234Th-derived particulate organic carbon export in the Prydz Bay,Antarctica

Hao Ma • Zhi Zeng • Jianhua He • Zhengbing Han •

Wuhui Lin • Liqi Chen • Jianping Cheng •

Shi Zeng

Received: 21 July 2013 / Published online: 19 November 2013

� Akademiai Kiado, Budapest, Hungary 2013

Abstract 234Th activities in sea water were measured

using Fe(OH)3 co-precipitation and beta counting at six

stations in Prydz Bay in March 2008 during the 24th

Chinese National Antarctic Research Expedition. Total234Th activities ranged from 0.96 to 2.44 dpm L-1 with an

average of 1.61 dpm L-1, showing an apparent deficit with

respect to 238U due to scavenging and export with particles.

With a one-dimensional steady state model, 234Th export

fluxes were converted to particulate organic carbon (POC)

export using bottle ratios of POC concentrations to par-

ticulate 234Th activities on suspended particles. POC

fluxes at the depth of 100 m varied between 33 and

297 mmol m-2 day-1, comparable to prior work in the

same region and higher than those of some other sea areas

in the Southern Ocean, and indicated efficient running of

biological pump in Prydz Bay. The results could be helpful

to expand the knowledge of carbon cycle in seasonally ice-

covered coastal regions around Antarctica.

Keywords Prydz Bay � Antarctica � 234Th �Particulate organic carbon

Introduction

Covering about 20 % of surface area of world oceans, the

Southern Ocean is an important CO2 sink region contrib-

uting 35 % of CO2 uptake of world oceans [1–4]. Further

studies indicate that continental shelf regions of the

Southern Ocean with high primary production and strong

upwelling play a significant role in effectively uptaking

CO2 from atmosphere and transport carbon downward to

the deep sea [5, 6].

Among many other processes, the particulate organic

carbon (POC) export from the upper ocean is regarded as a

critical index of the efficiency of biological pump and it has

often been used as a necessary measurement to determine

the biogeochemical cycling rates of particle-reactive ele-

ments and constituents in the ocean [7–9]. Most of studies

on POC export in the Southern Ocean were conducted in

west Antarctica and around Kerguelen Islands region, using

isotopes (e.g. 234Th) or sediment trap [10–19] with a long

term POC export record over the continental shelf of the

west Antarctic Peninsula [20]. However, less attention has

been paid to study POC export in Prydz Bay [21, 22].

Prydz Bay, the third largest embayment in the Southern

Ocean, lies in East Antarctica and is bounded on the south-

western side by the Amery Ice Shelf, on the southeast by the

Ingrid Christensen Coast, and by Mac. Robertson Land to the

west, ending in Cape Darnley. It is characterized by a broad

continental shelf, a steep shelf break, strong upper layer

stratification, and homogeneous deep waters. A closed

cyclonic gyre adjacent to the Amery Ice Shelf dominates

surface circulation in Prydz Bay [23–25]. Due to insufficient

H. Ma (&) � Z. Zeng � W. Lin � J. Cheng � S. Zeng

Department of Engineering Physics, Tsinghua University,

Beijing 100084, China

e-mail: mahao@tsinghua.edu.cn

H. Ma � Z. Zeng � J. Cheng

Key Laboratory of Particle & Radiation Imaging, Tsinghua

University, Ministry of Education, Beijing 100084, China

J. He � W. Lin � L. Chen

Key Laboratory of Global Change and Marine-Atmospheric

Chemistry (Third Institute of Oceanography), State Oceanic

Administration, Xiamen 361005, China

Z. Han

Laboratory of Marine Ecosystem and Biogeochemistry (Second

Institute of Oceanography), State Oceanic Administration,

Hangzhou 310012, China

123

J Radioanal Nucl Chem (2014) 299:621–630

DOI 10.1007/s10967-013-2842-y

research, particularly in seasonally ice-covered regions [26,

27], it is necessary to investigate POC export process in the

upper waters of Prydz Bay to improve our understanding of

the carbon cycle in coastal Southern Ocean.234Th, a radioactive nuclide which strongly absorbs onto

