| 研究実績の概要 |
Sea surface temperature (SST) and rainfall associated with the East Asian monsoon (EAM) play an important role in the society both in ancient and modern times. The intensity of EAM is potentially affected by global atmosphere-ocean systems such as El Nino Southern Oscillation (ENSO) and Intertropical Convergence Zone (ITCZ). Hermatypic corals are sensitive to both atmospheric and ocean changes. Trace elements and isotopic tracers in coral growth bands can track these changes. This research aims to reconstruct climate and ocean conditions beyond instrumental & historical data using fossil corals from the last 7,000 years.
Fossil corals from Kikaijima (n=9) and Northwest Luzon (n=13) were screened for diagenesis using XRD and SEM analyses. Samples with >1% calcite and secondary aragonite needles were not selected as material for paleoclimate reconstructions.
Radiocarbon dating of two fossil corals from Kikaijima, Japan yielded 5712±24 and 3235±20 yBP ages. Northwest Luzon corals have C14 age dates ranging from 2972±70 to 6285±79 yBP. NW Luzon corals have higher extension (10.2-17.6 mm/yr) and calcification rates (0.91-2.81 g/cm2yr) compared to Kikaijima corals. Conversely, Kikaijima corals have higher skeletal density (1.71-1.87 g/cm3) compared to NW Luzon corals. The significant difference of extension and calcification rates of NW Pacific corals indicate the strong influence of tropical-subtropical SST gradient in coral carbonate formation during Holocene. The highly varied coral growth in NW Luzon signify a more complex interplay of regional and local factors.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
1: 当初の計画以上に進展している
理由
The research has started on April 1, 2020. Just a month before that, the World Health Organization (WHO) declared Covid19 as pandemic. Because of that, several countries including Japan and the Philippines went into lockdowns and implemented strict health protocols. The plan was to conduct fieldworks in the Philippines to get modern and fossil corals. But due to the pandemic, non-essential travels were prohibited. Similarly, Japan also restricted mobility and non-essential travels. I devoted the time working on some previously gathered data from Kikaijima and Philippines. The existing data were analyzed and figures were prepared for possible submission for conference presentation. I communicated with several researchers asking for modern coral cores from the Philippines.
Fortunately, I was able to conduct a fieldwork in La Union, Northwest Luzon Island last November 2020 and collected several fossil samples for diagenetic screening. I also got two modern corals from Southwest Luzon that I can use for calibration. XRD analysis for mineral profiling was conducted for fossil corals. 3D Xray CT scan was done for the two modern corals for the examination of annual growth bands. Another fieldwork was conducted to collect modern coral core from Northeastern Luzon Island on March 9-12, 2021.
There were some delays of fieldwork and sampling collection, but generally, the research has good progress and I have now samples to work with.
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| 今後の研究の推進方策 |
Collected modern corals from Luzon Island, Philippines will be analyzed for Sr/Ca and oxygen isotopes. First, mineralogical and morphological analyses will be done using XRD, petrographic microscopy and Xray imaging. Another modern coral core will be collected from the Bolinao, Luzon Island near the sampling sites of fossil corals. The sampling will be carried out on April 2021. Fossil corals will be screened for diagenetic alteration using petrography, XRD and SEM.
Pristine fossil corals will be sent to Accelerator Mass Spectrometer Laboratory (AMS) for radiocarbon dating. We have dated 5 well-preserved fossil corals from Northwest Luzon. We are selecting additional well-preserved corals from 6 samples collected last year. From April to June 2021, XRD and petrographic analyses to know the aragonite-calcite content will be done. Fossils with good preservation will be used for geochemical analysis. Pristine samples will be sent to AMS laboratory for radiocarbon dating.
From June 2021 to March 2022, geochemical analyses of corals using ICP OES and IRMS coupled with Kiel IV carbonate device will be conducted. Modern and fossil corals will be cleaned and cut into 5 mm slabs. Linear extension rate, skeletal density and calcification rate of corals will be measured using Xray techniques. Microsampling using high-precision microdrill will be done in monthly resolution (at least). Sr/Ca, Mg/Ca and Ba/Ca will be analyzed using ICP OES and oxygen and carbon isotopes will be measured using IRMS (MAT 253+). Data analysis for paleoclimate reconstruction will be done.
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