Late quaternary environments of the Carpentaria Basin, Australia

PMaeogeography, Palaeoclimatology, Palaeoecology, 67 (1988): 245 261 Elsevier Science Publishers B.V., A m s t e r d a m

245

Printed in The N e t h e r l a n d s

LATE QUATERNARY ENVIRONMENTS OF THE CARPENTARIA BASIN, AUSTRALIA T. T O R G E R S E N 1, J. L U L Y 2, P. DE D E C K K E R 3, M. R. J O N E S 4, D. E. S E A R L E 4, A. R. CHIVAS s and W. J. U L L M A N 6 ~Department of Marine Sciences, The University of Connecticut, Groton, CT 06340 (U.S.A.) 2Department of Biogeography and Geomorphology, Australian National University, Canberra, A.C.T. 2601 (Australia) 3Department of Geology, Australian National University, Canberra, A.C.T. 2601 (Australia) "*Geological Survey of Queensland, GPO Box 194, Brisbane, Qld. 4001 (Australia) 5Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2601 (Australia) "College of Marine Studies, University of Delaware, Lewes, DE 19958 (U.S.A.) (Received J u n e 26, 1987: revised and accepted February 17, 1988)

Abstract 'l%rgersen, 7 , Luly, J., De Deckker, P., Jones, M. R., Searle, D. E.. Chivas, A. R. and Ullman, W. J., 1988. Late Q u a t e r n a r y e n v i r o n m e n t s of the C a r p e n t a r i a Basin, Australia. Palaeogeogr., Palaeoclimatol.. Palaeoeeol., 67: 245 261. Q u a t e r n a r y variations in the level of the sea have exposed the shallow ( < 70 m) Gulf of Carpentaria (Australia) a n u m b e r of times during the last glacial. Sedimentary, chemical and palynological analyses of cores from the Gulf are presented and interpreted to describe the p a l a e o e n v i r o n m e n t a l sequence of the Carpentaria Basin over the last < 40 kyr. The results show an early desiccation followed by the c o n t i n u o u s presence of a large (> 29,000 km 2) but shallow (maximum depth < 10 m) fresh-to-brackish Lake C a r p e n t a r i a from ~ 35 kyr to 12 kyr. Runoff[evaporation ratios were held at about half the present ratio. The e n v i r o n m e n t s of the Basin are shown to have been influenced by the level of the sea, Q u a t e r n a r y climatic/monsoonal variations and the tectonic diversion of the Fly River. Palynological a n a l y s e s indicate a continued s a v a n n a h - l i k e e n v i r o n m e n t in the Basin with few floristic introductions during the last < 40 kyr. The pollen assemblages closely resemble the black soil plains presently found along the s o u t h e r n Gulf. Thus, the biogeographic and climatic barrier across the A u s t r a l i a Papua New Guinea land bridge has remained largely intact in spite of the range of e n v i r o n m e n t a l conditions t h a t have occurred during the last < 40 kyr.

Introduction The Gulf of C a r p e n t a r i a , a shallow ( < 70 m) e p i c o n t i n e n t a l sea b e t w e e n A u s t r a l i a and Papua New Guinea, lies astride the b o u n d a r y between the hot, humid, tropical e n v i r o n m e n t to the n o r t h and the arid i n t e r i o r of A u s t r a l i a (Fig.l). The G u l f is c o n n e c t e d to the A r a f u r a Sea across a - 5 3 m sill and to the Coral Sea across the 12m-deep T o r r e s S t r a i t and is c o n t i g u o u s with the e x t e n s i v e shallow seas of the " M a r i t i m e C o n t i n e n t " of I n d o n e s i a (Ramage, 1968). The climate and the w a t e r b a l a n c e 0031 0182/88/$03.50

of the C a r p e n t a r i a (drainage) Basin are sensitive not only to the intricacies of tropical a t m o s p h e r i c c i r c u l a t i o n , and m o n s o o n a l rains, but also to the i n t e r a c t i v e m e t e o r o l o g i c a l effects of the S o u t h e r n Oscillation and the E1 Nifio p h e n o m e n a in general. D u r i n g the Q u a t e r n a r y , sea-level v a r i a t i o n s (e.g. Chappell, 1983) would h a v e s u b a e r i a l l y exposed the G u l f of C a r p e n t a r i a a n u m b e r of times (Fig.2). Nix and K a l m a (1972) identified a closed-basin w i t h i n the G u l f of C a r p e n t a r i a (Fig.l) and p r e d i c t e d the presence of a shallow b r a c k i s h lake (inferred p a r t l y from Phipps,

~ 1988 Elsevier Science Publishers B.V.

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Fig.l.A. The Carpentaria drainage basin showing the maximum extent of Lake Carpentaria and the Fly (-Strickland) Basin. Overlain on the geographic map are the contours of the present average annual runoff as deduced from the Atlas of Australian Resources (1977) and Smith (1973). Total average annual runoff to Lake Carpentaria under present conditions has been estimated to be 2.3 x 1011m3 yr- 1(Torgersen et al., 1985)•Total flow of the Fly River is 4.7 x 1011ma yr 1(Torgersen et al., 1983)• B. Core locations used in this study and the bathymetry of the Gulf of Carpentaria. Full bathymetry is given in Torgersen et al. (1983). Some core locations are given on the figure and the details are given in Table I. 1970) based on a w a t e r b a l a n c e d e d u c e d from t h e i r t e m p e r a t u r e , e v a p o r a t i o n , v e g e t a t i o n and p r e c i p i t a t i o n r e c o n s t r u c t i o n s . S m a r t (1977) m a t c h e d the ~4C d a t e d offshore e x p l o r a t i o n cores of C a n a d i a n S u p e r i o r M i n i n g Co. (Australia) and to the sea-level c u r v e of B l o o m et al. (1974). T o r g e r s e n et al. (1983) confirmed the closed c o n t o u r s of the G u l f of C a r p e n t a r i a , and identified the seismically-visible s h o r e l i n e of an a n c i e n t lake, w h i c h t h e y n a m e d L a k e C a r p e n t a r i a . T o r g e r s e n et al. (1985) r e p o r t e d the p r e l i m i n a r y results of a 1982 cruise w h i c h collected a n d identified the b r a c k i s h l a c u s t r i n e deposits of L a k e C a r p e n t a r i a a n d p r e s e n t e d a

p r e l i m i n a r y i n t e r p r e t a t i o n . A limited set of '4C dates from t h a t s t u d y suggests some disagreem e n t with the sea-level c u r v e of Chappell (1983) and the i n t e r p r e t a t i o n of S m a r t (1977). In s u b s e q u e n t studies, J o n e s and T o r g e r s e n (1988) e x a m i n e d the surface sediments, core lithologies and seismic profiles of the Gulf and proposed a d e p o s i t i o n a l history; De D e c k k e r et al. (1988) e x a m i n e d the p a l a e o n t o l o g i c a l r e c o r d of L a k e C a r p e n t a r i a , and provided a description of the p a l a e o e n v i r o n m e n t , p r o b a b l e w a t e r depths and s a l i n i t y v a r i a t i o n s based on the t r a c e - e l e m e n t r a t i o s of ostracods. This s t u d y r e p o r t s sedimentological, trace-

