Aridity in Australia: Pleistocene records of Palaeohydrological and Palaeoecological change from the...

Quaternary International 57/58 (1999) 35—47

Aridity in Australia: Pleistocene records of Palaeohydrologicaland Palaeoecological change from the perched lake sediments

of Fraser Island, Queensland, Australia

Maureen E. Longmore!,*, Hendrik Heijnis"

Archaeology and Natural History, Research School of Pacific and Asian Studies, Australian National University, Canberra, ACT 0200, AustraliaEnvironmental Radiochemistry Laboratory, Environmental Science Program, Australian Nuclear Science and Technology Organisation,

PMB 1 Menai, NSW 2234, Australia

Abstract

A Quaternary sedimentary sequence (ca. 600 ka) from a perched lake (Old Lake Coomboo Depression) on the World Heritage-listed coastal sandmass of Fraser Island has been analysed for dry bulk density, carbonised particles, pollen and chemistry.A chronology has been constructed for the organic sediments using a combination of radiocarbon and uranium/thorium disequilib-rium analysis. The lake basin is small (ca. 9 ha) with a restricted groundwater catchment, delimited by an aquitard, and minimalsurface runoff. It therefore acts as a sensitive raingauge with the perched groundwater-table, and hence the sediment facies depositedwithin the lake, reacting sensitively to any changes in the water budget. The sequence passes through a series of glacial cycles,demonstrating hydrologic and vegetation change. The record indicates a long-term, three-stage fall in the water-table from lake-fullca. 600 ka to an ephemeral lake in the Holocene, paralleled by a shift in the vegetation composition from predominantly rainforest tosclerophyllous components. The evidence for fire is minimal at the beginning of the record, increases from '350 ka through thesequence culminating at or before the LGM, is low during the LGM and is relatively high during the Holocene. Succession, fire andclimatic change, along with the accumulative effect of a series of 100 ka cycles, are believed to have driven the hydrologic andvegetation change and a human factor is not required to explain the record. Within the overall long-term increase in aridity recordedthrough about six glacial cycles, there appears to be a variation in the ‘dryness’ signal of glacial maxima, suggesting some form of‘supercycle’ phenomenon.

This record complements and extends the information available from some of the oldest pollen-analysed Quaternary lakesequences in Australasia, such as Lynch’s Crater in the Atherton Tableland of north Queensland and Lake George in the SouthernTablelands of New South Wales; and from some of the most sensitive lacustrine/ fluvial sedimentary records in the Lake Eyre basin.Signals for major environmental changes recorded in deep sea cores and the loess deposits of China may also be represented in therecord. The implications of the Depression record, if further corroborated, are significant in relation to climatic modelling, rainforestsurvival and faunal extinction. ( 1999 INQUA/Elsevier Science Ltd. All rights reserved.

1. Introduction

Quaternary palaeorecords from Australasian siteshave been used for the partial reconstruction of pastenvironments during the Holocene and over the late-Pleistocene period. Attempts have been made to drawtogether all the available biophysical palaeodata to pro-vide general models of climate change (Bowler et al.,1976; Bowler, 1982; Wasson and Clark, 1987; DeDeckker

*Corresponding author. Present address: Department of Geography,University of Adelaide, SA 5005, Australia.

et al., 1988; Chappell, 1991; Diaz and Markgraf, 1992), todetermine vegetation history (Walker and Singh, 1981;Singh, 1982; Hope, 1984; Dodson, 1994; Kershaw, 1995),to provide proxy data for climatic modelling (Chappelland Grindrod, 1983; Donnelly and Wasson, 1989) and tolink these with archaeological evidence to determine pasthuman/environment interactions (Dodson, 1992). Geo-graphically, the majority of terrestrial palaeoecologicalsites are concentrated in sections of the Australasianhumid perimeter (Fig. 1a). Long continuous records ofenvironmental and vegetation change beyond the lastinterglacial in Australasia are rare (Kershaw, 1986;Singh et al., 1981; Singh and Geissler, 1985). Overall, the

1040-6182/99/$20.00 ( 1999 INQUA/Elsevier Science Ltd. All rights reserved.PII: S 1 0 4 0 - 6 1 8 2 ( 9 8 ) 0 0 0 4 8 - 2

Fig. 1. (a) Quaternary site concentrations in Australasia and location of study sites mentioned in text. Modified after Kershaw (1995).

