Vol.5 No.2 2012

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Research paper : Paleoclimate reconstruction and future forecast based on coral skeletal climatology (A. Suzuki)−82−Synthesiology - English edition Vol.5 No.2 (2012) was observed between the past seawater temperature and the strength of ENSO for the several warm periods observed during the Middle HoloceneTerm 2 (about 6,000 years ago) and the Last Interglacial period (about 120 thousand years ago). These are in accord with the hypothesis that ENSO is affected by the average global climate conditions.The Pliocene Warm Period at about 4.6 million to 3 million years ago is considered to be most similar to the climate condition that may be caused by global warming (Fig. 2a). Although the Jurassic and Cretaceous Periods of the Mesozoic Era,Term 5 about one hundred million years ago when the dinosaurs existed, were also warm periods, the positions of the continents were totally different and simple comparison with the modern climate cannot be made. For the Pliocene Warm Period, a hypothesis has been proposed that the temperature gradient in the equatorial Pacific that caused El Niño disappeared, “permanent El Niño” where the seawater temperature of the entire region remained high occurred, and El Niño that occurred every few years ceased. On the other hand, there is a hypothesis that El Niño existed in the past, the temperature gradient between the east and west of Pacific increased, and El Niño occurred more powerfully and more frequently. The two hypotheses are based on the analysis of the core samples of the deep seafloor deposits with temporal resolution of several thousand years to several ten thousand years, and it was difficult to directly observe the El Niño phenomena that occur at several year intervals.The author’s research group discovered well-preserved fossil corals from the strata that corresponded to the warm period in Luzon Island, the Philippines, and succeeded in analyzing and obtaining the record of seawater temperature variation that is the oldest direct evidence of El Niño.[10] The corals secrete aragonite skeleton, but with passage of time, this changes into stable calcite depending on the temperature and pressure conditions on the earth surface. Normally, after 100 thousand years, the production of unaltered coral fossils is extremely rare. However, in this stratum, the coral fossils are surrounded by mudstone that is an impermeable stratum, and this is effective in preserving the primary aragonite skeletons. The oxygen isotope ratio composition (index of temperature and salinity) was analyzed for the two colonies of coral fossils collected, and the seasonal variation of the atmosphere and marine environment for 70 years and the chronological variation patterns were extracted (Fig. 9a).The marine region around the Philippines is strongly affected by El Niño, and it is known that the variation pattern of the oxygen isotope ratio of extant corals is an excellent record of the variation pattern of the current El Niño. When the results of analysis of the extant and fossil corals using the same method were compared, it became clear that the El Niño occurred at about the same cycle in the Pliocene Warm Period as the present (Fig. 9b).Fig. 10 Effect of large-scale bleaching event of August 1998 observed in the Porites skeleton of Ishigaki IslandThe x-ray photograph of the skeletons of the bleached colonies and of those that did not undergo bleaching, and the oxygen isotope ratio profile along the growth axis of the bleached coral skeleton The samples were collected in September 2002.Fig. 9 (a) El Niño during the Pliocene Warm Period recorded in two fossil coral colonies[10] This shows the annual variations for about 35 years for the two coral colonies (Coral 1 and Coral 2) in different periods about 3.5 million years ago. Black line is the variation of oxygen isotope ratio, red line is the seasonal pattern of average oxygen isotope ratio within the period, and blue line is the anomalies calculated by subtracting the average seasonal pattern from the oxygen isotope ratio variation. The period shaded by yellow color is estimated to be El Niño. (b) Power spectral density[10] The power spectral density shows at which cycle the variation is great in the chronological data, and offers a guideline for detecting the period variation ingredient. From left are the power spectral densities for oxygen isotope ratio of fossil corals (blue line = Coral 1; red line = Coral 2), oxygen isotope ratio of a modern coral, and El Niño index (Nino 3.4 index = anomalies for temperature of tropical Pacific; blue line = 1985~2010; red line = 1950~1984). There is a common peak at 0.3 cycle/year (3-4 year cycle).3-4 yearsCoral 2Coral 1Coral 1Coral 2 Cold, dryWarm, humidPower spectraldensityoxygen isotope ratio (per mil)El Niño during the Pliocene Warm PeriodEl Niño indexModern coralFossil coralCycle (cycle/year)Year (year)1950-19841985-2010ba35302520151050353025201510501246810000.5100.5100.50.100.1500.050.100.1500.05PSDPSD120.30.20.10-0.1-0.2-0.3-5.5-6.0-6.5-7.00.40.20-0.2-0.4-0.6-5.5-6.0-6.5-7.03-4 years3-4 yearsBleaching2 cm Bleached colony Oxygen isotope ratio(‰)Non-bleached colony

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