Stable oxygen isotope record and Sr/Ca ratios of corals from IODP Holes M0009D and M0023A

We present monthly resolved records of strontium/calcium (Sr/Ca) and oxygen isotope (d18O) ratios from well-preserved fossil corals drilled during the Integrated Ocean Drilling Program (IODP) Expedition 310 'Tahiti Sea Level' and reconstruct sea surface conditions in the central tropical South Pacific Ocean during two time windows of the last deglaciation. The two Tahiti corals examined here are uranium/thorium (U/Th)-dated at 12.4 and 14.2 ka, which correspond to the Younger Dryas (YD) cold reversal and the Bølling-Allerød (B-A) warming of the Northern Hemisphere, respectively. The coral Sr/Ca records indicate that annual average sea surface temperature (SST) was 2.6-3.1 °C lower at 12.4 ka and 1.0-1.6 °C lower at 14.2 ka relative to the present, with no significant changes in the amplitude of the seasonal SST cycle. These cooler conditions were accompanied by seawater d18O (d18Osw) values higher by ~0.8 per mill and ~0.6 per mill relative to the present at 12.4 and 14.2 ka, respectively, implying more saline conditions in the surface waters. Along with previously published coral Sr/Ca records from the island [Cohen and Hart (2004), Deglacial sea surface temperatures of the western tropical Pacific: A new look at old coral. Paleoceanography 19, PA4031, doi:10.1029/2004PA001084], our new Tahiti coral records suggest that a shift toward lower SST by ~1.5 °C occurred from 13.1 to 12.4 ka, which was probably associated with a shift toward higher d18Osw by ~0.2 per mill. Along with a previously published coral Sr/Ca record from Vanuatu [Corrège et al. (2004), Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dyras event. Nature 428, 927-929], the Tahiti coral records provide new evidence for a pronounced cooling of the western to central tropical South Pacific during the Northern Hemisphere YD event.

(Table 1) Trace element and age analysis from the Porites coral from IODP Exp 310

Climate responses and changes in marine environments during the last deglaciation have been controversial and few paleoceanographic data are available from the tropical South Pacific, though this region is crucial in the investigations of ocean-atmosphere interactions. Integrated Ocean Drilling Program Expedition 310 was conducted to establish the time course of the postglacial sea-level rise at Tahiti in the South Pacific. A principal objective of this expedition was to examine the variation of marine environments during the last deglaciation. As fossil Porites coral is ideal for assessing past marine environments, we selected only Porites specimens from the many coral specimens retrieved, examined them by XRD, and dated them by the 14C method. In all, we obtained 17 pristine Porites specimens composed of only aragonite with ages from 15 to 9 ka. Then, we measured Mg/Ca, Ba/Ca, and U/Ca ratios and Cd contents as proxies for upwelling and sea surface temperature. Higher Ba/Ca ratios and Cd content together with lower reconstructed SSTs using U/Ca ratios in the coral specimens between 12.6 and 9.8 cal ka compared to around 15 cal ka suggest that upwelling and/or entrainment of subsurface water into mixed layer was enhanced around Tahiti during this period. This finding is consistent with previous reports and supports the idea that the South Pacific was characterized by La Niña-like conditions at least from 12.6 to 9.8 cal ka.

(Table S1) Uranium and Thorium isotopes and U/Th ages of IODP Exp310 cores from offshore Tahiti

The timing of sea-level change provides important constraints on the mechanisms driving Earth's climate between glacial and interglacial states. Fossil corals constrain the timing of past sea level by their suitability for dating and their growth position close to sea level. The coral-derived age for the last deglaciation is consistent with climate change forced by Northern Hemisphere summer insolation (NHI), but the timing of the penultimate deglaciation is more controversial. We found, by means of uranium/thorium dating of fossil corals, that sea level during the penultimate deglaciation had risen to ~85 meters below the present sea level by 137,000 years ago, and that it fluctuated on a millennial time scale during deglaciation. This indicates that the penultimate deglaciation occurred earlier with respect to NHI than the last deglacial, beginning when NHI was at a minimum.