particles, is produced in situ from the decay of its parent238U with a constant rate in the ocean. 234Th can be

scavenged and removed rapidly with sinking particulate

matter, resulting in disequilibrium between 234Th and 238U,

especially in the upper water column. The deficiency of234Th with respect to 238U reflects the cycle, export and

remineralization of particles. Due to the relatively short

half-life of 234Th (24.1 days), it is very suitable for tracing

biogeochemical processes on the time scale similar to

particle dynamics in the upper ocean [28]. The 234Th

tracing technique has been widely used and demonstrated

to be a robust method in the estimation of particle export

fluxes from the upper waters in the ocean [29].

In this study, 234Th activities in the upper waters of

Prydz Bay were measured by Fe(OH)3 co-precipitation

method. POC export fluxes from upper 100 m seawater

were determined using 234Th and a one-dimensional steady

state model to further understand the POC export process

close to the end of main phytoplankton bloom period and

the role of seasonally ice-covered Prydz Bay in the carbon

cycle in high latitude coastal regions.

Sample collection and analysis

Water samples were collected in Niskin bottles at water

depths of 0, 25, 50, 100, 150 and 200 m from 6 stations

onboard R/V Xuelong at Prydz Bay in March 2008 during

the 24th Chinese National Antarctic Research Expedition

(Fig. 1; Table 1). Salinity and temperature of each depth

were automatically recorded at the same time by a Seabird

CTD profile (Mod: 17). Once the sample was collected, a

1 L portion of 20 L sample from each depth at six stations

was filtered through 0.7 lm glass fiber filters (WhatmanTM

GF/F), dried and brought back to the laboratory for POC

analysis. After HCl fumigation of the filters for 12 h, POC

concentrations were determined by a TC Analyzer (SSM-

5000A, SHIMADZU).234Th was analyzed by Fe(OH)3 co-precipitation tech-

nique, as employed by previous studies [30, 31] and reviewed

by Rutgers van der Loeff et al. [32]. Residual 19 L water

samples were filtrated to separate dissolved and particulate234Th ([0.45 lm) for analysis on board as soon as possible.

After filtration, water samples were immediately acidified

with concentrated HCl and spiked with*239.3 mBq 230Th as

a yield tracer. Then the pH was adjusted to 8 and uranium and

thorium isotopes were co-precipitated with Fe(OH)3 by add-

ing * 100 mg Fe3? and appropriate amount of concentrated

ammonium hydroxide. The precipitate was collected and

dissolved with 8 mol L-1 HCl. The solution was subse-

quently processed through an anion exchange column, and

thorium isotopes were purified and separated from other ele-

ments. The particulate sample on the filter membrane was

digested by HNO3–HF–HClO4 mixed solution with an addi-

tion of a 230Th spike and dissolved with 8 mol L-1 HCl. The

acidic solution was then processed following the above

radiochemical procedure. After radiochemical separation and

purification, the thorium isotopes were electrodeposited on a

stainless steel disc to be counted. A calibrated surface barrier

Fig. 1 Location of sampling

stations in Prydz Bay

(schematic map)

622 J Radioanal Nucl Chem (2014) 299:621–630

123

semiconductor (BH1216, CNNC Beijing Nuclear Instrument

Factory) was used to measure 234Th and 230Th simultaneously.

The detection efficiency of detector for both a and b radiation

is more than 20 %, and the b background is less than 0.5 cpm

[33]. The 234Th activities were finally corrected to the sam-

pling time, and in-growth of 234Th from 238U between the

sampling and separation time were taken into account, which

had been minimized by radiochemical separation of thorium

from uranium immediately after the pretreatment steps.

Uranium occurs in the toxic marine environment as the

soluble uranyl carbonate species UO2(CO3)34- and is conser-

vative with salinity [34–36]. Therefore, when carrying out

research on POC export and organic carbon cycling, 238U

activities are typically determined from the U-salinity

relationship: 238U (dpm L-1) = 0.07081 9 salinity according

to Chen et al. [35], instead of undertaking specific sample

analysis [32, 37]. The associated uncertainty of 238U activities

was in the vicinity of 3 % [38], and was also included when

calculating the combined uncertainty related to 234Th export

fluxes.

