Acta Amaz. - vol.16

RECONNAISSANCE STUDY OF M A C R O F O S S I L S F R O M T H E U P P E R PURUS RIVER - WESTERN A M A Z Ô N I A . R. E. Benchimol (*) K. Cooper (**) B . I . Kronberg (**) M . Powell SUMMARY

Voò&iJU

ofa iMood, bonz and tzzth

fauna

atong thzUppzA Puniu,

RÍVZA

o d Amazoni.a

WZAZ

òtudizd uòing convzntionat micAoócopy and òcanning zlzctAonmicAO&copy. Maòò òpzctomzthy

uxu, atòo uòzd to invzòtigatz minoA and thn.cz zlzmznt 6ÍgnatuAQj> o£ bonz àampizÃ.

Thz mícjioòopy òtudizi, òhowzd that thzAz

IAXLA

tittlz attzn.atA.on oá onl.QinaltQxtuh.zo.

I n thz faòòit uiood òomplzí*,

ld2

.nti.iizd in thin òzction OA> thopicat haAdwood

tnzzi>,

thz

nzplazzmznt e | thz ohigina matzhiat with òidzhitz òuggQAtA that faòòitization occuhe.din

òhaíloui òzdimzntf, in which int.zA

^tÁXiatmX2hò

^zAQ. hatuhatQdwith

hz

^pzctto iAon

In òamptzÁ o(, both fa&òitizzd bonz and wood, pnzcipitation

0$

òzcondahy ihon phaòZò w a ó

commonly obòZAvzd in ch.ac

.ko

and voldò. OthzA ózeondoAy phaòzò includzd òitica, inon

oxidzt,, manganz

-òz

caA.bona.tz. Thz intimatza&òociation oi thç^z òzeondaAy phaòzò mXh thz

ohiginaZ

biológica! òtAuztunzò coutd bz zvidzncz

^ O A

a micnobiological

AOZZ

in thz

^ohmxtion

0

(í

thzAz phaòzò. Thz òimitaAity in

AOAZ 2,aAth

zlzmznt (REE) òignatuAZA faA

2

faiòilbonz

òamplzò lhom difázAznt modznn locatíonò indicatzò thziA having àhaAzd òimitoA diagznztic

(vUtoAizò.

Thz vÍAtuati.y complzt

.2

pAzAQAvation o^ oAiginaJL tzxtuAZò òuggzòtò that mieAO.

5

O pie

òtudizi coutd bz ubzfaut in claòòi^ying faòòil and zvznin idznti^ying OAiginal matzhialò.

ROAZ

zaAth òignatuhzà in {,04&itizzd bonz may hz^-lzct gnound mtzA compoòitionò at thz

timz ofi

lois

&ilization.

(*) D e p a r t a m e n t o de G e o c i ê n c i a s , U n i v e r s i d a d e do A m a z o n a s , M a n a u s 69 0 0 0 .

(**) Department Of G e o l o g y , University of W e s t e r n O n t a r i o L o n d o n , C a n a d a , NOA 5 B 7 .

A C T A A M A Z O N ICA, 1 6/ 1 7 (n? u n i c o ) : 3 2 7 - 3 3 6 . 1 9 8 6 / 8 7 . 3 2 7

INTRODUCTION

M a c r o f o s s i 1 s are commonly found along the tributaries and c h a n n e l s of the A c r e and upper Purus Rivers (Figure I ) . Most foss i 1 s are ske 1 eta 1 rema i ns of rept i 1 es and m a m m a l s , that a r e being eroded frorn the Tertiary and P l e i s t o c e n e sediments which dominate the drainage basins of the Acre and Upper Purus R i v e r s . T h e fossils found in this region are

of special interest because Che e n v i r o n m e n t a1 and geologica 1 settings of Western Amazonia have been strongly influenced by the tectonic e v e n t s accompanying the formation of the Andes mountains in Cretaceous and Tertiary t ime and by per iods of dr ler cl imates as a result of major glacial advances in Pleistocene T i m e ,

A partial record of the changes in geological setting as the Amazon platform was tilted by the Andean uprising is found in the Acre Sed imen ta ry Bas i η , a ma j or sub-basin of upper Amazonia (Alto A m a i o n a s ) . The boundaries of the Acre Basin are the Iquitos Arch (to the n o r t h e a s t , east and s o u t h e a s t ) , the Brazilian shield (to t h e s o t h ) a n d the eastern Andean Cordillera (Cordillera O r i e n t a l ) in Peru (Miura, 1 9 7 2 ) . Preliminary studies show that the Acre Basin sediments (e.g., e v a p o r i t e s , marine and iπtracontinenta1 sediments) and their relation to each other are consistent w i t h the truncation of a continental margin to the west followed by the development of an intracontinenta1 q e o l o g i c a1 setting. By the late Tertiary an eastward draining fluvial sys tern w a s w e 1 1 es tab 1 i shed (Asnius and Porto, 1 9 7 2 ) .

