most part naturally occurring, produced from the decay of the earth since its formauranikn and tho&m tion (Fia. l). However, some radioactive chemicals have been adzed to the oceans from fallout from atomic weapons testing and releases from nuclear power plants and nuclear fuel reprocessing facilities. How have chemical oceanographers used radioactivity as a tool for determining growth rates of marine organisms? One of the first such attempts, carried out by Karl Turekian and colleagues at Yale University, was to determine the age at maturity of a small bivalve, Tindaria callistiformis (FiE. 2).which lives in the mud accumulating on the abyssal ocean floor (1).The low temperatures (2 'C) and high pressures (500 atm) in the abyssal ocean led marine biologists to predict that small bivalves like Tindaria grew slowly The strategy developed by Turekian and his colleagues was straightfornard. They sampled growth increments of Tindaria and analvzed them for the radioactive elements which, like a clo"ck, would tell how old the growth increments were. Two problems immediately presented themselves. The choice of an appropriate radionuclide and a method of sampling the shell. The choice of radionuclides was a relatively simple one. Because lEndaria lives in deep-sea muds, it made sense to choose a radionuclide that is released £rom sediment particles to sediment ore water and eventually to the overlying water column. '"Ra. with a half-life of 5.7 years, is such a tracer. Ra and Ca are chemically similar eiements, and once in solution 228Raatoms can substitute for Ca atoms in the CaCO? shell of Tindaria. Sam~line the shell roved more difficult.'nndaria prows to a maximum size ofHbout 1em (Fig. 2)and becauseif the sample size required for '"Ra analysis, it was not possible to sample successive m w t h increments within a single shell. 'fhe problem waHsolved by carefully sorting a population of several hundred Tindaria into size groups, and each size group was analyzed as a single sample. Larger, older shells contained older shell material than vounger, smaller shells. The '"Ra in this older shell material ;ad undergone radioactive decay, and thus the radioactivity of per gram of CaC03was lower in the older shells. The half-life of '"Ra (5.7 v) was used to calibrate a model of possible shell growth &sus size and the results indicated that Tindaria indeed grew slowly, reaching a size of about 1 cm in 100 years (1). The subsequent discovery of hydrothermal vents along the mid-ocean ridge system provided a new twist to the picture of slow growth of deep-sea bivalves. The vents are
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Flgure 2 Photo of Maria cacaN~stiIormrs.Thls deposlt feedmgbuaive laves in mud accumulat ng in the deep ocean floorand grows to a sue of about 1 cm. From (I). 750
Journal of Chemical Education
Figure 3. Photo of Calyptogena magnifica. This bivalve grows to a size of 30 cm and lives close to the hydrothermal vents associated with the mid-ocean ridge system. sites at which seawater heated by the volcanic processes associated with the ridge, emerges into the ambient deep sea. Hvdmthermal water emerges at tem~eraturesof a few degrees to 350 'C and the dissolved chemicals support a rich faunal assemblaee. Amone the vent fauna is a dam. Calyptogena magnifi-a, which&ows to a length of 30 c& (Fig. 3). How does the growth of Calyptogena compare with that of Tindaria? The vent clam is large enough so that individual specimens could be sectioned and analyzed for the trace amounts of radionuclides incorporated into the shell. The '"Ra chronometer used for Tindaria showed drastically different results for Calyptogena. The large vent clams grow rapidly and reach a size of 30 em in about 10 years (2-4).Such a rapid growth rate is well suited to the unpredictable vent environment, for subsequent studies showed that vents (and the clams' food supply) could become inactive on a time scale of decades. The Living Fossil
In the context of these deeo-sea studies. we tried to develop a strategy for determin& growth rates of the chambered nautilus (Fie. 4). The nautilus is a reclusive animal that has intrigued-scientists since Aristotle made the first recorded observations of it (5).Its importance to biologists
Figure 4. Photo d Nautilus pompilius. The shell has been art in half to reveal the chambers which the Nautilus constructs to maintain neutral buoyancy. The partitions between the chambers (septa)are growth increments that can be used to determine growth rates of the animal.
