The fiery face of the Arctic deep
Results from a German-American Arctic expedition to the Gakkel Ridge have implications for the understanding of the generation of new seafloor
The Gakkel ridge is a gigantic volcanic mountain chain stretching beneath the Arctic Ocean. With its deep valleys 5,500 meter beneath the sea surface and its 5,000 meter high summits, Gakkel ridge is far mightier than the Alps. This is the site of seafloor spreading that is actively separating Europe from North America, and was the goal of the international expedition AMORE (Arctic Mid-Ocean Ridge Expedition) with two research icebreakers, the "USCGC Healy" from USA and the German "PFS Polarstern". Aboard were scientists from the Max Planck Institute for Chemistry and other international institutions. The scientists had expected that the Gakkel ridge would exhibit "anemic" magmatism. Instead, surprisingly strong magmatic activity in the West and the East of the ridge and one of the strongest hydrothermal activities ever seen at mid-ocean ridges were found. These results require a fundamental rethinking of the mechanisms of seafloor generation at midocean ridges (Nature, January 16 and June 26).
The Gakkel ridge extends about 1800 kilometers beneath the Arctic Ocean from north of Greenland to Siberia. It is the northernmost portion of the mid-ocean ridge system, the global 75,000 kilometer long volcanic chain where the ocean crust is generated by erupting magma. Because of its extremely slow spreading rate of about one centimeter per year, the slowest rate of any mid-ocean ridge and 20 times slower than the better explored East Pacific ridge, Gakkel ridge is of particular interest for scientists. It shows a number of unique phenomena that could give more information about the generation of new oceanic crust.
Current theories of oceanic crustal production predict that volcanic activity deminishes as the spreading rate of the tectonic plates decreases and only little or no hydrothermal activity. Instead, the scientists found high levels of volcanic activity. "We expected the volcanic activity to gradually decrease from West to East as the magmatic systems shut down. Instead, approximately in the middle of the survey area, the magmatism shut down completely, then dramatically increased," says Dr. Jonathan Snow, the leader of the research group from the Max Planck Institute. This group was responsible for the petrological and geochemical investigations.
Hydrothermal hot springs on the seafloor were also far more abundant than predicted. "We expected this to be a hydrothermally dead ridge, and almost every time our water measurement instrument came up, they showed evidence of hydrothermal activity, and once we even 'saw' an active hot spring on the sea floor," noted Jonathan Snow. The biologists on the expedition theorize that Arctic hydrothermal vent communities have been cut off from the rest of the worlds oceans for long periods of time, and may have conserved archaic forms.
The central region without magmatic activity is unique in the worlds mid-ocean ridges, having no volcanic crust whatsoever. Here, scientists can directly sample rocks belonging to the Earths upper mantle, which is covered on every other part of the globe by thousands of meters of crustal rocks. Some of these mantle rocks were unusually well preserved, "I just about fell off my chair the first time I saw them in the microscope," says Jonathan Snow, "some of these samples looked just as if they had been brought right from the upper mantle by magic, not even a trace of alteration by seawater."
The observations made at the Gakkel ridge demonstrate that volcanic activity in certain regions is not only dependant on the spreading rate. Other factors such as the chemical composition and the temperature of the mantle at depth must be taken into account when describing the behavior of mid-oceans ridges.