- Email: [email protected]
Two 60-year records of 129I from coral skeletons in the South Pacific Ocean
592
Two 60-year records of 129I from coral skeletons in the South Pacific Ocean D.L. Biddulph* , J.W. Beck, G.S. Burr, D.J. Donahue Department of Physics and NSF-Arizona AMS Facility, The University of Arizona, Tucson, AZ 85721, USA Abstract 129 I is an important radionuclide tracer for certain natural and anthropogenic nuclear processes. 129 I has a half-life of 15.7 Myr and can be measured by accelerator mass spectrometry (AMS). This paper presents 129 I results made at
the University of Arizona with a NEC 3 MV Pelletron accelerator. For this study, we selected living corals from the Solomon Islands and Easter Island to monitor increases in anthropogenic 129 I in the surface waters of the Pacific Ocean. 129 I/127 I values were measured in cores taken from massive Porites head coral skeletons. Typical sample sizes for this study were 10 g for the Solomon Islands corals, and 30 g for the Easter Island corals. Temporal resolution was semi-annual at the Solomon Islands, and annual at Easter Island. Iodine was extracted from the corals without the use of carrier iodine. Results of our study produced records at both sites from roughly 1935 to 1996. While 129 I/127 I values have increased at both locations since the beginning of atmospheric nuclear weapons testing, different bomb-pulse curves at these sites suggest different transport mechanisms and/or 129 I inputs at the two sites. The implications of these measurements are discussed below. Keywords: 129 I, Accelerator mass spectrometry, Corals
1. Introduction Coral skeletons provide an archive of the chemical and physical conditions present in the surface waters of the ocean that existed when the material was deposited. Reef building corals live in shallow waters, generally within the upper 50 to 75 meters, and border islands or continental margins that bracket the latitudes 35◦ N to 30◦ S (Druffel, 1997). Coral records have been used extensively to study past changes in ocean circulation, ocean chemistry and climate. The analysis of uranium, thorium and 14 C in corals has been used to extend the atmospheric calibration curve used for 14 C dating beyond the dendrochronological limit of ∼ =12,400 years before present (Stuiver et al., 1998). The use of coral skeletons as a record of radioiodine has some advantages over other environmental iodine reservoirs, including: enhanced time resolution due to relatively rapid deposition (10 to 20 mm/year), the absence of mixing processes that commonly affect sediments * Corresponding author. Address: 1118 E. 4th St. PAS 81, Tucson, AZ 85721, USA; phone: (+520) 621 6825; fax: (+520) 626 9348; e-mail: [email protected]
RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08047-2
© 2006 Elsevier Ltd. All rights reserved.
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
593
(such as bioturbation), and the existence of dating techniques (Edwards et al., 1987), to establish reliable chronologies for particular specimens. For living corals, annual bands preserved in their skeletons can be counted for age identification. Ages of older corals can be determined by measuring the parent/daughter ratio of 234 U and 230 Th (Edwards et al., 1987). The purpose of this study is to assess the possibility of using coral skeletons as reliable recorders of the 129 I content of surface ocean waters during the time of skeletal formation. In order for this to work, the iodine must exist in substantial quantities (at least the ppb level) and be immobile within the coral skeleton once incorporated.
2. Site selection and sampling Both sampling sites were originally chosen as part of radiocarbon studies that looked at dissolved inorganic carbon (DIC) in surface ocean waters (Schmidt et al., 2004; Beck, 2004). The Solomon Islands skeletal core was retrieved in September 1994 from Marau Sound on the eastern tip of Guadalcanal, latitude 9.5◦ S and longitude 162◦ E. This location proves interesting for 129 I studies as it is presumably far from any point sources of anthropogenic iodine. In addition, it is located in the Southern Hemisphere where few anthropogenic iodine studies have been done to date. Easter Island is located at the center of the South Pacific gyre, latitude 27◦ S and longitude 109◦ W. Also far removed from anthropogenic sources of 129 I, this location was sampled as a comparative study site for the core drilled from the Solomons. This skeletal core was retrieved while the coral colony was still active, in the fall of 1996. The coral samples were obtained with an underwater hydraulic drill that was equipped with an 8 cm diameter bit. Cores were drilled perpendicular to the coral growth surface. A 5 mm slab was cut from each core and X-rayed to identify annual growth bands. These bands were marked for 129 I sampling in order to obtain a continuous, integrated record. The Solomon Islands core was sampled along growth bands for semiannual resolution, while larger samples were used for the Easter Island core to obtain annual resolution. The chronology for the Easter Island core was obtained by counting annual density bands where they were clearly identifiable in the X-ray record (the latest 15 years or so). Afterward, a linear growth rate was assumed. This chronology was then checked to ensure agreement with that obtained by counting seasonal cycles in the δ 18 O record from the same core (Beck, 2004; Mucciarone and Dunbar, 2003). The Solomon Islands chronology was initially determined by counting annual density bands only (Biddulph, 2004), but was subsequently adjusted to fit both the δ 18 O record and the 129 I data that was obtained from Easter Island.
