Palaeotrophic reconstruction and climatic forcing of mega-Lake Eyre inlate Quaternary Central Australia: a review

STEVEWEBB

BOREAS Webb, S. 2010 (April): Palaeotrophic reconstruction and climatic forcing of mega-Lake Eyre in late QuaternaryCentral Australia: a review. Boreas, Vol. 39, pp. 312–324.10.1111/j.1502-3885.2009.00120.x. ISSN 0300-9483.

Extreme Quaternary climatic variation in Australia brought radical environmental changes to various parts of thecontinent. In this article, I discuss these changes in terms of mega-lake development in Central Australia, and inparticular the southern Lake Eyre Basin (SLEB). The formation of these features, together with the fossil record ofthe region, throws light on the palaeoclimatic and palaeobiological relationships of megafauna and other animalgroups, and the trophic development required to support them. Australian continental drying during the lateQuaternary has been noted by many workers, but this process was punctuated by strong pluvial episodes of de-creasing strength from MIS 5e. Mega-lake development during MIS 5 resulted from unusual monsoonal and eva-porative patterns at that time. However, the climatic forcing behind mega-lake formation and the rate of lakegrowth is not well understood, although species composition in SLEB aquatic fossil fauna assemblages attests tothe size and development of these lakes and indicates their long-term persistence. The degree of trophic develop-ment and the maintenance of broad, well-bedded aquatic and terrestrial ecological frameworks and biotic varietysupport that conclusion. The fossil record contributes to our understanding of mega-lake and palaeoriverinetrophic complexity, the speed and duration of lake-fill and the intensity and persistence of the supporting in-tracontinental moisture balance. Although other more remote mega-lakes formed in Central Australia, they werenot populated by complex trophic systems or megafauna populations. This discrepancy between the various geo-graphic areas sheds light on the biogeography and population distribution of megafauna, thus helping form abetter picture of the reasons behind the final extinction of relict populations of this group in MIS 4.

Steve Webb (e-mail: stwebb@staff.bond.edu.au), Faculty of Humanities and Social Sciences, Bond University, GoldCoast, Queensland 4229, Australia; received 12th February 2009, accepted 18th August 2009.

Geomorphic study of late Quaternary deep-waterfacies, palaeosols, calcarious beach sands, beach ridges,palaeochannels, palaeodeltas and palaeoshorelinesaround large playa lakes throughout Central Australiais slowly establishing a palaeoclimatic pattern acrossthe continent (Chappell 1991; Williams et al. 1998).Study areas include lakes Gregory (Bowler et al. 2001),Eyre (Magee et al. 1995, 2004; Magee 1997; Magee &Miller 1998; Nanson et al. 1998, 2008; Croke et al.1999), Woods (Bowler et al. 1998), Amadeus (Chenet al. 1993), Lewis (English et al. 2001), the Finke River(Nanson et al. 1995) and Cooper Creek (Nanson et al.2008) and surrounding dune fields (Wasson 1989; Fitz-simmons et al. 2007) (Fig. 1). Mega-Lake Eyre was thelargest inland Australian lake, but up to now its bioen-vironmental construction and the possibility of differ-ential monsoon and evaporative patterns occurring inCentral Australia during MIS 5e have not been jointlydiscussed. While its palaeoshorelines attest to high-stand lake fillings, the time taken and climatic condi-tions required for their formation and maintenance ispoorly understood. Palaeoenvironmental and palaeo-climatic reconstruction are essential for correcting this,but so too is investigation of terrestrial and aquaticmegafauna population biology and biogeography(Webb 2008, 2009). In this article, I assemble a basicpicture of aquatic trophic complexity in order to assess

the size and durability of mega-lakes and of the climaticand hydrological mechanisms required for them toform. I begin by assessing the evidence for hydrologicaland climatic influences that caused mega-lake fillings,then move on to address lake size, filling rates and lakebiodiversity to determine the rate and duration of lakefills necessary to support the fossil animal communitiesthat are found there.

