A total of 147 samples from Holes 1179B and 1179C were taken at ~150-cm intervals, one sample per core section (Tables T1, T2); samples were initially collected for both calcareous nannofossil and diatom analysis. The samples were then prepared for examination under the light microscope for diatom content and abundance. Samples were prepared by treating ~2 cm3 of sediment with 20 mL of 30% H2O2 on a hot plate for 60 min to remove organic material. After cooling, 6 mL of 100% HCl was added to the solution and allowed to react until all carbonate dissolved. Samples were washed and centrifuged three times with distilled water to remove chemical residues from the solution. Samples were then centrifuged three times with a weak solution of Calgon (~5%) to suspend the clays. The diatom residue (0.5 mL) was diluted in 14 mL of distilled water, and 2 mL of the diluted solution was dried on a 22-mm x 40-mm coverslip. The diatoms were randomly dispersed over the entire coverslip as the solution dried. The coverslips were mounted onto glass slides using Norland optical adhesive-61 (refractive index = 1.56) mounting medium.
An Olympus BH compound microscope was used to examine the strewn slides at 1000x. Qualitative abundances of the present diatom species were estimated from 4 to 10 transects of the coverslip, depending on the abundance of diatoms in the sediment. Digital images were captured using either a Zeiss Axioscop with an Optronics camera or a Zeiss Axioscop II with a SPOT camera.
Relative abundance estimates of diatoms are based on the number of specimens observed per field of view at 1000x. These qualitative values presented in the distribution table are based on the following scale:
Preservation of diatoms were determined qualitatively and recorded as follows: