In this article, we present a study of seawater 129I/127I time-series data from several coastal sites in Taiwan, including Yehliu Geopark (north–northeast coast), Kaohsiung (southwest coast), and Zhuwei Fishing Village (northwest coast). The objective of this study was to document how 129I/127I responds to known seasonal variations in the surface ocean currents that carry 129I to each of these sites. The responses were shown to be quite distinct. The Zhuwei site, across Mainland China, had elevated spring and summer 129I/127I values, with abrupt peaks that reflected transient cross-strait currents that carry seawater with elevated 129I to the northwest coast of Taiwan. The Yehliu site, which receives all of its seawater from the Kuroshio Current, had relatively low and uniform 129I/127I values year-round. The Kaohsiung site showed a summertime minimum that likely stems from upwelling upstream induced by oceanic eddies associated with the Kuroshio intrusion. To our knowledge, these are the first continuous coastal 129I/127I time-series data published for Asian waters and the first to show large, abrupt, and regional 129I/127I changes in the surface seawater. We also documented 129I/127I values from multiple surface ocean sites in the South China Sea (SCS), including a vertical profile from the South East Asia Time-Series Station that extends to a depth of 3,700 m. The 129I from both coastal Taiwan and surface waters of the SCS is >98% anthropogenic, primarily released to the environment as a byproduct of nuclear fuel reprocessing.

Introduction

Anthropogenic radioisotopes produced by fission of uranium or plutonium are characteristic markers of the Anthropocene (Zalasiewicz et al., 2015). Among these, 129I occupies a special niche because unlike other fission products, it continues to be released to the environment as a by-product of nuclear fuel reprocessing, and the amount of 129I in the environment continues to grow. These releases are permitted because 129I is a relatively benign isotope with a very long half-life (15.7 × 106 year), low specific activity, and relatively low-energy decay products (Edwards, 1962). The first large-scale releases of anthropogenic 129I into the environment occurred in the late 1950s and early 1960s from aboveground nuclear weapons testing. These produced an estimated 90 ± 45 kg of 129I compared to a prenuclear marine inventory of about 130 kg (Snyder et al., 2010) produced naturally from fission of 238U and by interactions between cosmic rays and Xe in the atmosphere (Edwards, 1962; Fabryka-Martin et al., 1985). Aboveground nuclear weapons testing produced peak oceanic 129I/127I concentrations preserved in banded corals (Biddulph et al., 2006; Bautista et al., 2016; Chang et al., 2016; Sakaguchi et al., 2018) and in marine sediments (Fan et al., 2016; Zhao et al., 2019). In the ensuing years, after the era of atmospheric weapons testing, these marine archives also document a second, and much larger, anthropogenic source of 129I.

This second source comes primarily from nuclear fuel reprocessing plants that recover economic isotopes from spent nuclear fuel rods. On a global scale, the two most significant 129I sources to date are the Sellafield (England) and La Hague (France) commercial nuclear fuel reprocessing plants (Raisbeck and Yiou, 1999; Reithmeier et al., 2006; Hou et al., 2009; Hu et al., 2010b). The amount of 129I in the world’s oceans was recently estimated to have reached 7,310 kg (Chen et al., 2016). This estimate implies that as much as 98% of the 129I in the oceans is anthropogenic and that approximately only 1% of that anthropogenic 129I came from atmospheric nuclear weapons testing. Hence, the amount of 129I in the oceans has increased by about a factor of 60 from a prenuclear value of 130 kg, to 7,310 kg. According to the International Atomic Energy Agency (IAEA), from the onset of nuclear power production in 1954 to 2013, approximately 370,000 metric tons of heavy metal (t HM) in spent fuel has been consumed, and about one-third of that (120,300 t HM) has been reprocessed (IAEA, 2018). Most of the remaining spent fuel is stored at the 236 nuclear power stations where it was produced, and the 129I it contains has not yet been released to the environment (Hou et al., 2009; Hu et al., 2010b).

The increase in the amount of 129I in the oceans over the past 70 years makes clear that the rate of iodine removal by natural sinks is slower than the rate of its release by reprocessing plants. Anthropogenic marine 129I measurements from around the world also show that neither the shallow ocean nor the deep ocean is in equilibrium with 129I (Snyder et al., 2010). As a small subset of this global picture, we examined how seawater 129I values change with time in the vicinity of Taiwan in 2018 and 2019, with an emphasis on seasonal variations seen in time-series data collected regularly from several sites around the island.

Iodine is a very mobile element in nature that forms few minerals; its geochemistry is dominated by oceanic and atmospheric transport and storage (Fuge and Johnson, 1986). Iodine in seawater occurs as dissolved iodide, iodate, organic iodine, and particulate iodine (Wong, 1991; Santschi and Schwer, 2004). The concentration of dissolved iodine in seawater is about 60 µg L−1, and although iodine is a very mobile element, 129I can be used as a semiconservative −tracer in seawater to determine current pathways, transit times, and water mass mixing. A number of tracer studies of this type have been conducted in the North Atlantic Ocean and the Arctic Ocean, downstream from the release sites at Sellafield and La Hague (Raisbeck and Yiou, 1999; Alfimov et al., 2004; Smith et al., 2005), and in recent years, such studies have been refined by combining 129I with other anthropogenic isotopes (i.e., 137Cs, 99Tc, and 236U) to better discriminate between point sources and to determine more accurate transit times (Smith et al., 1998, 2011; Yiou et al., 2002; Christl et al., 2015; Casacuberta et al., 2018).

