Palaeomagnetism and the age of the Makapansgat hominid site

Earth and Planetary Sctence Letters, 44 (1979) 373-382 © Elsevaer Scientific Pubhshmg Company, Amsterdam - Printed m The Netherlands

373

[S]

PALAEOMAGNETISM AND THE AGE OF THE MAKAPANSGAT HOMINID SITE P L McFADDEN

Department of Phystcs, Umverstty of Rhodesia, Sahsbury (Rhodesta) A BROCK 1

Department of Physws, National University of Lesotho, Roma (Lesotho) and T.C PARTRIDGE

Palaeo-anthropology Research Group, Umverstty of the Wztwatersrand,Johannesburg (Repubhc of South Afrtca)

Received February 8, 1979 Revased version recewed May 1, 1979

The time of deposition of the Makapansgat Ltmeworks site in South Africa, contammg several hominid fossils, has an important bearing on hominid phylogeny m Africa Hence, in an attempt to determine this age, a palaeomagnetlc analysis of the site has been performed and the results are reported here Member 3 (from which most of the tmporrant hominid fossils have been recovered) appears to have an age greater than 2 90 Ma and possibly greater than 3 06 Ma but less than 3 32 Ma Prevaously it has not been possible to dehneate the age of this tmportant member with any degree of accuracy The most hkely position for the underlying Member 2 is m that section of the Gauss normal epoch predating the Mammoth event and havang age hmlts of 3 06-3 32 Ma

1 Introduction The Makapansgat Llmeworks site just south o f 15etersburg m the Transvaal (South Africa) has yielded a rich population o f fossds which includes at least ten hominid m&vlduals and the age o f the fossil-bearing deposits is therefore of considerable interest Unfortunately radlometnc and fission track dating methods have yielded no results Faunal and geomorphologlcal methods, although yielding tmportant age reformation, provide only wide hmlts In a prehmmary palaeomagnetlc study [ 1] it was shown that a polarity stratigraphy, wluch can be matched to the accepted polarlty time scale, exists m the deposit The slte was there1 Now at Apphed Geophysics Umt, Umverslty College of Galway, Galway, Ireland

fore revisited m an attempt to define the polarity stratigraphy m greater detad through intensive samphng Results o f the whole study are presented here

2 Geology The sxte has previously been described [2] but more detaded mapping by one of us (T C P ) has led to a revision o f the stratigraphy which will be briefly described here Full details will be pubhshed elsewhere The Makapansgat F o r m a h o n constitutes the cave fillmg at the Makapansgat Lmaeworks homlmd site, the cave being a solution cavity m dolomite o f the Malmare Subgroup, Transvaal Supergroup The Formation can be dwlded into five members The lowest member (Member 1) comprises the orlg-

374 anal lmmg to the walls and floor of the cave and consists of 0 5 - 1 5 m of white and pale brown banded calcite, locally contaminated with fine clamc sediments Noteworthy concentrations of bone occur m some locallhes The member was deposited on an irregular floor and is locally tilted and cross-bedded Member 2 consists of 2 - 1 0 m of well bedded and sometimes cross-bedded, browmsh-red, well calcified sediment, chiefly clayey sand-slit The majority of the sediments were water laid beneath a fluctuating water table while the unit as a whole has a very uniform upper surface at approx:mately 1458 m elevation Sedimentation occurred penecontemporaneously in two apparently separate depositories at the eastern and western ends of the cave separated by a high in the surface of Member 1 Faunal remains are locally frequent Member 3, which vanes m thickness from 0 5 to 2 m, consists of a densely packed accumulation of bone cemented by calmte w:th a relatwely small admixture of fine clastlc material The relahonshlp with Member 2 is unconformable and cross-beds m the former are truncated at the contact In most parts of the cave Member 4 unconformably overlies Member 2 and only near the southern end of the cave can it be seen to overhe remnants of Member 3 It is a thick ( 3 - 1 5 m) deposit containing angular chert and dolomite fragments in a pale brownish-red clayey silty sand matrix Boulders and large roof blocks increase m frequency upwards and extensive roof collapses are ev:dent In the upper part of the umt Member 5 consists of between 3 and 20 m of browmsh-red s:lty sand within which several discrete gravelly horizons are present The relatmnshlp between Member 4 and Member 5 is clearly unconformable and may represent a considerable lapse of t:me Most of the remains of Australoptthecus afncanus were recovered from Member 3 but the posterior part of one cranium was recovered from Member 4