Figure 5. The distribution of Nautilus in the South Pacific. From ( 1 1 ) . and paleontologists lies in the fact that it is a "living foss~l" -a link with past forms of chambered mollusks called ammonites. Hoth ammonites and closer ancestors of nautilus, the nautiloids, flourished in Mesozoic seas. Yet at the close of the ~ r e t a c e k period, s 65 million years ago, ammonites, like dinosaurs, became extinct. Some nautiloids survived the mass extinction, and their extant relatives are the chambered nautilus. Five species of Nautilus have heen identified and all live in restricted regions of the South Pacific (Fig.5). Unlike Tindarin or Calyptogena, Nautilus is a free-swimming mollusk confined to the upper 600 m of the water column. I t is found principally in deep water off the steep outer sides of the reefs of southwest Pacific islands. Because '"Ra, the chronometer used for Tindaria a n d C a "' l v ~ t o ~ e.nis a . added to the oceans from bottom sediments, this radionuclide is present in low cuncmtrntiona in the remote surface waters of che southwest Pacific and. therefore, could not be used in our study. Our search for a n alternate radionuclide to use a s a chmnometer led us to "Vb. Lead-210 is a naturally occurring radioactive form of lead, which decays with a 22-year halflife. I t is roduced in seawater by decay of another radionuclide, '%, which precedes it in the decay series (Fig. 1).Radon-222 is a noble gas that enters the atmosphere from continental soils, and a s a result, "T'b is produced in the atmosphere and added to the surface ocean by rain. Thus, 210Phis a tracer that is present in surface seawater and is naturally incorporated into the nautilus shell during growth. Once in the shell and isolated from its seawater source, the ""Ph atoms undergo radioactive decay, and the number of atoms remaining is a function of the time since deposition of the shell. A problem with this approach quickly became apparent. Nautilus is a swimming animal, and during its life can migrate both vertically and laterally in the oceans. The lead210 concentration of seawater also varies spatially. How were we to separate changes in "OPh due to changes in the nautilus' environment from those due to radioactive decay? We were able to take advantage of the fact that "Vb decays to another radionuclide, polonium-210, that in turn decays with a 138-day half-life. Polonium is not incorporated into the nautilus shell during growth, and when the nautilus forms its shell, the ratio of "90radio-
-
activity to that of '"Ph is zero. AS '"Pb decays, the radioactivity of '"Po increases until i t s r a t e of decay just matches that of 'lOPb. Thereafter, the two radionuclides are in radioactive e uilihrium or steady-state in which production of "% by 'l0Pb decay is balanced by decay of the '"Po itself. Mathematically, the relationship between the "@Po and 'lOPb radioactivitiesis represented by Ap,=l-e'-k%+hpy (1,
*Ph
where Apo,Aph= radioactivity of 210p. and 210Pb. respectively, expressed as disintegrations per minute (dpm) per gram of shell.
A, = decay constant for 2 1 0 ~(.693/half-life b = 8.5 x 104d") Ap, = decay constant for 210p, (.693/half-life= 5.0 x W3d-') t = time since deposition of a given shell increment
This relationship is plotted in Figure 6, which shows that it takes about two years for the 210Po/1'% ratio to
o 2 1 0 ~versus b Figure 6. Theoretical variation of the activity of 2 ' 0 ~ to time. Note that after about two years, the ratio asymptotically approaches its equilibrium value of 1.0. The curve assumes a closed system with no 2 ' 0 ~ oincorporated into the shell at the time of its formation. From (7). Volume 70 Number 9 September 1993
751
obtain its steady state or equilibrium value of 1.0. This means that shell material older than two years cannot be dated by this technique. However, for shell material younger than two years, the ratio of to "'Pb radioactivity is dependent solely on the time elapsed since the "OPb was incorporated into the shell. Thus, the 2'0P~210Pb pair forms a n "internal" chronometer that is independent of any variation in the initial '"Pb radioactivity incorporated into the shell (Fig. 6). Having chosen a chronometer, we sampled two small freshly collected nautilus (N. pompilius) from the Philippines (6).Unlike bivalves. the chambered nautilus has easily sampled growth increments -the partitions, or septa, which separate the chambers. Studies of the wav nautilus forms its'shcll show that the animal lrves in the ;utennojt chamber (the bods chamber,. During septal furmatiun, the animal moves forward in the body chamber and secreks a new septum from behind. The newly formed chamber is emptied of water by means of a connecting tube (the siphuncle). Chambers are formed a s the animal grows and serve to maintain its buoyancy. The septa thus are shell increments that preserve a record of the nautilus' growth throughout its life. Our first results were obtained for two juvenile nautilus that had been captured in the Philippines and died e n route to New York. We cut the shells in half to reveal the septa and chambers (Fig. 4) and carefully broke off septa