Tracer studies rely on time-series data to determine pathways, rates, and deposition of anthropogenic isotopes. Marine 129I time series can be derived from seawater (Chang et al., 2019), seaweed (Raisbeck and Yiou, 1999; Yiou et al., 2002), corals (Biddulph et al., 2006; Bautista et al., 2016; Chang et al., 2016; Sakaguchi et al., 2018), sediments (Fan et al., 2016; Zhao et al., 2019), or models (Alfimov et al., 2006; Smith et al., 2011). Seaweed and corals allow for time series with sub-annual to annual resolution. The time resolution of seawater samples is limited only by the rate at which samples are collected, which can be tailored to meet specific goals. Here, we chose bimonthly (every 2 weeks) resolution in order to document seasonal 129I variations. This seasonal variability complements 129I/127I time-series studies from corals, with annual resolution.

Study area

Open ocean sites in the South China Sea (SCS) were sampled in 2018, and coastal time-series sample collection began in 2018 or 2019 depending on the site. Three sites were investigated: (1) Yehliu Geopark (25.205N, 121.692E), (2) Kaohsiung (22.624N, 120.263E), and (3) Zhuwei Fishing Village (25.121N, 121.252E; Figure 1). None of these sites is located near a major river. The Zhuwei Fishing Village and Yehliu Geopark sites are only 45 km apart, but their seawater sources are very different. The Zhuwei site faces the northern end of the Taiwan Strait where it connects to the East China Sea (ECS), while the Yehliu site faces the Pacific Ocean. The Kaohsiung site, on the campus of National Sun Yat-Sen University (Figure S1), also faces the Taiwan Strait but on the southwest coast of Taiwan (Figure 1). Open ocean sites were sampled during a cruise of the R/V Legend (LGD-T11) from July 4 to 12, 2018. These included a number of surface sites (5-m depth) and a vertical profile from the South East Asia Time-series Study (SEATS) site (Figure 1).

Figure 1.

Seawater sampling sites. Open ocean sites were sampled from July 4 to 12, 2018. Map template from the Ocean Data View program (Schlitzer, 2020). DOI: https://doi.org/10.1525/elementa.2020.064.f1

Figure 1.

Seawater sampling sites. Open ocean sites were sampled from July 4 to 12, 2018. Map template from the Ocean Data View program (Schlitzer, 2020). DOI: https://doi.org/10.1525/elementa.2020.064.f1

Figure 2 shows the surface ocean currents that affect the coastal sites selected for this study. The most dominant current affecting Taiwan is the Kuroshio Current, a western boundary current of the North Pacific Ocean, which flows from south to north along the east coast of the Philippines and then along the east coast of Taiwan. The Kuroshio Current has a mean geostrophic volume transport of approximately 15 Sv (1 Sverdrup = 106 m3 s−1) as it passes the northern tip of Luzon Island, with maximum transport in spring and minimum transport in fall (Qiu, 2019). When the Kuroshio Current passes this point, a portion of its waters enters the SCS through the Luzon Strait (Chu and Li, 2000; Hu et al., 2000). This portion is known as the Kuroshio intrusion. The Kuroshio intrusion can follow several pathways after it diverts westward from the main stream of the Kuroshio Current: (1) along the shelf break and to the southwest, (2) northward through the Penghu Channel (the Kuroshio Loop Current extension), or (3) toward the northeast, where it rejoins the main Kuroshio stream (Hu et al., 2000, 2010a; Qiu et al., 2011). Nan et al. (2011) defined a Kuroshio SCS index (KSI) based on geostrophic vorticity to quantify analogous intrusion pathways as a leaking path, looping path, or leaping path. Although the Kuroshio intrusion can follow any of these paths at any time, the leaking and looping paths are most probable in autumn, winter, and spring, and the leaping path is most probable in summer (Nan et al., 2015). This seasonality is associated with the Southeast Asian monsoon. Regardless of the path, the Kuroshio intrusion contributes to the seasonal flow of water through the Taiwan Strait and is responsible for its northward flow year-round. Estimates of the westward volume transport of Kuroshio water through the Luzon Strait range from approximately 3 Sv to >10 Sv (Nan et al., 2015), while the northward volume transport through the Taiwan Strait is approximately 2.3 Sv in summer and approximately 0.8 Sv in winter (Hu et al., 2010a).

Figure 2.

Surface ocean currents that carry water to the coastal sites under study. Blue dots show coastal sampling locations. Currents include the Kuroshio Current, the Kuroshio Loop Current, the Kuroshio Loop Current extension, the South China Sea Warm Current, and the China Coastal Current. The dashed trajectory shows a transient path known to occur in winter and spring. Current trajectories are from Hu et al. (2010a) and Qiu et al. (2011). Map template from the Ocean Data View program (Schlitzer, 2020). DOI: https://doi.org/10.1525/elementa.2020.064.f2

Figure 2.

