In thermodynamic equilibrium with sea water the Sr/Ca ratio of aragonite varies predictably with temperature and the Sr/Ca ratio in coral have thus become a frequently used proxy for past Sea Surface Temperature (SST). However, biological effects can offset the Sr/Ca ratio from its equilibrium value. We report high spatial resolution ion microprobe analyses of well defined skeletal elements in the reef‐building coral Porites lutea that reveal distinct monthly oscillations in the Sr/Ca ratio, with an amplitude in excess of ten percent. The extreme Sr/Ca variations, which we propose result from metabolic changes synchronous with the lunar cycle, introduce variability in Sr/Ca measurements based on conventional sampling techniques well beyond the analytical precision. These variations can limit the accuracy of Sr/Ca paleothermometry by conventional sampling techniques to about 2°C. Our results may help explain the notorious difficulties involved in obtaining an accurate and consistent calibration of the Sr/Ca vs. SST relationship.

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Plots of magnetic susceptibility versus depth of sediment samples from both marine and terrestrial series demonstrate that, within a specified time interval constrained by magneto- and biostratigraphy, it is possible to interpret the pattern in terms of orbitally driven sedimentary cycles (Gilbert - or Milankovic cycles) with periodicities of around JOO and 20 Ka respectively. At present the method allows correlation of marine and terrestrial deposits with a time resolution of about 10 Ka. Some peaks have characteristic shapes and interference patterns that may be recognized over wide areas. Sediment series have been studied from central Poland, south Israel, south France, south China and Denmark. December 20th, 1992.