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Glaciers and beryllium

Chains of reasoning can be quite long, and quite tortuous, but they can join up the most surprising places. Consider beryllium-ten.

Almost all of the Earth’s beryllium is beryllium-nine, the isotope symbolized as ⁹Be and defined by having four protons and five neutrons in its nucleus. We also have a small stock of ¹⁰Be, which has an additional sixth neutron. Most of the ¹⁰Be is created in the atmosphere when incoming cosmic rays collide with gas molecules: it is a cosmogenic nuclide.

Cosmic rays are not rays but particles, mostly protons, that originate outside the solar system. Some are energetic enough to destroy the molecules with which they collide. For example, not only may a molecule of nitrogen be split into its two constituent atoms, but one of those atoms, a ¹⁴N (seven protons and seven neutrons), may be split into a helium (⁴He) and a ¹⁰Be.

¹⁰Be dissolves in rainwater, which is weakly acidic. When the rainwater falls on land, it becomes more alkaline by reacting with the surface minerals, which induces the ¹⁰Be to precipitate out. Unless it is carried away by running water or the wind, it accumulates.

The point of the story so far is simple: the longer a patch of surface has been accumulating the products of cosmic-ray collisions, the greater its stock of ¹⁰Be. If it remains in place, as is likely on the surfaces of large boulders, we can count the ¹⁰Be atoms, correct for the slow radioactive loss (¹⁰Be has a half life of 1.36 million years), and obtain the exposure age of the surface by a relatively straightforward calculation — as long as we know the rate of arrival of the cosmic rays.

But here we come to a tangle in the chain. The cosmic-ray flux is not constant. The list of corrections that have to be made is quite long, allowing for factors such as the varying strengths of the solar wind and the terrestrial magnetic field, the altitude and latitude of the exposed boulder, and the extent to which it is truly “exposed” and not shielded by the surrounding terrain, nearby obstacles, seasonal snow and so on.

The allure of that exposure age, however, has stimulated intense efforts over the past couple of decades. We have developed a good understanding of many of the required corrections, and, like the atoms on the boulders, reliable ¹⁰Be exposure ages are now accumulating in the literature.

Writing in Nature, Michael Kaplan and co-authors offer a new collection of ¹⁰Be ages from the terminal moraines of a glacier that once occupied a cirque in the Southern Alps of New Zealand. Three of the 37 ages are oddballs, but the remainder all cluster nicely, in groups from different moraines and morainic ridges.

The outermost moraine, about 2 km from the cirque headwall, has boulders that were first exposed to the atmosphere just before 11,000 BC. In sequence, the moraines nearer the headwall have ages of about 10,700 BC, 10,100 BC, 10,200 BC and finally, only a couple of hundred metres from the headwall, 9,500 BC. All of these ages are uncertain by 400-600 years.

The payoff of these observations is in the dates, which span very neatly the cold snap of the Younger Dryas. While abundant evidence for cooling was piling up in various northern palaeoclimatic archives, this little New Zealand glacier was dwindling into nothingness.

Kaplan and co-authors have thus nailed down, much more firmly than before, the conclusion that the hemispheres were out of sync at the end of the last ice age.

The atmospheric concentration of carbon dioxide increased during the Younger Dryas, ruling out a reduced greenhouse effect as an explanation for the northern cooling. The generally agreed explanation is that the north Atlantic received a rapid influx of buoyant fresh water from North America. This reduced the overturning of ocean water, and damped down the meridional circulation. But it also pushed the climate belts southwards, shifting the southern-hemisphere westerlies to higher southern latitudes at which they were better able to provoke oceanic upwelling. The resulting enhanced outgassing of deep-ocean carbon explains the increased atmospheric CO₂, and the ¹⁰Be atoms on the boulders show that the Younger Dryas was a time of net warming at least as far north as New Zealand.

So beryllium helps us to understand the hemispheric asymmetry of glacial climate. Long and tangled the chain of reasoning may be, but it does illustrate how complexity can be unravelled given doggedness and ingenuity.

Posted by Graham Cogley on October 11, 2010 3:00 PM | Permalink