Calcareous nannofossils from Leg 168; biochronology and diagenesis

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A study on late Pliocene and Quaternary calcareous nannofossils from the eastern flank of the Juan de Fuca Ridge was carried out to provide a detailed biochronology for sediments recovered during Leg 168 and to investigate the effects of hydrothermal circulation and turbidite activity on diagenesis of nannofossils. Through high-resolution stratigraphical analysis, 10 Quaternary nannofossil events were determined and a detailed biochronology of the Quaternary was obtained. Upper Pliocene sediments were also recovered but did not contain any upper Pliocene marker species, such as Discoaster species. However, an age of younger than 3.2 Ma was estimated for these sediments based on evidences of nannofossil assemblages. Based on the resolved biostratigraphy, sedimentation rates of sediment sequences at these 10 sites and their variations were determined. A sedimentation hiatus between basal sediments and basements in this young seafloor region was observed. Relative abundance of nannofossils in sediments is largely changed by dilution of turbiditic materials. Evidence suggests that variations in relative group abundance of nannofossils in the Juan de Fuca Ridge, affected by dilution of turbiditic materials, does not reflect the real variation in production of this group of microfossils. Observations of this study suggest that calcite overgrowth of nannofossils is controlled by temperature, heat flow, thermal gradient in sediments, and the variation in composition of pore water, as well as the supply of calcite materials in sediments. These results provide a detailed knowledge about effects of low-temperature hydrothermal alteration on diagenesis of nannofossils. Downhole variations in dissolution degree of nannofossils in sediment sequences at all Leg 168 sites were presented and compared with pH profiles. The correlation between these data indicates that nannofossils are very sensitive to variations in pH units in sediments. A reduction of pH will result in dissolution of nannofossils. Variations in pH units are induced by hydrothermal circulation as a result of a number of interactions between water /sediments and water/basement. The dissolution of nannofossils at Sites 1031 and 1032 were directly affected further by the pore-water upward flows at these two sites. A few records of nannofossil preservation suggest that the effects of hydrothermal activity on diagenesis of nannofossils might involve more complicated processes and mechanisms that we do not understand yet.

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Su, X. , Baumann, K. H. and Thiede, J. (2000): Calcareous nannofossils from Leg 168; biochronology and diagenesis , Proceedings of the Ocean Drilling Program, Scientific Results, 168 , pp. 39-49 .

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