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Tectonics and sediment geochemistry of Tuzo Wilson Seamounts, northeast Pacific Ocean Denton, Alexander W. S.


The Tuzo Wilson Seamounts are two submarine volcanic edifices that are located 50 km southwest of the Queen Charlotte Islands, offshore British Columbia. These geologically-recent features have formed on the boundary between the Pacific and Explorer plates in the vicinity of their triple junction with the North American plate. Seafloor photographs, acoustic images and conductivity-temperature surveys have been used to elucidate tectonic relationships and to detect hydrothermal activity. Seventy-five samples from seven short (<1.5 m) cores have been analyzed to characterize the sediments. Major elements, trace elements and halogens (I, Br, CI) were analyzed by X-ray fluorescence spectrometry (XRF). Carbon and nitrogen were determined with a CHN analyzer and carbonate carbon with a coulometer. Components of selected samples have been examined by scanning electron microscopy (SEM). X-ray diffraction (XRD) has been used to study the mineralogy of some samples with unusual geochemistry. The seamounts occur at the foot of the narrow continental slope, 30 km from the edge of the shelf, and consequently sedimentation is dominated by terrigenous input. The cores are located along a fault scarp which runs southwest from the shelf. Conductivity-temperature measurements indicate that discharge of hydrothermal fluids is occurring along this scarp. However, the geochemistry of the sediments does not reveal any evidence of hydrothermal activity, probably due to terrigenous dilution. Accelerator ¹⁴C dates indicate that core 3 penetrated the Pleistocene-Holocene transition. Abrupt changes in the depth distribution of Fe, Mg, Mn and the Rb/Zr ratio occur exclusively in this core, coincident with a shift in quartz/amphibole XRD peak-height ratios. These changes are attributed to a change in provenance and depositional conditions which occurred in response to climatic change and consequent rise in sealevel during deglaciation. A lower Holocene sedimentation rate for this core, suggested by its iodine distribution, explains why similar geochemical and mineralogical changes are absent in the other cores, which apparently have higher sedimentation rates. The distribution of iodine and bromine is closely related to that of organic carbon and the distribution of certain trace metals (Cu, Ni and Zn) may be influenced by biogenic productivity. Although evidence of hydrothermal activity was not detected in the sediments examined in this study, the Tuzo Wilson Seamounts are still considered a promising location for discovery of hydrothermal activity. Further research on the tectonic setting of the area and exploration for hydrothermal vent sites is strongly recommended.

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