From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
Black Carbon and other refractory forms in recent
sediments from the Gulf of Cadiz, Spain
F J González-Vila 1, J M de la Rosa and J A González-Pérez
Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS)-C.S.I.C.
Av, Reina Mercedes., 10, 41012-Seville, Spain.
E-mail: [email protected]
Abstract. Black carbon (BC) produced by incomplete combustion of fossil fuels and
vegetation is a ubiquitous form of refractory organic matter (ROM) widely distributed
in the environment. The stable non-reactive, mainly aromatic, BC has received
special interest in recent years as a possible carbon sink in soils and sediments. At
present, several methods have been used, with applications for soil and sediments,
although no single method has been accepted to detect BC forms, and therefore
providing the analytical data required to estimate BC contribution to terrestrial and
sedimentary carbon fluxes and the global carbon cycle. This study considers the
potential of the combined use of analytical pyrolysis (Py-GC/MS) and solid state 13C
NMR to determine the presence of BC and to characterize the refractory organic
matter in a set of recent marine sediments from the Southwest Atlantic coast of Spain
(Gulf of Cadiz). The content of BC-like material found in sediments (3.0 - 16.5 % of
total organic carbon) was in the range previously reported for this kind of samples
and decreases from coast to offshore. Solid state CP-MAS 13C NMR and Py-GC/MS
analysis of the isolated ROM revealed a conspicuous aliphatic domain remained in
the samples after chlorite oxidation. Our results suggests that, in recent sediments
from fluvial and marine environments, not only BC and refractory carbon forms with
aromatic nature may be sequestered in a stable form; it also seems probable that the
ROM pool contains a significant amount of C immobilized in the form of alkylic
compounds.
1. Introduction
Black carbon (BC), soot, elemental carbon and charcoal are different terms used to describe a
biologically refractory form of organic matter that remains as a residue from incomplete
combustion processes. Although BC is chemically heterogeneous, a major aromatic domain is
generally accepted for this material [1]. BC is ubiquitous in the atmosphere, ice, soils and
sediments due to its widespread production, and its stability and inertness in the environment
[2]. Moreover, it has been used as an indicator for the occurrence of forest fires and fossil fuel
emissions [3]. Thus BC can represent a significant carbon sink, because it represents carbon
that is transferred from the rapidly cycling biological-atmosphere carbon cycle to the longterm geological carbon cycle [4]. Considering the fact that several model calculations suggest
that more than 90 % of BC deposition occurs on continental shelves, undeniably, BC can
1
To whom any correspondence should be addressed.
c 2009 IOP Publishing Ltd
1
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
contribute greatly to the organic matter being buried in marine sediments [5], and we seek to
test this concept in sediments of the Gulf of Cadiz.
However, despite its importance to carbon-cycling dynamics, BC remains a component of
that large fraction of OC that is classified as biochemically uncharacterized, the main reason
for this is that BC is not a pure substance but rather a continuum of condensed aromatic
structures with different physicochemical properties [6] and its isolation and measurement is
subject to serious experimental constraints [7]. In Figure 1, taken from [8], BC materials are
shown as a continuum of combustion products, “ranging from slightly charred, degradable
biomass to highly condensed, refractory soot”.
Figure 1. The Black Carbon “continuum” (from Massiello [8]).
Various protocols exist for the identification and quantification of BC [9, 10, 11] utilizing
for example chemical, optical or thermal methods to distinguish BC from other organic and
inorganic carbon in combination with elemental analysis or spectroscopy. For a comparative
analysis of BC in soils and sediments it has been shown, however, that amounts of BC may
vary by up to two orders of magnitude depending on the applied method [9, 12]. Creation of
BC artifacts, removal of BC during pre-treatment and errors in the final estimation of BC after
pre-treatment have been discussed as reasons for the high variability among methods [13].
Depending on their selectivity, different methods probably isolate different fractions of the
BC continuum [14].
