Geology News
No hole at the Pole October 17, 2001
********************************************************
A major collaborative expedition to the waters of the North Pole has brought new insights into the structure of the Gakkel Ridge, and shown that rumours of the North Polar Ice Cap's death have been grossly exaggerated
The polar research vessels RV Polarstern and USCGC Healy returned after a ten week successful expedition to the high arctic regions.
Together with the new US Coast Guard icebreaker, the USCGC Healy, the Polarstern, run by the Alfred Wegener Institute for Polar and Sea Research (AWI) explored the Gakkel Ridge, the most northerly extension of the Mid-Atlantic submarine mountain chain during a combined AMORE-expedition (Arctic Mid-Ocean Ridge Expedition). Both ships managed to reach the North Pole - exactly 10 years after RV Polarstern and the Swedish icebreaker Oden had steamed to this magic location.
For the first time an important part of the geologically active Gakkel Ridge has been mapped and sampled – and with the highest precision, according to Jörn Thiede, director of AWI, and chief-scientist on the expedition.
The 1800km long Gakkel Ridge represents the most northerly extension of a 60,000 km long globe-encircling submarine volcanic mountain chain named the Mid Ocean Ridge System. Through the central fissures of the Mid Ocean Ridges magma forms in the earth's mantle and extrudes on to the sea floor, thus forming new oceanic crust.
The Ridge systems represent the borders of about a dozen of plates, making up the entire Earth's crust, which resembles the seamed surface of a soccer ball. Spreading rates of the diverging plate boundaries can be as much as 10 to 20cm per year, (East Pacific Ridge), though much lower rates are known.
The Gakkel Ridge, situated in the central part of the eastern north Polar Sea forms the most northerly boundary between Eurasia and North America. At the same time it represents the ridge segment with the lowest known spreading rates – less than 1cm per year on the average. The northern Atlantic ocean and the eastern Arctic ocean formed in early Tertiary period, i.e., during the last 55 Ma or so. These two oceans and the Northern boundaries of Europe are therefore geologically young, explains Jörn Thiede, a specialist in palaeoceanography.
For the first time, as a result of this expedition, ocean geologists now have precise maps of the Gakkel Ridge. “During the AMORE cruise the western part of the Gakkel Ridge has been mapped, measured and sampled precisely by both the Healy and the Polarstern in a combined effort. We now have a clear picture of this submarine mountain chain, which has for a long time been nameless and mysterious: below a deceptive, drifting Arctic sea-ice cover, a mountain chain is hidden which in places rises thousands of meters over the neighbouring deep sea basins. The crest is divided by an elongated, narrow and not continuous rift valley, where dramatic volcanic processes recently occurred.” says Thiede.
The region has not wanted for attention. In 1999, during the SCICEX-program (Scientific Ice Experiment), US nuclear submarines were used to do research under the Arctic sea-ice and studied the Gakkel Ridge using Side-Scan Sonar, discovering - in the centre of the Gakkel Ridge, at 85° East - evidence of submarine volcanic eruptions.
The Gakkel Ridge is the ideal place to test present models of global sea floor spreading on ultra slow ridges, says Dr. Wilfried Jokat (AWI). “A preliminary evaluation of seismic data shows pronounced variations in crustal thickness along the Gakkel Ridge. The crustal thickness values do not reflect any model predictions, varying between two and six kilometres. Apparently in a few places mantle rocks are displayed within the rift valley and are altered exclusively by penetrating seawater and resulting hydrothermal convection cells.”
Also spectacular are the seismic profiles through the bordering deep-sea basins. They allow, for the first time, a clear estimation of the variations in thickness of the oceanic basement, which is covered by kilometre-thick sediments. Following precise bathymetric measurements the Polarstern and the Healy have revealed the existence of complex volcanic cones in many areas of the Gakkel Ridge.
These measurements also clearly demonstrate, says Jokat, that the central Gakkel Ridge rift valley is deeper than any other mid-ocean ridge valley known on Earth. “This is clearly controlled by its slow spreading rate. Furthermore, it appears that an unexpectedly high number of volcanoes can be seen in the rift valley. These phenomena are revealed by the high quality of our bathymetric maps, which allow a surprising view of the structure of this ridge and thus clearly constitute one of the most spectacular results of our journey.”
The work of rock sampling and sediment coring in the vicinity of the Gakkel Ridge was truly "done by hand". Sampling rocks was done by dredging or with the help of a giant TV-controlled grab. Sediments were recovered using a gravity and kasten corer. The results of the sampling campaign were excellent, and helped to throw considerable light upon the origin and nature of the Gakkel Ridge. Some scientists had predicted that the Ridge had expected to sample mainly peridotites - rocks from the Earth’s mantle – emplaced on the sea floor by tectonic processes. “However, instead of peridotites the samples brought up mainly basalts, which struck me absolutely by surprise” says Dr. Jonathan Snow of MPI Mainz.
Sediment cores recovered from close to the Siberian shelf also give new insights into glacial history. It now appears that the glacial maximum in Northern Siberia happened some ten thousand years before the maximum observed in Scandinavia and the Alps, (which took place before 20,000 years) explains Dr. Robert Spielhagen from GEOMAR Research Centre in Kiel, Germany. “We intend, with our investigations on the sediment cores, to date the arrival of the glacial maximum on the Arctic Ocean and show that the resulting melting waters influenced oceanic circulation and the world's climate.”
Another discovery of the expedition was that the Central North Polar Sea is indeed permanently covered by sea ice. Last year, however, news focused public attention on a giant open water pit that opened at the North Pole – trumpeted (wrongly) as an impressive proof of the effects of global warming. AWI specialists believe that such open water pits, called polynia, exist every summer in great numbers within the sea ice cover, and are nothing unusual.
Therefore one of the goals of this expedition was the observation of variations in sea ice thickness and distribution in the Arctic Ocean, and to compare this to data from 1991 and 1998. “The ice is back. We saw normal conditions this summer on the North Pole, kilometre-wide large ice flows with numerous small and larger polynias in between. The ice was already covered by fresh snow and on the water new ice was forming relatively early compared to previous seasons. The sea ice thickness however, was less than our previous measurements, 2m compared to 2.5m in 1991” according to Dr. Christian Haas from the AWI, head of the ice researcher group.
