Carbon-14 in Coal Deposits

by Kathleen Hunt

Copyright © 2002

[posted: May 22, 2002]

The Problem:

Accelerator mass spectrometry (AMS), a sensitive radiometric dating technique, is in some cases finding trace amounts of radioactive carbon-14 in coal deposits, amounts that seem to indicate an age of around 40,000 years. Though this result is still too old to fit into any young-earth creationist chronology, it would also seem to represent a problem for the established geologic timescale, as conventional thought holds that coal deposits were largely if not entirely formed during the Carboniferous period approximately 300 million years ago. Since the halflife of carbon-14 is 5,730 years, any that was present in the coal at the time of formation should have long since decayed to stable daughter products. The presence of ¹⁴C in coal therefore is an anomaly that requires explanation.

The Solution:

Talk.origins' Kathleen Hunt wrote an e-mail to a noted expert on AMS and ¹⁴C dating. The results of her correspondence are reproduced below:

Hey, I really lucked out with my first email to an AMS researcher. Got a very informative reply right away.

The short version: the ¹⁴C in coal is probably produced de novo by radioactive decay of the uranium-thorium isotope series that is naturally found in rocks (and which is found in varying concentrations in different rocks, hence the variation in ¹⁴C content in different coals). Research is ongoing at this very moment.

(The fungi/bacteria hypothesis [that ¹⁴C in coal is produced by modern microorganisms currently living there --Ed.] may also be plausible, but would probably only contribute to inflation of ¹⁴C values if coal sits in warm damp conditions exposed to ambient air. There is also growing evidence that bacteria are widespread in deep rocks, but it is not clear that they could contribute to ¹⁴C levels. But they may contribute to ¹³C.)

The much longer version:

Over the weekend I emailed Dr. Harry Gove, an expert in the development of the AMS method of ¹⁴C dating. I picked him to bother with my emails because he had recently written some nice review articles about the AMS technique in the Radiocarbon journal. (Basically there are two ways of measuring ¹⁴C: (1) count the radioactive emissions, or, (2) a newer method, based on separating out the different carbon isotopes by their different masses via accelerator mass spectrometry [AMS] and counting the atoms themselves.)

Dr. Gove wrote back the very next day, as did one of his colleagues. By sheer coincidence, they are currently studying this exact question. It turns out that the origin and concentration of ¹⁴C in fossil fuels is important to the physics community because of its relevance for detection of solar neutrinos. Apparently one of the new neutrino detectors, the Borexino detector in Italy, works by detecting tiny flashes of visible light produced by neutrinos passing through a huge subterranean vat of "scintillation fluid". Scintillation fluid is made from fossil fuels such as methane or oil (plus some other ingredients), and it sparkles when struck by beta particles or certain other events such as neutrinos. The Borexino detector has 800 tons of scintillant. However, if there are any native beta emitters in the fluid itself, that natural radioactive decay will also produce scintillant flashes. (In fact that's the more common use of scintillant. I use scintillant every day in my own work to detect ¹⁴C and ³H-tagged hormones. But I only use a milliliter at a time - the concept of 800 tons really boggles the mind!). So, the physics community has gotten interested in finding out whether and why fossil fuels have native radioactivity. The aim is to find fossil fuels that have a ¹⁴C/C ratio of 10^-20 or less; below that, neutrino activity can be reliably detected. The Borexino detector, and other planned detectors of this type, must keep native beta emissions to below 1 count per ton of fluid per week to reliably detect solar neutrinos. (In comparison, my little hormone vials, here in my above-ground lab, have a background count of about 25 counts per minute for 3.5 milliliters.)

So, the physicists want to find fossil fuels that have very little ¹⁴C. In the course of this work, they've discovered that fossil fuels vary widely in ¹⁴C content. Some have no detectable ¹⁴C; some have quite a lot of ¹⁴C. Apparently it correlates best with the content of the natural radioactivity of the rocks surrounding the fossil fuels, particularly the neutron- and alpha-particle-emitting isotopes of the uranium-thorium series. Dr. Gove and his colleagues told me they think the evidence so far demonstrates that ¹⁴C in coal and other fossil fuels is derived entirely from new production of ¹⁴C by local radioactive decay of the uranium-thorium series. Many studies verify that coals vary widely in uranium-thorium content, and that this can result in inflated content of certain isotopes relevant to radiometric dating (see abstracts below). I now understand why fossil fuels are not routinely used in radiometric dating!

