PUBLICATIONS. Journal of Geophysical Research: Biogeosciences

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1 PUBLICATIONS Journal of Geophysical Research: Biogeosciences RESEARCH ARTICLE Key Points: Petrogenic POC content for Changjiang sediment ~0.46% Labile POC oxidized in the margin Dam-induced POC sequestration in Changjiang ~4.9±1.9 megatons biospheric POC per year since 2003 Supporting Information: Readme Text S1 Figure S1 Correspondence to: G. Li, Citation: Li, G., X. T. Wang, Z. Yang, C. Mao, A. J. West, and J. Ji (2015), Dam-triggered organic carbon sequestration makes the Changjiang (Yangtze) river basin (China) asignificant carbon sink, J. Geophys. Res. Biogeosci., 120, 39 53, doi: / 2014JG Received 14 FEB 2014 Accepted 22 NOV 2014 Accepted article online 4 DEC 2014 Published online 12 JAN 2015 Dam-triggered organic carbon sequestration makes the Changjiang (Yangtze) river basin (China) asignificant carbon sink Gen Li 1,2, Xingchen T. Wang 1,3, Zhongfang Yang 4, Changping Mao 1,5, A. Joshua West 2, and Junfeng Ji 1 1 Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China, 2 Department of Earth Sciences, University of Southern California, Los Angeles, California, USA, 3 Department of Geosciences, Princeton University, Princeton, New Jersey, USA, 4 School of Earth Sciences and Resources, China University of Geosciences, Beijing, China, 5 School of Earth Sciences and Engineering, Hohai University, Nanjing, China Abstract Worldwide dam building in large river basins has substantially altered the carbon cycle by trapping much of the riverine transported particulate organic carbon (POC) in terrestrial reservoirs. Here we take the Changjiang (Yangtze) River basin, in which ~50,000 dams were built over the past 50 years, as an example to evaluate the effect of dam building on POC sequestration. We report the characteristics (elemental composition, radiocarbon and stable carbon isotopic compositions, and Raman spectra) of bulk POC in the lower Changjiang from October 2007 to September 2008, and we estimate the POC sequestration induced by dam building since the 1950s for the Changjiang Basin. Using radiocarbon measurements, we quantify the fraction of biospheric POC (POC bio ) and petrogenic POC (POC petro ) in Changjiang POC. Over the study period, around 25% of the Changjiang POC is radiocarbon-dead POC petro ; the remaining is POC bio with a mean radiocarbon age of ~3.5 kyr. Studies on the East China Sea (ECS) shelf along with an oxidation experiment suggest that, prior to dam building, the Changjiang POC bio was significantly oxidized in the ECS margin. In contrast, high preservation of POC is observed in Changjiang reservoirs. Combining our POC data with hydrometric data sets, our study indicates that, over the past five decades, dam building may have largely shifted the Changjiang POC burial site from the ECS margin to terrestrial reservoirs. This shift in burial site preserved labile POC bio that would have been oxidized, suggesting a new temporary carbon sink. We estimate that dam building in the Changjiang has sequestered ~4.9 ± 1.9 megatons POC bio every year since 2003, approximately 10% of the global riverine POC burial flux to the oceans. 1. Introduction 1.1. Background Continental erosion and riverine transport of particulate organic carbon (POC) play central roles in the global carbon cycle [Blair and Aller, 2012; Galy et al., 2007; Hilton et al., 2012; Hedges et al., 1997]. POC transported by rivers is a mixture of carbon derived from the lithosphere (radiocarbon-dead POC petro ) and the biosphere (radiocarbon-enriched POC bio )[Blair et al., 2003; Galy et al., 2007; Clark et al., 2013; Kao et al., 2014]. Over geologic timescales, burial of POC bio in marine sediments works as a significant atmospheric carbon sink of the same magnitude as silicate weathering [Berner, 1990]; reburial of POC petro represents recycling of lithospheric carbon and has no sink effect, but its oxidation represents an important atmospheric carbon source [Bouchez et al., 2010; Galy et al., 2008; Lasaga and Ohmoto, 2002]. Continental margins work as the primary geologic site for organic carbon respiration and preservation [Burdige, 2005; Hedges and Keil, 1995]. During continental erosion and deposition in margins, the balance between preservation and oxidation of POC largely controls the net effect of fluvial POC transfer on the carbon cycle [Galy et al., 2008; Hilton et al., 2011]. Globally, about 70% of the total exported riverine POC returns to the atmosphere in continental margins and the remainder gets buried in marine sediments [Burdige, 2005; Hedges et al., 1997]. The riverine POC preservation efficiency in continental margins varies in different regions, but in general, tectonic settings exert a first-order control by regulating sedimentation rates and oxygen exposure time [Blair and Aller, 2012; Hartnett et al., 1998]. Mountainous rivers in tectonically active regions which drain directly to the oceans (e.g., Taiwan) as well as those that feed large rivers entering active continental margins (e.g., the Ganges-Brahmaputra or G-B) yield high sediment flux and sequester organic carbon at high efficiency LI ET AL American Geophysical Union. All Rights Reserved. 39

2 through rapid burial [Galy et al., 2007; Hilton et al., 2011; Kao et al., 2014]. In contrast, large river systems entering passive continental margins (e.g., the Amazon and Changjiang) are characterized by low sedimentation rates, long oxygen exposure time and high POC respiration [Burdige, 2005; Wu et al., 2013]. Our understanding of fluvial carbon transport relies on studies of modern rivers, yet anthropogenic activities have caused significant perturbation to continental erosion and associated carbon transport processes since pre-industrial times [Regnier et al., 2013]. Notably, during the past century, worldwide dam building has been carried out in nearly half of Earth s large river basins and has changed the global pattern of sediment and POC transfer remarkably [Syvitski et al., 2005; Mendonça et al., 2012; Raymond et al., 2013]. The resulting modification of fluvial POC transfer may have important implications for how we use modern river systems to understand organic carbon cycling over geologic time. At the same time, since