Results and discussion

Temperature, salinity and POC

Temperature, salinity and POC concentrations measured in

seawater of Prydz Bay are presented in Table 1. Station

Table 1 Temperature, salinity and POC concentrations in seawater of Prydz Bay

Station Latitude (S) Longitude (E) Layer (m) Temperature (�C) Salinity CPOC (mmolm-3)

P3–8 66.35� 73.17� 0 -1.80 33.322 12.84

25 -1.78 33.362 12.23

50 -1.58 34.180 6.13

100 -1.64 34.299 4.53

150 -1.50 34.347 3.34

P3–14 67.99� 72.93� 0 -1.79 33.749 23.47

25 -1.80 33.759 16.07

50 -1.70 34.379 8.10

100 -1.91 34.444 3.48

150 -1.91 34.461 3.03

200 -1.88 34.470 1.06

P3–15 68.45� 72.87� 0 -1.78 33.537 30.31

25 -1.77 33.550 21.97

50 -1.53 34.301 17.53

100 -1.83 34.402 4.07

150 -1.91 34.432 2.61

200 -1.99 34.447 5.27

P4–11 67.97� 75.41� 0 -1.82 33.637 14.29

25 -1.82 33.675 13.55

50 -1.76 33.833 11.47

100 -1.65 34.340 4.077

150 -1.70 34.374 3.26

200 -1.70 34.403 3.09

P4–12 68.50� 75.48� 0 -1.80 33.587 18.48

25 -1.80 33.614 17.73

50 -1.77 33.654 15.38

100 -1.58 34.350 13.83

150 -1.73 34.408 10.88

P4–13 68.96� 75.43� 0 -1.70 33.576 19.63

25 -1.69 33.585 17.34

50 -1.22 33.899 16.25

100 -1.62 34.356 16.61

150 -1.63 34.388 15.38

200 -1.75 34.416 10.77

J Radioanal Nucl Chem (2014) 299:621–630 623

123

B3-8 is located in the slope region of Prydz Bay, the mixed

layer of which was about 30 m deep. The thermocline and

halocline located between 30 and 50 m. Similar charac-

teristics of the mixed layer appeared in the rest 5 stations

located in the shelf region of Prydz Bay. Abnormal rise of

temperature of seawater was observed at a depth of about

50 m, possibly due to combined effect of solar warming

and ice melt.

Surface POC concentrations in seawater of the shelf

region of Prydz Bay ranged from 14.29 to 30.31 mmol m-3

with an average of 21.24 mmol m-3, higher than

12.84 mmol m-3 of slope region (Station P3–8), and the

horizontal distribution was characterized by an increasing

trend with decreasing offshore distance due to influence

from hydrographic situation, biological activities and ter-

restrial inputs. Vertical distribution of POC concentrations

appeared a decreasing trend with increasing depth, from

18.48 to 10.88 mmol m-3 at Station P4–12 for example,

indicating varying intensity of biological activities in water

column.

234Th distribution and fluxes

Dissolved, particulate and total (dissolved plus particulate)234Th activities and vertical distributions are presented with

1r statistical uncertainty in Table 2 and Fig. 2. Dissolved

Table 2 234Th activities, scavenging and export rates in Prydz Bay (1 dpm = 1/60 Bq)

Station Layer (m) ADTh (dpm L-1) APTh (dpm L-1) ATTh (dpm L-1) AU (dpm L-1) JTh (dpm m-3 day-1) PTh (dpm m-3 day-1)

