With the introduction of Australia’s Antarctic air link, scientists can investigate the link between solar activity and climate using short-lived radioactive isotopes in ice cores.
A cosmic ray, a proton far more energetic than our largest particle accelerators can produce, is created in the remnants of a distant supernova explosion. For thousands of years it travels through space at nearly the speed of light, passing the occasional nebula and galaxy, heading for a spiral arm of the Milky Way. It is on a collision course with our solar system and, as it nears, it penetrates the heliosphere and is deflected by the Sun’s magnetic field. But not enough. As it closes in on Earth its path is bent to follow a terrestrial magnetic field line towards the southern geomagnetic pole.
At about 20km above the Earth’s surface, within the stratosphere, the cosmic ray collides with atmospheric atoms, initiating a cascade of atomic and subatomic particles. These energetic secondary particles continue the process, smashing apart nuclei all the way to the Earth’s surface and a few metres below it. Eventually the huge energy of the cosmic ray is spent. One of the products of these nuclear reactions is the element beryllium — fragments of oxygen and nitrogen nuclei that were smashed into the smaller beryllium nucleus. The stable beryllium nucleus contains four protons and five neutrons and has an atomic mass of nine (beryllium-9). Beryllium has two radioisotopes of interest; beryllium-10, with an extra neutron and a half life of 1.5 million years, and beryllium-7, with only three neutrons and a half life of just 53 days (decaying to the more stable lithium nucleus).
Let us follow the fate of the cosmogenic (cosmic-ray-produced) beryllium-7 nuclei. Its solitary existence in the stratosphere is brief, before a small particle (perhaps a sulphate aerosol from a volcanic eruption years ago) traps it by adsorption to its surface. Now without any significant momentum of its own, it travels as a passenger on the small particle, at the whim of the air mass that carries it.
There is a barrier of stable air that separates the stratosphere from the troposphere beneath it, where weather patterns unfold. A disturbance causes a weakness in this stable layer that allows the particle and its beryllium-7 passenger to move into the troposphere. A pressure difference draws the air mass towards the Antarctic continent. Ice crystals jostle our particle and trap it. More ice mass is accumulated until the ice crystal feels the force of Earth’s gravity. It tumbles towards the summit of Law Dome, a small ice cap some 200km in diameter, on the edge of the main East Antarctic ice sheet, and the site of the Australian Antarctic Division’s (AAD) on-going ice core drilling and glaciology projects. Over time it is buried beneath snow layers.
Later, three scientists arrive at Law Dome. They drive a tube into the snow surface and extract an ice core containing the ice crystal and its cosmogenic beryllium-7 passenger. But this is not the only beryllium-7 atom in their ice core; there will be a few thousand others, among some 1022 water molecules, in each gram of ice. But the clock has been ticking since the isotope's formation in the atmosphere. All haste must be made to measure the beryllium-7 record before its short half life expires.
The scientists take five parallel ice cores and bundle them carefully into the back of a Hägglunds, taking them to Casey station, about 100km away. The new Antarctic air link is now indispensable. Within days of sampling the ice cores are flown to Hobart, where they are cut into sections representing the snowfall events that occurred over the past weeks and months. The ice samples are filtered to remove dust particles and micro-meteorites — other cosmic components that could confuse results. Beryllium-7 atoms from each sample are chemically trapped in small columns, which are flown by express freight to the Australian Nuclear Science and Technology Organisation (ANSTO) laboratories in Sydney. The ice samples with the beryllium-7 are now concentrated into a smaller volume and placed inside a gamma radiation detector for several days. By chance, the beryllium-7 atom that we have followed decays while it is inside the detector, becoming lithium-7 and emitting a gamma ray of characteristic energy. The gamma ray is detected and the (former) presence of our beryllium-7 atom is inferred. The epic journey of our cosmic ray across the universe has not been in vain! The scientists are also interested in the beryllium-10 signal, but this is not so urgent a matter. They have a few million years to make their measurement, but this time in a particle accelerator at ANSTO, using accelerator mass spectrometry.
Scientists are interested in beryllium radioisotopes for a number of reasons. The production rate of radio-beryllium in the atmosphere is governed by the intensity of the Sun’s magnetic field, which acts to deflect cosmic rays away from Earth. Furthermore, the Sun’s magnetic field is linked to solar activity, including total solar irradiance and the spectral distribution of energy from the Sun. Therefore the production of radio-beryllium in the atmosphere is intimately related to solar activity. Beryllium-10 retrieved from ice cores has become arguably the best indicator of solar activity over the past tens of thousands of years, way beyond the limit of the instrumental record. Of great interest to climate scientists is the possible relationship between changes in solar activity and climate change on Earth. It is widely agreed that solar activity is only a minor contributor to the observed warming of the last century (the period of anthropogenic global warming). However, over longer time scales some researchers have argued that changes in solar activity may have played a leading role in causing climate change on Earth.
The most significant area of uncertainty confronting research into beryllium-10 is a poor understanding of the processes that control its transport from the point of production in the atmosphere to its deposition at polar ice caps. For example, it is misleading to interpret beryllium-10 concentrations directly as ‘solar activity’ without accounting for the confounding influences of climate processes. To improve our understanding of solar influences on climate in the past, an improved understanding of these processes is sought.
Beryllium-7 is a useful tool to address this problem. Investigating the ratio of beryllium-10 to beryllium-7 provides information on the movement and residence time of beryllium-10 in the atmosphere. The very different decay rates of the two radioisotopes serve as a 'clock' for air mass age, which is particularly useful for studying the transport of gasses from mid-latitudes to the poles, and vertical gas exchange within the atmosphere. For example, high ratios of beryllium-10 to -7 are indicative of 'older' air from higher up in the atmosphere (i.e. the stratosphere) while lower ratios represent 'younger' air (i.e. with tropospheric origins).
Research into beryllium-7 has been difficult in the past, due to the long and infrequent sea voyages between Antarctica and Australia. The introduction of the air link allows ice cores to be extracted from Antarctica and measured for beryllium-7 within a couple of weeks. A preliminary study involving ANSTO, the AAD and the University of Tasmania, was carried out during the 2007–08 glaciology field campaign at Law Dome, using the air link to rapidly return samples from Law Dome summit to Australia. Processing of the data from this record is in progress and will be used to inform the strategy for a more targeted campaign to sample beryllium-7 at Law Dome in 2008–09.
JOEL PEDRO1 and ANDREW SMITH2
1 IASOS, University of Tasmania
2 Institute for Environmental Research, ANSTO