incomplete and weighted, both temporally and spatially,nature of the Australasian palaeorecords frequentlycreates reconstruction problems and leads to sustainedbias in the interpretation of palaeodata and the con-tained palaeoclimatic and palaeoecological signals.Globally, the core International Geosphere—BiosphereProgramme, Past Global Changes, seeks to integratedispersed palaeo-climatic information, so it is essentialthat palaeoenvironmental interpretations and their as-sumptions are continuously revisited and revised usingcurrent data to ensure robust interpretations. This paperreports long records from Fraser Island on coastal south-east Queensland (Fig. 1a) which are revealing further

information on past hydrological and ecological changesthrough the late-Pleistocene and the Holocene, increas-ing the latitudinal representation and combined value ofexisting long terrestrial records.

2. Core site and sediment analyses

2.1. Geomorphology, soils, vegetation and climate

Old Lake Coomboo Depression (hereafter called theDepression) is an ephemeral perched lake basin situatedon the western side of the main rainforest communities

36 M.E. Longmore, H. Heijnis / Quaternary International 57/58 (1999) 35—47

Fig. 1. (b) Location of study sites and distribution of rainforest communities (vine forest, tall closed forest and carrol scrub) and tall eucalypt forest onFraser Island sandmass. Modified after Map3. Queensland Government Great Sandy Region Management Plan (1994).

(Fig. 1b) in an inter-dune hollow of one of the oldest dunesystems (probably informal sedimentary unit ‘Qpa’,Ward, 1977 (Fig. 1c)) on the Fraser Island sandmass. Theisland sandmass above sea level consists of about nineparabolic dune series reaching a maximum elevation ofca. 250 m (Fig. 1d), believed to have been depositedepisodically during (Ward, 1977) or after (Thompson,1992) the periods of low glacial sea levels when sea levelfell to ca.!120 m (Chappell, 1994) (Fig. 1d), creating anintermittent chronosequence from east to west (Ward,1977). The dune deposits of the adjacent Cooloola sand-mass (Fig. 1b) are believed to date back to at least 700 ka(Tejan-Kella et al., 1990). The island would have been

joined to the Cooloola sandmass on the mainland duringlow sea levels, and the dune systems, due to the commonparent material, time of deposition and period of soil andlandform development, have geomorphological equiva-lents in both regions (C. Thompson pers. comm., 1996).The free-draining soils are podzols with the depth ofilluviation increasing with age to '20 m to the west(Thompson, 1992) and '13 m on Fraser Island (Ward,1977). Humic podzols occur where the water-table im-pedes the illuviation process. Parallel with the develop-ment of the soil at free-draining sites on Cooloola, thevegetation is believed to have followed successionthrough ‘maximum biomass’ stages (Fig. 1b) with tall

M.E. Longmore, H. Heijnis / Quaternary International 57/58 (1999) 35—47 37

Fig. 1. (c) Dune landforms and informal units. Modified after Ward (1977).

closed forests to nutrient deficient retrogressive stageswith stunted hardy vegetation communities adapted tofire and low nutrient status soils (Walker et al., 1981,1983). On Fraser Island the ‘maximum-biomass’ stagemay be represented by the vine forest, tall closed forestand carroll (Backhousia) scrub communities (Fig. 1b).The island is believed to have a mosaic of vegetationreflecting regional natural history and contemporarylocal edaphic factors (Coutts, 1995; Longmore, 1996b,unpublished data). Also, the island, being so intimatelyassociated with changing sea levels through the episodicbuild-up of dune systems during the glacials and theassociated evolving chronosequence of soils and vegeta-tion, is a unique record of climatic change. It is mostlikely that climatic change will have overidden suc-cessional trends operating in different parts of the timeseries. Climatic disturbance will change the boundaryconditions (Chappell, 1983) and may trigger or accelerate

a retrogressive or progressive response by the vegetation(Walker et al., 1983) depending on the frequency andmagnitude of climatic change and the antecedent condi-tion of the vegetation and soils.

The vegetation currently surrounding the lake is classi-fied as low open woodland and tall shrubland (4—10 m),an alliance of Eucalyptus signata, E. gummifera andBanksia aemula (Coutts, 1995). The younger dune sys-tems to the north and south carry open forest to 30 m, analliance of E. pilularis, Syncarpia hillii and Allocasuarinatorulosa (Coutts, 1995).

