of 8
Coal fly ash is a major carbon flux in the Chang Jiang
(Yangtze River) basin
Gen K. Li
a,b,1
, Woodward W. Fischer
b
, Michael P. Lamb
b
, A. Joshua West
c
, Ting Zhang
a
, Valier Galy
d
,
Xingchen Tony Wang
b,e
, Shilei Li (
)
a
, Hongrui Qiu
f
, Gaojun Li
a
, Liang Zhao
a
, Jun Chen
a
, and Junfeng Ji
a
a
Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;
b
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
c
Department of Earth Sciences, University of
Southern California, Los Angeles, CA 90089;
d
Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institute, Woods Hole, MA
02543;
e
Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467; and
f
Department of Earth, Environmental and
Planetary Sciences, Rice University, Houston, TX 77005
Edited by Donald E. Canfield, University of Southern Denmark, Odense M., Denmark, and approved March 25, 2021 (received for review December 9, 2019)
Fly ash
the residuum of coal burning
contains a considerable
amount of fossilized particulate organic carbon (FOC
ash
)thatre-
mains after high-temperature combustion. Fly ash leaks into natural
environments and participates in the contemporary carbon cycle,
but its reactivity and flux remained poorly understood. We charac-
terized FOC
ash
in the Chang Jiang (Yangtze River) basin, China, and
quantified the riverine FOC
ash
fluxes. Using Raman spectral analysis,
ramped pyrolysis oxidation, and chemical oxidation, we found that
FOC
ash
is highly recalcitrant and unreactive, whereas shale-derived
FOC (FOC
rock
) was much more labile and easily oxidized. By combin-
ing mass balance calculations and other estimates of fly ash input to
rivers, we estimated that the flux of FOC
ash
carried by the Chang
Jiang was 0.21 to 0.42 Mt C
·
y
1
in 2007 to 2008
an amount equiv-
alent to 37 to 72% of the total riverine FOC export. We attributed
such high flux to the combination of increasing coal combustion
that enhances FOC
ash
production and the massive construction of
dams in the basin that reduces the flux of FOC
rock
eroded from up-
stream mountainous areas. Using global ash data, a first-order esti-
mate suggests that FOC
ash
makes up to 16% of the present-day
global riverine FOC flux to the oceans. This reflects a substantial
impact of anthropogenic activities on the fluxes and burial of fossil
organic carbon that has been made less reactive than the rocks from
which it was derived.
coal
|
fly ash
|
carbon cycle
|
Chang Jiang (Yangtze River)
|
sediment transport
F
ossil particulate organic carbon (FOC) is a geologically stable
form of carbon that was produced by the ancient biosphere and
then buried and stored in the lithosphere; it is a key player in the
geological carbon cycle (1
7). Uplift and erosion liberate FOC
from bedrock, delivering it to the surficial carbon cycle. Some is
oxidized in sediment routing systems, but a portion escapes and
can be transported by rivers to the oceans (5, 8
10). Oxidation of
FOC represents a long-term atmospheric carbon source and O
2
sink, whereas the reburial of FOC in sedimentary basins has no
long-term net effect on atmospheric CO
2
and O
2
(1, 9, 11). Ex-
humation and erosion of bedrock provide a natural source of FOC
(2, 8), which we refer to as FOC
rock
. Human activities have in-
troduced another form of FOC from the mining and combustion
of coal. Burning coal emits CO
2
to the atmosphere but also leaves
behind solid waste that contains substantial amounts of organic
carbon (OC) that survives high-temperature combustion (12
14).
This fossil-fuel-sourced carbon represents a poorly understood
anthropogenic flux in the global carbon cycle; it also provides a
major source of black carbon, which is a severe pollutant and
climate-forcing agent (12
15).
Previous studies sought to quantify black carbon in different
terrestrial and marine environments and to distinguish fossil fuel
versus forest fire sources (14
18). In this study, we focused on fly
ash
the material left from incomplete coal combustion. As a
major fossil fuel, coal supplies around 30% of global primary
energy consumption (19, 20). Despite efforts to capture and utilize
fly ash, a fraction enters soils and rivers; the resulting fossil OC
from fly ash (FOC
ash
) has become a measurable part of the con-
temporary carbon cycle (14). FOC
ash
is also referred to as
un-
burned carbon
in fly ash (21
25); it provides a useful measure of
combustion efficiency and the quality of fly ash as a building
material (e.g., in concrete) (23
26). Industrial standards of FOC
ash
content in fly ash have been established for material quality as-
surance (23, 24, 26, 27). However, the characteristics and fluxes of
FOC
ash
released to the environment, and how these compare to
FOC
rock
from bedrock erosion, remain less well understood.
To fill this knowledge gap, we examined the Chang Jiang
(Yangtze River) basin in China
a system that allowed us to
evaluate the influence of FOC
ash
on the carbon cycle at continental
scales. In the 2000s, China became the largest coal-consuming
country in the world, with an annual coal consumption of over
2,500 Mt, equating to
50% of worldwide consumption (19, 20, 28).
