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Global Biogeochemical Cycles
RESEARCH ARTICLE
10.1002/2016GB005509
Key Points:
14
13
• Combined C and C study to
quantify black carbon in both
dissolved and particulate phases for
two largest rivers in China
• We identified two black carbon pools
with distinct C-14 ages transported in
the rivers
• River dissolved BC was younger and
mainly derived from biomass burning;
particulate BC was largely from the
fossil fuel combustion
Correspondence to:
X. Wang,
[email protected]
Citation:
Wang, X., C. Xu, E. M. Druffel, Y. Xue, and
Y. Qi (2016), Two black carbon pools
transported by the Changjiang and
Huanghe Rivers in China, Global
Biogeochem. Cycles, 30, doi:10.1002/
2016GB005509.
Received 20 AUG 2016
Accepted 12 NOV 2016
Accepted article online 16 NOV 2016
Two black carbon pools transported by the Changjiang and
Huanghe Rivers in China
Xuchen Wang1,2, Caili Xu1, Ellen M. Druffel3, Yuejun Xue1, and Yuanzhi Qi1
1
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao,
China, 2Qingdao Collaborative Innovation Center of Marine Science and Technology, Qingdao, China, 3Department of Earth
System Science, University of California, Irvine, California, USA
Abstract Major rivers play important roles in transporting large amounts of terrestrial organic matter from
land to the ocean each year, and the organic matter carried by rivers contains a significant fraction of black
carbon (BC). A recent study estimated that 0.027 Gt of BC is transported in the dissolved phase by rivers
each year, which accounts for ~10% of the global flux of dissolved organic carbon. The relative sources of
this large amount of riverine dissolved black carbon (DBC) from biomass burning (young, modern 14C) and
fossil fuel (old, 14C free) combustion are not known. We present radiocarbon measurements of BC in both
dissolved and particulate phases transported by the Changjiang and Huanghe Rivers, the two largest rivers in
China, during 2015. We show that two, distinct BC pools (young and old) were carried by the rivers. The
DBC pool was much younger than the particulate BC (PBC) pool. Mass balance calculations indicate that most
(78–85%) of the DBC in the Changjiang and Huanghe Rivers was derived from biomass burning, and only
15–22% was from fossil fuel combustion. In contrast, PBC from biomass burning and fossil fuel combustion
were approximately equal in these two rivers. Export of PBC and DBC by the rivers are decoupled, and
fluxes of PBC were 4.1 and 6.7 times higher than DBC in the Changjiang and Huanghe Rivers, respectively.
The 14C age differences of the two BC pools suggest that BC derived from biomass burning and fossil fuel
combustion are mobilized in different phases and on different time scales in these rivers.
1. Introduction
Black carbon (BC) is a group of combustion-derived, organic compounds that is widely dispersed in the natural environment, particularly in soils and marine sediments [Laflamme and Hites, 1978; Goldberg, 1985;
Masiello and Druffel, 1998]. BC is defined as condensed carbonaceous residue produced from incomplete
combustion of fossil fuel and biomass [Goldberg, 1985]. It is estimated that globally, about 0.05–0.27 Gt
(1 Gt = 1 × 1015 g) BC is produced annually by fossil fuel combustion [Kuhlbusch and Crutzen, 1995] and
0.012–0.024 Gt BC by biomass burning [Penner et al., 1993]. In their recent study, Santín et al. [2015] reported
that production of pyrogenic BC from boreal forest wildfires alone could reach 0.1 Gt yr and that is much
higher than the values previously estimated. After production, some of the BC is preserved in soils
[Czimczik and Masiello, 2007; Ohlson et al., 2009; Cusack et al., 2012], and some very fine particulate BC is transported long distances by rivers and as atmospheric aerosols, eventually being deposited in the ocean
[Masiello and Druffel, 1998; Mitra et al., 2002; Dittmar et al., 2012; Singh et al., 2012; Coppola et al., 2015].
Concerns regarding BC in the environment are largely based on its effects on natural ecosystems [Rust et al.,
2004; Lohmann et al., 2005; Moermond et al., 2005; Shrestha et al., 2010], health risks to humans [Na et al.,
2011], impact on climate, and importance as a sink for the global carbon cycle [Masiello and Druffel, 1998;
Druffel, 2004; Shrestha et al., 2010]. Because BC is chemically stable [Bird et al., 1999], it is believed to be transported mainly as fine particulate matter and aerosols. However, recent studies have suggested that a major
fraction of charcoal produced annually is lost from soils by dissolution and is transported in rivers [Dittmar
et al., 2012; Ding et al., 2013; Jaffé et al., 2013; Wagner et al., 2015]. Jaffé et al. [2013] estimated that about
0.027 Gt/yr of BC is transported in the dissolved phase by rivers, accounting for ~10% of the global flux of dissolved organic carbon (DOC) via rivers. Clearly, rivers play an important role in mobilizing BC and transporting
it from the land to the oceans. However, the sources of this large amount of riverine dissolved BC (DBC),
whether from recent biomass burning or fossil fuel combustion, are not known.
©2016. American Geophysical Union.
All Rights Reserved.
WANG ET AL.