starting. with the most recently formed. The septa were dissolved in hydrochloric acid. A yield monitor (209Po)was added to correct for any sample losses, and olonium was plated onto a l-cm diameter silver disk. ~ h e ~ ' ' radioac~o tivity was determined with a n aloha spectrometer that precisely measured the number a& energies of alpha decays arisim fmm both 210Poand the vield monitor 2WPo. sfcause th; alpha particles (doubly ionized He nuclcl, resulting from decay of the two isotopes have sliehtlv different b& character~sticenergies, i t is possible & determine the number of alpha decays corresponding to 210Poand to "%. '"Pb decays by beta decay, and its radioactivity cannot be measured with a n alpha spectrometer. Instead, we purified the "'Pb by chemically separating it from "9'0 and stored the purified " P b for about eight months to allow a fresh generation of "'Po to mow in. We then measured the al&a radioactivity of tge ingrown 210Po,&d knowing the time fur inerowth for each samde. calculated the initial "?b a c t i v i t g ~ h u sa, complete analysis of each sample for 210Poand " P b required about eight months. We used eq 1 to determine ages for the septa analyzed from the two juveniles (Fig. 7) and calculated that the average time for septa1 (and chamber) formation was about 60 days in specimen N.p.1 but only about 20 days in N.p.2. This difference suggested that nautilus. like other mollusks, may grow at-Gfferent rates depending on its size or environment. 111explore further the possibility that the growth rate of nautilus changes throughout its life span, we obtained a group of nautilus sprcimens freshly collected in I'alau by Bruce Saunders & y n Mawr college^. The specimens were and of a different soecies tN. helouensis, amd were larmr ~closer to matuhty than the juveniles we studied initially. The pattern of Z'OPo/2'0Pb in the mature specimens of N. belauensis showed that only the last septum had ratios less than 1(Fig. 8). This pattern was quite different from t h a t in the juvenile N. dompilius (Fig. 7). Indeed, the 21?op'0Pb results indicate that onlv the most recent sent u n of the mature N. belauensis was formed within tLe This sueeests that Nautilus. prior two Years of erowth (8). iike othe~mollusks,slows its growthas it reaches matu: rity. One final test of the 2'0Po/210Pbmethod remained -that of verifving the results on specimens whose erowth rate (or septal deposition rate) was independently known. An opportunity to perform such a test arose with nautilus specimens that h i d been maintained ili aquaria. John chamberlain (Brooklyn College) was s t u d -y i n-g t h e hydrodynamics ofjuvenlle naitilus using live specimens kept in the New York Aqu;trium. Several of the aquarium nautilus l~vedonly ahuut six months aflcr being placed in the a q u a r i u m , a n d we analyzed t h e s e specimens. Moroholwic features ofthe shell demarcated the shell mat e r i h deposited before and after the nautilus entered the aquarium and implied that the animals had deposited 3-5 septa in the aquarium. The Z''Po/2'0Pb results gave growth rates consistent with the momholoeic chanees and showed that septal deposition required 36-70 days (9). Interestingly, the 210Po/21~b chronometer applied to the aquarium specimens gave results similar to the initial measurements on the juvenile nautilus from the Philippines. ~
I
1
2
3
4 5 6 SEPTUM
7
8
9
Fioure 7. 2 ' 0 ~ o / 2 ' 0activity ~ b ratios in seota of two iuvenile Nautilus the ~hilioorn&. winis denote the measured ac~.~,~ Solid ,~~ t vlty ratios and venlca oars are mcena ntles ( I slandara devlat on, der ved fromco~ntlngerror. Tne dashed noruonla ine s the eqd 0r L m va Le of the ratqo.Septa are nLmoered wltn l ' des,gnaring me most recent y formeo ano so on Beca~sethe septa form a I me record of the shell's growth, this plot can be compared with the theoretical change In the ratio as a function of time (Fig. 6). Average times between successive septa are calculated to be 20-60 days for these specimens. From (6).
