Tritium, radiocarbon, 90Sr and 129I in the Pacific and Indian Oceans

Nuclear Instruments and Methods in Physics Research B 268 (2010) 1214–1218

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Tritium, radiocarbon,

90

Sr and

129

I in the Pacific and Indian Oceans

P.P. Povinec a,*,1, S.-H. Lee b,1, L. Liong Wee Kwong c, B. Oregioni c,2, A.J.T. Jull d, W.E. Kieser e, U. Morgenstern f, Z. Top g a

Comenius University, Faculty of Mathematics, Physics and Informatics, SK-84248 Bratislava, Slovakia Korea Research Institute of Standards and Science, Daejeon, Republic of Korea c International Atomic Energy Agency, Marine Environment Laboratories, MC-98000, Monaco d University of Arizona, Department of Physics, Tucson, AZ 85712-1201, USA e University of Toronto, IsoTrace Laboratory, Toronto, Canada M5S 1A7 f Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand g University of Miami, Rosentiel School of Marine and Atmospheric Sciences, Miami, USA b

a r t i c l e

i n f o

Article history: Available online 7 October 2009 Keywords: Tritium Radiocarbon 129 I Seawater Pacific Indian Ocean

a b s t r a c t Anthropogenic radionuclides have been widely used to investigate water circulation on regional and global scales. We observed that 3H, 14C, 90Sr and 129I concentrations in surface water of the Indian Ocean are similar to those measured in the North-western Pacific Ocean. This is due to the transport of water masses from the North-western Pacific via the Indonesian Seas to the Indian Ocean. The observed variations of radionuclide concentrations with latitude in the Indian Ocean are not due to deposition patterns of global fallout, but due to different water masses present in the region. Higher radionuclide concentrations observed in the South Indian Ocean in the latitudinal belt of 20–40°S are associated with the Indian Ocean Subtropical gyre, which acts as a reservoir of radionuclides. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Recent developments of accelerator mass spectrometry (AMS) methods for analysis of long-lived natural and anthropogenic radionuclides at ultra low-levels have opened doors for oceanic investigations which were not possible before either because of a lack of sensitivity, or availability of suitable samples [1–3]. Together with recent developments in the radiometrics sector, especially those using underground environment for analysis of medium and short lived radionuclides, we have at disposal necessary techniques which can be used for tracing radionuclides in the marine environment [3–5]. Anthropogenic radionuclides, mainly of global fallout origin and released from nuclear reprocessing facilities (e.g. 3H, 14C, 90Sr, 137Cs, 129 I, etc.) have been used as tools for tracing water masses in the open ocean in many investigations [6–9]. Their concentrations peaked in the atmosphere in 1963, after large-scale atmospheric nuclear weapons tests carried out during 1961–1962. The major deposition of global fallout occurred in the mid-latitudes of the northern hemisphere, and specifically in the Pacific region in the

* Corresponding author. Tel.: +421 260 295 544; fax: +421 265 425 882. E-mail address: [email protected] (P.P. Povinec). 1 Formerly at the International Atomic Energy Agency, Marine Environment Laboratories, Monaco. 2 Retired from IAEA. 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.136

North-western Pacific [10]. Tritium is an ideal tracer used extensively in oceanographic studies because it is directly incorporated into the water molecule, usually as HTO, and has suitable half-life of 12.32 y. It is produced either naturally by interactions of cosmic rays with nitrogen and oxygen in the upper atmosphere (Table 1), but it has also been produced in large amounts in atmospheric nuclear weapons tests (Table 2). Its peak concentration in the atmospheric moisture in 1963 was 1000 times higher than its natural cosmogenic background. It has also been released in large quantities from nuclear reprocessing facilities (Table 3). The penetration of bomb tritium from surface waters into deeper layers of the ocean has been used to study pathways and time scales of deep and bottom water formation [6,7]. Natural 14C levels in the environment have also been disturbed by bomb 14C. In 1963 they were by a factor of 2 higher than natural levels observed in atmospheric carbon dioxide. Because of its long half-life (5730 y), specific stratosphere–troposphere–biosphere mixing, exchange of carbon dioxide with surface ocean, and sequestration of carbon dioxide into the deep ocean, 14C became the most frequently studied environmental radionuclide, important for better understanding of climate change [11]. Other global fallout radionuclide tracers extensively used in water circulation studies are 90Sr (half-life 28.78 y) and 137Cs (half-life 30.17 y). Both were also released in large quantities (Table 3) from nuclear reprocessing facilities in Sellafield (situated at the western coast of England) and in La Hague (situated in the English Channel). They are found mostly in a dissolved phase, following