Surface ocean currents that carry water to the coastal sites under study. Blue dots show coastal sampling locations. Currents include the Kuroshio Current, the Kuroshio Loop Current, the Kuroshio Loop Current extension, the South China Sea Warm Current, and the China Coastal Current. The dashed trajectory shows a transient path known to occur in winter and spring. Current trajectories are from Hu et al. (2010a) and Qiu et al. (2011). Map template from the Ocean Data View program (Schlitzer, 2020). DOI: https://doi.org/10.1525/elementa.2020.064.f2

Methods

Total iodine

The method we describe below extracts dissolved iodine from seawater; in this study, “total iodine” and “total 129I/127I” are operationally defined as the iodine extracted using our technique. As noted above, dissolved iodine in seawater occurs in three forms: inorganic iodide (I), inorganic iodate ($IO3−$), and organic iodine. Our method extracts iodide, iodate, and organic species where iodine is loosely bound, such as methyl iodide. Many sensitive methods have been devised to extract and quantify individual dissolved inorganic 129I species (Hou et al., 1997) and dissolved organic species (Schwehr and Santschi, 2003; Zhang et al., 2010). These provide valuable tools to understand 129I pathways and their kinetics. However, in this study, we are concerned only with the bulk anthropogenic 129I carried in the seawater. Exchange between various species within a parcel of water will not change the total amount of anthropogenic iodine within it.

We also do not consider stable (127I) iodine concentrations because stable iodine is very uniform in the surface ocean (Fuge and Johnson, 1986; Wong, 1991; Chance et al., 2014). In one large study, for example, surface waters from a vast area of the Pacific (>20°N, 137–163°E) were shown to vary by less than 3% (Tsunogai and Henmi, 1971). This level of variation falls within the errors of our 129I/127I measurements and cannot explain variations larger than a few percentage.

Sampling

Surface water from open ocean sites was obtained during a cruise of the R/V Legend (LGD-T11) from July 4 to 12, 2018 (Figure 1). Samples for an 129I/127I depth profile were collected at the SEATS site (18.120°N, 116.052°E) on July 8, 2018. The depth profile samples were collected with Teflon-lined 12-L Go-Flo bottles mounted on a conductivity-temperature-depth/Rosette frame, which also measured fluorescence (Chlorophyll a) with depth. Upon recovery of the Go-Flo bottles, each open ocean sample was filtered through a 0.22-µm Polycap filter cartridge into a 1-L low-density polyethylene bottle. Surface water samples at 5-m depth were collected from a Teflon pipe in the wet lab on deck using a diaphragm pump. All the water samples were kept under 4°C before shipment to the laboratory. Two liters of seawater were collected from each coastal site (Figure S1) at 2-week intervals using 1-L high-density polyethylene bottles.

Analysis and uncertainties

Coastal seawater samples were filtered in the laboratory with 0.22-µm nylon membrane filters. Iodine was extracted from the filtered seawater in 1-L batches using the following steps (Chang et al., 2019): (1) the sample was placed in a 1-L separatory funnel, (2) it was acidified to pH < 2 using phosphoric acid (H3PO4), (3) 2.5 mL of 0.48 M sodium bisulfite (NaHSO3) was added and allowed to react overnight, (4) 5.4 mL of 1.0 M sodium nitrite (NaNO2) was added to the sample, and (5) I2 was extracted using chloroform in a separatory funnel as an immiscible solvent. Steps 4 and 5 were repeated 3 times to assure complete removal of the iodine produced using 0.4 mL of NaNO2 in each successive extraction. The chloroform was decanted into 20-mL vials with about 10 mg of silver powder, which reacted with the I2 to form AgI. The chloroform was allowed to evaporate, and the AgI was rinsed repeatedly and dried. No carrier was added to the sample. The reactions that occur during each step of the extraction are given in the Supplemental Material (Text S1) along with a photograph of the chloroform extraction step (Figure S2).

The accelerator mass spectrometric measurements were performed at the Micro Analysis Laboratory, Tandem Accelerator (MALT) facility, University Museum, University of Tokyo, using established measurement protocols (Matsuzaki et al., 2015). AgI samples were mixed with Nb powder and pressed into aluminum target holders for accelerator mass spectrometric analyses. The measurements were conducted at a terminal voltage of 3.48 MV, and particles in the 5+ charge state were selected. 129I/127I values were calculated relative to the S-Purdue 2 standard with a nominal 129I/127I ratio of 6.54 × 10−11. The machine blank for 129I/127I at the MALT facility is below 1.5 × 10−14. Uncertainties were calculated based on counting statistics (Burr et al., 2007) using the number of counts observed in the samples and standards. Machine instabilities were also taken into account.

Chlorophyll a was measured using a fluorescence meter mounted as an auxiliary detector on an SBE 9 plus conductivity-temperature-depth profiling recorder (Seabird Scientific) with a detection resolution of 1 datum/s. The fluorescence technique followed standard protocols for the method (Roesler et al., 2017). The instrument was calibrated at the Taiwan Ocean Research Institute immediately prior to the cruise. Chlorophyll a values and their uncertainties are given in Table S1 based on about 500 measurements at each depth interval.

