Intercolony δ18O and Sr/Ca variability among Porites spp. corals at Palmyra Atoll: Toward more robust coral-based estimates of climate

Abstract

Quantitative estimates of natural climate variability are required to detect anthropogenic climate trends in the tropical Pacific; however, instrumental records from this region are too short and scarce. Coral oxygen isotopic (δ18O) and strontium to calcium (Sr/Ca) records are often used to extend instrumental observations; however, differences in the mean Sr/Ca and δ18O values of Porites spp. colonies from the same reef can introduce large uncertainties in coral-based climate reconstructions. To quantify intercolony variability at Palmyra Atoll, we generate monthly resolved Sr/Ca and δ18O time series from five Porites spp. colonies that grew between 1980 and 2010. Monthly to interannual variability in Sr/Ca and δ18O is well-reproduced among different colonies; however, we document intercolony offsets in mean Sr/Ca of ±0.09 mmol/mol (1σ) or ~1 °C, and in mean δ18O of ±0.12‰ (1σ) or ~0.1 °C. The sensitivity of each proxy to climate also varies across colonies, with Sr/Ca-SST slopes ranging from −0.06 to −0.1 mmol mol−1 °C−1 and δ18O-SST slopes ranging from −0.25 to −0.35‰ °C−1. Intercolony variability in both coral Sr/Ca and δ18O reduces the reproducibility of coral-based δ18Osw reconstructions across overlapping colonies. Accounting for both intercolony variability and slope error suggests that SST reconstructions using Sr/Ca from a single Palmyra coral have an uncertainty of ±1.3 °C (1σ); however, replicating Sr/Ca records across multiple colonies can greatly reduce this uncertainty. A composite Sr/Ca record built using five modern cores, for example, offers a reduced error of ±0.6 °C (1σ) in mean SST reconstructions, ~2.5 times smaller than errors associated with reconstructions from single corals.