P.P. Povinec et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 1214–1218 Table 1 Inventories of natural radionuclides. Nuclide

Half-life (y)

Total activity (PBq)

Input to World Ocean (PBq)

3

12.32 5730 15.7  106

2200 1 0.6  103

2200 1 0.6  103

H 14 C 129 I

Table 2 Global fallout radionuclide inventories. Nuclide

Released activity (PBq)

Input to World Ocean (PBq)

Ocean inventory in 2010 (PBq)

3

186,000 213 620 950 0.4  103

113,000 130 380 600 0.3  103

8000 130 100 170 0.3  103

H C Sr 137 Cs 129 I 14 90

Table 3 Inventories of radionuclides released by reprocessing facilities. Nuclide

Released activity (PBq)

Input to World Ocean (PBq)

Ocean inventory in 2010 (PBq)

3

575 4 7 40 0.04

410 2.5 7 40 0.04

45 2.5 3 26 0.04

H C 90 Sr 137 Cs 129 I 14

well the movement of water masses, and their removal from the water column is mainly due to their radioactive decay and diffusion [8,9]. 129I has been introduced to the World Ocean mainly from nuclear reprocessing facilities (Table 3), and because of its long-halflife (15.7 million years) it represents an alternative tracer to global fallout radionuclides [12]. 2. Oceanographic background The North-western Pacific Ocean (Fig. 1) has been well known as the area with highest deposition of global fallout radionuclides

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[10]. The dominant current in the North-western Pacific is the Kuroshio Current which brings equatorial waters to the north [13]. Further, surface water masses from the Pacific Ocean flow via the Indonesian Seas to the North-eastern Indian Ocean, and then become part of the Indian Ocean Subtropical gyre, occupying the region between 20 and 40°S. Recent estimations show that the flow between the Pacific and Indian Oceans may be as high as 107 m3 s1, and may play important role in the ENSO and Asian monsoon climate phenomena [14]. Except the Subtropical gyre, typical for the South Indian Ocean is the banded structure of the Antarctic Circumpolar Current (ACC) consisting there of narrow jets associated with sharp fronts due to the presence of the Crozet and Kerguelen Islands [15]. Several frontal systems that meet there (Fig. 2), such as Agulhas Front (AF), Subtropical Front (STF), Subantarctic Front (SAF) and Polar Front (PF), represent narrow zones with sharp changes in temperature, salinity and oxygen content [16]. The distribution of tracers in the South Indian Ocean should be therefore controlled by the banded structure of these fronts. The International Atomic Energy Agency (IAEA) has recently completed international project Worldwide Marine Radioactivity Studies (WOMARS). The aim of the project was to study the distribution and behaviour of anthropogenic radionuclides in the world ocean, thus filling the existing gap in the WOCE programme [17]. This paper focuses on the evaluation of results obtained from the IAEA0 97 Pacific Ocean [9], Italica0 98 Indian Ocean [18], Arabian Sea [19,20] and ANTARES IV (ANTArctic RESearch), 1999 [21]) cruises, discussing the distribution of 3H, 14C, 90Sr and 129I in surface waters of the Indian Ocean.

3. Experimental methods 3.1. Samples Sampling in the North-western Pacific Ocean (IAEA0 97 cruise, Fig. 1) was carried out in the triangle Japan – Midway – Bikini/ Enewetak atolls with the aim to cover the main deposition area of global fallout radionuclides in the North-western Pacific Ocean [9]. Samples in the North Indian Ocean (Fig. 2) were collected in 1998 by revisiting the GEOSECS stations (1978) with the aim to

Fig. 1. Seawater sampling stations (IAEA´97 cruise) in the North-western Pacific Ocean (for exact position of sampling stations see [9]).

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(129I/127I = 1.174 ± 0.022) 1011). No background subtraction was required as the machine background was below 2  1014. Activity of a blank sample, prepared in a similar way as real samples was above the machine background only due to 129I contamination of water used for blank sample processing, therefore corrections were not applied. IAEA reference material, Irish Sea water [26], was analysed simultaneously with collected samples to ensure high quality of results. 4. Results and discussion 4.1. North-western Pacific Ocean

Fig. 2. Seawater sampling stations in the Indian Ocean ( – Italica cruise [18], G – revisited GEOSECS stations [19,20], o – ANTARES IV cruise [21]) with a simplified structure of the main surface currents (AF – Agulhas Front; STF – Subtropical Front; SAF – Subantarctic Front; PF – Polar Front (modified after [16,30])).

