Reconstructing 20th century SST variability in the southwest pacific: A replication study using multiple coral Sr/Ca records from New Caledonia

Coral-based climate reconstructions typically have not used multiple cores from a region to capture and replicate a climate signal largely because of concerns focused on coral conservation, analytical expense, and time constraints. Coral Sr/Ca reproducibility through the 20th century was investigated using three intra-colony and three inter-colony coral records, from the reefs offshore of Amédée Island, New Caledonia. Different sampling resolutions were examined in coral Sr/Ca (fortnightly and monthly) and delta 18O (fortnightly, monthly, and seasonally) as well as similar scale subsampling of the daily in situ SST record. The mean coral Sr/Ca, delta 18O, and daily SST values do not change as a function of sampling resolution. The coral Sr/Ca signal is highly reproducible; the average absolute offset between coeval Sr/Ca determinations between any two coral Sr/Ca time series is 0.036 mmol/mol (approximately 0.65°C), which is less than twice the analytical precision of the coral Sr/Ca measurements. The stack average of the monthly coral Sr/Ca variations and monthly anomalies are significantly correlated with monthly in situ SST (r equals -0.95, -0.56, respectively) for the period 1967 to 1992 and monthly 1-degree gridded SST data product (r equals -0.95, -0.53, respectively) for the period 1900 to 1999. The coral Sr/Ca-SST reconstruction exhibits decadal-scale fluctuations that exceed those observed in the gridded SST time series, which may reflect true differences between the SST at a shallow reef site and those averaged over a 1-degree grid box or they may reflect inadequacies in the methodology used to create the gridded SST product when few observations are available. A warming trend of approximately 0.6°C is observed in the coral Sr/Ca-SST record. Monthly coral Sr/Ca records and seasonally resolved coral delta 18O record from this site share variance in the latter half of the 20th century, but not in the early 20th century, suggestive of a change in seawater delta 18O