Although reef coral skeletal carbon isotopes (δ 13 C) are routinely measured, interpretation remains controversial. Here we show results of a consistent inverse relationship between coral δ 13 C and skeletal extension rate over the last several centuries in Porites corals at Fiji, Tonga, Rarotonga and American Samoa in the southwest Pacific. Beginning in the 1950s, this relationship breaks down as the atmospheric 13 C Suess effect shifts skeletal δ 13 C > 1.0‰ lower. We also compiled coral δ 13 C from a global array of sites and find that mean coral δ 13 C decreases by −1.4‰ for every 5 m increase in water depth ( R = 0.68, p < 0.01). This highlights the fundamental sensitivity of coral δ 13 C to endosymbiotic photosynthesis. Collectively, these results suggest that photosynthetic rate largely determines mean coral δ 13 C while changes in extension rate and metabolic effects over time modulate skeletal δ 13 C around this mean value. The newly quantified coral δ 13 C-water depth relationship may be an effective tool for improving the precision of paleo-sea level reconstruction using corals.

The Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleo‐application of Isopora coral‐derived SST calibrations to a suite of 25 previously published fossil Isopora from the central GBR spanning ~25–11 kyr BP. The resultant multicoral Sr/Ca‐ and δ18O‐derived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5–5.5°C at Noggin Pass (~17°S) and ~7–8°C at Hydrographer's Passage (~20°S) (Sr/Ca‐derived) during the LGM, in line with previous estimates (Felis et al.,  2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9–11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with near‐modern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil Isopora SSTA data from the GBR provide new insights into the deglacial reef response, with near‐modern warming during the YDC, since the LGM.

Isopora (Acroporidae) is a genus of often encrusting, branching to submassive corals that are common in many shallow habitats on modern and fossil Indo‐West Pacific reefs. Although abundant, Isopora is largely absent from paleoceanographic literature. The scarcity of large Porites and the abundance of Isopora retrieved from the Great Barrier Reef (GBR) on Integrated Ocean Drilling Program Expedition 325 focused paleoceanographic attention on Isopora. Here we provide the first independent high‐resolution calibration of both Sr/Ca and δ18O for temperature analyses based on Isopora and demonstrate its consistency with Porites records. We developed modern skeletal Sr/Ca‐ and δ18O‐sea surface temperature (SST) calibrations based on five modern Isopora colonies from Heron Island in the southern GBR. Pairing the coral Sr/Ca record with monthly SST data yielded Sr/Ca‐SST sensitivities from −0.061 ± 0.004 (centered) to −0.083 ± 0.007 (raw) mmol/mol °C−1 based on reduced major axis regressions. These sensitivities are in the middle of the range of published Porites values and overlap most published values for Isopora, −0.075 ± 0.011 to −0.065 ± 0.011 mmol/mol °C−1. The δ18O‐SST sensitivities range from −0.184 ± 0.014 (centered) to −0.185 ± 0.014 (raw) ‰ °C−1, assuming that all seasonal variation in δ18O was due to SST. These δ18O‐SST sensitivities are smaller than the widely accepted value of −0.23‰ °C−1 for biogenic aragonite but are at the upper end of high‐resolution Porites‐defined sensitivities that are consistently less than the aforementioned established value. Our results validate the use of Isopora as an alternative source of paleoceanographic records in habitats where large massive Porites are scarce or absent.