The samples studied here include petrified w o o d , a croccodi I e tooth a n d a sample of fossilized bone from the Upper Purus River. Another fossilized bone sample found along the Acre River was used for comparison of rare earth element s i g n a t u r e s . The intention in this study was to e x p l o r e the usefulness of optical and scanning electron microscopy as well as minor and trace element studies i n o b t a i n i n g information on pa 1e o e n v i r o n m e n t s .

Experimental Work

a) Petrified Wood - In these samples the original c e1 Iu1 a r c h e m i c a I components were replaced mainly by s'lderite ( F e C O ^ ) . From observations both in thin section and with scanning electron microscopy (SEM) there is no evidence for physical alteration of the or i g i na 1 ce Π f rairiework and tissue s t r u c t u r e d u r i n g f o s s i l i z a t i o n ( P l a t e l , P h o t o A ) . M i n o r secondary phases w e r e comr.only observed infilling voids (Plate 1 , Photo B) and these in­ cluded iron sulphide ( a ) , potassium a 1 u m1 nosi1icate ( b ) , mixed iron/silica phases ( c ) , tranganese associated w i t h iron phases (d) , and silica (e) . The samp 1 es of pet r i f i ed wood were tentatively identified as tropical h a r d w o o d s due to the lack of growth rings and the presence of multicelled rays and vessels (Plate 1 , Photo A ) .

b) Croccodile Tooth - Carbonate hydroxyapa t i te i s the major component o f this sample and replacement of the original (material by secondary carbonate fluorapatite was also evident. In thin section both the enaniel and the d e n t i n e appear intact and the latter is observed as a system of minute tubules radiating o u t w a r d s . Plate 1 , Photo C shows a

fracture infilled with a carbonate phase ( p r e s u m a b1y r i ch iη i r o n ) , surrounded by opaque phases and iron stained m a t e r i a l . The dentine on either side of the fracture a p p e a r s as pareilel tubules whose physical arrangement is unaltered from the origina I materia 1 . In SEM observations (Plate I, Photos D, E) there was little ev i dence of chem r ca 1 replacement of the original m a t e r i a l . M i c r o f r a c t u r e s (Photo 0 ) wi th i η the a.pa t i te framework (a) were

i nf iI 1ed by i ron su lphide (b) , iron carbonate (c) and iron o x i d e . Cu and Pb w e re detected as minor components of iron sulphide phases (d) . Photo Ε shows a cross-sect ion of dentine

tubules, some of w h i c h are infilled by iron sulphide p h a s e s . The intimate association

between these phases and the a p a t i t e framework suggests that iron ajpbi 1ization and the formation o f secondary iron phases may have been influenced by microbiological activity,

c) Bone Samples - I η the òonc sjin.p I E S (one f r Qui thf; Upper Purus and the other from the Acre R i v e r ) , the major phases are c a r b o n a t e f I u o r a p a t i t e , carbonate byòroxyapatite and calcite, M a r c a s i t e is a minor c o m p o n e n t . In thin sect ion cu t perpend i cu 1 a r to the osteon c a n a l s , the fossil bone s t rue ture appea rs as. pa ra 1 1 e I I y Β I i gned subsn icroscop Ϊ c b i re f r i ng?n t crystallites. Secondary phases are found as prec ί ρ i ta tes w i th i η the voi ds of t he or i g i na I structure. For e x a m p l e , Photo F shows a void infilled with a carbonate phase and sur­ rounded by a mare opaque (iron rich) p h a s e . These voids appear as o p e n i n g s in fractures (upper left of photo F ) , w h i c h p r e s u m a b l y served as pathways for fluids during fossii-ization. The doubly refracting c y s t a l l i t e s surrounding voids indicates that the frame­ work of the original material is intact and suggests that only minor postmortal a l t e r ­ ation has o c c u r r e d . In SEM o b s e r v a t i o n s (Plate I, Photo G) some osteon voi ds were observed to be filled with a zoned p r e c i p i t a t e of iron and m a n g a n e s e carbonate in the core ( a ) , surrounded by iron sulphide (b) and unaltered apa t ite ( c ) . Photo Η shows another osteon void infilled with a barium sulphur phase ( p r e s u m a b1y b a r i t e ) ,