Results and discussion

We report three types of 129I/127I results: (1) open ocean surface water (5-m depth; Table 1), (2) a vertical profile from the SEATS site (Table 2), and (3) surface seawater time-series results from the coastal sites (Table 3). All of the measured values are given in the tables along with site details, collection dates, and number of 129I particles detected. The Kaohsiung time-series results were reported previously in the Proceedings of the 8th East Asia AMS Symposium (Burr et al., 2020) and are included here to facilitate comparison.

Table 1.

Surface ocean 129I/127I results for samples at 5-m depth from the South China Sea. DOI: https://doi.org/10.1525/elementa.2020.064.t1

Lab (Site) IDLocation (°N, °E)Date Collected129I/127I × 10−12 (± 1σ)129I Particles Detected (n)
NTU 24 (TS a) 22.833, 120.057 July 4, 2018 62.50 ± 6.96 115
NTU 25 (LZ a) 20.831, 119.306 July 5, 2018 52.23 ± 2.84 474
NTU 26 (LZ b) 20.016, 119.009 July 5, 2018 47.36 ± 2.67 460
NTU 31 (DS) 20.518, 116.461 July 10, 2018 46.60 ± 1.46 1,917
NTU 32 (NSCS) 21.873, 117.587 July 11, 2018 50.97 ± 2.34 646
NTU 33 (TS b) 22.419, 120.228 July 12, 2018 53.24 ± 3.00 408
Lab (Site) IDLocation (°N, °E)Date Collected129I/127I × 10−12 (± 1σ)129I Particles Detected (n)
NTU 24 (TS a) 22.833, 120.057 July 4, 2018 62.50 ± 6.96 115
NTU 25 (LZ a) 20.831, 119.306 July 5, 2018 52.23 ± 2.84 474
NTU 26 (LZ b) 20.016, 119.009 July 5, 2018 47.36 ± 2.67 460
NTU 31 (DS) 20.518, 116.461 July 10, 2018 46.60 ± 1.46 1,917
NTU 32 (NSCS) 21.873, 117.587 July 11, 2018 50.97 ± 2.34 646
NTU 33 (TS b) 22.419, 120.228 July 12, 2018 53.24 ± 3.00 408

NTU = National Taiwan University; DS = Dongsha Island; LZ = Luzon Strait; NSCS = Northern South China Sea; TS = Taiwan Strait.

Table 2.

129I/127I depth profile results from the Southeast Asian Time-series Study site (18.120°N, 116.052°E) collected July 8, 2018, in the South China Sea. DOI: https://doi.org/10.1525/elementa.2020.064.t2

Lab IDDepth (m)129I/127I × 10−12 (± 1σ)129I particles Detected (n)
NTU 34 3,700 1.67 ± 0.26 72
NTU 35 3,500 6.04 ± 0.48 246
NTU 36 3,000 1.19 ± 0.15 99
NTU 37 2,500 1.19 ± 0.14 125
NTU 38 2,000 1.23 ± 0.14 138
NTU 39 1,500 2.22 ± 0.27 121
NTU 40 1,000 7.44 ± 0.48 498
NTU 41 800 12.03 ± 0.38 1,508
NTU 42 600 18.23 ± 0.65 1,244
NTU 43 400 29.05 ± 2.23 370
NTU 44 200 44.04 ± 1.21 2,424
NTU 45 100 43.11 ± 1.35 1,978
NTU 46 65 42.97 ± 1.56 1,916
NTU 47 10 50.82 ± 1.51 2,061
Lab IDDepth (m)129I/127I × 10−12 (± 1σ)129I particles Detected (n)
NTU 34 3,700 1.67 ± 0.26 72
NTU 35 3,500 6.04 ± 0.48 246
NTU 36 3,000 1.19 ± 0.15 99
NTU 37 2,500 1.19 ± 0.14 125
NTU 38 2,000 1.23 ± 0.14 138
NTU 39 1,500 2.22 ± 0.27 121
NTU 40 1,000 7.44 ± 0.48 498
NTU 41 800 12.03 ± 0.38 1,508
NTU 42 600 18.23 ± 0.65 1,244
NTU 43 400 29.05 ± 2.23 370
NTU 44 200 44.04 ± 1.21 2,424
NTU 45 100 43.11 ± 1.35 1,978
NTU 46 65 42.97 ± 1.56 1,916
NTU 47 10 50.82 ± 1.51 2,061

NTU = National Taiwan University.

Table 3.

129I/127I results from coastal sites in Taiwan. DOI: https://doi.org/10.1525/elementa.2020.064.t3

