Reef-building corals provide seasonally resolved records of past climate variability from the ocean via variations in their oxygen isotope composition (δ18O). However, a variety of non-climatic factors can influence coral δ18O including processes associated with coral biomineralization and post-depositional alteration of the coral skeleton, which add uncertainty to coral based paleoclimate reconstructions. These uncertainties are especially large in mean climate reconstructions developed from coral δ18O values due to the large variability that exists in mean skeletal δ18O signatures. We present a novel framework to minimize this uncertainty in mean coral δ18O records based on a regression model that uses four commonly measured properties in coral skeletons and associated coral δ18O records. We test the ability of the model to reduce noise in a Holocene climate reconstruction comprised of 37 coral δ18O records from Kiritimati in the equatorial Pacific. Up to 43% of the variance in the detrended Holocene dataset is accounted for by a combination of four predictors: (1) mm-scale variability in a coral δ18O record, (2) the physical extent of diagenetic alteration, (3) coral extension rate, and (4) the mean coral δ13C value. Once these non-climatic artifacts are removed from the reconstruction, the weighted variance of the Holocene dataset is reduced by 46% and the uncertainty in the trend of coral δ18O over time is reduced by 26%. These results have important implications for the climate interpretation of this Holocene data set. This framework has the potential to improve other paleoclimate records based on ensembles of coral δ18O records.

Coral Sr/Ca ratios provide quantitative estimates of past sea surface temperatures (SST) that allow for the reconstruction of changes in the mean state and climate variations, such as the El Nino-Southern Oscillation, through time. However, coral Sr/Ca ratios are highly susceptible to diagenesis, which can impart artifacts of 1-2˚C that are typically on par with the tropical climate signals of interest. Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the sampling of primary skeletal material in altered fossil corals, providing much-needed checks on fossil coral Sr/Ca-based paleotemperature estimates. In this study, we employ a set modern and fossil corals from Palmyra Atoll, in the central tropical Pacific, to quantify the accuracy and reproducibility of SIMS Sr/Ca analyses relative to bulk Sr/Ca analyses. In three overlapping modern coral samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1s). We demonstrate high fidelity between 3-month smoothed SIMS coral Sr/Ca timeseries and SST (R = -0.5 to -0.8; p<0.5). For lightly-altered sections of a young fossil coral from the early-20th century, SIMS Sr/Ca timeseries reproduce bulk Sr/Ca timeseries, in line with our results from modern corals. Across a moderately-altered section of the same fossil coral, where diagenesis yields bulk Sr/Ca estimates that are 0.6mmol too high (roughly equivalent to -6˚C artifacts in SST), SIMS Sr/Ca timeseries track instrumental SST timeseries. We conclude that 3-4 SIMS analyses per month of coral growth can provide a much-needed quantitative check on the accuracy of fossil coral Sr/Ca-derived estimates of paleotemperature, even in moderately altered samples.

Coral oxygen isotopes (δ18O) from the central equatorial Pacific provide monthly-resolved records of El Niño-Southern Oscillation (ENSO) activity over past centuries to millennia. However, calibration studies using in situ data to assess the relative contributions of warming and freshening to coral δ18O records are exceedingly rare. Furthermore, the fidelity of coral δ18O records under the most severe thermal stress events is difficult to assess. Here, we present six coral δ18O records and in situ temperature, salinity, and seawater δ18O data from Kiritimati Island (2°N, 15°W) spanning the very strong 2015/16 El Niño event. Local sea surface temperature (SST) anomalies of +2.4±0.4°C and seawater δ18O anomalies of -0.19±0.02‰ contribute to the observed coral δ18O anomalies of -0.58±0.05‰, consistent with a ~70% contribution from SST and ~30% from seawater δ18O. Our results demonstrate that Kiritimati coral δ18O records can provide reliable reconstructions even during the largest class of El Niño events.