1

Jim Brinson CLFS 619A (Molecular Spectroscopy) Dr. Paul Mazzocchi 11/20/09

Applications, Advantages, and Problems with Accelerator Mass Spectrometry Radio Carbon Dating

Introduction:

There are two basic techniques that allow scientists to date archaeological

samples using radiocarbon chemistry. One technique is the conventional detection of beta

particles from the decay of C-14 atoms to measure the number of half lives (and thus the

relative age of the sample), and the other is via the use of an accelerator mass

spectrometer to actually count the number of C-14 atoms in the sample (Hedges &

Gowlett, 1984; Linick et al., 1989).

C-14 is a radioactive isotope of carbon found in the environment. It is constantly

found in the upper atmosphere when neutrons bombard N-14 atoms:

14N + 1n --> 14C + 1H

Then, via beta decay, the C-14 turns back into N-14, and this decay has a half-life

of 5730 years:

14C --> 14N + e-

This continuous formation and decay results in a nearly constant equlibrium amount of

C-14, which ends up forming CO2, and gets incorporated into plants through

photosynthetic pathways. These C-14's then make their way through the food chain, and

become dispersed through all living organisms. So, it makes sense that living organisms

2

maintain a constant ratio of C-14 to C-12 in their tissues that matches their atmosphere.

This carbon exhange ceases, obviously, upon death, and this ratio begins to decrease with

a half life of 5730 years. So, anything once living or made of once-living material can be

dated accordingly. For example, if an ancient artifact has a C-14 to C-12 ratio that is 25%

of that found in living organisms, it must be 2 half-lives old, or 11,460 years old. This of

course rests on the assumption that there is no large variance of atmospheric C-14 levels

over time.

Accelerator mass spectrometer. Photo courtesy of Japan Atomic Energy Agency (www.jaea.go.jp).

Methods for Using Accelerator Mass Spectrometry Dating:

There are clear advantages to using accelerator mass spectrometry (AMS) for

dating purposes as opposed to using conventional techniques, the biggest being that you

need only a very small sample size (20 – 500 mg, compared to 10 – 100 g) (Currie et al.,

1985). This obviously is very useful when one needs to preserve an archaeological

sample that is already small. AMS analysis of samples is also much faster than

3

conventional methods, taking only a few hours for a sample, compared to one or two

days with conventional methods (Linick et al., 1989). In addition, AMS measurements

are usually more precise with less background (a method has recently been developed to

detect C-14 in a sample while ignoring the more abundant isotopes that obscure the C-14

signal). However, AMS has its disadvantages, namely the cost of operation. Set-up and

maintenance of an AMS can cost millions of dollars (Hellborg & Skog, 2008). And,

contamination of sample is also problematic due to the small size of the samples used.

Consequently, the pre-treatment of samples to remove contaminants prior to testing is

thus a rigorous process (Hellborg & Skog, 2008).

As described by Litherland (1984), the premise of mass spectroscopy is that

atoms in a sample can be separated according to their atomic weights. In AMS, the

analysis begins by accelerating ions in the sample to very high kinetic energies. This is

usually accomplished with either a cycloctron or else a tandem electrostatic accelerator.

For pre-treatment, the C-14-containing sample is converted to graphite by first

combusting it to produce carbon-containing CO2, and then treating it with zinc to house

the carbon in the form of CO (Fifield, 1999). With iron as a catalyst, this CO can be

converted into graphite, which can be introduced into the AMS. It should be noted,

however, that the combustion process makes the sample susceptible to contamination

from common atmospheric atoms, such as N-14, which is particularly problematic in

mass spectrometry since its atomic weight is the same as that of C-14 (Fifield, 1999).