study the development of radionuclide concentrations with time [19]. Sampling in the South Indian Ocean was carried in the Crozet Basin between 32–48°S and 51–70°E, in the confluence of the AF, STF and SAF fronts (Fig. 2). Further samples were collected on transect from the Australian coast to the Red Sea [18]. Surface samples for radionuclide analysis were collected by pumping about 100 L of seawater from an average depth of 4 m. Samples for tritium, radiocarbon and 129I analyses were stored in 1L glass bottles with air-tight covering so no exchange with the surrounding air was possible. Radiocarbon samples were poisoned by adding mercury chloride to prevent any biological activity. 3.2. Analysis After filtering (0.45 lm mesh), onboard pre-concentration procedures were carried out to separate radionuclides from the collected water samples [22]. 90Sr was separated by co-precipitation with oxalic acid, and determined in IAEA-MEL by 90Y in-growth method followed by beta-counting. Tritium was measured either by the 3He in-growth method at the University of Miami (USA) or by liquid scintillation spectrometry (after an electrolytic enrichment) at the Institute of Geological and Nuclear Sciences (Lower Hutt, New Zealand). Tritium results are expressed in Tritium Units (1 TU = 118 m Bq L1 of water). Radiocarbon in the form of dissolved inorganic carbon was extracted either in the Arizona University AMS facility [1] or in IAEA-MEL [23] by acidification of the water sample to pH3. The released carbon dioxide was then (in a flow of high purity oxygen gas) collected in a trap cooled by liquid nitrogen. After purification the carbon dioxide was finally converted to graphite over a Fe catalyst. The radiocarbon activity in seawater samples is expressed by D14C defined as

D14 C ¼ ðF m  1Þ103 ; where Fm (a fraction of modern carbon) is the measured AMS ratio of 14C to 13C, normalised to D14C of 25‰, as defined in [24]. Iodine samples were prepared either in the IsoTrace Laboratory or in IAEAMEL following the procedure described in [25]. NaI carrier (2– 10 mg) was added to the sample, and after several steps AgI was obtained which was used as a target for AMS measurements carried out in the IsoTrace Laboratory. The AMS measurements were normalised with respect to ISOT-2 reference material

3 H, 14C, 90Sr and 129I water profiles observed in 1997 in the North-western Pacific Ocean are presented in Fig. 3. The subsurface 3 H maxima are seen at depths between 100 and 300 m with maximum levels between 1.2 and 1.4 TU. Tritium at stations close to Bikini and Enewetak atolls (St. 6 and 7, respectively) has penetration depths only down to about 500 m, while at the other stations it is down to about 1500 m. 14 C water profiles are more uniform than 3H profiles, having subsurface (between 100 and 300 m) maxima between 90 and 110‰ and minima at around 2000 m, with a slightly increasing down to the bottom. The surface concentrations decreased by 50–80‰, and increased by about the same amount in intermediate waters when compared with GEOSECS data [27]. In deep waters (below 1000 m) the observed radiocarbon concentrations were similar to GEOSECS values. As 90Sr is dissolved in water, its water profiles look like 3H profiles with maxima between 100 and 300 m and maximum concentrations between 1.7 and 2.2 m Bq L1, with penetration depths down to 500 (St. 6 and 7) and 1000 m (the other stations). 129 I water profiles differ substantially between the stations. The highest level (0.3  108 atom L1) was observed at Enewetak atoll (St. 7) at 200 m (0.2  108 atom L1 at Bikini atoll). Both stations show a concentration minimum at around 500 m, and a secondary maximum at about 1000 m. The open ocean stations (St. 2 and 3) show a smooth decrease down to 1000 m, and then negligible levels down to the bottom. Although the global fallout inventory of 129 I is estimated to be by about 2 orders of magnitude lower than that from nuclear reprocessing (Table 2 and 3), the presented 129I profiles may demonstrate a local impact of nuclear weapons testing carried out on the Enewetak and Bikini atolls during the early fifties. Since iodine is strongly associated with organic matter, 129 I concentration can also be influenced by transport of organic material. However, the investigated area is oligotrophic, with low biological production. Unfortunately, we can not compare the 129I profiles with historical data due to absence of previous measurements.