Lab IDDate Collected129I/127I × 10−12 (± 1σ)129I Particles Detected (n)
Kaohsiung (22.624°N, 120.263°E)a
NTU 88 May 26, 2019 44.94 ± 1.91 628
NTU 87 June 9, 2019 48.07 ± 2.05 622
NTU 90 June 25, 2019 44.01 ± 1.65 835
NTU 103 July 9, 2019 41.48 ± 2.28 356
NTU 105 July 22, 2019 38.52 ± 2.11 359
NTU 106 August 13, 2019 41.48 ± 2.28 639
NTU 108 August 28, 2019 42.42 ± 1.10 2,165
NTU 109/110 September 12, 2019 44.02 ± 1.58 2,665
Yehliu Geopark (25.205°N, 121.692°E)
NTU 69 March 26, 2019 60.42 ± 3.06 496
NTU 75 April 9, 2019 45.70 ± 1.34 1,537
NTU 78 April 23, 2019 46.80 ± 2.62 342
NTU 80 May 20, 2019 53.92 ± 1.53 1,691
NTU 82 June 4, 2019 40.68 ± 2.77 226
NTU 84 June 16, 2019 45.62 ± 1.88 628
NTU 89 June 29, 2019 49.67 ± 2.11 629
NTU 92 July 16, 2019 46.13 ± 1.91 669
NTU 94 July 22, 2019 43.97 ± 2.22 428
NTU 95 July 29, 2019 51.52 ± 2.84 353
NTU 97 August 12, 2019 54.27 ± 1.98 887
NTU 100 August 26, 2019 47.16 ± 1.82 776
NTU 101 September 9, 2019 52.43 ± 2.53 618
NTU 112 September 23, 2019 50.90 ± 1.18 3,473
NTU 114 October 7, 2019 50.61 ± 1.25 2,817
NTU 115 October 21, 2019 44.93 ± 1.06 3,242
NTU 118 November 4, 2019 48.52 ± 2.85 313
Zhuwei Fishing Village (25.088°N, 121.176°E)
NTU 48 November 6, 2018 49.87 ± 2.80 1,725
NTU 49 November 25, 2018 47.72 ± 1.87 346
NTU 51 January 14, 2019 42.53 ± 1.57 2,004
NTU 54 January 28, 2019 53.50 ± 2.71 716
NTU 57 March 1, 2019 61.36 ± 7.00 100
NTU 62 March 18, 2019 82.06 ± 6.59 225
NTU 74 April 3, 2019 69.68 ± 3.03 743
NTU 77 April 19, 2019 117.10 ± 2.57 3,734
NTU 79 May 13, 2019 52.71 ± 1.54 1,554
NTU 81 May 26, 2019 61.89 ± 2.14 1,020
NTU 83 June 10, 2019 66.94 ± 4.12 1,375
NTU 85 June 24, 2019 112.83 ± 3.54 1,309
NTU 91 July 8, 2019 51.91 ± 2.18 648
NTU 93 July 21, 2019 52.63 ± 2.39 542
NTU 96 August 10, 2019 58.30 ± 2.57 576
NTU 99 August 18, 2019 58.35 ± 2.19 835
NTU 101 September 3, 2019 46.81 ± 2.00 618
NTU 111 September 17, 2019 45.09 ± 1.38 1,449
NTU 113 September 30, 2019 47.32 ± 1.34 1,806
NTU 116 October 16, 2019 51.32 ± 1.70 1,184
NTU 117 October 28, 2019 52.84 ± 1.33 1,972
Lab IDDate Collected129I/127I × 10−12 (± 1σ)129I Particles Detected (n)
Kaohsiung (22.624°N, 120.263°E)a
NTU 88 May 26, 2019 44.94 ± 1.91 628
NTU 87 June 9, 2019 48.07 ± 2.05 622
NTU 90 June 25, 2019 44.01 ± 1.65 835
NTU 103 July 9, 2019 41.48 ± 2.28 356
NTU 105 July 22, 2019 38.52 ± 2.11 359
NTU 106 August 13, 2019 41.48 ± 2.28 639
NTU 108 August 28, 2019 42.42 ± 1.10 2,165
NTU 109/110 September 12, 2019 44.02 ± 1.58 2,665
Yehliu Geopark (25.205°N, 121.692°E)
NTU 69 March 26, 2019 60.42 ± 3.06 496
NTU 75 April 9, 2019 45.70 ± 1.34 1,537
NTU 78 April 23, 2019 46.80 ± 2.62 342
NTU 80 May 20, 2019 53.92 ± 1.53 1,691
NTU 82 June 4, 2019 40.68 ± 2.77 226
NTU 84 June 16, 2019 45.62 ± 1.88 628
NTU 89 June 29, 2019 49.67 ± 2.11 629
NTU 92 July 16, 2019 46.13 ± 1.91 669
NTU 94 July 22, 2019 43.97 ± 2.22 428
NTU 95 July 29, 2019 51.52 ± 2.84 353
NTU 97 August 12, 2019 54.27 ± 1.98 887
NTU 100 August 26, 2019 47.16 ± 1.82 776
NTU 101 September 9, 2019 52.43 ± 2.53 618
NTU 112 September 23, 2019 50.90 ± 1.18 3,473
NTU 114 October 7, 2019 50.61 ± 1.25 2,817
NTU 115 October 21, 2019 44.93 ± 1.06 3,242
NTU 118 November 4, 2019 48.52 ± 2.85 313
Zhuwei Fishing Village (25.088°N, 121.176°E)
NTU 48 November 6, 2018 49.87 ± 2.80 1,725
NTU 49 November 25, 2018 47.72 ± 1.87 346
NTU 51 January 14, 2019 42.53 ± 1.57 2,004
NTU 54 January 28, 2019 53.50 ± 2.71 716
NTU 57 March 1, 2019 61.36 ± 7.00 100
NTU 62 March 18, 2019 82.06 ± 6.59 225
NTU 74 April 3, 2019 69.68 ± 3.03 743
NTU 77 April 19, 2019 117.10 ± 2.57 3,734
NTU 79 May 13, 2019 52.71 ± 1.54 1,554
NTU 81 May 26, 2019 61.89 ± 2.14 1,020
NTU 83 June 10, 2019 66.94 ± 4.12 1,375
NTU 85 June 24, 2019 112.83 ± 3.54 1,309
NTU 91 July 8, 2019 51.91 ± 2.18 648
NTU 93 July 21, 2019 52.63 ± 2.39 542
NTU 96 August 10, 2019 58.30 ± 2.57 576
NTU 99 August 18, 2019 58.35 ± 2.19 835
NTU 101 September 3, 2019 46.81 ± 2.00 618
NTU 111 September 17, 2019 45.09 ± 1.38 1,449
NTU 113 September 30, 2019 47.32 ± 1.34 1,806
NTU 116 October 16, 2019 51.32 ± 1.70 1,184
NTU 117 October 28, 2019 52.84 ± 1.33 1,972

