1. LAURENCE KULP Lamont Geological Obsematory, Palisades, New York
ONE of t,he most important scientific discoveries of the past decade is the existence of measurable amounts of carbon 14 in the atmosphere, hydrosphere, and hiosphere by W. F. Libby and his associates (1). since the half-life of carbon 14 is reasonably long (5568 + 30 years) and since a constant concentration is present in living things, air, and surface water, the time of removal from the carbon dioxide life cycle can be measured qualltitatively if the carbon 14 concentration is was devised which perdetelmilled, ~h~~ a mits relat.ively prerise age determination of archeological and geological materials from the present back to tens of thousands of years ago, ~h~ tirne dimellsioUis nniquely important to the sciences of arc.eology, allt,hropology, and geology, and as no quantitative method was previously available for this tirne interval, the application of this method is causing major advance in these sciences. At the present time some 400 dates have accumulated from work at chicago and Columbia ( 1 , 8 , 3). Additional contributions from other laboratories are expected during the coming year, while a fe,,. remain unsolved, the agreement bet,veen different laboratories, the selfconsistency of the data, and the agreement of the ages measured ,,.ith external checks provided by geology and archeology assure the general applicability of the method. PRINCIPLES The rarhon-14 dating method is based on the following considerations. Neutrons produced cosmic rays in the ~lpperatmosphere react with nitrogen 14 atoms to give carbon 14 atoms. These are oxidized t o carbon 14 dioxide which then enters the carbon dioxide life cycle of air, water, plants, and animals. Since the cosmic ray flux is essentially stant ant on the 10% time scale involved, the reservoir of carbon into which the carbon 14 is diluted is a constant, and the rate of decay of the carbon 14 is a constant, a sample of carbon taken from ally part of the carbon cycle dmuld yield a constant concentration of carbon 14. This deduction has been verified experimentally. Now if carhon is removed from this rycle by the death of a Plant or animal, the sedimentation of a carbonate or the submergence of surface ocean water, that carbon receives no more carbon 14 a d the amount which it had is decaying away according to the known half-life. Therefore, the carbon 14 concentration forever after will be a qnant,itative indicator of the time since d a t h or removal from the cycle.
TECHNIQUE
The measurement of the extremely l o d e v e l beta activity found in natural carbons requires special instrumentation to reduce the background to sufficiently low levels (4, 5 ) . Carbon 14 can be measured in principle by the ion chamber, Geiger counter, proportional counter; or srintillation counter. All of the published results to date have been obtained by counting elemental carbon mounted on the inside of a large-volume Geiger tube. The Process consists of preparing a sample of pure carbon from the natural carbon-baring material, mounting this in a special screen-wall Geiger counter, and counting it against the background for several days. The key to t'he success of the procedure is the reduction of the unshielded background of the counter to c.P.m. the use of (about 350 elaborate shielding and anticoincidence counters and
The Preparation of the P ~ r rarbon e requires careful attention. The carbon in carbon-bearing material IS converted to carbon dioxide either by acidification Or combustion. I t is then trapped with liquid air, expanded into ammonia solution, and precipitated CaC03, which is thoroughly washed. The carbon dioxide is released from this purified carbonate with acid, trapped again in liquid air, and reduced over magnesium turnings a t 1000' C. The reaction products from the reduction must be lached away from the carbon. It appears that, although the products of reduction (except carbon) are readily soluble in HCI (Mg, Mg&, MgO), an extended leaching with boiling concentrated HCI will notrednce the ash below 10-20 per cent. Spectrographic analysis of the ash shows only a pattern of periclase (MgO). I t turns out that the graphite flakes form L'tentsnaround the M ~ O particles and +,hat these "tents" be hi^ can be done by a 10-dinute leach in boiling concentrated nitric acid. The process oxidizes about 5 per cent of the carbon hut statistirally eats a hole into each "tent" reducing the ash immediately to 1 per cent. The purified carbon is made into a slurrv distilled materand a trareof agar and coated on the inside of a sample cylinder is used in the screen-wall counter. After the counter has been evacuated filled, it is placed ill 12-to11 iron shield, ~ h iron , removes mostof the gamma soft components and reduces the hackground to about 120 c.p.m. in our laboratory. The mesons 1 Present,& at the 122nd Meeting of the ~merieanchemiea~ which pass through the iron shield trigger a tube of the anticoincidence ring a t the same instant that they Society, Atlantic City, September, 1952. 432
