For U-Th analysis, 0.3 to 1g of sponge material was drilled along visible growth layers. The sample size was chosen so that the width and depth of the sampling groove was equivalent to a period of about 20 years, given a growth rate on the order of 0.2 mm/a. 11 samples were anayzed for U-Th. The external reproducibility of the U/Th dating results was determined as about i¿½20 years (2sem) by five replicate analyses of sample Ce96-40 and confirmed by analysis of three closely spaced sample pairs (Ce96-45/51, Ce96-170/172, Ce96-123/40). For one sample pair (Ce96-184/186) U/Th ages could not be confirmed within external reproducibility. As the age data of this specimen deviate significantly from a linear growth curve, we fitted a third order polynomial to the U/Th ages.

The observation that sponges precipitate aragonite in chemical and isotopic equilibrium indicates that the composition of the precipitating fluid is hardly influenced by the sponge metabolism. Thus pH changes in the ambient seawater may be directly monitored by the d18O record of the sponge aragonite. This hypothesis is supported by the boron isotopic composition of sponge skeletons, which indicates mineralization under seawater pH conditions [Bohm et al., 2000b].

Finnigan Mat 252 mass spectrometer with Kiel Device

Archive team- in inferred material, need to differentiate between surface and subsurface water.

; climateInterpretation_seasonality changed - was originally -12 -11 -10 -9 -8 1 2 3 4 5 6 7; archiveType changed - was originally Sclerosponge (SS)

We conclude that, together with variations in seawater d18O, the pH effect on d18O of carbonates can obscure the temperature signal in the sponge d18O record sufficiently to explain the divergent trends of the Sr/Ca and the oxygen isotope records. These results indicate that for temperature reconstructions using C. nicholsoni skeletons, Sr/Ca is a more robust proxy than d18O.

The d18Owater salinity relation in the Caribbean is on the order of 0.2 to 0.3%/psu [Ruhlemann et al., 1999; Watanabe et al., 2001]. Oxygen isotope records of these sponges do notreveal reproducible temperature signals. Variations in thed18O of the ambient water and the pH effect on the oxygenisotopic composition of carbonates [Zeebe, 1999] mayobscure temperature signals in thed18O records.