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Accelerator mass spectrometry; tracking carbon in the marine environment.

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. The solid graphite is then placed into the cathode of a device called a cesium sputter ion source. Negative ions are emitted when the graphite is bombarded by cesium.

Carbon atoms and molecules such as CH and |CH.sub.2~ (called hydrides) emerge from the ion source as a beam of negatively charged ions. A series of magnets splits the beam, permitting only particles with masses 12, 13, and 14 having a single negative charge to be injected into the accelerator. The three masses are simultaneously injected into the first acceleration region where they are accelerated to an energy of 2.5 million electron volts. There they pass through an argon gas canal where collisions with the argon atoms cause four electrons to be removed from each atom. Any molecules (such as hydrides) are also broken down into atoms at this point. Losing electrons converts each atom into a triple-charged, positive ion. The ions then pass into the second half of the accelerator region and obtain a kinetic energy of 10 million electron volts and a velocity 4 percent of the speed of light (12,000 kilometers per second). At this point the ions exit into the analyzer region. The beam is magnetically split again: Positively charged carbon-12 and carbon-13 ions are detected in charge collectors (called Faraday cups) while carbon-14 ions pass through more electrostatic and magnetic filtering and into an ultrasensitive gas ionization detector. There they are individually counted. The counting rate for a modern sample is about 80 per second (or 288,000 per hour), compared to 1 beta decay per hour using the old technique.

The AMS system at Woods Hole became operational in 1991, and has already demonstrated 0.5 percent precision for measuring modern carbon-14/carbon-12 ratios. Only 1 milligram of carbon is required, an amount small enough to make AMS useful in a wide range of applications. For instance, 240-liter barrels of seawater are no longer necessary for radio carbon analysis, as they were 20 years ago. Also, the dating of small quantities of plankton and Foraminifera is now possible.

The most ambitious AMS sampling program to date is under way with the World Ocean Circulation Experiment (WOCE). Scientists in this program are collecting seawater samples from many depths at stations across the world's oceans. Dissolved inorganic carbon dioxide is extracted from water samples as small as 100 milliliters, then reduced for analysis to a few milligrams of graphite. These carbon-14 measurements will provide scientists with a three-dimensional picture of ocean circulation, as well as details on ocean/atmosphere exchange processes.

In addition to seawater samples, the AMS system has been used to measure carbon-14 concentrations in coastal and lake sediments, shell carbonates, and corals. Together with known temperature ranges of certain marine species, these data can yield climate-change information going back almost 50,000 years, the current limit of the technique. As an example, planktonic Foraminifera extracted from different depths in a box core collected from the Norwegian-Greenland Sea were dated to identify the time and locus of Northern Hemisphere deglaciation.

All of these data will provide marine scientists and those who model climate change with the ability to measure carbon-exchange rates between the environmental reservoirs. The facility was established and is supported by the National Science Foundation, Division of Ocean Sciences.

Robert J. Schneider is a Senior Research Specialist and Glenn A. Jones is an Associate Scientist in the Geology and Geophysics Department at Woods Hole Oceanographic Institution.
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Author:Schneider, Robert J.; Jones, Glenn A,
Date:Dec 22, 1992
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