A few milligrams of the graphite, along with a sample of reference material, are

pressed onto a metal target disk, which will then rotate in order to be analyzed in

sequence (Jull et al., 1983). As described by Donahue (1995), cesium ions are shot at the

4

target wheel in order to ionize the carbon atoms, which are then focused and injected via

a magnet system, ultimately reaching the tandem accelerator. At the tandem acclerator, a

voltage difference of 2,000,000 V accelerates the ions to the positive terminal. At this

point, carbon ions continue on to a gas or metal foil to be converted into cations, while

other unstable anions are unable to reach the detector. After contacting the foil, the

carbon cations are now triple positively charged, which allows for separation since

molecules cannot exist with carbons in such a state (Matteson et al., 1992). These triple

charged carbons accelerate away from the positive terminal and are refocused to be mass

analyzed. A magnetic field deflects the trajectories of these moving ions, with the lightest

particles being deflected the most, and detectors can then count the separated particles.

The numbers of C-14 atoms in the orginal sample can then be compared to the C-12 and

C-13 atoms present, and isotope ratios can be determined, and the relative age can thus be

determined.

Applications to Archeaology:

An example of the application of AMS to archaeological or geochemical analysis

involves the carbon dating of bone samples. The basic idea behind bone dating is that

since bone tissue comes from once-living organisms, carbon exchange with the

atmopshere and environment occurred throught the duration of the organism’s life. Part

of the carbon exchanged would be C-14, and on average, C-14 amounts in an organism’s

tissues should be relatively consistent with levels found in their immediate environment

However, once the organism dies, exchange no longer occurs (radiocarbon age = zero).

So, “time-width” of the sample becomes important, since it gives a ratio of growth to

5

duration of exchange in an individual, and allows approximation of the sample’s age

(Gillespie et al., 1984). However, these samples are easily contaminated, since most

bones are found surrounded by other organic matter, which obviously contain much

carbon. For example, if limestone was near the sample and is a source of contamination,

since limestone is of a geological origin and thus far more older than living organims, it

could lead to a far older age than the true age of the sample. On the other hand, roots

Fish bones. Photo courtesy of University of Bradford (www.brad.ac.uk).

from plants interacting with a bone sample could lead to carbon isotope ratios for a

younger age than the true age. Soil acids, such as those from microbes, could work either

way depending on the age of the source of these acids. So, pre-treatment of bone samples

is very, very important.

Radiocarbon dating timescale. Courtesy of CSG Network (www.csgnetwork.com)

Recently, AMS bone dating was able to prove that human remains collected in the

1920’s from northwestern China that were presumed to be from the Late Pleistocene

were, in fact, only about 200 years old (Keates et al., 2006). This was the first dating of

6

its kind for a Chinese fossil hominid, and it has proven that the same methodology needs

to be applied to other samples thought to have been of Pleistocene age. Also, AMS was

used to identify the oldest human remains in Siberia that has been directly dated

(Kuzmin, 2009).

Physical processes associated with such pre-treatment include visual inspection

for obvious contaminants, removal of all roots and rootlets, crushing the sample (to

increase surface area for chemical treatment), and scraping of surfaces to clear off debris.

Chemical processes then include a repeated dilute HCl wash, followed by a NaOH wash

to neutralize any residual organic acids (Yuan et al., 2000).

AMS can also be used to date shells, though the procedure for such is more

problematic. The potential contaminants of shell samples depends on the type of shell and

from where it comes (the ocean, freshwater, etc.), and the implications of this were

demonstrated in they study by Nakamura et al. (2007). The researchers pointed out that

shellfish mostly obtain their organic carbon from plants and their inorganic carbon from

atmospheric CO2 and dissolved HCO3- ions. But this inorganic carbon is the most

important, since the shellfish deposit calcium carbonate within a protein matrix, which is

present in far fewer amounts than the calcium carbonate. However, formation of this shell

still incorporates C-14 from the environment, so it is capable of being analyzed for

dating.

Shell samples. Photo courtesy of Natural Resources Canada (www.nrcan.gc.ca).

7

The biggest problem, though, is that since this form of inorganic carbon is

soluble, chemical exchange often occurs with the environment, which can impact C-14

ratios in a given sample. Other problems with dating shells includes the fact that

atmospheric CO2 exchange equilibrium occurs more rapidly at the surface of water, so C-

14 levels could already be in decay by the time outgoing CO2 from deeper waters is

exchanged with surface water. The CO2 has actually been show to linger in oceans for

thousands of years, compared to only 6 – 10 years in the atmosphere (Bard et al., 1989).