Radiocarbon measurement of BC is powerful for identifying the sources of BC produced from biomass burning (modern 14C age) compared with BC generated from fossil fuel combustion (>50,000 14C years).
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10.1002/2016GB005509
Figure 1. Map showing the Changjiang (Yangtze) River and the Huanghe (Yellow) River basins and the sampling stations in
2015.
Ziolkowski and Druffel [2010] reported Δ14C values of BC in high molecular weight DOC (HMW-DOC) isolated
from the Suwannee River in Florida (USA) in 1999 and the ocean. They found that the 14C age of DBC in the
Suwannee River (410 280 years B.P.) was much younger than the DBC age (18,000 3000 years B.P.) in the
Atlantic and Pacific Oceans. This suggests that the DBC transported by rivers could be more labile than presently believed [Ziolkowski and Druffel, 2010]. However, since there was only a single sample from a local
small river, it remains unknown whether this is representative of larger river systems.
In this paper, we present the results of 14C measurements of BC in both dissolved and particulate phases
transported by the Changjiang and the Huanghe Rivers. Two BC pools with distinct 14C ages were identified,
and their sources and geochemical cycling in the rivers are discussed.
2. Methods
2.1. Study Site and Sample Collection
Water samples were collected from the Changjiang (Yangtze) River Estuary and the lower reach of the
Huanghe (Yellow) River (Figure 1). The Changjiang and Huanghe Rivers are the two largest rivers in China
and the third and sixth longest rivers in the world [Milliman and Meade, 1983; Cai et al., 2008]. Together, they
drain one third of Chinaˈs land (~3.0 × 106 km2) and deliver about 0.0019 and 0.0016 Gt of particulate organic
carbon (POC) and DOC annually to the coasts of the East China Sea (ECS) and Bohai Sea, respectively
[X. C. Wang et al., 2012].
Water samples (15–50 L) were collected in the lower reach of the Changjiang River, its estuary, and the coast
of ECS in July and October 2015. Monthly water samples (15 L) were collected from the lower reach of the
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10.1002/2016GB005509
Huanghe River during 2015. Subsurface (1.5 m) water samples from the Changjiang River Estuary and the
coast were collected using an in situ pump (SFP-900P) on board the R/V Dongfanghong-2 (a fishing boat used
for the river water collection), filtered immediately through 0.7 μm Whatman GF/F filters (precombusted at
550° for 4 h), and acidified with high-purity HCl to pH 2. The acidified water samples were stored in precleaned polyethylene bottles at low temperature for further processing. The suspended particles retained
on the filter were kept frozen for solid phase BC analysis. The river water was transported back to the laboratory within 1 day and extracted immediately, and the acidified coastal water samples were extracted within
1 week [Xu et al., 2016].
2.2. DBC and PBC Measurement
To determine DBC, water samples were first extracted for DOC using the solid phase extraction (SPE) method
based on Dittmar et al. [2008] and Coppola et al. [2015]. We used prepacked PPL cartridges (Agilent
Technologies Mega Bond Elute) that contained 5 g styrene divinyl benzene polymer as sorbent (pore size
150 Å). The extraction efficiency and detailed description of the sample processing are described in Xu
et al. [2016]. For the dissolved phase, we measured 14C of total DOC, SPE-extracted DOC (SPE-DOC), DBC
and permeate-DOC (the filtrate passing through the sorbent for two Huanghe River samples). For the solid
phase, we measured 14C of POC and PBC (particulate BC).
Both DBC and PBC were determined using the thermal oxidation method based on Gustafsson et al. [2001].
Previous studies have shown that this method is reproducible for BC analyses of standards, marine sediment,
and particles [Gustafsson et al., 2001; Reddy et al., 2002; Hammes et al., 2007; Wang and Li, 2007]. The thermal
oxidation method has also been used to determine DBC in HMW-DOM samples [Mannino and Harvey, 2004].
However, it should be stated that there is no single standard method used to separate and quantify BC in
both particulate and dissolved phases. Different techniques used in different studies give a wide range of
BC concentrations [Currie et al., 2002; Hammes et al., 2007]. For BC determination in the dissolved phases, a
chemical oxidation method using benzene polycarboxylic acids (BPCAs) has been used recently [Dittmar,
2008; Ziolkowski et al., 2011; Coppola et al., 2015]. The BPCA method chemically oxidizes condensed aromatic
BC in SPE-DOC to produce individual BPCAs that are subsequently analyzed using high-performance liquid
chromatography [Dittmar, 2008] or preparative capillary gas chromatography [Ziolkowski et al., 2011]. For
the thermal oxidation method, highly condensed aromatic structures in BC are measured, usually giving
lower BC concentrations [Hammes et al., 2007].
For our study, 1.0–2.0 mL of SPE-DOC was put into a 9 mm OD × 200 mm quartz tube (precombusted at 850°C
for 2 h) and dried with high purity N2 gas. The quartz tube was then placed in an oven and thermally oxidized
at 375°C for 24 h with a continuous air supply to remove non-BC OC. The organic carbon that remained after
thermal oxidation was operationally defined as DBC. For PBC, suspended particles were dried at 60°C and
ground to a fine powder. About 1.0 g powder was placed in a very thin layer (2–3 mm) in a petri dish and
heated at 375°C for 24 h in an oven with a continuous air supply. Before and after thermal oxidation, particles
were acidified with 10% HCl to remove inorganic carbon. DBC, PBC, and POC were combusted in evacuated
9 mm OD quartz tubes (with CuO and Ag wire added) at 850°C for 2 h [Druffel et al., 1992]. The resultant CO2
was collected cryogenically and quantified manometrically on a vacuum line. The purified CO2 was flame
sealed in a 6 mm OD glass tube for isotope analysis.
