Vol.5 No.2 2012

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Research paper : Paleoclimate reconstruction and future forecast based on coral skeletal climatology (A. Suzuki)−81−Synthesiology - English edition Vol.5 No.2 (2012) oxygen isotope ratio of the Ishigaki coral skeletons in winter is determined by temperature, and the minimum winter temperatures reconstructed from the oxygen isotope ratio for 1971~1987 showed good correlation with the monsoon indices that express the strength of the seasonal winter winds. In contrast, the seawater temperature of Ishigaki after the regime shift decreased its correlation with the monsoon index. Instead the correlation with the southern oscillation index (SOI) became clearer than the monsoon index. This is an interesting phenomenon where Ishigaki that is categorized as the subtropical zone seemed to shift to the tropical zone. A period of low seawater temperature was observed in the coral sample from around 1900.[6] January 1902 was recorded as the winter with prevailing powerful Siberian Highs, and this was the year when the tragic incident occurred where the entire regiment of the Imperial Japanese Army was lost at Mt. Hakkoda during the winter training.On the other hand, the reconstruction of temperature and salinity for about 130 years using the coral records at Ogasawara was the first true application of the dual proxy method[7] in the Northwestern Pacific region. For this coral sample, the U/Ca ratio, which is considered to be an excellent index of seawater temperature as in Sr/Ca ratio, was analyzed (Fig. 7). The good match of the reconstruction results of temperature and salinity by the combinations of oxygen isotope ratio and either the Sr/Ca or U/Ca ratio demonstrated the high reliability of the coral records (Fig. 8). The reconstructed seawater temperature corresponded with the Pacific Decadal Oscillation.[8] What was more interesting was that a rapid decline in salinity was observed around 1905-1910. The change in estimated salinityTerm 4 was about 1, and while the adequacy of scale remains under question, it was the most notable change in the coral records over 130 years. No sign such as diagenic alteration was observed in the skeleton during this period. The cause of salinity decrease at the beginning of the 20th century in Ogasawara was assumed to be caused by the decreased evaporation volume due to the weakening of the Ogasawara High caused by the decline of the westerlies at the time. The relationship with the low temperature event shown in the coral record of Ishigaki Island is also interesting.4 Reconstruction of El Niño by fossil corals of the Pliocene Warm PeriodEl Niño phenomenon that occurs every few years in the equatorial Pacific region plays an important role in the current climatological system. With the progression of global warming, how will El Niño-Southern Oscillation (ENSO) change in the future? The Fourth Assessment Report of the IPCC predicts a frequent occurrence of powerful El Niños[9] but there is much opposition. The investigations of coral skeletons were conducted actively to address this issue.[1] Through the analysis of coral records over 500 years including the period before the Industrial Revolution, it was found that the strength of El Niño was correlated to the average seawater temperature, and El Niño was active as the temperature increased. This indicates that ENSO is affected by average global climate conditions, and implies the possibility that ENSO may change due to future warming. Also, correlation Sr/Ca ratio(mmol/mol)U/Ca ratio(µmol/mol)Oxygen isotope ratio(δ18Oc, ‰)Year (AD)9.49.29.08.81.51.41.31.21.1-3.6-4.0-4.4-4.8-5.220001980196019401920190018801905-1910Coral δ18OCoral Sr/CaCoral U/CaFig. 7 Variation of oxygen isotope ratio, Sr/Ca ratio, and U/Ca ratio of the corals from Chichi Island, Ogasawara Islands[7]The time from 1905 to 1910 when rapid increase in oxygen isotope ratio was seen is shaded.Oxygen isotope ratio deviation( δ18Oc, ‰)High salinityLow salinityEstimated salinity deviationSalinity(SSS)Temperature(SST,℃)Sr/Ca ratio (mmol/mol)U/Ca ratio (mmol/mol)Year (AD)200019801960194019201900188034.8534.8023.524.00.40.20.09.39.29.19.01.351.301.251.201.151.00.50.0Coral U/CaCoral Δδ18O(U/Ca)Coral Δδ18O(Sr/Ca)Coral Sr/Ca1905-1910SSSSSTFig. 8 Variations of sea surface temperature (SST) and sea surface salinity (SSS) reconstructed from the coral from Ogasawara Islands[7]For salinity, the amount that contributed to the salinity variation of the oxygen isotope ratio calculated by dual proxy method from the coral skeleton is converted to salinity, by using the relationship of oxygen isotope ratio and salinity of the seawater in the Northwestern Pacific region (increase of salinity 1.0 equivalent to increase of oxygen isotope ratio 0.42 ‰). The deviation is labeled as estimated salinity deviation from the recent value (right axis). The results of the two combinations of oxygen isotope ratio with Sr/Ca ratio (blue line) and with U/Ca ratio (red line) are shown. The measured temperature and salinity are also shown (black line). The region of 1905 to 1910 when rapid decrease in salinity was observed is shaded.

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