So, anything that results in more mixing of surface and deeper waters will affect the

results of C-14 dating for shells.

Physical pre-treatment of shells includes obvious cleaning and reduction of

sample size (crushing), and a drill is used to remove the outer layer of the shell that may

be the result of recrystallization. Then, a dilute HCl wash is employed to remove any

residual exterior (Dallimore et al., 2005).

Applications to Dendrochronology:

Charcoal and wood provide a third example of substances that can be dated by

AMS. Samples of this nature require far less pre-treatment and yield much more accurate

results. The process and C-14 exhange theory for dating is pretty much the same as the

bone example mentioned above, except in this case the time-width is determined by the

number of tree rings selected in the dating process when considering a wood sample (it is

hard to quantify this in charcoal samples) (Hogg et al., 2006). An inner ring behind a new

ring thus represents a cessation of carbon exchange, and this provides time-width (Hogg

et al., 2006). Possible contaminants are the same as those listed above, and the physical

8

pre-treatment is the same as well. Chemical preparation is similar to aforementioned

techniques---HCl wash to remove carbonates, and NaOH for neutralizing the organic

acids.

Balsam fir (Abies balsamea) tree rings. Photo courtesy of H.D. Grissino-Mayer, University of Tennessee.

Dendrochronology (the science of tree-ring dating) is built on the fact that trees

usually grow by the addition of rings. Growth ring patterns of trees and aged wood can

actually be analyzed to determine the exact calendar year each tree ring was formed.

Such applications and findings were of immense importance in the early days of

radiocarbon dating in that they provided a reference standard for carbon 14 dating

method accuracy. During the late 1950s, scientists used dendrochronological dating to

confirm the discrepancy between radiocarbon ages and calendar ages in tree samples (van

der Plicht, 2004), and this calibration technique is still used today. Reference libraries of

tree ring data showing different calendar ages (going back nearly 11,000 years) are

available to researchers. The most commonly referenced tree samples in the United States

are of the bristlecone pine (Pinus aristata).

Recent studies have applied AMS to tree dating of Beaver pond deposits in

Yellowstone National Park to help study long term effects of beavers and climate change

9

on Holocene fluvial activity (Persico & Meyer, 2009). Beaver damming obviously affects

stream flow and thus ecology of mountain stream valleys, and AMS dating of wood

deposits helped determine years of Beaver occupation within the last 4,000 years, but

gaps in Beaver populations from about 2200 – 1800 and 950 – 750 years ago. These

correspond with severe droughts that likely caused low to ephemeral discharges in

smaller streams, as in modern severe drought.

The Problem with “Bomb” Carbon:

Two major assumptions are made when it comes to radiocarbon theory—(1) the

amount of C-14 at any global atmospheric location at any given time has not significantly

fluctuated over time, and (2) that due to equilibrium of radio carbon concentrations (see

formation process discussed in the introduction), atmospheric C-14 levels have remained

equivalent to biospheric C-14 levels (Baxter & Walton, 1971). When radiocarbon dating

was originally developed, other sources of C-14 were not really considered or accounted

for. Currently, however, scientists must recalibrate the dating to consider increased

atmospheric C-14 levels due to dramatically increased nuclear weapons testing in the

1950’s and 1960’s (Stuiver, 1965; Suess, 1979). On the other hand, the fact that humans

have exponentially increased the burning of fossil fuels, which has actually decreased the

atmospheric concentrations of C-14 (Suess, 1965), also provides a source of fluctuation

that must be accounted for.