3. Sample preparation To remove external contamination, the coral samples were acid washed and sonicated for 45 minutes, then dissolved in a five to one solution of distilled water and 85% H3 PO4 , with an acid to coral ratio of 2.1 ml per gram of sample. This leaves the solution with a low pH, which is required to oxidize existing I− ions to I2 molecules. Particulates were removed by filtration through a 0.22 µm cellulose acetate membrane. After filtration, the solution was poured into
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a separatory funnel with the addition of 5–10 ml of CHCl3 and one drop of 1 M NaNO2 . The solution was shaken for one minute, and time was allotted for the CHCl3 plus I2 mixture to settle to the bottom of the funnel. This oxidation and extraction process was repeated three times, or until no color change due to dissolved I2 was noticeable in the CHCl3 . At this point, we took advantage of the fact that silver powder has an affinity for I2 molecules (Yiou et al., 2004). The CHCl3 plus I2 mixture was drained into a glass vial containing 10–15 mg of 120 mesh Ag powder. Within a couple of hours the I2 is transferred from the CHCl3 to the Ag powder, and this transfer is evident as the CHCl3 loses its purple hue and the Ag powder darkens in color. Upon completion, the CHCl3 was allowed to evaporate in a fume hood, and the Ag plus I2 mixture was rinsed three times in distilled H2 O and allowed to dry. This material was then pressed into an aluminum cathode for AMS measurement. Carrier iodine was not used for target preparation. A 50 ml aliquot of solution was saved for each coral sample for stable iodine measurements with an iodide selective electrode. The measured iodine content in the Solomon Islands coral samples varied little between individual samples from a mean of (3.4 ± 0.1) ppm. The biophile nature of iodine is evident in these samples, as this iodine concentration is elevated nearly 100 times with respect to the iodine content of surface seawater (50 ppb). Stable iodine measurements of the Easter Island samples are pending, but results from chemical extraction and ion source current yields suggest that the concentrations are similar to those measured from the Solomon Islands.
4. AMS measurements The AMS measurements were performed at The University of Arizona with a NEC 3 MV Pelletron accelerator. Nominal 127 I− ion source currents produced from the coral samples were in the 500–1000 nA range. Measurements were made in the +5 charge state, at a terminal voltage of 2.75 MV with a transmission of 4.0%. Each sample was measured six times, and the weighted average of the measurements was calculated. Errors include statistical uncertainties and a random machine error of 4.3%. A blank correction factor of 129 I/127 I = (1.0 ± 0.1) × 10−12 due to the chemical extraction and filtration process was applied to all coral samples. A more detailed description of the AMS measurements performed at this facility has been published previously (Biddulph et al., 2000; Biddulph, 2004).
5. Results and discussion The results of the radioiodine measurements for both skeletal cores are plotted together in Fig. 1. Both records show equivalent pre-bomb pulse levels of 129 I/127 I ≈ 5 × 10−13 , confirming the expectation of ocean homogeneity given the very long half-life of 129 I (15.7 Ma) relative to an ocean mixing time of roughly 1000 years (Fabryka-Martin et al., 1984). Approximately half of this natural 129 I comes from atmospheric spallation reactions involving cosmic rays and 129 Xe, with the rest coming from fission of natural uranium. 129 I increases at similar rates in both records from the late 1950s to about 1970 in consequence of fallout from atmospheric nuclear weapons testing which disseminated large
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
595
Fig. 1. 129 I/127 I values from coral skeletons in the South Pacific Ocean.