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activate the sample Geiger tube in the center. The correlations differed in absolute time by some 300 circuit is arranged to cancel such a pulse in the sample years. This date corresponds exactly with one of these Geiger tube. This further reduces the background to and thus fixes the Mayan culture in history. An important recent discovery to Biblical and Semitic about 5 c.p.m. About half of this residual count is from gammas from the 12-ton iron shield. I t turns out scholars mas a manuscript scroll of the Old Testament that 1 in. of mercury placed on the inside of the anti- hook of Isaiah. Debate ensued as to whether it was coincidence ring absorbs 85 per cent of these and re- written a century or two B.c., in the second century duces the background to about 2 c.p.m. This situation a.n., or whether it was a medieval forgery. The makes it possible to date materials back to about 30,000 carbon-14 date proved that it was genuine, but there was inadequate precision to decide between the first years before the present. two alternatives. RESULTS Specimens from England, Tunisia, Iran, and France Knowns. Of primary importance in the early stage indicate the time range for certain prehistorical culof development of the carbon-14 dating method was tures. The Lascaux cave is the oldest date on a human the measurement of samples of known age. Table 1 site so far tested. It is the site of the famous cave paintings of the extinct woolly mammoth. The woven sandals from Oregon and several other TABLE 1 dates from Western caves suggest man has been in Known Samples North America a t least 10,000years. Sample Known Carbon-I4 Among the more exotic specimens tested was the M. Desc~iption aue am stomach contents of the recently extinct Moa bird *L-123C Inca temple 444 & 25 450 3z 150 of New Zealand. 880 3z 15 930 i 100 L-108A Sequoia tree ring 1377 i 4 1430 i 150 L-108B Sequoia tree ring One of the most interesting specimens measured so C-62 Mummy coffin, Egypt 2280 & 5 2190 + 450 far in this group is the flesh and hair of one of the widely Funerary ship, SesosC-81 publicized superbison frozen in the permafrost ground 3750 & 10 3620 i 180 tris tomb Cypress beam, Sneferu C-12 of northern Alaska. I n both Alaska and Siberia such tomb 4575 i 75 4802 & 210 animals have been found in such an excellent state of a L = Lamont Geological Observatory Columbia. University. preservation that dogs have eaten the flesh. The C = Institute for Nuclear Studies, ~ n i v e k i t yof Chicago. fact that this one was frozen over 28,000 years ago and preserved ever since is probably the longest recorded shows the results on a few of the calibration samples of deep freeze yet reported. known historical date. I n all cases the carbon-14 Geological Applicadions. Table 3 shows some selected date agrees with the known date within the probable dates of geological interest. One of the most imporerror of the counting statistics. Since this has been tant geological problems is the time of retreat of the found to hold for many other samples, it appears that last continental ice sheet. The first two samples of the counting precision is still the limiting experimental extinct forest now overlain by the latest drift shows that factor. the last continental glarier was still in Wisconsin 11,000 Archeological Applications. Many specimens of years ago. Many additional samples are making it archeological interest have been submitted for carbon14 dating. Table 2 contains a few of these dates TABLE 3 selected to show the kinds of problems to which the Geological Samples method has been applied. Descriptia Age, ~.~ years B. P. A Mayan lintle was discovered on which a Mayan Sample no. (2-308 Two Creeks, spruce wood 10,880 i 740 date was inscribed. The two proposed Mayan calendar (2-366 L-1251
TABLE 2 Archeological Samples Sample no. L-113 C-576 C-602 L-134 (2-524 C-579 C406 C428 L-120 L-127
Deseriplzon
Mayan lintel with carved date Isaiah manuscript Stonehenge, Englmd, late neolithic Tunisia, early neolithic Belt cave, Iran, mesolithic France, upper Paleolithic Laficaux cave Oregon sandals Stomach contents of extinot Moa, New Zeeland Suoerbieon flesh and hair. klaska
L-125G L-125A
Age, yeam B. P.