Coal contributed over 60% of China
s national primary energy
consumption through the 2000s. A significant portion of this coal
(approximately one-third) was consumed in the Chang Jiang
(CJ) basin, where China
s most populated and economically
developed areas are located (29). Significant amounts of fly ash
and FOC
ash
continue to be produced and consumed in the CJ
basin. To determine the human-induced FOC
ash
flux, we inves-
tigated the FOC
ash
cycle in the CJ basin. We characterized OC in
a series of samples including fly ash, bedrock sedimentary shale,
Significance
Coal combustion releases CO
2
but also leaves behind solid
waste, or fly ash, which contains considerable amounts of car-
bon. The organic carbon sourced from fly ash resists chemical
breakdown, and we find that it now contributes nearly half of
the fossil organic carbon exported by the Chang Jiang
the
largest river in Asia. The fly ash flux in this basin is similar to the
natural sediment flux to the oceans because dam building has
reduced sediment transport, while increased coal consumption
generates abundant fly ash. Our results show that fly ash is an
important component of the present-day carbon load in rivers
and illustrates that human-driven carbon cycling can match the
pace of the geological carbon cycle at decadal timescales.
Author contributions: G.K.L., W.W.F., M.P.L., A.J.W., J.C., and J.J. designed research;
G.K.L., T.Z., X.T.W., and S.L. performed research; G.K.L., T.Z., V.G., and J.J. contributed
new reagents/analytic tools; G.K.L., W.W.F., M.P.L., V.G., X.T.W., S.L., H.Q., G.L., L.Z., and
J.J. analyzed data; and G.K.L., W.W.F., M.P.L., A.J.W., T.Z., V.G., X.T.W., S.L., G.L., L.Z., J.C.,
and J.J. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
Published under the
PNAS license
.
1
To whom correspondence may be addressed. Email: ligen@caltech.edu.
This article contains supporting information online at
https://www.pnas.org/lookup/suppl/
doi:10.1073/pnas.1921544118/-/DCSupplemental
.
Published May 17, 2021.
PNAS
2021 Vol. 118 No. 21 e1921544118
https://doi.org/10.1073/pnas.1921544118
|
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AND PLANETARY SCIENCES
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and river sediment through multiple geochemical analyses. We
then estimated the CJ-exported FOC
ash
flux and evaluated how
human activities modulated FOC transfer at basin scales. We
found that in the CJ basin, coal combustion and dam construction
have conspired to boost the FOC
ash
flux and reduce the FOC
rock
flux carried by the CJ; as a result, these two fluxes converged over
an interval of 60 y.
The CJ Basin
The CJ basin is located in central China and has a drainage area
of around 1,800,000 km
2
nearly 20% of the total terrestrial
area of China (30, 31). Originating from the Tibetan Plateau, the
CJ drains mountainous areas in its upper reach and alluvial
plains in its lower reach, before emptying into the Eastern China
Sea (Fig. 1). The regional climate is controlled by the East Asian
monsoon with peak precipitation and associated floods occurring
from May to September (30, 31). The bedrock geology of the CJ
basin is mainly composed of sedimentary rocks, with minor ig-
neous and metamorphic exposures (Fig. 1
A
) (32, 33).
Over the past 60 y, the CJ basin has undergone significant an-
thropogenic change including the massive construction of indus-
trial and hydrological infrastructure (e.g., coal-fired power plants
and dams) and increased coal combustion (Fig. 1
B
and
C
)
(34
36). More than 50,000 dams, including the world
slargest
the Three Gorges Dam
have been built in the CJ basin; these
dams trap sediment in the resulting reservoirs and have signifi-
cantly reduced the CJ sediment export to the ocean (Fig. 2) (34,
37). In the process, they profoundly altered the natural transfer of
carbon (38, 39). The growth in China
s coal consumption over the
past decades has also fundamentally altered the CJ basin, and the
annual production of fly ash is, remarkably, now comparable in
total mass to the entire CJ sediment flux (Fig. 2). It is not a sur-
prise, then, that fly ash particles and associated black carbon have
been observed in recent river and delta sediment in the CJ basin
(16, 40
43).
Materials and Approaches
We sampled fly ash, shale, river sediment, and plant debris (leaves of typical
plants) in the middle-lower CJ basin and collected samples from a core drilled
30° N
25° N
35° N
100° E
110° E
120° E
Water body
Carbonate
Sedimentary
Igneous
Metamorphic
3
0° N
25° N
35° N
30° N
25
° N
35° N
100° E
110° E
120° E
3
0
° N
2
5
° N
35
° N
3
0
° N
2
5
° N
3
5
° N
3
0
° N
2
5
° N
35
° N
Reservoir
3
capacity (km )
0.1-1.0
1.0-10
>10
A
CJ basin
B
C
J
i
n
s
ha J
iang
Y
a
l
o
n
g J
i
a
n
g
D
a
d
u He
M
i
n Ji
a
n
g
J
i
a
l
i
n J
i
a
ng
Han Jiang
Wu J
i
an
g
X
ian
g Jia
n
g
Yu
a
n J
ia
n
g
Three Gorges Dam
Coal-fired
power plant
capacity (kMW)
0.1-1.0
1.0-3.0
>3.0
Fly ash
Chang Jiang river sediment
Holocene sediment
Shale
Biomass
Sample
Fig. 1.
Maps of the CJ (Yangtze River) basin including (
A
) bedrock geology and (
Inset
) regional context (33, 38), (
B
) the distribution of reservoirs
(capacity
>
0.1 km
3
) with the major tributaries of the CJ denoted (details in
SI Appendix
), and (
C
) the distribution of major coal-fired power plants (capacity
>
100 MW)
(details in
SI Appendix
).
