss Alps, 46819
0
N, 16848
0
E); and
(iv) in the vers chez le Brandt cave, inside a wide
chamber at 100 metres from the entrance (Swiss
Jura Mountains, 46856
0
N, 6828
0
E). At the first two
sites (soils developed on scree slopes), samples
exhibit two different morphologies directly visible
in the outcroup: (i) cotton-ball-like NFC that
accumulates in the soil pores; and (ii) coatings
on grains and centimetric to decimetric cryoclasts.
When wet, these coatings constitute a plastic
paste, which becomes pulverulent when dry. At
the third and fourth sites (caves), only moonmilk
deposits were sampled in the form of a wet plastic
wall coating, up to 30 cm thick. In addition to crys-
tals, fungal strands associated with different macro-
scopic morphologies of NFC have been sampled for
electron microscope observations. Strands have
been taken from cotton-ball-like NFC, associated
with rock fragment coatings, or free in the soil
pores. All samples were collected using polypropy-
lene tweezers and stored in sterilized 50 mL tubes at
low temperature.
Bulk samples were analysed by X-ray diffrac-
tion (XRD) using a Scintag diffractometer in order
to determine their mineralogical nature. In each
sample, quartz powder was added in order to nor-
malize and compare samples with each other.
In order to detect organic from mineral material,
prior to observations, each macromorphological
sample was stained using a 4% osmium aqueous
solution from a modified Pearson et al. (2004)
protocol. However, using this labelling method,
the presence of the organic matter can be deter-
mined but not its nature. Samples were gold-coated
(10 nm) and observed using a Phillips ESEM-FEG
XL30 Field Emission Gun Scanning Electron
Microscope (FEG-SEM). Osmium staining was
detected with an EDAX Energy Dispersive
Spectrometer (EDS) coupled to the electron micro-
scope. With natural non-flat samples and absence
of a standard, EDS spectra only give qualitative
information.
Results
XRD analysis of the three types of samples
(cotton-ball-like structures, coatings on grains, and
moonmilk) shows that their mineralogy is calcitic
in nature (Fig. 2a-c). Moreover, the absence of
shift (expected in presence of Mg in the crystal
lattice) after normalization with quartz powder
characterizes
the presence of
low magnesium
calcite (LMC) at these sites.
SEM observations of soil samples show recur-
sive associations between NFC, unidentified nanofi-
bres, and fungal strands. NFC is characterized
by a width between 0.5-2.0 mm and a length
,100 mm. Some epitactic growths are present but
no important development is observed (no big
euhedral crystals due to epitactic cementation).
The NFC is present either as random meshes, or as
bundles, 3-30 mm in diameter (average 10 mm;
Fig. 3a). This microscopic feature constitutes the
macroscopic cotton-ball-like structures. Bundles
contain some nanofibres, occasionally associated
with amorphous matter assumed to be an organic
veil (Fig. 3b). These nanofibres are rarely observed
on NFC when the latter are randomly oriented
and/or strongly modified by epitactic growth. At a
microscopic scale, coatings from soil grains exhi-
bit various amounts of NFC and nanofibres, in
which needles are less represented than in the
cotton-ball-like morphology. NFC often shows ran-
dom orientations, whereas nanofibres are packed in
clusters. This morphology shows great similarity
with microstructures of moonmilk samples,
in
which NFC is even less represented.
SEM measurements show that nanofibres are up
to 6 mm long (the shortest is 0.2 mm long) with an
average width of 78.6 nm (standard deviation of
22.5 nm, based on 106 measurements; Fig. 3c).
They are characterized by a high flexibility (as
mentioned by Borsato et al. 2000), leading to spec-
tacular curvature (Fig. 3c). They appear smooth
under TEM. Two kinds of structures are observed:
(i) a randomly-oriented framework of nanofibres,
in which widespread putative organic veils and cal-
citic micro-aggregates are present; these meshes are
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