Particulate organic carbon export flux by 234Th/238U disequilibrium in ...

Acta Oceanol. Sin., 2013, Vol. 32, No. 10, P. 67-73 DOI: 10.1007/s13131-013-0303-7 http://www.hyxb.org.cn E-mail: [email protected]

Particulate organic carbon export flux by 234Th/238 U disequilibrium in the continental slope of the East China Sea BI Qianqian1 , DU Jinzhou1∗ , WU Ying1 , ZHOU Jing1 , ZHANG Jing1 1

State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China

Received 7 July 2012; accepted 25 February 2013 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2013

Abstract 234 Th is widely used to quantify the magnitude of upper ocean particulate organic carbon (POC) export in oceans. In the present work, the rates of particulate organic carbon export were measured based on the distribution patterns of 234 Th/238 U disequilibrium in the water column within the continental slope of the East China Sea (ECS) during May 2011. The profiles of particulate and dissolved 234 Th activities at all three stations showed a relative deficit with respect to 238 U in the upper 100 m of the water column. The dissolved 234 Th scavenging rates and the particulate 234 Th removal rates and their residence times were calculated by a one-dimensional steady state model. The results showed that the dissolved 234 Th scavenging rates and the particulate 234 Th removal rates ranged from 12.4–61.4 dpm/(m3 ·d) and from 3.8–21.8 dpm/(m3 ·d), respectively. The residence times of dissolved and particulate 234 Th were in the range of 3.4–158 d and 63.7– 96.5 d, respectively. Combined with the measurement of POC/234 Th ratios of suspended particles, POC export flux (calculated by carbon) from the euphotic zone was estimated in the study region, which ranged from 4.14–14.7 mmol/(m2 ·d), with an average of 8.21 mmol/(m2 ·d), occupying 35% of the prime productivity in the study area. The results of this study can provide new information for better understanding the carbon biogeochemical cycle within the continental slope of the ECS. Key words: 234 Th/238 U disequilibrium, residence time, POC export, East China Sea (ECS) Citation: Bi Qianqian, Du Jinzhou, Wu Ying, Zhou Jing, Zhang Jing. 2013. Particulate organic carbon export flux by 234 Th/238 U disequilibrium in the continental slope of the East China Sea. Acta Oceanologica Sinica, 32(10): 67–73, doi: 10.1007/s13131-0130303-7

ical aquatic processes over time scales ranging from days to weeks (Coale and Bruland, 1985; Buesseler et al., 1992, 1998, 2006). For example, 234 Th deficiency relative to 238 U is used to calculate the removal flux of 234 Th through scavenging on sinking particles, which can be used to study particle dynamics and estimate POC export flux from the ocean surface to the deep sea (Bacon et al., 1996; Buesseler et al., 1998; Cai et al., 2002; Charette et al., 1999; Cochran et al., 1995; Coppola et al., 2005). Waples et al. (2006) summarized the application of 234 Th as a radiotracer in marine environmental processes by estimating the total mass of the sinking particles from the upper ocean water column and calibration of sediment traps, calculating the rate of particle remineralization in the mesopelagic zone, determining the rate of sediment resuspension/mixing/deposition near the benthic boundary layer, and evaluating both adsorption/desorption and aggregation/disaggregation of differing particle sizes (e.g., large, small, colloidal). The East China Sea (ECS) is a typical marginal sea in the western Pacific Ocean, where there can be large amounts of materials both from the upwelling of subsurface waters of the Kuroshio Current (Wu et al., 2007) and the Changjiang (Yangtze) River (Chen et al., 1999; Wu et al., 2003). To date, the magnitude of POC export has been little studied in the ECS continental margin. Hung and Gong (2007) first measured POC export

