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| Fig. 1: A diver drilling a coral core from a Porites coral. (Source: Wikimedia Commons) |
Corals are highly important keystone species upon which much marine biodiversity occurs. One of the reasons for this dependence is the formation of habitat that coral provides. Sclerectinia corals are reef-builders which precipitate calcium carbonate to form hard skeletons which form the complex structures that comprise coral reefs. Because corals are colonial organisms capable of producing both sexually and asexually, a single coral colony can be extremely long-lived. As old polyps die, new polyps bud and take their place as a colony continues to grow. Because of this, the limits on colony lifespan are primarily environmental disturbance. Massive bouldering Porites spp. can grow extremely large and act as living records of geochemical history spanning hundreds of years with resolution on the scale of a month. [1] Researchers use underwater drills to collect calcium carbonate cores created by these corals (Fig. 1). Fossilized Porites provide records as old as 10 million years. [2]
I-129 is a long-lived fission product produced anthropogenically and environmentally released from nuclear bomb testing, nuclear fuel reprocessing, and nuclear accidents. Because it is long-lived it is an effective tracer in the marine environment and I-129 to I-127 isotopic ratios in coral cores provide valuable data. While it is produced naturally, the inventory of pre-anthropogenic I-129 (1,640 GBq) has been dwarfed by release from nuclear bomb testing, airborne reprocessing facility release and liquid reprocessing facility release contributing 617, 5,146, and 27,410 GBq. The Chernobyl and Fukushima accidents also released 39 and 8 GBq, respectively. [3] These events can be identified in the isotopic ratios of coral cores from the South China Sea (Parola Atoll). Two Porites spp. coral cores show a pre-nuclear-testing I-129/I-127 ratio ≤ 5 × 10-12. A peak of bomb-released I-129/I-127 (33 × 10-12) is identifiable in 1962, the year with the highest recorded amount of I-129 from nuclear testing. Peaks are identifiable in correlation to other events in nuclear testing, most notably reaching a peak of 37 × 10-12 in 1986 (the year of the Chernobyl Reactor accident). Coral cores from the Pacific Ocean (Baler) showed a 9-11 year lag, suggesting that while that corals in the South China Sea are more influenced by atmospheric transport and corals in the Pacific Ocean are more influenced by prevailing ocean currents. Isotopic ratios of I-129/I-127 were measured using inductively coupled plasma mass spectrometry and AgI accelerator mass spectrometry analysis. Age models were constructed using x-rays of annual growth rings. [3]
In more stable deep sea environments, individual living coral colonies have been dated as old as 4,265 years old, with an average of 970 years. Proteinaceous deep-sea corals such as Leiopathes spp. and Gerardia spp. still uptake carbon (Fig. 2). While C-14 does occur naturally, anthropogenic activities have led to an increase of bomb C-14, produced by atmospheric nuclear testing. This very same 4,265 year old coral colony has been analyzed to show a record of bomb C-14 that is traceable to atmospheric testing beginning in 1957. Skeletons grown prior to 1950 had a ΔC-14 value of less than −50‰ relative to the 1950 standard, which quickly jumps to +150‰ by 1970. [4] Taking a 1.176 × 10−12 value of C-14/C-12 from the 1950 reference, this correlates to 1.1172 × 10−12 C-14/C-12, which increases to 1.3524 × 10−12 C-14/C-12. Interestingly, the skeleton reflects the same uptake of C-14 as that of surface corals with little to no delay. This suggests that the primary mode of C-14 transport to the depths is not ocean circulation which could take years, but instead particulate and sediment fall, which occurs on the scale of weeks. Other radionuclides, such as Sr-90, Pu-239, and Pu-240, can similarly be measured to peak in the coral record after nuclear testing begins, with minor variation depending on geography. [5]
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| Fig. 2: A Leiopathes spp. deep sea coral. (Source: Wikimedia Commons) |
There is little research into the effect of radionuclides on coral, aside from that of tracer uptake in skeletal structures. It is likely that radionuclide deposition in the ocean becomes so diluted as to have little to no physiological effect on coral, aside from very concentrated hotspots due to nuclear testing or waste release. Bikini Atoll was a major testing site for the U.S. nuclear program. Between 1946 and 1958 23 nuclear weapons tests were conducted at the atoll. In 1946- 1947 and 1950, the U.S. The Department of the Interior conducted a geological survey which included an in-depth analysis of the coral community composition on Bikini Atoll and the neighboring Marshall Islands. There are complexities in comparison due to changes in coral taxonomy as a result of cryptic species and normal taxonomic evolution, but an analysis published in 2008 found remarkable strong recovery of the coral biodiversity in the more than 50 years since testing began. While there was a comparable amount of biodiversity, there was a genuine loss of 28 species (accounting for species lost to taxonomic synonymy) from the assemblage at Bikini Atoll, the majority of which were lagoon species. Due to the testing of nuclear bombs, the Atoll sustained significant physical damage. One of the largest Bikini tests, Bravo was a 15 megaton detonation of a shallow reef in 1954 which destroyed three islands and millions of tons of material to become airborne. Coral, which host photosynthesizing algae that provide the majority of their energy, are particularly vulnerable to sedimentation. The effects of this were likely centered inside of the lagoon, where there is less current to sweep away sediments and replace clouded waters. Bikini Atoll has, on the other hand, been protected from common anthropogenic coral threats that other reefs face more recently, such as fishing and tourism, as a result of these very same tests. The authors do concede that the natural stochasticity of coral reef assemblages provides a factor of uncertainty, and it is difficult to attribute ecological drift to any specific cause. [6]
© Max Shen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. B. Dunbar et al., "Eastern Pacific Sea Surface Temperature Since 1600 A.D.: The δ18O Record of Climate Variability in Galpagos Corals," Paleoceanog. Paleoclimatol. 9, 291 (1994).
[2] T. Brachert et al., "Porites Corals From Crete (Greece) Open a Window into Late Miocene (10 Ma) Seasonal and Interannual Climate Aariability," Earth Planet. Sci. Lett. 245, 81 (2006).
[3] A. T. Bautista VII, H. Matsuzaki, and F. P. Siringan, "Historical Record of Nuclear Activities From 129I in Corals From the Northern Hemisphere (Philippines)," J. Environ. Radioact. 164, 174 (2016).
[4] E. Roark et al., "Extreme Longevity in Proteinaceous Deep-Sea Corals," Proc. Natl. Acad. Sci. (USA) 106, 5204 (2009).
[5] C. Purdy, E. Druffel, and H. Livingston, "Anomalous Levels of 90Sr and 239,240Pu in Florida Corals: Evidence of Coastal Processes," Geochim. Cosmochim. Acta 53, 1401 (1989).
[6] Z. T. Richards et al., "Bikini Atoll Coral Biodiversity Resilience Five Decades After Nuclear Testing," Mar. Pollut. Bull. 56, 503 (2008).
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