Evidence of tertiary forest in the Falkland Islands (Islas Malvinas)

Palaeogeography, Palaeochmatology, Palaeoecology, 55 (1986) 45--53

45

Elsewer Science Publishers B V , Amsterdam -- Printed m The Netherlands

EVIDENCE OF TERTIARY (ISLAS MALVINAS)

FOREST

IN THE FALKLAND

ISLANDS

J F BIRNIE and D E ROBERTS

Department of Geography, Unwerszty of Aberdeen, Aberdeen AB9 2UF (U K ) (Received September 8, 1983, revtsed and accepted October 21, 1985)

ABSTRACT Blrme, J F and Roberts, D E , 1986 Ewdence of Tertiary forest in the Falkland Islands (Islas Malwnas) Palaeogeogr , Palaeochmatol , Palaeoecol, 55 45--53 Recent exammatmn of the sediments and mmrofossfls of a wood-bearing deposit m West Falklands (Lat 51 ° 21'S, Long 60°42'W) mdmates that it represents an m-sltu forest environment and not driftwood, as omgmally assumed following ~ts description m 1912 Palynomorphs m the orgamc matrix are consistent w~th wood ]dentifmations, and record a local palaeoenvironment of podocarp-type forest w~th some Nothofagus and tree ferns Physical characteristics of the sediment matrix also mdmate a terrestrml rather than coastal origin The immense contrast of this forest with the treeless oceamc heath of the Falklands at the present t~me makes a Quaternary date extremely unhkely, despite the excellent preservatmn of micro- and macrofosslls In the context of present knowledge of the Southern Hemisphere palaeochmate, a date of deposltmn prmr to Antarctic cooling and the initiation of the circumpolar current ts suggested Palaeobotamcal characterlstms of the palynomorph assemblage also suggest pre-Late Tertmry date INTRODUCTION T h e F a l k l a n d I s l a n d s / I s l a s M a l v m a s he s o m e 6 5 0 k m t o t h e e a s t o f T m r r a del Fuego and 1200 km north of the Antarctic Peninsula The present chmatm charactemstms, with strong westerly winds, a mean annual temperature of only 5 9°C, high cloud cover and yet relatively low rainfall (60 cm per a n n u m ) , r e s u l t f r o m i t s o c e a m c s i t u a t i o n m t h e w e s t e r l y b e l t , b u t m t h e lee o f t h e A n d e s T h e n a t u r a l c h m a x v e g e t a t i o n is o c e a m c h e a t h ( M o o r e , 1 9 6 8 ) and this has been the case for at least the past 9300 years (Barrow, 1978) No evidence of indigenous tree growth has been found m Quaternary sediments m the Falklands, with the exceptmn of the Forest Bed described by Halle m 1912, and reexamined here If Halle's interpretation of the material as l o c a l a n d Q u a t e r n a r y is c o r r e c t , a n i n t e r g l a c i a l c h m a t e r a d i c a l l y d i f f e r e n t f r o m t h a t o f t h e p r e s e n t d a y w o u l d be m d m a t e d , a f a c t o r o f c o n s i d e r a b l e slgmhcance m the reconstruction of Southern Hemisphere palaeochmates The wood-bearmg deposit was exposed at the back of a beach m a sheltered b a y m W e s t P o i n t I s l a n d ( F i g 1) T h e s t r a t i g r a p h y , as d e s c r i b e d b y H a l l e , is 0031-0182/86/$03 50

© 1986 Elsewer Scmnce Pubhshers B V

46

Isouth ~40

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o

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Falkland

a

site of forest bed Islands \ t

60 S~

West

/

L,,/~-

,s,and

~West L

5kin

J

~,nt,~rctl c~

Falkland

(

Fig 1 Location map

shown m Fig 2. At that time wood samples were ldentffmd as being similar to Podocarpus ch~hna (now P sahgnus) and Lzbocedrus (now Austrocedrus) chllensls Halle assumed a Quaternary date because of its excellent state of preservation The accumulation of densely-packed wood material m a soil

On

Og

15

_LLL LLL _LLL LLLL

Yellowish clay, unst rat f fled, some sand and small stones

V~°L

Big angular blocks and ') s c a t t e r e d pebbles I SOLIFLUCTION cemented by clay DEPOSIT

...-L L ~

Clay c o n t a l n 0 n g l e n s e s and s t r i p s of black soil, t r e e t r u n k s m lower part

"Y'L 24

trans~hon ( ......