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metal and stable-isotope studies of the calcareous laminae of Unit III, thus establishing the chemical environment of their deposition and providing an authigenic indicator of water salinity and chemistry with which to i nt er pr et the ostracod/salinity studies of De Deckker et al. (1988). We f u r th e r report new pollen data to define the terrestrial envi r onm ent and biogeography of this important land bridge. Extensive 1~C dates provide a detailed chronology of the Gulf-Lake system and its relation to Q u a t e r n a r y sea-levels (e.g. Chappell, 1983). The study assesses the physiography and palaeoenvironments of the Carpentaria Basin over the last < 40 kyr and describes the hydrologic and physiographic conditions controlling the filling of Lake Carpentaria. Methods and results

Stratigraphy, distribution and 14C dating of Late Quaternary sediments The Reef Seeker cruise of 1982 collected thirty-five cores (see Fig.l) with good seismic

control of which twenty-two have been examined and logged in detail. The distribution of cores is representative of all bathymetric depths but most cores were taken from water depths > 6 0 m . Torgersen et al. (1985) have described the primary units of the stratigraphic section and the preliminary i nt erpret at i on of these sediments. We elaborate on those descriptions and include additional 14C dates and relevant additional descriptions from Jones and Torgersen (1988) and De Deckker et al. (1988). The units are numbered from top to bottom of the type section (GC-2). Several core logs pert i nent to this paper are shown in Fig.3. Unit I is a grey-green soft shelly and sandy mud containing marine molluscs, bryozoans, and reworked euryhaline marine ostracods (Polycope, Bairdia, Cyprideis) as well as reworked euryhaline marine foraminifera

(Pseudorotalia inflata, Heterolepa subhaidingeri, Neoeponides schreibersi, Ammonia beccarii, A. convexa. Quinqueloculina philippinensis, Textularia agglutinans). This unit was deposited in the marine environment of the modern central Gulf. Some Cyprideis shells are cracked, broken or worn indicative of substantial reworking and all Cyprideis exhibit a broad range of Mg/Ca and Sr/Ca ratios TABLE I Locations of some cores from the Gulf of C a r p e n t a r i a Core

Date

Latitude

Longitude

Water depth

(m) SPR-1 GC-1 GC-2 GC-3 GC-4 GC-5 GC-10A GC-14B GC-21B GC-28 GC-33 GC-41 a GC-69

19-3-82 12 22.50'S 13-10-82 12 34.09'S 1 3 - 1 0 - 8 2 12~31.18'S 13-10-82 12~'33.79'S 14-10-82 12/33.11'S 14-10-82 12130.12'S 16-10-82 13~04.40'S 17-10-82 13' 36.67'S 19-10-82 13//52.32'S 20-10-82 14/13.41'S 21-10-82 13i: 15.70'S 23-10-82 10'39.47'S 1-11-82 1£~01.00'S

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(De D e c k k e r et al., 1988)• T h e s e d i m e n t o l o g i c a l o c c u r r e n c e of ooids, and alluvial q u a r t z o s e sand with s u b a n g u l a r g r a i n shapes, dull surface l u s t e r and yellow-brown s t a i n i n g (Jones and T o r g e r s e n , 1988) suggest t h a t U n i t I has a s t r o n g r e l i c t c o m p o n e n t . 14C dates for surface sediments are in the r a n g e 4700-7800 y r B•P. (8890_+ 110 y r B.P. for surface ooids; see T a b l e II)o T h e lowest section of the sedimento-

logically defined U n i t I has a 14C date of 7240_+160 y r B.P. (ANU-4801) and the top of U n i t II has a 14C date of 9800_+340 yr B•P. (ANU-4802)• Thus, the lithologic t r a n s i t i o n from U n i t II to U n i t I o c c u r r e d ~ 8500 y r B•P• U n i t II is a d a r k g r e y firm mud c o n t a i n i n g o s t r a c o d s (principally Ilyocypris and Cyprideis) and f o r a m i n i f e r a (A. beccarii, A. convexa, A.

Tepida, P. inflata, H• subhaidingeri, Elphidium

249

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of b r a c k i s h w a t e r . Rec a l c i f i e d o s t r a c o d s ( i n d i c a t i v e of s u b a e r i a l e x p o s u r e ) w e r e i d e n t i f i e d i n U n i t II of s o m e cores collected in w a t e r s h a l l o w e r t h a n the - 6 0 m c o n t o u r . I n c o r e s GC-2 a n d GC-10A ( b o t h f r o m - 67 m), n o e v i d e n c e w a s f o u n d for s u b a e r i a l e x p o s u r e of U n i t II. O p e n m a r i n e f a u n a are p r e s e n t in the u p p e r sections of U n i t II (68 70 c m a n d 0 50 c m i n GC-2; 0 - 4 5 c m i n GC-10A) a n d i n d i c a t e a ( m a r i n e / e s t u a r i n e ) transitional environment, whereas the lower s e c t i o n s of t h i s u n i t a p p e a r to h a v e b e e n deposited by a b r a c k i s h - w a t e r lake whose depth and surface area varied between the (nominally) -57 m and -60m c o n t o u r s as judged from recalcified remains. The faunal i n d i c a t o r s for U n i t II i n f e r a f r e s h - t o - b r a c k i s h lake which merged continuously into transit i o n a l - e s t u a r i n e d e p o s i t s (De D e c k k e r et al., 1988). T h e d e p o s i t i o n of U n i t II o c c u r r e d