1 Introduction The coastal ocean, which comprises only 7% of the ocean surface area, is known to contain a large percentage of the total oceanic primary production (15%–30%, Walsh et al., 1988; Walsh, 1991; Wollast, 1993, 1998). The continental margins, which may be potential carbon sinks, play an equally important role in the global carbon cycle (Walsh et al., 1988; Liu et al., 1999). Through a process known as the biological pump, phytoplankton transform atmospheric CO2 into biomass through photosynthesis and store it in the deep ocean. Most of the CO2 absorbed by phytoplankton is recycled near the surface, but a variable and vital amount of biomass, often referred to as marine snow, leaves the euphotic zone and sinks into deep waters (Ducklow et al., 2001). Finally, less than 1% of the biomass reaches the seafloor (Feely et al., 2001; Berelson et al., 2002). The export flux of particulate organic carbon (POC) from the euphotic zone to the deep ocean is considered to be a crucial index of the efficiency of the biological pump. With a rapid turnover rate, the variable POC export flux has great significance for the carbon cycle with regard to global climate change. Recently, 234 Th (T1/2 =24.1 d), which is produced continuously in seawater from the decay of its parent 238 U (T1/2 =4.5×109 a), has been found to be a useful radiotracer of the kinetics of a wide variety of biological, chemical, and phys-

Foundation item: The National Key Basic Research Program from the Ministry of Science and Technology of China under contract Nos 2011CB409801 and 2010DFA24590; the National Natural Science Foundation of China under contract No. 41240038. *Corresponding author, E-mail: [email protected]

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BI Qianqian et al. Acta Oceanol. Sin., 2013, Vol. 32, No. 10, P. 67-73

fluxes, which ranged from 2.2–5.6 mmol/(m2 ·d) off the east coast of Taiwan, by drifting sediment traps in the main stream of the Kuroshio Current. More recently, Hung et al. (2010) made a comparison between northeastern Taiwan and the Gulf of Mexico, indicating that the POC export flux was 6.2–18.8 mmol/(m2 ·d) and 4.0–5.0 mmol/(m2 ·d) in upwelling and oligotrophic oceanic regions, respectively. In this study, a steady state model was applied to estimate POC export fluxes from the euphotic zone in the ECS continental slope (water depth ∼1 000 m) based on 234 Th/238 U disequilibrium.

(Fig. 1 and Table 1). At each station, approximately 10 L of seawater was collected with a large capacity (80 L) hydrophore. After collection, 6 L of seawater was filtered through 47 mm 0.70μm Whatman glass fibers filters (GF/F) (precombusted at 450 ◦ C for 4–5 h to reduce background carbon levels). After filtration, samples were frozen and stored in a freezer to take back for POC analysis. The remaining 4 L of seawater was filtered through a 47 mm 0.45-μm mixed cellulose ester membrane (MCEM) to separate dissolved and particulate 234 Th.

2 Materials and methods 2.1 Study area The ECS, well known for high biological productivity, has a total area of 740 000 km2 with an average depth of 300 m, making it one of the largest marginal seas in the western Pacific (Wong et al., 2000). Two major sources of nutrients of the ECS shelf are from the upwelling of the Kuroshio Current subsurface waters (Chen, 1996) and from tremendous river runoff of the Changjiang (Yangtze) River. Riverine carbon fluxes (calculated by carbon) are 32 Tg/a (Chen and Wang, 1999). Our study stations were located in the southeastern ECS, and were strongly influenced by the Kuroshio Current, which flows continuously off the east coast of Taiwan toward Japan along the continental slope of the ECS. The southeastern ECS is characterized by warm, high salinity and nutrient-depleted water, with low chlorophyll a levels. 2.2 Sample collection The samples were collected at three stations using the R/V Shiyan 3 during a spring survey cruise to the ECS in May 2011

Fig.1. Location of sampling sites in the ECS, May 2011, showing the collection sites of 234 Th samples (circles) and POC samples (triangles and circles).