2.3. DOC Oxidation, DOC Concentration, and Isotope Measurements
Total DOC and permeate-DOC of the samples were processed separately using a modified UV-oxidation
method for carbon isotope measurement [Xue et al., 2015]. Briefly, about 200 mL (river) or 400 mL (coast)
of filtered and acidified water was placed into 2 to 4 custom-made 150 mL quartz reaction tubes specially
designed to interface directly with a vacuum extraction line. Samples were first purged with ultrahigh purity
(UHP) helium gas for 30 min to remove dissolved inorganic carbon and then irradiated using a 1.2 kW
medium-pressure mercury arc UV lamp (Hanovia Co.) for 5 h. Following UV-irradiation, CO2 generated from
UV oxidation of DOC was purged again with UHP helium gas through the vacuum line and collected cryogenically and measured manometrically. The purified CO2 was flame sealed in 6 mm OD Pyrex tubes for
δ13C and Δ14C analyses. The oxidation efficiency and blanks associated with the UV-oxidation of DOC were
tested using high purity Milli-Q water and a DOC standard (oxalic acid OXI) solution, yielding high oxidation
efficiency (~95%) of DOC and considerably low blanks (<5 μg C) [Xue et al., 2015]. The concentrations of DOC
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were analyzed by the high-temperature combustion method using a Shimadzu total organic carbon (TOC)-L
analyzer equipped with an ASI-V autosampler. DOC concentration was calibrated using a five-point calibration curve generated from a DOC standard prepared using potassium hydrogen phthalate and UV-oxidized,
high purity Milli-Q water. Instrument blank and standard validation for DOC were checked against lowcarbon, reference water, and deep seawater reference material (University of Miami, Rosenstiel School of
Marine and Atmospheric Sciences). Blank subtraction was carried out using high purity Milli-Q water that
was analyzed before each sample. The average blank associated with DOC concentration measurements
was about 5 μM, and the analytic precision on triplicate injections was 3%.
δ13C and Δ14C measurements were made at the National Ocean Sciences accelerator mass spectrometry
(NOSAMS) facility at Woods Hole Oceanographic Institution. A small split of CO2 was analyzed for δ13C using
a VG isotope ratio mass spectrometer, and the rest of the CO2 was graphitized for 14C analysis using AMS.
Values of δ13C are reported in ‰ relative to the Vienna Peedee belemnite standard and the 14C measurements were reported as modern fraction [McNichol et al., 1994]. The conventional radiocarbon ages (years
before present, B.P.) were calculated based on Stuiver and Polach [1977]. The errors of Δ14C (1σ) measurements determined from duplicate sample analyses are 6.0‰ and 3.5‰ for DOC and POC [Xue, 2016],
respectively; errors were 5.0‰, 7.0‰, and 4.5‰ for SPE-DOC, DBC and PBC measurements,
respectively.
3. Results and Discussion
3.1. SPE-DOC, DBC, and PBC Concentrations
The SPE extraction efficiency of DOC using a PPL cartridge was 59–61% (n = 16) for river water and 42 2%
(n = 3) for seawater [Xu et al., 2016]. These values are in good agreement with the results reported by
Dittmar [2008] and Coppola et al. [2015] using the same resin. In Table 1, we summarize the DOC and
POC concentrations and the DBC/DOC and PBC/POC ratios determined for the Changjiang and Huanghe
Rivers. The average distributions of SPE-DOC, DBC, permeate-DOC, POC (non-BC fraction), and PBC determined for the total DOC and bulk POC samples in the Changjiang and Huanghe Rivers, and ECS coastal
samples are plotted in Figure 2. The DBC accounted for (with standard deviation) 3.0 0.4% and
4.8 3.6% of the DOC pools, and PBC accounted for 13 0.4% and 22 11% of the POC in the
Changjiang and Huanghe Rivers, respectively (Table 1). Detailed discussion of the concentrations of DOC
and POC and the concentrations and fluxes of DBC and PBC in the two rivers is presented in a separate
publication [Xu et al., 2016].
Our results are comparable to those reported in earlier studies. Surface seawater was reported to contain
2.6% and 0.9% of DBC from the Gulf of Mexico and the Atlantic Ocean [Dittmar, 2008] and 4.2 1.0% DBC
in the northeast Pacific [Coppola and Druffel, 2016]. Also, using the SPE and BPCA methods, Wagner et al.