dam systems have a lifespan from tens to thousands of years (e.g., the 2200 year old Dujiangyan irrigation system in the upper Changjiang River, central China, still remains in regular operation today) [Tilt et al., 2009; Yang et al., 2007], the resulting impact on fluvial POC erosion and mobilization can last from decadal to millennial timescales over the Anthropocene. The effect of dams on fluvial carbon transfer is thus important not only for interpreting carbon cycle processes on geologic timescales but also for understanding present-day biogeochemical cycles of nutrient elements and for accurately assessing anthropogenic perturbation of the carbon cycle Dam Building and Its Impact on the Carbon Cycle One significant consequence of dam building that is clear from previous studies is that a significant amount of riverine transported POC has now been sequestered in artificial reservoirs under high sedimentation rates [Battin et al., 2009; Dean and Gorham, 1998; Stallard, 1998]. However, the overall effects of dam building on the carbon cycle still remain to be well understood. Previous observations report that reservoirs may emit large amounts of CO 2 and CH 4 and thus represent a new carbon source [Barros et al., 2011; Fearnside and Pueyo, 2012]. Carbon burial in reservoirs, which can be important in compensating such emissions, has been largely neglected and remains poorly constrained [Battin et al., 2009; Mendonça et al., 2012]. Recent observations and model results suggest that terrestrial reservoirs trap POC at high burial efficiency with limited exposure to oxygen [Battin et al., 2009; Dean and Gorham, 1998; Stallard, 1998]. Specifically, this recognition is important for regions drained by passive-margined river systems (e.g., Changjiang), where POC burial efficiency may be much lower offshore [Blair and Aller, 2012]. If POC is efficiently buried in terrestrial reservoirs, instead of being intensively oxidized in continental margins, the shift of POC burial sites from margins to reservoirs would then represent an atmospheric carbon sink over the Anthropocene [Mendonça et al., 2012; Regnier et al., 2013]. Evaluation of this potential carbon sink is complicated by the composition of buried POC, which is a mixture of biospheric and petrogenic carbon. Since POC petro is essentially a recycled lithospheric component and has no effect on contemporary atmospheric carbon drawdown [Berner, 1990; Galy et al., 2007], it is important to quantify the proportions of POC bio relative to POC petro when estimating the effects on the carbon cycle from dam building. Although studies have attempted to quantify burial and degradation degassing of POC in individual reservoirs [e.g., Chen et al., 2011; Sobek et al., 2012; Knoll et al., 2013; Mendonça et al., 2014], efforts to integrate up to the large river basin scale [e.g., Syvitski et al., 2005], as well as to differentiate between biospheric versus petrogenic components, have been rare. Since 1950, around half of world s large dams (dams with a height of 15 m or more from foundation, or of 5m 15 m height and a reservoir volume over 3 million m 3 ) have been constructed in China [World Commission on Dams, 2000]. The Changjiang River (CR) Basin has around 50,000 dams at present, including the world s largest, the Three Gorges Dam or TGD [Yang et al., 2005] (Figure 1b). Dramatic declines in sediment and POC fluxes have been observed from the lower CR as a result of dam building and associated secondary human activities aimed at soil and water conservation (e.g., afforestation) [Chen et al., 2008; Wu et al., 2007; Yu et al., 2011]. The dense dam building, substantial changes in POC transport, and the continuous hydrometric record together make the CR an ideal case for evaluating sequestration of POC resulting from dam building in river basins that deliver sediments to passive margins. Until now, the effects of dams on CR carbon transfer have not been carefully studied. Moreover, there is little previous information about the sources and composition (petrogenic versus biospheric) of CR POC. Here we characterize POC (elemental/isotopic compositions and Raman spectrum) in monthly suspended sediment LI ET AL American Geophysical Union. All Rights Reserved. 40

3 Journal of Geophysical Research: Biogeosciences samples collected from the lower CR between October 2007 to September 2008, investigate its fate in the East China Sea (ECS) margin versus in reservoirs, and estimate changes in the CR POC cycle over the last 50 years. We first distinguish between POCpetro and POCbio based on radiocarbon measurements. Reported studies of POC degradation from the CR delta and the ECS margin, together with a laboratory oxidation experiment on river sediments, allow us to demonstrate the preferential, significant oxidation of POCbio in the margin. The natural fate of CR POC in the ECS margin is then compared to observations from reservoirs. Low degassing flux and high preservation of POC in the CR reservoirs suggest a temporary carbon sink triggered by dam building. Finally, using a hydrological regression model, we quantify this sink effect, in terms of the total sequestrated POCbio. 2. Materials and Methods Figure 1. Study sites and maps of the CR topography, main tributaries, reservoir distribution, and bedrock geology. (a) Topographic map of the lower Changjiang River (CR) and study sites; the stars are the sampling sites of CR sediments in this study (Nanjing) and in Wang et al. [2012] 8 3 (Datong). (b) Distribution of major reservoirs (with capacity > 1 10 m ) and main tributaries in the CR basin, modified after Yang et al. [2005]; the white circles represent reservoir locations with sizes proportional to reservoir capacities; the black lines indicate the boundary of the CR basin; the red lines represent the boundaries of the subcatchments; the blue lines indicate the Changjiang River and the major tributaries; and the background is a grayscale-shaded topographic map. (c) Bedrock geology map of the CR basin over a grayscale-shaded topographic map. Data from Hartmann and Moosdorf [2012] and modified after Chetelat et al. [2008]. Located in central China, the CR Basin covers an area of around km2, ~20% of the terrestrial area of China (Figure 1). The upper reach, from the