ANALYTICAL DETAILS

Pure aragonite samples of 0.09 to 1.0 g were spiked with a ²²⁹Th-²³⁶U mixed spike to give an expected ²³⁶U/²³⁵U ratio of 5 and ²²⁹Th/²³⁰Th ratio of 2 to 40 depending on sediment age. Samples were then covered with distilled water and dissolved by drop-wise addition of 7.5M HNO₃. U and Th were separated using techniques described before (e.g. Edwards et al., 1987). After co-precipitating with Fe, they were loaded on 2 mL of 1 x 8 anion-exchange resin in 7.5M HNO₃, washed with 7.5M HNO₃, the Th eluted with 12M HCl, and the U with 0.01M HCl. Both the U and Th separates were cleaned further using 100 µL of 1 x 8 anion-exchange resin. Thorium separates were loaded in 7.5M HNO₃, washed with 7.5M HNO₃, and eluted with 1M HNO₃. U separates were loaded in a 90% acetone-10% 6M HCl mix, washed with the same mixture, and eluted with 1M HCl. Chemical yields for the entire procedure were ~80% for both U and Th.

U and Th were analyzed as the metal by thermal ionization mass spectrometry using a VG 54-30 mass spectrometer (Edwards et al., 1987). Thorium-229, ²³⁰Th, and ²³²Th were measured by single-collector peak switching using the daly ion-counting system. Uranium-234, ²³⁵U, and ²³⁶U were also measured by ion counting, but ²³⁶U was additionally monitored in a faraday cup during the daly-²³⁴U collection step, and ²³⁸U monitored in the same faraday during the daly-²³⁶U collection step. This enables in-run assessment of the daly/faraday gain and measurement of the ²³⁸U/²³⁵U to correct the analyses for mass fractionation.

Uranium was measured at 200 counts per second of ²³⁴U and 200 ratios were taken for each sample. Thorium runs were more variable depending on the sample size but at least 80 ratios were collected for each sample. Machine performance was assessed during the collection of this data by repeated measurement of the U-500 standard, which yielded an average ²³⁵U/²³⁴U of 95.79 ± 0.35 (2 SD; N = 16), within error of the certified value of 95.95 (Garner et al., 1971). Two total-procedure blanks (from dissolution to analysis) were measured as 68 and 22 pg ²³⁸U and 77 and 55 pg ²³²Th.