[2015] reported that 3.2–6.6% of the riverine DOC pool was DBC in the Cache La Poudre River in the
Colorado Rocky Mountains (USA). Using an ultrafiltration technique and thermal oxidation method,
Mannino and Harvey [2004] reported that DBC comprised 8.9 6.5% of the DOC in the Delaware Estuary
and 4–7% in the Atlantic coastal waters. Using ultrafiltered DOC and BPCAs methods, Ziolkowski and
Druffel [2010] reported that DBC accounted for 0.5–3.5% of the DOC pool in the Suwannee River and
Atlantic seawater. These similarities in DBC abundance in different aquatic environments may suggest that
similar mechanisms or processes control the dissolution and cycling of BC in the DOC pool. As demonstrated
by Jaffé et al. [2013], a strong, linear correlation (r2 = 0.95) exists between DBC and DOC concentrations in the
15 large rivers they studied, suggesting that rivers play an important role in mobilizing and transporting soil
BC to the ocean.
The PBC/POC abundances in the Changjiang (12.2–13.0%) and Huanghe (9.0–45.2%) rivers were higher than
the DBC/DOC abundances (Table 1 and Figure 2). This is in good agreement with the results reported previously for the BC content in the coastal sediments of Chinaˈs marginal seas [Wang and Li, 2007; Kang
et al., 2009]. In the Bohai Sea, which is influenced largely by the Huanghe River, BC content in the surface sediments ranged from 27 to 41% of the sedimentary TOC, higher than values (5–26%) in the ECS coastal sediments [Kang et al., 2009]. This indicates that most PBC transported by the rivers was deposited and
preserved in the coastal sediments and is consistent with the recent study of Tao et al. [2016]. In their earlier
study, Masiello and Druffel [2001] measured BC in POC samples collected in the Santa Clara River in California
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a
Table 1. Concentration and Calculated Fluxes of DOC, POC, DBC, and PBC in the Changjiang and Huanghe Rivers
Concentration
Sample
DOC
(μM)
POC
(mg/g)
Flux
DBC/DOC
(%)
PBC/POC
(%)
DOC
POC
9
(× 10 g C)
DBC
PBC
PBC/DBC
Flux Ratio
1520
47.4
192
4.1
0.99
1.20
1.54
5.88
8.13
6.52
14.4
0.97
0.20
0.48
0.27
0.64
41.2
0.06
0.15
0.14
0.59
0.12
0.04
0.23
0.27
0.07
0.02
0.02
0.01
1.72
0.19
0.26
0.33
2.46
1.76
2.95
3.27
0.17
0.03
0.01
0.05
0.07
11.5
3.2
1.7
2.4
4.2
14.7
73.8
14.2
0.6
0.4
0.5
2.5
7.0
6.7
Changjiang River
CR01 (32.13°N,
118.78°E)
CR02 (32.35°N,
118.92°E)
CR03 (31.39°N,
121.57°E)
CR04 (32.35°N,
118.92°E)
Mean/Annual
121 2
2.8 0.1
116 2
2.7 0.2
193 6
11.2 0.3
3.5 0.2
13.0 0.9
118 7
9.3 0.4
3.1 0.4
12.2 0.7
137 37
10.2 1.0
3.0 0.4
12.6 0.4
b
1580
b
c
ECS Coastal Water
P02 (31.35°N,
122.55°E)
P04 (30.66°N,
123.49°E)
P06 (30.00°N,
124.49°E)
Mean
139 5
2.9 0.2
80 3
3.3 0.4
72 2
4.1 0.4
97 36
3.4 0.6
Huanghe River (37.60°N, 118.53°E)
January
February
March
April
May
June
July
August
September
October
November
December
Mean/Annual
236 2
225 4
215 19
220 4
131 4
123 13
214 40
219 27
284 20
244 2
188 5
219 17
210 45
3.5 0.4
3.7 0.3
4.0 0.6
3.2 0.2
3.1 0.4
3.1 0.3
2.6 1.1
5.8 0.6
2.9 0.4
10.7 1.2
6.0 0.5
2.0 0.4
4.2 2.4
2.6 0.2
5.7 0.3
8.2 0.4
13.0 0.3
3.6 0.4
1.9 0.2
2.8 0.3
9.3 0.5
4.8 0.3
2.9 0.2
2.1 0.2
1.2 0.1
4.8 3.6
19.4 1.4
21.9 0.8
21.5 1.5
41.9 2.5
21.6 1.0
45.2 0.6
22.7 2.7
17.9 1.0
13.4 0.7
9.0 0.7
16.7 0.2
10.5 1.5
21.8 11.1
2.28
2.57
1.69
4.56
3.32
1.92
8.11
2.94
1.45
0.81
0.73
1.06
31.4
a
The highest fluxes in July in the Huanghe River was caused by a man-made flood event when the water reservoir gate
at Xiaolangdi Dam was opened to wash away the sediments deposited on the river bed along the low river basin [Xu
et al., 2016].
b
Annual DOC and POC fluxes for the Changjiang River are based on X. C. Wang et al. [2012].
c
POC and PBC were not measured for the ECS coastal water.
(USA) and found that BC accounted for 7.9–17% of the POC. Using the thermal oxidation method, Mitra et al.
[2002] reported that 16–17% of river POC, and 25–28% of the shelf water POC was PBC in the Mississippi River
Estuary. The comparable PBC values determined in different rivers and coastal waters using different methods may suggest that a similar mechanism controls the transport and cycling of BC in the solid phase. Strong
adsorption of BC onto particles could be an important process. Association with minerals is a major control of
BC preservation in soils [Cusack et al., 2012]. Strong adsorption of aromatic compounds such as polyaromatic
hydrocarbons onto marine sediments is also common [Khodadoust et al., 2005].