source region to Yichang, contributes ~55% of the drainage area and is characterized by mountainous topography. The middle and lower reaches mainly consist of fluvial plains and inland water systems [Chen et al., 2008]. Controlled by a subtropical monsoon climate, around 70 80% of precipitation and runoff are in the flood season from May to October, accompanying higher monthly sediment load. The sediment delivered to the East China Sea (ECS) is mainly transported as suspended load [Yang et al., 2002]. To characterize the POC sources in the CR suspended sediment, suspended sediments from the lower CR were collected monthly at a depth of 30 cm in the middle of the main channel near the city of Nanjing (N32.093, E ; Figure 1a) from October 2007 to September L of river water was collected in acid-cleaned containers and filtered through 0.45 μm Millipore membranes. Suspended sediment was collected on the filters. This monthly, high temporal resolution sampling covered one complete hydrological year and featured both flood and dry seasons. The sampling strategy helps us to constrain the compositional range of the CR POC, as carbon sources may change significantly under different hydrologic regimes [Clark et al., 2013; Hilton et al., 2008], and to characterize POC compositions of sediments with varying grain sizes, as sediment grain size varies with flow conditions and exerts a primary control on POC loading LI ET AL American Geophysical Union. All Rights Reserved. 41

4 Figure 2. Hydrologic and POC characteristics of the suspended sediment samples from the lower CR in (a) Water discharge (Q w ) and sediment flux (Q s ) over the sampling period. (b) Fm and C org of suspended sediment samples over the sampling period; error bars show 1σ uncertainties for C org ; errors are smaller than the symbol size for Fm. (c) Power law correlations between POC concentration and water discharge (Q w ) in , and suspended sediment concentration (SSC) and water discharge (Q w ) in during our sampling period (grey circles) and in (white circles) as reported in Bulletin of Changjiang Sediment [2003, 2004, 2005, 2006]; similar power law correlations have been observed in Taiwan [Hilton et al., 2008; Hovius et al., 2000]. (d) Linear correlation between monthly sediment flux (Q s ) and POC flux (Q POC ). [Bouchez et al., 2014; Galy et al., 2007]. While depth profile sampling has been conducted in other large river studies and often captures variations of POC compositions along vertical water columns [Bouchez et al., 2010; Galy et al., 2007], depth profile samples were not obtained over the river cross section in this study. To constrain this potential variation with depth, one bed sediment sample was also collected in December 2008 at the same site, and a depth correction was applied (see supporting information). All sediment samples were oven dried at 40 C and homogenized using agate pestle and mortar. Complementary hydrological data were taken from the annual report of the CR water resources commission [Bulletin of Changjiang Sediment, 2003, 2004, 2005, 2006, 2007, 2008]. Riverine POC is a mixture of carbon from different sources characterized by various recalcitrance and various ages [Blair et al., 2003; Galy and Eglinton, 2011; Rosenheim and Galy, 2012]. To examine the recalcitrant nature of the CR POC, a chemical oxidation experiment was conducted following Helfrich et al. [2007]. In brief, 0.5 g of sediment sample, together with 20 g Na 2 S 2 O 8 and 22 g NaHCO 3, was dispersed in 250 ml distilled water and heated on a magnetic stirrer. The experiment was set at 80 C and run for 48 h. For each sample before and after chemical oxidation, decarbonation was performed using 1 M HCl [Wu et al., 2007]. Total organic carbon content (C org, wt %), nitrogen content (N, wt %) and stable carbon isotope composition (δ 13 C org, ) of bulk POC were measured by a Vario Macro-CHNS element analyzer (EA) and a MAT-251 isotopic ratio mass spectrometer, respectively, at the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences. The precision of δ 13 C analysis was ±0.1 (1σ standard deviation), determined from measurements of standards in the same analytical conditions. The reproducibility (1σ) for C org and N were 6% and 4% of the measured mean values, respectively, based on replicate sample measurements. For major element analysis, samples were prepared using lithium tetraborate/metaborate fusion and measured by a ARL9800XP + X-ray fluorescence spectrometer, with precision (1σ) better than 5%, at the Center of Modern Analysis, Nanjing University. Radiocarbon composition (reported as fraction of modern carbon, Fm) [Mook and van der Plicht, 1999] was analyzed using the accelerator mass spectrometer (AMS) facility at the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. The precision (1σ) of AMS analysis was within 5%. Raman spectroscopy analysis was conducted to characterize POC petro, using a Renishaw RM2000 Raman spectrometer at the State Key Laboratory for Mineral Deposits Research, Nanjing University. Raw samples were directly run for the Raman detection, with the synchroscan band from 100 to 2000 cm 1, covering both the carbonaceous matter band ( cm 1 ) and the band for common silicate/carbonate minerals ( cm 1 ). Every time before analysis, the position of the standard silicon wafer was measured for calibration. LI ET AL American Geophysical Union. All Rights Reserved. 42