.o&oiliusfrom .~~~
752
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Journal of Chemical Education
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A Total Picture Our studies with 210Poand "'Pb provided evidence that the nautilus mew more rapidly a s a juvenile than when it approached maturity, aid we were able to determine growih rates fur the last two years of gruwth, at these different growth stages. Yet we had no chnmometer that spanned the entire ~ ~ o whistor) th of a single specimen and
that would allow us to determine the age a t maturity of this animal. A good candidate for such a chronometer is bomb radiocarbon. Radiocarbon, or 14C(half-life = 5730 y), is produced
naturally a s cosmic rays bombard the earth's atmosphere, and also a s a consequence of atomic weapons testing. The atmospheric testing of atomic weapons carried out i n the 1950's and early 1960's released large quantities of radiocarbon into the atmosphere, overwhelming the natural signal. The atmospheric I4C quickly combines with oxygen to form carbon dioxide 2) and is taken up by organisms on land. It
06
0
0.4
~ . b4.
N b. 5 SubmDlun
2 0.2
0.2
(via gas exchange) and is incorporated into marine organisms. The radioactivity of 14Calways is expressed a s a ratio to the abundance of the non-radioactive or stable isotope, 12C.The 14C112Cratio in a sample is further compared to that of a carbon standard (generally NBS oxalic acid) which wrre-
--
sampleand standard are small (but analvticallv " sienificant) and.. for convenience, are multiplied and ex---- ------- pressed as "parts per thousand"byor1000 "per mil" (abbreviated " '?Em'). Thus, a sample with a 14C/12C ratio identical to the standard has a A14C value of 0 %o. Old carbon in which "C has decreased by radioactive decay has negative A14C values and samples containing bomb radiocarbon have Nb. 3 Motwe positve A14C values. The pattern of recent radiocarbon change with time in the atmosphere and oceans is re0.0 9.0 corded in tree rings andannual growth bands of SEPTUM SEPTUM corals, respectively, and Figure 9 shows the record from a coral in the South Pacific (10).The Figure 8. 210~o/Z10~b activity ratios in septa of four mature or nearly mature Nautilus pattern is one of constant [and negative A14C) belauensisfromPalau. Note that in com arison with data from the juvenile specimens until about 1960 when the A14C value began to ~ ' than ~ ~ 1.0 b (henceless than two increase indicating the presence of bomb radioshown in Figure 7, these animals ~ h o w ~ P o /less years since deposition) only in the most recently deposited one or two septa. This carbon in water in which the coral grew, indicates that growth slows as maturity is approached, a pattern not uncommon in A, a its calcium carbonate other mollusks. For explanation of symbols, see Figure 7. From (8). shell. it adds carbon-14 alone with the stable or non-radioactive carbon present a s carbonate ions in seawater. Our strategy to use 14Ca s a chromometer was to measure the carbon-14 content of nautilus septa and to compare the pattern of radiocarbon in the shell with that i n seawater (as recorded by corals). If we could determine the point in the shell growth that bomb radiocarbon first becomes evident, we would be able to date that shell material as having formed about 1960. Matching the AI4C L
-1
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[---
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YEAR
Figure9. Radiocarbon in the annual growth bandsof a reef coral from Australia, plotted as a function of the year in which the band was deposited. "cis expressed as the parts per thousand (%.) difference (A) between the ' 4 ~ / 1ratio 2 ~ in a samDle relative to a standard. ~ d m bradiocarbon is apparent in CaCO, deposited by the coral after 1959. The constant values to- 1959 -..Drior ,~ - - - denote .. - the - ors-bnmb - ... levels of raoiocarbon in water in wnlch tne coral grew. The snarp Increase begmnmg n 1959 and con1 nL ng to the early 1970's ref ens Ihe mpJt of raa~ocarbonproa~cedby atmospher c alomic weapons testng to tne s~rface ocean and to tne coral sneleton Vai~eseve1 ofl and decline after about 1975,because atmospheric testing of atomic weapons is no longer significant. The pattern in the coral growth bands is similiar to that seen in tree rings, but lags the tree ring pattern because of the time necessary to transfer the bomb radiocarbon signal from the atmosphere to the surface ocean. From (10). ~
