5:15 PM - 7:15 PM
[MGI25-P03] ENSO variability during Medieval Climate Anomaly reconstructed using δ18O and Sr/Ca ratio of fossil Porites coral skeleton from Bora Bora, French Polynesia.
Keywords:coral skeleton, French Polynesia, ENSO, Sr/Ca ratio, oxygen isotope ratio
The period from 950s to 1250s AD is called as “the Medieval Climate Anomaly (MCA)”. Humans expanded into remote Oceania (Eastern Polynesia) during the MCA period. One of the factors of this event has been identified as the El Niño Southern Oscillation (ENSO) (Anderson et al., 2006). There have been few previous studies regarding monthly high resolution about per month climate reconstructions in the South Pacific during the MCA, which conflict previous studies about ENSO variability (Newton et al., 2006; Rustic et al., 2015; Moy et al. 2002; Rein et al., 2004).
Geochemical composition in massive coral skeletons (CaCO3) is known to provide high time resolution and continuous record of the environment during skeletal formation. Strontium/Calcium ratio (Sr/Ca ratio) in coral skeleton is known to have strong negative correlation with Sea Surface Temperature (SST) SST , oxygen isotope ratio (δ18Ocoral) on the other hand being affected by both paleo SST and seawater oxygen isotope ratio(δ18Osw) can be used reconstruct sea surface salinity (SSS).
In this study, from 42 fossil Porites samples were collected from 7 sites located in Bora Bora, French Polynesia coasts by using the MCA sea level (Hallman et al., 2018) as a reference,
Coral skeleton were first processed into slices in a way to preserve annual extension direction. In fossil corals, Sr/Ca ratios and δ18Ocoral happened to be altered with secondary aragonite (Hendy et al., 2007) or calcite (McGregor and Gagan, 2003) due to diagenesis processes. When affected by diagenesis. In order to confirm the state of the sample regarding diagenesis processes by scanning election microscopy (SEM) multiples small portions have also been cut out from the skeleton slices. Calcite/aragonite ratio have also been obtained by using X-ray diffraction (XRD). Well-preserved samples were dated by Carbon 14 method. Soft X-ray images were taken to confirm annual rings, and powder samples for geochemical analysis were subsampled along the annual growth direction. The Sr/Ca ratio of powder samples were analyzed using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). to reconstruct SST from using equation converting Sr/Ca ratio and SST (Farley et al., 2024). The error of the Sr/Ca ratio was ±0.071 mmol/mol(1σ;n=22). With duplicate measurement, δ18Ocoralof powder samples have also been analyzed as a SST and SSS fluctuation proxy through automated carbonate device coupled with Dual Inlet Isotope Ratio Mass Spectrometer (DI-IRMS). The error of the δ18Ocoral ratio was ±0.070 ‰VPDB(1σ;n=13). The oxygen isotope ratio of seawater (δ18Osw), which is approximated by salinity, was determined from the reconstructed SST and δ18Ocoral .
The modern marine environment around Bora Bora was known with the Integrated Global Ocean Service System (IGOSS) SST (within 1° × 1° centered at 16.5°S, 151.5°E). As for SSS, was used to the UK Met Office EN4 salinity data set (within 1° × 1° of 16.5°S - 17.5°S, 151.5°E - 152.5°E). The Southern Oscillation Index, one of the El Niño indicators, is based on a data set published on the JMA website (https://www.data.jma.go.jp/gmd/cpd/db/elnino/index/soi.html).
One of a well-preserved sample by Carbon dated indicate a growth between 879 to 1013 AD (1σ). First geochemical analysis of seasonal variabilities in Sr/Ca ratio and δ18Ocoral data yield on a period of 7 years. SST was reconstructed based on the results of Sr/Ca ratio analysis. Average values were found to be 0.78℃ lower than the present SST.
Sea surface salinity anomalies (SSSA) were calculated from the EN4 observational data for Bora Bora, excluding interannual variability and long-term trends, and found large decreases in 1997 and 2005, when a strong El Niño occurred and the SOI decreased significantly.