The mean annual rainfall for the area ranges fromca. 1300 (Sandy Cape, 1871—1982) to 1700 mm (Ungowa,1915—1982), mean minimum and maximum temperaturesare 22 and 29 (Sandy Cape), 22 and 27°C (Double IslandPoint) in January and 14 and 21, 14 and 19°C in July,respectively. Evaporation exceeds precipitation in latewinter and spring at Cooloola (Walker et al., 1981) but


Fig. 1. (d) Topographic spot heights (m) and bathymetric contours (m) down to 300 m.

near the core site estimated total evaporation was16—50% the total rainfall over 1—3 month monitoringperiods 25/12/83—18/9/84 (Longmore, 1996a).

2.2. Old Lake Coomboo Depression and Hidden Lake

The perched lake basin floor of the ephemeral De-pression is at an altitude of ca. 86.4 m above sea level andhas a relatively small basin (ca. 9 ha) with a restrictedperched groundwater catchment delimited by the mor-phology of the relic Bh horizon, which acts as anaquitard, of the humus podzol (Longmore, 1997a). Thereis minimal surface runoff due to the high hydraulic con-

ductivity of the soils, and most water and nutrient inputarrives either directly from the rain and or from ground-water flow over and through the upper layers of theB horizon (Longmore, 1997a). The basin appears to haveexperienced severe erosion some time in the past due tothe exposure and reworking of the B horizon of the soilpodzol and the shallow depths of the A2 horizon (Fig. 2).Seepage zones occur at the intersection of the slope withthe exposed indurated Bh horizon. The water-table pres-ently fluctuates between about 1 m below and severalmetres above the present lake floor; it is believed to havebeen much higher in the past as indicated by a series ofpalaeo-shorelines evidenced by wave-cut cliffs incised


Fig. 2. Old Lake Coomboo Depression: cross-section of soils and sediments, location of palaeo water-tables and core sites.

into the Bh-horizon on the north shore (J. Jennings pers.comm., 1984) and beach dunes on the west, east andsouth shores located at ca. 3.5 and 7.5 m above thepresent lake floor (Fig. 2).

The main sedimentary materials in Hidden Lake(Fig. 1), a perched lake in near lake-full condition (8 mdepth) 100 m above sea level, are allochthonous organicmaterial arriving in the lake in solution and flocculatinginto organic colloids in the lower pH of the lake water(J. Head, pers. comm., 1986), decomposing remains oflittoral vegetation and siliceous sand. The amount ofsand decreases rapidly and linearly towards the centreof the lake and by four metres depth has been replaced byhighly organic sediment (Longmore, 1997a). The sedi-ment facies deposited in these perched lake basins aresensitive to changes in the lake level and hence the waterbudget. Hidden Lake acts as a guide for conditionsand processes operating in other perched lakes, withsimilar depths and morphometry, at present and inthe past. It is presently ca. 8 m deep, dystrophic,thermally and chemically stratified from ca. September toApril, has a low pH (ca. 4.0), a low conductivity (ca.60—70 lS, 18°C), and low visibility (Secchi 0.75—3.5 m)(Longmore, 1986). Similar conditions and processes mayhave occurred in the Depression when the lake levelswere higher, and these may be modified today in anephemeral state.

At the Depression, sediment cores were taken at thelowest central location and in the littoral zone of theephemeral perched lake basin (Fig. 2) during a dry phasein 1985. At the centre, a 6 m core reached the base of the

lacustrine sediments and penetrated the underlying sandusing a Jacro hydraulic drill rig. At the littoral site thedrill rig reached the basal sands at about 5 m. The re-mainder of the transect illustrated in Fig. 2 was investi-gated with a sand auger. The central core was sliced at1 cm intervals. One cm3 sub-samples were taken fromeach interval, weighed and dried at 105°C and weighedagain to determine dry bulk density. An additional 1 cm3

was used for pollen and carbonised particle analysis andcounting was carried out at ca. 10 cm intervals. Selectedintervals were analysed for midge larvae head capsulesand diatoms but none were found. Dry sediment samplesat 10 cm intervals were analysed for the major ele-ments using standard total silicates digestion techniques(J. Caldwell, pers. comm., 1986).

3. The quaternary record

3.1. Chronology

The sequence was dated by a combination of radiocar-bon and U/Th disequilibrium analysis of the organics(Heijnis, 1992; Heijnis and van der Plicht, 1992).