b e t w e e n (post) 26,960 + 580 y r B.P. (top of U n i t I V GC-10A) a n d 7 2 4 0 + 160 y r B.P. ( b o t t o m of U n i t I, GC-2). A g e s of 9800_+ 340 y r B.P. (GC-2, 31 35 cm; A N U - 4 8 0 2 ) a n d 1 4 , 1 7 0 + 3 9 0 y r B.P. (GC-2, 68 78 cm; ANU-4804) o c c u r w i t h i n t h e u n i t i n GC-2 g i v i n g a s e d i m e n t a t i o n r a t e of 8 cm/1000 yr. This implies approximately 14,560 y e a r s to a c c u m u l a t e t h e 120 cm of U n i t II i n GC-2 o r a d e p o s i t i o n b e t w e e n ~ 8500 y e a r s a n d ~ 2 3 , 0 0 0 y e a r s B.P. U n i t I I B is a m u d d y s a n d , i n s o m e s e c t i o n s o o l i t i c , a n d o f t e n s u b a e r i a l l y w e a t h e r e d to v a r y i n g degrees. It c o n t a i n s n u m e r o u s weathe r e d a n d b r o k e n s h e l l s as w e l l as c a r b o n a t e c o n c r e t i o n s a n d t y p i c a l l y lies d i r e c t l y b e n e a t h Unit I above the -53 m contour. This deposit is p r o b a b l y i n d i c a t i v e of a s h o r e l i n e or r e w o r k e d a n c i e n t d e p o s i t . 14C d a t e s ( T a b l e II) on c a r b o n a t e c o n c r e t i o n s from U n i t IIB give a g e s of 21,600_+ 300 y r B.P. a n d 22,460 +_340 y r

TABLE II Radiocarbon results used in this study to delineate the Carpentaria Basin chronology. The ages reported are in years before present (5730 yr 14C half-life) and have been corrected for isotopic fractionation ANU No.

Core

Depth, description

Age (yr B.P.)

3266 3268 3269

SPR-1 SPR-1 SPR-1

Surface 80 cm, concretion, Unit IIB 80 cm, concretion, Unit IIB

5540 + 90 21,600+310 22,460+340

3693 3695 3697 3698 3701 3702 3703

GC-1 GC-2 GC-4 GC-5 GC-28 GC-41 GC-69

Surface Surface Surface Surface Surface Surface Surface

4800 4801 4802 4804

GC-2 GC-2 GC-2 GC-2

0 7 cm, Unit I 26 31 cm, Unit I 31 35 cm, Unit II 68 70 cm, Unit II

5170+ 130 7240_+ 160 9800+_340 14,170_+390

3694

GC-1

134 141 cm, Unit IV

31,680 + 1770

3691 3692 3690 3699

GC-10A GC-10A GC-10A GC-14B

95 106 cm, Unit IV 106 115 cm, Unit IV 115 126 cm, Unit IV 129 135 cm, Unit IV

26,960_+580 35,440+1050 35,330_+ 1520 35,080_+1160

4805

GC-5

Core catcher, Unit V, C-rich

34,510+ 720

sediment sediment sediment sediment sediment sediment, ooids sediment

4720_+100 6290 +_120 7050 + 1l0 5780 +_110 5020 + 90 8890 + 110 7740 + 120

250

B.P. suggesting that Unit IIB is the onshore equivalent of the primary lacustrine series, Unit II-III which developed at low sea stands. The distribution of Unit II can be inferred from the 1600 km of seismic profiling (see Jones and Torgersen, 1988, for tracks) obtained in conjunction with this study (Torgersen et al., 1983; Torgersen et al., 1985). The Gulf sediments are extremely flat-lying with few deep incisions in the latest sedimentary sequences. There is a close correspondence between the (cored) presence of Unit II and a ubiquitous seismic shoreline near the - 5 3 m contour. Based on this correspondence and the topographic analysis (Torgersen et al., 1983), this seismic shoreline is inferred to be the position of the Unit II-IIB lateral contact and is probably related to the latest phase in the Carpentaria Basin when sea-level crested the Arafura Sill ( - 53 m). Unit III is a finely laminated mud containing calcite crystals 10-20 pm across (Fig.4), which are probably the result of authigenic calcite precipitation. The unit is 5-25 cm thick and usually occurs near the base of Unit II or between Unit II and IV. A. beccarii tepida are predominant; Cyprideis show definite marks of bleaching, dissolution, and iron sulphides implying anoxic sediments. Deposition began

Fig.4. Scanning-electron photomicrograph of the calcareous "mush" comprising the light-colored bands in the laminated sequence of Unit III. The spacing of the marks at the bottom of the photo is 10 pm.

soon after 26,960 yr B.P. (top of Unit IV, GC10A) and possibly around 23,000 yr B.P. when sea level was well below the Arafura Sill and lacustrine conditions were probable. The Sr/Ca and Mg/Ca ratio of the ostracod shells increase 2-3 x during the depositional period of Unit III (De Deckker et al., 1988). The substantial amount of authigenic calcite precipitation which resulted in Unit III is likely the cause of this relative enrichment in the Sr and Mg content of the lake water. It was generally observed that Unit III was present near the bottom of Unit II and only at water depths greater than 62 m. In addition, very thin, less developed and less obvious laminated sequences (Unit III?) also occurred higher in Unit II, indicating that such an environment was recurrent several times during the history of the Lake. Two cores, GC-5 and GC-4 in 60 and 63 m of water respectively, contain only a thin (< 20 cm) accumulation of Unit II deposited directly over Unit IV. This suggests that Unit III was confined to the deeper water sections (>63 m) in the eastern Gulf and its deposition and/or preservation may be morphologically controlled. Unit III is probably analogous to the ~Seekreide" of the Black Sea (Hsfi, 1978) and the laminae of Green Lake (Brunskill, 1969) and Lake Zfirich (Kelts and Hsfi, 1978). Seismic profiling (see Jones and Torgersen, 1988, for tracks) also provides evidence for a continuous '~shoreline" near the - 6 3 m contour by which we infer the areal extent of Unit III. This limited area of calcareous laminae accumulation again suggests some degree of morphological control by the restricted deeper basin (Fig.l). Whether such laminae are preserved due to a lack of bioturbation caused by anoxic sediments (a high organic input and reduction) or by an anoxic stratified water column cannot be readily assessed. However, fluid dynamical arguments (Torgersen and Ivey, unpublished) suggest that a stratified water column is unlikely to be maintained in this shallow "playa-like" morphology. Unit IV is a shelly mud composed mainly of Americanna, Gyraulus and Corbiculina shells.