P3–8 0 1.06 ± 0.08 0.13 ± 0.05 1.19 ± 0.09 2.36 ± 0.07 37.4 ± 3.0 33.8 ± 3.4

25 1.22 ± 0.08 0.54 ± 0.04 1.76 ± 0.09 2.36 ± 0.07 32.9 ± 3.2 17.3 ± 3.4

50 1.01 ± 0.10 0.28 ± 0.05 1.29 ± 0.12 2.42 ± 0.07 40.6 ± 3.6 32.5 ± 3.9

100 1.46 ± 0.08 0.35 ± 0.06 1.81 ± 0.10 2.43 ± 0.07 27.9 ± 3.1 18.0 ± 3.5

150 1.98 ± 0.07 0.21 ± 0.04 2.19 ± 0.08 2.43 ± 0.07 13.0 ± 2.8 6.9 ± 3.0

P3–14 0 0.66 ± 0.03 0.39 ± 0.05 1.05 ± 0.06 2.39 ± 0.07 49.9 ± 2.3 38.7 ± 2.7

25 0.82 ± 0.06 0.15 ± 0.03 0.96 ± 0.06 2.39 ± 0.07 45.3 ± 2.6 41.1 ± 2.8

50 1.73 ± 0.08 0.28 ± 0.05 2.01 ± 0.09 2.43 ± 0.07 20.2 ± 3.1 12.2 ± 3.4

100 0.90 ± 0.08 0.16 ± 0.05 1.06 ± 0.09 2.44 ± 0.07 44.4 ± 3.0 39.8 ± 3.3

150 1.67 ± 0.08 0.21 ± 0.04 1.88 ± 0.09 2.44 ± 0.07 22.1 ± 3.1 16.0 ± 3.3

200 2.13 ± 0.09 0.35 ± 0.05 2.48 ± 0.11 2.44 ± 0.07 8.9 ± 3.4 -1.2 ± 3.7

P3–15 0 0.60 ± 0.09 0.41 ± 0.04 1.02 ± 0.10 2.38 ± 0.07 51.0 ± 3.3 39.1 ± 3.5

25 1.16 ± 0.05 0.37 ± 0.03 1.53 ± 0.06 2.38 ± 0.07 35.0 ± 2.5 24.5 ± 2.7

50 0.95 ± 0.04 0.48 ± 0.03 1.43 ± 0.05 2.43 ± 0.07 42.6 ± 2.4 28.7 ± 2.5

100 1.25 ± 0.02 0.12 ± 0.02 1.38 ± 0.03 2.44 ± 0.07 34.1 ± 2.2 30.5 ± 2.3

150 1.04 ± 0.06 0.59 ± 0.03 1.63 ± 0.07 2.44 ± 0.07 40.3 ± 2.8 23.4 ± 2.9

200 1.86 ± 0.03 0.56 ± 0.03 2.42 ± 0.04 2.44 ± 0.07 16.7 ± 2.3 0.6 ± 2.5

P4–11 0 1.20 ± 0.06 0.22 ± 0.04 1.42 ± 0.08 2.38 ± 0.07 34.1 ± 2.8 27.6 ± 3.0

25 0.94 ± 0.07 0.08 ± 0.04 1.02 ± 0.08 2.39 ± 0.07 41.7 ± 2.9 39.4 ± 3.1

50 – – 1.44 ± 0.07 2.40 ± 0.07 – 27.6 ± 3

100 1.83 ± 0.08 0.32 ± 0.04 2.14 ± 0.09 2.43 ± 0.07 17.5 ± 3.1 8.4 ± 3.3

150 1.20 ± 0.07 0.18 ± 0.04 1.38 ± 0.08 2.43 ± 0.07 35.6 ± 3 30.3 ± 3.1

200 1.99 ± 0.08 0.21 ± 0.04 2.21 ± 0.09 2.44 ± 0.07 12.7 ± 3.1 6.6 ± 3.3

P4–12 0 1.08 ± 0.09 0.10 ± 0.05 1.18 ± 0.10 2.38 ± 0.07 37.4 ± 3.3 34.4 ± 3.6

25 0.88 ± 0.08 0.21 ± 0.06 1.09 ± 0.10 2.38 ± 0.07 43.1 ± 3.2 37.2 ± 3.6

50 1.32 ± 0.09 0.09 ± 0.06 1.41 ± 0.10 2.43 ± 0.07 32.0 ± 3.3 29.4 ± 3.6

100 1.95 ± 0.07 0.24 ± 0.03 2.19 ± 0.07 2.44 ± 0.07 13.9 ± 2.8 7.0 ± 3.0

150 2.23 ± 0.07 0.21 ± 0.03 2.44 ± 0.08 2.44 ± 0.07 6.0 ± 3.0 -0.1 ± 3.1

P4–13 0 0.97 ± 0.07 0.13 ± 0.03 1.10 ± 0.08 2.38 ± 0.07 40.6 ± 2.9 36.7 ± 3.0

25 0.84 ± 0.08 0.32 ± 0.04 1.16 ± 0.08 2.38 ± 0.07 44.2 ± 3.0 35.0 ± 3.2

50 1.66 ± 0.09 0.12 ± 0.03 1.78 ± 0.09 2.40 ± 0.07 21.3 ± 3.2 17.9 ± 3.4

100 1.78 ± 0.08 0.13 ± 0.04 1.91 ± 0.09 2.43 ± 0.07 19.0 ± 3.2 15.1 ± 3.3

150 1.22 ± 0.10 0.10 ± 0.04 1.32 ± 0.10 2.44 ± 0.07 35.0 ± 3.5 32.1 ± 3.7

200 2.00 ± 0.09 0.30 ± 0.04 2.30 ± 0.10 2.44 ± 0.07 12.6 ± 3.4 4.1 ± 3.6

624 J Radioanal Nucl Chem (2014) 299:621–630

123

Temperature(C)

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

T

SP3-8

Temperature(C)

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

TS P3-14