¹he ºranium/¹horium disequilibrium (º¹D) datingmethod: The natural isotope 238U decays to 234U, whichin turn decays with a half-life of 248 ka into 230Th(Ivanovich and Harmon, 1992). This is the only naturalsource of 230Th which decays with a half-life of 75 ka.As long as the production of 230Th exceeds its radio-active decay, 230Th accumulates and the 230Th/234U is a


measure of age until after 350 ka when equilibrium,where the decay of 230Th balances its production, isreached. Lake organics, especially humic, humin andfulvic acids, are able to absorb large amounts of uraniumfrom groundwater and, together with clay minerals, formlarge stable complexes. Since thorium is insoluble inwater, the 230Th concentration should be zero at the timeof deposition of the lake organics. The observed presentday 230Th concentration is therefore solely the result ofthe decay of its radioactive precursor 234U. Any violationof this assumption will be traced by the presence of thelong-lived isotope 232Th, which has a half-life of1.39]1010 a, and corrections made using isochron plotsdescribed by Schwarcz and Latham (1989). The reliabilityof the UTD depends on a closed lake sediment system,where the compaction and burial by younger sedimentsisolates the organic sediments from further U uptake.The chemical procedures are described in Heijnis andvan der Plicht (1992).

The radiocarbon ages were believed to be con-taminated by younger carbon below at least 158 cm(J. Head, pers. comm., 1986) which was dated to31,700$1370 yr BP (ANU 4283), although this samplemay prove to be contaminated. The detailed U/Th analy-sis of samples from the Depression yielded the followingresults. Samples from 158, 160, 161 and 162 cm werecombined and plotted in an isochron plot to make cor-rections for detrital thorium. The results indicated anopen system behaviour with respect to uranium and wererejected. (Due to the estimated U/Th ages of the follow-ing underlying levels, and the pollen sequence, this level isbelieved to be older than the ca. 32 ka radiocarbon age,probably around the last interglacial.) Samples from 249,252 and 254 cm were combined and plotted in an isoch-ron plot to make corrections for detrital thorium. Theslopes in the isochron plots gave corrected ratios forcorrected 230Th/234U and 234U/238U of 0.86749$0.09and 0.9659$0.11 respectively. The corrected ratios asderived from the isochron plots gave an apparent age of225#75!45 ka. Samples from 354, 355, 356, 358 cmwere combined and plotted in an isochron plot to makecorrections for detrital thorium. The slopes in the isoch-ron plots gave corrected ratios for corrected 230Th/234Uand 234U/238U of 1.02499$0.11 and 0.6864$0.07, re-spectively. The corrected ratios as derived from the isoch-ron plots gave an apparent age of '350 ka.

The series of 14C and U/Th dates have been plotted onFig. 3. Although constant sediment accumulation ratescannot be assumed, extrapolation of the corrected U/Thapparent ages to the base of the sequence gives an ap-proximation of the age range of the palaeorecord. Thebasal date has been estimated to be ca. 600 ka. confirm-ing that the sequence spans a series of glacial cycles. Therelic beach dunes representing palaeo-shorelines are cur-rently being dated by Optically Stimulated Lumines-cence (N. Spooner, pers. comm.).

3.2. Stratigraphy and bulk density

The fluctuation of dry bulk density values (Figs. 3 and4) reflects the variations in the proportion of organicmatter and minerogenic material in the sediment andhence the variation in the relative water depth overa series of glacial cycles.

The pronounced minerogenic layer at 100—130 cm(Figs. 2 and 3) is older than 12 160$450 yr BP (ANU4281) and is assumed to represent the period spanningthe Last Glacial Maximum (LGM) and ‘late-glacial’ withorganic sedimentation resuming at ca. 12 ka. Underlyingthe minerogenic layer is a 10 cm layer of large carbonisedparticles (Fig. 3), extending across the lake floor sug-gesting the burning of vegetation in situ. A 2 cmorganic layer immediately above this was dated to21 800$1270 yr BP (ANU 4282) (Fig. 3). (A new series ofaccelerator 14C dates through this and the overlying sandlens is currently being processed.) It appears that at sometime between the last interglacial and the LGM, perhapsthe onset of the last glacial (ca. 70 ka), there was a periodof extreme fire where vegetation which had colonisedthe dry lake floor was burnt in situ. (Currently, young¸ophostemon suaveolens colonise the lake floor duringdry periods.) This was followed by a deeper water phase(ca. 22 ka) before the onset of extremely dry conditionsduring the LGM and late-glacial.