5 Table 1. Sampling Information and Data of the Lower CR Sediments a Type Sample ID Sampling Date C org (%) ±C org (%) (1σ) Fm ±Fm (2σ) δ 13 C org ( ) ±δ 13 C org (1σ) ( ) N/C Mass Ratio ±N/C (1σ) Al/Si Mass Ratio ±Al/Si (1σ) Suspended sediment CJSS /25/ at 30 cm depth CJSS /21/ CJSS /26/ CJSS /22/ CJSS /27/ CJSS /25/ CJSS /24/ CJSS /26/ CJSS /26/ CJSS /25/ CJSS /26/ CJSS /25/ Bed sediment CJKSJS01 12/ NA Na 2 S 2 O 8 -oxidized suspended sediment at 30 cm depth a NA = not available. ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA ACJSS NA NA 3. Results 3.1. Elemental and Isotopic Compositions The mean C org of suspended sediment samples over the study period was 1.78 ± 0.38% (n = 12), close to the previously reported value of 1.91 ± 0.43% for CR suspended sediment samples collected from the middle-lower reaches in 2006 [Yu et al., 2011]. C org ranged from 1.24% to 2.51%, with lower values in flood months and higher in dry months (Figure 2a and Table 1). Fm of samples averaged ± (5.9 ± 0.9 kyr), ranging from to 0.569, and was to first-order positively correlated with C org (Figures 2a and 4a). The δ 13 C org averaged 25.5 ± 0.4 (n = 12), ranging from 26.0 to 24.9 with heavier isotopic compositions in flood months. Power law correlations were observed (Figure 2c) for both POC concentration (mg L 1 ) versus water discharge (Q w,m 3 s 1 ) and suspended sediment concentration (SSC, mg L -1 ) versus water discharge. The relationship between monthly POC flux (Q POC, megaton (Mt) month 1 ) and sediment flux (Q s, Mt month 1 )defined a linear trend (Figure 2d). C org and Al/Si values of the samples were positively correlated (Figure 3a), suggesting a grain size control on sediment POC loading [Blair and Aller, 2012; Galy et al., 2007]. To resolve the biospheric and petrogenic end-members of the CR POC, we employed a binary mixing model [Galy et al., 2008]: Fm bio C bio þ Fm petro C petro ¼ Fm C org (1) C bio þ C petro ¼ C org (2) where Fm, Fm bio, and Fm petro are the radiocarbon compositions of bulk POC, POC bio, and POC petro respectively, and C org,c bio, and C petro are the contents of bulk POC, POC bio, and POC petro in sediments, respectively. Since POC petro is radiocarbon-dead (Fm petro = 0), rearrangements of equations (1) and (2) give Fm C org ¼ Fm bio C org C petro (3) Based on a linear fit between Fm C org and total C org, the POC petro content was determined from the x axis intercept to be 0.46 ± 0.10% (~25% of the total C org ), which was constant and insensitive to grain size LI ET AL American Geophysical Union. All Rights Reserved. 43

6 Figure 3. Characteristics of POC and major elements for the lower CR sediments collected in (a) Al/Si (mass ratio) versus bulk POC content C org (%); the first-order positive correlation shows the grain size control on sediment POC loading [Galy et al., 2007]; the grey squares are the suspended sediments; and the yellow square indicates the bed sediment sample. (b) Modern C org (C org Fm) versus C org for the suspended sediment samples; the solid lines shows the best fit: Y = (0.645 ± 0.037)X +( ± 0.060), R 2 = 0.93, with a slope (Fm bio ) of ± 0.037, and a Fm bio = 1 line; the intercept at the x axis defines C petro as 0.46 ± 0.10%. (c) Modern C org (C org Fm) versus C org δ 13 C org for the suspended sediment samples; the solid line shows the best fit: Y =( ± 0.001)X + ( ± 0.056), R 2 = All the dashed lines represent 95% confidence bands. Error bars show 1σ uncertainties. variations (as revealed by Al/Si). The Fm of POC bio was given by the slope (0.645 ± 0.037), defining a mean POC bio radiocarbon age (τ bio ) of 3.5 ± 0.5 kyr (Figure 3b). A similar binary mixing calculation was performed for partitioning δ 13 C org : δ 13 C bio C bio þ δ 13 C petro C petro ¼ δ 13 C org C org (4) where δ 13 C bio and δ 13 C petro refer to the stable carbon compositions of POC bio and POC petro end-members, respectively. Combination of equations (1), (2), and (4), with Fm petro = 0, gives Fm C org ¼ Fm bio =δ 13 C bio δ 13 C org C org δ 13 C petro C petro (5) Based on a linear fit between δ 13 C org C org and Fm C org (Figure 3c), δ 13 C bio was determined as 26.9 ± 2.2 and δ 13 C petro as 21.5 ± 7.0. After oxidation, the composition of the residual POC was very close to the calculated POC petro end-member (Figure 4a): average C org decreased to a relatively constant value of 0.53 ± 0.08%. Radiocarbon activity was sharply depleted, to Fm = ± (24.6 ± 1.0 kyr). Based on isotopic mass balance, we calculated Fm and C org for the oxidized POC and performed Fm-based binary mixing (equations (1) and (2)). Results from the binary mixing clearly indicated that almost no POC petro was oxidized (Figure 4b). Since POC petro was mostly Figure 4. Characteristics of POC in the residual Changjiang sediments after persulfate oxidation. (a) Fm and C org of the suspended sediment samples before and after chemical oxidation; the black diamond indicates the calculated POC petro end-member. (b) Modern C org (C org Fm) versus C org for the oxidized mass of the suspended sediment samples; the solid line shows the best fit: Y = (0.647 ± 0.038) X + (0.020 ± 0.042), R 2 = 0.90, demonstrating that POC petro is statistically insignificant in the oxidized POC. All the dashed lines represent 95% confidence bands. Error bars show 1σ uncertainties. LI ET AL American Geophysical Union. All Rights Reserved. 44

7 Figure 5. Representative Raman spectra for the CR suspended sediments samples. The G band represents the main graphite band. Spectra for POC petro display forms as individual particulate graphitic carbon (CJSS0809) and mineral-associated graphitic carbon (CJSS0804 and CJSS0807). preserved in the residual sample material after oxidation, the difference between the residual C org (0.53 ± 0.08%) and C petro (0.46 ± 0.10%), as well as the nonzero residual Fm (0.047 ± 0.006), could be attributed to a mixture between POC petro and a refractory, radiocarbon-aged POC bio component. We conducted another mass balance calculation: Fm rbio C rbio þ Fm petro C petro ¼ Fm res C res (6) C rbio þ C petro ¼ C res (7) where C rbio and C res are contents of the persulfate-resistant POC bio and the residual POC, and Fm rbio and Fm res represent radiocarbon compositions of the persulfate-resistant POC bio and the residual POC, respectively. Results indicated that the refractory, aged POC bio is ~10% of the bulk POC bio,with a mean radiocarbon age of ~8.7 kyr (Fm rbio = ± 0.146) Raman Spectral Characteristics In the Raman spectra, both primary and disordered graphitic carbon bands were detected in the lower CR suspended sediment. Graphitic carbon appeared either as individual particles or in association with minerals (Figure 5). Compared to the Raman spectra of river sediments from the Himalayan Ganges-Brahmaputra (G-B) [Galy et al., 2007] and the Amazon rivers [Bouchez et al., 2010], the Changjiang sediment Raman spectra showed broader graphite G bands (around 1580 cm 1 ) and overlapped defect bands (around 1350 cm 1 ), suggesting a relatively lower degree of metamorphism and graphitization. 