Temperature(C)

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

TS P3-15

Fig. 2 234Th vertical distributions accompanied with temperature and salinity at all sampling stations in Prydz Bay

J Radioanal Nucl Chem (2014) 299:621–630 625

123

Temperature(C)

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

TS P4-11

Temperature(C)

-2.0 -1.9 -1.8 -1.7 -1.6 -1.5

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

T

SP4-12

Temperature(C)

-2.0 -1.8 -1.6 -1.4 -1.2

Dep

th(m

)

0

50

100

150

200

Salinity

33.0 33.5 34.0 34.5 35.0

T

SP4-13

Fig. 2 continued

626 J Radioanal Nucl Chem (2014) 299:621–630

123

234Th activities ranged between 0.604 and 2.230 dpm L-1

with an average of 1.351 dpm L-1. Particulate 234Th

activities varied from 0.080 to 0.589 dpm L-1, represent-

ing 6.5–40.6 % of total activities.

Vertical profiles of 234Th activities showed an apparent234Th deficit relative to 238U in upper 100 m, implying that234Th was scavenged and removed from upper water col-

umn by particles at all investigated stations. For weaker

biological activities below 100 m depth, 234Th deficit ten-

ded to be smaller, and 234Th activities came back in bal-

ance with those of 238U at most stations.

The change in activity of 234Th with time was deter-

mined by [39, 40]:

oATh

ot¼ k� AU � ATh½ � � PTh þ V ð1Þ

where AU and ATh represent the activities (dpm L-1) of 238U

and 234Th, and k is the 234Th decay constant (0.02876 day-1).

The term PTh (dpm m-3 days-1) represents the export rate of

particulate 234Th due to particle sinking and V, the physical

contributions to the 234Th fluxes.

The steady state model is applicable when the temporal

change of 234Th activities is little and the most commonly used

in this type of study [40]. However, when the temporal change

is rapid and significant, for instance, in the period of phyto-

plankton bloom or within physically dynamic regions, a non-

steady state model should be applied [41]. In our cruise,

however, we did not have opportunity to reoccupy the stations

due to limited ship time and bad weather. Consequently, in this

study, we used the steady state model and assumed the tem-

poral change of 234Th activities to be neglected.

Physical processes, such as upwelling, could also

influence the calculation of 234Th fluxes. In Prydz Bay, a

strong seasonal thermocline located at the depth of 200 m

during our cruise made the above water column steady

without apparent upwelling [23]. In addition, given the

small differences observed in total 234Th activities, we

assumed the horizontal contributions to the 234Th fluxes to

be neglected. Therefore, the V term of Eq. (1) was assumed

to be neglected in this study.

