METHODS

The diatom biostratigraphy presented here was constructed based on the examination of strewn slides, viewed using plane and differential interference contrast light microscopy at a magnification of 625x (Zeiss Apo 40x, NA = 0.65). Sediments recovered at Site 1140 were diatom nannofossil ooze, silty diatom ooze, foraminifer-bearing nannofossil ooze, and nannofossil chalk. To select samples for further diatom analysis in this study, shipboard reports of moderately abundant diatoms in core catcher smear slides (prepared for nannofossil analysis) were used to bracket intervals with abundant diatoms.

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 had been 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.

Individual diatom species relative abundances were recorded as follows:

A = abundant; 11-100 specimens per 10 fields of view (FOV).

C = common; 6-10 specimens per 10 FOV.

F = few; 1-5 specimens per 10 FOV.

R = rare; 1 specimen per longitudinal (40 mm) traverse.

The same definitions were used for estimations of total diatom abundance of each sample. Preservation of the diatom assemblage was determined as follows:

G = good: individual specimens exhibit little or no dissolution or fragmentation; diagnostic characteristics are preserved and nearly all specimens can be identified to the species level.

M = moderate: individual specimens show evidence of dissolution or fragmentation; some specimens cannot be identified to the species level.

P = poor: individual specimens exhibit considerable dissolution or fragmentation; many specimens cannot be identified to the species level.

Each author prepared and examined every third slide in this sequence with the results tabulated in a composite range chart (Table T1). Estimated abundances presented in the distribution table should be considered qualitative abundances, reflecting that three separate people carried out preparation and examination. Before examination began, the authors agreed on key diatom characteristics used to define species concepts.

The biostratigraphy employed in this paper is that of Harwood and Maruyama (1992) and is presented in Figure F2. The ages of diatom datums (Table T2) are from ODP Leg 177 Shipboard Scientific Party (1999).

Diatom species considered in this paper are listed in the "Appendix" where they are arranged alphabetically by generic epithet. Bibliographic references can be found in Harwood and Bohaty (2001), Scherer et al. (2000), Harwood et al. (1998), Harwood and Maruyama (1992), Harwood and Bohaty (1991), or Schrader and Fenner (1976).