Trace element fingerprinting of the bone samples w a s carried out using spark source T,5SS spec t romet ry (SSMS) (Table 1 , A p p e n d i x 1) and rare earth e I emenl s (REE) w e r e analysed quantitatively using inductively coupled plasma m a s s spec t rox.e t ry ( I CPMS) (Figure 2 , A p ­ pendix l ) . About half of the m i n o r and trace elements surveyed by SSMS (B, C l , S c , V, Cr, Hi, C u , G a , B r , R b , Z r , N b , S n , C s , L a , C e , P r , N d , H f , P b , Th) have levels below those found in average crustal rocks. As and U are the most s t r o n g1y en r i ched (100 times relative to crustal a b u n d a n c e ) , followed by Sr and Ba (> 10 t i mes crus tal abundance), Most olhe r e 1 emen ts surveyed have crustal c o n c e n t r a t i o n s . Qu i te s imi 1 ar REE pa t terns (construe ted from the ICPMS data) were found for bone samples from the Upper Purus and A c r e Rivers (Figure 2 ) ,

DISCOSSIOH

The most remarkable feature of the fossils studied here is the preservation of original t e x t u r e s . This would imply that these foss i 1 s have never beer, deep I y bur ied. The fossil wood samples are the- most e x t e n s i v e l y chemically altered and the replacement of the original material mainly by siderite would indicate that the pore w a t e r s of the enclosing sediments at the time of fossiIization were s a t u r a t e d w i th respect to s i d e r i t e ,

In other work on lake sediments has been shown that interstitial waters only 2 0 - 3 0 cm below the sediment surface w e r e nearly ten-fold supersaturated w i t h respect to siderite (Emerson , 1 9 7 6 ) ,

The preservation of primary a p a t i t e m i n e r a l s forming fossil bones and teeth and the apparently small amount of secondary replacement would imply that more deta i I ed chemica 1

information using minor and trace element signatures could pr o vid e information on e n ­ vironmental parameters at the time o f formation or f o s s i I i z a t i o n . Lambert ct a l . ( I 9 8 1 )

have pointed out that tcoth enamel is much rrore compact than bone and thus chemical re­ placement is slower. Thus minor and [race element levels i η foss i 1 i zed teeth niay ref l e d dietary conditions at the time of formation. Lambert et a t . ( 1 9 8 ^ ) h a v e a l s o ;.hc;wn from work on human fossils that Sr and Zn may be the elements 1 eas t mob Î 1 e during d i a g e n e s i s . Sr has been shown to be an integral part οΓ the hyd roxyapat i te I a 11 i ce (Pa j'kt r and T o o t s ,

1 9 7 0 ) , e ?S i " / S r ratios in m a r i n e fossils have been used to mon i tor va r i a t i ons i η seanater over geological timies (Koepnick et a ) . , I 9 8 5 ) , and th i s parameter may be useful înmonitorinq

the changes in geological setting and climate in upper A m a z o n i a .

Rare earth clement (REE) patterns (Henderson e t a l . , 1 9 8 3 ) in fossil human bonei have also been to suggest that REE are incorporated from ground w a t e r s during fossiI ization. The REE patterns they ob ta i ned showed s t ronger î i (jli t BE Ε en r ί chine π t -and heavy REE dep 1 c t i on wl th respect to chondritic values than did the patterns obtained h e r e . A ma jor d i f f erence was the lack of a Ce anomaly in our d a t a , corroborating the o x y g e n poor diagenetic en vironnient suggested by the presence of s i d e r i t e .

The m i c r o s c a l e variations in chemistry underscore s the comp I ex d i s t r i bu t i on of chein t a 1 gradients during d ' a g e n e s i s and the o b s e r v a t i o n s documented here would suggest a high degree of microbiological activity during d i a g e n e s i s .

CONCLUSIONS

Both conventional microscopy and scanning electron mi c roscopy we re used in studies of fossils found along the Upper Purus River of Upper A m a z o n i a . The nios t sir iking features of these samples is the preservation of original t e x t u r e s . This sugoests the usefu 1 ness of m i c r o s c o p i c studies in fossil classification and possibly in identification of original m a t e r i a l s . From the minor and trace el er.en t stud ici, it appears [fiat rart earth el entent signatures may be useful in identifying fossils with similar diagenetic h i s t o r i e s . High strontium levels in fossilized bone samples i n d i c a t e t ha t strontiurn isolope signature merit further investioation .

