Western Pacific corals reveal Common Era changes in ENSO-diversity, thermal stress, and volcanic cooling

How El Niño–Southern Oscillation variability and diversity have changed over the Common Era, and how corals have responded to ENSO-induced temperature anomalies, remains unclear. Here, Porites microatolls from Arno Atoll, Marshall Islands, in the western Pacific were analyzed across three periods: 484–560 ± 6 CE (Late Antique Little Ice Age; LALIA), 1273–1332 ± 4 CE (Early Little Ice Age; early LIA), and 1954–2017 CE (modern). The early LIA record indicates significantly colder conditions and reduced Sr/Ca-derived temperature seasonality compared to today, consistent with contemporaneous Central Pacific δ¹⁸O records. Negative Sr/Ca-temperature anomalies in the LALIA record likely relate to the 536 and 540 CE volcanic eruptions. Comparison with Central Pacific records suggests that Central Pacific El Niño activity increased between the first and second millennium of the Common Era, while Eastern Pacific El Niño activity remained largely stable, or may even have been stronger than today. Thermal anomalies impacted only modern coral calcification, indicating reduced tolerance when close to the population’s thermal threshold, regardless of potential adaptation to local thermal regimes.