L-111A
Two Creeks, peat Mississippidelta, weathered surface 273 ft. Mississippi delta, shell 73 ft. Miasissippi delta, wood 25 ft. Bermudsdrowned forest 7W
11,100 i 600
Older than 30,000 9,000 i 200 2.900 i 500
1,470 i 120 1,920 i 200 3,800 3z 275 8,400 i 400 10,560 & 1200 13,000 i 560 15,500 i 900 9,050 + 350 1,800
+
150
Older than 28,000
possible to understand the absolute time of the advance and retreat of the ice during the last part of the ice age. The Mississippi delta and Bermuda samples show the worldwide change in sea level accompanying the last glacial period. The weathered surface a t 273 ft. corresponds to the maximum seaAevel lowering. Then
JOURNAL OF CHEMICAL EDUCATION
434 ---..
Loenlion
54'35'N
41°W
TABLE 4 Deep Ocean Water Depth C. p. rn. Surface
(1) 6.82 i 0.09 (2) 7.74f 0.07 Av.
5S019'N 32"56.8'W
Average modern shells 1829 m.
53-52.6'N 21a06'W
2743 m.
6.ii f 0.06 6.72 i 0.10 5.38 i 0.07 5.60 i 0.11
AV.
5.45 i 0.06 5.31 i 0.12 5.36 0.12 5.34 0,
*
as the ice melted back we have a complete record of sea-level rise as a function of time. It was about 80 ft. lower than now 11,000 years ago, 70 ft. a t 9000, 50 ft. a t 7000, and 25 ft. a t 3000. Several samples of peat a t the bottom of the peat section from different parts of the Florida Everglades have been dated. They all give ages around 5000 years, suggesting that -just -prior to this the lower ueninsula of Floridawas under water. On the California coast steplike terraces are very well defined. Since this was the last large geomorphic event in the area it was of interest to find out if the lowest, last terrace was within 30,000. It apparently is not. An entirely different application of the carbon-14 method can be made in oceanography. The cold water a t the polar region sinks, moves along the bottom, and in a more or less complicated manner makes its way to the surface near the equatorial regions. One of the most basic parameters of oceanography which affects calculations on biological-oxygen economy as well as heat flow from the suboceanic crust is the rate of this mass ocean turnover. Since the carbon 14 in the carbonate of the polar surface water is in equilibrium with the cycle, i t has the constant modern value of carbon 14 concentration; however, when it leaves the surface it is sealed off from the source and hence the time since it left the surface, and consequently the rate of flow along the bottom, can be measured. Two-hundred gallon samples of ocean bottom water (2000 to 4000 meters) were obtained, the carbonate converted to carbon dioxide, and extracted from the water on board ship. Later in the laboratory this carbon dioxide was purified, reduced to carbon, and the element counted. Some of the data on the first samples measured are shown in Table 4. It appears that a t latitude 50-55' the water is 1500 to 2000 years old. This is highly significant and suggests a slower circulation rate than had been anticipated. Many additional samples have been taken on scientific cruises in the summer of 1952, including intermediate-depth samples. These are in process. As soon as they are dated, a reasonably accurate picture of the rates and direction of deep ocean movements should be available.