ACKNOWLEDGEMENTS

The authors are indebted to John Forth and Walter Harley (University of Western Ontario) for thuir skill and time in the preparation of thin s e c t i o n s . Photographs are by Ian Craig (University of Western O n t a r i o ) . Will Ooherty and A U n n d e r Voet (Ontario Geological Survey) are acknowledged for providing the rare earth element data obtained by ICPHS. T h e • :hors are indebted to Megan Cook (University of Wei-tern Ontario) for critical reading the m a n u s c r i p t . T he f i e 1 d work a n d S a n p 1 e collecting excursions for this

study w e r e financed by the D e p a r t a m e n t o Nacional d a P r o d u c a o Mineral (D.N1.P.M.) and the authors wish to extend a special acknowledgement to Or. José Bel fori dos Santos Bastos

(D.N.P.M., DrasTlia) for his interest and his efforts in making this research p o s s i b l e . 3 3 0 R. Ε . Benchimol et al.

RESUMO

Fósseis de m a d e i r a , ossos e dentes a c h a d o s no Estado do A m a z o n a s , ao lor Purus e d o rio A c r e , foram e s t u d a d o s , u s a n d o - s e m í c r o s c õ p i a convencional e m i c r varredura. A espectrometria d e m a s s a foi utilizada para investigar os níveis «

tos menores e traços em algumas a m o s t r a s d e o s s o f o s s i l i z a d o . Os e s t u d o s m i c r o s c ó p i c o s mostram que há pouca a l t e r a ç ã o das texturas o r i g i n a i s . As amostras d e m a d e i r a fossili-zada foram identificadas em lâmina delgada c o m o madeira d e lei. Nestas a m o s t r a s a subs tituição da m a t é r i a original por siderita sugere que a fóssilizaçio ocorreu em s e d i m e n -tos r a s o s , nos quais as águas intersticiais foram saturadas pelo c a r b o n a t o d e f e r r o . A precipitação de fases secundarias d e ferro foi observada freqüentemente nas rachaduras e v ã o s , nas amostras tanto d e osso* f o s s i l i z a d o como de m a d e i r a f o s s i l i z a d a . Outras fases secundárias incluem a s í l i c a , ó x i d o s de ferro e c a r b o n a t o d e m a n g a n ê s . A a s s o c i a ç ã o ín tima dessas fases secundárias com as estruturas b i o l ó g i c a s o r i g i n a i s pode ser evidência para um papel m i c r o b i o l õ g i c o na formação dessas f a s e s . Os padrões de c o n c e n t r a ç ã o seme lhante d e terras raras em dois ossos em localidades m o d e r n a s diferentes indicam históri as diagenéticas s e m e l h a n t e s . A p r e s e r v a ç ã o virtualmente completa das texturas o r i g i -nais sugere que e s t u d o s m i c r o s c ó p i c o s podem ser úteis na c l a s s i f i c a ç ã o dos f ó s s e i s e a t é na identificação d e m a t e r i a i s o r i g i n a i s . Padrões de c o n c e n t r a ç ã o d e terras raras em o £ sos fossilizados podem refletir as c o m p o s i ç õ e s d e água subterrânea na época de f ó s s i l i -zação.

F i g . 3 . P h o t o m i c r o g r a p h s of fossilized w o o d , teeth and b o n e .

P h o t o A - fossilized wood (80 χ ) ; Β - scanning electron micrograph of f o s s i l ­ ized wood (1160 x ) ; C - fossilized tooth (200 x ) ; D - scanning electron m i c r o graph of fossilized tooth (1240 χ ) ; Ε - scanning electron micrograph of fossil_ ized tooth (2940 x ) ; F - fossilized bone (200 x ) ; G - scanning electron m i c r o ­ graph of fossilized bone (1029 χ ) ; Η - scanning electron m i c r o g r a p h of fossil^ ized bone (1028 x ) .

Tabela I. Spark Source Mass Spectrometry (SSMS) Data - Bone Samples

Element Concentrat ion (ug

g

Β 0 . 5 F

_>kyo

S >39o CI 50 Sc 3 V 10 Cr 0 . 5 Ni 15 Ou 20 Zn 120 Ga 0 . 5 Ge 2 As >20C Se 0.1 8r 0 . 0 5 Rb 0 . 5 Sr 3300 _h Y

_W)

e

[W

b Zr 60 Nb 1 Μα 1 Ag 0.Ί Sb 3.2 0.3 Ba 6750 La (9) [19] Ce (2k) [38] Pr (3) [ 5 ] Nd (15) [2Ί] Sm

_(Ό

_[51

Eu (1) [2] Gd (7) [8] Tb

_(D

[1] Dy (7) [ 6 ] Ha

(!)

