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Putting One Thing on Top of Another

Superposition means putting one thing on top of another. Nature does it all the time, but it is only in the past thousand years or so that we have worked out how to exploit it. Ibn Sina, an 11th-century Persian better known in the West as Avicenna, understood how Nature piles younger sediment on top of older, and Leonardo came very close. The first person to articulate the Principle of Superposition clearly, though, was a 17th-century Dane, Steno: In any pile of sediment, the youngest is on top and the oldest is on the bottom.

It is an idea so blindingly obvious as to sound stupid, but for a long time the obviousness blinded us to its potential: depth in the pile is equivalent to time before the date of the top. With patience and hard work, there is a historical record waiting there for us to decode.

There are exceptions that prove the rule. For example folding can overturn the layers. Little beasts that live just under the sea floor can blur the layers by burrowing. In glaciers, where the accumulating snow is a sediment just as much as the mud on the sea floor, the main problems are flow, which stretches and squeezes the layers, and refreezing of meltwater, which mixes this year’s accumulation with that of earlier years. A satisfactory solution is to drill at the summit of an ice sheet, where there is no melting and the flow rate is negligible.

The payoff has been invaluable. Ice cores give us our most detailed picture of the Earth’s history over the past million years. We have barely begun to unravel the story. The wealth of incident in the story is so rich that it is hard to know how to pick and choose, but a recent technical advance by Elizabeth Thomas and colleagues makes a good start. They cut slices just 2 millimetres thick from a 4.5-metre section of a core from the interior of the Greenland Ice Sheet. This section, 2070 metres beneath the surface, is estimated to represent the years from 36,401 to 36,169 BC - at a rate of 7 to 11 samples per year. The assignment of calendar years is a bit dodgy. The dates could be out by more than 1400 years. But the relative error, from bottom to top of the section, is only about three years, and the march of the seasons all those years ago can be seen distinctly in the varying concentrations of dissolved ions. We also learn interesting facts such as that 36,263 BC was a rather dry year, while 36,262 BC was so-so and 36,261 BC rather snowy.

There is more to this work than minute detail. It tells the story of the transition from a full glacial state to the warm climatic stage DO-8. The last ice age is peppered with these DO or Dansgaard-Oeschger events, warmer episodes that lasted 1000-1500 years and began abruptly.

The authors are properly cautious about interpretation. Their aim was more to show what attention to detail can uncover than to write the last word about the transition to DO-8. But they do suggest that the transition lasted just 21 years, during which snowfall increased by a half and temperature rose by 11.4 °C. This last number calls for particular caution. It needs to be seen in context, because it probably represents a local rather than a global change, and there are some technical complications to be sorted out. But at face value it implies warming at 0.5°C per year, a hundred times faster than the global warming of the 20th century and ten times faster than some extreme predictions for the 21st century.

Dansgaard-Oeschger transitions are not like the warming that is about to happen this century. For one thing, they are almost certainly not due to increases in greenhouse-gas concentrations, at least not primarily. They are more probably related to abrupt changes in the circulation of the north Atlantic Ocean. But they do share the attribute of abruptness with our near future, and that makes them intensely interesting. Avicenna and Leonardo would have understood why.

Posted by Graham Cogley on June 29, 2009 3:00 PM | Permalink