Vol.5 No.2 2012

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Research paper : Paleoclimate reconstruction and future forecast based on coral skeletal climatology (A. Suzuki)−79−Synthesiology - English edition Vol.5 No.2 (2012) coral fossils from the Little Ice AgeTerm 1 and the Medieval Warm Period, as well as the HoloceneTerm 2 or the PlioceneTerm 3 Warm Period of 3.5 million years ago. The research method using the coral skeleton may also help clarify the coral bleaching events that are caused by abnormally high temperatures and ocean acidification phenomena.In this paper, the research for reconstructing the past climate changes using the modern and fossil coral skeletons will be described, along with the examples of the latest research that is progressing through the combined evaluation of various indices, under the recent breakthroughs. Also, the methodology for enhancing the accuracy of global environment prediction will be discussed.2 Knowing the temperature and salinity from the chemical composition of coral skeletonsSome of the massive colonies of the Porites corals that are distributed widely in the shallow waters of the tropical and subtropical zones continue to grow over the past several hundred years while secreting skeletons, whose main ingredient is calcium carbonate, at 1~2 cm thickness per year (Fig. 1). The skeletons consist of areas of high and low density layered on top of each other, and annular bands are thus normally formed. When a columnar sample is collected from a coral whose colony surface is alive, the year of skeleton formation can be known accurately by counting the annular bands. By cutting and analyzing the minute samples at 0.2~0.4 mm intervals along the growth axis of the skeleton, the paleoclimate can be constructed at high resolution of monthly or higher time units.Oxygen isotope ratio is used frequently in the researches for the chemical composition of the coral skeletons. Normally, the oxygen isotope ratio is expressed as 18O by calculating the per mil (‰) of the isotope ratio in the sample (18O/16O) against the standard sample. The isotope ratio of oxygen in calcium carbonate and the isotope ratio of oxygen in seawater are given small letters c and w, and are expressed 18Oc and 18Ow, respectively. The oxygen isotope ratio of calcium carbonate (18Oc) is dependent on the temperature when precipitation occurred and the oxygen isotope ratio of the seawater (correlated to salinity) (Fig. 4). To estimate the water temperature from the oxygen isotope ratio of the skeleton, it is preferable to use the relational expression obtained by comparing the oxygen isotope ratio of the upper part of the colony and the seawater temperature observation records. Also, to avoid the effect of skeletal growth rate on the chemical composition, the analysis should be done along the maximum growth axis of the colony where the growth rate is 5 mm y−1 or higher. In the area where the salinity variation is small throughout the year, the oxygen isotope Fig. 3 Change in the number of papers on skeletal annular band of reef-building coralsFrom the number of papers listed in the Coral Banding Bibliography, AUSCORE on the website of the Australian Institute of Marine Science (http://www.aims.gov.au/pages/auscore/auscore-08.html). The papers contributed by AIST are marked as black bars.0100200010-2-42004000Years (10,000 years ago)1000Year (AD)0-4-8Pliocene Warm Period10 thousand years(c)(b)(a)Little Ice AgeDegree of data match (%)Temperature (°C)Estimatedtemperature (°C)Gas analysisOrganic material analysisElement analysisIsotope ratio analysisTime rangeSubject of analysisLocalityElementaltechnologyTemporal resolution1 year~10~100 years~ 1 monthSpeleothemWide rangeAdvancement of global environmental predictionLake depositLowlatitudeHighlatitudePresent100 years1,000 years100 thousand years3 million yearsIce core analysisCoral skeletonAnnular tree ringsSeafloor depositFig. 2 Subject used in paleoclimate reconstruction and the positioning of coral skeletal research(a) Estimation of temperature variation for the past 5.5 million years.[2] The past temperature can be estimated since the oxygen isotope ratio of the carbonate shell of the benthic foraminifers obtained from the core sample of deep seafloor deposits is a good index of global ice volume. It is an estimate of the temperature difference compared to present in the South Pole region, and the absolute value of the temperature difference differs greatly according to the latitudes and regions. (b) Temperature variation in the past 1200 years (from Fig. 6.10.c of Reference[1]). The deviation from the average value for 1961-1990 is shown and the degree of match among several researches is shown by darkness of color. (c) The producing range, analysis method, and temporal resolution of the subject used in the paleoclimate reconstruction using coral skeleton and others are presented schematically.20001990198019701960195019401930Year publishedSr/Ca ratioDual proxy methodX-ray photography of annular bandsOxygen isotope ratioDiscovery of annular bandsNumber of papers0102030405060

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