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].

Three transects were sampled as a test for reproducibility (Figures 5 and 10b). Correlation between the transects was carried out by following the growth bands of the skeleton. The trends and larger features are well-reproduced in the three transects. However, transect 1 is offset from the other two transects by about 0.1%.

d18O calibration to T assuming constant d18Ow

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 [Bo�hm et al., 2000b].

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. 4 samples were anayzed for U-Th. 10 samples for radiocarbon analysis were removed. They compared D14C data with a D14C curve measured on a Florida surface water coral [Druffel, 1989]. The fit was constrained by assuming that the growth rate was constant in the relevant part of the sponge skeleton. They further assume that the timing of the radiocarbon increase at 125 mbsl was similar to surface waters in that region because the amplitude of change in D14C is similar to the surface water change. For specimen Pb19 a mean growth rate of 0.19 mm/a was calculated from the slope of a linear fit to the dated tie points. A lifespan from about 1400 A.D. to 1996 A.D. was determined by extrapolating the mean growth rate to the base of the skeleton. The U/Th chronology is independently confirmed by the correct localization of the nuclear weapon test radiocarbon increase at a depth of about 5.9 mm below the surface of the skeleton (Figure 6, Table 2). Further independent support for the reliability of our chronologies is provided from the fit of d13C data of the investigated sponge skeletons and atmospheric CO2 records [Bo�hm et al., 2002].