Geoscience Reference
In-Depth Information
Mires, especially bogs with
Sphagnum
moss
and peat, are strongly acidic, typically in the pH
range 3.5 to 4.5 and reaching as low as 3. Cation
exchange is a principal source for hydrogen
ions, moss releases organic acids, and suli de
oxidation may form sulfuric acid (Charman
2002).
Sphagnum
is particularly effective at
obtaining cations from solution, which are nec-
essary nutrients for survival in ombrotrophic
situations, and releasing hydrogen ions. This
cation exchange is facilitated by uronic acids
that are held in cell walls and make up 10-30
percent of the dry mass of
Sphagnum
, and phe-
nolic compounds also aid cation exchange (Aus-
tralian Bryophytes 2008). The spreading rope
rush (
Empodisma minus
) in New Zealand raised
mires is another plant that has similar high
cation exchange capacities in its root layer
(Charman 2002).
In general warm, tropical sea water of the
continental shelf environment favors precipita-
tion of calcium carbonate, which may form the
mineral calcite or aragonite depending on
subtle differences in other chemical constitu-
ents. For example, corals build aragonite skele-
tons, oysters construct multilayered calcite and
aragonite shells, and brachiopod shells are
calcite (Blatt, Middleton and Murray 1972).
Ground water in contact with limestone and
dolostone is often highly charged with dis-
solved calcium and magnesium carbonate, so-
called hard water. Water hardness is generally
reported as ppm (or mg/L) CaCO
3
equivalent
(40 ppm Ca
2
+
Figure 4-20.
Hot springs at Uunartoq, southwestern
Greenland. Helium, nitrogen, argon and other gases
bubble out of the water (Persoz, Larsen and Singer
1972). The helium is presumably a byproduct of
deep-seated radioactive decay. Photo courtesy of P.
Jensen.
value is lower. For example vinegar has a
pH
3. Fresh water in contact with the atmos-
phere absorbs CO
2
, which creates carbonic acid
and lowers pH to 5.6. In basic solutions with
less H
+
, the pH value is higher; for example,
standard sea water pH
=
8.3. Note that this scale
is logarithmic; each whole value represents a
10-fold increase or decrease from the next
higher or lower value.
Many chemical reactions depend upon pH;
for example the behavior of calcium carbonate,
which is among the most common dissolved
solids of both fresh and sea water. The following
reactions are related to its stability:
=
=
2
+
+
3
2
−
(
CaCO
Ca
CO
carbonate
)
3
100 ppm CaCO
3
). When hard
water emerges from springs, dissolved CO
2
is
lost and lime precipitates. In some cases,
impressive deposits of tufa or travertine may
accumulate (Fig. 4-21).
In addition to pH, the oxidation potential of
water is extremely important for many chemical
reactions. Oxidation or redox potential (Eh)
refers to changes in electrical valence of ions in
solution and is measured as a voltage required
to force a change in valence. Iron is among the
most common and important elements for
wetland environments. Iron exists in two valence
states
-
ferrous (reduced), which is relatively
soluble, and ferric (oxidized), which is highly
insoluble. The standard potential is:
=
+= +
+
2
+
−
(
CaCO
H
Ca
HCO
bicarbonate
)
3
3
+
=
+
+
−
=
CO
H O
H
HCO
H CO
(
carbonic acid
)
2
2
3
2
3
In most waters, thus, the stability of calcium
carbonate depends mainly on how much CO
2
is
dissolved, which affects acidity (pH). More CO
2
drives the reaction toward solution; less CO
2
favors precipitation of solid CaCO
3
. The reaction
also depends on temperature, which affects
the solubility of CO
2
and other gases, and the
content of other dissolved solids such as sodium
and potassium (Davis and DeWiest 1966). None-
theless, pH is the primary controlling factor for
calcium carbonate in most suri cial and shallow
natural waters.
