Australia: The Land Where Time Began

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Gakkel Ridge in the Arctic Ocean – Discovery of Abundant Hydrothermal Venting on the Ultra-slow Spreading Ridge

Along mid-ocean ridges submarine hydrothermal venting is an important contributor to ridge thermal structure (Chen, 2000), and the distribution on a global scale of such vents has implications for the heat and mass fluxes (Baker, German & Elderfield, 1995) from the crust and mantle of the Earth as well as for the biogeography of organisms that are endemic to vents (van Dover et al., 2002).

It has been predicted by earlier studies that the hydrothermal venting incidence would be extremely low on ridges that are ultraslow spreading, i.e., ridges with full spreading rates of less than 2 cm/year, which comprise 25 % of the total global ridge length, in ultramafic as well as volcanic rocks (Baker, Chen & Phipps-Morgan) would host such vent systems. In this paper Edmonds et al. present evidence for active hydrothermal venting on the Gakkel Ridge, which, at 0.6-1.3 cm/year, is the slowest spreading mid-ocean ridge and also the least explored. Edmonds et al. identified hydrothermal plumes dispersing from 12, at least, discrete vent sites, based on water column profiles light scattering, temperature and manganese concentration along 1,100 km of the rift valley. A reassessment of the geologic conditions controlling such hydrothermal circulation in ultraslow-spreading ridges is required, as a result of their discovery of such abundant venting, as well as its apparent localisation near centres of volcanism.

Plumes resulting from such high temperature venting on the floor of the ocean ascend through the oceanic water column, which entrain surrounding seawater until they reach the neutral buoyancy level some hundreds of metres above the seafloor (Lupton et al., 1985). A means by which new thermal vent sites can be located (Baker, German & Elderfield, 1995; Klinkhammer, Rona Greaves & Elderfield, 1985; Sheirer; Baker & Johnson, 1998; Charlou & Donval, 1993) is provided by subsequent spreading laterally of these plumes and their associated signatures in temperature, chemical and suspended particle material.

The unknown makeup of chemosynthetic faunal communities surrounding hydrothermal vents is one of the factors that drive the continuing search for hydrothermal vents on the Gakkel Ridge. The faunal inhabitants of the known vent sites in the Atlantic and Pacific oceans differ markedly from each other, and there are also distinct biogeographical differences within each of these oceans (van Dover et al., 2002; Tunnicliffe & Fowler, 1996). There is no modern ridge-depth deep water connection between the Gakkel Ridge and other parts of the mid-ocean ridge system south of Iceland. That there is no connection between the Arctic Ocean and the other major ocean basins during its history is suggested by current knowledge of the tectonic history of the Arctic Ocean (Johnson, Pogrebitsky & Macnab, 1994; Lawler et al., 1990). It is therefore likely that new species of vent biota, which have evolved isolated from those in other oceans, will therefore be found at hydrothermal vent sites on the Gakkel Ridge (van Dover et al., 2002). Future studies which locate these vent sites, and return images and samples from them, will provide insights into the evolution of vent organisms, and the routes they used to migrate into these sites.

Sources & Further reading

1. Edmonds, H. N., et al. (2003). "Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean." Nature 421(6920): 252-256.