ANNUAL CYCLICITY IN HIGH RESOLUTION SR RECORDS FROM SCLEROSPONGES

Sclerosponges form hard calcareous skeletons which contain 100's of years of geochemical records and may be potentially important as paleoclimate proxies. They inhabit cryptic environments in the tropics and occur in a wide range of water depths (0 to 150 m). Using a combination of laser ablation mass spectrometry, micro sampling and stable isotope mass spectrometry, we have demonstrated the presence of annual cyclicity in the Sr/Ca ratio and the oxygen isotopic record of sclerosponges obtained from the Bahamas. No cyclicity is evident in the carbon isotopic composition or in Mg/Ca, Ba/Ca, U/Ca, or Pb/Ca ratios. As the oxygen isotopic composition is positively correlated with the Sr/Ca ratio, it can be assumed that temperature is inversely correlated with the Sr concentration, as in the case of scleractinian corals. However, based on this study, changes in the Sr/Ca ratio of sclerosponges are greater than those observed in scleractinian corals. The number of annual cycles counted using Sr/Ca and oxygen isotopic values in a 100 year sample agrees with independent age estimates made using uranium series methods, comparisons with the changes in the carbon isotopic composition of the atmosphere (C-13 Suess affect), and the variations in the concentration of lead in the oceans. These studies reveal that the growth rate of the sclerosponges may be highly variable (100 to 400 mm/yr). If sclerosponges are to be useful for correlation with decadal climate variations, ages provided by uranium series and carbon-14 methods alone are clearly insufficient and the measurement of annual cyclicity using Sr/Ca ratios provides a more accurate method of age determination.