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Li et al.
https://doi.org/10.1073/pnas.1921544118
Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin
Downloaded at California Institute of Technology on May 19, 2021
into the CJ estuary delta recovering Holocene sediment (Fig. 1). To characterize
OC in the samples, we measured OC content and conducted chemical oxida-
tion experiments, ramped pyrolysis oxidation (RPO) analysis, and Raman
spectral analysis (details in
SI Appendix
).
Characteristics of Fly-Ash-Sourced OC.
The CJ fly-ash samples we have studied
have an FOC
ash
content ([FOC]
ash
) of 2.25
+1.63
/
1.18
% (Fig. 3
A
, reported as the
median and 16th and 84th percentiles)
a value 2 to 100 times higher than
the FOC content in global river sediments (38, 44). The measured FOC
ash
content is comparable to the Chinese industrial standard (GB/T 477-2008) of
5% for use in concrete and cement and to the standards of FOC
ash
content in
other major coal-consuming countries (
SI Appendix
, section S6
) (24, 26, 27).
In general, FOC
ash
comprises a spectrum of carbon species with different
forms and origins (21
24). Substantial effort in prior work has been spent on
imaging, characterizing, and separating FOC
ash
of different carbon species
(e.g., refs. 23, 24, 45
48 and
SI Appendix
, section S6
). Notably, studies using
electron microscopy found that FOC
ash
includes nanometer-scale soot par-
ticles, micrometer-sized char particles, and carbon associated with inorganic
minerals (23, 24, 48
50). The relative abundances of the different carbon
forms in fly ash vary due a range of factors including coal rank and com-
bustion conditions (22, 23, 51, 52), as well as separation method (see more
discussion in
SI Appendix
, section S7
). A study using liquid-suspension gravity
separation found that soot represents a nonnegligible component in fly ash,
contributing
35% carbon mass to the total FOC
ash
(53).
Raman spectral analyses of our CJ samples provided insight into the chemical
moieties of FOC
ash
, specifically the presence of graphitic structure and its as-
sociation with minerals (Fig. 3
B
and
SI Appendix
, Figs. S2 and S3
). Graphitic
carbon was indicated by characteristic G and D bands, at
1,350 cm
1
and
1,600 cm
1
, respectively (Fig. 3
B
and
SI Appendix
, Figs. S2 and S3
), with the G
band corresponding to graphite and the D band induced by defects (5, 54).
Nongraphitic carbon, or carbon bonded to heteroatoms (e.g., nitrogen and
oxygen), likely caused the high background florescence observed in some
samples (
SI Appendix
,Fig.S2
) (55). Mineral-associated carbon featured spectral
peaks of minerals and graphitic carbon, and often high background fluores-
cence (Fig. 3
B
and
SI Appendix
, Fig. S2
). The Raman analyses also resolved
FOC
ash
as individual carbon particles, displaying no close association with
minerals (Fig. 3
B
and
SI Appendix
, Figs. S2 and S3
)
textures consistent with
other observations of char and soot particles in coal ash (23, 24). We collected
a total of 30 Raman spectra in seven of the CJ ash samples: 20 of these dis-
played mineral-associated carbon containing both graphitic and nongraphitic
carbon, 5 revealed graphitic carbon not associated with minerals, and the
remaining 5 displayed mixtures of graphitic and nongraphitic carbon not in
association with minerals (
SI Appendix
, Figs. S2 and S3
).
Complementing the Raman data, we collected new RPO and chemical
oxidation data, which showed that FOC
ash
is much more recalcitrant than
shale-derived FOC
rock
(Fig. 3
C
and
D
). The RPO results were reported as
thermograms and translated to spectra of activation energies (E
a
) (56). In the
thermograms, FOC
ash
has a higher fraction of carbon decomposed and more
CO
2
released at high temperatures (e.g.,
>
700 °C) than FOC
rock
(
SI Appendix
,
Fig. S6
). Converting the thermograms to the spectra of activation energies, we
foundthatflyashfeaturedmuchhigherE
a
than the shale (Fig. 3
C
) and other
FOC
rock
-dominated river sediment samples (9, 56), revealing a high thermal
stability and an extremely refractory phase of FOC
ash
(E
a
>
220 kJ/mol). The
chemical oxidation experiments employed sodium persulfate (Na
2
S
2
O
8
)
a
chemical with a very high oxidation potential (standard oxidation
reduction
potential is E
0
2.01V) comparable to O
3
(E
0
2.07V) and much higher than O
2
(E
0
1.23V) (57). This agent has been used in soil studies to simulate oxidation
in natural environments (58). The results from oxidation experiments are
reported as
f
ox
, the mass fraction of OC that gets oxidized in the experiments.
Alow
f
ox
value means the sample was difficult to oxidize and contains a high
proportion of OC that is recalcitrant. For the fly ash samples,
f
ox
is low (0 to
0.25), as expected since FOC
ash
has undergone pedogenesis, petrogenesis, and
high-temperature incineration, leaving behind the most refractory OC class
(14). For shale samples,
f
ox
values were much higher (0.4 to 0.85), implying that
a significant portion of FOC
rock
is reactive and labile, which is consistent with
field observations of substantial loss of FOC
rock
in soils and sediment routing
systems (5, 59, 60). Expectedly, the plant samples were highly labile, with a
high
f
ox
of 0.9 to 1.