Therefore, the SSS in the waters surrounding Bora Bora is expected to decrease with the onset of El Niño. Comparing the standard deviation of salinity variability predicted from fossil coral skeletons with the standard deviation of SSSA from the current EN4, the salinity variability of fossil coral skeletons was larger than the standard deviation of current salinity for the last 40 years, suggesting that ENSO variability around 1000years ago may have been larger than that of present.
Geochemical composition in massive coral skeletons (CaCO3) is known to provide high time resolution and continuous record of the environment during skeletal formation. Strontium/Calcium ratio (Sr/Ca ratio) in coral skeleton is known to have strong negative correlation with Sea Surface Temperature (SST) SST , oxygen isotope ratio (δ18Ocoral) on the other hand being affected by both paleo SST and seawater oxygen isotope ratio(δ18Osw) can be used reconstruct sea surface salinity (SSS).
In this study, from 42 fossil Porites samples were collected from 7 sites located in Bora Bora, French Polynesia coasts by using the MCA sea level (Hallman et al., 2018) as a reference,
Coral skeleton were first processed into slices in a way to preserve annual extension direction. In fossil corals, Sr/Ca ratios and δ18Ocoral happened to be altered with secondary aragonite (Hendy et al., 2007) or calcite (McGregor and Gagan, 2003) due to diagenesis processes. When affected by diagenesis. In order to confirm the state of the sample regarding diagenesis processes by scanning election microscopy (SEM) multiples small portions have also been cut out from the skeleton slices. Calcite/aragonite ratio have also been obtained by using X-ray diffraction (XRD). Well-preserved samples were dated by Carbon 14 method. Soft X-ray images were taken to confirm annual rings, and powder samples for geochemical analysis were subsampled along the annual growth direction. The Sr/Ca ratio of powder samples were analyzed using Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). to reconstruct SST from using equation converting Sr/Ca ratio and SST (Farley et al., 2024). The error of the Sr/Ca ratio was ±0.071 mmol/mol(1σ;n=22). With duplicate measurement, δ18Ocoralof powder samples have also been analyzed as a SST and SSS fluctuation proxy through automated carbonate device coupled with Dual Inlet Isotope Ratio Mass Spectrometer (DI-IRMS). The error of the δ18Ocoral ratio was ±0.070 ‰VPDB(1σ;n=13). The oxygen isotope ratio of seawater (δ18Osw), which is approximated by salinity, was determined from the reconstructed SST and δ18Ocoral .
The modern marine environment around Bora Bora was known with the Integrated Global Ocean Service System (IGOSS) SST (within 1° × 1° centered at 16.5°S, 151.5°E). As for SSS, was used to the UK Met Office EN4 salinity data set (within 1° × 1° of 16.5°S - 17.5°S, 151.5°E - 152.5°E). The Southern Oscillation Index, one of the El Niño indicators, is based on a data set published on the JMA website (https://www.data.jma.go.jp/gmd/cpd/db/elnino/index/soi.html).
One of a well-preserved sample by Carbon dated indicate a growth between 879 to 1013 AD (1σ). First geochemical analysis of seasonal variabilities in Sr/Ca ratio and δ18Ocoral data yield on a period of 7 years. SST was reconstructed based on the results of Sr/Ca ratio analysis. Average values were found to be 0.78℃ lower than the present SST.
Sea surface salinity anomalies (SSSA) were calculated from the EN4 observational data for Bora Bora, excluding interannual variability and long-term trends, and found large decreases in 1997 and 2005, when a strong El Niño occurred and the SOI decreased significantly.
Therefore, the SSS in the waters surrounding Bora Bora is expected to decrease with the onset of El Niño. Comparing the standard deviation of salinity variability predicted from fossil coral skeletons with the standard deviation of SSSA from the current EN4, the salinity variability of fossil coral skeletons was larger than the standard deviation of current salinity for the last 40 years, suggesting that ENSO variability around 1000years ago may have been larger than that of present.