Central Australian palaeofloods and climaticsystems

Modern Lake Eyre receives water from a 1.14 millionkm2 endoreic catchment comprising the Georgina(205 000 km2) and Diamantina (160 000 km2) riversand Cooper Creek (306 000 km2) sub-basins (Kot-wicki 1986; McMahon et al. 2005; Costelloe et al.2006) (Fig. 1). The Diamantina contributes 64% ofinflow, with 17% from the Cooper and 19% fromother sources (Morton et al. 1995). Under currentconditions, only 68% of the catchment directly con-tributes water to Lake Eyre, with high transmissionlosses in the middle and lower sections (McMahonet al. 2008; Nanson et al. 2008). In addition, modernrunoff and lake ponding are subject to high evapora-tion rates ranging from 2400mm/a in the north to

DOI 10.1111/j.1502-3885.2009.00120.x r 2009 The Authors, Journal compilation r 2009 The Boreas Collegium

3600mm/a near the lake (Kotwicki 1986; Nansonet al. 2008). Lake Eyre has a 9690 km2 surface area,1718 km shoreline and supports a volume of 30.1 km3

(McMahon et al. 2005).Palaeoshorelines signal an MIS 5e filling to between

110 and 115mAustralian Height Datum (AHD) from�16m maximum depth, producing a 25 260 km2 lake,25–30m deep and supporting 332 km3 of water (Magee1997; DeVogel et al. 2004). At 19m AHD, water couldflow from Lake Eyre through the WarrawoocarraChannel into the mega-Lake Frome system, which in-cluded lakes Callabonna, Blanche and Gregory thatthen comprised an expanded system measuring�34 250 km2 (DeVogel et al. 2004; Nanson et al. 2008)(Fig. 2). Twomajor mega-Lake Eyre fillings occurred at125 ka and �85 ka, the latter excluded the mega-Fromesystem and covered 19 500 km2 (Magee 1997; DeVogelet al. 2004). At its maximum, mega-Lake Eyreexpanded east and north along the Cooper–Warburton–Diamantina and Kallakoopah palaeoval-leys, respectively, with major flood-outs incorporatingplaya lakes along the main channels. Flood levels ofbetween 110 and 115m AHD expanded the lakenorthwest along the Warburton palaeovalley andthrough Billicoorinna Creek, filling modern LakeWarranderinna. Similarly, the Kallakoopah palaeoval-

leys overflowed southeast into modern lakes north andsouth of the main channel.

Other Central Australian lakes experienced lateQuaternary highstands as a series of temporally andspatially staggered high moisture signifiers against abackground of fluctuating wetting and drying (Nansonet al. 1992; Webb 2008). Evidence from Lakes Gregoryand Woods indicates mega-lake episodes during MIS 7and 8. MIS 6 brought a �30 000 year dry period of se-vere aridity that excavated to the Miocene EtadunnaFormation around Lake Eyre (Magee 1997). Recentinvestigations of prominent palaeoshorelines on thewestern side of a 3000 km2 playa lying 1000 km north-west of Lake Eyre reflect a lake highstand earlier(�130–140 ka), but similar, to those on Lakes Eyre,Frome, Callabonna, Gregory, Woods and Lewis. Thisevidence marks an abrupt return to wetter times fol-lowing the MIS 6 hiatus, but predates the 125 ka mega-Lake Eyre event, which also filled Lake Frome. A wetevent at 92–105 ka is recorded at lakes Lewis, Gregoryand Woods, together with a full flood down the FinkeRiver and flows into Lake Eyre via the WarrawoocarraChannel and Strzelecki Creek (Nanson et al. 1995,2008; Bowler et al. 1998; English et al. 2001) (Fig. 2).The smaller �85 ka Lake Eyre coincides with an inter-stadial between 86 and 79 ka. Cooper Creek flooded at

Fig. 1. Lake Eyre basin, Australia, showingmain lake systems, MIS 5e southern monsoonlimit, Western Pacific Warm Pool (WPWP) in-fluence and contributory and surroundingcatchments: C-s=Cooper–Strzelecki Creekcatchment; D=Diamantina–Warburton Rivercatchment; F=Finke River catchment;G=Georgina River catchment; H=HaleRiver catchment; T=Todd River catchment.

BOREAS Mega-Lake Eyre in late Quaternary Central Australia 313

�75 ka and Lake Eyre and Cooper Creek received floodwaters at �65 ka, but corresponding wet events are notrecorded on other Central Australian lakes, suggestinglocalized events.

Studies of lake-level fluctuations throughout theworld highlight the question of whether they are afunction of increased or more effective precipitation ordecreased evaporation – or all of these (Bradley 1999).Although palaeoshorelines like those of Lake Eyre arenot featured on all Central Australian playas, theirpresence during MIS 5e on several lakes testifies to ex-tensive, high volume but temporally and spatially stag-gered intra-continental and possibly constant rainfall.The above discussion invokes discrete summer mon-soon (SM) lows acting along a trough as deep as 23.51Sto cover Lake Mackay and enter the northern LEBcatchment, but it may not be that clear (Fig. 1). It hasbeen proposed that while the monsoon was aroundduring MIS 5e it turned off at MIS 5a, at least innorthwest Australia (Johnson et al. 1999; Magee et al.