amounts of fissiogenic 129 I throughout the atmosphere. Whereas atmospheric nuclear testing virtually ceased in 1963 as a result of the Nuclear Test–Ban Treaty, 129 I levels in both records increase continuously at the same linear rate between 1963 and 1970. Beginning in 1970 129 I/127 I values remained nearly unchanged in both records, until 1975, when the Solomon Islands record began to rise at a rapid rate. In contrast, 129 I/127 I in the Easter Island record has remained nearly constant, or has risen only slightly since 1975. The response asymmetry between the Solomon Islands site and Easter Island suggests that the large signal observed at Guadalcanal after 1975 resulted from one or more point source releases of radioiodine into the ocean mixed layer. We believe that at least some of the rapid rise observed after 1975 in the Solomon Islands record may be due to contaminated wastewater releases into the Columbia River from the Hanford nuclear fuels reprocessing center in the state of Washington (Heeb et al., 1996). The signal could have arrived at the Solomons by propagating southward via the coastal California Current system into the North Equatorial Current which in turn helps to feed the Pacific Equatorial Countercurrent, some of which passes the vicinity of the Solomon Islands. Weak cross-equatorial advection of these water masses would explain the much-attenuated post-1975 radioiodine signal reaching Easter Island, which is located near the center of the South Pacific gyre. This being stated, we must emphasize the difficulties involved in locating specific iodine sources responsible for the signals seen in the South Pacific. Other possible sources may include nuclear test remnants from regional island testing in the 1970s, or point discharge sources from the eastern Asian coastline. In contrast to the behavior exhibited after 1975, both records exhibit virtually identical behavior between 1958 and 1970. Because these two sites have strongly dissimilar hydrographic settings, we could not explain this coherence if 129 I were being transported from a point source via ocean water masses. Instead an atmospheric transport pathway must be invoked, which can produce a fairly homogeneous 129 I distribution. The residence time of iodine in the troposphere is quite short, however (ca. two weeks) (Kocher, 1981), which
596
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would not only lead to a heterogeneous deposition pattern, but we could not also explain the decade long period of coherence exhibited in these records between 1960 and 1970. Thus, most of the 129 I deposited in the Pacific Ocean mixed-layer during this time interval may have been stratospheric bomb pulse fallout, where iodine could have a much longer residence time. Using 36 Cl as an analogue for bomb 129 I, we would expect the stratospheric residence time to be on the order of two to four years (Synal et al., 1990; Elmore et al., 1982). However, such a short residence time would not produce the nearly linear rate of increase in 129 I/127 I observed for the period 1963 to 1970. On the other hand, if the residence time were longer, we would not be able to explain the period between 1970 and 1975 during which 129 I/127 I was not increasing in either record. This somewhat puzzling finding suggests that there was likely more than one source of atmospheric 129 I during this period. One of these must have been stratospheric fallout from bomb testing, but the other is less certain. Tropospheric 129 I emissions from Hanford, Sellafield and La Hague were substantial during this time interval (Heeb et al., 1996; Wagner et al., 1996; Lopez-Gutierrez et al., 2004), and it is possible that these releases conspired to mask the exponential decay in the stratospheric fallout signal expected from bomb 129 I. Additional records may help clarify this situation. It is instructive to compare these plots with their radiocarbon records, Fig. 2 (Schmidt et al., 2004; Beck, 2004). In striking contrast to the 129 I records, the 14 C values for the waters around Easter Island are elevated roughly 25% as compared to the Solomon Island waters. These results are consistent with ocean surface water measurements (Linick, 1980; Key et al., 2004) of 14 C, which show a north–south 14 C gradient with maxima at roughly 30◦ N and 30◦ S, and a minimum in the equatorial regions. The contrast between the radiocarbon and radioiodine curves at these two sites is a clear indication of different sources and transport mechanisms for these two anthropogenic isotopes.
Fig. 2. 14 C values from the same corals as in Fig. 1.
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
597
6. Conclusion Coral skeletons preserve an iodine record in the tropical surface waters of the ocean. Once deposited within the skeleton, the iodine is fixed and may be retrieved with straightforward chemical extraction techniques. Comparison of anthropogenic iodine concentrations in skeletons from the Solomon Islands and Easter Island suggests different input functions and transport mechanisms for these two locations. Additional radioiodine records of corals at alternate locations should aid in the understanding of these transport phenomena, and may help pinpoint the locations of the most important point sources of the signals being observed. Coral skeletons, which are common throughout the Pacific, represent a valuable resource for understanding anthropogenic inputs and the transport of iodine in surface ocean currents of the world’s oceans.