251

The foraminiferal assemblage comprises abundant A. beccarii and A. convexa and rare, possibly reworked Pseudorotalia inflata. Cyprideis is the predominant ostracod. The unit has been I~C dated from 26,960_+580 yr B.P. to 35,330 _+_1520 yr B.P. (Table II; GC-10A, GC-1), a time when the sea (Fig.2) may have inundated the Gulf. However, the fauna indicate fresh water conditions without an open connection to the sea and Mg/Ca ratios of 1 to 3 in the water column (De Deckker et al., 1988). Freshwater conditions with a variable water depth and high biological productivity are suggested for this unit. Unit V is eL slightly mottled dark-grey, finegrained mud with fresh to brackish water (principally reworked Cyprideis) and truly marine (Cytherella, Monoceratina, Uroleberis) ostracods and a foraminiferal assemblage comprising P. inflata, A. beccarii, and A. convexa. Unit V is the most consolidated of all the units encountered and probably limited corer penetration. Mild pedogenesis occurs within the unit indicating subaerial weathering. One ~4C date from an organic carbon-rich Unit V sediment from the core catcher of GC-5 gave a ~4C date of 34,510_+720 yr B.P. which is not inconsistent with the 35,080_+ 1160 yr B.P. date on the shell material at the bottom of Unit IV. The ostracod inferred (De Deckker et al., 1988) Mg/Ca ratio of the waters depositing Unit V is less than half the seawater ratio although the inferred Sr/Ca ratio of the water is near seawater. The waters of Unit V may have inherited some solute of marine derivation.

Chemical studies Chemical studies on the sediments and porewaters of several cores were u n d e r t a k e n to provide f u r th er detail on the salinity and chemistry of the Lake Carpentaria system. Both porewater chlorinity profiles (e.g. Calvert and Batchelor, 1978) and total sulfur/organic carbon profiles (e.g. Berner and Raiswell, 1984; Leventhal, 1983) gave equivocal results. The diffusive exchange of porewater, relatively

thin sedimentary units and low organic carbon concentrations, present the primary problems in this specific application. Thus, analysis of the palaeoenvironment must rely on other chemical and environmental indicators.

The laminated sequence: stable isotopes and trace metals The laminated sequence (Unit III) is best developed in GC-2 where over 100 individual laminae have been counted between 150 cm and 174 cm. It is established t hat the Unit was deposited over a period of<4000 years by 14C dates discussed above. Kelts and Hsfi (1978) have identified the necessary conditions for the formation and preservation of fine-scale laminations and especially lacustrine carbonates. These include: (a) the absence of sediment grazers and burrowers which usually requires anoxia, (b) a seasonally differentiated particulate flux, (c) limited clastic input which suggests a separation from large-scale, near-shore sediment sources, (d) minimal bottom currents and thus some protection from the effects of wind, and (e) limited bubble ebullition. Scanning electron microscope and X-ray diffraction studies of the laminae in GC-2 indicate a low-Mg calcareous ~mush" with irregular crystals of 10 20 pm in length (Fig.4); no aragonite or dolomite was detected. This indicates an authigenic fresh-to-brackish water precipitate (Folk and Land, 1975). The physical appearance and absorbance of the dark layers suggests a higher organic carbon content which was confirmed by direct measurement (0.7 1.1°/O organic C in bulk Unit IIl compared with ~0.3% in most of Unit II). Energy dispersive analyses on the electron microprobe using a Li-drifted silicon detector (Ware, 1981) indicate the light-colored (calcitic) layers have a Ca/(Si+A1) ratio of ~1:4 whereas the dark layers have Ca/(Si+Al) ratios of ~ 1:70. The low total abundance (i.e. 4100%) of elements with atomic numbers greater than eleven (Z > 11) determined by this method can be used to infer organic matter (C, N, O, H) and these measurements suggest a

252 dominant organic component in the dark layers. The Mg/Ca ratio of both light (high CaCO3) and dark (high organic) layers is indistinguishable and is much less t ha n one. The laminated sequence of GC-2 was subsampled at a scale of 2-3 mm for stable isotope and trace metal studies. Oxygen and carbon isotope samples were prepared using standard phosphoric acid dissolution of whole-sediment samples after removal of organic m a t t e r by plasma ashing. Replicate analyses were performed (by A.R.C.) on a triple-collector CO2 mass spectrometer and the results (Table III) have an overall u n c e r t a i n t y of +0.05%0. The &160 values (Fig.5, Table III) are practically constant t h r o u g h the entire sequence of (GC-2) Unit III. The mean value, (~180pD B = - - 3 . 0 5 _ 0.29°/oo is consistent with authigenic precipitation from a not highly evaporated tropical meteoric source. The mean 513CpDB= --0.09+0.33°/oo is consistent with a source water close to equilibrium with atmospheric CO 2. The slightly lighter and more variable 513C values above 162 cm in core GC-2 could be caused by (1) a decrease in primary productivity (thus leaving more dissolved 12C in the water column; McKenzie, 1984) or (2) the

Stable-isotope and trace-metal ratios for calcites in the laminated sequence of Unit III, Core GC-2 Depth (cm)

Mg+/Ca2+

Sr+/Ca2+

128.8

0.093

&ISOpD B 5~3CpoB

0.0033

- 1.96

- 0.33

0.057

0.0023

- 3.39

- 0.49

158.6

0.074

0.0023

- 3.22

- 0.11

159.5

0.044

0.0026

- 3.05

- 0.61

160.2

0.061

0.0022

- 3.24

- 0.13

161.1

0.045

0.0025

- 3.20

- 0.69

161.9

0.077

0.0023

- 3.07

+ 0.24

162.9

0.078

0.0021

- 3.36

+ 0.11

165.3

0.077

0.0021

- 3.26

+ 0.17

166.7

0.067

0.0024

- 2.74

+ 0.21

169.0

0.067

0.0021

- 2.96

÷ 0.08

169.8

0.082

0.0022

- 2.60

+ 0.35

170.4

0.087

0.0029

- 3.05

- 0.18

171.4

0.067

0.0028

- 2.46

- 0.28

_+0.05

_+0.05

Typical

error

_i?~8

, -3~0

, -2i2

, -1;4

..//

0 ., 0 0 2

01003

011004

128" 13 O;

15e: 158E

160'

•'r" 162" Ia. 164. i,i I=1 166' 168' 170' 172.

8'~C -0,6-0.4-0.2 0.0 0.2 0.4 0.6

. . . . . . .

128 13q

166

Mg'VCE" ,~,..// 0.4. . . 0,6 . . . 0,8 .

1.0

I

158 ~ 160 X 162 a. 164 t,~ 1 6 6 168 170 172

Fig.5. Trace-metal and stable-isotope results for the calcareous fraction of the laminae of Unit III in core GC-2.

