Contrasting Central Equatorial Pacific Oxygen Isotopic Signatures of the 2014/2015 and 2015/2016 El Ni?o Events

Paleoclimate reconstructions of El Nino/Southern Oscillation (ENSO) behavior often rely on oxygen isotopic records from tropical corals (delta 18O). However, few reef-based observations of physical conditions during El Nino events exist, limiting our ability to interpret coral delta 18O. Here we present physical and geochemical measurements from Palmyra Atoll (5.9 degrees N, 162.1 degrees W) from 2014-2017, along with a data assimilation product using the isotope-enabled Regional Ocean Modeling System (isoROMS). Coral delta 18O signals are comparably strong in 2014-2015 and 2015-2016; notably, over 50% of the signal is driven by seawater delta 18O, not temperature. If a constant seawater delta 18O:salinity relationship were present, this would imply a comparable salinity anomaly during both events. However, salinity changes are much larger during 2014-2015, indicating a highly nonstationary relationship. isoROMS then shows that advection strongly influences delta 18O during both the 2014-2015 and 2015-2016 El Nino, driving differences in the salinity/seawater delta 18O relationship. This demonstrates the need for considering ocean dynamics when interpreting coral delta 18O. Coral reefs record past El Nino events through changes to their oxygen isotopic composition, but since very few observations exist of local reef conditions during El Nino, the exact effects of El Nino on local delta 18O are not well known. We measured oceanographic conditions at Palmyra Atoll during two El Nino events, along with the isotopic compositions of seawater, rainwater, and corals themselves. Salinity changes were much stronger during the smaller 2014-2015 El Nino, but the coral and seawater delta 18O changes were comparable-similar coral delta 18O changes came about for different reasons during these two events. Ocean dynamics appears to be the dominant factor in coral and seawater delta 18O anomalies. This indicates that properly interpreting results from coral delta 18O records requires considering how ocean circulation has varied in the past. A new set of physical oceanographic and oxygen isotopic observations was collected at Palmyra Atoll, spanning two El Nino eventsPalmyra experienced stronger seawater delta 18O changes during the weak 2014-2015 El Nino than during the strong 2015-2016 El NinoOceanic dynamics strongly affect coral and seawater delta 18O, and must be considered when interpreting coral-based reconstructions