3.2. Isotopic Signatures of DBC and PBC
We report δ13C and Δ14C values for DOC, DBC, POC, and PBC samples and for SPE–DOC and permeate-DOC in
selected samples (Table 2). Mass balance calculations based on the concentration and isotope measurements
of SPE-DOC and permeate-DOC showed 94.5 0.7% recovery of bulk DOC and offsets of 0.6 0.2‰ and
14 4‰ from the bulk δ13C-DOC and Δ14C-DOC values, respectively. Figure 3 shows δ13C versus Δ14C values
for DBC and PBC samples, and distinct BC pools transported by the Changjiang and Huanghe Rivers are
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Figure 2. Percent of OC separated in SPE-DOC, permeate-DOC and DBC for the total DOC, and POC and PBC in the particulate phase in the Changjiang (n = 2) and Huanghe Rivers (n = 12), and ECS seawater (n = 3).
clearly seen. River DBC with high Δ14C and low δ13C values is in one pool (yellow shaded area), which is different from the river PBC that has lower Δ14C values (green shaded areas). The Huanghe River PBC is separated into two isotopic groups: PBC-1 with relatively high Δ14C and δ13C values and PBC-2 with low Δ14C
and δ13C values. DBC in the offshore seawater had low Δ14C and high δ13C values, quite different from the
river DBC (Figure 3), and consistent with the study of Ziolkowski and Druffel [2010].
In Figure 4, we compare the δ13C values and 14C ages for total DOC, SPE-DOC, DBC and permeate-DOC fractions separated for two Huanghe River samples and find three interesting differences. First, the SPE-DOC had
similar δ13C values but were significantly younger (average 1060 14C years B.P.) than the total DOC (1380 14C
years). Second, the permeate-DOC had high δ13C values (average 22.4‰) but much older 14C ages (2070–
3200 14C years) than both total DOC and SPE-DOC. Third, DBC had similar δ13C values but was younger
(1030 70 14C years) than total DOC and similar to the ages of SPE-DOC. These patterns were similar for
all of the samples we measured for Δ14C of DOC, SPE-DOC, and DBC (Table 2).
In Figure 5, we compare the monthly 14C ages of DOC and DBC, POC and PBC, and DBC and PBC in the
Huanghe River. We note that the ages of DBC (945–1510 14C years) are younger than those of DOC (1320–
1680 14C years) during all months (Figure 5a). The PBC, however, shows different seasonal patterns
(Figure 5b). PBC samples collected in the winter months (December–March) were much older (9290–
12,600 14C years) than the bulk POC (4300–4630 14C years), and PBC collected from April to July and
September were younger (3100–3970 14C years) than the POC (4520–5060 14C years). Comparison between
DBC and PBC indicates that PBC samples were much older than DBC during all months, with an even larger
difference during the winter months in the Huanghe River (Figure 5c). These are consistent with the isotopic
results shown in Figure 3, where PBC in the four winter months had low δ13C ( 24.1‰ to 24.4‰) and low
Δ14C ( 688‰ to 794‰) values (PBC-2 group), whereas PBC in the nonwinter months had relatively high
δ13C ( 20.5‰ to 22.0‰) and high Δ14C ( 278‰ to 395‰) values (PBC-1). Based on the isotopic values,
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13
Table 2. Radiocarbon (Δ C) and Stable Carbon (δ C) Isotopic Values Measured for the Samples From the Changjiang and Huanghe Rivers and ECS
DOC
13
14
δ C
Sample Location
Changjiang (CR03)
ECS-P02
ECS-P04
Huanghe River
January
February
March
April
May
June
July
August
September
October
November
December
Mean
Δ C
SPE-DOC
Age
137
277
269
1120
2540
2460
26.4
22.6
21.0
26.7
27.0
26.4
25.3
22.4
22.6
24.6
24.5
26.1
24.6
164
188
195
186
159
158
164
187
167
166
192
178
175
1380
1610
1680
1600
1330
1320
1380
1600
1410
1390
1650
1520
1490
14
Δ C
δ C
a
13
Age
25.2
22.7
166
293
1390
2720
26.8
128
1040
26.6
26.8
132
118
1070
940
26.5
133
1080
26.5
26.5
26.9
26.6
152
156
176
142
1260
1300
1500
1170
Age
13
δ C
3200
2070
Δ C
26.3
25.5
Age
14
δ C
Δ C
PBC
Age
(‰)
13
δ C
Δ C
14
Age
(‰)
25.9
22.1
19.0
64.9
152
283
475
1230
2620
24.2
384
3840
19.0
437
4550
25.0
24.7
24.3
25.3
24.6
24.5
24.6
23.1
23.3
23.5
22.6
22.9
24.0
120
132
118
126
121
131
135
137
147
160
178
162
139
965
1080
945
1020
970
1070
1100
1120
1230
1340
1510
1360
1140
23.6
23.1
24.1
22.9
23.1
25.0
23.8
22.8
23.7
23.7
24.3
24.3
23.7
442
439
427
452
435
432
471
243
436
217
324
419
394
4630
4590
4420
4760
4520
4480
5060
2170
4530
1900
3080
4300
4040
24.1
24.4
24.3
20.7
21.0
21.4
21.9
20.5
20.8
21.2
22.0
24.1
22.2
688
778
731
395
327
360
375
278
286
300
331
794
470
9290
12050
10500
3970
3120
3500
3710
2550
2675
2800
3200
12600
5830
13
14
δ C
(‰)
334
233
Δ C
POC
13
Measured
14
δ C
14
(‰)
Mass balance
(‰)
22.4
22.5
Δ C
DBC
13
(‰)
26.2
26.4
27.9
13
14
δ C
(‰)
Permeate-DOC
January
July
13
Δ C
(‰)
192
184
26.4
25.3
164
164
14
Values of δ C and Δ C for DOC and POC were from Xue [2016]. Samples left empty were not measured; the isotopic mass balance was calculated as the sum
of SPE-DOC and permeate-DOC based on the extraction efficiency of SPE-DOC [Xu et al., 2016]. The values are compared with measured bulk DOC values. The
14
errors (1σ) of Δ C measurements from duplicate samples are 6.0‰ and 3.5‰ for DOC and POC [Xue, 2016]; 5.0‰, 7.0‰, and 4.5‰ for SPE-DOC,
DBC, and PBC, respectively.