Previous high dry bulk densities in older sections ofthe core are believed to represent lower lake levels andreduced effective precipitation. Intervening sedimentarylayers have low bulk densities and high loss on ignitionvalues (Fig. 3), and are believed to represent deeper waterand higher effective precipitation. The sedimentary re-cord indicates a fluctuation between relatively deep water(about 4—7 m) (Longmore, 1997a) and shallow or absentopen water over a period of ca. 600 ka.

The Holocene sediments are markedly different fromthose of previous dated (penultimate) and inferred (last)interglacials, with fluctuating dry bulk densities, on aver-age, about double (Fig. 3). Also, the areal extent oforganic sediments representing the Holocene is muchreduced (Fig. 2) suggesting a smaller, shallower lake.Presently, the lake is ephemeral in nature and the lakehad several metres of water in 1977 (A. Georges, pers.comm.), 1992 and 1995; in the intervening periods theperched water-table was at or below the lake floor. Thehigh dry bulk density values suggest the lake has beenephemeral in nature for most of the Holocene in contrastto the apparent permanent deep water lake of previousinterglacials/interstadials.

Nevertheless, within the Holocene, fluctuations in thewater-table have occurred as evidenced by changes fromsand to organics through the Holocene sequence of thelittoral core in the Depression. It is believed these water-level fluctuations are of much lower magnitude thanduring the Pleistocene due to the continously restricted


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Fig. 4. Old Lake Coomboo Depression central core: stratigraphy, dry bulk density, carbonised particles and summary pollen and chemistry diagramwith zones.

nature of the Holocene sediment body (Fig. 2). TheHidden Lake sequence, which also records water-level changes through the Holocene, indicates a mid-Holocene ‘Dry’ which is anticorrelated with southeasternAustralian records and supposedly those of northQueensland (Longmore, 1997a). It is possible that thetiming of the climatic signals during the Pleistocene inthe Depression sequence may not be correlated withother records in Australia but, given the accuracy andprecision of dating techniques for organic sedimentsprior to the 14C dating range, this would be difficult todetermine.

3.3. Carbonised particles, pollen and chemistry

A summary of the major changes in bulk density,carbonised particles, pollen and sediment chemistry ispresented in Fig. 4. The full pollen data are reported inLongmore (1997b) and chemical data in Longmore(1996c, unpublished data). The Dry sclerophyll categoryincludes E. signata type and E. intermedia/Angophora spp.type. Rainforest includes Syncarpia hillii/¸ophostemonspp. type, Podocarpus sp. and Araucariaceae. ¼et scler-

ophyll includes E. pilularis/ Melaleuca spp. type pollen.Araucariaceae includes Araucaria cunninghamii type andAgathis robusta type. The Groundwater elements categoryis dominated by total aluminium, but includes iron, man-ganese, phosphorus, sodium and potassium, and Otherelements includes total chloride, sulphur, calcium andmagnesium. The relative proportions of these two chem-ical element categories are believed to reflect the relativedominance of groundwater influence against rainfall con-tribution (Longmore, 1996c, unpublished data).

There appear to have been major changes in the veg-etation and hydrology: the vegetation composition shiftsfrom predominantly rainforest, with Syncarpia hillii,¸ophostemon spp. and Araucariaceae, surrounding a deepwater lake at ca. 600 ka to a dryer rainforest, with pre-dominantly Araucariaceae and Podocarpus sp., with me-dium lake levels after ca. 350 ka. A more sclerophyllous,Casuarinaceae-dominated forest occurs until probablythe last interglacial, when Araucariaceae pollen fre-quency increases before falling dramatically. After themajor fire episode and after the return of open water at21 800$270 yr BP, dune vegetation changes from openforest to probably the sturdy association of stunted


Eucalyptus signata, Banksia spp. and myrtaceous shrubscharacteristic of the older dune systems to the west of theisland, and lake levels fall probably below the lakefloor. After the LGM open forest with Casuarinaceaereturns, but Araucariaceae pollen frequencies neverrecover and the lake becomes an ephemeral system witha fluctuating water-table in the Holocene. Poaceae andJuncaceae pollen frequencies increase parallel with theapparent fall in the water-table (Fig. 4) (Longmore,1997b).