TABLE III

158.0

Sr"/Co"*

8180

+0.006

_0.0001

import of organic matter, possibly via rivers, and its subsequent oxidation. If an increased organic carbon input was transported by riverine flow it would likely be accompanied by a greater clastic input which is not reflected in the sediments. If an increased organic carbon input is the result of local sources (shoreline shrubs, death of floating plants), it signals a change in vegetation or salinity. Thus, the variability in 513C (above 162 cm, GC-2) suggests an instability and/or decline in biologic carbon production which may be related to the cessation of laminae preservation and/or production. Trace-metal studies on the same laminae subsamples were conducted to examine their depositional environment. Because of their size (Fig.4), it was not possible to separate the

253 detrital CaCO 3 from the sedimentary clays and organic particles; therefore, chemical separation was used. Dry whole-sediment samples ( < 8 m g ) were reacted with 0.5ml of EDTA (alkali-free), filtered through glass fiber filters and rinsed with 3 ml of distilled water. The resulting solution was diluted to 5 ml and analyzed using a high resolution Inductively Coupled Argon-Plasma Emission Spectrometer (Shelley and Taylor, 1981). The limits of detection are ~<0.04 ppb for Ca, Mg and Sr and have a procedural reproducibility of better than 1°/0 for Mg and Sr and better than 5% for Ca. The method was tested using mixtures of reagent grade CaCO 3 mixed with clastic sediment and was found to reflect only CaCO 3 dissolution with no measurable effect from ion exchange with clay minerals. The trace-metal ratios (Table III and Fig.5) of the authigenic calcite precipitate (0.045 ~
Lake Carpentaria (20-130; ~160 during the deposition of Unit II, one point only) is significantly less than the seawater ratio (Mg/Sr of seawater ~ 592). The combination of Mg/Ca, Sr/Ca and Mg/Sr ratios and :so measured in the authigenic calcite precipitates therefore support the concept of a continental water mass during the deposition of Unit III and not seawater or a diluted seawater condition as might be inferred by the sea-level curves of (e.g.) Chappell (1983).

Pollen analyses The preservation of pollen in lacustrine sediments can provide information on both the subaerial environment and the aquatic environment. To this end, pollen samples were taken at 5-cm (or less) intervals from Core GC-2 and processed using a combination of techniques outlined by Gray (1965). Clays were dispersed in 10% sodium tripolyphosphate before treatment with hot hydrofluoric acid to remove silicates. Organic material was recovered from the residue by density separation in ZnBr 2 and acetylised following the method of Faegri and Iversen (1964). Samples were mounted in silicon oil. Pollen in the preparations were sparse, but generally well-preserved and charcoal is abundant throughout the core. The results (J. L.) are presented in Fig.6. Unit V (pollen zone LC-7) contains a grass dominated assemblage with few other statistically significant pollen types. Preservation at this level is poor due to subsequent mild pedogenesis. Unit IV (pollen zone LC-6B) contains a comparatively high concentration of pollen, with high percentages of pollen from aquatic taxa, principally Typha and Myriophyllum. The assemblage suggests a lake surrounded by swampy flood plains and intense wet season rainfall. In Unit III and the lower portions of Unit II (pollen zone LC-6A, from 164 to 140 cm of Core GC-2), the abundance of aquatic pollen decrease and Cyperaceae dominate the pollen assemblage. The highest pollen concentrations

254 LAKE CARPENTARIA

- C O R E G C - 2 [percent of total pollen]

,~ .Oo~, ,~+" ~~

DRY LAND

,%o # : ~ , , , ' :

,

)o

2

/"

4c 6

e 1o

2

4

~

e

TAXA

AQUATIC

,o ~2 ?a

)o ~

o<"

3o~4o so 6o ~o

1o 20 30 #o so 60 7o

TAXA

ALGAE

OTHER

MICROFOSSILS

1o

)0

.-=.-

Unit T

iCc:I Unit~

"

~ , . I~ "

tO, .

Umt ~

i

Lc 1 2o -m--

LC6)~o a ~60

.

.

.

.

.

'

.

m

I .....

m- .........

1--_i-t: Fig.6. The pollen data for core GC-2, see text for details. occur near the Unit III-IV boundary. As will be discussed later, the preservation of the laminae in this sequence was probably aided by high biological productivity and anoxic sediments. However, algal colonies and pollen from aquatic plants do not increase proportionally in this section, thus suggesting t h a t bluegreen algae (which do not preserve well) may have been responsible for any increase in biological activity. In the lower sections of the wholly lacustrine deposits of Unit II (140-115 cm, pollen zone LC5), Cyperaceae and Poaceae remain relatively constant in abundance while Myrtaceae pollen increase and Myriophyllum disappears, implying drier conditions with an increasingly savannah-like vegetation in the Lake Carpentaria catchment. In the primary Unit II sequence (115-75 cm, pollen zone LC-4), minimal Typha occur with no Myriophyllum; Cyperaceae persist but decrease in abundance towards the top of the lacustrine zone while grasses and Callitris increase. From 75 to 32 cm in GC-2 (pollen zone LC-3) (which encompasses the lacustrine/estuarine/ marine transition of the Unit II sequence defined by De Deckker et al., 1988), grasses again dominate the pollen spectra while Cyper-

aceae contract and Casuarina increases in abundance. An increase in the proportion of Typha indicates a wetter regional environment and a reactivated seasonal waterflow to the lake/embayment. There is a comparative lack of mangrove-type pollen assemblages which may say more about the dispersal capacity of mangrove pollen than the ability of the mangrove communities to settle across the basin. Relatively low salinity conditions in this transitional sequence may have also prevented the development of modern mangrove assemblages. The 32-0 cm section (pollen zones LC-2 and LC-1) which comprises most of the Unit I sequence of fully marine sediments, contains only occasional pollen. This assemblage probably reflects the extent of the modern Gulf and contemporary deposition patterns. Thus, the overall character of the pollen spectra remains surprisingly constant throughout most of the core. Probably of greatest significance is the near-total absence of taxa with New Guinea affinities (there is one occurrence of Nothofagus (brassii group) and a few Podocarpus pollen grains, but little else), suggesting that the climatic biogeographical barrier between Papua New Guinea and Aus-