2004), although Cooper Creek floods post-date this(Nanson et al. 2008). The timing of Central Australianmega-floods does not align precisely, but it has beensuggested that this may be an artifact of low-level geo-chronological precision rather than real temporal dif-ferentials (Magee et al. 2004). However, thisdiscrepancy could also indicate multiple forcing forLEB runoff and a different SM pattern from today.

Another proposal is that trapping of a western Pa-cific warm pool (WPWP) along the eastern Australiancoastline during oceanic lowstand might have con-tributed to Lake Eyre mega-fillings with lows movingwest through coastal Queensland (Burdekin and Fitz-roy catchments) into the eastern Lake Eyre catchment(Nanson et al. 2008) (Fig. 1). The palaeoflood signifierssuggest a SM with differential continental penetration,variable east–west extension and temporally and spa-tially differing strengths during MIS 5e. The LakeMackay evidence indicates an efficient SM deliveringenhanced monsoon rains over the lake (231S) because

Mega-Lake Frome

Mega-Lake Eyre CooperPalaeovalley

WarburtonPalaeovalley

KallakoopahPalaeovalley

WarrawoocarraChannel

LakeGregory

LakeBlanche

LakeCallabonna

Lake Frome

Warburton-DiamantinaInflow

Cooper Inflow

Strzelecki Creek Inflow

50 km

Lake Umaroona Group

Lake Warranderinna

Fig. 2. The mega-Lake Eyre system with pa-laeovalley flooding and its association with themega-Lake Frome system (base map after De-Vogel et al. 2004).

314 Steve Webb BOREAS

its inefficient catchment would be unable to deliver en-ough inflow to maintain lake levels high enough to forma palaeobeach of the dimensions observed. Lake Lewis,350 km to the east, does not reflect this at that time, butit is difficult to believe that a SM strong enough to fillLake Mackay did not reach Lewis, although it may re-flect a reduced intensity that far east. It is also difficultto firmly link the Mackay filling with later fills at LakeEyre (�125 ka), although their temporal proximitymakes it tempting to do so. It is worth mentioning thatwhile Lake Mackay experienced high lake levels, thelack of terrestrial and aquatic fossil fauna, includingGenyornis egg shell, suggests that the presence of waterdoes not naturally mean the presence of megafauna.The placement of Lake Mackay obviously put it be-yond the reach of these animals, probably because ofthe lack of suitable migration corridors.

To trigger a large enough inflow to Lake Eyre fromthe same SM would require a 1000 km easterly exten-sion to reach the Georgina–Diamantina catchment, inwhich case it would cover Lake Lewis. Alternatively, adiscrete depression could enter the LEB from the northor northeast over 600 km east of Lewis, leaving itunaffected. The upper Finke River catchment(63 000 km2) in the West MacDonnell Ranges abuts the22 800 km2 Lake Lewis catchment. A Finke flood ataround 100 ka indicates an SM intrusion to at least23.51S, which coincides with a 495 ka filling of LakeLewis, but it would also fill Lake Mackay (Nansonet al. 1995; English et al. 2001).

Morphological changes to major LEB catchmentstreams have probably taken place during the last100 000 years, and aerial and ground observation of thedry major upper catchment channels (Georgina, Dia-mantina, Hamilton, Burke) and their tributaries sug-gests a system built by massive and widespread runoffin the past. The immediate broadening of headwaterchannels also indicates a past weather system that tar-geted the area. The possibility of increased rates of cy-clonic regeneration tracking into mid-lower sections ofthe LEB catchment from the east during MIS 5e couldalso have contributed to mega-lake development. Al-though cyclones develop into rain depressions overland, 12% of the time they regenerate into cyclonesagain (McBride & Keenan 1982). Increased cyclone/deep low frequency and regeneration with longer cy-clone seasons during MIS 5e could have enhanced pre-cipitation across the Thomson–Barcoo system feedingCooper Creek. Without this, Cooper Creek would nothave a runoff that contributed much to building themega-Lake Eyre. Conversely, with the SM switched offand the WPWP extending inland, Lake Eyre could re-ceive water but mega-Lake Frome would be the great-est recipient with filling through Strzelecki and CooperCreeks (Nanson et al. 2008). Fillings of 110 to 115m,however, would probably require a large contributionfrom both the SM and WPWP. It seems that the 125 ka

mega-Lake Eyre filling was probably caused by a verydifferent climate system than that of today, and while itseems logical to assume a SM cause, a multiple systemmight be a better explanation. Whatever it was, it wasmore active, complex, of long duration, regular andexpanded south causing continuous stream flowthrough a more efficient system.