aAs reported at the 8th East Asia Accelerator Mass Spectrometry (AMS) Symposium (Burr et al., 2020).

The SCS is the largest marginal sea in Southeast Asia with water depths of more than 5,000 m and a very broad continental shelf along its western border. As the basin is largely restricted and the climate is monsoonal, the surface circulation in the SCS is driven by the monsoon, with a seasonal reversal of surface current directions (Wyrtki, 1961). The mean circulation of the upper layer (0–200 m) of the SCS is cyclonic in winter and anticyclonic in summer (Hu et al., 2000). Circulation is also dependent on the Kuroshio intrusion as discussed above. Most of the seawater that replenishes the SCS ultimately comes from the Kuroshio Current, with a net westward flow through the Luzon Strait (Hu et al., 2010a; Nan et al., 2015). In addition to seasonal variations, both the Kuroshio intrusion and Kuroshio Current exhibit significant interannual variability in terms of pathways and volume transport, and the Kuroshio intrusion causes the formation of mesoscale eddies that promote vertical mixing, especially in winter (Caruso et al., 2006; Hu et al., 2010a; Nan et al., 2015; Qiu, 2019). As shown below, 129I/127I is very sensitive to vertical mixing.

Open ocean results

The locations of the open ocean sites sampled for this study are shown in Figure 1. Taiwan Strait sites (TS a and TS b) receive water from the Kuroshio Loop Current extension in the Penghu Channel (Figure 2). The Dongsha Island site (DS) and Northern South China Sea site (NSCS) are situated in the path of the SCS Warm Current (Figure 2). The Luzon Strait sites (LZ a and LZ b) are located in the Luzon Strait, where water from the Kuroshio intrusion enters the SCS. Despite their hydrographic differences, the 129I/127I values at these surface water sites were statistically indistinguishable as shown in Figure S3. That is, all individual measured values overlap with the weighted mean for the group: 129I/127I = 49.1 ± 2.4 × 10−12 (2σ external variance) during the sample collection period from July 4 to 12, 2018. This average gives us a reference value for the northern SCS, upstream from the Taiwan Strait, to compare with our coastal sites further north, at Kaohsiung and Zhuwei Fishing Village. Our SCS surface ocean results are typical of the open ocean in the region and overlap with southeastern ECS values in the range of 20–60 × 10−12 reported elsewhere (Liu et al., 2016; Wang et al., 2019). They also overlap with surface water values measured at the SEATS site, discussed next.

Vertical profile from the SEATS site

The SEATS site is located in a deep basin, south of the open ocean sites (Figure 1). Results were obtained from 14 samples at progressive depths from 10 to 3,700 m (approximately 100 m above the seafloor) on July 8, 2018. 129I/127I values ranged from deep water values of approximately 1.2 × 10−12 to surface water values of approximately 50.8 × 10−12 (Table 2 and Figure 3). The deep water values are typical prenuclear marine values and agree with those reported by Yi et al. (2018) from a nearby site (Figure 3) and from several deep water sites off the northeast coast of Japan (Suzuki et al., 2010, 2018). Our SEATS profile shows that surface water 129I/127I values in 2019 were enriched by a factor of about 50 above SCS deep waters. Hence, 129I is a very sensitive upwelling indicator. The 129I/127I depth profile we observed is typical of marine depth profiles, with most of the 129I contained in the surface layer of the ocean. The profile observed by Yi et al. (2018), measured from a nearby site 4 years prior to our sampling, is very different with respect to depth in the water column. However, the overall shape of the two profiles is similar, and they have overlapping surface and deep water values. The interannual variability of mesoscale eddies associated with the Kuroshio intrusion north of the site could contribute to the vertical mixing observed in 2014. As these are the only two 129I/127I depth profiles currently available, conjecturing further would be premature.

Figure 3.

Depth profiles of 129I/127I and Chlorophyll a from the South East Asia Time-series Study (SEATS) site. (a) 129I/127I depth profile from the SEATS site (18.120°N, 116.052°E) sampled in 2018 for this study (solid line) and a profile from 2014 (dashed line) from a nearby site (18.004°N, 116.006°E) reported by Yi et al. (2018). (b) Chlorophyll a and 129I/127I results from the upper 500 m of the SEATS profile sampled on July 8, 2018. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f3

Figure 3.