An explanation for the progressive fall in the perchedwater-table could be the movement of the coastline awayfrom the lake site as successive parabolic dune seriesaccumulated on the eastern side of the island during orafter the fall in sea level with each glacial. The resultingalteration in the extent of the regional water-table duringthe following interglacial may have impacted on theperched water-tables. Nevertheless, it appears the coast-line returned to a similar location during the majority ofthe interglacials (Ward, 1977). Additionally, the verystrong control of the Bh-horizon on the movement ofgroundwater in the Hidden Lake and Depression catch-ment, and the fact that the Depression lake is at a lowerelevation and closer to the coastline than near lake-fullHidden Lake suggests a lateral shift in the coastline isunlikely to be a major cause. Undoubtedly, there will besome relationship between perched and regional water-tables and between these and sea level; it is believed thatthe ultimate control on perched lake levels given a par-ticular evaporation/precipitation regime will be thegeometry and permeability of the B-horizon. Detailedaugering, at the site of a perched water-table interfacewith the B-horizon, located sharp (within 15 cm) lateralchanges in the saturated zone confirming the low lateralhydraulic conductivity of the indurated Bh horizon(Longmore, 1997a). As a result of the hydraulic control ofthe perched water-table by the aquitard, these perchedlakes appear to behave very much in the manner ofrainfall gauges, responding sensitively to the waterbudget.

Alternatively, edaphic factors such as the continueddevelopment and deepening of the weathered A2 horizonon free-draining slopes may have allowed the water-tableto fall, as the lowest elevation of the B-horizon woulddrop. Given that the B-horizon is at or near the surfacetoday due to the removal of the A2 horizon, and giventhat the wavecut-cliff (shaped from that Bh-horizon) iscurrently preserved 7.5 m above the now ephemeral lakefloor (Fig. 2) (indicating a once high lake level within thecurrent basin), the erosion exposing the B-horizon mustpre-date the high lake level. This evidence suggests majorsoil development was interrupted. Evidence for olderpalaeosols, also truncated, occur in the basin (Fig. 2).

The B-horizon itself is unlikely to be removed exceptunder extreme erosional conditions. Also, if the erosionwas so severe that it removed the upper layers of the

B-horizon within the basin, it would be very probablethat the lake sedimentary record would have been re-moved also. Further, given that the pollen evidence fromthat record supports the theory, arising from the prevail-ing parabolic dune forms, that vegetation cover persistedthrough the glacial periods (Thompson, 1992), erosion isunlikely to have had a major effect on the upper, let alonethe lower, soil horizons during the period represented bythe sedimentary sequence. Nevertheless, at some time inthe past erosion has been so severe such that the B hor-izon has been exposed. It is believed by the authors thatthis occurred prior to the existing sedimentary recordand that the lake basin has been deflated by wind erosionout of a pre-existing palaeobasin. The shallow soilsresulting from the deflationary episode would haveexposed the relatively nutrient-rich B-horizon near thesurface and this soil seems to have been able to supportrainforest communities (Longmore, 1997b).

Overall, there is evidence for a progressive long-termshift in the water budget with increasing ‘dryness’, ground-water dominance, intensity or frequency of fire, sclero-phyllous vegetation from zone I, swamp and sedgelandvegetation and decreasing rainforest. The primarilydeep-water lake surrounded by rainforest has been grad-ually replaced, through a series of glacial/interglacialcycles, by an ephemeral lake with a dominantly sclero-phyllous vegetation; it is believed that successional pro-cesses, along with fire, played a role but that retrogressivestages were accelerated by climatic change (Longmore,1997b). The evolution of the lake and catchment can beexplained by a combination of succession, fire and cli-mate change. Anthropogenic burning, although possiblya contributor (Kershaw, 1986, 1994; Kershaw andNanson, 1993), is not required to account for the reduc-tion in rainforest at this site. The independent evidencefor the reduction in effective precipitation through thesediment chemistry shows three distinct stages (ZonesK-I, H-E and D-A) (Fig. 4) in the evolution of dryerconditions from ca. 600 ka to the present, which mayhave led to the increased severity of the fire regime andthe eventual demise of the araucarians. The reduction inAraucariaceae appears to have occurred in two stages atthe end of Zone F and the end of Zone D (Fig. 4).This record may parallel the regional reduction inAraucariaceae recorded in the deep sea core off the coastof northern Queensland (Kershaw et al., 1993) at ca.150—130 ka and the local reduction on the Athertontableland recorded in Lynch’s Crater at ca. 38 and 26 ka(Kershaw, 1986). In the Depression area, Araucariaceaemay have retreated to a number of edaphically favour-able refugias (Dodson, 1989; Kershaw et al., 1993;Kershaw, 1995) during the second major drying in ZoneE, expanding in the more humid Zone D (ca. 120 ka)and then was decimated in those refugia at the thirdmajor drying at the onset of the last glacial (Zone C)(Longmore, 1997b).