Seeking for an accurate analytical protocol for quantifying BC stored in different
environmental complex matrices, Simpson & Hatcher (2004) [15] developed a procedure
based in the removal of mineral fractions by treatment with hydrofluoric acid, followed by
chemical oxidation with sodium chlorite to remove non-BC aromatic compounds (mainly
lignin derived). Therefore, this study considers the potential of the combined use of analytical
pyrolysis (Py-GC/MS) and solid state 13C NMR to determine the presence of BC and to
characterize the refractory organic matter in a set of recent marine sediments from the
Southwest Atlantic coast of Spain (Gulf of Cadiz)
2
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
2. Materials and methods
In the current study, we applied the Simpson & Hatcher procedure to study the molecular
features of ROM and to quantify BC content in recent sediment samples taken from the
Southwest Atlantic coast of Spain (Gulf of Cadiz), a site of geological and environmental
interest [16 and references therein]. Surface sediment samples (0 to 20 cm depth) were
collected of-shore and in the estuaries of the Guadiana, Tinto, Odiel and Piedras rivers (Fig.
2). The area encompasses approximately 90000 km2 of considerable deposition. Details of
equipment and methods applied as well as sampling details and location are described in
detail in de la Rosa et al. (2007) [17].
The quantification of the BC involves the use of solid-state CP-MAS 13C NMR and the
integration of the aromatic regions of the spectra after HCl/HF and NaClO2 treatments.
However, aromatic lignin signals overlap with those of BC and the removal of lignin is
necessary for estimating BC contents. Analytical pyrolysis (Py-GC/MS) in the presence of
tetramethylammonium hydroxyde (TMAH) was used to investigate the molecular
composition of ROM as to test for the presence of lignin derived moieties after chlorite
oxidation. This method has been found useful for the detection of lignin biomarkers [18].
Figure 2. The sampling area. Red arrow indicates the selected
transect. Sample code corresponds to location as follows R: fluvial;
E: estuarine; M: marine
3. Results and discussion
After chlorite oxidation, the analytical pyrolysis in presence of tetramethylamonium
hydroxide (TMAH Py-GC/MS) confirmed an evident reduction of lignin derived moieties
(methoxyphenols) that could interfere with NMR aromatic signal overestimating BC content.
The content of BC as estimated by CP MAS 13CNMR is shown in Table 1. BC-like
material values for sample R-25 are very high (45.7 % TOC). This sample is located in the
Odiel River and has conditions for accumulation of sediments with charred material from the
several forests located upstream. On the other hand, this place is also affected by heavy ships
traffic due to the nearby port of Huelva. The rest of the values obtained are in the usual ranges
3
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
reported for BC-like materials in marine sediments [4, 19], ranging from the 3.0% BC/TOC
obtained in the E-218 sample located at the end of the Guadiana estuary to the 16.5% in the
R/E-211 sample (located in the mouth of the Guadiana river).
When analyzing a selection of samples that forms transect from NW to SE (Fig. 2), it is
observed that BC content is independent of the TOC in the sample and that BC decreases
from coast to offshore sediments (Fig. 3).
Table 1. Sediment BC content as estimated by the Simpson & Hatcher [15] method.
Ref.
R-25
R/E-28
R-35
R-39
R-40
E-202
E-209
R/E-211
E-213
E-218
E-226
M-131
M-155
gBC/Kg
BC/TOC %
8.09
0.78
0.90
0.81
0.87
0.47
0.47
1.76
1.87
0.52
0.59
0.64
0.67
45.7
4.3
8.2
7.8
7.9
4.1
6.1
16.5
15.3
3.0
5.0
5.5
6.1
CP MAS 13C-NMR analysis also shows that after severe sample oxidation with chlorite.
there are conspicuous signals at 110-60 and 45-0 ppm that corresponds to O-alkyl and C-alkyl
moieties respectively (Fig. 4). This accounted for c. 60% of total NMR signal (O-alkyl 27.5 ±
2.83 and C-alkyl 31.9 ± 4.54).