4. Discussion 4.1. Flux and Characteristics of the CR POC The POC flux over the study period was calculated by integrating monthly POC flux. Our sampling strategy (surface suspended sediment) did not take into account probable variation of sediment grain size and C org with depth in the river [Bouchez et al., 2014; Galy et al., 2007], and so may have been biased to the fine grain size-sediment end-member. Though we cannot directly estimate the depth profile-integrated POC flux due to a lack of depth profile samples, recently published data on the CR sediment depth profile at Datong [Luo et al., 2012], another site in the lower CR, is employed here to constrain the sampling bias in the POC flux. We define η as the ratio between surface suspended sediment-based POC flux (F ss ) and depth profile-based POC flux (F dp ), i.e., η = F dp /F ss. Using the data from the Datong depth profile and assigning the POC petro content from this study to the C org of bed sediment gives a conservative estimate of η ~0.7 over the study period (see supporting information for details). After this correction, the exported bulk POC flux is calculated as ~1.3 ± 0.8 Mt POC yr 1, and the exported POC petro flux is ~0.5 ± 0.1 Mt POC petro yr 1. The present-day, post-damming CR POC petro flux is comparable to the flux from the G-B (~ Mt POC petro yr 1 )[Galy et al., 2008] and slightly lower than the flux from Taiwan (~0.9 Mt POC petro yr 1 )[Hilton et al., 2011]. The CR POC petro flux in predam building times may have been higher due to elevated sediment yield. The high POC petro content and millennial POC bio radiocarbon age of the CR sediments highlight the role of the CR basin as an important system in the global carbon cycle, both in terms of (i) significant POC petro export and (ii) prolonged sequestration of POC bio within the basin. Mountainous rivers are thought to be major sources exporting POC petro, because of high POC petro content in sediments (~ %) [Blair and Aller, 2012; Hilton et al., 2012] and large sediment yields [Blair and Aller, 2012]. In this work, we observed consistent, high POC petro content (0.46 ± 0.10%) for the CR sediments, which is comparable to typical mountainous rivers and nearly 1 order of magnitude higher than other large rivers like the G-B and the Amazon (<0.05%) [Galy et al., 2007; Bouchez et al., 2010] (Figure 6). Widespread POC petro -enriched carbonate-interlayered black shale formations in the CR basin [Jiang et al., 2012] are likely a major petrogenic source (Figure 1c) [Hartmann and Moosdorf, 2012]. POC associated with industrial fly ash released from local coal-fired power plants LI ET AL American Geophysical Union. All Rights Reserved. 45

8 Figure 6. Fm bio and C petro of POC from our data, along with previously studied river systems: the lower Ganges- Brahmaputra, the Himalayan range, lower Amazon, Andes, Taiwan, and lower Changjiang [Bouchez et al., 2010; Galy and Eglinton, 2011; Hilton et al., 2008; Hilton et al., 2011; Wang et al., 2012; this study]. Error bars indicate 1σ uncertainties. may also contribute to the CR POC petro [Li et al., 2011]. Ancient paddy rice soil in the lower CR Basin can also be important because of its enrichment of radiocarbon-depleted, old POC [e.g., Cao et al., 2006]. Characterized by intensive physical erosion, mountainous rivers are also generally thought to be efficient systems exporting POC bio with limited oxidation during rapid transfer and burial. The millennial POC bio radiocarbon age of the CR sediments emphasizes the importance of the CR basin in the global carbon cycle in terms of prolonged sequestration of POC bio. The mean POC bio radiocarbon age (~3.5 ± 0.5 kyr, Fm bio = ± 0.037) of the CR sediments is close to that found for the G-B (~2 3.5 kyr, Fm bio ~ ) [Galy and Eglinton, 2011] and much longer than for the Amazon (~1.5 kyr, Fm bio ~ ) and Taiwan (~0.8 kyr, Fm bio ~1) [Bouchez et al., 2010; Hilton et al., 2012] (Figure 6). The ~3.5 kyr POC bio radiocarbon age indicates millennial storage of POC bio in the CR basin, which can be attributed to stable tectonics, moderate erosion rate (262 t km 2 yr 1 in , inferred from Chen et al. [2008], ~100 times lower than Taiwan at 21,700 ± 3900 t km 2 yr 1 during , [Dadson et al., 2003]) and temperate climatic conditions in the basin. A radiocarbon-depleted, refractory POC bio component associated with aged soil may also contribute to this old mean POC bio radiocarbon age, as observed in the G-B [Galy and Eglinton, 2011]. The resolved biospheric and petrogenic δ 13 C org end-members from mass balance calculations (as shown in section 3.1) are supported by other related observations. δ 13 C bio is determined as 26.9 ± 2.2, within the range typical of C3 vegetation [Goñi and Eglinton, 1996], which dominates the biomass in the CR basin, and within the range of surface soil from the region [Wu et al., 2007; Wu et al., 2013]. For δ 13 C petro, a much wider range is defined ( 21.5 ± 7.0 ). Due to the lack of a complete bedrock δ 13 C org data set, we cannot directly compare the calculated petrogenic end-member to all of the CR geologic sequences. However, our findings are consistent with the observed highly variable ranges for POC petro, including various types of kerogen ( 24 to 30 ) [Goñi et al., 2005; Blair et al., 2003], ancient soil in the CR basin ( 22 to 28 ) [Wu et al., 2013], and the POC petro end-member in river suspended sediments from Taiwan ( 20 to 25 ) [Hilton et al., 2010]. Jiang et al. [2012] showed a large range of δ 13 C org ( 25 to 35 ) for Cambrian black shales in the CR basin. Heavier δ 13 C org has also been observed for some Carboniferous kerogen (approximately 14 ) [Eglinton et al., 1991] and noncarbonate graphitic carbon in carbonaceous chondrites (low to 10 ) [Ueno et al., 2004, and references therein]. It is clear that our calculated δ 13 C org is within these observed ranges and thus represents a reasonable estimate of the POC petro end-member for the CR sediments. The complicated hydrological and geological heterogeneity over the whole CR Basin [Chen et al., 2002] suggests potential for significant spatiotemporal variation in the CR POC characteristics. Wang et al. [2012] reported a 2009 data set for CR POC from Datong, ~200 km upstream of Nanjing, our sampling site (Figure 1). Compared to our Nanjing samples, the 2009 Datong samples show lower C org values (average 1.31 ± 0.32%, n = 3), higher bulk POC-Fm (average ± 0.014, n = 3) and Fm bio (0.912 ± 0.230), and lower but more variable POC petro content ( / 0.03%) (where Fm bio and POC petro are determined from the same end-member mixing method as developed in section 3.1, with equations (1) (3)). This discrepancy might be caused by enhanced input of ancient and fossil carbon during transfer from Datong to Nanjing, because of the widespread black shale formations and coal-fired power plants along the pathway. Alternatively, this difference may also indicate strong interannual variation. Long-term monitoring of the CR POC transfer would be necessary to reconcile the discrepancy. Nevertheless, since the LI ET AL American Geophysical Union. All Rights Reserved. 46