References Beck, J.W. (2004). Unpublished data, personal correspondence. Physics Department, The University of Arizona. Biddulph, D.L. (2004). PhD dissertation. The University of Arizona. Biddulph, D.L., Beck, J.W., Burr, G.S., Donahue, D.J., Hatheway, A.L., Jull, A.J.T. (2000). Measurement of the radioisotope 129 I at the NSF-Arizona AMS laboratory. Nuclear Instruments and Methods in Physics Research B 172, 693–698. Druffel, E.R.M. (1997). Geochemistry of corals: Proxies of past ocean chemistry, ocean circulation, and climate. Proceedings of the National Academy of Sciences USA 94, 8354–8361. Edwards, R.L., Chen, J.H., Wasserburg, G.J. (1987). 238 U–234 U–230 Th–232 Th systematics and the precise measurement of time over the past 500,000 years. Earth and Planetary Science Letters 81, 175–192. Elmore, D., Tubbs, L.E., Newman, D., Ma, X.Z., Finkle, R.C., Nishiizumi, K., Beer, J., Oeschger, H., Andree, M. (1982). The 36 Cl bomb pulse measured in a shallow ice core from Dye 3, Greenland. Nature 300, 735–737. Fabryka-Martin, J., Bentley, H., Elmore, D., Airey, P.L. (1984). Natural iodine-129 as an environmental tracer. Geochimica et Cosmochimica Acta 49, 337–347. Heeb, C.M., Gydesen, S.P., Simpson, J.C., Bates, D.J. (1996). Reconstruction of radionuclide releases from the Hanford Site, 1944–1972. Health Physics 71, 545–555. Key, R.M., Kozyr, A., Sabine, C.L., Lee, K., Wanninkhof, R., Bullister, J.L., Feely, R.A., Millero, F.J., Mordy, C., Peng, T.-H. (2004). A global ocean carbon climatology: Results from GLODAP. Global Biogeochemical Cycles, in press. Kocher, D.C. (1981). A dynamic model of the global iodine cycle and estimation of dose to the world population from releases of iodine-129 to the environment. Environmental International 5, 15–31. Linick, T.W. (1980). Radiocarbon 22 (3), 599. Lopez-Gutierrez, J.M., Santos, F.J., Garcia-Leon, M., Schnabel, Ch., Synal, H.-A., Ernst, Th., Szidat, S. (2004). Levels and temporal variability of 129 I concentrations and 129 /127 I isotopic ratios in atmospheric samples from southern Spain. Nuclear Instruments and Methods in Physics Research B 223/224, 495–500. Mucciarone, D., Dunbar, H. (2003). Stable isotope record of El Niño-Southern Oscillation events from Easter Island. In: Loret, J., Tanacredi, J.T. (Eds.), Easter Island, Scientific Exploration into the World’s Environmental Problems in Microcosm. Kluwer Academic/Plenum Publishers, New York, NY, pp. 113–132. Schmidt, A., Burr, G.S., Taylor, F.W., O’Malley, J., Beck, J.W. (2004). A semiannual radiocarbon record of a modern coral from the Solomon Islands. Nuclear Instruments and Methods in Physics Research B 223/224, 420–427. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., Van Der Plicht, J., Spurk, M. (1998). Intcal 98 radiocarbon age calibration, 24,000–0 cal BP. Radiocarbon 40 (3), 1041–1083. Synal, H.A., Beer, J., Bonani, G., Suter, M., Wölfli, W. (1990). Atmospheric transport of bomb-produced 36 Cl. Nuclear Instruments and Methods in Physics Research B 52, 483–488.
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Wagner, M.J.M., Dittrich-Hannen, B., Synal, H.-A., Suter, M., Schotterer, U. (1996). Increase of 129 I in the environment. Nuclear Instruments and Methods in Physics Research B 113, 490–494. Yiou, F., Raisbeck, G., Imbaud, H. (2004). Extraction and AMS measurement of carrier free 129 I/127 I from seawater. Nuclear Instruments and Methods in Physics Research B 223/224, 412–415.