Although current methods cannot delineate the microscale (nanometer to
micrometer) characteristics of FOC
ash
and directly quantify all the different
carbon forms in fly ash (
SI Appendix
,sectionS2
), our Raman and RPO results
together provided constraints on the carbon species composing FOC
ash
in our
samples
and this compositional information helped explain results from the
oxidation experiments. First, the Raman data indicated the presence of mul-
tiple different carbon species, including the dominance of mineral-associated
carbon. Second, considering that graphitic carbon is thermally decomposed at
temperatures of 700 to 800 °C, the RPO thermograms designated that 20 to
50% of FOC
ash
is composed of graphitic carbon (
SI Appendix
,Fig.S4
) (61).
Altogether, these analyses of our fly ash samples reveal that graphitic carbon is
the major constituent in FOC
ash
andnongraphiticcarbonoccursmostlyinas-
sociation with minerals. These two forms of carbon
graphitic and mineral-
associated
hinted at two plausible mechanisms for preserving FOC
ash
during
high-temperature combustion: selective preservation as stable graphitic car-
bon and mineral protection (62, 63). Nongraphitic carbon is less recalcitrant
than graphitic carbon (5, 47). Thus, the relative proportions of graphitic versus
nongraphitic carbon may also explain the natural variation in
f
ox
observed in
our fly ash samples (Fig. 3 and
SI Appendix
, Figs. S2 and S3
). In any case, the
low
f
ox
of FOC
ash
underscored its recalcitrance; simply put, it is a less reactive
component in the surficial carbon cycle than rock-derived FOC.
OC in the CJ River Sediments.
Chemical oxidation procedures, in conjunction
with radiocarbon analyses, also helped resolve the bulk composition of riv-
erine OC. OC in river sediment represents a mixture of radiocarbon-enriched
biospheric OC and radiocarbon-dead FOC (11, 64). In
f
ox
-1/OC space, CJ
suspended sediment sits on a mixing trend between a biospheric OC end
member and a radiocarbon-dead FOC end member (Fig. 3
D
and
SI Appen-
dix
). Coupling this mixing relationship with prior chemical oxidation results
reveals that CJ river FOC could not be oxidized via the chemical treatment
method and made up 85% of the residual OC after oxidation (38).
The
f
ox
of the core sediment samples from the CJ estuary had a range
similar to the CJ suspended sediment samples (0.5 to 0.8). Those samples
came from floodplain and fluvial sedimentary facies (65), representing ash-
uncontaminated, preindustrial CJ sediment. If we assumed a behavior of FOC
in those preindustrial samples similar to that found during the oxidation ex-
periment (i.e., riverine FOC cannot be oxidized and makes up 85% of the re-
sidual OC after oxidation), we estimated a FOC
preindustrial
contentof0.15
±
0.02%
avalue6to30timeslowerthan[FOC]
ash
(2.25
+1.63
/
1.18
%). We treated
this estimate as an approximation of rock-derived FOC in the modern CJ sedi-
ment (FOC
CJ0
) without FOC
ash
contamination. Note that FOC
CJ0
represents the
final product of FOC
rock
after alteration during erosion and transfer before
entering the sedimentary OC pool, thus it is not necessarily equivalent to the
fresh FOC
rock
derived from rocks within the CJ basin. To validate our estimate of
[FOC]
preindustrial
, we defined a similar OC mixing trend for preindustrial CJ sed-
iment using our inferred FOC end member (
f
ox
=
0and[FOC]
=
0.15
±
0.02%)
and the biospheric OC end member (Fig. 3
D
). The Holocene sediment samples
fitthismixingtrend(Fig.3
D
), suggesting our estimate of [FOC]
preindustrial
is robust.
Examining the geochemical data, we also identified hydrologic and sedi-
ment transport controls on the degree of mixing observed between FOC
ash
and
FOC
CJ0
. When separating the CJ sediment samples by river discharge (flood
versus dry season) (38), distinct trends emerged (Fig. 3
E
and
SI Appendix
,Fig.
Mater
i
al f
l
ux (M
t/y
r)
200
400
600
Ye a r ( A D )
1950
1970
1990
2010
CJ sediment
Fly ash produced
Fig. 2.
Time series of annual sediment export fluxes by the CJ plotted
alongside fly-ash production in the basin during 1950 to 2010 (details in
SI
Appendix
).
Li et al.
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Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin
https://doi.org/10.1073/pnas.1921544118
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
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S2): The dry season featured an FOC-enriched end member, whereas the flood
season marked an FOC-diluted end member. The difference could be
explained by sediment transport conditions varying with the hydrograph. Fly
ashismainlycomposedoffinecenosphereparticles(
1s to 10s
μ
m) with a
density close to or lower than typical river sediment (25, 28) and can be easily
entrained by rivers during low flow. The proportion of fly ash in sediment,
therefore, is lower in the flood season than in the dry season, because flooding
can entrain coarser and denser sediment. The high fraction of coarse sediment
and its low [FOC]
CJ0
(assuming it is equivalent to the preindustrial [FOC]) would
lead to a mixed [FOC] during flooding that is lower than the dry season. Thus,
our observation indicated some hydraulic control on the composition of riv-
erine FOC as a mixture of high-[FOC] fly ash and low-[FOC] natural sediment,
corroborating the low [FOC]
CJ0
seen in Holocene sediment.
Fly-Ash-Sourced OC Flux.
During 2007 to 2008, around 130 Mt of fly ash was
produced in the CJ basin (36% of the total fly-ash production in China;
SI
Appendix
). Taking an ash utilization rate of 67% estimated for China (28),
40 Mt of fly ash was released to the environment in the CJ basin; this is a
remarkable flux given that the total CJ sediment flux is around 130 Mt
·
y
1
.