To elaborate on the “bomb” effect, it rests on the premise that “artificial” C-14

was produced by the nuclear bomb reactions, and that the levels of this C-14 have thus

begun to accumulate in the atmosphere. A massive release of neutrons in the atmosphere

10

means more neutron bombardment of N-14, and thus a higher (unnatural) rate of

formation of C-14. Research suggests that since the 1950’s, the atmospheric levels of C-

14 have nearly doubled compared to measurements taken in the year 1965 (Stuiver, 1978;

Nydel & Lovseth, 1983). Furthermore, and amazingly, the levels of “bomb” C-14

between 1963-1965 were 100% above normal.

Nuclear testing explosion, 1952. Photo courtesy of United States Air Force.

The most significant implication of this “bomb” carbon discussion is, of course,

the necessity of an organic C-14 reference sample that predates these nuclear (and fossil-

fuel consuming) events. The accepted date of such a sample is pre-1950 (meaning 1950 is

the zero point on the radio carbon dating scale), and the accepted reference is an oxalic

acid sample from beets held by the U.S. National Bureau of Standards since it should, in

theory, contain the same amount of C-14 as a wood sample from pre-1950.

The Marine Reservoir Effect:

The atmosphere, biosphere, and the ocean are all reservoirs of carbon, though

each stores slightly different amounts from one another (Siegenthaler & Sarmiento,

1993). Carbon from the atmosphere is dissolved in the ocean as CO2, and subsequently

11

taken up by plants for photosynthesis (both in water and on land), thereby entering the

food chain (Farquhar et al., 1989). It follows, then, that marine consumers, as well as

terrestrial organisms, eventually ingest and incorporate C-14 in their own biochemical

systems. However, the levels of C-14 near the surface of the waters are not the same (due

to mixing between the different reservoirs) as those measured in the deeper parts of the

ocean, so not all marine organisms possess the same C-14 content (Siegenthaler & Wenk,

1984). Oceans are huge C-14 reservoirs, though, and the distribution of C-14 comes not

only from surface water interaction with the atmosphere, but also from mixing of surface

waters with deeper waters (that has radioactive decay producing C-14 at its own rate)

(Ennis-King & Paterson, 2005).

This is important, because when analyzing a sample, one must consider the C-14

content of an organism relative to its environment. For example, due mostly to diet, an

animal living at, in, or near the ocean would have a different level of C-14 than a further

inland terrestrial animal, so when their tissue chemistry is analyzed, they will appear to

have different radiocarbon ages, even if they are actually of the same age. In fact, it

appears that the age discrepancy between terrestrial and marine organisms is about 400

radiocarbon years, with marine organisms appearing much older than their terrestrial

counterparts (Ascough et al., 2005).

This reservoir effect also varies with location. The mixing of surface and deeper

waters (called “upwelling”) is dependent largely on latitude, occurring most drastically at

or near the equator (Friederich, 2008). However, coastal geography and climatology also

affect this phenomenon (Ver et al., 1999). Mangerud (1972) analyzed shell carbonates to

show that the global variation of the marine reservoir effect was “due to the incomplete

12

mixing of upwelling water of ‘old’ inorganic carbonates from the deep ocean where long

residence times of more than 1,000 years cause depletion of carbon 14 activity through

radioactive decay, resulting in very old apparent carbon 14 age.”

Carbon cycle. Image courtesy of Natural Resources Canada (www.nrcan.org).

When testing samples possibly affected by this reservoir effect, there are three

major techniques applied. One is to make sure your samples are pre-1955 specimens that

were collected live, and that they are of a known age. A second technique is to pair your

results with an analagous shell or charcoal sample test from assumedly contemporary

archaeological contexts. Lastly, you can pair your analysis with uranium-thorium dating

with corals or shells that are old but have clear growth bands (Hall & Henderson, 2001).

13

There are also correction factors that can be applied when comparing terrestrial

and marine samples. Due to the global variance in the carbon radio chemistry, correction

factors relative to location can be found in an online database maintained by the Marine

Reservoir Correction Database (http://intcal.qub.ac.uk/marine/). It is worth noting,

however, that the corrections are complex, and are even further dependent on periodic

local ocean circulation patterns. Additional computer programs are available to help with

such calculations (Stuiver & Reimer, 1993; Bronk-Ramsey 1995). The database also does

not include samples beyond a depth of 75 meters.