Dr. Gove and his colleagues are currently trying to improve AMS technology to be able to identify certain fossil fuels that have extremely low ¹⁴C content. Current AMS techniques have a ¹⁴C/C detection limit of about 10^-15 (corresponding to 60,000 yrs), and Dr. Gove's current research, this year, is aimed at improving detectability to 10^-18 (110,000 yrs).

Their ultimate goal is to reliably measure ¹⁴C/C ratios down to the unbelievably low levels of 10^-22 (180,000 yrs). This AMS technology would then be used to identify certain oils that have very low ¹⁴C levels, and then those oils would be the ones used in the neutrino detectors.

(This research is part of the "Old Carbon Project" funded by the U.S. National Science Foundation's Particle and Nuclear Astrophysics Program and also by Canada's Natural Science and Engineering Research Council. The team will be presenting results to date this September at the 9th International Conference on Accelerator Mass Spectrometry in Japan.)

Finally, I did also get a copy of David Lowe's 1989 Radiocarbon paper. It is a short paper. A summary:

(1) old coal often has a little more ¹⁴C than expected - instead of having the expected ratio of ¹⁴C/C at or beyond the detectable limit of 10^-15 (corresponding to ~60,000 yrs old), it often has detectable ¹⁴C/C of 10^-14 or 10^-13.

(2) radioactive elements hypothesis: Lowe briefly discussed the possibility that native radioactive elements can create new ¹⁴C by radioactive decay. He only discussed radium, and discounted this as a major effect based on low concentrations of radium in coal (and yet my own brief stint of research has turned up many abstracts showing that concentrations of radionuclides vary widely in coal - some of these are pre-1989 so I don't know why Lowe didn't address this more carefully).

(3) bacteria/fungi hypothesis: Lowe then makes a reasonable case for fungi and bacteria - there are fungi that can degrade lignite (Polyporus versicolor and Poria montiola), as well as autotrophic "thiobacillus-like" bacteria that oxidize pyrites in coal, and he points out that bacteria have been found 3km underground apparently living on granite. Lowe states that fungal and bacterial activity is particularly likely in warm, damp coal exposed to air, and he points out that microbial action only has to result in the deposition of ~0.1% by weight of modern carbon in the coal to produce an apparent age of 45,000 years for the specimen.

Since Lowe's paper, there have been many more reports of deep subterranean bacteria, which apparently form a heretofore unrecognized ecosystem deep below the earth in rocks and in oils (abstracts below). Presumably most of these bacteria never interact with the "modern" ¹⁴C of the atmosphere. But some deep bacterial activity apparently can result in increased concentrations of ¹³C.

(4) Lowe goes on to make recommendations about using only freshly mined dry coal stored under inert gas, and other recommendations about choice of "background" for radiocarbon labs.

So, it looks like in-situ production of new ¹⁴C is the best-supported hypothesis; but research is ongoing, and I look forward to seeing the results of the Old Carbon Project and new research on the deep subterranean bacteria.

References

A great general introduction to carbon-14 dating:
http://www.c14dating.com/

General information on the many types of neutrino detectors:
www.slac.stanford.edu/gen/meeting/ssi/1997/wojcicki4.pdf

A very nice in-depth discussion of the three new neutrino detectors and how they work (scroll almost to the end to read about Borexino):
http://www.sns.ias.edu/~jnb/Papers/Popular/Nextgensolar/nextgensolar.html

A diagram of the Borexino (Italy) neutrino detector - notice the enormous shielding to protect it from radiation from the surrounding rock:
http://almime.mi.infn.it/html/borexinod.html

More about the Borexino detector from Princeton University:
http://pupgg.princeton.edu/~borexino/welcome.html

The original paper which raised this "old coal" issue:
"Problems associated with the use of coal as a source of ¹⁴C-free background material." D.C. Lowe, Radiocarbon 31(2):117-120 1989.