9 POC petro content from our study is much higher than other large rivers and similar to POC petro -enriched mountainous river systems, our estimate likely sets an upper bound for the POC petro content of sediment exported by the CR Oxidation of POC in the ECS Margin Several studies have investigated the preservation of POC in the ECS margin [Aller, 1998; Li et al., 2012; Wang and Li, 2007; Wu et al., 2013; Yang et al., 2011], where most CR sediment accumulates and the CR POC dominates the buried terrestrial POC (POC terr )[Deng et al., 2006; Milliman et al., 1985]. In general, the POC preserved in the ECS sediments is highly reduced and characterized by much older radiocarbon ages compared to riverine or marine-derived organic carbon [Li et al., 2012; Wang and Li, 2007; Wu et al., 2013; Zhu et al., 2013]. The CR POC burial efficiency is estimated at around 25% (based on a sediment POC loading proxy [Wu et al., 2013], lignin biomarker studies [Li et al., 2012; Li et al., 2013], and sedimentation rate-burial efficiency scaling [Aller, 1998]). Such significant oxidation of riverine transported terrestrial POC has also been observed for other passive margin rivers [e.g., Burdige, 2005; Hedges et al., 1997; Keil et al., 2004]. Here we consider the oxidation potential of the CR sediments as another constraint on the potential for POC degradation. Specifically, we use our chemical oxidation method to evaluate the recalcitrance of the CR POC, and we find that these results are consistent with previous estimates of POC loss from the ECS margin sediments. In soil studies, chemical oxidation methods have been used extensively to isolate the inert soil carbon pool resilient to oxidation during pedogenesis and petrogenesis [Eusterhues et al., 2005]. These methods have been shown to provide effective tools to evaluate chemical reactivity of organic matter in natural environments [Helfrich et al., 2007; Mikutta and Kaiser, 2011]. For our CR POC treatments, we adopted one of the most effective methods, characterized by the greatest radiocarbon-depletion of soil samples after oxidation [Helfrich et al., 2007]. This method generates a highly oxidative environment by using sodium persulfate (Na 2 S 2 O 8 )asthe oxidant, which is a strong radical-based oxidant with high-standard oxidation-reduction potential (E 0 ~ 2.01 V), that is, comparable to ozone (E 0 ~ 2.07 V) and much higher than oxygen (E 0 ~1.23V)[Huang et al., 2002]. After persulfate oxidation, the CR POC is significantly reduced in both POC content (~75% loss) and radiocarbon age (from ~6 kyr to ~25 kyr or Fm from ~0.48 to ~0.05), similar to the observed POC degradation and radiocarbon-aging in the ECS margin [Li et al., 2012; Li et al., 2013; Wang and Li, 2007; Wu et al., 2013; Zhu et al., 2013]. For example, Li et al. [2013] reported substantial predepostional loss (~85%) of lignin-phenol (Λ 8 ), an effective terrestrial POC biomarker, during sediment deposition and resuspension for ECS sediments collected in 2009 and 2010, and we find that a similar proportion (~90%) of POC bio is removed during the oxidation experiment. These similar observations demonstrate that POC bio is relative labile and tends to get oxidized significantly during transport and deposition Recalcitrance of Petrogenic and Biospheric POC Our persulfate oxidation experiment also suggests that the resistant POC pool, characterized by older radiocarbon age, is likely a mixture of radiocarbon-dead POC petro and radiocarbon-depleted POC bio (section 3.1). Compared to most POC bio, POC petro is expected to be relatively more recalcitrant, because POC petro is typically a composite of kerogen and graphite, representing a highly refractory residual that remains after pedogenesis, diagenesis, metagenesis, and crustal exhumation [Blair et al., 2003; Galy et al., 2008]. Notably, in addition to POC petro, we also detected a persulfate-resistant, radiocarbon-depleted POC bio component in the residual after oxidation. This persulfate-resistant POC bio component is likely preserved during transfer and deposition due to its recalcitrance. As calculated in section 3.1, this persulfate-resistant component has a much older mean radiocarbon age (~8.7 kyr) compared to the bulk CR POC bio (~3.5 kyr) and may capture part of a radiocarbon-depleted POC pool associated with aged soil [Blair et al., 2003; Galy and Eglinton, 2011]. Recent studies identifying riverine POC radiocarbon age structure and thermochemical stability also show a refractory, radiocarbon-aged POC bio (up to 17.4 kyr) pool in sediment samples from the G-B [Galy and Eglinton, 2011; Rosenheim and Galy, 2012], which is consistent with our CR results. This recalcitrant, radiocarbon-aged POC bio portion is likely typical for large river systems draining various, complex geological units and soil/vegetation coverage, which integrate POC from heterogeneous sources and are capable of providing long residence time for POC degradation and radiocarbon-aging [Rosenheim and Galy, 2012]. LI ET AL American Geophysical Union. All Rights Reserved. 47