4. Continental and regional trends in aridity

The overall trend in climate indicated by the FraserIsland palaeorecord seems to parallel the climatic recordin the aeolian and fluvial sediments in the Cooper Creekand Diamantina River region (Nanson et al., 1992) andthe Finke region, which is at a similar latitude (Fig. 1a), ofarid central Australia since Oxygen Isotope Stage 7(Nanson et al., 1995) and also Lake Eyre (Fig. 1a) wherethe lake levels seem to have been falling at least fromStage 5 (Magee et al., 1995). Lake George (Singh andGeissler, 1985) and Lynch’s Crater (Fig. 1a) (Kershaw,1986) also record the move to more sclerophyllous andfire promoting vegetation during the last glacial cycleand in Lake George there appears to be a move towardsa more ephemeral system (Singh et al., 1981; Bowler,1982). In the Holocene there are records of dune activityalong the east coast of Australia (Lees et al., 1990; Bryantet al., 1994) but not in the previous interglacial (Bryantet al., 1994). Dust deposits off the coast of eastern Austra-lia increase in cores in the Tasman Sea (Hesse, 1994) andin the north Pacific (Hovan et al., 1989) at around aboutStage 12 and continue for the next four glacial cycles. Thechange in the climatic trend in Australia in the last 400 kawas proposed by Bowler (1983) with an increase in theamplitude and frequency of the glacial cycles at the end ofa long term increase in aridity since the Oligocene. Otherauthors have hypothesised a trend towards greater arid-ity since ca. 700 ka and 900—1600 ka (An et al., 1986;Chen and Barton, 1991, respectively). The increasingaridity recorded in the Depression may be a continentalfeature of Australia and perhaps more extensively as partof a lower frequency cycle such as the 413 ka theoreticalvariation in orbital eccentricity, a frequency componentidentified in the Chinese loess deposits in the 1.65—2.5 Maperiod (Ding et al., 1994). On the other hand, the increas-ing aridity in the late Pleistocene may be due to thecumulative impact of a series of 100 ka glacial cycles(Bowler, 1983) which is reflected in vegetation changes(Walker, 1982; Dodson, 1989), beginning about 600 ka.

The Last Glacial Maximum appears from the Depress-ion record to have been ‘dryer’ than any of the previouspossible glacials in the sequence on the basis of the depthof the sand deposit, the apparent degree of groundwaterseepage, the apparent severity of the fire regime and thesignificant nature of the vegetation change before,during and after the LGM, in particular the demise ofAraucariaceae. The parallel change from open water toprimarily ephemeral conditions suggests natural causes.Other authors have commented on the apparent extreme‘dryness’ of this glacial due to the environmental impactsrecorded (e.g. Hope, 1984; Kershaw, 1989). Dust wasentrained from a number of sources across Australia’sexpanded arid zone (Bowler, 1982) and transported off-shore (McTainsh, 1989) with a brief, high peak in massaccumulation rate at several sites in the Tasman Sea (but

interestingly not at the deep sea core site to the east ofFraser Island) (Hesse, 1994). The expansion of sand duneactivity to the coast during this extreme arid period isevident from the minerogenic facies deposited in the lakebasin and the apparent marked alteration in the forestcommunity. It accords with coastal records in easternAustralia (Thom et al., 1994) and the chronologicallyprogressive spread of aridity coastwards during theLGM (Nanson et al., 1992; Bryant et al., 1994). Prelimi-nary ages of parabolic dunes on the adjacent Cooloolasandmass dated by thermoluminescence and mor-phological development indicate dune activity aroundabout this period (Tejan-Kella et al., 1990). The Depres-sion record suggests this may have been the ‘dryest’glacial signal since the sequence began.

The original formation of the present lake basin byerosion, possibly by deflation, also suggests that there hasbeen an arid glacial phase ‘dryer’ than the LGM. Guidedby extrapolation of the U/Th ages, this may have beenOxygen Isotope Stage 16. According to Shackleton(1987) Stage 12 and 16 were significant glacials withgreater ice volumes and lower sea levels than Stage 2. Inthe aeolian deposits of China exceptional thicknesses ofloess were deposited in isotope Stages 2—4, 6, 12, 16, 22,38 and WS4 (Kukla, 1987). Although there may seem tobe a tenuous theoretical link with the region from thesingular Depression record, it does seem reasonable tohypothesise from the present evidence that the erosion ofthe lake basin was triggered by an arid period moreextreme than occurred at any glacial up to the present orby antecedent conditions favourable to deflation (Magee,pers. comm., 1996). Given the apparent sensitivity of thesite to hydrologic, and hence climate, change, it is pos-sible that the signal was widespread. Investigation offurther long records in the region will confirm or refutethe representivity of this site. The actual timing andforcing mechanisms need further investigation at a var-iety of sites with long records. Nevertheless it is believedthat some combination of factors has resulted in a moresignificant environmental impact at the time of formationof the lake basin.