[2] Er

_W

_W

Tm (0.5) [ 0 . 6 ] Yb (3) [3] Lu (0. Ί ) [ 0 . 6 ] Pb 2 U 170

(a) REE data O obtained by ICPMS for bone sample from Purus R i v e r . (b) REE data [] obtained by I CPUS for bone sample from Acre River.

S n , C s , Hf and Th c o n c e n t r a t i o n s did not exceed 1 ug g \

A p p e n d i x - Analytical M e t h o d s Used

X-ray Dlffration (XRD) - T h e d i f f r a c t i o n of x - r a y s produced by a Cu-Κα source through sample material w a s used to d e t e r m i n e the m a j o r m i n e r a l s p r e s e n t .

Spark Source Mass Spectroscopy (SSMS) - T h i s technique is used for multi-element

( 5 O - 6 O e l e m e n t s ) fingerprinting down to part per billion l e v e l s . One a d v a n t a g e is the simplicity of preparation of solid samples by mixing w i t h g r a p h i t e (Kronberg et a l . , in preparat i o n ) .

Scanning Electron Microscopy (SEM) - Using SEM it is possible to obtain micron scale images of surfaces of solid m a t e r i a l s . Simultaneous q u a l i t a t i v e ana 1ysis i s possj_ ble of the sample if probed w i t h x-rays ( E D S ) .

Inductively Coupled Plasma M a s s Spectrometry ( I C P M S ) : In this study ICPMS w a s used to determine the c o n c e n t r a t i o n s of rare earth e l e m e n t s in bone s a m p l e s . Rare earth elements w e r e concentrated by c h r o m a t o g r a p h i c extraction from s o l u t i o n . The analytical solutions are then aspirated into a plasma and analysed by q u a d r a p o l e m a s s spectrometry (Bolton et a l .,

1583).

References

A s m u s , Η. E . & P o r t o , R. - 1 9 7 2 . C l a s s i f i c a ç ã o das b a c i a s sedimentares b r a s i l e i r a s se-gundo a tectonica de p l a c a s . I n : A n a i s C o n g r e s s o B r a s i l e i r o d e G e o l o g i a , 26 Belém, 1 9 7 2 . v. 2: 6 7 - 9 0 .

Bolton, Α.; H w a n g , J.; Vander V o e t , A . - 1 9 8 3 . T h e determination of scandium, yttrium and selected rare earth elements in geological m a t e r i a l s by inductively coupled plasma optical emission spectrometry. S p e c t r o c h i m i c a A c t a , 3 8 B : 1 6 5 - 1 7 4 .

Emerson, S. - 1976. Early d i a g e n e s i s in anaerobic laks s e d i m e n t s : chemical equilibria in interstitial w a t e r s . G e o c h i m et C o s m o c h i n A c t a , 4 0 : 9 2 5 - 9 3 4 .

Henderson, P.; M a r l o v , C. Α . ; M o l l e s o n , Τ. I.; W i l l i a m s , C . T, - 1 9 8 3 . Patterns of c h e ­ mical change during bone f o s s i l i z a t i o n . N a t u r e , 3 0 6 : 3 5 8 - 3 6 0 .

K o e p n i c k , R. B . ; B u r k e , W . H.; D e n i s o n , R. E . ; H e a t h e r i n g t o n , Ε . Α . ; N e l s o n H. P.; Otto J. B.; W a i t h e , L . E . - 1 9 8 5 . Construction of the seawater curve for the Cenozoic and C r e t a c e o u s : supporting data. Isotope G e o s c i e n c e , 4 ( 1 ) : 5 5 - 8 1 .

K r o n b e r g , Β . I.; M u r r a y , F. H.; D a d d a r , R.; F y f e , W . S. - 1986. E v a l u a t i o n of spark source m a s s spectrometry for semiquantitative analysis of geological m a t e r i a l s . Int J M a s s Spec a n d Ion P r o c . (in p r e p a r a t i o n ) .

Lambert, J. B.; Simpson, S. V.; S z p u n a r , C . B.; B u i k s t r a , J. E . - 1984. Ancient h u m a n diet from inorganic analysis of b o n e . Acc C h e m R e s . , 17: 2 9 8 - 3 0 5 .

M i u r a , K. - 1972. P o s s i b i l i d a d e s petrolíferas da bacia do A c r e . I n : A n a i s C o n g r e s s o B r a s i l e i r o de G e o l o g i a , 26, Belém, 1 9 7 2 . v. 3: 1 5 - 2 0 .