There is currently no systematic storage or treatment of the unutilized ash,
suggesting much of this may be released into the environment. From the
budget of production alone, it is unclear how much of the wasted fly ash
and FOC
ash
enter rivers, but we can use three lines of evidence to constrain
the riverine-transported FOC
ash
flux.
First, we conducted mass balance calculations using the preindustrial [FOC]
(0.15
±
0.02%) as an approximation of the [FOC] of ash-free sediment. As-
suming the modern-day CJ sediment is a mixture of fly ash and ash-
uncontaminated sediment, we calculated the flux of FOC
ash
:
[
FOC
]
ash
×
(
f
sed
ash
)
+
[
FOC
]
CJ0
×
(
1
f
sed
ash
)
=
[
FOC
]
CJ
,
[1]
where
f
sed-ash
is the mass fraction of fly ash in the ash-contaminated CJ sedi-
ment and [FOC]
CJ
represents the modern-day CJ FOC. We took 2.25
+1.63
/
1.18
%
for [FOC]
ash
,0.15
±
0.02% for [FOC]
CJ0
, and 0.45
±
0.10% for [FOC]
CJ
(38), to
resolve
f
sed-ash
and the fraction of FOC
ash
in the CJ-exported FOC,
f
FOC-ash
.We
then employed Monte Carlo random sampling techniques to propagate errors
and estimate uncertainties (
SI Appendix
), reported as medians and the 16th to
84th percentiles of the sampling results. We obtained a
f
FOC-ash
of 72
+8
/
12
%,
with a FOC
ash
riverine flux of 0.42
+0.14
/
0.15
Mt C, whereas the total CJ-exported
FOC flux is 0.58
±
0.13 Mt C
·
y
1
(38). We also estimated a
f
sed-ash
of 13
+18
/
7
%,
which is 13
+18
/
7
% of the total produced fly ash and 39
+54
/
21
% of the ash
wasted in the basin (Fig. 4
A
).
Second, we referred to a prior study that estimated a
f
sed-ash
of 7% based
on the changes in the magnetic susceptibility (MS) of river sediment and islet
deposits. An abrupt increase in the MS of the CJ sediment has been observed
in recent years and attributed to the input of fly ash, which has MS
30 times
higher than ash-free sediment (40). Using a
f
sed-ash
of 7%, a CJ sediment flux
of 130 Mt (66), and an FOC content of 2.25
+1.63
/
1.18
% in our fly ash samples,
we estimated a FOC
ash
flux of 0.21
+0.15
/
0.11
and a
f
FOC-ash
of 37
+40
/
19
%of
the CJ FOC export. In this case, 21% of the unutilized fly ash in the CJ basin
enters the rivers (Fig. 4
A
).
Third, we noticed that the FOC content (0.45
±
0.10%) in our sediment
samples from the lower reach of CJ was typically higher than in the samples
(0.10 to 0.20%) from regions upstream of the Three Gorges Dam and areas
of intense coal consumption (39, 67). Attributing this downstream increase
in FOC content to ash input resulted in estimates of
f
FOC-ash
of
60 to
80%
values similar to those achieved via mass balance. To formally dem-
onstrate that FOC content increases downstream as a result of fly ash input,
one would need more depth-profile sampling to capture variations in riv-
erine OC across hydraulic gradients, and additional accounting for the
bedrock and landscape heterogeneities within the CJ basin (e.g., variations
in lithology and contributions from different tributaries) (11, 38, 39, 67).
Nonetheless, this first-order estimate was similar to that achieved with our
two other approaches for quantifying
f
FOC-ash
.
The estimate of
f
FOC-ash
from the change in MS (37
+40
/
19
%) is somewhat
lower than the estimate from mass balance calculations (72
+8
/
12
%) and the
downstream trends in FOC content (
60 to 80%). This discrepancy can be
explained because the mass balance calculations assume FOC
ash
is the only
anthropogenic FOC input to the CJ FOC pool. As there may be other an-
thropogenic sources of fossil carbon, e.g., from petrochemicals (68
70), our
-1
Raman shift (cm )
1/[OC ] (1/%)
Relative intensity
f
ox
200
600
1000
1400
1800
0
1
23
4
5
6
7
0.2
0.4
0.6
0.8
1
0
Graphitic C
Mineral
[OC] (%)
Frequency
0
1
23
4
5
6
1
2
3
4
5%
95%
50%
Me
a
n
84%
16%
A
Fly ash
-1
E (kJ mol )
a
-
1
Probabi
lity density (mol kJ
)
0.01
0.02
0.03
100
140
180
220
260
300
B
Fly ash
Fly ash
Shale
Plant
CJ sediment
2007-2008
CJ FOC
2007-2008
Holocene core
sediment
CJ FOC
pre-industrial
D
M
i
xing tr
en
d
m
ode
rn
Mix
ing trend
p
r
e-industria
l
0
12
3
1/[OC]
0.2
0.6
1.0
E
Flood
se
a
son
Dry
sea
s
on
f
ox
C
Fly ash
Shale
5
0
0
D
G
Fig. 3.
