| I have discovered annual variations in the 238U/ 40Ca ratio of Porites coral skeletons. Measurements were made using thermal ionization mass spectrometric techniques, yielding precisions of ±2‰ (2σ) for 238U/ 40Ca and ±1‰ (2σ) for 88Sr/ 40Ca, on samples of ∼2 mg coral. As annual U/Ca cycles appear to be a general feature of primary coralline aragonite, the preservation of such features will be important in identifying unaltered coral for U-series dating studies. U/Ca variations mimic and are in phase with annual variations in 88Sr/40Ca. For a given fractional shift in Sr/Ca, the fractional shift in U/Ca is about 6 times larger. For two modern corals, 238U/40Ca is strongly anti-correlated with measured temperature, suggesting that annual variations in 238 U/40Ca are largely controlled by temperature and 238U/40Ca has potential as a proxy for sea surface temperature. There is some indication in the modern data for some other factor besides temperature, which may also affect 238U/40Ca.; Fossil coral sequences were drilled at a number of sites on the islands of Urélapa and Espiritu Santo, Vanuatu, with the aim of reconstructing past sea surface temperatures. 230Th dates show that we reached Last Glacial Maximum (LGM) corals in 2 cores. I estimated past SSTs using Sr/Ca and δ18O thermometry, both applied to Porites corals. The seasonal amplitude in Sr/Ca, U/Ca and δ 18O in a LGM coral is larger than that in the late Holocene. The difference in mean annual Sr/Ca temperatures between a ∼22 ky coral and an average of a modern and late Holocene coral is 3.5°C. The mean annual δ 18O values of Porites show a glacial to interglacial amplitude of 1.72‰. By removal of the temperature component from the coral δ18O signal, with Sr/Ca thermometry, the estimated total shift in δ18O of seawater during the last glacial-interglacial cycle is ∼1.0‰ in West Pacific surface water, which is similar to the average for the world oceans. Our data show clear evidence for LGM cooling in the West Pacific Warm Pool. Our estimate of the magnitude of tropical cooling is moderate, less than many land-based estimates but larger than values based on foram transfer functions. |