3 Samphng and measurement A total of ninety-three palaeomagnetxc samples were collected Owing to the very varied physical nature of the rocks a variety of collecting techniques had to be used Seventy-four of the samples were cored and ten (all Member 1 samples, one In Member 2 and three m Member 4) were collected as blocks using a block

onenter described by McFadden [3], all of these samples were oriented with a theodohte cahbrated from sun shots Nine samples from soft red muds (Member 2) In the deepest part of the cave were encased in small plastic cubes and oriented magnetically, a correction being made for the magnetic declination as determined from the theodolite sun shots It is considered that the samples represent six horizons in Member 1, fourteen In Member 2 (ten from the western depository and four from the eastern), nine m Member 4 and fourteen in Member 5 Member 3 was not sampled at all due to the poor mechanical properties of the rock at the accessible locahtles Within Member 4 both the size and percentage content of chert and dolomite fragments increases upwards to the point (within the upper half of this member) where it was not possible to obtain large enough portaons of matrix to constitute a useful palaeomagnetic sample Samphng was therefore restricted to the lower half of Member 4. Samples from Member I were weakly magnetized, their natural remanent magnetization lying m the range 7 X 10 -4 to 4 X 10 -2 A/m As would be expected from the rock types, samples from the other three members were generally more strongly magnetized (their natural magnetizations lying m the range 4 X 10 -3 to 7 A/m) All measurements were made in the Sahsbury laboratory using a spinner magnetometer described by van Oorschot and RIdler [4] Many samples showed evidence of rotational remanent magnetization (RRM) [5,6] Hdlhouse [7] has shown that a double demagnetlzatmn at the same peak field, with the spec:men rotating m opposite directions for each demagnetization, will effectwely remove RRM ff the dlrecnon of magnetization of the specimen is considered to be the mean of the direction after each of the demagnet:zatlons The demagnetization equipment m Salisbury does not allow for reversal of the direction of rotation The effects of RRM have however been mm:mlsed by a process of double demagnetlzatmn effectwely producing a reversal of the dlrechon of rotation Using such procedure routinely a stable component was ~solated at peak fields of typically 20 mT Stable dlrectmns were arbitrarily classified as normal (N) or reversed (R) ff they were within 40 ° of the relevant dipole field dlrectmn and as lntermedmte (I) ff they fell outside this hmlt Samples wh:ch showed

375 erratic demagnet]zatlon behawour, or which exhibited internal d~sagreement between specimens, were rejected If samples from a single horizon exhibited disagreement then this horizon was rejected (X) Deta~led d~rect~onal data may be obtained on request

20-

18

4 Results 1.2

Member 1 The NRM directions of magnehzatlon and the stable directions after alternating field (AF) demagnetization are shown m Fig la and b respectively Demagnetization curves for each of the samples are gwen m Fig lc The mean direction of magnetization of these samples after cleaning (D = 185 1°, I = +37 3 °) is 6 1° from the reverse dipole field direction (D = 180 0 °, I = +42 0 °) while the c]rcle of 95% confidence

Mira. 1.o

o.o.

O.4

OE.

/ O REVERSE ADI POLE

) ~

1•

/ 2-0

mT

O0

)

~

PEAKF I E L D

Fig 1 Dtrectlons of magnetization (a) before and (b) after AF demagnetization (with circle of 95% confidence) and (c) the demagnetization curves for Member I samples Northseeking vector stereographlc projections Open symbols, upper hemisphere, sohd symbols, lower hemisphere

NORMAL O)eOLt

t

•

' ÷ ~

(

b--

'

'

t

m the mean direction (ags) has a semi-angle of 8.8 ° [8] These two directions are therefore not slgmficantly different On correcting the stable dlrectlons of magnetization for the local distortions of the member the estimate, k, of the precision parameter as gwen by Fisher [8] is reduced from 58 4 to 5.1. This reduction is significant [9] and suggests that the observed magnehzat~on was acquired after the d~stortlon of Member 1 As the samples were collected from horizons umformly distributed throughout the exposure of Member 1, it is concluded that the whole of this member possesses a stable magnetization m the reverse dipole dlrect~on