Filling mega-Lake Eyre

The modern monsoon has a variable season length andrainfall intensity and covers 400 000km2 of northernAustralia (Suppiah 1992). It is forced by two continentalheat lows over the west Australian Pilbara and Cloncurryin western Queensland. Onset is dependent on global-scale circulation changes, solar insolation, Pacific seasurface temperature, the presence–absence of El-Ninoevents, poleward displacement of the Southern Hemi-sphere tropical jet-stream and propagation of low-levelwesterlies over the Indonesian–Arafura Sea, amongothers (Suppiah 1992). Differences in any of these in thepast could have increased or decreased monsoonstrength and its geographical position. Coming to termswith this level of complexity in the past is daunting butstarting at the other end of the puzzle might help.

Palaeoshorelines are proof of a mega-Lake Eyre, buthow long did it take to fill and how long did it exist?Various inflow and evaporation rates are presentedhere to test possibilities for filling a mega-Lake Eyre tothe 125 ka level (332 km3) (DeVogel et al. 2004) (Tables1, 2). Each includes inflow rates of 3000 and 6000m3/s,similar to those recorded in the 1974 flood, in conjunc-tion with three evaporation rates of 500, 1000 and1900mm/a, the last-mentioned calculated during thesame flooding (Tetzlaf & Bye 1978; Kotwicki 1986).Modern evaporation rates (2400–3600mm/a in the up-per and lower catchment, respectively) were also used,but immediately eliminated as being too high to estab-lish a mega-lake. The formula V/109�E=WL, whereV=lake volume (km3), E=evaporation rate andWL=water body loss (km3), has been used to calcu-late lake fill rates. Results show, first, that lake growthceases as inflow rates are matched by evaporation.Therefore, a water body of finite size is constructed de-pending on evaporation rate rather than the size of in-flow. Second, the largest mega-lakes (4245 km3) formonly with annual evaporation rates around 500mm/aand constant inflow of 6000m3/s maintained over manyyears (Table 1C). Third, fill speed is naturally greaterwith large flow rates, and a water body of�416 km3 canbe achieved with a constant fill rate of 6000m3/s oversix years and an evaporation rate of 500mm/a. A con-stant fill rate of 3000m3/s over 11 years would achieve amaximum lake size of 337.9 km2 at the same evapora-tion rate. Lake size drops to a maximum of 212 km2

achieved in eight years at 1000mm/a evaporation

BOREAS Mega-Lake Eyre in late Quaternary Central Australia 315

Table 1. Various annual evaporation and flow rates and filling times during MIS 5e modelled to produce variously sized mega-lakes.Italics=Maximum lake size possible at modelled filling and evaporation rates.

A: 1900mm annual evaporation rate

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

189.2 68.0 94.6 16.22 121.2 78.4

310.4 183.1 178.4 60.53 127.3 117.9

316.5 190.3 215.5 85.84 126.2 129.7

224.3 95.65 128.7

223.3 94.76 128.6

B: 1000mm annual evaporation rate

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

189.2 35.8 94.6 8.92 153.4 85.7

342.6 117.4 180.3 32.53 225.2 147.8

414.4 171.7 242.4 58.84 242.7 183.7

431.9 186.5 278.3 77.55 245.4 200.8

295.4 87.36 208.1

302.7 91.67 211.1

305.7 93.58 212.2

C: 500mm annual evaporation rate

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

189.2 17.9 94.6 4.52 171.3 90.1

360.5 65.0 184.7 17.13 295.5 177.6

484.7 117.5 272.2 37.14 367.2 235.1

556.4 154.8 329.7 54.45 401.6 275.3

590.8 174.5 369.9 68.46 416.3 301.5

396.1 78.47 317.7

412.3 85.08 327.3

421.9 89.09 332.9

427.5 91.410 336.1

430.7 92.811 337.9

316 Steve Webb BOREAS

Table 2. Mega-lake growth at various evaporation and flow rates and filling times when inflow occurs only during any four consecutive monthsannually. Italics=Maximum lake size possible at modelled filling and evaporation rates.

