Environmental Engineering Reference
In-Depth Information
of decomposition, including stable humic substances,
and elemental forms of carbon in highly carbonized
compounds such as charcoal, coal and graphite
(Nelson & Sommers 1996).
The organic matter in marine sediments is a sig-
nifi cant source of nutrients for benthic organisms,
and the study of organic enrichment (natural or
anthropogenic) is important to evaluate disturbance
in the benthos (Luczak
et al.
1997). Furthermore, in
estimating the global carbon budget it is fundamen-
tal to measure organic carbon correctly in sediments
(Byers
et al.
1978). Another important goal of marine
sedimentological study is to distinguish between
marine and terrestrial organic matter contributions,
which can be achieved using carbon isotope analysis.
The source of carbon for photosynthesis in marine
phytoplankton is bicarbonate dissolved in seawater,
whereas land plants take carbon from atmospheric
carbon dioxide (Schubert & Nielsen 2000). The
stable carbon isotope ratio
13
C/
12
C is represented by
In soils, quantifi cation of carbonates based on
these reactions can be obtained either by analysis of
the amount of reactant H
+
consumed or the products
Ca
2+
and Mg
2+
or CO
2
released.
In studies of the organic carbon in sediments and
aquatic particulates, determination of total organic
carbon content must be preceded by the removal of
carbonates. There are several methods involving dis-
solution of carbonates by acid-treatment, which
causes the release of carbonate carbon as carbon
dioxide (Schubert & Nielsen 2000). Ideally, the
removal of carbonate should be complete and the
amount and composition of organic carbon pre-
served. However, it may be impossible to achieve
both conditions simultaneously. Liquid acidifi cation
with HCl is commonly used, although it may be
accompanied by solubilization of a signifi cant
portion of the organic carbon (Galy
et al.
2007).
Another method was proposed to replace the liquid
acidifi cation based on vapor acidifi cation (Hedges &
Stern 1984). According to Galy
et al.
(2007), the
vapor acidifi cation method appears to work for cal-
cite-rich sediments, but it was not tested for sedi-
ments containing appreciable quantities of dolomite
or siderite. Because the acid digestion of calcite is
much faster than that of dolomite, it is unlikely that
vapor acidifi cation is an effi cient method for the
dissolution of dolomite.
Working on the determination of total organic
carbon (TOC) content and
δ
(Coplen 1996):
13
()
=
(
)
−×
δ
C
‰
[
R
R
1
]
1000
(1)
sample
standard
13
C value of 0‰.
VPDB (Vienna PeeDee Belemnite), the reference
standard for
Bicarbonate in seawater has a
δ
δ
13
C, and atmospheric carbon dioxide
7‰ (Schubert & Nielsen 2000;
Dickens
et al.
2006).
Particulate inorganic carbon (C
inorg
) and particu-
late organic carbon (C
org
) have distinct isotopic sig-
natures (Lorrain
et al.
2003). Therefore it is necessary
to remove carbonates from organic carbon samples
to avoid contamination of the isotopic signal. The
procedure used to remove this carbonate should seek
to preserve not only the organic matter content but
also its composition. The elucidation of the molecu-
lar structure of organic matter in sediments allows
some assessment of its age or origin, e.g. lignin is an
indicator of terrestrial origin.
Differentiation between C
org
and C
inorg
is usually
based on preliminary decarbonation performed by
leaching the sample with acetic or hydrochloric acid
(Galy
et al.
2007). Acid dissolution of carbonates can
be summarized in the reactions below, for calcite and
dolomite minerals (Loeppert & Suarez 1996):
have
δ
13
C values of
−
13
C in carbonate-rich
detrital sediments, Galy
et al.
(2007) proposed an
analytical procedure based on liquid acidifi cation
that completely digests both calcite and dolomite,
while the amount of C
org
solubilized during acid
leaching is estimated. After carbonate removal, the
acid insoluble C
org
present in the leached sediment is
measured by combustion in an element analyzer. The
amount of acid soluble C
org
remaining in the leachate
is determined using a TOC analyzer. Galy
et al.
(2007) studied river sediments from the Himalaya-
Bengal Fan system, where the amount of calcite and
dolomite (from weathering of ancient sedimentary
Himalayan rocks) can be as high as 50% and the
TOC is generally low (
δ
1%). They also measured the
C
org
content, before and after acid leaching in sedi-
ments from the Amazon River, where carbonate is
absent. Comparison of sediments from the Himalayan
and Amazonian Rivers showed that the proportion
of C
org
solubilized during acid leaching was relatively
<
CaCO
+→ + +
2
H
+
Ca
2
+
CO
H O
3
2
2
CaMg CO
(
) +→ + +
4
H
+
Ca
2
+
Mg
2
+
2
CO
+
2
H O
3
2
2
2