Surface water productivity at Ocean Drilling Program Site 907 has been assessed from total diatom abundance and Chaetoceros resting spore frequency. The sediments are divided into four intervals based on relative diatom abundance. Intervals 1 and 4 are mainly barren of diatoms. Interval 2 shows a highly fluctuating abundance pattern, whereas Interval 3 is a biosilica-rich middle to late Miocene (13.7-10.1 Ma) sediment containing abundant diatoms dominated by Chaetoceros resting spores. Resting spores of Chaetoceros are classified according to their forms. The round to elongate spores with smooth surfaces or spines/ridges dominate the assemblage throughout the biosilica-rich interval. The transition from the diatom barren interval to the biolsilica-rich interval at about 13.7 Ma is characterized by extremely high (100%-85%) Chaetoceros spore content. The late Miocene to late Pliocene (10.1-3.0 Ma) interval with highly variable diatom content is characterized with lower Chaetoceros percentages than in the biosilica-rich interval. We infer that the period between 13.7 and 10.1 Ma reflects high surface water productivity at Site 907. The high percentage of Chaetoceros resting spores at this interval suggests upwelling of nutrient-rich subsurface waters during this period.

Date of initial receipt: 7 July 1995
Date of acceptance: 11 December 1995

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