Also TMAH Py-GC/MS analysis revealed a highly aliphatic nature of the ROM isolated
from marine sediments, even in the sample with the highest value estimated for BC (R-25)
(Fig. 5 and Table 2). The chromatograms are dominated by series of methyl and dimethyl
esters of fatty acids including branched molecules. Also present are benzene and naphthalene
derivatives.
Both CP MAS 13C NMR spectra and TMAH Py-GC/MS chromatograms shows that after a
severe oxidation of, a conspicuous lipidic and peptidic domain still remains in the
sedimentary OM as recalcitrant fraction. That fraction of ROM could be considered out of the
so called continuum concept to define BC material. Our results point to a strong recalcitrance
of aliphatic moieties in such environments that, together with the condensed BC forms, these
moieties are main components of fluvial and marine ROM fraction and that as previously
suggested [10] this refractory, non-oxidable, aliphatic carbon pool, may represent a stable OM
fraction with influence in C fluxes and therefore in the global biogeochemical C cycle.
4
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
18
16
BC (% TOC)
14
y = -3,23x + 19,37
2
12
R = 0,8008
10
8
6
4
2
0
R/E-211
E-213
E-209
E-226
M-131
1,4
1,2
TOC (%)
1
y = 0,014x + 1,036
0,8
2
R = 0,015
0,6
0,4
0,2
0
R/E-211
E-213
E-209
E-226
M-131
Figure 3. BC (% TOC) and TOC (%) in a transect from NW to SE
Guadiana River in the gulf of Cadiz as illustrated in Fig 2..
40,00
STD
Mean
35,00
30,00
25,00
20,00
15,00
10,00
5,00
0,00
Ketone C,
aldehyde C
carboxyl C,
amide C
aromatic C
O-alkyl-C
N-alkyl C,
methoxyl C
alkyl-C
Figure 4. Solid state CP MAS 13C NMR integration regions after
HCl/HF and NaClO2 treatments. Results are given in % and n=13.
5
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
Figure 5. Example of TMAH Py-GC/MS chromatogram of sediment (R-25) after chlorite
oxidation. The corresponding biomarker abbreviations are listed in Table 2. Methyl esters of
fatty acids are identified as FAMEn, dimethyl diesters of fatty acids are identified as DAMEn,
where “n” is the length of the carbon chain. LG: Lignin derived compound.
Table 2. Peak identification of TMAH Py-GC/MS non lignin origin released compounds.
Label refers to peks in Figure 5.
Label
Non-Lignin Compounds released by Py-GC/MS
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Dimethyl sulfone
N.N-dimethyl-t-butylamine
Glycine-N-acetyl methyl ester
Benzoic acid, methyl ester
Dimethyl, methyl fumarate
Benzoic acid, 3-methyl-methyl ester
Naphthalene, 2-methyl
1.3.5-Triazine-2.4.6 (1H.3H.5H)-trione-trimethyl
1.2-Benzenedicarboxilic acid dimethyl ester
1.2.4-Benzenetricarboxylic acid, trimethyl ester
N.N-Dimethyl-propanamide
Phenol
1.2.3-Trimethylbenzene
m-Cresol
Glicine.N( 2-methoxy-2-oxoethyl)-methyl ester
Benzoic acid, 3-methyl-methyl ester
N-methylpyroglutamic acid, methyl ester
2-Naphthalenecarboxylic acid, methyl ester
2.4.6-Trimetoxybenzonitrile
2.6-Naphthalenedicarboxylic acid, dimethyl ester
1.2.4.5-Bencenetetracarboxylic acid, tetramethyl ester
6
From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
IOP Publishing
IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
Acknowledgements
We thank Ms Trinidad Verdejo for assisting us in the Py-GC/MS spectroscopy.The
SpanishMinistry ofEducation provided financial support for this research (fellowship BES2003-1900 and project CGL2006-12730-C03-C01). We would also like to thank Dr. E.
López-Capel for helpful comments when reviewing the manuscript.
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From Deep-sea to Coastal Zones: Methods and Techniques for Studying Paleoenvironments
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IOP Conf. Series: Earth and Environmental Science 5 (2009) 012009
doi:10.1088/1755-1307/5/1/012009
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