REVERS[ ) Member 2 A schematic of the samphng scheme Is given m Fig 2 Deposition m two distinct depositories makes accurate temporal correlation difficult However, It is felt that the 1448-m level m the eastern depository correlates approxtmately with the 1450 5-

376 Absolute elevQtlon (metre) 1458

West ,6,g

East 1456

' 35,36

tlon is more than twice as far (17 °) from the present earth's field direction (D = 343 ° , I = - 6 4 °) at Makapansgat Typical demagnetization curves and directions of magnetization before and after A F demagnetization are given In Fig 3

Member 4 Samples from this member were collected from both the eastern wall o f the entrance quarry and

15,16,17 1454

5,6,7

1452 3"/,38 3 g 40,41' ,26,27,28--, 33.34

Trons~tton ; V Zone~

10,32 11,31 29,30

OE 1450 ~

89,~)0,Q1

(a)

12,13,14

L_.. 86,8Z88

1448

Fig 2 Schematic of samphng scheme for Member 2 m level in the western depository and it IS reasonable to assume that the deposition rates were approximately equal The palaeomagnetlc results do not contradlct this hypothesis and Interpretation o f the polarIty stratigraphy has been made on this basis The six samples from the three lowest horizons In the western depository of Member 2 display some inconsistency within horizons, inconsistency between horizons and discrepancy with the directions of either the reversed or normal dipole field directions These three horizons have been interpreted as representing a transition zone (T) from reversed upwards to normal All other horizons in Member 2 gave consistency within horizons showing that the rock type is capable o f retaining a stable direction These eleven horizons were all classified as normal (the greatest deviation between an horizon mean direction and the normal dipole field direction being 24 2 ° ) The overall mean of these eleven horizon directions (/9 = 354 6 ° , I = - 4 8 . 2 °) is 7 3 ° from the normal dipole field direction (D = 0 ° , I = - 4 2 °) while ags is 6 1° Although this indicates a significant difference between the two directions the mean dIrec-

MIMo

04

0.2

oo,

5

lO

PEAK FIELD

ill mT

2o

25

N

Fig 3 (a) Demagnetization curves and (b) demagnetization paths for 4 typical Member 2 samples classified as normal The circle of 95% confidence is for the upper l 1 honzons Plotting convention as In Fig l

377 from the western side of the cave entrance The stratP graphic relation between the eastern and western sampies is not clear due to the presence of a fault between the two sections A schematic of the samphng is gwen in Fig 4, because ln&Vldual &scusslon of the horizons is necessary they have been alphabetically labelled Samples within horizon H gave lncons]stent results wh:le those from horizon J gave a mean direction 84 ° from that of the normal dipole field Since the stratp graphical relaUonshlp of these samples with the mare body of samples is unclear these essentmlly non-meaningful results were excluded from further consideration Horizon A gave an intermediate direction while horizon B gave a very clear reversal w~th mean direction (D = 174 20,• = +31 9 °) only 11 1° from the reversed dipole field direction The demagnetization curves and &rectlons of magnetization before and after demagnetization for the two samples from this

horizon are gwen in Fig 5 Horizons C, D and E gave normal mean directions Horizon F again gave an lntermedmte direction and the samples from horizon G indicate the presence of a reversely magnetized reg]on The demagnet:zanon

10,

08

06

(a)

A bs olute elevation Western side of cave entrance

Eastern wall

04

(metre) 1464

52.53,54(J)

02

1462 55,56,57(H) (G) 7 7,78

1460

OO~

5

,

(F)Sl

PEAK

110 FIELD mT

,

~,

115

I 20

1458 (E)4g,50

(D)47,48 1456 (C)44,45,46 (B)42,43 (A)20021,22 1454

Fig 4 Schematic of samphng scheme for Member 4 Horizon ldenttficatlons are given m parentheses (see text)

Fig 5 (a) Demagnetlzatmn curves and (b) demagnetization paths for the 2 samples of horizon B Plotting convention a s mFlg 1

378 curves and darectlons of magnetization during demagnetization for the two samples from this horizon are given m Fig 6 Th:s horizon is interpreted as being right at the bottom of a transmon from normal to reversed Unfortunately this cannot be checked as it proved unposslble to obtain samples from this member any higher than this horizon

08

Member 5 Samples from this member showed incon-

OS

sistency both within and between horizons It :s therefore concluded that the rock type of this member is incapable of retaunng a stable magnetlzat~on It :s apparent that the reversed and mtermedmte horizons previously reported from this member [1 ] were misinterpretations allowed by the very low samphng denslty m this member Intenswe samphng has now shown that no temporal reformation may be deduced from the magnet~zatlon of this member

Ca) 0.4

02

QCo

~

*'o PEAK

~'. FIELD

~o

5. Discussion

mT

5 1 0 n g m ofmagnettzanon

Ibl

÷

"'-.

