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Identification of microfossils found in the core was used to help date the sediment. Radiolarian zones are given in the North Pacific zonation of Kamikuri et al. (2004, 2007) wherever possible. Absolute ages of bioevents are also provided by Kamikuri et al. (2004, 2007).

Biostratigraphy analyses were generally carried out on material from the core catcher for each core, although a few cores did not have a suitable amount or quality of sample to enable microfossil identification.

Sample preparation for microscopic examination followed the standard techniques described by Sanfilippo et al. (1985). Samples were treated with hydrogen peroxide (20% H2O2) and sodium pyrophosphate (5% Na4P2O7) and heated to boiling. Hydrochloric acid (HCl) was added to dissolve calcareous components where necessary. Disaggregated particles were wet-sieved through a 63 µm mesh sieve. Remaining residues were removed and dried. Undisaggregated sediment was treated again. The clean particles were strewn on glass slides and mounted with Entellan-New. Slides were examined with a transmitted light microscope at a magnification of 100× to 400×. The first 100 specimens encountered on one slide were counted, after which slides were scanned to determine whether other taxa were present.

Preservation of the radiolarian assemblage was based on the following categories:

  • G = good (radiolarians show no sign of dissolution with only minor fragmentation).

  • M = moderate (radiolarians show evidence of moderate dissolution with obvious fragmentation).

  • P = poor (radiolarians show signs of a high degree of dissolution with very few intact specimens).