The Lomonosov ridge, a mountain range under the Arctic Ocean, gained unusual notoriety in August, when a Russian submarine expedition planted a rust-proof titanium flag there to reinforce the country's Arctic territorial claims.
Now Brian Haley and colleagues report in Nature Geoscience that the ridge furnishes evidence of Russia's past influence on the region — at least, on its ocean circulation (B. A. Haley et al. Nature Geosci. doi:10.1038/ngeo.2007.5; 2007). They study neodymium (Nd) isotopic ratios in marine sediments in a core of sediments drilled from the Lomonosov ridge near the North Pole, at a sea depth of 1,250 metres.
The core represents a historical sketch of Arctic oceanography over the past 65 million years. The authors' big news is that the ratio 143Nd/144Nd of deep Arctic water that is preserved in the sediments was consistently far higher in the Neogene period between 15 million and 2 million years ago than it is now, indicating the influence of young, mantle-derived rock. In the past 2 million years, similarly high ratios are found only during short ice ages.
Haley et al. argue that the only credible source for such a signal is material from the Putorana basalts of the Central Siberian Plateau. But how did a surface-water signal from the Siberian coastal shelf sink to the great depths of the Lomonosov ridge?
The authors see the answer in the wider global climate, and the opening up of the Fram Strait 17.5 million years ago. This strait is the only deep passage for water to and from the Arctic, and runs roughly along the Greenwich meridian between Greenland and the Norwegian island of Spitsbergen. Its breaching allowed warm, saline waters to flow into the Arctic from the Atlantic. As these waters entered colder climes, evaporation increased, causing more precipitation and the growth of extensive floating ice shelves at northerly latitudes — including along the Siberian coast.
As sea water freezes, it rejects salt. Haley et al. propose that the resulting denser, briny water that sank beneath the developing ice sheet carried Russian sediment to the Lomosonov ridge during the Neogene, as it similarly does off Antarctica today.
Waters from the North Atlantic drift have low neodymium ratios. The authors estimate that, to maintain the ratios of the Neogene core record after the initial influx following the Fram breach, the flow of warm water from the Atlantic into the Arctic could have been no more than half of that today. They suggest that the Atlantic conveyor belt must at that time have stopped at a more southerly point than it does today. This is a well-established prediction for recent ice ages, which indeed also produce high neodymium ratios in the Arctic cores. But how it would have been maintained as an equilibrium state during the entire Neogene is unclear.
The model of an Arctic circulation dominated by seawater subduction off the Russian coast contrasts with today's picture, in which the Arctic is under a steady North Atlantic influence. In sketching the Arctic's sensitivity to past climate change, Haley et al. underscore its vulnerability to further change today.
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Webb, R. Siberian salt in the cellar. Nature 450, 808 (2007). https://doi.org/10.1038/450808a