ISamples t a k e n I Jfrom this level -lln 1979

to .........

b l a c k , v e g e t a b l e soil, '~ variable sand content, with densely packed wood, including t r u n k s over 1 5m and twigs less than 5ram in diameter Bed found over distance of 550m ~long coast, 30m inland

Fig 2 Stratigraphy (after Halle, 1912)

FOREST BED

47 matrLx was interpreted as m-s~tu remains of a forest, preserved beneath a wet-landshp deposit, and mdmatmg a milder and wetter chmate than at present The blocky overburden was taken to be a sohfluctlon deposit datmg from a penglacml phase Scepticism led subsequent workers to dismiss the Forest Bed as driftwood (Baker, 1924), although it was not re-examined m the field In 1979 D Roberts had an o p p o r t u m t y to wslt the site, and sampled wood and its sedimentary matrix from the upper part of the Forest Bed, then exposed by coastal erosion The samples were subjected to sediment, pollen and radiocarbon analysis with the mm of checking Halle's interpretation ANALYSIS Partmle size charactenstms of the sediment matrix showed great similarity to the fmer portion of both the overlying sohfluctlon deposit and to sohfluctlon material widespread throughout the Falkland Islands, where such matenal comprises the characteristic Quaternary deposit (Roberts, 1983) With 56% sand, 29% silt and 15% clay, the West Pomt Island sediment is too poorly sorted to be a typical beach deposit, and samples prepared for examination of marine mmrofossfls proved to be barren Analysis therefore suggests that the material is a terrestrial slope deposit, and Halle's interpretation is reasonable A sample of the sediment was prepared for pollen and spore analysis using standard procedures dlsaggregatmn and solution of humlc colloids, solutmn of silicates, and acetylatmn (Faegn and Iversen, 1975} Addltmn of a known quantity of exotic grams to a measured volume of sample prior to preparation permitted assessment of actual pollen and spore concentratmns (Stockmarr, 1971, 1973) Prepared samples were m o u n t e d m sflmone off A count of two slides at traverses 2 mm apart gave a total of 767 grams The assemblage bears no similarity to post-glacial and mterstadml spectra from the Falkland Islands (Table I) The West Point Island deposit is dominated by podocarptype pollen 1, mcludmg some Dacryd~urn-type grams, and Dzcksonza-type spores are a major c o m p o n e n t Nothofagus fusca-type comprises 6% of the total, and the rest of the assemblage mcludes small quantities of a variety of pollen forms, notably Asteha-type, Myrtaceae-type and Proteaceae-type The pollen and spore assemblage appears to be of local ongm Long-distance transport, for example from South Amerma, is unhkely because the assemblage includes spores which are not transported easily, and because the concentratmn of pollen and spores is of the same order as that of Postglacml deposits whmh are of at least 95% local ongm {Table I) A low concentration would have suggested a long-distance omgm or secondary derivation Thus the pollen and spore assemblage m the West Point Island materml may be taken to mdmate local podocarp-dommated forest, of a type whmh has its 1All palynomorphs have been described m terms ot the present-day forms to which they show the closest affinity, pending more detailed comparison w~th fossil materml

I

aLocatlons

and stratlgraphm

Podocarp Dacrydzum (frankhnu) N o thofag us fus ca f o r m Nothofagus obhqua form Dzcksonla Other Flhcales Myrtaceae Asteha Proteaceae Type A dlcot Type B dmot Undafferentlated Eneaceae and Empetrum Grammeae Cyperaceae Composltae Acaena Gunneraceae Maytenus Other ldenhfled herbs Concentration of palynomorphs X 103/cc sed