In addition, aside from the overall increase in aridityand the significant signal for the LGM (Stage 2) and ca.Stage 16, there appears to be a variation in the strength ofthe signal for glacial periods in the interim period. This ishighlighted by the strength of the ‘dry’ signals below ca.Stage 8 (300—400 cm) and the weakness of the signal forca. Stage 6 (180—250 cm) in the bulk density curve. Whathas become apparent from the Depression sequence isthat all glacial maximums may not be equal in terms of‘dryness’ and the concept of a ‘Supercycle’, which wasalluded to by Kukla on a re-visting of the Pleistocenedeposits of Europe and China (Kukla, 1995), becomesa possibility. Identification of these atypical cyclic fea-tures has only now become possible in terrestrial se-quences with the advent of a dating technique for old


organic sediments (Heijnis, 1992) deposited earlier thanthe reliable radiocarbon range.

5. Conclusions

The sedimentary records in the perched lakes are re-sponsive to changes in water level, and hence effectiveprecipitation. The long term move towards aridityalluded to by Bowler (1982) and framed within cycles ofmillions of years (Frakes et al., 1987) may be partlyrecorded in the perched lake sequence beginning at a sig-nificant glacial ca. Stage 16. Major deflationary episodesmay have removed all sedimentary sequences on theIsland prior to ca. Stage 16 but some special conditionsmay have protected older records from removal at par-ticular sites. The LGM (Stage 2) appears more extremethan any intervening glacial and, along with the appar-ently drier Holocene, may be part of a low-frequencycycle of increasing aridity or the cumulative effect ofa series of glacial cycles.

The role of humans in driving major environmentalchange appears overshadowed in the light of this appar-ent long-term trend towards aridity. Within the FraserIsland sequence, the apparent fall in the perched water-table (representing independent evidence of climaticchange) accompanying vegetation change and the factthat these preceded apparent changes in the fire regime,suggests the ecological changes were dominantly climat-ically-driven or initiated. Succession, fire and climatechange are believed to be the primary factors responsiblefor the vegetation and hydrological changes recorded.

It is apparent that not all glacials have equal ‘dryness’signals and there may be a form of ‘supercycle’ operatingwithin the longer-term aridity cycle.

The implications of this record, if substantiated byfurther studies in the region, are significant in regardto climatic modelling, rainforest survival and faunalextinctions.

Acknowledgements

J. Jennings (deceased), R. Leech, C. Lovell, J. Luly, J.Neale, I. Prosser, K. Shaw, E. Wallensky for assistancewith drilling and augering; R. Coutts for plant identifica-tion; J. Head for radiocarbon dating; AINSE for fund-ing U/Th disequilibrium analysis; R. Chisari ANSTOfor U/Th analysis; G. Atkin for pollen preparations;J. Caldwell (deceased) for chemical analysis; J. Magee forassistance with identification of heavy minerals; E.Geissler, G. Hope, J. Luly, M. Macphail, D. Moss andJ. Owen for assistance with pollen identification;D. Thiriet for exploratory chironomid analysis; J.Richardson for exploratory diatom analysis; D. O’Deafor preparation of diagrams on TILIA; S. Murray for

drafting of diagrams; D. Walker and J. Jennings for earlyencouragement; I. Bayley, C. Thompson, J. Walker andB. Ward for introducing MEL to the area; J. Chappell forvaluable discussion and criticism of work at severalstages; S. Haberle, G. Hope, M. Macphail, G. McTainsh,B. Pillans, J. Stevenson and C. Thompson for valuablecomments on the data; M. Bourke and D. Questiaux formoral support while writing; P. Kershaw, J. Bowler, B.Boyd, J. Magee and an anonymous referee for detailedand valuable comments on the submitted paper; and allMEL’s colleagues, students and staff, at ANH and Uni-versity of Adelaide for their support.

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