Journal of Conference Abstracts

Sediment Sources and Metal Accumulation Rates at the Mid-Atlantic Ridge South of the Azores (37°-39°N)

Thomas O. Richter Graduiertenkolleg, GEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany

trichter@geomar.de

Erwin Suess GEOMAR Research Center, Wischhofstr. 1-3, 24148 Kiel, Germany

Gérard A. Auffret DRO/GM IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

Michael Sarnthein Geologisch-Paleontologisches Institut, University of Kiel, 24118 Kiel, Germany

Introduction

Reconstruction of past hydrothermal activity at mid-ocean ridges can only be based on continuously accumulated pelagic sediments in the vicinity of hydrothermal sources. We present geochemical data on Mid-Atlantic Ridge sediments south of the Azores (37°-39°N) sampled during the GEOFAR cruise of RV Le Noroit in the framework of the FARA program, in an area where the Lucky Strike and Menez Gwen hydrothermal fields have been recently discovered. Metal accumulation rates (Fig. 1) have been determined in three cores dated by oxygen isotope stratigraphy.

Fig. 1: Temporal variability of Fe and Mn accumulation rates in core GEOFAR KF13 (37°35'N 31°50'W). Mn fluxes multiplied by 10 for scaling reasons. Surficial fluxes in oxidized surface layers (not shown) amplified by secondary diagenetic remobilization. Benthic oxygen isotope curve shown for stratigraphic reference. Supporting Mn data from Thomson et al., (1984; Nares Abyssal Plain, NW Atlantic) and Barrett et al., (1987; DSDP Leg 92, East Pacific Rise).

Factor analysis of geochemical data: Principal sediment sources

Sediments are dominantly pelagic oozes with 60-90% CaCO₃; hence biogenic carbonate is the dominant sediment source in time and space. Factor analysis (157 samples, 21 elements) indicates the presence of four principal non-biogenic sources: Factor 1 has high loadings on Si, Al, Fe and V and slightly lower loadings on Zn, Nb, Sc and Cr; highest factor scores occur on volcanic ash layers and samples from the flank of the Lucky Strike seamount containing abundant volcanic glass. Hence, this factor indicates a volcanic contribution which may be derived from the Mid-Atlantic Ridge and/or the Azores islands. Factor 2 has high loadings on Mn and P and lower loadings on As, V, Fe and Cu. Fe, Mn and Cu may be derived from hydrothermal solutions; As, P and V are known to be scavenged by hydrothermal oxyhydroxides in the water column (e.g. Trefry and Metz, 1989; Feely et al., 1991). Highest factor scores on samples from oxidized surface layers indicate diagenetic remobilization of metals and coprecipitated elements and reprecipitation under oxidizing conditions. The third factor has high loadings on K and Y and lower loadings on Ba, Zn, Rb and Pb and is interpreted as representing continentally-derived detritus. Highest scores on this factor occur during glacial periods and especially during Termination I. Factor 4 has high loadings on Mg, Cr and Ni. These are elements typical of ultramafic rocks; highest factor scores on samples from an allochthonous sedimentary serpentine layer in the central part of the 38°05'N fracture zone, which is derived from alteration of a marine ultramafic protolith.

Magnitude and spatial variability of metal accumulation rates

Close to the sediment surface, metal concentrations are amplified by the effects of secondary diagenetic remobilization. Underneath oxidized surface layers, metal accumulation rates vary from 12-170 mg/cm²*kyr Fe, 1.3-10 mg/cm²*kyr Mn, and 40-560 mg/cm²*kyr Cu, Zn and Ni. Maximum values are in the range of published data from hydrothermally influenced environments such as the East Pacific Rise (McMurtry et al., 1981; Barrett et al., 1987) and Lau basin (Cronan et al., 1986), whereas minimum values are comparable to values from the NW Atlantic Nares Abyssal Plain (Thomson et al., 1984) without hydrothermal input. By means of a simple rate calculation correcting for detrital and hydrogenic sources of Mn (McMurtry et al., 1991), we estimate that 0-60% of total Mn in sediments may be of hydrothermal origin. Maximum metal accumulation rates during the Holocene and Oxygen Isotope Stage 2 have been observed at 37°N south of the Lucky Strike hydrothermal site, suggesting ongoing hydrothermal activity in this area since at least 20,000 years ago.

Temporal variability of metal accumulation rates: Possible paleoceanographic controls

Peaks of metal accumulation rates at the base of the Holocene are probably caused by remobilization of these elements during Stage 2 and reprecipitation at the stage boundary in response to a decreasing supply of organic carbon. Further peaks of iron accumulation rates during Termination I, associated with increased Al accumulation rates, suggest the sporadic influence of detrital (eolian and possibly ice-rafted) sources of iron. Low accumulation rates of Mn during glacial stages 3 and 4 and increased fluxes during interglacial stage 5 might be related to different dispersal patterns of hydrothermal effluent caused by changes in bottom current direction. Alternatively, a more homogenous Mn distribution throughout the glacial ocean as a result of a longer residence time of Mn in the water column may be invoked. This was similarly proposed for the Galapagos microplate, and has been ascribed to decreased bottomwater oxygenation (Mangini et al., 1994; Frank et al., 1994).

Conclusions

The record of temporal variability of hydrothermal fluxes in pelagic sediments in the study area is modified by post-depositional diagenetic processes due to changing supply of organic carbon and maybe by paleoceanographic parameters such as changes in bottom currents or bottomwater oxygenation. However, time-integrated average fluxes strongly suggest a continuous influence of hydrothermal activity on sediment geochemical composition at least during the last 20,000 years, with maximum fluxes occurring at 37°N south of the Lucky Strike hydrothermal field.

References

Barrett, T.J., Taylor, P.N. & Lugowski, J., Geochim. Cosmochim. Acta 51, 2241-2253 (1987).

Cronan, D.S., Hodkinson, R., Harkness, D.D., Moorby, S.A. & Glasby, G.P., Geo-Mar. Lett. 6, 51-56 (1986).

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

Frank, M., Eckhardt, J.-D., Eisenhauer, A., Kubik, P.W., Dittrich-Hannen, B., Segl, M. & Mangini, A., Paleoceanogr. 9, 559-578 (1994).

Mangini, A., Rutsch, H.-J., Frank, M., Eisenhauer, A. & Eckhardt, J.-D., NATO ASI Ser. I 17, 87-104 (1994).

McMurtry, G.M., Veeh, H.H. & Moser, C., Geol. Soc. Am. Mem. 154, 211-249 (1981).

McMurtry, G.M., De Carlo, E.H. & Kim, K.H., Mar. Geol. 98, 271-295 (1991).

Thomson, J., Carpenter, M.S.N., Colley, S., Wilson, T.R.S., Elderfield, H. & Kennedy, H., Geochim. Cosmochim. Acta 48, 1935-1948 (1984).

Trefry, J.H. & Metz, S., Nature 342, 531-533 (1989).