Interaction between hemipelagic sediment and a hydrothermal system: Middle Valley, northern Juan de Fuca Ridge, subarctic northeast Pacific

Abstract

Slightly hydrothermally altered sediments were recovered from a sedimented rift valley with active hydrothermal discharge. The sediment consists mainly of terrigenous material with a low content of biogenic components. In spite of the alteration, glacial and non-glacial periods are still discernable based on lithology, mineralogy, carbonate content and bulk sediment geochemistry. In the vicinity of the core studied, extremely high heat flow anomalies and the presence of active black smokers suggest a rapid discharge of hydrothermal fluids along discrete pathways such as coarse-grained turbidite layers or faults. In the biogenic-rich fine-grained sediments, diffuse transport can supply the components and allow the time necessary for the formation of authigenic phases such as carbonate concretions, gypsum, barite, often Mg-rich amorphous silica and Mg-rich silicates (stevensite). Mg-rich amorphous silica could be the precursor of the Mg-rich silicates commonly found associated with hydrothermal deposits. Carbon and oxygen isotopic results on the carbonate nodules suggest (1) that the carbon in the nodules could be partly derived from methane which would have been oxidized at the oxic-anoxic boundary and in the sulphate reduction zone and (2) that successive cycles of hydrothermal fluid migration through the sediment could explain the variability in the results.