Fisher, A., Davis, E.E., and Escutia, C. (Eds.), 2000 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 168 39 4. CALCAREOUS NANNOFOSSILS FROM LEG 168: BIOCHRONOLOGY AND DIAGENESIS 1 X. Su, 2,3 K.-H. Baumann, 4 and J. Thiede 2 ABSTRACT A study on late Pliocene and Quaternary calcareous nannofossils from the eastern flank of the Juan de Fuca Ridge was car- ried out to provide a detailed biochronology for sediments recovered during Leg 168 and to investigate the effects of hydrother- mal circulation and turbidite activity on diagenesis of nannofossils. Through high-resolution stratigraphical analysis, 10 Quaternary nannofossil events were determined and a detailed biochro- nology 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 materi- als, 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 nanno- fossils. 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 hydrother- mal circulation as a result of a number of interactions between water/sediments and water/basement. The dissolution of nanno- fossils 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. INTRODUCTION The Juan de Fuca Ridge is a seafloor spreading center lying 300– 400 km off the coast of North America (Vine and Wilson, 1995). Hy- drothermal activity in the region has been observed (Baker et al., 1987; Davis and Fisher, 1994; Ginster et al., 1994). The eastern flank of the Juan de Fuca Ridge is covered by upper Pliocene and Quater- nary sediments. These sediments seal and host hydrothermal fluid flow, confining fluids in the crust at high average temperatures and leading to hydrothermal alteration of minerals in sediments (Lister, 1970; Davis et al., 1992; Davis, Mottl, Fisher, et al., 1992; Davis and Currie, 1993; Davis and Fisher, 1994). The objective of Leg 168 was to study several common hydro- thermal regimes that occur on the eastern flank of the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997). During Leg 168, 12 holes at 10 sites were drilled along a transect spanning oceanic crustal ages from 0.6 to 3.6 Ma with sediment thicknesses varying from 50 to 620 m (Table 1; Figs. 1–3). These sediments contain calcareous nanno- fossils with a variety of abundance and preservation states. Previous studies of calcareous nannofossils from the high lati- tudes of the northeast Pacific dealt with nannofossil zonations (Gart- ner, 1970; Bukry and Bramlette, 1970; Bukry, 1971a, 1971b, 1973, 1981; Haq and Lipps, 1971; Hay, 1971; Wise, 1973; Worsley, 1973). Mao and Wise (1994) studied calcareous nannofossils from the Mid- dle Valley of the Juan de Fuca Ridge and determined two upper Qua- ternary biostratigraphic zonations for sediment sequences of Leg 139. Very few previous studies dealt with diagenesis of nannofossils subjected to hydrothermal activity. Mao and Wise (1994) first ob- served that nannofossils were strongly dissolved due to hydrothermal activity when high temperatures ranged from about 65° to higher than 200°C. In the area of Leg 168, on the eastern flank of the Juan de Fuca Ridge, basement temperatures are low, varying from 15° to 62°C (Davis, Fisher, Firth, et al., 1997). Leg 168 materials allow us to investigate the effect of low-temperature hydrothermal alteration on diagenesis of nannofossils. Sediments from the eastern flank of the Juan de Fuca Ridge are dominated by turbiditic depositions. The effects of turbidites on the postdepositional preservation of nannofossils should be considered and examined. For the reasons above, the nominal objective of this study is to provide a detailed biochronology for Pliocene and Quaternary sedi- ments of Leg 168. Another aim of this study is to investigate the ef- fects of hydrothermal circulation and turbidite activity on the post- depositional variations of nannofossils. MATERIALS AND METHODS Materials The sediment region on the eastern flank of the Juan de Fuca Ridge belongs to the Cascadia Basin off the coast of North America. 1 Fisher, A., Davis, E.E., and Escutia, C. (Eds.), 2000. Proc. ODP, Sci. Results, 168: College Station TX (Ocean Drilling Program). 2 GEOMAR Research Centre for Marine Geosciences, Wischhofstraße 1-3, D-24148 Kiel, Germany. 3 Present address: China University of Geosciences, 29 Xueyuan Road, Beijing 100083, Peoples Republic of China. xsu@cugb.edu.cn 4 FB Geowissenschaften, Universität Bremen, Postfach 330440, D-28334 Bremen, Germany.