Vol.5 No.2 2012

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Research paper : Paleoclimate reconstruction and future forecast based on coral skeletal climatology (A. Suzuki)−80−Synthesiology - English edition Vol.5 No.2 (2012) ratio of the coral is a good index of the seawater temperature. For example, the oxygen isotope ratio of the corals from Ishigaki Island (Ishigaki-jima), the Ryukyu Islands corresponds well with the seawater temperature.[3]For the coral skeletons, other useful indirect indices (proxies) have been found other than the oxygen isotope ratio. The strontium/calcium (Sr/Ca) ratio and uranium/calcium (U/Ca) ratio of the coral skeletons change according to seawater temperature only.The oxygen isotope ratio of coral skeletons is dependent on both the seawater temperature and salinity (more accurately, the oxygen isotope composition of seawater), while the Sr/Ca ratio is dependent only on temperature. Therefore, by estimating the temperature from the skeletal Sr/Ca ratio, and then subtracting the variation by temperature from the variation of the skeletal oxygen isotope ratio, the difference will indicate the variations of the oxygen isotope ratio composition of the seawater or the variations in salinity.[4] This is the dual proxy method using oxygen isotope ratio and Sr/Ca ratio of the coral skeleton (Fig. 5). The U/Ca ratio can also be used instead of Sr/Ca ratio.3 Near past climate change in Ishigaki and Chichi Islands reconstructed from coral skeletonsThe long-length coral research in the Northwestern Pacific region around Japan has not been done actively compared to overseas. Our research group conducts chemical analyses of the long columnar samples of Porites that are over 100 years old from Ishigaki Island (Ishigaki-jima,24° N, 124° E) of the Ryukyu Islands and Chichi Island (Chichijima, 27° N, 135° E) of the Ogasawara Islands (Fig. 6).In the Pacific, a rapid change in the climate condition called the regime shift is known to occur,[5] and the event of 1988/1989 is notable in the Southern Ryukyus.[3] Before this regime shift, the seawater temperature during winter in the shallow waters of the coral reef of Ishigaki Island was sensitive to the Siberian High, and good correlation was seen with the monsoon index (pressure difference between Irkutsk and Nemuro) that indicates the strength of the monsoon. The (δ18Oc-δ18Ow, ‰)TemperatureTemperature(SST, ℃)01020-6-4-20230SalinitySSS Difference in oxygen isotope ratioof coral and seawater SST(℃)SST(℃) δ18Owδ18Oc= (δ18Oc-δ18Ow)+abFig. 4 Oxygen isotope ratio (18Oc) of the coral skeleton and sea surface temperature (SST)The oxygen isotope ratio is expressed as 18O by calculating the per mil (‰) of the isotope ratio (18O/16O) in the sample against the standard sample. The oxygen isotope ratio in calcium carbonate is expressed with a small letter c. By calculating the relational expression by comparing the oxygen isotope ratio of the surface of the coral colony and the seawater temperature observation record, the seawater temperature at that time can be estimated from the oxygen isotope ratio of the past skeletons. Strictly speaking, it is affected by the oxygen isotope ratio of seawater (18Ow), but this can be neglected in the marine region with small variation in sea surface salinity (SSS). The salinityTerm 4 is expressed without units according to the convention of oceanography.TimeOxygen isotope ratio of seawater (δ18Ow)(δ18Oc)Oxygen isotope ratio of coralsSr/Ca ratio of coralsCorresponding value for temperature is substitutedTemperatureSalinitySalinityTemperatureFig. 5 Conceptual diagram of the dual proxy method using oxygen isotope ratio and Sr/Ca ratio of the coral skeletonThe reconstruction of the seasonal variation of temperature and salinity is presented. 1850 1900 1950 2000 Year (AD)-3 -5 -4 -6 Cold, dryWarm, humidIshigaki(124°E, 24°N)Chichi, Ogasawara Islands(142°E, 27°N)Oxygen isotope ratio (δ18Oc, ‰)190519001988Fig. 6 Oxygen isotope ratio record of the coral skeletons collected from Ishigaki Island, Ryukyu Islands and Chichi Island, Ogasawara Islands[6][7]The seasonal variations of temperature, etc. can be reconstructed at 1~2 month temporal resolution. The low temperature period at about 1900 for the Ishigaki coral discussed in this paper, the regime shift in 1988/89, and the salinity shift at around 1905 for the Ogasawara coral are marked with arrows.

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