MATERIALS AND METHODS

Two individual barnacle fragments, each composed of multiple plates, were recovered from a >5-m-thick well-consolidated massive muddy diamictite at 262.63 meters below seafloor (mbsf) (Sample 178-1103A-28R-5, 93-95 cm) and 262.98 mbsf (Sample 178-1103A-28R-5, 127-129 cm) (Pl. P1). The recovered fragments are of the same unidentified species and were not attached to a hard substrate when recovered.

A review of the strontium isotope dating technique, including diagenetic considerations, is presented by Lavelle and Armstrong (1993) and McArthur (1994). In summary, surficial contaminants were removed from the fragmented shell surface by repeated 10-s ultrasound treatment in 1-M acetic acid and quartz-distilled water. All samples were visually inspected using a binocular microscope, and homogeneous and well-preserved macrofossil specimens were divided into working and archive splits. The archive fractions were examined using a scanning electron microscope (SEM) to identify original shell ultrastructure at the submicron scale (Pl. P2). For archive samples that were identified as homogenous and well preserved, the matching working halves were rinsed in distilled water in an ultrasonic tank and dissolved in quartz-distilled 1.75-M HCl.

Strontium was extracted using standard ion-exchange techniques and was loaded onto a tantalum filament as a nitrate. Isotope measurements were carried out using a VG Sector 54 mass spectrometer at the Department of Earth Sciences, University of Cambridge. ⁸⁷Sr/⁸⁶Sr ratios were normalized to our long-term laboratory value of NIST-987 = 0.7210249 (N = 107; 2 = 0.000017). Measurements were also normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194. Analytical blanks were typically <100 pg Sr. Corrected mean isotope measurements were converted to best-fit age and uncertainty using the LOWESS fit to the marine Sr curve of Howarth and McArthur (1997) (look-up table version 3: 10/99). As we have no long-term laboratory average ⁸⁷Sr/⁸⁶Sr value for modern biogenic carbonate, the long-term precision value for NIST-987 was used to calculate the 95% confidence limits on the best-fit age. No statistical attempt has been made to reduce sampling and analytical uncertainty below that of the long-term standard deviation value (17 x 10^-6). The Sr isotope ages are calibrated to the timescales of Shackleton et al. (1994) (0-7 Ma) and Cande and Kent (1995) (7-72 Ma).