~

" ..~.

,

REVERSE AOIPOLE

Fig 6 (a) Demagnetazatlon curves and (b) demagneUzataon paths for the 2 samples of horizon G Plotting convention as mFlg 1

The reversed magnetization of Member 1 was apparently acquired post-d~stort~on and the tune of acquisition is therefore uncertain However, the transition zone from reversed upwards to normal at the base of Member 2 is consistent with an hypothesis that the magnetization of Member 2 was acquired subsequent to the acquisition of the observed magnetlzahon of Member 1 The upper 11 horizon dlrect]ons m Member 2 cluster around the normal dipole direction rather than the dxrectlon of the present earth's field In addition, there is no red,cation In the demagnetization curves of any recent magnetlzat~on The observed magnet~zat:on is therefore thought to have been acquired at the t:me of deposition of th:s member The existence of reversed and intermediate horizons m Member 4 shows that this rock type is capable of retaining a stable magnetization for a s]gmficant period of tune There is no reason to suspect that the horizons classified as normal have been preferentmlly remagnet~zed m the recent past and the observed magnetizations are therefore interpreted as being those acquired at the tune of formation of the rock

379

5 2 Polarity strattgraphy and mterpretatton

TABLE 1 Polarity data

The relevant polarity data are presented in Table 1 and Fig 7 The presence of homlmd remains and geomorphological evidence (see later) suggests that the deposit is not only Calnozolc but is younger than 5 Ma Thus It is reasonable to compare the observed polarity pattern with the geomagnetic polarity time scale of Cox [10] The principal observation is that most of the section is normally magnetized and hence most probably dates from a normal epoch containing at least two reversed events Given the upper lmalt of 5 Ma on the age this effectively identifies the normal portions as being within the Gauss normal epoch The observed polarity pattern is then most simply related to the polarity time-scale by postulating that the reversed Member 1 lies within the last period of the Gilbert reversed epoch The transition low In Member 2 then corresponds to that between the Gilbert and Gauss epochs at 3 32 Ma * and the remainder of Member 2 lies entirely within that section of the Gauss epoch predating the reversed Mammoth event The simplest Interpretation (Interpretation A, Fig 8) of the Member 4 pattern is that the two reversed zones represent the Mammoth and Kaena events within the Gauss normal epoch, thereby placing this part of Member 4 between about 2 8 and 3 06 Ma If this IS correct the presence of the transition zone low in Member 4 implies that the bone-bearing Member 3 must predate the lower boundary of the Mammoth event at 3 06 Ma However, the break in the record as a result of the unconformities between Members 2, 3 and 4 and the lack of samphng in Member 3 allows interpretation B (Fig 8) of the pattern In Member 4 In this interpretation the time covered by the unconformities and the laying down of Member 3 covers the Mammoth event entirely The reversal low m Member 4 Is then the Kaena event and the apparent transition from normal to reversed IS the transmon from the Gauss normal to the Matuyama reversed epoch Thus Member 3 would predate the Kaena event at 2 90 Ma

* All of the ages quoted are those given by Cox [10] To obtain the ages according to the decay constants given by Stelger and Jager [23 ] each of the ages quoted here must be multlphed by 1 027

Unit

Sample number by horizons

Polarity of horizons

Member 4

77, 78 (G) 51 (F) 49, 50 (E) 47, 48 (D) 44, 45, 46 (C) 42, 43 (B) 20, 21, 22 (A)