A: 1900mm annual evaporation rate

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

63.1 7.6 31.5 1.92 55.5 29.6

118.6 26.7 61.1 7.13 91.9 54.0

155.0 45.7 85.5 13.94 109.3 71.6

172.4 56.5 103.1 20.25 115.9 81.9

179.0 60.9 113.4 24.46 118.1 89.0

120.5 27.67 92.9

124.4 29.48 95.0

125.5 29.99 95.6

B: 1000mm annual evaporation rate

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

63.1 2.0 31.5 1.02 61.1 30.5

124.2 15.4 62.0 3.83 113.8 58.2

176.9 31.3 89.7 8.14 145.6 81.6

208.7 43.6 113.1 12.85 165.1 100.3

228.2 52.1 131.8 17.46 176.1 114.4

239.2 57.2 145.9 21.37 182.0 124.6

245.1 60.1 156.1 24.48 185.0 131.7

163.2 26.69 136.6

168.1 28.310 139.8

171.3 29.311 142.0

173.5 30.112 140.4

C: 500mm annual evaporation

Filling time (years) Filling rate 6000m3/s Filling rate 3000m3/s

Beginning of year Lake size (km3) Total annualevaporation (km3)

Lake size (km3) Total annualevaporation (km3)

63.1 2.0 31.5 0.52 61.1 31.0

124.2 7.7 62.5 2.03 116.5 60.5

179.6 16.1 92.0 4.24 163.5 87.8

226.6 25.7 119.3 7.1

BOREAS Mega-Lake Eyre in late Quaternary Central Australia 317

(Table 1B). Fourth, by increasing flow rates, the lakegrows more quickly but does not reach significantlylarger proportions. For example, an 8000m3/s flowachieves a lake of �416 km3 at an evaporation rate of500mm/a, but only 112 km3 at an evaporation rate of1900mm/a (Table 1A). Finally, reducing flow rate tofour months annually limits maximum lake size to287 km2 in 12 years or 215 over 17 years; any higherevaporation rate reduces the lake size significantly(Table 2).

The above suggests that mega-lakes formed onlywith constant, high inflow over many years at eva-poration rates far less than those of today. Therefore,the 125 ka (332 km3) event could have been achievedonly with an evaporation rate �500mm/a and a flowrate between 5000 and 6000m3/s maintained for atleast a decade. Incorporating the mega-Frome group(430 km3), a constant flow of around 7000–8000m3/s isrequired for at least four years at the same low eva-poration rate. With transmission losses in the lowercatchment similar to those of today, however, uppercatchment flow had to be far higher than6000–8000m3/s, which stretches credibility with regardto a climate system that could deliver and maintainsuch flows for years on end winter and summer. With amoisture pattern that influenced both the Diamantinaand Cooper systems, flow to the lake would be sharedby these systems. Moreover, lower annual ambienttemperatures and southern catchment and over-lakeprecipitation have not been considered, nor have thevegetation mosaics required for maintaining bothaquatic and terrestrial fauna feeding and breeding ha-bitats. Lower annual ambient temperatures and localrainfall levels are also vital unknowns and would alter

the picture. Lacustrine hydrological balance is main-tained through evaporation versus precipitation/run-off. Evaporation is affected by cloudiness, solarinsolation, wind, lake depth and salinity, humidity aswell as temperature (Bradley 1999). Similarly runoff iscontrolled by precipitation frequency, intensity andtype, vegetation cover, ground temperature and slopegradient. Therefore, the most logical picture for mega-Lake Eyre formation seems to be constant catchmentflow for extended periods. This produced a deep(30m), largely freshwater, lake, higher humidity, high-er frequency of cloudy days and widespread vegetationcover throughout the catchment and around the lake,all of which lowered ground temperature. Jointly, mostof these factors would have acted to reduce evapora-tion and help maintain lake levels with lower catch-ment inflows. It was noted that, during Lake Eyre’s1974–76 flood, precipitation rates were higher andevaporation rates lower, a phenomenon thought to beassociated with a feedback relationship occurringwhen the full lake (31 km3) sustains a more humid me-teorological regime during total flooding (Bye & Will1989). If this is so, a lake eight times as big probablyhad an even firmer feedback mechanism that helpedmaintain it.

The palaeo-aquatic fauna of the mega-LakeEyre system

Like islands, lake size dictates its faunal population sizeand biological diversity (MacArthur & Wilson 1967;MacDonald 2003). The fossil record testifies to thewidespread and complex fauna that developed in and

5 200.9 112.2264.0 34.8 143.7 10.3

6 229.2 133.4292.3 42.7 162.9 13.3

7 249.6 149.6312.7 49.2 181.1 16.4

8 263.5 164.7326.6 53.3 196.2 19.3

9 273.3 176.9336.4 56.6 208.4 21.7

10 279.8 186.7342.9 58.8 218.3 23.8

11 284.1 194.5347.2 60.3 226.0 25.5

12 286.9 200.5232.0 26.9

13 205.1236.6 28.0

14 208.6240.1 28.8

15 211.3242.8 29.5

16 213.3245.0 30.0

17 215.0

318 Steve Webb BOREAS

around mega-Lake Eyre. It therefore begs questionsconcerning the formation and trophic developmenttimes required for a trophic complexity to form thatincluded a permanent fauna consisting of large terres-trial and aquatic animals such as crocodiles, large tur-tles and large fish.

