Magazine article Oceanus

Accelerator Mass Spectrometry; Tracking Carbon in the Marine Environment

Magazine article Oceanus

Accelerator Mass Spectrometry; Tracking Carbon in the Marine Environment

Article excerpt

The latest in accelerator technology is now available to marine scientists interested in the oceanic carbon cycle. At the National Ocean Sciences Accelerator Mass Spectrometry Facility at Woods Hole Oceanographic Institution, scientists can trace oceanic circulation, determine the age of seafloor sediments, and track nutrient flow from surface waters to the benthic environment with unprecedented precision and accuracy. Known as accelerator mass spectrometry (AMS) since its development at the University of Rochester in 1977, this technique combines classical mass spectrometry, which separates atoms or molecules by mass, with a high-voltage accelerator, which dissociates them into atomic ions of different charge states. AMS actually counts atoms of a selected mass and charge state from suitably prepared samples. It is practically 1,000 times more sensitive than counting radioactive decays from a carbon-containing sample, the method developed by Willard Libby in 1950.

In the global carbon cycle, the major reservoirs are the oceans (including ocean sediments), the terrestrial biosphere (plus sedimentary rocks), and the atmosphere. Carbon occurs in nature almost entirely as carbon-12, with only 1.1 percent as carbon-13 and one part in 1,000,000,000,000 as carbon-14 for modern materials. The origins of these three isotopes are quite different. The stable isotopes, carbon-12 and carbon-13, are derived from Earth's mantle, and released when carbon-bearing rocks weather at the surface. Organic matter deposited millions of years ago and used today as fossil fuel (oil, natural gas, and coal) contains only these two carbon isotopes. Small differences in the stable-isotope ratios result from diffusion and photosynthesis, and lead to lower carbon-13 levels in fossil fuels and vegetation than in the atmosphere and oceans. Stable-isotope mass spectrometry measures these subtle differences, a few parts per thousand, and distinguishes organic from inorganic carbon in the marine environment.

The unstable isotope, carbon-14, is produced continuously in the upper atmosphere, mostly by the interaction of cosmic rays with nitrogen. In modern times, it has also been produced as a byproduct of nuclear power and nuclear weapons testing. Its half-life of 5,730 years makes it useful as a tracer and for dating purposes, with limits set only by the available concentration and the sensitivity of the measurement technique. Since the 1960s, the carbon-14 (and also tritium or hydrogen-3) injected into the atmosphere and ocean by nuclear weapons tests has been useful for studying oceanic/atmospheric mixing and ocean circulation. A 1-milligram sample of organic carbon from the "prebomb days" contains about 50,000,000 carbon-14 atoms. In an hour, on average, only one of these atoms decays back to a nitrogen atom. Radiocarbon dating by counting the electrons emitted during these so-called beta-decays therefore requires a large sample and a long counting time to obtain enough detector counts for a precise age estimate.

The modern AMS technique is equivalent to counting the atoms in the sample and sorting them into three bins, according to their mass and charge. This requires them to be dissociated from any molecules and ionized into a unique charge state. It relies on the fact that carbon-14 readily forms a negative ion, whereas nitrogen-14, its closest competitor for mass selection, does not. (In fact, the early workers were looking for negatively charged nitrogen-14 when they realized the importance of this fact.) Samples can be prepared from any carbon-containing material, whether a shell carbonate, dissolved inorganic carbon dioxide in seawater, an organic fraction from a sediment, or an archaeological artifact. The sample is acidified or combusted to produce carbon dioxide, which is then completely reduced to solid graphite at high temperature in a small reactor. …

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