Two 60-year records of 129I from coral skeletons in the South Pacific Ocean D.L. Biddulph* , J.W. Beck, G.S. Burr, D.J. Donahue Department of Physics and NSF-Arizona AMS Facility, The University of Arizona, Tucson, AZ 85721, USA Abstract 129 I is an important radionuclide tracer for certain natural and anthropogenic nuclear processes. 129 I has a half-life of 15.7 Myr and can be measured by accelerator mass spectrometry (AMS). This paper presents 129 I results made at
the University of Arizona with a NEC 3 MV Pelletron accelerator. For this study, we selected living corals from the Solomon Islands and Easter Island to monitor increases in anthropogenic 129 I in the surface waters of the Pacific Ocean. 129 I/127 I values were measured in cores taken from massive Porites head coral skeletons. Typical sample sizes for this study were 10 g for the Solomon Islands corals, and 30 g for the Easter Island corals. Temporal resolution was semi-annual at the Solomon Islands, and annual at Easter Island. Iodine was extracted from the corals without the use of carrier iodine. Results of our study produced records at both sites from roughly 1935 to 1996. While 129 I/127 I values have increased at both locations since the beginning of atmospheric nuclear weapons testing, different bomb-pulse curves at these sites suggest different transport mechanisms and/or 129 I inputs at the two sites. The implications of these measurements are discussed below. Keywords: 129 I, Accelerator mass spectrometry, Corals
1. Introduction Coral skeletons provide an archive of the chemical and physical conditions present in the surface waters of the ocean that existed when the material was deposited. Reef building corals live in shallow waters, generally within the upper 50 to 75 meters, and border islands or continental margins that bracket the latitudes 35◦ N to 30◦ S (Druffel, 1997). Coral records have been used extensively to study past changes in ocean circulation, ocean chemistry and climate. The analysis of uranium, thorium and 14 C in corals has been used to extend the atmospheric calibration curve used for 14 C dating beyond the dendrochronological limit of ∼ =12,400 years before present (Stuiver et al., 1998). The use of coral skeletons as a record of radioiodine has some advantages over other environmental iodine reservoirs, including: enhanced time resolution due to relatively rapid deposition (10 to 20 mm/year), the absence of mixing processes that commonly affect sediments * Corresponding author. Address: 1118 E. 4th St. PAS 81, Tucson, AZ 85721, USA; phone: (+520) 621 6825; fax: (+520) 626 9348; e-mail: [email protected]
RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08047-2
© 2006 Elsevier Ltd. All rights reserved.
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
593
(such as bioturbation), and the existence of dating techniques (Edwards et al., 1987), to establish reliable chronologies for particular specimens. For living corals, annual bands preserved in their skeletons can be counted for age identification. Ages of older corals can be determined by measuring the parent/daughter ratio of 234 U and 230 Th (Edwards et al., 1987). The purpose of this study is to assess the possibility of using coral skeletons as reliable recorders of the 129 I content of surface ocean waters during the time of skeletal formation. In order for this to work, the iodine must exist in substantial quantities (at least the ppb level) and be immobile within the coral skeleton once incorporated.
2. Site selection and sampling Both sampling sites were originally chosen as part of radiocarbon studies that looked at dissolved inorganic carbon (DIC) in surface ocean waters (Schmidt et al., 2004; Beck, 2004). The Solomon Islands skeletal core was retrieved in September 1994 from Marau Sound on the eastern tip of Guadalcanal, latitude 9.5◦ S and longitude 162◦ E. This location proves interesting for 129 I studies as it is presumably far from any point sources of anthropogenic iodine. In addition, it is located in the Southern Hemisphere where few anthropogenic iodine studies have been done to date. Easter Island is located at the center of the South Pacific gyre, latitude 27◦ S and longitude 109◦ W. Also far removed from anthropogenic sources of 129 I, this location was sampled as a comparative study site for the core drilled from the Solomons. This skeletal core was retrieved while the coral colony was still active, in the fall of 1996. The coral samples were obtained with an underwater hydraulic drill that was equipped with an 8 cm diameter bit. Cores were drilled perpendicular to the coral growth surface. A 5 mm slab was cut from each core and X-rayed to identify annual growth bands. These bands were marked for 129 I sampling in order to obtain a continuous, integrated record. The Solomon Islands core was sampled along growth bands for semiannual resolution, while larger samples were used for the Easter Island core to obtain annual resolution. The chronology for the Easter Island core was obtained by counting annual density bands where they were clearly identifiable in the X-ray record (the latest 15 years or so). Afterward, a linear growth rate was assumed. This chronology was then checked to ensure agreement with that obtained by counting seasonal cycles in the δ 18 O record from the same core (Beck, 2004; Mucciarone and Dunbar, 2003). The Solomon Islands chronology was initially determined by counting annual density bands only (Biddulph, 2004), but was subsequently adjusted to fit both the δ 18 O record and the 129 I data that was obtained from Easter Island.