DBC, PBC-1, and PBC-2 were derived from different sources. BC produced from biomass burning should have
modern 14C ages compared with BC generated from fossil fuel combustion (>50,000 14C years) [Ziolkowski
14
13
Figure 3. Plot of C (‰) versus C (‰) values for DBC and PBC in the Changjiang and Huanghe Rivers and ECS seawater
(DBC only). For the river DBC: (solid triangle) DBC in the Huanghe River; (open triangle) DBC in the Changjiang River; (dia14
mond) DBC in the nearshore water (P02). The errors of C (1σ) measurements, determined from duplicate sample analyses,
13
are 7.0‰ and 4.5‰ for DBC and PBC, which are in general smaller than the symbols. And the errors of C (1σ)
measurements are 0.5‰ and 0.3‰ for DBC and PBC, respectively.
WANG ET AL.
BLACK CARBON IN RIVERS
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10.1002/2016GB005509
14
Figure 4. Plot of C ages ( C years B.P.) for the total DOC, SPE-DOC, permeate-DOC, and DBC for the two Huanghe River
13
water samples. The C (‰) values are also shown.
and Druffel, 2010]. Because river DBC had relatively high Δ14C (average 139‰, 1140 years B.P.) and low δ13C
(average 24.0‰) values, it originated mostly from biomass burning likely from C3 plants that have similar
δ13C values ( 22‰ to 34‰) [Vogel, 1993]. This BC could have been stored in soils and preaged before
entering the riverine DBC pool. For the PBC-2 group, which had old 14C ages (average 11,100 years) and
low δ13C values (average 24.2‰ similar to fossil fuels, mainly coals used in China) [Xu and Shen, 1990], it
is evident that fossil fuel combustion generated BC is a major contributor to PBC-2. For PBC-1, these samples
likely contained a mixture of BC from both fossil fuel combustion and biomass burning.
If we assume that all BC produced from biomass burning had a postbomb Δ14C value of 100‰ [Ziolkowski
and Druffel, 2010] and BC derived from fossil fuel combustion had a 14C-free Δ14C value of 1000‰, we
can estimate the contribution of each source to the DBC and PBC pools using an isotopic mass balance
approach. By doing this, we calculate that 85 5% and 78 12% (n = 12) of the DBC in the Changjiang
and Huanghe Rivers, respectively, were derived from biomass burning, and 15 4% and 22 12% of the
DBC were derived from fossil fuel combustion. For all PBC samples, we estimate that 51 7% and
48 19% were from biomass burning, and 49 6% and 52 19% were from fossil fuel combustion in the
Changjiang and Huanghe Rivers, respectively. If we calculate the percentages for the PBC-1 and PBC-2 groups
separately, we find that 34–45% is from fossil fuel produced BC, and 55–66% is from biomass burningproduced BC for PBC-1 (nonwinter), and 72–81% is from fossil fuel BC, and 19–28% is from biomass burning
BC for PBC-2 (winter), respectively. For the Changjiang River and offshore (P02) DBC samples (Figure 3), they
originated mainly from biomass burning-produced BC as well (85% and 77%, respectively). These calculations
are overly simplistic. We assumed only two end-members (modern BC from biomass burning and old BC from
fossil fuel combustion) and did not take into account other sources, such as BC from soils or groundwater. The
Changjiang River PBC and offshore DBC samples had high δ13C values ( 19.0‰), not consistent with others,
and we are not able to provide an adequate explanation. More isotopic data for DBC and PBC are needed for
the Changjiang River and ECS to resolve this issue.
