18O values. When this taxon was rare, Morozovella crassata, Acarinia praetopilensis, or Globigerinatheka mexicana were chosen as the best alternatives.
For isotopic analyses, 2.8-cm3 samples from Hole 1051B were examined at 10-cm intervals. All samples were dried, weighed, soaked in a Calgon/peroxide solution overnight, and wet sieved on a 63-µm mesh. The >63-µm size fraction was then oven dried at <50°C and weighed again to obtain a measurement of percentage coarse fraction (>63 µm). For planktonic foraminiferal analyses the >63-µm fraction was dry sieved into three different size fractions: >355 µm, 250-355 µm, and <250 µm. Isotopic examination was conducted on planktonic foraminifers from the 250- to 355-µm size fraction. A narrow size fraction was selected to overcome vital and ontogenetic effects on stable isotopic interpretation (Shackleton et al., 1985; Corfield and Cartlidge, 1991; Pearson et al., 1993).
The mixed-layer dweller Morozovella spinulosa was selected for analysis because of its ease of identification, abundance within the samples, and light 18O values. When this taxon was rare, Morozovella crassata, Acarinia praetopilensis, or Globigerinatheka mexicana were chosen as the best alternatives.
For planktonic isotopic investigation, multiple specimens of the same species were picked for each sample studied, normally 5-20 individuals depending on abundance. Analysis of multiple specimens provides results that are nearer to the species mean stable isotopic value than analyses conducted on single specimens. However, using this method, data on intraspecific deviation are lost (Pearson and Shackleton, 1995). Sample weights were usually 0.17 ± 0.03 mg. Before analysis, specimens were placed in methanol and cleaned ultrasonically to dislodge attached fine calcite particles. Ultrasonic cleaning was repeated when visual examination proved this to be required.
All planktonic foraminifer samples were analyzed isotopically using a VG Isogas Prism III mass spectrometer at the University of Edinburgh, Scotland. Normal corrections were employed, and results of stable isotope measurements are expressed in parts per thousand relative to the Peedee belemnite (PDB) standard reference carbonate of zero (Craig, 1957). Silver Mine (SM) calcite powdered standard was measured concurrently (mean = 0.20 mg) to record analytical precision and instrument calibration. Replicate analyses of standards gave rise to standard deviations of 0.09
for 18O and 0.05 for 13C.
The benthic foraminifer Nuttalides truempyi was picked from the >150-µm fraction and analyzed isotopically with a Finnigan MAT252 mass spectrometer and associated automated carbonate device ("Kiel Device") in the Department of Geology and Geophysics at the Woods Hole Oceanographic Institution. Samples (average weight = 60 µg) were lightly crushed in the reaction vessels to ensure complete reaction with phosphoric acid at 70°C. Six standards were run with each set of 40 unknowns; standards included Carrara Marble, Atlantis II deep-sea coral, and B-1 marine carbonate. Results were corrected to VPDB (PDB estimated by analysis of NBS-19) followed by a second correction for sample gas volume. Replicate analyses of all three standards yield standard errors of 0.08 for 18O and 0.04 for 13C as averages for reproducibility on the A and B lines in the Kiel Device.
Leg 171B material has never been deeply buried, resulting in the excellent preservation of foraminifers. Light and scanning electron microscopy shows the foraminifers to be devoid of carbonate infilling and visible dissolution. Pores on the outer and inner test walls are plainly visible with no surficial overgrowth. Preservation of primary calcite is shown by cross sections through the test walls where pores are open and smooth. A minor amount of fine carbonate debris (mainly coccoliths) are seen attached to the test surfaces. Good preservation is also confirmed by the difference in 18O between the planktonic species and between planktonic and benthic species, whereas uniform 18O values are forecast from models of bulk carbonate diagenetic alteration (Killingley, 1983; Schrag et al., 1995). Our results will contribute to the debate on mechanisms of climate change by establishing subtropical SSTs from well-preserved and complete material, without the consequences of diagenic alteration.