Pollen a n d spores types

context

212

49 5 6 1 14 5 2 1 -}5 4 7

of Postglaclal

peats

1 1 66 11 1 13 1 + 1

1 12 55

and Interstadial

481

14

1

sediments

from

489

7 73

Lowland Peat ( .3 6 8 0 + 4 0 y r B P )

1983

Quaternary

given In Roberts,

221

3

11

and

Lowland Peat ( 4 2 1 0 + 4 0. y r B P )

deposit

1

2 +

Upland Peat (5900 + . 60 yr B P ) .

Island

Postglamal deposits

West Point

Site

of the

West Point Island

Comparison of pollen and spore assemblages expressed as percentages of counted totals

TABLE Islands a,

37

3

94 3 1

Lowland Peat ( 2 6 , 0 6 0 + 4003s0y r B P )

Interstadaal deposat

the Falkland

o0

49 closest parallels at the present day in the North Island of New Zealand and in northern South America The present southern hmlts of Podocarpus sahgnus and Austrocedrus chzlenszs are 42°S and 45°S respectively. The present vegetation of the Falklands (51°S), oceamc heath dominated by Cortaderla (tussock grass) and Empetrum (Moore, 1968) is typically represented by a pollen spectrum of 70% Grammeae with the only arboreal pollen being Nothofagus fusca-type of long-distance origin, comprising less than 2% of the total The source of this pollen is most likely to be forested areas in Tmrra del Fuego or the Andean region, where Nothofagus antarcttca is the dominant tree Podocarp-type pollen has not been found m Postglaclal deposits m the Falklands A wood sample collected m 1979 was examined by Dr J A Petty (Forestry Department, Aberdeen Umverslty) and appears to be very similar to some contemporary Nothofagus specms The wood structure was very well preserved, and the principal dmgnostlc characteristics include diffuse porous arrangement of vessels, either sohtary or in pairs, mean vessel w~dth about 40 t~m, with considerably more than 25 vessels per mm 2 of transverse surface, perforation plates predominantly s~mple, but a few scalarfform, heterogeneous rays mainly blsermte, about 30 #m wide and up to 20 cells high, vertical parenchyma sparse, diffuse and possibly boundary in distribution This indicates similarity to a group of species including Nothofagus fusca, N rnenztesu, N procera, and N cunnmghamn, whilst Nothofagus dombeyn, N obhqua, N mooreL N truncata and N chffordmldes are probably excluded Podocarpus and Austrocedrus-type wood, as described by Halle, were n o t recorded m the hmlted sample taken on this occasmn The identification of Nothofagus- and Podocarpus-type macrofosslls is consistent with the pollen record, and confirms that both tree types were present at the site, although the relative pollen representatmn indicates Podocarpus dommance Austrocedrus-type pollen has n o t yet been identified Part of the wood sampled in 1979 was submitted for radmcarbon dating Analysis of the cellulose content gave an ostensibly flmte date of 35,960 -+ 900 810 conventional 14C yr B P (SRR-1784), but the evidence of the palaeobotamcal record suggests that this is hkely to be a spurious age, which could have resulted from modern carbon contamlnatmn of less than 1% (D D Harkness, personal commumcatlon, 1981) A second attempt at 14C analysis will be made when the site can be rewslted for further samphng CONCLUSIONS The extent of the West Point Island deposit, the preservation of fme twigs (Halle's observations), the non-marme nature of the sediment and the consistency of the matrLx pollen and spore assemblage with the wood macrofosslls contained within it indicate that the w o o d y deposit is most unlikely to be drifted Rather it appears to represent an ln-sltu record of a major environmental change in the Falkland Islands Quaternary interglacial deposits m the

lip

J

!