The fossil fuel BC contributions to POC in the Huanghe and Changjiang Rivers were higher compared with the
Mississippi River (USA), for which Mitra et al. [2002] reported that 27% of POC-BC was from fossil fuel origin. In
China, fossil fuel consumption, especially coal burning, has increased dramatically in the last 20 years. Data
from the National Bureau of Statistics of China (www.stats.gov.cn) indicated that in 2015, 4.3 Gt coal C were
combusted in China. In the northern region of China, coal is the major source of heating fuel during winter, so
large amounts of BC are produced from coal combustion. The fine BC particles that are produced could enter
rivers via wind deposition, wet deposition, and runoff [Raymond, 2005; Avery et al., 2013; Chen et al., 2015;
Wang et al., 2016]. This is clearly demonstrated in the very old PBC (average 11,100 1500 14C years) measured in the Huanghe River during the four winter months (December, January, February, and March) compared to the rest of the months in 2015 (average 3240 520 14C years) (Figure 5b). During the nonwinter
months, more biomass was burned than coal, producing younger BC that was added to the PBC pool in
WANG ET AL.
BLACK CARBON IN RIVERS
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Global Biogeochemical Cycles
Figure 5. Comparison of monthly changes of the
Huanghe River during 2015.
10.1002/2016GB005509
14
C ages of DOC and DBC, POC and PBC, and DBC and PBC in the
the Huanghe River. During the field sampling in the late summer and fall seasons, we observed that agriculture residue stalk burning in the farm lands along both sides of the river is a very common phenomenon. In
their recent study, Zhao et al. [2011] estimated that 0.033 Gt biomass carbon emission was produced from the
farm land grain crop residues each year in China. R. Wang et al. [2012] reported that about 0.00196 Gt BC was
produced annually in China in recent years. They estimated that 22.2%, 3.1%, and 0.14% were from residential
biomass (firewood and crop residue) burning, open agricultural crop waste burning, and wildfire burning,
respectively. Residential coal combustion that occurred mostly in the winter months contributed 27.5% of
the BC produced each year. This trend has been clearly demonstrated by our 14C results. The relatively high
δ13C values ( 20.7‰ to 22.0‰) of the PBC in the spring, summer, and fall months may suggest that some
BC is from C4 plants, such as corn stalk burning.
As the largest two rivers in China, the Changjiang and Huanghe are impacted by human activities, such as
anthropogenic inputs and dam construction. The flow rate of the Changjiang River is usually 40–50 times
WANG ET AL.
BLACK CARBON IN RIVERS
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10.1002/2016GB005509
higher than that of the Huanghe
River throughout the year, and the
fluxes of DOC and POC in the two
rivers were determined to be 50
and 4 times higher, respectively, in
the Changjiang River than in the
Huanghe River (Table 1) [X. C. Wang
et al., 2012]. Despite the large flux
differences, the comparable 14C
ages of DBC and PBC determined in
the two rivers may suggest similar
processes and mechanisms of mobilization of BC from land to the ocean
by rivers [Dittmar et al., 2012; Jaffé
et al., 2013; Wagner et al., 2015].
3.3. Fluxes of DBC and PBC in
the Rivers
Based on the flow rates of water and
concentrations of DOC, POC, DBC,
and PBC determined for the
Changjiang and Huanghe Rivers,
we calculated the fluxes of DBC and
PBC for the rivers (see Table 1). For
the Huanghe River, the monthly
fluxes of DBC and PBC ranged from
0.01 to 0.59 × 109 g C and 0.01 to
3.27 × 109 g C, respectively. The
monthly flux variations were likely
affected by the hydrological differences. Higher flow rate in the
Huanghe River usually occurs in the
later spring and summer (rainy season), and rainwater runoff could
mobilize soil BC into the river and
result in relatively higher DBC and
PBC fluxes (Table 1). The annual
fluxes of DBC and PBC in the
Huanghe River in 2015 were
1.7 × 109 g C and 11.5 × 109 g C,
which accounted for 5.5% and 28%
Figure 6. Plot of fluxes of DBC versus DOC, PBC versus POC, and DBC versus
PBC for the Huanghe River. Solid lines are liner regression of the data.
of the annual fluxes of DOC and
POC in the river, respectively
(Table 1). For the Changjiang River, we used the average DBC and PBC values of the two river samples and
the annual flux values of DOC and POC in 2009 determined in our previous study [X. C. Wang et al., 2012].
The annual fluxes of DBC and PBC are calculated to be 47.4 6.3 × 109 g C and 19.2 6.1 × 1010 g C, which
accounts for 3% and 13% of the annual DOC and POC fluxes in the Changjiang River, respectively. The PBC
flux was 4.1 and 6.7 times higher than the DBC fluxes in the Changjiang and Huanghe Rivers, suggesting that
the PBC flux was the dominant process for mobilization and transport of BC from land to the ocean by the
two rivers. We calculated that together, the Changjiang and Huanghe transported 12.7% of the 0.00196 Gt
BC produced annually in China [R. Wang et al., 2012]. These large fluxes of DBC and the great age of PBC transported by the Changjiang and Huanghe Rivers demonstrate a significant sink of anthropogenic carbon,
which likely plays an important role in the cycling of carbon in Chinaˈs marginal seas.
WANG ET AL.