Based on the above discussion, a steady state model

without physical process contributions was implemented

and Eq. (1) is reduced to:

PTh ¼ k� AU � AThð Þ ð2Þ

Considering dissolved and particulate phase of 234Th with,

the scavenging rate of dissolved 234Th onto particles can be

defined and Eq. (2) is rewritten:

JTh ¼ k� AU � ADThð Þ ð3ÞPTh ¼ JTh � k� APTh ð4Þ

where ADTh and APTh represent activities (dpm L-1) of

dissolved and particulate 234Th, and JTh (dpm m-3 day-1)

represents the scavenging rate of dissolved 234Th onto

particles. JTh and PTh were thus calculated and listed in

Table 2.

The scavenging rate of 234Th ranged between 6.0 and

51.0 dpm m-3 day-1 with an average of 30.5 dpm m-3 day-1

in seawater of Prydz Bay. The export rates varied from -1.2

to 41.1 dpm m-3 day-1, averaging 23.1 dpm m-3 day-1.

Export rates of -1.2 ± 3.7 dpm m-3 day-1 at 200 m depth

of station P3–14 and -0.1 ± 3.1 dpm m-3 day-1 at 150 m

depth of P4–12 implied that little scavenging and export of234Th occurred at these two positions, which was consistent

with the re-equilibrium between 234Th and 238U indicated in

Table 2 and Fig. 2.

As shown in Table 2, the 234Th export fluxes (FPTh)

were calculated by depth integral of the export rates

through upper 100 m water columns and ranged from

2,382 to 2,963 dpm m-2 day-1 in the Prydz Bay, compa-

rable to the results of previous work in the same region

(1,017–2,736 dpm m-2 day-1) [21] and those in Pacific

sector of Southern Ocean along 170�W (1,800–

3,500 dpm m-2 day-1) [12]. For comparison, a 234Th

export of 3,200 dpm m-2 day-1 was seen at Antarctic

Polar Front north of Weddell Sea [10]. In the Indian sector

of Southern Ocean, the 234Th export fluxes in Polar Front

Zone ranged from 735 to 1,831 dpm m-2 day-1 and 311 to

1,254 dpm m-2 day-1 in subtropical zone [16]. The 234Th

export fluxes during natural iron bloom in the Southern

Ocean region of the Crozet Islands fell into two groups, and

varied from 576 to 821 dpm m-2 day-1 and from 1,340 to

2,467 dpm m-2 day-1, respectively [18].

POC export fluxes

With the empirical method advanced by Buesseler et al.

[41], the POC export flux (FPOC) at the depth of interest

can be estimated by multiplying the 234Th export fluxes and

the ratio of POC concentration (CPOC) to 234Th activities on

sinking particles:

FPOC ¼ FPTh � CPOC=APTh ð5Þ

The uncertainty of POC export significantly depends on the

CPOC/APTh ratios. Variations of the ratios have been found

in previous studies and the processes controlling the ratios

of marine particles have been reviewed [29]. Briefly, the

ratios’ variations can be caused by various marine geo-

chemical and biological mechanisms, such as changes in

volume to surface area ratio, particle aggregation and dis-

aggregation, solution chemistry, decay of 234Th and so on

[42].

In general, large size particles are considered to be more

likely to sink, CPOC/APTh ratios on more than 53 lm par-

ticles are used to convert 234Th export into POC fluxes.

However, because of lack of ship time and bad weather

J Radioanal Nucl Chem (2014) 299:621–630 627

123

conditions, the devices sampling large size particles, such

as in situ pumps or sediment traps, were not used in this

study. In practice, the ratios on suspended particles col-

lected by Niskin bottles were measured and used to cal-

culate the POC fluxes at the depth of 100 m. The results are

listed in Table 3, and compared with those of previous

studies based on 234Th in Table 4.