r~

-3

51 Falklands are n o t available for comparison, but any such change must also have affected South Amerma, and so far, interglacial sites there show no evidence o f a slgmfmant southerly advance of podocarp forest (Auer, 1958, 1970, Heusser, 1974) Ocean core evidence from the region of the Antarctm Convergence suggests t ha t only minor latitudinal shifts of the Polar F r o n t t o o k place during the Quaternary (Kennett, 1978), giving little reason to e x p e c t major chmatm change in the Falklands The present pattern of ocean current c~rculatlon m the vicinity of the Falklands was estabhshed with the initiation of the circumpolar current around the Antarctm cont m e nt , which probably occurred in the middle Ohgocene (Kennett, 1978) Boersma and Shackleton (1977) give ewdence f o r a major decrease m near-surface ocean t e m p e r a t u r e m the South Atlantic f r o m 20 to 9 ° C durmg the late Eocene Fr om environmental evidence, therefore, the West Point Island deposit could represent condltmns prior to the initiation of th e circumpolar current, and possibly prmr to late Eocene cooling The strengthening latitudinal z o n a t m n o f climate could explain a change from a warm, humid chmate m the Tertmry to the cool, dry climate of the Quaternary, when the Falkland e n w r o n m e n t became governed by its poslt~on m the lee o f South Amerma The Cordilleran orogeny of the Andes m the late Phocene and early Pleistocene may then have further enhanced the rainshadow effect However, on their own, no known Quaternary changes would appear to be sufficient to explain a change from ralnforest to oceamc heath m the Falklands. In the c o n t e x t of the known palaeobotany of the southern hemisphere, a Tertmry age would also seem more likely than a Quaternary age for the West Point Island materml In the Tertmry, Nothofagus forest appears to have e x t e n d e d far to the south of Its Quaternary hm~ts, with evidence from the Ross Sea region (Kemp and Barret, 1975, Brady and Martin, 1979}, East Antarctma (Kemp, 1972), S e y m o u r Island (Cranwell, 1959) and even Alexander Island ( T h o m s o n and Burn, 1977) glwng dates ranging from the Eocene to the middle Miocene Nothofagus fusca-type pollen has been recorded f r o m Fuegm in deposits from the Eocene and early Ohgocene, but n o t f r o m the later Tertmry (Flenley, 1979) The p o d o c a r p - d o m l n a t e d forest o f West Point Island, with Nothofagus fusca-type present would be consistent with this southerly extensmn of forest limits by some 10--15 ° of latitude Interestmgly, the Proteaceae are less well represented m Falkland than In these Antarctm sites, which may be a function of the m ore nort herl y environm e n t , or may indicate a later date of deposltmn PLATE I

1 Dzckson~a-typespore, each side 55 t~m in length 2 3 4 5 6

Podocarp-type pollen, 48 am across breadth Podocarp-type pollen, 48 X 15 am Same podocarp gram as m 3, different focus Gram hke Dacrydmm frankhnn, side vmw, 38 t~m top to bottom Similar to 5, front view, mare body 45 um length

52 Aspects of the West Point Island assemblage itself also suggest a Tertiary date on the basis of the evolutmnary h~story of plant types The complete absence of Grammeae and Composltae suggests a pre-Mmcene date, but the presence of As tel~a may mean that it is at least early Ohgocene (Flenley, 1979) More detailed comparison with other fossil materml may lead to closer dating of this deposit, but it already seems hkely that the Dtckson~a-type spores and Dacrydmm-type pollen (Plate I) are related to Tnhtes tubercul~form~s Cookson and Phylloclad~tes Cookson respectively, descmbed from early Tertmry deposits of southern Chile (Cookson and Cranwell, 1967) The palaeochmatm and palaeobotamcal context of the West Point Island deposit, and prehmmary assessment of the afhmtms of the wood and mmrofossil assemblage, p o m t to the materml bemg an exceptmnally well preserved record of a Tertmry, possibly early Ohgocene forest enwronment m the Falkland Islands The site itself seems to have considerable further potentml m terms of its contrlbutmn to knowledge of Tertiary plant hfe and environm e n t m the South Atlantm regmn, and comprehensive samphng through the full depth of the deposit, together with detailed compamson of plant remains, pollen and spores with fossil materml from other Southern Hemisphere Tertmry sites, would do more justice to its slgmfmance The authors hope that the results of the hmlted re-mvestlgatmn presented here will lead on to a more comprehensive e x a m m a t m n of the site by interested partms ACKNOWLEDGEMENTS Dave Roberts wishes to acknowledge financial support from the Natural Environment Research Council and logistic support from the British Antarctm Survey, and the hospltahty of R o d d y and Lily Napmr of West Pomt Island during fmldwork Jacky Blrme wishes to acknowledge the Department of Geography, Umverslty of Aberdeen for making faclhtms for analysis available Both authors are grateful to Dr J A. Petty for the wood ldentffmatmn, Dr. D. D Harkness for radmcarbon analysis, and Dr D E Sugden for adwce and encouragement REFERENCES