255

tralia remained largely in place over the last < 40 kyr. Discussion

From the data presented, it can be concluded that a brackish water Lake Carpentaria occupied a large portion of the Carpentaria Drainage Basin during much of the last <40 kyr. Although the water balance of the region was drier (Torgersen et al., 1985), the regional terrestrial environment, as indicated by the pollen analyses, suggests little if any major vegetational change with the assemblage closely resembling that generated by vegetation of the black soil plains (Luly, in prep.) currently distributed around the southern margins of the Gulf of Carpentaria. These plains are dominated by sedgegrass communities with aquatic taxa in the low-lying swamps and woodland on the higher, well-drained ground. Seasonal burning is frequent but the vegetation is probably kept more or less herbaceous due to saline groundwater or underlying marine/lacustrine deposits which excludes deep-rooted taxa. The black soil plains tend to be characterized by seasonal flooding and waterlogged soils. The growing seasons are directly tied to the onset of the wet season and the vegetation must be particularly tough and resilient to survive the environmental stress imposed by fire, sub-surface salinity, seasonal waterlogging and seasonal drought. The specific succession of events and climates which are recorded in the sediments of the Carpentaria Basin, however, require an integrated interpretation based on sea-level variations and the palaeoenvironmental indications of various pollen, sediment, and ostracod analyses. Chappell (1983) (see Fig.2) probably represents the most up-to-date Quaternary sea-level curve for the region and will be used to frame this discussion. Unit V can be characterized as a fresh-tobrackish water sediment deposited prior to 35 kyr. Given the probable inflow of the Fly River (4.7 × 1011 m3/yr; Torgersen et al., 1983) to the Carpentaria Basin during this time

(Blake and Ollier, 1971), and the relatively low but oscillating sea-levels between 80 and 30 kyr, Unit V could represent both low-salinity estuary and/or lake deposits since both Cyprideis and truly marine ostracods are found together (although they are not necessarily contemporaneous). It seems improbable that flow of the Fly River through the Carpentaria Basin, with its proclivity for seasonal floods and high flow rates, which would have increased freshwater flow to the Basin by a factor of 3 x (Torgersen et al., 1982; Torgersen et al., 1985), could be maintained without flooding the deepest topographic basin in the eastern Gulf. Even if the Fly River had been channelized away from the deepest section of the Basin (~ 70 m), the latest incisions of the Arafura Sill (over 300 km downstream) occur only to depths of - 6 2 to - 7 5 m (Jones and Torgersen, 1988). Thus, the pedogenesis which occurs in Unit V provides positive evidence for the diversion of the Fly River from the Lake Carpentaria Basin by the Oriomo Uplift (Blake and Ollier, 1971) prior to ~35 kyr. If the Fly River diversion had occurred prior to the sealevel high (see Fig.2) at ~40 kyr, the entrapment of seawater would have precipitated halite upon evaporation to the - 61 m contour. Since the Gulf sediments reflect no evaporite deposits and fresh-to-brackish water (De Deckker et al., 1988), it is most probable that the Fly River flow flushed the seawater salts from the Basin through an incision in the Arafura Sill and was then diverted sometime between the previous sea-level high (~40 kyr) and 35 kyr. The present topography of the Arafura Sill (Torgersen et al., 1983) could then result from aeolian action and/or subsequent sea-level deposition and levelling which filled minor incisions. This is significantly prior to the time indicated for diversion of the Fly River to the Coral Sea by Gardner (1970) and Blake and Ollier (1971). Five dates have been obtained from Unit IV (Table II) in three different cores and indicate that this distinct shelly horizon was deposited between 35 and 26 kyr with a mean age of 32 kyr. Both the gastropod and the ostracod

256

analyses indicate fresh-to-brackish water conditions with no connection to an open marine environment (De Deckker et al., 1988). This unit occurs at a time when Chappell (1983) has indicated a sea-level t h a t would have crested the Arafura Sill ( - 5 3 m) (see Fig.2). Several alternatives can be considered that might account for this discrepancy: (1) The Carpentaria Basin existed as a freshto-brackish estuary of sufficient size that the fauna left no record of this connection to the sea in the cores studied. (2) The Arafura Sill was higher at ~ 32 kyr. (3) Sea-level did not reach the level indicated by Chappell (1983). Given the stability of the biogeographical zones indicated by the pollen analyses of this study and the water balance of Torgersen et al. (1985) which indicates a runoff to evaporation ratio about half the present value, it seems unlikely t h a t sufficient freshwater flow could have been available to maintain a low-salinity estuary near the - 4 5 m contour (see 32 kyr, Fig.2). The only possible mechanism for maintaining such a water body would have to include the inflow of the Fly River of Papua New Guinea. This is clearly not the case since subaerial weathering occurring within the deposits of Unit V, indicate t h a t the Fly River had already been diverted from the Carpentaria Basin by the Oriomo Uplift (Blake and Ollier, 1971). The existence of an Arafura Sill much higher than the present - 5 3 m would have required the construction of a large topographic feature over the previously incised terrain. Although erosional discontinuities (indicated by seismic profiling) exist across the Arafura Sill, they give no evidence of the thickness of sediment removed. However, the overall structure of the Arafura Sill appears to be constructional in nature with flat-lying bedded sediments. It seems unlikely that a Sill lowering post-~28 kyr could produce the extremely flat morphology of the Sill. In order to avoid incision of the Sill, it would also require a near coincidence of the sea-level lowering with the diversion or termination of large-scale freshwater flow

needed to maintain the fresh-to-brackish water of Unit IV. The Fly River is the only possible source of such freshwater flow and since the Fly River had already been diverted to allow pedogenesis in Unit V (see above), the environmental indicators of Unit IV in the Carpentaria Basin from 26 to 35 kyr suggest a lower sea-level " h i g h " t h a n t h a t indicated by the sealevel curve of Chappell (1983; Fig.2). The wide range of Mg/Ca and Sr/Ca in ostracod shells from Unit IV (De Deckker et al., 1988) suggest a variable lake volume and the pollen assemblage suggests swampy flood plain surrounding the lake and intense wet season rainfall. Even a lower sea-level "high" than that indicated by Chappell (1983) could have re-established some shallow seas in the Maritime Continent of Indonesia and suggests the possibility of increased monsoonal activity in moderately wetter climate during this time. The ostracod analyses of De Deckker et al. (1988) indicate a distinct increase in the Mg/Ca and Sr/Ca ratios of ostracod shells across the Unit IV/Unit III boundary ( ~ 23 kyr). Such a change might indicate either an increase in salinity (and thus a decrease in the lake level) or a change in lake chemistry due to authigenic calcite precipitation. The sediment of Unit IV contains pollen which are suggestive of a seasonally boggy, marsh environment in the lake catchment and contain brackish fossil remains. Unit III has been described (Torgersen et al., 1985; De Deckker et al., 1988) as a fresh-to-brackish water lacustrine sediment. The trace-metal and stable-isotope analyses of this study confirm t h a t interpretation with a Mg/Ca ratio in the water column of 1.5~Mg/Ca~3.1, an authigenic calcite precipitate and a ~lsO consistent with a tropical meteoric source. We favor a change in water chemistry across the Unit IV/III boundary and possibly increasing water depth rather t h a n a simple increase in salinity based on the following discussion. There appears to be a first-order positive correlation between /~13C and Mg/Ca ratios and between 51sO and Sr/Ca (see Fig.5). Low values of/~3C are correlated with low Mg/Ca

