Coral-Based Approaches to Paleoclimate Studies, Future Ocean Environment Assessment, and Disaster Research
Global warming causes serious harm to the Earth’s environment. A more sophisticated and accurate climate model can be developed by reconstructing climatic change since the Industrial Revolution and for other past periods of global warming. Coral skeletons are an important archive of past climate changes, and advances in the ability to read sea surface temperature and salinity in the coral record have been made by applying state-of-the-art technology. Coral skeletal climatology has been successfully applied to characterize both the recent global warming trend in the Western Pacific and the mid-Pliocene warming that occurred 3.5 million years ago, and it has also been used to investigate biological and environmental issues such as ocean acidification and coral bleaching, which is caused by unusually high seawater temperatures. Coral skeletal climatology methods have also been used to study Porites boulders cast ashore by historical tsunamis; such studies have high social value from the perspective of regional disaster prevention. Nevertheless, aspects of coral skeletal climatology still need clarification, including the basic mechanism by which seawater temperature is recorded in coral skeletons, and further research on biomineralization will improve predictions of the future responses of marine calcifying organisms to ocean acidification.
KeywordsCoral Global warming Ocean acidification Coral bleaching Tsunami
29.2 Coral Skeletal Climatology
Conditions during the Pliocene warm period, about 4.6–3 million years ago, are thought to be similar to the climate conditions expected to result from global warming in the near future. Watanabe et al. (2011), who compared analysis results obtained by the same method between modern corals and well-preserved fossil corals from Luzon Island, the Philippines, showed that El Niño occurred on about the same cycle during the Pliocene warm period as at present. Their study is an example of the successful application of coral skeletal climatology to the distant past.
29.3 Application to Environmental Issues
Coral skeletal climatology can also be applied to the investigation of biological and environmental issues such as coral-bleaching events and ocean acidification.
Coral bleaching at a scale unseen before occurred in coral reefs around the Ryukyu Islands in August 1998, and another major coral bleaching event occurred in the southern Ryukyu Islands, especially around Ishigaki Island, in summer 2016. Suzuki et al. (2003) examined skeletal records of bleached corals and observed an abrupt rise, corresponding to the bleaching period, in the δ18O profile analyzed at high resolution along the growth axis of the skeleton. They interpreted this jump to reflect a cessation of coral skeletal growth for a few months immediately after bleaching. As global warming progresses and high seawater temperatures occur more frequently, environmental conditions can be expected to further inhibit coral growth.
Another good proxy for the pH of seawater, or, more precisely, that of the calcifying fluid of the organism, is the boron isotope ratio of the coral skeleton. Kubota et al. (2017) conducted high-precision boron isotope measurements of two coral cores collected from Kikai Island (Ryukyu Islands) and Chichijima Island (Ogasawara Islands) and reported that the ratios from the two islands decreased over the long term, indicating decreasing pH. Interestingly, in both cases, the rate of decline increased in the latter half of the twentieth century. Although seawater pH changes have been observed by shipboard measurements since 1985, the coral record confirms the existence of an ocean acidification trend in the Western Pacific.
29.4 Application to Disaster Research
29.5 Future Directions
This paper is based on joint research with H. Kawahata, A. Iguchi, Y. Ohno, M. Inoue, D. Araoka, T. Watanabe, and many other collaborators. To all of them, I express my sincere gratitude. This study was supported by KAKENHI grant 15H02813 and 18H03366 to AS.
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