Intensification of the meridional temperature gradient in the Great Barrier Reef following the Last Glacial Maximum

Abstract

Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and δ(18)O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1-2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.

Figure 1. Map of the south-western Pacific Ocean.

Locations of Integrated Ocean Drilling Program (IODP) Expedition 325 drilling sites at Noggin Pass (NOG) and Hydrographer’s Passage (HYD) in the central Great Barrier Reef are superimposed on annual mean sea surface temperature (SST) and surface ocean circulation patterns in the study area (SEC, South Equatorial Current; EAC, East Australian Current; TF, Tasman Front). For reference, the modern coral site at Heron Island (HER) in the southern Great Barrier Reef and sediment core MD97-2125 in the southeastern Coral Sea are shown. The Coral Sea is the region off the northeast coast of Australia that is bounded by New Guinea to the north and several smaller islands in the east. The Western Pacific Warm Pool is defined as the region where annual mean SST is greater than 28 °C (ref. 62).

Figure 2. Great Barrier Reef coral Sr/Ca and δ¹⁸O and western tropical Pacific temperatures.

(a) Western Pacific Warm Pool (WPWP) sea surface temperature (SST) anomaly (left y axis) and southeastern Coral Sea SST (right y axis) reconstructed from planktonic foraminiferal Mg/Ca. Modern annual mean WPWP SST is >4 °C warmer than SST in the southeastern Coral Sea. (b) Mean Sr/Ca of individual Isopora palifera/cuneata corals from Noggin Pass (NOG) and Hydrographer’s Passage (HYD), central Great Barrier Reef (GBR), and Heron Island (HER), southern GBR (approximate latitude indicated). Weighted least-squares regression lines are shown for NOG and HYD, utilizing data variances as weights. HYD coral Sr/Ca is significantly different from NOG coral Sr/Ca (Methods and Supplementary Fig. 5). The coral-based SST anomalies are not adjusted for changes in seawater Sr/Ca, and thus provide upper estimates of the magnitude of cooling. However, the effects of seawater Sr/Ca changes are similar at both sites, so the reconstructed SST differences between sites are not affected. For reference, the fossil coral-based Sr/Ca-SST anomalies are plotted relative to average Sr/Ca at HER (dashed green line). Modern mean SST at NOG (26.6 °C) and HYD (26.0 °C) is shown relative to SST at HER (24.5 °C) and scaled using the mean coral Sr/Ca-SST relationships of −0.084 mmol mol⁻¹ per °C (ref. 34) (solid red and blue lines) and −0.140 mmol mol⁻¹ per ^°C (ref. 32) (dashed red and blue lines). (c) As in b, but for mean coral δ¹⁸O corrected for the influence of changes in ice volume using a global compilation of benthic foraminifer δ¹⁸O records (for uncorrected coral δ¹⁸O see Supplementary Fig. 8). The resulting coral δ¹⁸O-SST anomalies are shown relative to average δ¹⁸O at HER (dashed green line) using the average of three mean coral δ¹⁸O-SST relationships (−0.22‰ per °C, refs 32, 33, 34). The larger cooling inferred from coral δ¹⁸O compared with Sr/Ca suggests a positive seawater δ¹⁸O anomaly. Modern mean SST at NOG and HYD is shown relative to HER (solid red and blue lines). The grey shading indicates the timing of the Last Glacial Maximum (LGM).