Crocodiles

Crocodiles were at the top of the mega-Lake Eyretrophic pyramid. The remains of very large specimensare found throughout the region from KallakoopahCreek to Cooper Creek (Webb 2009). Previously re-ported as Crocodylus porosus, this animal could alsohave been the freshwater giant Pallimnarchus pollens;other remains indicate the possibility of a smaller spe-cies P. gracilis (Willis & Molnar 1997; Molnar 2004).Described as an aquatic ambush predator, Pallim-narchus sp. has been found on headwaters of the LEBcatchment and upper reaches of rivers entering the Gulfof Carpentaria. While an average length of 4–6m isnormal for an adult animal, an upper limit of 8m mayhave been possible (Webb & Manolis 1989). A lack ofenemies probably allowed specimens to achieve this sizeregularly. Crocodylus could have been larger because oftheir tendency to be much heavier in freshwater en-vironments than in tidal rivers (Webb &Manolis 1989).

For these animals to be present, mega-Lake Eyrewould have to provide a myriad of niches and habitatssimilar to those inhabited by modern estuarine or salt-water crocodiles (C. porosus). Their diet includes insectsand crabs at the edge of the bank, particularly favouredby juvenile specimens. Larger animals eat fish, smallmammals, birds and turtles in freshwater environ-ments, together with lizards and snakes. Terrestrialmegafauna may have been prey to the biggest speci-mens when frequenting the river bank or lake side,particularly if muddy conditions hampered a quick es-cape. Crocodile tooth marks in diprotodontid longbones attest to such predation, but carrion is also takenby crocodiles, which will move several hundred metresto collect a carcass and return to the water with it.While crocodiles are at the top of the aquatic foodchain, they usually have only 50 big feeds annually.Population densities for estuarine crocodiles in theNorthern Territory range from 0.5/km2 to 1.5/km2 peranimal depending on the type of ecosystem surveyed(Messel & Vorlicek 1989; Harvey & Hill 2003). Usingthe latter figure, a 30 000–34 000 km2 lake could sup-port a crocodile population of 20 000 animals. With anaverage length of 5m these would require 50 000 tonnesof prey annually, equivalent to 20000 adult diproto-dontids (Webb 2009).

An estuarine crocodile needs replenishment of water,a suitable breeding environment (nesting sites) con-taining sedges and cane grass (Phragmites sp.), rafts of

vegetation and other aquatic plants as well as largerplant species for refuge areas (Webb & Manolis 1989).Pallimnarchus probably had the same requirements,and thus associated vegetation must have been in placefor these reptiles to have thrived. Moreover, large iso-lated billabongs and waterways in the SLEB could havesupported crocodiles during drier periods, but droughtwould need to be short-lived with regular rainfall highenough to trigger river flows to flush the system. Apermanent population could not be maintained withoutgood rainfall, and, therefore, on both these counts,across-lake precipitation must have been much higherthan today. Moreover, widespread fossil evidence oflarge specimens, separated by as much as 300 km, sug-gests a widespread population reaching into every cor-ner of the mega-lake.

Turtles

An environment that supported large crocodiles wouldnaturally support turtles, and mega-Lake Eyre has re-vealed a wide range of fossil turtle species. The largestMeiolania sp. (M. platyceps) is represented by horncores and vertebrae (Fig. 3). While the type specimen isfrom Lord Howe Island, other Pleistocene exampleshave been found in southeastern and northern Queens-land (Gaffney & McNamara 1990; Gaffney 1996). Thepresence of Meiolania at Wyandotte Creek, high in theBurdekin catchment and close to the headwaters ofCooper Creek, suggests it could have followed water-ways into Lake Eyre catchment from northeasternQueensland (Fig. 1). Meiolania may not have been asaquatic as other turtle species, however, in that its sizeand morphology, including elephant-like feet ratherthan the webbed feet of other turtles, suggests beha-viour more like that of the giant Galapagos tortoises(Geochelone elephantopus) that have a largely terrestrialhabitat (Gaffney 1985). This would also helpMeiolaniamove between basins even if it was slow. Nevertheless,Lake Eyre meiolanids are found in association with

Fig. 3. Meiolania sp. horn core retrieved from the Warburton pa-laeovalley.