Geochemical characteristics of CJ sediment and fly-ash samples. (
A
) Histograms of OC content in CJ fly-ash samples with a box-and-whisker plot
showing the 5th, 16th, 50th, 84th, and 95th percentiles and mean of OC content for fly-ash samples. (
B
) Selected Raman spectra of CJ fly-ash samples showing
a mix of organic and inorganic mineral phases with mineral and graphitic carbon peaks (D and G bands) denoted (for more data see
SI Appendix
, Figs. S2 and
S3). (
C
) Probability density of activation energy (E
a
) of a fly-ash sample and a shale sample derived from RPO results, indicating a higher modal E
a
of the fly ash
sample than the shale sample. (
D
) Oxidation fraction
f
ox
versus 1/[OC] (reciprocal of OC content) for studied samples, with two dashed lines indicating mixing
trends between the fossil OC and biospheric OC end members. (
E
) Mixing relationships for the CJ sediment collected in the dry season (yellow) versus in the
flood season (blue), defined by least-squares linear regression.
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Li et al.
https://doi.org/10.1073/pnas.1921544118
Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin
Downloaded at California Institute of Technology on May 19, 2021
estimate based from mass balance likely sets an upper bound on
f
FOC-ash
and
FOC
ash
flux.
Overall, the multiple approaches yield results of a similar magnitude,
suggesting this estimate of FOC
ash
flux is robust to a first order. Combining
the results above, we concluded that in 2007 to 2008, fly ash contributed a
FOC
ash
flux of 0.21 to 0.42 Mt C
·
y
1
, making up 37 to 72% of the CJ-exported
FOC (Fig. 4
B
). For comparison, the estimated atmospheric carbon drawdown
via silicate weathering in the basin is 2.29
±
0.53 Mt C
·
y
1
(Fig. 4
B
) (32).
Fossil Carbon Flux Perturbed by Human Activities.
Globally, the total fly ash
production during the 2000s was around 750 Mt
·
y
1
(28, 71), with annual
global river sediment export estimated at 17,800 Mt
·
y
1
(72). Thus, while the
mass flux of fly ash in the CJ now matches the scale of natural sediment
transfer in this basin (Fig. 2), the same is not true globally. The globally av-
eraged utilization rate of fly ash is not well determined, but the major coal
consumers (China, United States, and India, accounting for 70% of the total
consumption) reported an average utilization rate of
50% during 2007 to
2008 (28), leaving
375 Mt
·
y
1
of fly ash that was not utilized. A global-
average FOC
ash
content is challenging to estimate accurately, because FOC
ash
depends on a range of factors including coal types and combustion conditions
(see expanded discussion in
SI Appendix
,sectionS6
)(23,24,45,51)
conditions that likely vary from region to region. For a first-order constraint,
we compiled data on FOC
ash
content for 247 samples from different regions
and found an FOC
ash
content of 4.70
+9.69
/
3.40
%(medianand16thto84th
percentiles;
SI Appendix
, section S7 and Fig. S6
), which was on the same order
of magnitude as the FOC
ash
content (2.25
+1.63
/
1.18
%) measured in our CJ fly-
ash samples. Note that most global ash FOC content data were estimated via
the loss on ignition
a method that can overestimate the true FOC content
and thus may partially explain why the CJ FOC
ash
content is lower than esti-
mated globally (see more discussion in
SI Appendix
,sectionS7
). Considering
the global data compilation, the CJ FOC
ash
content likely represents a
conservative estimate of the actual carbon content in fly ash. Notably, FOC
ash
contents we measured in CJ ash samples and those from the compiled global
dataset were similar to the industrial standards of FOC
ash
content of 5 to 10%
in different countries and regions (
SI Appendix
, section S6
)(23,24,26,27);this
lent confidence to the overall estimates of the FOC
ash
flux. If [FOC]
ash
found in
the CJ samples is typical, a global FOC
ash
yield of 8.43
+6.11
/
4.43
Mt C
·
y
1
can be
expected. Assuming 20% of the unutilized fly ash is transported by rivers (we
found that this number was 13 to 27% for the CJ basin), the global riverine
FOC
ash
flux to the oceans is then 1.69
+1.22
/
0.89
Mt C
·
y
1
,makingup
3.9
+12
/
3.2
% of the modern-day riverine FOC flux (43
+61
/
25
Mt C
·
y
1
)(44).Note
that this estimate of global FOC
ash
flux carries large uncertainties, and further
studies of FOC
ash
contents and ash supply to rivers in different regions will be
required to improve upon it. Nonetheless, our first-order estimate of the
global-average
f
FOC-ash
of
4% is lower than the CJ case (37 to 72%), meaning
that the CJ basin likely represents an upper end member in the distribution of
FOC
ash
production and export. So why does the CJ basin have such a high
f
FOC-
ash
, and such a dominant overall flux of fly ash? We attributed this to two of
the major anthropogenic modifications of the CJ basin: increasing coal con-
sumption and dam construction.
First, coal consumption in China has substantially increased over the past 60
y, boosting fly ash production (Figs. 2 and 4
C
). Hosting China
smosteco-
nomically developed and populated areas, the CJ basin has witnessed intense
construction of coal-fired power and steel plants (Fig. 1
C
), providing a major
source of FOC
ash
, especially in its middle and lower reaches (35, 70, 73). In the
2000s, China
sconsumptionincreasedtomorethan50%oftheglobalcoal
consumption. Alone, all the provinces in the CJ basin comprise 36% of China
s
coal consumption
a value equivalent to 18% of the global coal consumption.
Thus, the CJ basin represents a major locus of fly ash production.