R (9) I N N N R I

Member 2 east

15, 16, 17 89, 90, 91 12, 13, 14 86, 87, 88

N N N N

Member 2 west

8, 9 35, 36 5,6,7 37, 38, 39 40,41 26, 27, 28 33, 34 10, 32 11, 31 29, 30

N N N N N N N Tl T ~transmonal T

Member 1

2 93

R R

4

R

3 92

R R

1

R

A third Interpretation (mterpretatlon C, Fig 8) is obtained by postulatmg that the reversed Member 1 hes within the Mammoth reversed event The transition low in Member 2 then corresponds to that at 2 94 Ma The reversal low in Member 4 is then, as In interpretation B, the Kaena event The conclusion for the age of Member 3 Is then the same as for interpretation B However this third interpretation allows only 40,000 years for the laying down of Members 2 and 3, the base of Member 4 up to the first reversal (approximately 1 m) plus the ttme involved in the unconformmes Hence this interpretation is considered unhkely Clearly by postulating several hiatuses and rapid, large scale fluctuations o f the deposition rates within each member a fit against Cox's scale could be made

380 Member

Time

Polamty

Scale

Matuyama r * ] _

.

O~

R9

@

N

@ @ 0 @

N N R T

4 E

I

i.~

0

3

>

|

N

o

N

2

!'

.

.

._

.

.

.

.

C

.

.

.

.

.

.

.

.

N • R 0 T (~

N~

I.J >

5

.

B

2 94 .......

Member" 4

3M0a6m m o t h

~,

~

._

N N N

Fig 7 Summarlsed polarity data for Members 1, 2 and 4 of the Makapansgat Pormatlon and interpreted magneto-zones

almost anywhere However, there is no geological evidence to support any such hypotheses The (reasonable) interpretations discussed above were made essentially independent of faunal evidence

5 3 Faunal and geomorphologwal evtdence Using evidence of nlckpolnt migration and valley widening Partridge [11] calculated an approxmaate age for the cave opening at Makapansgat of 3 7 Ma This is the time from which cave filling became possable and is therefore a maximum age for the deposit The pubhshed faunal evidence is meagre and owing to the relative concentrations of faunal remains, faunal age esttmates for Makapansgat are essentially age estimates for Member 3 Cooke [12], later supported by Magho [ 13 ], found the closest affinity of Makapansgat sulds and elephantlds to lie with corresponding forms in the middle Shungura (Omo) and lowest

.

1

.

.

.

.

~

.

.

Gauss__ G,Ibert

3 32 - - -

Fig 8 Relatmn between the geomagnetic polarity ttme scale and the reasonable interpretations The shaded areas indicate normal polarity

Koobl Fora (East Rudolf) formations and therefore suggested tentatively an age of 2 5 - 3 0 Ma for Makapansgat White and Harris [14] have recently suggested that "the Makapansgat 'graclle' australopltheclnes are approximately equivalent m age to Shungura Member C" The palaeomagnetlc stratigraphy for the Shungura Formation [ 15] places the base of Member B, the base of Member C and the top of Member C at about 3 1, 2 65 and 2 4 Ma respectively Vrba (personal communication) has used bovlds to suggest tentatively that the Sterkfonteln type site is "somewhere between 2 and 3 Ma" whilst Wells [16] feels that some of the many points of difference between the faunas of the two sites might plausibly suggest that Makapansgat is older than Sterkfonteln Colllngs et al [17] infer from Fehdae studies that Members 2 and 3 "seem to fall within the lmaltS set by zones 1 and 2 at the East Rudolf localities, with clearly a trend beyond the older date being probable" From their Hyaemdae studies they conclude that Member 5

381 "could have a maximum age o f 2 75 Ma" However, K-Ar dating b y Curtis et al [18] has raised doubts about the age of the crucial KBS tuff and Hillhouse et al [19] have had to propose two alternative palaeomagnetic interpretations Hence ages based on correlations with East Rudolf must be considered equivocal In reviews o f the evidence Tobias [ 2 0 - 2 2 ] gives a tentative faunal age o f 2 5 - 3 0 Ma This faunal age is entirely consistent with the interpretatiors gwen for the observed polarity stratigraphy Owing to the lack of precision in a faunal date it is not possible (and probably never will be) to use faunal evidence to distinguish between the interpretations

6 Conclusions Fairly definite conclusions m a y be drawn regarding the ages o f Members 1 and 2 Member 1 may be placed entirely within the Gilbert reversed epoch and therefore has an age in excess o f 3 32 Ma The lowest horizons o f Member 2 indicate a transition zone which may be correlated with the transition from the Gilbert reversed to the Gauss normal epochs while the whole of the rest of the member Is of normal polarity Member 2 m a y therefore be placed in that section o f the Gauss normal epoch predating the Mamm o t h event and hence m a y be gwen age hmits o f 3 0 6 - 3 32 Ma Lack of continuity in the temporal record allows two strnple interpretations o f the remainder of the observed polarity stratigraphy, one implying that the bone-bearing Member 3 predates the lower boundary o f the Mammoth event at 3 06 Ma and the other that it predates the lower boundary o f the Kaena event at 2 90 Ma. It may therefore be concluded that Member 3 was most hkely laid down during the Gauss normal epoch but predating the Kaena event

