Coral Oxygen Isotopic Records Capture the 2015/2016 El Nino Event in the Central Equatorial Pacific

Coral oxygen isotopes (delta O-18) from the central equatorial Pacific provide monthly resolved records of El Nino-Southern Oscillation activity over past centuries to millennia. However, calibration studies using in situ data to assess the relative contributions of warming and freshening to coral delta O-18 records are exceedingly rare. Furthermore, the fidelity of coral delta O-18 records under the most severe thermal stress events is difficult to assess. Here, we present six coral delta O-18 records and in situ temperature, salinity, and seawater delta O-18 data from Kiritimati Island (2 degrees N, 157 degrees W) spanning the very strong 2015/16 El Nino event. Local sea surface temperature (SST) anomalies of +2.4 +/- 0.4 degrees C and seawater delta O-18 anomalies of -0.19 +/- 0.02 parts per thousand contribute to the observed coral delta O-18 anomalies of -0.58 +/- 0.05 parts per thousand, consistent with a -70% contribution from SST and similar to 30% from seawater delta O-18. Our results demonstrate that Kiritimati coral delta O-18 records can provide reliable reconstructions even during the largest class of El Nino events. Plain Language Summary Oxygen isotope anomalies in coral skeletons are a well-established proxy for changes in tropical Pacific Ocean temperature variations, which have a profound impact on weather extremes around the planet. However, only a handful of calibrations exist that quantify the relationship between ocean temperature and coral oxygen isotopic composition at a given site, especially across extreme events where this relationship may vary most strongly. Here we compare ocean temperature data from loggers installed on the reef at Kiritimati Island (2 degrees N, 157 degrees W) to coral oxygen isotopic records spanning the record-breaking 2015/16 El Nino event. We find that oxygen isotopes in corals provide accurate reconstructions of ocean temperature extremes during this very strong El Nino event, with similar to 70% of the signal originating from ocean temperature and the remainder from increased rainfall.