Second, the continued construction of dams and reservoirs in the CJ basin
has decreased fluxes of sediment and FOC
rock
. After the impoundment of the
Three Gorges Reservoir in 2003, the CJ sediment export has reduced to
100
f
sed-ash
f
FOC-ash
0
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
0
-1
C
a
rbo
n flux (Mt C yr )
0
1
2
3
Year (AD)
-1
Fossil OC flux (Mt C yr )
1950
1970
1990
2010
0
0.4
0.8
2.0
FOC
-FOC
ash
CJ0
mixing trend
Ash release
Ash addition
(mass balance)
Ash addition
(MS increase)
Released FOC
ash
(Produced - Utilized)
Riverine FOC
rock
1.2
1.6
1
2
Released FOC
ash
(20
0
7-2008)
Riv
e
rin
e FO
C
ash
(2
00
7-20
0
8)
Riverine FOC (1950s)
R
iv
eri
ne F
OC (2
00
7-
200
8)
R
i
v
erine FOC
(
20
07-20
0
8)
rock
-1
M
a
t
erial flux (Mt y
r
)
100
200
300
400
500
600
0
1950-1970
pre-damming
2004-2010
post-damming
Ri
v
er sediment
Ri
ver se
dim
en
t
Fly ash released
Fly as
h p
ro
d
uction
F
ly a
s
h pr
o
du
c
tio
n
Si
l
icat
e w
eath
e
ring
(
200
6)
A
B
C
D
Fig. 4.
Fluxes of FOC, fly ash, and sediment in the CJ basin. (
A
)
f
FOC-ash
(fraction of fly ash-sourced fossil OC in the total flux of fossil OC exported by CJ) as a
function of
f
sed-ash
(mass fraction of fly ash in the total sediment flux) (curves with uncertainty bands determined from Monte Carlo simulations;
SI Appendix
),
with the mixing trend between FOC
ash
and FOC
CJ0
(FOC in ash-free sediment), fly-ash release (dashed line, difference between produced and utilized fly ash),
and estimates of
f
sed-ash
(red line from changes in MS of the CJ sediment, and black line and gray range from mass balance calculations). (
B
) Different types of
carbon flux in the CJ basin where
1
and
2
denote riverine FOC
ash
flux estimated from mass balance calculations and from changes in MS of CJ sediment,
respectively. The released FOC
ash
flux is estimated from this study as the product of the released fly ash flux and the FOC
ash
content. The 2007 to 2008 riverine
FOC flux is estimated in ref. 35, and the silicate weathering flux (2006) is from ref. 25. The riverine FOC
rock
flux (2007 to 2008) and the riverine FOC flux (1950s)
are quantified from this study. (
C
) Changes of riverine FOC
rock
flux versus released (produced
utilized) FOC
ash
flux in 1950 to 2010. (
D
) Sediment flux and fly-
ash flux in the CJ basin in 1950 to 1970 (predamming) and 2004 to 2010 (postdamming).
Li et al.
PNAS
|
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Coal fly ash is a major carbon flux in the Chang Jiang (Yangtze River) basin
https://doi.org/10.1073/pnas.1921544118
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
Downloaded at California Institute of Technology on May 19, 2021
Mt
·
y
1
from
500 Mt
·
y
1
before the dams were emplaced (Fig. 4
C
and
D
)
(34), contributing
<
1% to the global riverine sediment flux to the oceans
(72). Since FOC
rock
is mainly eroded from upstream areas and scales with
sediment flux (38, 44), a significant proportion of FOC
rock
is being seques-
tered in the reservoirs, leading to a reduction in the riverine-carried FOC
rock
flux in the lower CJ reach where many large-scale power and steel plants,
the major sources of FOC
ash
, are located (Fig. 1
C
).
Transport and Fate of Fossil OC.
Our analysis revealed that FOC
ash
and FOC
rock
have systematically distinct reactivities. When compared overall to biospheric
carbon, the low
f
ox
and high E
a
values we observed indicated that FOC
ash
is
expected to be significantly more recalcitrant and conserved during transport
and storage compared to FOC
rock
. These differences will affect their fates
during fluvial transport and storage, leading to different impacts on the car-
bon cycle. For FOC
ash
, the graphitic component is less reactive and is expected
to be more conserved as it transits the landscape, whereas the nongraphitic
carbon is more reactive and thus more likely to interact with active carbon-
cycle processes in surface and subsurface environments (5, 47).
ThefateofFOC
rock
as it becomes exposed to surface processes depends on its
chemical properties and the geomorphic setting. We saw that FOC
rock
in the CJ
basin contains a large fraction that is labile and prone to oxidation (Fig. 3),
whereas FOC in river sediments is comparatively recalcitrant (38). This suggested
the preferential loss of the labile component in FOC
rock
during denudation,
fluvial transport, and sediment storage in the CJ system
a pattern consistent
with prior observations of significant oxidation of FOC
rock
in large floodplain
systems (e.g., Amazon) (59). Although laboratory experiments have intimated
slow reaction kinetics of FOC
rock
oxidation (e.g., first-order kinetic coefficients
on the order of 10
3
to 10
4
y
1
) (74), the long transit time of OC and sediment
(e.g.,
10
4
to 10
5
y) in large floodplain systems (75, 76) matches or exceeds the
characteristic reaction timescales of FOC
rock
and thus would allow sufficient
reaction time for FOC
rock
oxidation. In contrast, FOC
rock
oxidation might be
kinetically limited in river systems with
smaller catchment sizes and shorter
transit timescales such as the rivers in
mountainous islands (8). Overall, we hy-
pothesized that the differences in the reactivity of FOC
rock
and FOC
ash
translate
into the differences in their fates most profoundly in large alluvial systems (e.g.,
CJ and Amazon) with long transit times,
and such differences are expected to
be dampened in smaller catchments with shorter transit times.