4.2. Indian Ocean 4.2.1. North Indian Ocean Fig. 4 shows surface distributions of 3H, 14C, 90Sr and 129I with latitude in the Indian Ocean. The 3H values are much higher than expected decay corrected values from the GEOSECS programme [28]. Observed 3H and 90Sr concentrations are comparable with surface values found in the North-western Pacific Ocean (Fig. 3). The surface D14C values are between 30‰ and 40‰, by about a factor of two lower than the corresponding values in the North-western Pacific (Fig. 3), and similarly lower than the observed values in the GEOSECS programme, reflecting the vertical transport of anthropogenic carbon in the Arabian Sea [29]. Although latitudinal variations in anthropogenic radionuclide concentrations related to global fallout deposition have been re-

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1.50 St. 1 St. 2b St. 2 St. 3 St. 4

0.75

Sr (mBq L-1)

1.00

St. 5 St. 6 St. 7 St. 8 St. 9

3

H (TU)

1.25

90

0.50 0.25 1000

2000 Depth (m)

200

St. 1

St. 5

St. 2b

St. 6 St. 7 St. 8 St. 9

St. 2 St. 3 St. 4

0

4000

-100

129

Δ

14

C (o/oo)

100

3000

I (108 atoms/L)

0.00 0

St. 1 St. 3 St. 6

St. 7 St. 8

1000 2000 3000 4000 5000 6000 7000 Depth (m) St. 2 St. 3 St. 6 (Bikini) St. 7 (Enewetak)

0.28 0.23 0.18 0.13 0.08

-200 -300 0

2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 0

1000

2000

3000

4000

5000

0.03 0

6000

1000

2000

Fig. 3. Tritium,

14

C,

90

Sr and

129

3000

4000

5000

6000

Depth (m)

Depth (m)

I in the water column of the North-western Pacific Ocean (sampling stations as in Fig. 1).

0.10 1.6

120 0.09 100

1.2

-1

I (10 atom L )

0.6

0.07

129

60

Δ

14

0.8

0.08

8

80

C (‰)

1.0

0.4

3

14

90

129

H Sr

3

90

-1

H (TU); Sr (mBq L )

1.4

0.2

C I

40

0.06

20

0.05

0.0 -50

-40

-30

-20

-10

0

10

20

30

Latitude Fig. 4. Tritium,

14

C,

90

Sr and

129

I concentrations in surface water of the Indian Ocean vs. latitude.

ported in several oceanic studies, e.g. [6,9], the observed southern Hemisphere latitudinal effect is much weaker than in the northern Hemisphere. As most of nuclear weapons tests with global radionuclide deposition were carried out in the northern Hemisphere, and due to the specific stratosphere–troposphere mixing, the distinct latitudinal variations of the integrated global fallout density have been observed mainly in the northern Hemisphere [10]. Following the global deposition patterns, higher radionuclide concentrations are expected at sampling areas around 45°S, with decreasing values to the Equator and the South Pole. However, the latitude distribution of radionuclides (Fig. 4) follows only partially these patterns, as they have been considerably influenced by current systems of the Indian Ocean.

4.2.2. South Indian Ocean The distribution of radionuclide concentrations in surface waters reveals a strong latitudinal variation defined by the water fronts. The surface 3H and 90Sr concentrations at stations situated between 32 and 43°S are similar, as they are influenced by the AF. These tendencies change from the south of 43°S, where the STF dominates, and especially south of 46°S, where decreasing radionuclide levels were observed due to the influence of SAF. It is clear that the observed 3H and 90Sr levels do not follow the global deposition pattern, as they were significantly influenced by the water fronts. Surface 3H, 14C, 90Sr and 129I concentrations observed in the North-western Pacific in 1997 (Fig. 3) are comparable with the

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Subtropical gyre data (at 20 and 40°S) in the South Indian Ocean (Fig. 4). We may conclude that higher radionuclide concentrations observed there are associated with Subtropical gyre which acts as a reservoir of radionuclides transported from the North-western Pacific Ocean via Indonesian Seas to the Indian Ocean. 5. Conclusions By comparing recent radionuclide (3H, 14C, 90Sr and 129I) levels in the North-western Pacific Ocean with those observed in the North and South Indian Ocean it has been found that they are of common origin as these radionuclides were mostly transported from the North-western Pacific via the Indonesian Seas to the Indian Ocean. The radionuclide variations with latitude observed in the Indian Ocean are not due to deposition patterns of global fallout, but due to different water fronts present in the region. Higher radionuclide concentrations observed in the South Indian Ocean in the latitudinal belt of 20–40°S are associated with Subtropical gyre which acts as a reservoir of radionuclides. Acknowledgements We acknowledge an assistance of colleagues participating in water sampling and in pre-treatment of samples. This work was carried out in the framework of the IAEA international project Worldwide Marine Radioactivity Studies (WOMARS), and collaboration with several laboratories in Denmark, France, Germany, India, Italy, Japan, Republic of Korea, New Zealand, Sweden, UK and USA is greatly acknowledged. The International Atomic Energy Agency is grateful to the Government of the Principality of Monaco for support provided to its Marine Environment Laboratories.

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