257

ratios and thus intuitively " f r es he r " water. However, there is no evidence for reduced salinity and fresher water conditions in the Sr/Ca ratio or the ~lsO. This correlation between 513C and Mg/Ca is thus probably the result of an increase in biological productivity over Unit III inducing a precipitation of calcite which depletes Ca in the water column preferentially to Mg. 51sO and Sr/Ca would be little affected by calcite precipitation and the productivity change. Since the preservation of the distinct laminae of Unit III requires the absence of sediment burrowers, sediment anoxia fed by this higher biological productivity is suggested. The decreased 313C values in the upper sections of Unit III suggest t ha t decreasing biological productivity (McKenzie, 1984) may have contributed to the t e r m i n a t i o n of Unit III. It: is concluded t ha t the Mg/Ca ratio and calcite precipitation in Lake C ar pe nt a r i a during the deposition of Unit IH were probably biologically controlled and thus, the Mg/Ca ratios may therefore reflect conditions ot her t h a n the evaporation/precipitation balance. Between the primary laminated sequence (Unit III) of GC-2 (between 156 cm and 172 cm) and the isolated set of calcite laminae at 128.8 cm in Unit II (see Fig.5 and Table III), there is a substantial increase in the Mg/Ca and Sr/Ca ratios, a minor increase in Mg/Sr of the water (,~160) and a significantly heavier 51sO signature. This indicates a more chemically evolw~d and evaporatively dominated lakewater and an enr i c hm ent of Sr relative to Mg. Such an evolution could be caused by increased evaporation and chemical precipitation of calcite. The trace-metal data on ostracods (De Deckker et al., 1988) show a < 50% increase in Sr/Ca between Unit III and Unit II (128.8cm in GC-2) in agreement with the authigenic calcite trace-metal analyses. On the other hand, the increase in Mg/Ca of the ostracods and the authigenic calcite ( ~ 3 x ~1.5 x ) is significantly larger. Thus, analyses of the inorganic precipitates of this study concur with the fossils and infer a Unit III to Unit I! transition towards a more saline, evaporative]y dominated and chemically

evolved Lake Carpent ari a derived in part by calcite precipitation. These conclusions drawn from the tracemetal and stable-isotope data of the authigenic calcite, emphasize the importance of closedbasin lakewater evolution on the chemistry of the lake (see Torgersen et al., 1986). They f u r t h e r suggest t hat first-order evaluation of Mg/Ca and Sr/Ca ratios as indicators of variable salinity may be oversimplified. For this extremely shallow system, the biologically moderated precipitation of calcite and the i nt eract i on of the sediments with the water column can alter the chemistry of the lake in a m a n n e r c o n t r a r y to simple models. However, in this case, Mg seems to be affected to a greater extent t han Sr and thus the palaeoenvironmental reconstructions based on the Sr cont ent of ostracod shells (De Deckker et al., 1988) can be interpreted with minor qualifications. During the deposition of Unit II, the Sr/Ca ratios of De Deckker et al. (1988) show a variability in salinity of < 3 x over both longand short-term. Choosing a more lenient salinity variability of 4 x for any one horizon, the variability in the surface extent and depth of Lake Carpent ari a can be estimated using the morphologic data of Torgersen et al. (1983). To maintain continuous water cover at - 6 2 m and below as inferred from the cores, the lake would have varied in surface extent between the - 62 m and the ~-- - 59 m contours. The area of Lake Carpentaria was therefore on the order of 29,000 km 2 and extending to 71,000 km 2. This estimate of a Lake Carpentaria contained within the - 62 to - 59 m contours is conservative because Unit II ostracod sequences from GC-33 (58 m depth) show evidence of an ephemeral lacustrine presence. It would appear t hat Lake Carpentaria (Unit II) most likely varied in extent between the - 6 0 m cont our and the - 5 7 m c o n t o u r and t h a t Unit II sequences between - 5 7 m and - 5 3 m most likely represent an upper t ransi t i onal / est uari ne sequence. Thus, the runoff to evaporation ratio of Lake Carpentaria exhibited considerable short-term variability (a factor of 2) and averaged approxi-

258 mately half the present-day value (Torgersen et al., 1985, fig.4). It may also have been highly seasonal in nature. The palaeontological data (De Deckker et al., 1988) and the interpretation presented above indicate an evolution from the fully lacustrine regime of Unit II to a transitional/ estuarine phase (near - 7 0 c m , GC-2, and continuously from - 5 0 cm to 0 cm, GC-2) to a fully marine phase with significant reworking and the preservation of relict sediment features. The relatively shallow seas of the Indonesian region during this transitional/estuarine phase may have contributed to increased monsoonal activity, seasonal rainfall and a wetter climate as indicated by Typha pollen abundances. The 14C date of 14.2 kyr for 68-70 cm in GC-2 (Table II) would place these marine biota in the Gulf significantly before the breaching of the Arafura Sill as indicated by the sea-level curve of Chappell (1983). This also raises the question of how to reestablish lacustrine conditions on a continuously rising sea-level. On the other hand, the continuous presence of marine biota above - 50 cm (GC-2) occurs at an interpolated age of 11.8 kyr which is more in agreement with the sea-level curves. Thus, it is concluded t h a t marine biota at ----70 cm in GC-2 may represent a limited reworking of sediments although there are dissenting authors. The Unit I-II transition between 9.8 kyr and 7.2 kyr suggests t h a t fully marine conditions may have been established when sea-level crested the Torres Strait ( - 12 m). During the deposition of Unit II, the development of Unit IIB would have been accomplished and streams draining the continental margin would have extended their alluvial fans over the exposed Carpentaria Basin between the 0 m and - 53 m to - 57 m contours. The rise of sea level and near shore depositional processes suggests t h a t Unit IIB and upper sections of Unit II in this region now interfinger and have been reworked and mixed with the modern Gulf sediment, Unit I. This could explain the relict nature and old ages of surficial Unit I sediments.

