Journal of Conference Abstracts

Sediment Deposits and Tracers of Hydrothermal Activity South of the Azores (37°-39°N)

G. A. Auffret Département Géociences Marines IFREMER, Centre de Brest, B.P.70 29280 Plouzané Cédex, France

gauffret@ifremer.fr

B. Dennielou Département Géociences Marines IFREMER, Centre de Brest, B.P.70 29280 Plouzané Cédex, France

A. Boelaert Département Géociences Marines IFREMER, Centre de Brest, B.P.70 29280 Plouzané Cédex, France

H. D. Needham Département Géociences Marines IFREMER, Centre de Brest, B.P.70 29280 Plouzané Cédex, France

T. Richter Graduiertenkolleg, GEOMAR, Wischofstrasse 1-3, Kiel, Germany

C. Organo CNRS/CEA, avenue de la Terrasse, 91198 Gif/Yvette Cédex, France

J. L. Reyss CNRS/CEA, avenue de la Terrasse, 91198 Gif/Yvette Cédex, France

C. Müller 6 bis rue Haute, 92500, Rueil-Malmaison, France

P. Watremez Antenne BRGM, Centre IFREMER Centre de Brest, B.P.70 29280 Plouzané Cédex, France

Introduction

It is presently recognised that the impact of hydrothermal activity on the oceanic budget of a number of elements must be taken into consideration. This is particularly true for Mn, Li, Rb, Cs, Fe and Mn but also for Ba, Ca and Si; these last two elements, and possibly Fe, are involved in the organic carbon cycle (Martin, 1990). While the evaluation of present day hydrothermal activity is based on in-situ sampling of hydrothermal plumes, the assessment of the historical variability of the hydrothermal activity relies on the sedimentary record or on hydrothermal precipitates. A pre-requisite is that diagenetic processes do not completely obliterate the record of hydrothermal fluxes. As a contribution to the FARA program, we have investigated the distribution of the sedimentary cover close to the ridge axis, in order to test whether it could constitute a suitable record of past environmental changes (including hydrothermal activity variations) for the last tens of thousand years. During the GEOFAR cruise of RV Le Noroit (July 20th-August 17th 1993), we took about 25 coupled box cores and piston cores along the MAR south of the Azores (Fig. 1). The composition of surface sediments has been determined by neutron activation analysis and calculated on a carbonate free basis. Nannofossil assemblages have been investigated and oxygen isotope stratigraphy of selected cores has been established.

Fig. 1: Morphological framework, location of samples and tracer distribution.

Nature of the sediment deposits

Sediments are dominantly nannofossil ooze with carbonate contents ranging from 60 to 90% and were deposited during both glacial and interglacial periods. Only a few cores present turbiditic deposits (Fig. 2). Correlation of nannofossil assemblages and colour (grey level) variations with oxygen isotope stratigraphy of cores SU 9008 (Grousset et al., 1993), SHOM91 KS4, GEOFAR KF13 and KF9 allows us to fix the depth of the Last Glacial Maximum (18.3 kyr) in three E-W core transects across the ridge axis (Fig. 2).

Fig. 2: Lithostratigraphy, (_*) is an indicator for cores where oxygen isotopic studies have been made.

Distribution of manganese, arsenic and barium in surface sediments

Mn is a tracer of the activity of hydrothermal sources (black smoker type) which can be spread at an oceanic scale (Klinkhammer and Hudson, 1986). The highest Mn concentrations are observed north of the Lucky Strike segment and within this segment south of the Lucky Strike volcano. This hydrothermal input is presumably derived from the Lucky Strike hydrothermal field (Klinkhammer et al., 1995). It may include a contribution from other hydrothermal sites located to the south of our study area (German et al., 1996).

Maximum As concentrations are also observed in the southern part of the Lucky Strike segment. As is scavenged from sea water and co-precipitates with iron oxides within hydrothermal plumes (Birolleau, 1987; Feely et al., 1991). Noteworthy is the good correlation (0.88) between As and Fe concentrations which suggests that most of the iron present at the interface is of hydrothermal origin.

Maximum Ba concentrations are located south of the Pico Fracture Zone and near the Axial Mount 38°20'N. Ba has been frequently used as a tracer of paleoproductivity in pelagic sediments (Rutch et al., 1995) but is also present in hydrothermal fluids (von Damm, 1990). Hydrothermal vents emitting Ba-rich solutions have been recently discovered at the Menez Gwen site (Fouquet et al., 1994). Feely et al. (1994) have shown that Ba and Mn are present in plumes from the EPR, scavenged by particulate matter which has probably been resuspended. In the present case we notice, in Mn rich sediments (Mn >5000 ppm), a positive correlation (0.80) between Ba and Mn, which suggests that similar resuspension and adsorption processes take place in the Lucky Strike segment. The origin of Ba on the Axial Mount 38°20'N is as yet unclear. No active hydrothermal activity has been reported there, so its origin may be related to the early diagenetic precipitation of barite (in relation with the opaline components) or to a volcanic source (associated with Sc) as on the Lucky Strike volcano.

References

Birolleau, A., Thèse Doc., ENSC. Rennes, 261 p. (1987).

Feely, R.A., Trefry, J.H., Massoth, G.J. & Metz, S., Deep Sea Res. 38, 617-623 (1991).

Feely, R.A., Gendron, J.F., Baker, E.T. & Lebon, G.T., Earth Planet. Sci. Lett. 128, 19-36 (1994).

Fouquet, Y., Charlou, J.-L., Costa, I., Donval, J.-P., Radford-Knoery, J., Pellé, H., Ondréas, H., Lourenco, N., Segonzac, M. & Tivey, M.K., InterRidge News 3, 14-17 (1994).

German, C.R., Parson, L.M. & HEAT Scientific Team, Earth Planet. Sci. Lett. 138, 93-104 (1996).

Grousset, F., Labeyrie, L., Sinko, M., Cremer, M., Bond, G., Duprat, J., Cortijo, E. & Huon, S., Paleoceanogr. 8, 175-192 (1986).

Klinkhammer, G. & Hudson, A., Earth Planet. Sci. Lett. 79, 241-249 (1986).

Klinkhammer, G., Chin, C.S., Wilson, C. & German, C.R., Geol. Soc. Spec. Publ. 87, 87-96 (1995).

Martin, J. H., Paleoceanogr. 5, 1-13 (1990).

Rutsch, H.-J., Mangini, A., Bonani, G., Dittrich-Hannen, B., Kubik, P.W., Suter, M. & Segl, M., Earth Planet. Sci. Lett. 133, 129-143 (1995).

Von Damm, K.L., Ann. Rev. Earth Planet. Sci. 18, 173-204 (1990).