15:30 〜 15:45
[MGI25-05] ENSO-driven hydrological variability at 4.2 ka BP recorded by corals from Northern Luzon, Philippines
キーワード：ENSO, fossil corals, 4.2 ka , Luzon Island, carbonate geochemistry
The El Niño Southern Oscillation (ENSO) controls the interannual climate variability affecting regional and global rainfall patterns. It is defined by the temperature anomalies in the western, eastern, and central equatorial Pacific. Rainfall in the Philippines shows a robust negative correlation with ENSO. The interannual ENSO affects the marine hydrological dynamics as reflected by sea surface salinity (SSS) and sea surface temperature (SST). Geochemical proxies (e.g. Sr/Ca, δ18O) in corals are natural recorders of SSS and SST variabilities and are often used to track climate-driven marine hydrological changes.
Sr/Ca ratio and oxygen isotopes (δ18Ocoral) were measured from modern, 4.2 ka BP, 4.3 ka BP, and 6.1 ka BP Porites coral colonies collected from recent and emerged reef terraces of La Union and Ilocos Norte, Philippines. The oxygen isotope of seawater (δ18Osw) was estimated by paired coral Sr/Ca and δ18Ocoral by removing the SST component from the δ18Ocoral. Analytical and slope errors were propagated in the SST and δ18Osw using the simple Monte Carlo in R software to estimate the uncertainty of δ18Osw. ENSO signals were extracted from monthly-resolved δ18Osw records by filtering the 2-8-year bandpass frequency.
The δ18Osw threshold values for different ENSO intensities were determined using the statistically significant positive correlation (r=0.75; r2=0.56; p<0.000) of 2-8-year bandpass filtered δ18Osw and the ERSST v4 NINO 3.4 SSTA from 1982 to 2012. The distinct rainfall-SSS seasonality during neutral and El Niño states was evaluated using modern δ18Osw records. The rainfall was reduced during the strong El Niño in 2015-2016 relative to the neutral state, resulting in higher SSS as indicated by higher δ18Osw. The 2-8-year bandpass filtered δ18Osw records of mid-Holocene corals also show ENSO cycles. Using the threshold values from the regression analysis, El Niño and La Niña signals were identified in each fossil coral record. The change of standard deviation of δ18Osw records from Kikai fossil corals revealed a 20% to 50% ENSO reduction relative to the 2011-2017 benchmark. A mid-Holocene weakening, particularly around 4.2 ka BP, was confirmed from the ENSO variance of compiled coral records from the western and central Pacific. The 4.2 ka BP event was marked by abrupt climate change, resulting in the social disruption and collapse of ancient East Asian civilisations.
Sr/Ca ratio and oxygen isotopes (δ18Ocoral) were measured from modern, 4.2 ka BP, 4.3 ka BP, and 6.1 ka BP Porites coral colonies collected from recent and emerged reef terraces of La Union and Ilocos Norte, Philippines. The oxygen isotope of seawater (δ18Osw) was estimated by paired coral Sr/Ca and δ18Ocoral by removing the SST component from the δ18Ocoral. Analytical and slope errors were propagated in the SST and δ18Osw using the simple Monte Carlo in R software to estimate the uncertainty of δ18Osw. ENSO signals were extracted from monthly-resolved δ18Osw records by filtering the 2-8-year bandpass frequency.
The δ18Osw threshold values for different ENSO intensities were determined using the statistically significant positive correlation (r=0.75; r2=0.56; p<0.000) of 2-8-year bandpass filtered δ18Osw and the ERSST v4 NINO 3.4 SSTA from 1982 to 2012. The distinct rainfall-SSS seasonality during neutral and El Niño states was evaluated using modern δ18Osw records. The rainfall was reduced during the strong El Niño in 2015-2016 relative to the neutral state, resulting in higher SSS as indicated by higher δ18Osw. The 2-8-year bandpass filtered δ18Osw records of mid-Holocene corals also show ENSO cycles. Using the threshold values from the regression analysis, El Niño and La Niña signals were identified in each fossil coral record. The change of standard deviation of δ18Osw records from Kikai fossil corals revealed a 20% to 50% ENSO reduction relative to the 2011-2017 benchmark. A mid-Holocene weakening, particularly around 4.2 ka BP, was confirmed from the ENSO variance of compiled coral records from the western and central Pacific. The 4.2 ka BP event was marked by abrupt climate change, resulting in the social disruption and collapse of ancient East Asian civilisations.