The POC export at 100 m depth were efficient and

ranged from 33 mmol m-2 day-1 in slope region to

297 mmol m-2 day-1 in shelf region of Prydz Bay with an

average of 112 mmol m-2 day-1, lower than those of prior

study in the same region in 2006 (99.5-515.7 mmol m-2

day-1) [21]. For comparison, the POC export fluxes in Ross

Sea ranged from 7.4 to 91.0 mmol m-2 day-1 [13]. In

Atlantic sector of Southern Ocean, POC fluxes were esti-

mated to be 20–40 mmol m-2 day-1 in 1992 [10] and

8.8 mmol m-2 day-1 in 1996 [11]. During the GEOTRACES

expedition ZERO and DRAKE, POC fluxes varied from 3.1 to

13.2 mmol m-2 day-1 [43]. Low POC export were found in

Australian sector (0.1–0.3 mmol m-2 day-1) [15] and Indian

sector (0.1–2.5 mmol m-2 day-1) [16]. During the natural

iron bloom around Crozet Islands, the POC export fluxes

varied from 4.9 to 23.2 mmol m-2 day-1 [18]. The difference

between present work and other studies mentioned above may

be to a large extent caused by CPOC/APTh ratio on particles with

different size used to estimate POC export and different

sampling periods and regions.

Conclusion

The role of coastal ocean in carbon cycle is poorly con-

strained, particularly in seasonally ice-covered regions. It is

difficult to understand the carbon cycle in high latitude

coastal regions due to lack of field measurements and

complicated physical and biological processes occurring in

shelf regions [27]. In present study, POC export fluxes

from upper 100 m were estimated in Prydz Bay during the

24th Chinese National Antarctic Research Expedition

using natural radionuclide 234Th. Vertical distribution of234Th activities showed an apparent deficit of 234Th with

respect to 238U. The scavenging and export rates were

calculated to characterize the particle dynamics and

determine the 234Th export fluxes. POC export fluxes were

determined by CPOC/APTh ratios on small suspended par-

ticles instead of sinking materials and ranged from 33 to

297 mmol m-2 day-1, implying efficient export and high

productivity in the studied region. The results can expand

the knowledge of carbon cycle, especially POC export

processes, in seasonally ice-covered coastal regions, par-

ticularly in east Antarctica, and be helpful to provide useful

insight into carbon cycle research around Antarctica.

Acknowledgments This work was supported by the National Nat-

ure Science Foundation of China (11205094, 41106167 and

41076134) and Tsinghua University Initiative Scientific Research

Program (2010Z07108). We appreciate the assistance of Jiuxin Shi,

Table 3 234Th and POC export fluxes at 100 m depth in Prydz Bay

Station Layer (m) FPTh (dpm m-2 day-1) CPOC/APTh lmol dpm-1 FPOC (mmol m-2 day-1)

P3–8 100 2,524 ± 159 13 ± 2 33 ± 6

P3–14 100 2,963 ± 139 22 ± 6 64 ± 18

P3–15 100 2,938 ± 110 33 ± 5 97 ± 16

P4–11 100 2,575 ± 135 13 ± 2 33 ± 5

P4–12 100 2,637 ± 148 57 ± 8 150 ± 22

P4–13 100 2,382 ± 143 125 ± 34 297 ± 83

Table 4 Comparison of 234Th-derived POC export fluxes in the upper ocean around Antarctica

Region Time POC fluxes

mmol m-2 day-1References

Prydz Bay Feb 2006 99.5–515.7 [20]

Ross sea Oct–Nov 1996 7.4–91.0 [13]

Atlantic sector of southern ocean Nov 1992 20–40 [10]

Dec 1995 8.8 [11]

GEOTRACES expedition ZERO and DRAKE

(weddell sea, drake passage, zero meridian)

Feb–Apr 2008 3.1–13.2 [42]

Indian sector of Southern Ocean Feb 1999 0.1–2.5 [16]

Crozet islands Nov 2004–Jan 2005 4.9–23.2 [18]

Prydz Bay Mar 2008 33.0–297.4 This study

628 J Radioanal Nucl Chem (2014) 299:621–630

123

Renfeng Ge, Chuanyu Hu and other colleagues of 24th Chinese

National Antarctic Research Expedition with CTD data and sample

collection. We also give thanks to Wenliang Wei and Quan Shen

along with the crew of the R/V Xuelong for their help during the

cruise. We are grateful to two anonymous reviewers for their con-

structive comments on manuscript.

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