We summarize the last < 4 0 k y r of the Carpentaria Basin with the following chronology (Fig.7): (1) Sometime between 35 kyr and ~40 kyr, the Fly River, which had maintained a large fresh-to-brackish waterbody in the Basin, was diverted from the Carpentaria Basin by the Oriomo Uplift. Mild pedogenesis occurred in the deposits of Unit V during the low sea-level 40-35 kyr ago. (2) Beginning about 35 kyr and continuing until ~ 2 6 k y r , the net water balance of the Basin became wetter with aquatic pollen indicating intense wet-season precipitation. A shelly fresh-to-brackish sediment (Unit IV) was deposited. A sea-level below the crest of the Arafura Sill ( - 53 m) was maintained which is less than indicated by the sea-level curve of Chappell (1983). The " h i g h " sea-level at this time may have contributed to this wetter climate. (3) Commencing near 26 kyr, Lake Carpentaria was a permanent feature of the landscape at least to the - 63 m contour and periodically extended to the vicinity of the ~ - 5 7 m contour. The lakewater was distinctly continental in character and preserved authigenic calcite laminae under anoxic conditions possibly defined by a seismic shoreline at - 6 3 m. High biologic productivity indicated by 513C (possibly blue-green algae), contributed to calcite precipitation in the lake with morphology providing an additional factor in the extent of anoxic sediments and the preservation of calcite laminae. A change in water chemistry and an increase in salinity marked the transition to Unit II. (4) Between ~ 23 kyr and ~ 11.8 kyr, a freshto-brackish Lake Carpentaria covered 2.9-7.1x104 km 2 with shorelines confined approximately between the - 57 m and - 60 m contours. The dominance of terrestrial grasses in the pollen and high charcoal counts suggests a cyclical wet-dry sequence which may have been seasonal. The region was characterized by savannah-like environment with little if any significant vegetational change in the highly resilient black soil plains taxa, al-

259

-0

-10

TORRES STRAIT

~

-

m~m

Ill

~o

-20 er

~°

:=} {D

~

~

I

~

-30

I,LI -40

'

!

.,I =

-50 '

~

~

ARAFURA SILL

-60

-70

|

5

.

10

•

15

20

25

30

35

40

kyr Fig.7. Physiographic history of the Carpentaria Basin based on the results of this study and those of others. The Roman numerals refer to the sedimentological units of the cores shown in Fig.3. Unit IIe refers to the upper stages of Unit II which indicates an open connection to the sea and implies an estuarine/transitional phase during the latest sea-level rise. See Table II for 14(2 dating control and the text for details.

though the water balance indicates drier conditions. A t no t i m e did P a p u a N e w G u i n e a flora c o n s t i t u t e a s u b s t a n t i a l c o m p o n e n t of t h e pollen s p e c t r a . (5) B e g i n n i n g n e a r 12 kyr, sea-level h a d r i s e n sufficiently t h a t a p e r m a n e n t a n d o p e n connection to the sea h a d b e e n established. O s t r a c o d s a n d f o r a m i n i f e r a i n d i c a t e t h e p r e s e n c e of this connection and the ostracod trace-element d a t a (De D e c k k e r et al., 1988), f u r t h e r confirm this m a r i n e c o n n e c t i o n . P r e s u m a b l y , the salinity r e m a i n e d low due to r e s t r i c t e d w a t e r e x c h a n g e o v e r the A r a f u r a Sill a n d possibly w e t t e r c l i m a t i c c o n d i t i o n s aided by t r o p i c a l

cyclones. T h e d e p o s i t i o n of the e s t u a r i n e / t r a n sitional s e q u e n c e m a y coincide w i t h t h e sealevel c u r v e of C h a p p e l l (1983). Thus, it a p p e a r s t h a t i s o s t a t i c r e b o u n d a c r o s s t h e G u l f (due to s u b a e r i a l e x p o s u r e ) was m i n i m a l . This e s t u a r y p r o b a b l y s e r v e d as a n e x t e n s i v e fluvial sediment trap and the -53 m "seismic shoreline" m a y be t h e r e s u l t of e s t u a r i n e processes. It is u n l i k e l y t h a t the - 5 3 m seismic s h o r e l i n e is the r e s u l t of l a c u s t r i n e processes. (6) A r e t u r n to fully m a r i n e c o n d i t i o n s o c c u r r e d n e a r 8 k y r as i n d i c a t e d by t h e deposition of U n i t I a n d a p p e a r s to coincide w i t h a c h a n g e in t h e w a t e r b a l a n c e a n d h y d r o g r a p h y

260

of the Gulf as a result of Coral Sea water entering across Torres Strait. The rate of sealevel rise between 12 kyr and 5 kyr must have been extremely rapid as indicated by the estuarine conditions at ~ 12 kyr ( - 5 3 m), the ooids at 8.5kyr ( - 4 5 m ) , the fully marine conditions at ~ 7 kyr (presumably - 10 m) and the extent of relict sediments in the Gulf. The rates of sea-level rise, 1-3 mm/yr, would certainly have had a dramatic effect on the redistribution of fauna and flora in the drainage basin.

Conclusions A palaeoenvironmental reconstruction of the Carpentaria Basin has been made using sedimentary, palaeontologic, and palynologic profiles as well as chemical analyses of authigenic calcite precipitates and ostracods. The record, fixed by a 14C chronology, indicates a succession of terrestrial and lacustrine environments from 40 kyr to the present that has been controlled by sea-level, climatic and monsoonal variations, and the tectonic diversion of rivers. We conclude t h a t Lake Carpentaria and its "fluvial" freshwater precursor have dominated the environment of low-latitude Australia and the Papua New Guinea land bridge for > 40 kyr. Of major importance is the confirmation of a continued savannah-like environment in the Basin during the Last Glacial Maximum with runoff to evaporation ratios about half of the present value as was first suggested by Nix and Kalma (1972). The biogeographical vegetational assemblages seem to have been in reasonable equilibrium since < 40 kyr with some variations in abundance but there are few floristic introductions. Thus, the climatic biogeographical barrier between Papua New Guinea and Australia remained largely intact.

Acknowledgements We wish to t h a n k the technical staffs of the Research School of Earth Sciences, the Depart-

ment of Biogeography and Geomorphology and the Carbon-14 Laboratory of the Australian National University, particularly B. McDonald, E. Kiss, A. Chapman, J. M. G. Shelley, J. Cali, M. Malikides, N. Ware, C. J. Radnell and M. J. Head. We also wish to t h a n k M. Livingstone and the captain and crew of the Reef Seeker for their cooperation and enthusiasm. M. R. J. and D. E. S. publish with permission of the Chief Government Geologist, Department of Mines, Queensland. This work was supported by the Research School of Earth Sciences and the Department of Biogeography and Geomorphology of the Australian National University and by a Marine Sciences and Technology grant Number 80/2029 from the Australian government.

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