Conclusion:

The relatively recent discovery of radiocarbon dating has undoubtedly had a huge

impact on man’s understanding of the history of our world and our place within it. Be it

in the fields of archaeology, geology, hydrology, geophysics, atmospheric science,

oceanography, paleoclimatology, or biomedicine, the applications and importance of C-

14 dating to the substantiation or refutation of competing scientific theories cannot be

over emphasized. Accelerator mass spectrometry offers a better and more efficient way to

measure C-14 concentrations, since unlike other instrumental techniques, it does not

measure or count beta particle emission, but rather the actual number of carbon atoms

within the sample and the isotopic proportions, and requires only a small sample size,

thus allowing for sample preservation.

14

Literature Cited

Ascough, P., G. Cook, and A. Dugmore. (2005). Methodological approaches to determining the marine radiocarbon reservoir effect. Progress in Physical Geography 29(4): 532-547.

Bard, E., R. Fairbanks, M. Arnold, P. Maurice, J. Duprat, J. Moyes, and J.C. Duplessy.

(1989). Sea-level estimates during the last deglaciation based on δ18O and accelerator mass spectrometry 14C ages. Quaternary Research 31(3): 381-391.

Baxter, M.S., and A. Walton. (1971). Fluctuations of atmospheric C-14 concentrations

during the last century. Proceedings of the Royal Society of London A 321: 105- 127.

Bronk-Ramsey, C. (1995). Radiocarbon calibration and analysis of stratigraphy: The

OxCal program. Radiocarbon 37(2): 425-430. Currie, L.A., G.A. Klouda, D. Elmore, and H.E. Gove. (1985). Radiocarbon dating of

microgram samples: Accelerator mass spectrometry and electromagnetic isotope separation. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 12(3): 396-401.

Dallimore, A., R.E. Thomson, and M.A. Bertram. (2005). Modern to Late Holocene

deposition in an anoxic fjord on the west coast of Canada: Implications for regional oceanography, climate and paleoseismic history. Marine Geology 219(1): 47-69.

Donahue, D.J. (1995). Radiocarbon analysis by accelerator mass spectrometry.

International Journal of Mass Spectrometry and Ion Processes 143: 235-245. Ennis-King, J., and L. Paterson. (2005). Role of convective mixing in the long-term

storage of carbon dioxide in deep saline formations. Journal of the Society of Petroleum Engineers 10(3): 349-356.

Farquhar, G.D., J.R. Ehleringer, and K.T. Hubick. (1989). Carbon isotope discrimination

and photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 40: 503-537.

Fifield, L.K. (1999). Accelerator mass spectrometry and its applications. Reports on

Progress in Physics 62(8): 1223-1274. Friederich, G.E., J. Ledesma, O. Ulloa, and F.P. Chavez. (2008). Air-sea carbon dioxide

fluxes in the coastal southeastern tropical Pacific. Progress in Oceanography 79(2): 156-166.

15

Gillespie, R., R.E.M. Hedges, and J.O. Wand. (1984). Radiocarbon dating of bone by accelerator mass spectrometry. Journal of Archaeological Science 11(2): 165-170.

Hall, B.L., and G.M. Henderson. (2001). Use of uranium-thorium dating to determine

past 14C reservoir effects in lakes: Examples from Antarctica. Earth and Planetary Science Letters 193(3): 565-577.

Hedges, R.E.M, and J.A.J. Gowlett. (1984). Radiocarbon dating: Accelerating carbon

dating. Nature 308: 403-404.

Hellborg, R., and G. Skog. (2008). Accelerator mass spectrometry. Mass Spectrometry Reviews 27(5): 398-427.

Hogg, A.G., L.K. Fifield, C.S.M. Turney, J.G. Palmer, R. Galbraith, and M.G.K. Baillie.

(2006). Dating ancient wood by high-sensitivity liquid scintillation counting and accelerator mass spectrometry. Quaternary Geochronology 1(4): 241-248.

