Super instrumental El Nio events recorded by a Porites coral from the South China Sea

The 2-7-year periodicities recorded in fossil coral records have been widely used to identify paleo-El Nio events. However, the reliability of this approach in the South China Sea (SCS) has not been assessed in detail. Therefore, this paper presents monthly resolution geochemical records covering the period 1978-2015 obtained from a Porites coral recovered from the SCS to test the reliability of this method. The results suggest that the SCS coral reliably recorded local seawater conditions and the super El Nio events that occurred over the past 3 decades, but does not appear to have been sensitive enough to record all the other El Nios. In detail, the Sr/Ca series distinctly documents only the two super El Nios of 1997-1998 and 2014-2016 as obvious low values, but does not match the Oceanic Nio Index well. The super El Nio of 1982-1983 was identified by the growth hiatus caused by the coral bleaching and subsequent death of the coral. Three distinct stepwise variations occur in the delta C-13 series that are coincident with the three super El Nios, which may be related to a substantial decline in endosymbiotic zooxanthellae density caused by the increase in temperature during an El Nio or the selective utilization of different zooxanthellaes that was required to survive in the extreme environment. The increase in rainfall and temperatures over the SCS during El Nios counteracts the effects on seawater delta O-18 (delta O-18(sw)) and salinity; consequently, coral Delta delta O-18 series can be used as a proxy for delta O-18(sw) and salinity, but are not appropriate for identifying El Nio activity. The findings presented here suggest that the method to identify paleo-El Nio activity based on the 2-7-year periodicities preserved in the SCS coral records might not be reliable, because the SCS is on the edge of El Nio anomalies due to its great distance from the central equatorial Pacific and the imprints of weak and medium strength El Nio events may not be recorded by the corals there.