BOREAS Mega-Lake Eyre in late Quaternary Central Australia 319

fluvial sediments but reworking of fossil remains mayaccount for this.

Because of continental dryness, modern Australianturtles are limited largely to the continental periphery,but fossil evidence from the SLEB indicates that lateQuaternary turtle demography extended far inland.Fossil turtle carapace and plastron fragments fromsmaller species emerge from Katipiri Formation sedi-ments (MIS 5). Various marginal and costal shell pat-terns and morphology indicate a wide range of speciesand variously sized animals. Without morphologicalcharacteristics readily identifiable to species, the rangeof shell and margin shape that takes place duringgrowth makes accurate identification of shell fragmentsto species level tentative at best, and this is widely re-cognized (Gaffney 1981; Legler & Georges 1993). An-other approach to knowing what species may have livedthere is to extend modern turtle ranges from catch-ments adjacent to the Lake Eyre basin (LEB). Rangeextension would have been possible through floodedsub-catchments of the Georgina, Diamantina andCooper streams during Pluvial episodes. Turtles do notmigrate, so their entrance into the SLEB must havebeen on a permanent basis each wet period. They hi-bernate, but not for extended periods of drought, so re-colonization of the mega-lake system must have takenplace following any extended dry time (Wyneken et al.2008).

Three Chelodina species now live in catchments ad-jacent to the LEB. These include C. logicollis, C. non-aeguineae and C. expansa in the upper Cooper, Bulloo-Bancannia, Warrego, Burdekin and Fitzroy catchments(Cann 1998). They are joined by three Emydura species(E. macquarii macquarii, E. krefftii and E. macquariiemmotti, the last-mentioned being the Cooper Creekturtle that still resides there). Four Elsaya species (El.dentata, El. irwini, El. latisternum and El. albagula) andRheodytes leukops also inhabit the Burdekin and Fitz-roy catchments that abut the Cooper catchment (Cann1998). Most of these species grow to between 35 and45 cm in length, producing moderately large animalswith a plastron thickness of 15mm.Other fragments ofshell and carapace from the SLEB reflect this size ofanimal and may also indicate great longevity. Toothgouges in some fragments attest to predation by croco-diles. All of the above species could have entered theLEB catchment anywhere from the Gulf of Carpentariato the central Queensland coast by crossing the BarklyTableland and Great Divide, respectively. However,permanent free-flowing water was required for them tobe able to cross basin boundaries and move into thecatchment system. As with crocodile remains, the widedistribution, variety and common occurrence of fossilspecimens suggests that the LEB turtle populationduring MIS 5e was large, broadly distributed acrosshundreds of square kilometres, and residentially longterm.

Large fish

Formal identification of fish species from Katipiri fossilassemblages has not been completed. For the purposeof this study, however, such identification is hardly ne-cessary with large vertebrae indicating the great sizeattained by a couple of species. Using the formula LogY=0.913812.4497 (log X), where X is the vertebraldiameter (Casteel 1976), a range of fish sizes has beencalculated up to 69.0 kg. Fish attained large size andgreat age in the mega-lake. The size of some vertebraesuggests three possible species, including Golden Perch(Macquaria ambigua), Murray Cod (Maccullochellapeelii peelii) and Barramundi (Lates calcarifer). GoldenPerch can reach 75 cm and 23 kg, but are more regularlyaround 5 kg (www.nativefish.asn.au 2008). Their dietincludes yabbies, shrimps, insects, molluscs and smallfish. Extant Perch in the Murray–Darling Basin growlarger than those in the neighbouring LEB, but this si-tuation may not have applied 100 000 years ago. Thelargest Murray Cod caught weighed 113 kg and mea-sured 183 cm; however, a normal upper limit is around125 cm and 45 kg, although this size is considered raretoday. Murray Cod are top predators with a diet ofother fish, crayfish, yabbies, shrimp, freshwater mus-sels, frogs, water fowl, small mammals, turtles andother reptiles. Barramundi could fit the bill for size andbe suited to the Lake Eyre basin habitat during pluviallake phases, but they normally require access to estu-aries and coastal waters for spawning, so have beeneliminated until a positive identification is made.Lungfish (Neoceratodus sp.) tooth pates are also foundin fossil assemblages around Lake Eyre, some indicat-ing large specimens. Lungfish reach 1.5m and 40 kg andare capable of living in stagnant water during droughtbut generally require deep pools, slow-moving waterand vegetation along river banks (Kemp 1991). Today,they are confined to a small area of the Burnett on theeast coast of Queensland.