Dam building in the CJ basin has probably influenced the fate of FOC
rock
during fluvial transit as well. Previous studies suggested that the high sedi-
mentation rate in the reservoirs would limit oxygen exposure time of
carbon-bearing particles and promote their preservation (11, 38, 39, 64).
Thus, the reservoirs in the CJ basin might be expected to help sequester and
preserve FOC
rock
from upstream CJ, buffering its oxidation in downstream
floodplains and estuaries (38). In addition, although there has been an in-
creased supply of FOC from coal ash in the middle-lower CJ basin; one prior
study hypothesized that the emplacement of the Three Gorges Dam in 2003
would lead to younger and fresher OC exported by the CJ (39). We do not
have upstream samples during our study time interval to delineate the
downstream changes of riverine OC in 2007 to 2008. However, we noticed
that the average proportion of FOC in our 2007 to 2008 sediment samples
(
25%) was higher than the FOC proportion in the middle-to-lower CJ
sediment samples (
10%) collected 1 to 2 y later after our sampling time.
This difference suggested a temporal shift toward a lower proportion of FOC
in the CJ-exported OC
a trend consistent with the proposed change toward
a younger and fresher riverine OC flux after the impoundment of the Three
Gorges Reservoir. Nonetheless, continued monitoring and systematic sam-
pling of the whole CJ fluvial network are needed for a more detailed picture
of how hydraulic engineering impacts carbon cycling in this system (38, 39).
FOC
ash
and FOC
rock
are also carried by particles of different sizes, which
can affect their fate via transport processes. FOC
ash
is mostly encapsulated in
micrometer-sized fly-ash particles, whereas FOC
rock
is bound to coarser (e.g.,
sand-sized) grains. Thus, FOC
ash
could more easily bypass dams during flow
release, whereas FOC
rock
is likely to be sequestered in reservoirs. When de-
livered to the CJ estuary and the East China Sea margin where hydraulic
conditions and sediment transport and storage processes are complex, the
fine-grain-carried FOC
ash
may have more dynamic behavior (e.g., floccula-
tion settling, suspension, and dispersion) and may be spread over a larger
depositional area than FOC
rock
(77). The fine particle sizes that carry FOC
ash
can also be more efficiently transported by aeolian processes, which can
deliver FOC
ash
to remote areas beyond riverine transport within a given
catchment (78). The aeolian flux of FOC
ash
both within and outside of the CJ
basin requires further assessment, but we anticipated that these fluxes are
minor compared to the riverine flux, considering the dominance of the wet,
monsoonal climate in the basin that limits ash transport by aeolian processes
(79, 80). In northern China where a drier climate dominates, aeolian pro-
cesses may well play a more important role transporting FOC
ash
(80, 81).
Conclusions and Implications
The CJ basin illustrates how human activities have significantly
altered the carbon cycle at continental scales. In the CJ basin, fly
ash contributes a remarkable 37 to 72% of the riverine fossil OC
exported to the oceans. Driven by the human pursuit of energy,
the riverine-carried FOC
ash
flux has increased while the riverine
FOC
rock
flux decreased
and as a result, these two fluxes have
converged over an interval of 60 y to amplify the concentration
of FOC
ash
on the landscape. This serves as an example of how
the pace of the human-induced alteration of the carbon cycle can
catch up with nature-sourced carbon at decadal timescales and
demarcates another dimension of the human imprint on the
short-term carbon cycle beyond that directly associated with CO
2
emission during fossil fuel combustion (19, 82).
Our results showed that not all fossil OC is made equal:
FOC
rock
has a significant fraction that is labile and can be oxi-
dized during transport, whereas FOC
ash
is highly recalcitrant
(i.e., unreactive) and can be conserved during transport. While
coal burning is a leaky process with respect to OC, the way that
carbon is transformed by incomplete combustion means that the
FOC
ash
that escapes this process is much less likely to end up as
CO
2
compared to the FOC
rock
naturally derived from erosion.
Furthermore, its fossil origin means FOC
ash
is radiocarbon-dead
(7, 11). With increasing coal consumption and ash production
(19), FOC
ash
flux is expected to increase and to contribute to a
greater proportion of the total riverine FOC flux to the oceans.
With this growing human-induced carbon flux, caution will need
to be taken when interpreting radiocarbon-based material flux as
well as records from recent offshore sediments. With this ob-
servation in mind, given that the magnitude of fly ash release can
match natural sediment fluxes at regional scales (e.g., in the CJ
basin), the unique properties of FOC
ash
make it a useful tracer of
anthropogenic impacts on the OC cycle. By reflecting the history
of coal consumption, FOC
ash
in sedimentary cores and other
archives could provide a distinct marker of the Anthropocene
(78, 83).
Data Availability.
All study data are included in the article and/or
supporting information.
ACKNOWLEDGMENTS.
This project was funded by the National Key R&D
Program of China (Grant 2017YFD0800300). G.K.L. acknowledges support from a
California Institute of Technology Geology Option Postdoctoral Fellowship and a
National Ocean Sciences Accelerator M
ass Spectrometry Laboratory Graduate
Intern Fellowship. W.W.F. and M.P.L. acknowledge support from Foster and Coco
Stanback, California Institute of Technology
s Terrestrial Hazard Observation and
Reporting Center, and the Resnick Sustainability Institute. We thank Yuliang
Chen for help with data compilation.
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