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Another uerennial uroblem in neochemistry and geology has been the origin of petroleum. Most theories reauired a long period of time, e. g., 1,000,000years, for Recent the hydrocarbon molecules to be prepared; then came migration and accumulation. I t had further been frequently stated that recent marine sediments contained no hydrocarbons. Dr. Paul Smith a t the Esso Laboratories (6)began a new search for such hydrocarbons using modern organic 1600 f 130 techniques in brilliant fashion and discovered hydrocarbons in appreciable quantities. Kot only did he find enough 1750 150 hydrocarbons to account for known petroleum reserves, but the range of compounds was similar to crude oil. The problem remained as to whether these hydrocarbons had diffused up from oil-hearing horizons below or whether they really form now. Although the extremely small quantities of hydrocarbon available required the development of special techniques, it was possible to demonstrate that the hydrocarbons mere recent geologically and thus the problem of the origin of oil becomes t,he problem of accumnlatinn nnlv. .~.. PROBLEMS
Several interesting side problems have developed in the course of this work. I t appears, for example, that along coastlines with deep troughs offshore, such as along Japan, Peru, the Aleutian arc, and to a less extent California, upwelling of the deep "old" water sufficiently reduces the carbon-14 concentration of the carbonate so that shells of living animals show apparent ages of 1000 to 3000 years. This effect is not observed on mid-ocean island beaches or along a coast with a broad continental shelf such as the Atlantic. It also appears that certain types of lake plants may utilize carbonate carbon from the water which in part is derived from rock erosion, hence anomalous carbon14 roncentrations appear. These, fortunately, are a negligible proportion of the plant kingdom. No difficulties have been encountered from wood, charcoal, or the other more common materials used for dating. One of the basic assumptions of the carbon-14 dating method is the constancy of cosmic ray flux. The calibration measurements on historically dated material prove that no appreciable change has occurred in the last 10,000 to 15,000 years. Further, modern theory of cosmic ray origin would suggest that it has been essentially constant. However, since the technique goes hack to 30,000 and since the ice age was drawing to a close a t just the time our calibrations give out, it was important to obtain an independent proof of this assumption. An experiment was conducted which did just this. Highly homogeneous deep sea sediment cores were selected. On such a core, one can determine the age of any particular layer by the ionium method, by measuring the radium content, as a function of depth. The ionium method (7) is based on the preferential
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435
chemisorptiou of iooium (thorium 230) by these sedi- of 100 or more in either precision or sensitivity. I t ments and its subsequent approach to equilibrium and seems feasible to measure back to 100,000 years ago hence is independent of the cosmic ray flux. Carbon-14 when this technique is developed-or to an accuracy dates call he obtained on the carbonate in the same of + 5 to 10 years, 2000 years ago. This would not cores, but these directly depend on the neutron pro- only vastly extend the application to geological procduction in the upper atmosphere from cosmic rays and, esses and archeological detail but would probably hence, the cosmic ray flux. Three different cores have permit studies of natural isotopic fractionation in :he given the same dates within experimental error for the carbon cycle. To chemists this fine structure which two methods on samples ranging from 5000 to 25,000 mould he revealed mould probably present the most years before the present. I t can be concluded that the entrancing problems of all. Global tracer expericosmic ray flux, integrated over several centuries, has ments could be carried out with home-grown carbon-14 been the same within 10 per cent for the past 30,000 atoms. The possibilities seem almost limitless. years.
*
LITERATURE CITED FUTURE POSSIBILITIES
The future possibilities for carbon-14 dating are a t least as exciting as the past accomp~ishments,particularly with regard to advances in technique. While the ion chamber can be used to detect carbon-14 activity, it offers no improvement in sensitivity over the Geiger method. Proportional counting might yield several more half-lives with a considerable expenditure of effort hut severe stability problems wmdd remain. Liquid srintillat,ion counting, however, offers a factor
(1) LIBBY,W '.
F.,Radiocarbon Dating," University of Chicago Press, Chicago, 1952. (2) KULP, J. L., H. W. FEEI,~, AND L. E. TRYON, Scimce, 11% 565-8 (1951).
J. L., L. E. TRYOX,w. ECKELMANN, A X D W. A. SNELL, ibid, 116,409-14 (1952). (4) KULP, J. L., L. E. TRYON, AND H. W. FEELY,Tram. Am. Geophys. Union, 33,184-92 (1951). (5) KUIP, J. L., AND 1., E. TRYON, Rev. Sci. Inst., 23, 296-7 (3)
KULP,
(1952). (6) SMITH, P. Science, November (1952). (7) URRY, IT. D., Am. J. Set., 247,257-75 (1949).
w.,