3. Sample preparation To remove external contamination, the coral samples were acid washed and sonicated for 45 minutes, then dissolved in a five to one solution of distilled water and 85% H3 PO4 , with an acid to coral ratio of 2.1 ml per gram of sample. This leaves the solution with a low pH, which is required to oxidize existing I− ions to I2 molecules. Particulates were removed by filtration through a 0.22 µm cellulose acetate membrane. After filtration, the solution was poured into
594
D.L. Biddulph et al.
a separatory funnel with the addition of 5–10 ml of CHCl3 and one drop of 1 M NaNO2 . The solution was shaken for one minute, and time was allotted for the CHCl3 plus I2 mixture to settle to the bottom of the funnel. This oxidation and extraction process was repeated three times, or until no color change due to dissolved I2 was noticeable in the CHCl3 . At this point, we took advantage of the fact that silver powder has an affinity for I2 molecules (Yiou et al., 2004). The CHCl3 plus I2 mixture was drained into a glass vial containing 10–15 mg of 120 mesh Ag powder. Within a couple of hours the I2 is transferred from the CHCl3 to the Ag powder, and this transfer is evident as the CHCl3 loses its purple hue and the Ag powder darkens in color. Upon completion, the CHCl3 was allowed to evaporate in a fume hood, and the Ag plus I2 mixture was rinsed three times in distilled H2 O and allowed to dry. This material was then pressed into an aluminum cathode for AMS measurement. Carrier iodine was not used for target preparation. A 50 ml aliquot of solution was saved for each coral sample for stable iodine measurements with an iodide selective electrode. The measured iodine content in the Solomon Islands coral samples varied little between individual samples from a mean of (3.4 ± 0.1) ppm. The biophile nature of iodine is evident in these samples, as this iodine concentration is elevated nearly 100 times with respect to the iodine content of surface seawater (50 ppb). Stable iodine measurements of the Easter Island samples are pending, but results from chemical extraction and ion source current yields suggest that the concentrations are similar to those measured from the Solomon Islands.
4. AMS measurements The AMS measurements were performed at The University of Arizona with a NEC 3 MV Pelletron accelerator. Nominal 127 I− ion source currents produced from the coral samples were in the 500–1000 nA range. Measurements were made in the +5 charge state, at a terminal voltage of 2.75 MV with a transmission of 4.0%. Each sample was measured six times, and the weighted average of the measurements was calculated. Errors include statistical uncertainties and a random machine error of 4.3%. A blank correction factor of 129 I/127 I = (1.0 ± 0.1) × 10−12 due to the chemical extraction and filtration process was applied to all coral samples. A more detailed description of the AMS measurements performed at this facility has been published previously (Biddulph et al., 2000; Biddulph, 2004).
5. Results and discussion The results of the radioiodine measurements for both skeletal cores are plotted together in Fig. 1. Both records show equivalent pre-bomb pulse levels of 129 I/127 I ≈ 5 × 10−13 , confirming the expectation of ocean homogeneity given the very long half-life of 129 I (15.7 Ma) relative to an ocean mixing time of roughly 1000 years (Fabryka-Martin et al., 1984). Approximately half of this natural 129 I comes from atmospheric spallation reactions involving cosmic rays and 129 Xe, with the rest coming from fission of natural uranium. 129 I increases at similar rates in both records from the late 1950s to about 1970 in consequence of fallout from atmospheric nuclear weapons testing which disseminated large
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
595
Fig. 1. 129 I/127 I values from coral skeletons in the South Pacific Ocean.