Table 1. Vertical distributions of temperature and salinity as well as the activities of 234 Th and 238 U at three stations in the ECS Sampling date

2.3

Location

Depth/m

Temperature/◦ C

Salinity A Thp

±error

Activities/dpm·L−1 A Thd ±error A Tht

±error

AU

26.2 26.1 23.8 14.1 10.3

34.4 34.4 34.8 34.5 34.3

1.39 0.50 0.30 0.28 0.40

0.42 0.37 0.19 0.09 0.05

0.21 1.68 1.95 1.83 1.73

0.04 0.35 0.39 0.58 0.85

1.60 2.18 2.25 2.11 2.13

0.43 0.51 0.44 0.58 0.85

2.36 2.36 2.39 2.37 2.35

2011-05-23

CJ (28.13◦ N, 127.05◦ E)

0 30 100 280 390

2011-05-19

D9 (26.97◦ N, 126.12◦ E)

0 30 80 160 400

25.0 24.9 24.4 21.8 10.5

34.6 34.6 34.7 34.8 34.3

1.20 1.10 0.33 0.50 0.30

0.41 0.38 0.25 0.24 0.09

0.68 0.85 1.90 1.64 1.91

0.26 0.48 0.79 0.78 0.80

1.88 1.95 2.23 2.14 2.21

0.49 0.61 0.83 0.82 0.80

2.37 2.37 2.38 2.38 2.35

2011-05-16

G8 (25.48◦ N, 122.92◦ E)

0 50 200 400

26.2 25.6 17.5 11.4

34.6 34.8 34.5 34.4

0.80 0.70 0.60 0.65

0.29 0.23 0.36 0.20

1.16 1.39 1.31 1.56

0.39 0.37 0.88 0.62

1.96 2.09 1.91 2.21

0.48 0.44 0.95 0.65

2.37 2.38 2.37 2.36

234 Th

analysis Our analysis of dissolved samples used the small volume MnO2 co-precipitation method (Cai et al., 2006). Briefly described, 0.5 ml of 3.0 g/L KMnO4 and 0.5 ml of 8.0 g/L MnCl2 ·4H2 O solutions were added to 4 L seawater to form a suspension of MnO2 . After stirring adequately, samples were heated in a water bath (>80◦ C) for 2–3 h. The samples were then taken out of the hot water to cool. Subsequently, the suspension was filtered onto a 47 mm 0.45-μm MCEM filter. The dissolved and particulate samples were dried and covered with two layers of aluminum foil and taken back to a land-based lab-

oratory for beta counting. All samples were measured with a gas-flow proportional low level MPC 9604 beta counter, manufactured by the American Protean Instrument Company. The background count rate was approximately 0.40–0.46 cpm. In this study, the detector efficiency of the beta counter was calibrated by adding a known quantity of 238 U–234 Th equilibrium solution to the MnO2 co-precipitation. The recovery experiment was performed by collecting seawater in the bottom of the water column at Stas CJ and D9, where there were a small difference between 238 U and 234 Th activities. Each sample was beta-counted five times during a 60-d period using the “multi-


count” method (Buesseler et al., 2001). The data was corrected by the decay curve to the midpoint of the MnO2 filtration. 2.4

238

U analysis The activities of 238 U were estimated from the relationship between salinity and 238 U as 238 U (dpm/L)=0.068 6× salinity×desinity (Chen et al., 1986).

2.5 POC analysis The contents of organic carbon were determined through a Vario ELIII CHNOS Elemental Analyzer after removing the carbonate fraction. The analytical precision of the experimental procedure was estimated to be ±2% (Zhang et al., 1998; Wu et al., 2007).

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3 Results and discussion 3.1 Hydrological and hydrochemical settings: T, S, and POC The vertical profiles of temperature and salinity are presented in Fig. 2. The sea surface temperature (SST) ranged from 25.0–26.2 ◦ C, with an average of 25.8 ◦ C. There was a maximum salinity of 34.8 at the depths of 50–160 m, which may be a modified North Pacific Tropical Water. In addition, at 400 m, there was a minimum salinity of 34.4, which may be modified North Pacific Intermediate Water. In short, the water was characterized by high temperature and salinity. Furthermore, the water column was stratified indistinctly. As shown in Fig. 3, the concentration of POC was generally

Fig.2. Vertical profiles of 234 Th and 238 U activities, temperature, and salinity at Stas CJ, D9, and G8 in the water column of the ECS, May 2011.