Recent references from the ongoing "Old Carbon Project":
"The measurement of very old Radiocarbon ages by AMS." 2001. A. E. Litherland and K. H. Purser. In: Proc. Conf. on the Fundamental Aspects of Modern Physics, 2000, Luderitz, Namibia. Published by World Scientific Publishing Company in 2001.

"Progress in AMS Research at IsoTrace." 2000. A. E. Litherland, R. P. Beukens, J. P. Doupe, W. E. Kieser, J. S. Krestow, J. C. Rucklidge, I. Tomski, G. C. Wilson and X-L. Zhao. Nucl. Instr. and Meth. B172:206-210.

"Ion Beam Preparation Systems for Atomic Isobar Reduction in Accelerator Mass Spectrometry." 2001. A.E. Litherland, J.P. Doupe, I. Tomski, J. Krestow, X-L. Zhao, W.E. Kieser and R. P. Beukens. In: Proceedings of the 16th Int. Conf. on the Applications of Accelerators in Research and Industry, J. L. Duggan and I. L. Morgan (Eds.), Conf. Proc. of the Am. Inst. of Physics 575:390-393.

On subterranean bacteria:
"Deep life in the slow, slow lane." Richard A. Kerr, Science, May 10 2002: 1056-1058.

More about subterranean bacteria:
"A hydrogen-based subsurface microbial community dominated by methanogens." 2002. F.H. Chapelle, K. O'Neill, P.M. Bradley, B.A. Methe, S.A. Ciufo, L.L. Knobel, and D.R. Lovley. Nature, 17 January, 2002; 415 (6869): 312-315.
ABSTRACT: The search for extraterrestrial life may be facilitated if ecosystems can be found on Earth that exist under conditions analogous to those present on other planets or moons. It has been proposed, on the basis of geochemical and thermodynamic considerations, that geologically derived hydrogen might support subsurface microbial communities on Mars and Europa in which methanogens form the base of the ecosystem. Here we describe a unique subsurface microbial community in which hydrogen-consuming, methane-producing Archaea far outnumber the Bacteria. More than 90% of the 16S ribosomal DNA sequences recovered from hydrothermal waters circulating through deeply buried igneous rocks in Idaho are related to hydrogen-using methanogenic microorganisms. Geochemical characterization indicates that geothermal hydrogen, not organic carbon, is the primary energy source for this methanogen-dominated microbial community. These results demonstrate that hydrogen-based methanogenic communities do occur in Earth's subsurface, providing an analogue for possible subsurface microbial ecosystems on other planets.

A new species of bacteria found in deep, hot fossil fuels:
"Isolation and characterization of Thermococcus sibiricus sp. nov. from a Western Siberia high-temperature oil reservoir." 2001. M.L. Miroshnichenko, H. Hippe, E. Stackebrandt, N.A. Kostrikina, N.A. Chernyh, C. Jeanthon, T.N. Nazina, S.S. Belyaev, and E.A. Bonch-Osmolovskaya. Extremophiles 5(2):85-91.
ABSTRACT: Anaerobic organotrophic hyperthermophilic Archaea were isolated from five of eight samples from oil wells of the Samotlor oil reservoir (depth, 1,799-2,287m; temperature, 60-84 degrees C). Three strains were isolated in pure cultures and characterized phylogenetically on the basis of comparison of the 16S rRNA gene sequences. All strains belonged to a new species of the genus Thermococcus, with Thermococcus litoralis, Thermococcus aggregans, Thermococcus fumicolans, and Thermococcus alcaliphilus being the nearest relatives (range of sequence similarity, 97.2%-98.8%). Strain MM 739 was studied in detail. The new isolate grew on peptides but not on carbohydrates. Elemental sulfur had a stimulatory effect on growth. The temperature range for growth was between 40degree and 88degreeC, with the optimum at 78degreeC; the pH range was 5.8 to 9.0, with the optimum around 7.3; and the salinity range was 0.5% to 7.0%, with the optimum at 1.8%-2.0%. The doubling time at optimal growth conditions was about 43 min. The G+C content of the DNA was 38.4 mol%. The DNA-DNA relatedness between strain MM 739 and T. litoralis was 27%; between strain MM 739 and T. aggregans, it was 22%. Based on the phenotypic and genomic differences with known Thermococcus species, the new species Thermococcus sibiricus is proposed. The isolation of a hyperthermophilic archaeum from a deep subsurface environment, significantly remote from shallow or abyssal marine hot vents, indicates the existence of a subterranean biosphere inhabited by indigenous hyperthermophilic biota.