amounts of fissiogenic 129 I throughout the atmosphere. Whereas atmospheric nuclear testing virtually ceased in 1963 as a result of the Nuclear Test–Ban Treaty, 129 I levels in both records increase continuously at the same linear rate between 1963 and 1970. Beginning in 1970 129 I/127 I values remained nearly unchanged in both records, until 1975, when the Solomon Islands record began to rise at a rapid rate. In contrast, 129 I/127 I in the Easter Island record has remained nearly constant, or has risen only slightly since 1975. The response asymmetry between the Solomon Islands site and Easter Island suggests that the large signal observed at Guadalcanal after 1975 resulted from one or more point source releases of radioiodine into the ocean mixed layer. We believe that at least some of the rapid rise observed after 1975 in the Solomon Islands record may be due to contaminated wastewater releases into the Columbia River from the Hanford nuclear fuels reprocessing center in the state of Washington (Heeb et al., 1996). The signal could have arrived at the Solomons by propagating southward via the coastal California Current system into the North Equatorial Current which in turn helps to feed the Pacific Equatorial Countercurrent, some of which passes the vicinity of the Solomon Islands. Weak cross-equatorial advection of these water masses would explain the much-attenuated post-1975 radioiodine signal reaching Easter Island, which is located near the center of the South Pacific gyre. This being stated, we must emphasize the difficulties involved in locating specific iodine sources responsible for the signals seen in the South Pacific. Other possible sources may include nuclear test remnants from regional island testing in the 1970s, or point discharge sources from the eastern Asian coastline. In contrast to the behavior exhibited after 1975, both records exhibit virtually identical behavior between 1958 and 1970. Because these two sites have strongly dissimilar hydrographic settings, we could not explain this coherence if 129 I were being transported from a point source via ocean water masses. Instead an atmospheric transport pathway must be invoked, which can produce a fairly homogeneous 129 I distribution. The residence time of iodine in the troposphere is quite short, however (ca. two weeks) (Kocher, 1981), which
596
D.L. Biddulph et al.
would not only lead to a heterogeneous deposition pattern, but we could not also explain the decade long period of coherence exhibited in these records between 1960 and 1970. Thus, most of the 129 I deposited in the Pacific Ocean mixed-layer during this time interval may have been stratospheric bomb pulse fallout, where iodine could have a much longer residence time. Using 36 Cl as an analogue for bomb 129 I, we would expect the stratospheric residence time to be on the order of two to four years (Synal et al., 1990; Elmore et al., 1982). However, such a short residence time would not produce the nearly linear rate of increase in 129 I/127 I observed for the period 1963 to 1970. On the other hand, if the residence time were longer, we would not be able to explain the period between 1970 and 1975 during which 129 I/127 I was not increasing in either record. This somewhat puzzling finding suggests that there was likely more than one source of atmospheric 129 I during this period. One of these must have been stratospheric fallout from bomb testing, but the other is less certain. Tropospheric 129 I emissions from Hanford, Sellafield and La Hague were substantial during this time interval (Heeb et al., 1996; Wagner et al., 1996; Lopez-Gutierrez et al., 2004), and it is possible that these releases conspired to mask the exponential decay in the stratospheric fallout signal expected from bomb 129 I. Additional records may help clarify this situation. It is instructive to compare these plots with their radiocarbon records, Fig. 2 (Schmidt et al., 2004; Beck, 2004). In striking contrast to the 129 I records, the 14 C values for the waters around Easter Island are elevated roughly 25% as compared to the Solomon Island waters. These results are consistent with ocean surface water measurements (Linick, 1980; Key et al., 2004) of 14 C, which show a north–south 14 C gradient with maxima at roughly 30◦ N and 30◦ S, and a minimum in the equatorial regions. The contrast between the radiocarbon and radioiodine curves at these two sites is a clear indication of different sources and transport mechanisms for these two anthropogenic isotopes.
Fig. 2. 14 C values from the same corals as in Fig. 1.
Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean
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6. Conclusion Coral skeletons preserve an iodine record in the tropical surface waters of the ocean. Once deposited within the skeleton, the iodine is fixed and may be retrieved with straightforward chemical extraction techniques. Comparison of anthropogenic iodine concentrations in skeletons from the Solomon Islands and Easter Island suggests different input functions and transport mechanisms for these two locations. Additional radioiodine records of corals at alternate locations should aid in the understanding of these transport phenomena, and may help pinpoint the locations of the most important point sources of the signals being observed. Coral skeletons, which are common throughout the Pacific, represent a valuable resource for understanding anthropogenic inputs and the transport of iodine in surface ocean currents of the world’s oceans.
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