Jull, A.J.T., D.J. Donahue, and T.H. Zabel. (1983). Target preparation for radiocarbon

dating by tandem accelerator mass spectrometry. Nuclear Instruments and Methods In Physics Research 218(1): 509-514.

Keates, S.G., G.W.L. Hodgins, Y.V. Kuzmin, and L.A. Orlova. (2007). First direct dating

of a presumed Pleistocene hominid from China: AMS radiocarbon age of a femur from the Ordos Plateau. Journal of Human Evolution 53(1): 1-5.

Kuzmin, Y.V., P.A. Kosintsev, D.I. Razhev, and G.W.L. Hodgins. (2009). The oldest

directly-dated human remains in Siberia: AMS 14C age of talus bone from the Baigara locality, West Siberia. Journal of Human Evolution 57(1): 91-95.

Linick, T.W., P.E. Damon, D.J. Donahue, and A.J.T. Jull. (1989). Accelerator mass

spectrometry: The new revolution in radiocarbon dating. Quaternary International 1: 1-6.

Litherland, A.E. (1984). Accelerator mass spectrometry. Nuclear Instruments and

Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 5(2): 100-108.

Mangerud, J. (1972). Radiocarbon dating of marine shells, including a discussion of

apparent ages of Recent shells from Norway. Boreas 1: 143-72. Matteson, S., D.L. Weathers, Y.D. Kim, A.M. Arrale, F.D. McDaniel, J.L. Duggan, J.M.

Anthony, and M.A. Douglas. (1992). Molecular ion stability and populations in tandem accelerator mass spectrometry. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 64(1): 330-335.

16

Nakamura, T., I. Nishida, H. Takada, M. Okuno, M. Minami, and H. Oda. (2007). Marine reservoir effect deduced from 14C dates on marine shells and terrestrial remains at archeological sites. Nuclear Instruments and Methods in Physics Research, B 259(1): 453-459.

Nydal, R., and K. Lövseth. (1983). Tracing bomb 14C in the atmosphere 1962-1980.

Journal of Geophysical Research 88: 3621-3642. Perisco, L., and G. Meyer. (2009). Holocene beaver damming, fluvial geomorphology,

and climate in Yellowstone National Park, Wyoming. Quaternary Research 71(3): 340-353.

Siegenthaler, U. and T. Wenk. (1984). Rapid atmospheric CO2 variations and ocean

circulation. Nature 308: 624-626. Siegenthaler, U., and J.L. Sarmiento. (1993). Atmospheric carbon dioxide and the ocean.

Nature 365: 119-125. Stuiver, M. (1965). Carbon-14 content of 18th and 19th century wood: Variation correlated

with sunspot activity. Science 149: 533-535. Stuiver, M. (1978). Atmospheric carbon dioxide and carbon reservoir changes. Science

199: 253-258. Stuiver M., and P.J. Reimer. (1993). Extended 14C database and revised CALIB

radiocarbon calibration program. Radiocarbon 35: 215–230. Suess, H.E. (1965). Secular variations of cosmic ray produced carbon-14 in the

atmopshere. Journal of Geophysical Research 70: 5937-5952. Suess, H.E. (1979). A calibration table for conventional radiocarbon dates. In

Radiocarbon Dating, R. Berger and H.E. Suess, eds. Berkely, CA: University of California Press.

van der Plicht, J. (2004). Radiocarbon calibration – past, present and future.

Proceedings of the Ninth International Conference on Accelerator Mass Spectrometry, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 223-224: 353-358.

Ver, L.M.B., F.T. Mackenzie, and A. Lerman. (1999). Carbon cycle in the coastal zone:

effects of global perturbations and change in the past three centuries. Chemical Geology 159(1): 283-304.

Yuan, S., X. Wu, S. Gao, J. Wang, L. Cai, K. Liu, K. Li, and H. Ma. (2000). Comparison

of different bone pretreatment methods for AMS 14C dating. Nuclear Instruments

17

and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 172(1): 424-427.