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in the range of 2.24–8.20 μmol/L, with an average of 4.22 μmol/L. It was generally greatest in the surface layer and decreased with depth, but there were some exceptions. For example, at Sta. G8, the surface POC was approximately 4 μmol/L but the anomalously high POC concentration was 8 μmol/L at 100 m, which may be due to the increased biomass. At 870 m, there was another abnormal POC concentration in the bottom water, resulting from a sampling error that the hydrophore hit the ground. In comparison, the vertical profile of POC concentrations at Stas CJ and D9 decreased with depth, which may be

due to degradation of the organic matter. Compared to the POC concentration of Sta. G8 in the surface water, Stas CJ and D9 were slightly higher (about 6 μmol/L). In the bottom water, the POC concentrations were lower than 2 μmol/L. As presented in Fig. 4, the surface distribution of POC in the continental slope was in the range of 5.7–14.6 μmol/L and decreased offshore. Stations D7, E7, F7, G7, and C9 were located at the shelf edge of the ECS. The maximum POC was at Sta. C9.

Fig.3. Depth profiles of POC at Stas CJ, D9, and G8 in the water column of the ECS, May 2011. late 234 Th decreased with depth. High particulate 234 Th concentrations may be related to the high concentration of suspended particulate matter (SPM) (Cai et al., 2002). In conclusion, the depth profile of total 234 Th was similar with dissolved 234 Th, which was the opposite of particulate 234 Th. 3.3 Scavenging rate, removal rate, and residence time of 234 Th To estimate rates of 234 Th scavenging onto suspended particles and 234 Th removal via sinking particles, 234 Th activity balance equation was used (Buesseler et al., 1992; Coale and Bruland, 1985; Cochran et al., 1995, 2000; Savoye et al., 2006): dA Th = A U λ − (A Thd λ + A Thp λ) − PTh + V, dt

dA Th is the change of total 234 Th activity with time, A U is dt the activity of 238 U, A Thd and A Thp are the activities of measured dissolved and particulate 234 Th, respectively, λ is the decay constant for 234 Th (0.028 8 d−1 ), PTh is the net loss of 234 Th on sinking particles, and V is the sum of advection and diffusive terms. In the open ocean, the magnitude of PTh is most often driven by the extent of 234 Th/238 U disequilibria. The term V stands for physical processes other than particle settling, such as vertical and horizontal advection and eddy diffusion. To solve Eq. (1), some assumptions must be made. First, a single measurement necesdA Th = 0), sitates the assumption of a steady state (SS, that is, dt which has been demonstrated to be an adequate resolution of 234 Th, except during plankton blooms when significant 234 Th removal can occur (Buesseler et al., 1992, 1998). Second, we ignored the physical processes of advection and diffusion, since the vertical transport of 234 Th through sinking particles and radioactive decay were predominant (Aono et al., 2005; Thomalla et al., 2006). The net scavenging rate of dissolved 234 Th [ J Th , dpm/(m3 ·d)] and the net removal rate of total 234 Th on sinkwhere

Fig.4. Surface distributions of POC at Stas D7, E7, F7, G7, C9, G8, D9, and CJ, May 2011. 3.2 Distributions of dissolved, particulate, and total 234 Th in the upper water column Measured 234 Th activities are shown in Table 1. 234 Th activities were 0.21–1.95 and 0.28–1.39 dpm/L for dissolved and particulate fractions, respectively. As typical of oceanic profiles, dissolved 234 Th was the dominant form, ranging from 44%–87% in the water column, except in the surface water, which was similar with Benitez-Nelson et al. (2000). The maximum and minimum dissolved 234 Th were generally observed at subsurface and surface depths, respectively. In surface waters, 234 Th was deficient relative to its parent 238 U at three stations, suggesting that 234 Th was scavenged and removed from the upper water column by sinking particles. The activity of particu-

(1)

71


ing particles [PTh , dpm/(m3 ·d)] are given by Coale and Bruland (1985) and Moran and Buesseler (1993), respectively: J Th = (A U − A Thd )λ,