~
~
~
~
.-80
bomb
YEAR
Figure 10. Radiocarbon in the shell of a nautilus from the South Pacific. The curve is an estimate of the radiocarbon content of the water in which the nautilus lived (0-360 m), based on the coral record shown in Figure 9. Measured radiocarbon valuesfor three septaand the growing margin of a mature specimen of Nautilus collected in 1970 are indicated by the filled circles and the vertical bars denote the analytical unceltainty of each measurement ( l o precision).The times of deposition of the analyzed septa are determined by matching the measured radiocarbon value with the corresponding value in the water (given by the curve). From (10). Volume 70 Number 9 September 1993
753
record in the nautilus to that in the seawater (coral) provided a means of dating septa deposited after 1960. Because the maximum chanee in 14C in surface seawater occurred in the mid to late r960's, we analyzed septa from a mature specimen of nautilus that had been caught around 1970 (10). The results showed that the bomb radiocarbon sirmal clearlv was Dresent in the nautilus shell bv the 20th septum a n d that septa deposited subsequentiy all had bomb radiocarbon (Fig. 10). By matching the pattern of 14C versus time recorded in South Pacific corals (Fig. 9) with that observed in the nautilus, we were able to determine that the nautilus shell was formed over about 10 to 13 y e a n (Fig. 10). Thus, the animal lived for about 10 years, a much longer lifespan than the octopus or squid, which live only about 3 years (10). Because nautilus is a reclusive animal that lives poorly in captivity, such results are difficult to obtain by direct observations. Indeed the results for Nautilus, as well as for Tindaria a n d Calyptogena, demonstrate t h a t radionuclides incorporated into the shells of marine organisms provide powerful tools with which to measure growth rates. Acknowledgement This paper was presented at the 11th Biennial Conference on Chemical Education, August 5-9, 1990, Atlanta,
754
Journal of Chemical Education
GA. The authors work on Nautilus has been supported by the National Geographic Society and the National Science Foundation. This is contribution number 856 from the Marine Sciences Research Center. This paper is part ol'a series to appear in The Journal of Chemical Education on nuclrar and radiochemixtry. It was prepared for the Committee on Nuclear and ~adidchemistry of the National Academy of Sciences-National Research Council. Literature Cited 1. Tureldan,K. K;Cochran, J. K: Kharkar, D.P: Cerrato,R. M.; Vaisnya.J. R.: Sanders,H.L.;Grassle,J.F:Mlen, J.APmc.NoLAeod Sci.USA1576,72,2829-2832. 2. Tureldan. K K:Cochrsn. J. K Selonee 1881.214,909411. 3. Tureldan,K K.;Coehran, J. K.; Naaki. YNmNre 1919,280,38&387. 4. Tureldan.K K.;Cochran, J. K: Bennett,J. T Nature 1989,303,5556. 5. Landman, N.Dismuery 1982.16.20-23, 6. Cochrsn.. J. K: Rve. D. M.:. Lsndman. N. H. Pokobiol. lU81.7.469-480. 7. Landmsn, N. H.: Cochran. J. K. 1nNoutilus:The BiolagyondPoieobidogvofaLiving Fossil; Saundera, W. B.; Landman, N. H., Eds, Plenum: New York,1987,pp 401420. 8. Coehran, J.K:Landmsn, N. H.Nolure 1984,308,725-727. 9. Landman, N. H.;Cochran, J. K; Chamberlain, J A : Nirschberg, D. J. Mar B i d 1989.102.65-72. 10. L a n d m q N . H.;Druffruffl,BR.M.;Cochran,JK;Donehue,D.J.:dull.A.J.T&~h Plomt. Sci. Lett. 1988.89,2 W 4 . 11. Hause. M.R.1nNoutilus:TkBiologyandPoleobiolagyofaLiuingFassil:Saundem, W B.; Landman, N. H.,Eds; Plenum: New Ymk,1987,pp 53-64.