10 Figure 7. Changes of the CR sediment and POC fluxes since the 1950s. Grey circles represent annual sediment flux [Chen et al., 2008]; red squares represent documented annual POC flux (Table 2); blue lines are multiyear averaged sediment flux; and red lines are multiyear averaged POC flux from the regression model. (a) 1950 to late 1980s, with less human disturbance. (b) , after intensive dam construction, and implementation of water - soil conservation projects [Chen et al., 2008]. (c) Linear correlation between POC and sediment fluxes during nondrought years. In addition to the well-constrained magnitude of POC burial in the ECS margin, our chemical oxidation approach provides another constraint on the recalcitrance of different POC components in the CR sediments. Both our laboratory experiment and other field studies observed significant POC loss and radiocarbon-aging during oxidation. Moreover, we detected a radiocarbon-depleted, refractory POC bio component from our oxidation experiment, which shows distinct characteristics to the bulk POC bio pool and is likely to be preserved during transport and deposition. Thus, our oxidation method not only evaluates the oxidation potential of the bulk POC but also provides insights into resolving various components characterized by distinct radiocarbon compositions and recalcitrance, promising a better understanding of riverine POC transport and deposition Sequestration of POC bio in the Changjiang River Basin Since 1950s Previous studies have been conducted to investigate POC burial in individual reservoirs via seismic surveys and sediment coring [e.g., Knoll et al., 2013; Mendonça et al., 2014; Teodoru et al., 2013]. To quantify the POC sequestration resulting from dam building for the CR basin, we would ideally integrate POC sequestration in each individual reservoir within the basin. However, because of the substantial number (~50,000) of dams and reservoirs in the CR, this method is not realistic. Instead, taking the difference between riverine exported POC fluxes before and after dam building provides the basis for a first-order estimate. Over the past 50 years, massive dam and reservoir construction, along with widespread water and soil conservation projects, has caused a sharp decline in the CR sediment flux, from 472 Mt yr 1 in the 1950s to ~100 Mt yr 1 in recent years (Figure 8) [Chen et al., 2008]. Though only a few POC flux measurements were made in 1950s to 1980s, a linear correlation describes the relationship between the documented POC flux and sediment flux in nondrought years (within the normal range of annual sediment flux variation, [Dai et al., 2008; Bulletin of Changjiang Sediment, 2006, 2007, 2008]) (Figure 7c). The drought years deviate from the fitted line (Figure 7c), most likely because of enhanced proportion of fine sediment and thus higher POC loading [Galy et al., 2007] in drought years, and higher sensitivity of the linear relation between POC flux and sediment flux to sediment flux fluctuation at the lower end. Since the time period with fewer direct POC Table 2. Annual Fluxes of POC and Sediment for the Changjiang River Year POC (Mt yr 1 ) Sediment (Mt yr 1 ) Reference a , , , b 206 1, b 86 1, this study, 6, 7 a 1[Chen et al., 2008]; 2 [Milliman et. al., 1985]; 3 [Cauwet and Mackenzie, 1993]; 4 [Duan, 2000]; 5 [Yu et al., 2011]; 6[Bulletin of Changjiang Sediment, 2007]; and 7 [Bulletin of Changjiang Sediment, 2008]. b POC fluxes in 2003 and 2006 are calculated by multiplying the POC content and the sediment flux in the cited references. LI ET AL American Geophysical Union. All Rights Reserved. 48

11 Figure 8. POC bio and POC petro fluxes exported by the CR river before and after dam building (grey arrows: CR POC flux; green arrows: POC bio flux; black arrows: POC petro flux; unit: Mt yr 1 ). POC oxidation fluxes during fluvial transport are not directly addressed in this study. POC petro and POC bio flux calculations use the POC petro content of 0.46 ± 0.10% and the factor η (~0.7) for C org depth-variation correction. The oxidation fraction of the CR POC in the ECS margin is assigned as 75% based on previous study [Wu et al., 2013]. measurements is dominated by nondrought years [Chen et al., 2008], the observed good correlation between sediment flux and POC flux for these conditions should be applicable for resolving the past POC flux. Based on the observed correlation and documented sediment flux over the past 50 years [Chen et al., 2008], we reconstructed the past CR POC flux (Figure 8). The reconstruction indicates that POC flux has decreased from 10.9 ± 2.6 Mt yr 1 during , a time period defined by less anthropogenic perturbation [Chen et al., 2008], to 1.9 ± 1.0 Mt yr 1 during , after massive construction of dams and reservoirs, and the impoundment of the TGD reservoir. This implies that 9.0 ± 2.8 Mt POC has been retained in the CR Basin every year after extensive damming. If we take the extremely low POC petro content ( / 0.03%) from the 2009 Datong data [Wang et al., 2012], the relevant captured POC bio flux is 8.7 ± 2.9 Mt yr 1.Ifthe average POC petro content over the past 50 years is close to the calculated POC petro content in this study (0.46 ± 0.10%), which is likely to be an upper bound, then the resulting more conservative estimate of sequestrated POC bio flux is 6.9 ± 2.8 Mt yr 1. When evaluating the dam building effect on the carbon cycle, another key question is what happens to this POC that is now trapped in reservoirs, as opposed to being delivered to the margin where it is mostly oxidized. Due to the restricted accommodation space in reservoirs compared to vast margin regions, reservoirs generally have higher sedimentation rates. Sedimentation rate largely determines the oxygen exposure time, a central parameter controlling POC preservation [Blair and Aller, 2012; Galy et al., 2007; Hartnett et al., 1998], so high sedimentation rates in reservoirs indicate less POC oxidation during burial. Moreover, due to intensive stratification in reservoirs, downward transport of atmospheric oxygen is limited, causing low oxygen levels in reservoir bottom-waters, which also contributes to more effective POC burial [Mendonça et al., 2012]. Efficient preservation has been observed in many reservoirs worldwide [Dean and Gorham, 1998; Mendonça et al., 2012; Stallard, 1998]. For the CR Basin, the best data for assessing dam building-induced riverine POC burial and preservation come from the TGD. For the TGD reservoir, the sedimentation rate is currently as high as 15 g cm 2 yr 1 (or ~10 cm yr 1, sediment flux trapped in the TGD reservoir: 172 Mt yr 1 [Hu et al., 2009]; reservoir area: 1084 km 2 [Wang, 2000]), nearly 10 times the sediment accumulation rate on the