(2)

PTh = [A U − (A Thd + A Thp )]λ, A Thd , τd = J Th A Thp , τp = PTh

(3)

of dissolved and particulate 234 Th, respectively. J Th , PTh , τd , and τp were calculated and are shown in Table 2. The scavenging rates of 234 Th ranged from 12.4–61.4 dpm/(m3 ·d), with an average of 32.7 dpm/(m3 ·d). The removal rates ranged from 3.8–21.8 dpm/(m3 ·d), averaging 10.2 dpm/(m3 ·d). As presented in Table 2, the residence times of dissolved and particulate 234 Th ranged between 3.4–158 d and 63.7–96.5 d, with an average of 62.9 d and 80.6 d, respectively. A comparison with other study areas is displayed in Table 3. The scavenging rate of 234 Th in this study was slightly slower, but the removal rate was opposite. The residence time of particulate 234 Th was considerably longer. However, the residence time of

(4) (5)

where A Thd and A Thp are the activities of dissolved and particulate 234 Th, respectively. τd (d) and τp (d) are the residence times

Table 2. Scavenging rates, removal rates, and residence times of 234 Th in the ECS Station

Depth/m

J Th /dpm·m−3 ·d−1

PTh /dpm·m−3 ·d−1

τp /d

τd /d

CJ

0 30 100 0 30 80 0 50

61.4 19.4 12.4 48.2 43.3 13.7 34.6 28.4

21.8 5.2 3.8 14.0 11.9 4.3 11.8 8.4

63.7 96.5 78.5 85.7 92.2 76.9 67.9 83.3

3.4 86.4 158 14.1 19.7 139 33.5 49.0

D9

G8

Table 3. Comparison of 234 Th export fluxes and residence times in the euphotic zone with other research areas Study area

Sampling time

J Th /dpm·m−3 ·d−1

PTh /dpm·m−3 ·d−1

τp /d

τd /d

Reference

Southeastern ECS

May 2011 April 2007

Bering Sea

July 2003

Central South China Sea

May 2002

3.8–21.8; average 10.2 2.6–38.6; average 18.9 4.7–20.0; average 12.3 3.1–37.7; average 17.3


3.4–158; average 62.9 12.5–50.8; average 80.6 37.2–104.7; average 70.9 22.4–198.8; average 69.5

this study

Northwestern South China Sea


dissolved 234 Th was in accord with other results. 3.4 Estimating POC export fluxes from the euphotic zone The particulate 234 Th [FPTh , dpm/(m2 ·d)] from the euphotic zone has been provided by Cai et al. (2002):

z =h

FPTh =

PTh dz ,

(6)

z =0

where z represents the depth and h is the depth of the export interface. The export flux of POC [FPOC , mmol/(m2 ·d)] from the euphotic zone can be estimated using the following equation (Buesseler et al., 1992, 1998): FPOC = FTh (

POC )the export depth , A ThP

(7)

POC ) the export depth is the measured POC/234 Th ratio of A ThP particulate matter at the export interface. Here, the depth of where (

Ma et al. (2011) Ma et al. (2009) Ma et al. (2008)

the euphotic zone is defined as the depth of 1% light penetration. In previous papers, the depths of the euphotic zone in May were located at 75, 85, and 60 m near the Stas CJ, D9, and G8, respectively (Li et al., 2003). Considering some objective reasons in this study, we chose 100, 80, and 50 m as the export depths at Stas CJ, D9, and G8, respectively. As shown in Table 4, POC export fluxes from the export layer were 4.14, 14.7, and 5.77 mmol/(m2 ·d) at Stas CJ, D9, and G8, respectively, with an average of 8.21 mmol/(m2 ·d). A comparison with other marginal seas is presented in Table 5. Compared with the southern South China Sea (Cai et al., 2008), the Beaufort Sea (Moran and Smith, 2000), the slope of the Chukchi Sea (Yu et al., 2012), as well as the Arabian Sea (Buesseler et al., 1998), our study results were slightly elevated, but lower than northeastern Taiwan, the upwelling regions of the ocean, and the oligotrophic Gulf of Mexico (Hung et al., 2010). We presumed that the major factor was sized-fraction filtration. Hung et al. (2010) considered that 234 Th (and POC) mainly preferred to attach onto medium (10–15 μm) rather than larger-sized (50–150 μm) suspended particles. Note that we