Auer, V, 1958 The Pleistocene of Fuego-Patagoma Part II, The history of flora and vegetation Ann Acad Scl Fenn Ser A III, 50, 269 pp Auer, V, 1970 The Pleistocene of Fuego-Patagoma Part V Quaternary problems of Southern South Amerma Ann Acad Scl Fenn Ser A III, 100, 194 pp Baker, H A, 1924 Final report on geologmal investigations m the Falkland Islands 1920--1922 C S O, Stanley, 33 pp Barrow, C J , 1978. Postglaclal pollen diagrams from South Georgia and West Falkland Island J Blogeogr, 5(2) 251--274. Boersma, A and Shackleton, N, 1977 Tertiary oxygen and carbon isotope stratigraphy Site 357 (Mid Latitude South Atlantic) In P R Supko et al , Imtlal Reports of the Deep Sea DrdhngProject, Vol 39 U S Government Printing Office, Washington, D C, pp 911--924

53 Brady, H and Martin, H , 1979 Ross Sea region m the middle Miocene A ghmpse into the past Scmnce, 203 437--438 Cookson, I C and Cranwell, L M , 1967 Lower Tertiary mmroplankton, spores and pollen grams from southernmost Chile Mmropaleontology, 13(2) 204--216 Cranwell, L M , 1959 Fossd pollen from Seymour Island, Antarctica Nature, 184 1782 Faegrl, K and Iversen, J , 1975 Textbook of Pollen Analysis Blackwell, Oxford, 295 pp Flenley, J R , 1979 The Equatorial Ram Forest A Geologmal History Butterworths, London, 162 pp Halle, T G , 1912 On the geological structure and history of the Falkland Islands Bull Geol I n s t , Umv Uppsala, 11 115--229 Heusser, C J , 1974 Vegetation and chmate of the southern Chilean Lake District during and since the last rater-glaciation Quat R e s , 4 290--315 Kemp, E M , 1972 Reworked palynomorphs from the West Ice Shelf area, East Antarctica, and their possible geological and palaeochmatologmal significance Mar G e o l , 13 145--157 Kemp, E M and Barrett, P J , ]975 Antarctm glaciation and early Tertiary vegetation Nature, 258 507--508 Kennett, J P , 1978 Camozolc evolutmn of clrcumantarctm palaeoceanography In E M Van Zmderen Bakker (Editor), Antarctic Glacial History and World Palaeoenvlronments Balkema, Rotterdam, pp 41--56 Moore, D J , 1968 The vascular flora of the Falkland Islands Br Antarct Surv Scl Rep , 60 202 pp Roberts, D E , 1983 The Quaternary history of the Falkland Islands Thesis, Aberdeen Umverslty (unpubhshed) Stockmarr, J , 1971 Tablet~ with spores used m absolute pollen analysis Pollen Spores, 13(4) 615--621 Stockmarr, J , 1973 Determmatmn of spore concentration with an electromc particle counter Dan Geol Unders Arbog, 1972 8 7 - 8 9 Thomson,M R A and Burn, R W , 1977 Angiosperm fossils from latitude 70°S Nature, 269 139--141