Sedimentary fluxes at the Mid-Atlantic Ridge: sediment sources, accumulation rates, and geochemical characterisation

Abstract

Oxygen isotope stratigraphy was established, and various geochemical and mineralogical investigations were performed on Mid-Atlantic ridge crest sediments at 37-39°N in the vicinity of the Lucky Strike and Menez Gwen hydrothermal sites, in order to characterize sediment sources in time and space and to describe and quantify hydrothermal input in sediments. Multivariate statistical analysis of geochemical data demonstrated that volcaniclastic, ultramafic, detrital and hydrothermal sediment sources are present in addition to biogenic carbonate, which dominates the chemical composition of most samples. The hydrothermal contribution includes metals derived from hydrothermal solutions (Fe, Mn, Cu) and elements scavenged from the water column by hydrothermal oxyhydroxides (P, V, As). The distribution of hydrothermal factor scores and the pattern of metal accumulation rates suggest maximum hydrothermal input at 3rN south of the Lucky Strike vent field, which may include contributions from other presently undiscovered vent fields south of the area of the present study. Metal accumulation rates are significantly higher than at abyssal plains remote from the ridge axis, and comparable to other hydrothermally influenced environments. While the downcore distribution of hydrothermal tracers was obviously modified by early diagenetic processes, time-integrated average fluxes clearly indicate continuous hydrothermal influence in the study area at least down to 55,000 years B.P .. Barium appears to be dominantly of hydrothermal origin in sediments on the flanks of the Lucky Strike seamount, where it may be derived from redeposited hydrothermal precipitates containing abundant barite. A minor contribution of plume-derived fine-grained sulfides and possibly barite may be present in distal sediments based on the occurrence of a cubic phase, probably sphalerite, in gravity concentrates (>3.0g/cm1 ). However, biogenic sources seem to predominate in these sediments. Ba maxima, particularly during early Tennination I, were tentatively interpreted as indicators of productivity spikes which could result from a combination of incursions of cold surface water and enhanced terrigenous input. An allochthonous chrysotile-dominated serpentine layer is intercalated within pclagic sediments in a basin in the central part of the 38°05'N fracture zone. This material was derived from alteration of ultramafic rocks and may represent a low-temperature hydrothermal deposit associated with this rock type. It was apparently transported down the walls of the fracture zone in this area of steep relief. Cr/Ni-ratios suggest an olivine-enriched protolith, which is in accord with the results of previous petrologic studies of pcridotites at the Mid-Atlantic Ridge, and related to the proximity of the Azores hot spot. The predominance of chrysotilc suggests that serpentinization took place at temperatures <200°C. Smectite/chlorite mixed layer phases and chlorite formed at the same or slightly higher temperatures. The accessory phases talc and tremolite indicate greenschist facies conditions, which may have occurred prior to serpentinization during a history of retrograde metamorphosis. Alternatively, talc and tremolite were derived from another source area on the walls of the fracture zone.