Table 4. 234 Th scavenging and POC export fluxes of the study area in the ECS Station

Export layer/m

FPTh /dpm·m−2 ·d−1

POC/μmol·L−1

POC/A Thp /μmol·dpm−1

FPOC /mmol·m−2 ·d−1

CJ D9 G8

100 80 50

513 836 493

4.04 5.82 8.20

8.07 17.6 11.7

4.14 14.7 5.77

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Table 5. Comparison of POC export fluxes from the euphotic zone with other research areas Study area

Sampling time

Range of POC export fluxes/mmol·m−2 ·d−1

Reference

Continental slope of ECS Southern China Sea

16–23 May 2011 28 April–25 May 2004

Beaufort Sea

August–September 1995

4.14–14.7 (–10.7±1.5)–(12.6±1.1) (average 3.8±4.0) ∼(0.7±0.1)–(2.4±0.22)

Chukchi Sea

12 July–22 September 2008

Arabian Sea

March–April 1995

(2.1±0.5) (slope)–(29.5±23.0) (shelf) 0.9–8.5

Northeastern Taiwan

15–17 May 2009

6.2–18.8

Gulf of Mexico

1–14 April 2005; 30 April–11 May 2006

2.3–9.7

this study Cai et al. (2008) Moran and Smith (2000) Yu et al. (2012) Buesseler et al. (1998) Hung et al. (2010) Hung et al. (2010)

have ignored the physical processes, which may result in increasing the uncertainty of the calculation of the export fluxes of POC (Buesseler et al., 1995; Chang et al., 2009). The higher concentration of 234 Th carried by the the upwelling of the Kuroshio prossibly resulted in higher export of POC (Savoye et al., 2006). In comparison to the annual primary productivity [285 mg/(m2 ·d), calculated by carbon] (Gong et al., 1999) of Kuroshio waters, 35% of carbon can be transferred from the ocean surface to the deep ocean. 4 Summary Based on the 234 Th activities and POC concentrations in the upper 400 m of the water column in the ECS continental slope shelf, the following conclusions can be drawn: (1) 234 Th was observed to have a significant deficiency relative to 238 U in the surface water, which had a subsurface maximum possibly due to the accumulation of suspended particles before sampling. (2) POC export fluxes from the euphotic zone ranged from 4.14–14.7 mmol/(m2 ·d), and accounted for approximately 35% of prime productivity during the spring within the ECS continental slope shelf. (3) In order to better understand the distribution of 234 Th with an accurate spatial resolution, more intensive sampling layers, especially in the upper 100 m of the water column, are needed. Finally, future studies should investigate the fluxweighted POC/APTh ratio for elucidating POC flux, and explore the influence of physical processes in detail. Ac k now l e d g e m e n t s The authors wish to thank all colleagues of Marine Chemistry in the State Key Laboratory of Estuarine and Coastal Research, East China Normal University, as well as all crews of the R/V Shiyan 3 for their assistance during the entire cruise. References Aono T, Yamada M, Kudo I, et al. 2005. Export fluxes of particulate organic carbon estimated from 234 Th/238 U disequilibrium during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS 2001). Progress in Oceanography, 64(2): 263–282 Bacon M P, Cochran J K, Hirschberg D, et al. 1996. Export flux of carbon at the equator during the EqPac time-series cruises estimated from 234 Th measurements. Deep-Sea Research II, 43(4): 1133–1153 Benitez-Nelson C R, Buesseler K O, Crossin G. 2000. Upper ocean carbon export, horizontal transport, and vertical eddy diffusivity in the southwestern Gulf of Maine. Continental Shelf Research, 20:

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