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Xin Su, Karl-Heinz Baumann, Jörn Thiede:
Diagnesis of calcareous nannofossils in sediments from the Juan de Fuca Ridge, ODP Leg 168
(Poster)

The eastern flank of the Juan de Fuca Ridge is covered with Upper Pliocene and Quaternary turbidite sediments. These sediments seal and host hydrothermal fluid flow, confining fluids in the crust at high average temperatures, leading to hydrothermal alteration of minerals in sediments. A study of Late Pliocene and Quaternary calcareous nannofossils from the eastern flank of the Juan de Fuca Ridge was carried out to investigate the effects of hydrothermal circulation and turbidite activity on the diagenesis of nannofossils.

Relative abundance of nannofossils in sediments is largely changed by dilution by turbiditic materials. Evidence suggests that variations in relative group abundance of nannofossils in the Juan de Fuca Ridge, affected by dilution by 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, the variation in composition of pore-water, as well as the supply of calcite materials in the sediments. These results provide a detailed knowledge about the effects of low-temperature hydrothermal alteration on the diagenesis of nannofossils. Downhole variations in dissolution degree of nannofossils in sediment sequences at seven 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 the effects of hydrothermal activity on the diagenesis of nannofossils might involve more complicated processes and mechanisms, which we do not understand yet.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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