Example of high uranium content in certain coals:
"Anomalous trace element abundances in Tertiary coal ash from East Garo Hills, Meghalaya, India." 1996. I.V. Sastry, M.S. Deshpande, and K.K. Dwivedy. Journal of Atomic Mineral Science 4:75-79.
ABSTRACT: Coal ash of Thakmari-Wathregithem Tertiary coals from East Garo Hills record anomalous concentrations of uranium (up to 0.219% U₃ O₈), vanadium (up to 9.3%), chromium (up to 1.7%), copper (up to 1.17%), manganese (up to 0.15%), cobalt (up to 1.15%), strontium (up to 0.38%), barium (up to 0.30%), and lead (up to 0.75%). Economic aspects as well as possible implications of such high concentrations are briefly evaluated.

Example of high uranium content in fossil fuels also distorting other radiometric methods of dating:
"The lead isotope geochemistry and the uranium-lead apparent ages of the sandstone-type uraniferous ore of Domiasiat, Meghalaya, India." 1998. R.R. Das, K.K. Dwivedy, T.R. Mahalingam, and C.K. Mathews. Journal of the Geological Society of India 51(6): 817-825.
ABSTRACT: The relative abundance of radiogenic lead varies depending on the mineralogical characteristics of the ore. In high uranium containing coaly matter, radiogenic lead is >80% of the total, whereas in a composite ore, as much as 60% of the total is of the initial common lead. The uranium-lead "apparent ages" are in the range of 25 Ma to 95 Ma and are related to the mineralogical assemblage in the ore. The most recent of the uranium mineralization, of "apparent age" -25Ma is with the coaly matter separates that contain considerable amounts of pyrite. The variations in the abundance and distribution of radiogenic lead isotopes and the resultant orderly changes in "apparent ages" of uranium mineralizations in different type of mineral separates are attributed to the existing favourable conditions in the ore for movement of uranium from the rock and preferential concentration and retention in the coaly matter, leading to more than one stage uranium mineralization.

Old (but still good) reference on variation in uranium content of coals:
"Geology of uranium in coaly carbonaceous rocks." 1962. J.D. Vine. U. S. Geological Survey Professional Paper. Pages 113-170.
ABSTRACT: U is one of many inorganic constituents associated with coal, impure coal, coaly shale, and carbonaceous shale. These rocks normally contain less than 0.0001% U. At least 0.005 and as much as 0.1% U is known to occur locally in alpine-meadow peaty soils, in carbonaceous shale beds, and as large low-grade deposits in lignite and subbituminous coal in the Rocky Mountain and Great Plains regions. Bituminous and anthracitic coal is only rarely known to contain as much as 0.005% U even locally. Economically significant tonnages of impure lignite and lignitic shale contain as much as 0.1% U in at least 14 areas in the United States. Nearly half of these areas occur in the Fort Union lignite region of western North and South Dakota. Most of these deposits occur in thin beds of impure lignitic rocks that have been weathered to soft earthy material containing about 50% moisture. U occurs chiefly as an epigenetic constituent of the plant remains in coaly rock, chemically fixed by cation exchange on the humic matter. The localization of deposits is dependent on the availability of U in ground-water solutions, the structure and permeability of the adjacent rock, and especially on the permeability and chemical susceptibility of the coaly rock. The peculiar set of conditions necessary to form an economically significant deposit of U in coaly carbonaceous rocks is rarely met and follows no easily predictable pattern.