ANNUAL CYCLES OF U CA IN CORAL SKELETONS AND U/CA THERMOMETRY/

We have discovered annual variations in the U-238/Ca-40 ratio of Porit es coral skeletons. Measurements were made using thermal ionization ma ss spectrometric techniques, yielding precisions of +/-2 parts per tho usand (2 sigma) for U-238/Ca-40 and +/-1 parts per thousand (2 sigma) for Sr-88/Ca-40. Coralline aragonite subsamples weighed similar to 2 m g, enabling submillimeter sampling resolution corresponding to similar to monthly temporal resolution. The annual nature of the cycles was c onfirmed by comparison with annual banding observable in X-radiographs . For two modern and one fossil sample, the amplitude of the U/Ca vari ation ranges from 6 to 23%, well outside of analytical error. As annua l U/Ca cycles appear to be a general feature of primary coralline arag onite, the preservation of such features will be important in identify ing unaltered coral for U-series dating studies. U/Ca variations mimic and are in phase with annual variations in Sr-88/Ca-40. For given fra ctional shift in Sr/Ca, the fractional shift in U/Ca is about 6 times larger. For the two modem corals, U-238/Ca-40 is strongly anticorrelat ed with measured temperature, suggesting that U-238/Ca-40 has potentia l as a paleothermometer. If temperature is the only significant contro l on coralline U-238/Ca-40, We reach the following conclusions from an alyses of fossil samples: a Vanuatu sample, which grew halfway through the last deglaciation, gives U/Ca temperatures 4 to 5 degrees C below modern values. U/Ca thermometry applied to published data for Barbado s corals indicates that (1) temperature generally correlates with sea level, (2) glacial temperatures (stages 2 and 4) were 4 to 6 degrees C lower than interglacial temperatures (stages 7a, 5e, and 1), and (3) temperatures rose from glacial to interglacial values early in the las t deglaciation. Thermometry applied to Papua New Guinea corals indicat es that (1) temperatures were 5 to 6 degrees C lower than interglacial temperatures from similar to 13 to similar to 10 Ky BP, then rose to present values similar to 9 Ky BP, (2) the temperature depression betw een similar to 13 and similar to 10 Ky BP is consistent with low tempe ratures observed in Vanuatu during the same time interval, and (3) the Papua New Guinea deglacial temperature history differs from that of B arbados. The results generally support estimates of tropical temperatu res obtained from Sr/Ca thermometry and snow line elevation data, but disagree with those based on foram transfer functions. A thermodynamic model suggests that coralline U-238/Ca-40 may also be sensitive to ma rine carbonate ion concentration, raising the possibility that some of the observed glacial-interglacial U-238/Ca-40 variation may result fr om glacial-interglacial carbonate ion changes. However, the key experi ments that might establish a coralline U-238/Ca-40-carbonate ion relat ionship have yet to be performed.