Depth profiles of 129I/127I and Chlorophyll a from the South East Asia Time-series Study (SEATS) site. (a) 129I/127I depth profile from the SEATS site (18.120°N, 116.052°E) sampled in 2018 for this study (solid line) and a profile from 2014 (dashed line) from a nearby site (18.004°N, 116.006°E) reported by Yi et al. (2018). (b) Chlorophyll a and 129I/127I results from the upper 500 m of the SEATS profile sampled on July 8, 2018. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f3

An interesting feature in our record is an increase in 129I/127I (from 1.2 to approximately 6 × 10−12) near the seafloor at 3,500 m depth. This increase suggests remobilization of particulate iodine associated with sinking organic matter or particulate iodine. Based on the work of Brewer et al. (1980), Wong (1991) estimated that about 4% of the particulate iodine in surface ocean waters can settle to the deep ocean, and as much as 95% of that iodine can be dissolved at the water–sediment interface, providing a potential pathway to explain our observed deep water 129I signal (Figure 3). An inflection point is also observed in our 129I/127I profile between 65 and 200 m depth (Figure 3). Iodine is a biophilic element known to be scavenged by marine organisms, especially seaweed (Fuge and Johnson, 1986; Hou et al., 1997; Santschi and Schwer, 2004). Our measured peak Chlorophyll a concentrations (approximately 77 m depth) coincide with the 129I/127I inflection seen in our profile (Figure 3).

Bimonthly time series from coastal sites

Measured 129I/127I time series from Yehliu Geopark, Kaohsiung (Burr et al., 2020), and Zhuwei Fishing Village are given in Table 3. The 1σ precision of the measurements was <5% and improved as the study progressed with refinements to the sample processing protocol that included larger sample volumes (up to 2 L) and longer reaction times (overnight) after the addition of sulfite. With reference to the surface ocean currents summarized in Figure 2, we can follow the marine pathways of iodine in the vicinity of Taiwan. The most straightforward of these pathways involves the Yehliu site, which is dominated by the Kuroshio Current as it passes the northern tip of Taiwan. Over the entire observation period (March–November 2019), the Yehliu data are very consistent, with no seasonal trend. At two standard deviations, the average value varies by less than 4%, with a weighted average of 129I/127I = 48.61 ± 1.90 × 10−12 (2σ external variance). These low values reflect the large volume of water carried by the Kuroshio Current, which passes no significant anthropogenic 129I source in its path and is apt to inherit low-129I upwelled waters from its parent current, the North Equatorial Current, and from mesoscale eddies (Qiu, 2019).

Kaohsiung has a direct connection with the Kuroshio Current by way of the Kuroshio Loop extension that flows northward through the Penghu Channel along the southwestern coast of Taiwan (Figure 2). This region is the only part of the west coast of Taiwan where surface currents flow northward year-round, every year (Qiu et al., 2011). Upstream from Kaohsiung is an area where seasonal cold core eddies form, driven by the Kuroshio intrusion (Li et al., 1998). Cold core eddies induce vertical mixing and can dilute surface water 129I. This effect is what we observed during the summer of 2019 at Kaohsiung, where surface water 129I/127I dropped from 48.07 ± 4.10 × 10−12 on June 9 to 38.5 ± 4.2 × 10−12 (2σ) on July 22 during the summer monsoon. This minimum is well below our SCS surface water average value, 49.1 ± 2.4 × 10−12, and is the lowest value among seven consecutive measurements that define a smooth minimum over a 3-month period (Figure 4). On a longer time scale, our Kaohsiung time-series data are consistent with 129I/127I measurements from the same site, which ranged from approximately 36 to 53 × 10−12 between 2011 and 2015 (Chang et al., 2019).

Figure 4.

129I/127I time series from coastal Kaohsiung, Taiwan. 129I/127I time series Kaohsiung (open symbols) showing a summertime minimum at Kaohsiung. The shaded band shows the measured average 129I/127I value for South China Sea surface waters with 2σ uncertainties. The complete data set and locations are given in Table 3. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f4

Figure 4.

129I/127I time series from coastal Kaohsiung, Taiwan. 129I/127I time series Kaohsiung (open symbols) showing a summertime minimum at Kaohsiung. The shaded band shows the measured average 129I/127I value for South China Sea surface waters with 2σ uncertainties. The complete data set and locations are given in Table 3. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f4

At Zhuwei Fishing Village, we recorded the highest 129I/127I values of any site, as much as 2.4 times the average for Yehliu, which is only 45 km away (Figure 5). Clearly, this high level of 129I did not come from the direction of Yehliu because of the low values there, as well as the southern ECS in general (Liu et al., 2016). However, Liu et al. (2016) identified high 129I/127I values (up to approximately 200 × 10−12) in the Yangtze River Estuary and in coastal waters that extend to the south. The China coastal current flows toward the south, off the coast of Mainland China, and carries water into the Taiwan Strait. During the winter and spring, transient cross-strait surface flow can develop (Liao et al., 2018), shown as the dashed current trajectory in Figure 2. Hence, we have a known source and known pathway that can explain the two abrupt 129I/127I peaks at Zhuwei that we observed, and the relatively higher spring and summer values as mixtures of water from the China Coastal Current and surface seawater from the south. Although Zhuwei is located at the narrowest stretch of the Taiwan Strait (130 km), surface water need not flow directly across to arrive there. Water can cross at any point to the north or south, because although coastal waters along the northwest coast of Taiwan generally flow northward, they have also been observed to flow to the south at times (Qiu et al., 2011). Our 129I results demonstrate that 129I can serve as a useful indicator of this cross-strait flow, which was strongest in the spring and summer of 2019 at Zhuwei Fishing Village.