ECS shelf (~0.19 g cm -2 yr -1 [Deng et al., 2006]), and of the same magnitude as the Bengal Fan (~15 75 g cm 2 yr 1 [Suckow et al., 2001]), which is known for near-complete POC preservation [Galy et al., 2007]. The POC burial flux in the TGD reservoir is calculated as ~2.1 Mt POC yr 1 (burying sediment flux: 172 Mt yr 1 [Hu et al., 2009]; POC content of the upper stream sediment: 1.2% [Wu et al., 2007]), which is around 30% of the total POC sequestration flux within the CR basin. In situ studies indicate that the TGD CH 4 emission flux is ~ Mt Cyr 1 [Chen et al., 2011], similar to the modeling results for CO 2 and CH 4 fluxes of ~ Mt C yr 1 and ~ Mt C yr 1, respectively [Lo, 2009]. These emissions are about 3 orders of magnitudes smaller than the 2.1 Mt yr 1 POC burial flux, indicating that the TGD is a very efficient carbon sink. For the whole CR Basin, limited studies on other reservoir degassing have been conducted. However, upstream reservoirs in mountainous regions at high elevations are where the majority of the sequestrated sediment is buried, partly resulting from intensive mass wasting [e.g., Li et al., 2014]. These reservoirs have high sedimentation rates and LI ET AL American Geophysical Union. All Rights Reserved. 49

12 low temperatures, which are unfavorable for POC degradation [Burdige, 2011], so we can expect equally high POC preservation as in the TGD reservoir. For the total dam building-associated POC sequestration over the past 50 years, if we take the estimate of POC bio flux (6.9 ± 2.8 Mt yr 1 ) based on this study (POC petro = 0.46 ± 0.10%) and consider sediment C org depth-variation by multiplying the correction factor η (~0.7 in this study), the sequestrated POC bio flux in the CR Basin is then 4.9 ± 1.9 Mt yr 1 since 2003, after massive dam and reservoir construction and the impoundment of the TGD reservoir. This first-order estimate is conservative because we assume generally high POC petro content for CR (i.e., relatively lower POC bio content) and η is likely an underestimate since we assign the POC petro content as a boundary constraint (see supporting information). Moreover, this flux only considers the river sediment-associated POC, neglecting any contribution from coarse woody debris, which can also be significant, especially in flood seasons [e.g., West et al., 2011] and thus could increase the estimated POC sequestration. However, for simplification, we refer to the conservative estimate (POC bio sequestration flux: 4.9 ± 1.9 Mt POC bio yr 1 ) in the following discussion. If all the sequestrated POC bio is preserved, then the POC bio burial in the basin is equivalent to ~10% of the global terrestrial POC burial flux in the oceans [Blair and Aller, 2012], comparable to the burial flux in the Bengal Fan (3.8 ± 0.4 Mt POC bio yr 1 [Galy et al., 2007]), and the silicate weathering flux within the CR Basin (~2.3 Mt C yr 1 [Chetelat et al., 2008]). Actual preservation may be somewhat lower, which could decrease the calculated sequestration. The overall carbon budget within the CR basin is determined by the imbalance between POC bio burial, silicate weathering, and POC petro oxidation [Galy et al., 2008], but we are not able to address this overall budget here due to a lack of constraints on POC petro oxidation, which counteracts POC bio burial. However, dams decrease the fluvial connectivity and shorten the pathway of carbon transfer. Instead of undergoing long-distance transfer accompanying substantial oxidation [Galy et al., 2008; Bouchez et al., 2010], POC petro is rapidly reburied in reservoirs after being eroded from bedrock, so a decline in POC petro oxidation during transport is expected. In that case, dam building would decrease the carbon source from POC petro oxidation, equivalent to a further enhanced carbon sink. Overall, it is clear from our results that dam building in the CR Basin has generated a significant, new carbon sink, because of the shift of POC burial from the ECS margin to terrestrial reservoirs and the distinct carbon preservation between these environments (Figure 8). With increasing demand for freshwater and electricity, more dams and reservoirs are being built, leading to greater impacts on the carbon cycle [Teodoru et al., 2013]. Our CR work provides a typical case study for evaluating carbon sequestration resulting from dam building at large river basin scales. Other observations have shown that reservoirs can work as an important carbon source (greater carbon emission than burial), either in tropic or boreal regions [Mendonça et al., 2014; Teodoru et al., 2013]. Our study indicates that construction of dams and reservoirs along the CR, in a subtropical large river basin, has led to overwhelming carbon burial over emission. Compared to tropical reservoirs (e.g., Amazon) where carbon emissions exceed burial rates, the CR reservoirs have lower temperature and higher sedimentation rate (e.g., ~10 cm yr 1 in the TGD reservoir versus ~0.51 cm yr 1 in the Mascarenhas de Moraes reservoir, Brazil [Mendonça et al., 2014]). The observed high POC preservation in the TGD is consistent with a first-order conceptual model that nontropical reservoirs are dominated by POC burial [Mendonça et al., 2012]. However, care must be taken when applying the CR case to the global scale, as preservation of POC in reservoirs depends on several regional factors including sedimentation rate, oxygen exposure time, temperature, microbial activity, and POC properties [Sobek et al., 2012; Cardoso et al., 2014], which may vary significantly in different cases. Moreover, as highlighted in this study, predamming processes and compositions of carbon (biospheric versus petrogenic) also require careful consideration. When accounting for these factors, the CR result may be applicable to other similar regions (e.g., under high sedimentation and moderate-low temperatures). Overall, more observations and long-term monitoring are required to unravel the role of dams and reservoirs in the carbon cycle. 5. Conclusion In this study, we take the Changjiang River (CR) as an example to examine the carbon sink effect resulting from dam building on rivers entering oxidative, passive margins. High POC petro content (0.46 ± 0.10%) was LI ET AL American Geophysical Union. All Rights Reserved. 50