Acknowledgements We thank Professor P V Tobias for access to Makapansgat and for his continual enthusiastic support We thank the Bernard Price Institute for Palaeontologlcal Research for permission to collect samples at the site We would hke to thank Drs D L Jones and R D Met-

calfe, for without their drllhng and rock-cltmbing skills many of the crucial samples would not have been collected Financial support was provided by the WennerGren Foundation for Anthropological Research and the University of Rhodesia

References 1 A Brock, P L McFadden and T C Partridge, Prehmlnary palaeomagnet~c results from Makapa~asgatand Swartkrans, Nature 266 (1977) 249 2 C K Brain, The Transvaal ape-man-bearing cave deposlts, Trans Mus Mem 11 (1958) 1 3 P L McFadden, A palaeomagnetlc study of Cretaceous klmberhte occurrences m the Northern Cape, D Phd Thesis, Umverslty of Rhodesia, Salisbury (1975) 4 B P J van Oorscbot and P F Rldler, A sensltwe spinner magnetometer using a cod detector, Geophys J 45 (1976) 569 5 R L Wdson and R Lomax, Magnetic remanence related to slow rotation of ferromagnetic material m alternating magnetic fields, Geophys J 30 (1972) 295 6 A Brock and W lies, Some observations of rotational remanent magnetization, Geophys J 38 (1974) 431 7 J W Hlllhouse, A method for the removal of rotational remanent magnetization acqutred during alternating field demagnetization, Geophys J 50 (1977) 29 8 R A Fisher, Dispersion on a sphere, Proc R Soc London, Ser A, 217 (1953) 295 9 G S Watson, Analysis of dispersion on a sphere, Mon Not R Astron Soc, Geophys Suppl 7 (1956)153 10 A Cox, Geomagnetic reversals, Science 163 (1969) 237, 11 T C Partridge, Geomorphologlcal dating of cave opening at Makapansgat, Sterkfontem, Swartkrans and Taung, Nature 246 (1973) 75 12 H B S Cooke, Notes from members, Bull Soc Vertebr Palaeontol 90 (1970) 2 13 V J Magho, Origin and evoluUon of the Elephantldae, Trans Am Phdos Soc (N S )63 (1973)1 14 T D White and J M Harris, Stud Evolution and correlation of African Hominid locahtles, Soence 198 (1977) 13 15 F H Brown, R T Shuey and M K Croes, Magnetostratlgraphy of the Shungura and Usno Formations, southwestern Ethlopm new data and comprehenswe re-analysis, Geophys J 54 (1978) 519 16 L H Wells, Faunal subdlWSlOnof the Quaternary in southern Africa, S Afr Archaeol Bull 24 (1969)93 17 G E Collmgs, A R I Crmekshank, J M Magmre and R M Randall, Recent Faunal studies at Makapansgat hmeworks, Transvaal, South Africa, Ann S Afr Mus 71 (1975)153 18 G H Curtis, R F Drake, T E Cerhng and J H Hampel, Age of KBS Tuff m Koobl Formation, East Rudolf, Kenya, Nature 258 (1975) 395

382 19 J W HtUhouse, J W M Ndombl, A Cox and A Brock, Additional results on palaeomagnet:c stratigraphy of the Koobl Fora Formanon, east of Lake Turkana (Lake Rudolf), Kenya, Nature 265 (1977) 411 20 P V Tobias, Imphcatlons of the new age estimates of the early South African Hominids, Nature 246 (1973) 79 21 P V Tobias, New developments m hominid palaeontology m south and east Africa, Ann Rev Anthrop 2 (1973) 311

22 P V Tobias, New evidence on the dating and the phylogeny of the Pho-Plelstocene hommldae, Quat Stud (1975) 289 23 R H Stelger and E Jager, Subcommlsslon on Geochronology Convention on the use of decay constants m geoand cosmochronology, Earth Planet Scl Lett 36 (1977) 359