Figure 5.

Comparison of 129I/127I time series from coastal sites at Yehliu and Zhuwei Fishing Village, Taiwan. 129I/127I time series from Yehliu Geopark (circles) and Zhuwei Fishing Village (diamonds). The complete data set and locations are given in Table 3. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f5

Figure 5.

Comparison of 129I/127I time series from coastal sites at Yehliu and Zhuwei Fishing Village, Taiwan. 129I/127I time series from Yehliu Geopark (circles) and Zhuwei Fishing Village (diamonds). The complete data set and locations are given in Table 3. Error bars are 2σ. DOI: https://doi.org/10.1525/elementa.2020.064.f5

Conclusions

In this study, we have documented surface seawater 129I/127I from a large area in the SCS and found uniform values that reflect well-mixed waters. We documented a vertical 129I/127I profile to 3,700 m depth from the SCS SEATS site that shows prenuclear values in deep waters, a typical diffusive curve, and an 129I/127I enrichment of approximately 50 times in surface waters compared to deep waters. We followed the path of anthropogenic 129I in ocean currents from the SCS to the east and west coasts of Taiwan. At Yehliu, all of the water comes from the Kuroshio Current and with no distinct seasonal effects. At Kaohsiung, we observed a summertime 129I minimum likely associated with seasonal cold core eddies induced by the Kuroshio intrusion. At Zhuwei Fishing Village, we observed high spring and summer 129I/127I values with sporadic short-lived 129I/127I peaks that record transient periods of cross-strait flow with water likely coming from the China Coastal Current. To our knowledge, these data are the first continuous coastal 129I/127I time series published for Asian waters and the first to show abrupt changes in surface water 129I/127I in the coastal waters of Taiwan. In addition to revealing such local changes, studies of this type, and of anthropogenic isotopes in general, are useful for quantifying and understanding human impacts that define the Anthropocene.

Data accessibility statement

A complete list of 129I/127I results cited in this study is included in the main body of the manuscript (Tables 13). Chlorophyll a data measured at the SEATS site are given in Table S1 of the Supplemental Material.

Supplemental files

Figure S1. Coastal sampling sites. Photographs of the three coastal sites studied in this article: (a) National Sun Yat-Sen University, Kaohsiung, (b) Yehliu Geopark, and (c) Zhuwei Fishing Village.

Figure S2. Extraction of iodine from seawater. Immiscible chloroform at the base of a separatory funnel with artificial seawater above. The purple color is from dissolved iodine from 20-µL methyl iodide dissolved in 1 L of artificial seawater.

Figure S3. Plot of 129I/127I values from open ocean sites in the South China Sea. The dashed line is the weighted average of these values, and the shaded error represents the 2σ uncertainty of the weighted average (49.1 ± 2.4 × 10−12). All samples overlap with the average within errors.

Table S1. Chlorophyll a results.

Text S1. Docx.

Acknowledgments

Sincere thanks to Chin-Wen Yang, at National Sun Yat-Sen University, who assisted with sample collection at Kaohsiung. We also thank the anonymous reviewers of our article who offered constructive criticisms that led to significant improvements in the final version. Many thanks to Professor Lin Li-Hung (NTU) and his many students for their assistance in the laboratory and technical advice. We are very grateful to the technical team and crew of the R/V Legend for their skillful support during the LGD-T11 cruise. We would also like to recognize the bus drivers of the number 1068 and 953 buses who provided reliable transportation and support.

Funding

We are grateful to the Taiwan Ministry of Science and Technology (MOST 107-2116-M-002-001-MY3) for funding, and funds cofinanced by the European Union and the State of Hungary in support of AJTJ, through the European Regional Development Fund project (GINOP-2.3.2-15-2016-00009 “ICER”).

Competing interests

The authors have no competing interests to declare.

Author contributions

Substantial contributions to conception and design: GB, HM, BSW, CCC, CHL.

Acquisition of data: GB, HM, BSW, HT, TY, SLW.

Analysis and interpretation of data: GB, HK, TLY.

Drafting the article or revising it critically for important intellectual content: GB, HM, BSW, AJTJ.

Final approval of the version to be published: GB, AJTJ.

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How to cite this article: Burr, George S, Matsuzaki, H, Wang, B-S, Kusuno, H, Tokuyama, H, Yamagata, T, Yu, T-L, Wang, S-L, Chang, C-C, Jull, AJT, Lo, C-H. 2020. Anthropogenic 129I in the South China Sea and coastal waters around Taiwan. Elem Sci Anth 8: 1. DOI: https://doi.org/10.1525/elementa.2020.064.

Domain Editor-in-Chief: Jody W. Deming, School of Oceanography, University of Washington, Seattle, WA, USA

Associate Editor: Yusuke Yokoyama, Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan

Knowledge Domain: Ocean Science

Part of an Elementa Special Feature: Pan-Pacific Anthropocene

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