BLACK CARBON IN RIVERS
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10.1002/2016GB005509
In Figure 6, we examined the relationships between DBC and DOC
fluxes, PBC and POC fluxes, and
DBC and PBC fluxes in the
Huanghe River. It appears that a
significant positive correlation
(r2 = 0.73,
p < .0002)
exists
between DBC and DOC fluxes if
we eliminate the highest DOC flux
in July (Figure 6a). This high DOC
flux was caused by a man-made
flood event. Since 2002, annual
water management regulations
for the Huanghe River require
opening the dam gate at the
Xiaolangdi Reservoir (600 km
14
upstream) during June–July, to
Figure 7. Comparison of the C ages of DOC and DBC for the three rivers
14
remove the sediment deposited
(Changjiang, Huanghe, and Suwannee) that were studied. A 50 50 C
14
years was used for the Suwannee River DOC to represent a Δ C value
on the river bed during previous
(153‰) that was postbomb.
months [Xu et al., 2016]. We did
not see a high DBC flux during this
high flow period. It is possible that some DBC could be adsorbed onto sediment particles because the flux of
particles was the highest during July (Table 1). For PBC and POC fluxes (Figure 6b), there is a strong positive
correlation (r2 = 0.84, p < .0001) showing that the flux of PBC increased with the flux of POC in the river. The
fluxes of DBC and PBC, however, were not significantly correlated (Figure 6c) (r2 = 0.20, p < .11). This supports
our observations indicating that the DBC and PBC transported in the river were derived from different
sources; therefore, the fluxes of DBC and PBC are decoupled in the Changjiang and Huanghe Rivers. This finding is consistent with the conclusion made by Wagner et al. [2015].
4. Implications
Our study reveals evidence that two BC pools were transported by the Huanghe and Changjiang Rivers during 2015. BC transported in the riverine dissolved phase (DBC) is significantly younger than BC in the solid
phase (PBC), indicating a higher percentage of BC from biomass burning. Though Δ14C measurements of
DBC in rivers are few, we compare results from three rivers (Changjiang, Huanghe, and Suwannee Rivers in
the USA) in Figure 7. DBC in the Changjiang River (475 150 years B.P.) had a similar 14C age to that in the
Suwannee River (450 280 years B.P.) collected in 1999 [Ziolkowski and Druffel, 2010]. The DOC in the
Suwannee River is modern (Δ14C = 153‰) and likely derived from terrestrial plants that are concentrated
along the river basin. This compared with the old ages of DOC in the Changjiang (900 282 14C years) and
Huanghe (1489 133 14C years) rivers, which were heavily influenced by preaged OM in soils [X. C. Wang
et al., 2012; Tao et al., 2015; Xue, 2016] and anthropogenic inputs [Zhang et al., 2013]. The young DBC found
in these rivers also supports the speculation of Ziolkowski and Druffel [2010] that riverine DBC could be more
labile than presently believed.
Another interesting finding in our study is that the 14C ages of permeate-DOC (average 2640 560 14C years)
were much older than the total DOC, SPE-DOC, and DBC in the Huanghe River, while their δ13C value
( 22.4 0.5‰) was higher and close to that of marine DOC ( 20.4‰ to 22.4‰) [Druffel et al., 1992;
Bauer et al., 1998]. Because the PPL cartridge uses a more polar sorbent, the 42% of the riverine DOC that
passed through the resin could contain a significant fraction of nonpolar, smaller-sized DOC molecules. It
is not clear why the permeate-DOC fraction is so old. We expect that the SPE-DOC fraction is younger and
more labile and could be decomposed more rapidly by both bacteria [Amon and Benner, 1996; Benner and
Biddanda, 1998] and photodegradation [Spencer et al., 2009] during river transport and in estuarine and
coastal waters. The nonpolar, smaller-sized riverine DOC thus could be more refractory and possibly contribute to the very old oceanic DOC pool (4900–6400 years B.P.) [Druffel and Griffin, 2015]. In their recent study,
WANG ET AL.
BLACK CARBON IN RIVERS
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10.1002/2016GB005509
Coppola and Druffel [2016] reported that at least two distinct DBC pools exist in the ocean. A younger pool of
DBC (4800 620 14C years) cycles on centennial time scales, and an ancient DBC pool (23,000 3000 14C
years) cycles on > 105 year time scales. They suggest that river input of DBC is an important process, though
most DBC delivered via rivers is not accumulating in the ocean. Riverine DBC could be removed by processes
such as UV oxidation [Stubbins et al., 2012] and microbial degradation [Coppola and Druffel, 2016].
Nevertheless, our study brings up more questions than it answers. Considering the limited data base for
14
C measurements of DBC and DOC in rivers, larger data sets for more rivers are crucial to nail down the
sources and geochemical cycling of river transported DOC and their roles in oceanic DOC cycling and
its budget.
Acknowledgments
We thank Coppola at UCI (now
University of Zurich) for sharing her
detailed SPE method procedure for DOC
extraction. We thank the staff and colleagues at the National Ocean Science
accelerator mass spectrometry
13
14
(NOSAMS) facility for δ C and Δ C
measurements of the samples. We
appreciate the Editor, Associate Editor,
and three anonymous reviewers who
provided valuable and constructive
comments. Financial support for this
work was provided by the National
Natural Science Foundation of China
(grants 91428101, 41476057, and
41521064) and Ocean University of
China (201312004). Any additional data
may be obtained from XCW (e-mail:
[email protected]).
WANG ET AL.
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