Mega-Lake Eyre’s fish in the 14–25 kg range indicateseveral species, but those in the 26–69 kg range arelikely to have been Murray Cod. They all (and Barra-mundi) favour river snags, require water temperaturesabove 231C to spawn, can survive drought and Perchand Cod can migrate 1000 km. Their presence also in-dicates a secure trophic system that included a broadvariety of producers in order for them to have thrived.Large fish vertebrae are found widely across the oldmega-lake landscape.

Trophic status and palaeoaquatic colonizationof mega-Lake Eyre

Systems ecology has long recognized that trophic pyr-amids are more complex than simple levels of stackedanimal groups. While it remains a useful concept to

320 Steve Webb BOREAS

present a basic picture of community relationships, inreality such pyramids reflect complex food webs con-sisting of subtle relationships between different organ-isms with energy going in various directions, notnecessarily vertically from simple producers to carni-vore consumers on top. Therefore, in order to appreci-ate community structure, function and its response tochange and disturbance, it is necessary to understandthe food webs involved (MacDonald 2003). However,gathering enough data to build a basic food web in ex-tant communities is a considerable task, but to do thisfor extinct animal communities and environments isprobably impossible. There are some inferences thatcan be made, however, even though the complete SLEBcommunity of creatures is still unknown.

Mega-Lake Eyre morphology must have included aeuphotic zone that extended from the littoral to a levelwhere photosynthesis ceases (MacDonald 2003). Witha maximum lake depth of 30m (from �15m to 115mAHD) there was also a disphotic zone suitable for fishand other large aquatic species but not plant growth.Lake salinity is unknown, but using 1974 flood figuresshows that minimum salinity (11%) occurred at thesouth end and deepest (�12 to�10m depth AHD) partof the lake, with 300–330% in a 20 cm brine layer on thebottom (Magee 1997). In the 1974 event, wave mixingdid not involve the bottom layers. Salinity also declinedfrom south (11%) to north (o0.5%), at the same level,at the shallow end of the lake (Magee 1997). Withdepths from �10 to 110 the mega-lake would haveconsisted of almost pure fresh water in over 90% of itsupper depth in most places. This helps put the varietyand composition of the fossil aquatic fauna and sup-portive vegetation in perspective.

It took time to grow a full mega-Lake Eyre ecosys-tem with its own unique constellation of ecological at-tributes. Rapid filling was not accompanied by trophicgrowth and this would result in a long lag-time betweenstanding deep-water environments and the arrival oflarge fauna. Presumably, as the lake filled it passedthrough three standard stages: oligotrophic (a low nu-trient lake), mesotrophic (developing), eventuallyreaching a eutrophic or ‘well-nourished’ lake with highnutrient levels and associated vegetation (Leveque &Mounolou 2003). The lag-time involved the establish-ment of planktonic organisms and plant colonizers inthe shallow littoral zone at the edge of the primary lake.Lake growth increased the littoral, as shorelines ex-tended to at least double the modern length, perhapsreaching 3500 km. The deepening lake drove salinity tothe bottom profundal zone, where plants did not grow.Organic detritus, phytoplankton and algae built up inthe littoral that fed micro-fauna (zooplankton) andsmall consumers which, in turn, supported larval fishand other varieties of small invertebrates. Interaction ofplant photosynthesis and respiration was required toestablish a large food base of the plant primary produ-

cers necessary even to support relatively few larger fish.Other lake fauna found in fossil assemblages includeshellfish (Unio sp.), crustaceans (Cherax sp.) and mol-luscs (Viviparidae, Thiaridae, Coxiella sp.). The pre-sence of fish, therefore, indicates a fairly completetrophic pyramid consisting of primary producers (phy-toplankton) and consumers (zooplankton) and sec-ondary consumers (small fish, shell fish, frogs andtoads). Only then would tertiary consumers (large fish,crocodiles and turtles) slowly move in, but these alsorequired supporting terrestrial vegetation mosaics that,in turn, indicate climatic conditions far wetter thanthose found there today.

Mega-lake phases were optimum times for broadoccupation by terrestrial-based megafauna acrossextensive riverine and peri-lacustrine environments,but climatic conditions limited or enhanced the mi-gration–colonization process. Our present under-standing of Australia’s megafauna distributionindicates that 32 species lived in the southeast, 26 ofthem in the western half of that area, placed directlysouth of Lake Eyre (Webb 2009). Twenty species livedin the SLEB in addition to Pallimnarchus and Meiola-nia, while both these and nine other species livednortheast of the LEB. The main migration corridor forterrestrial megafauna in and out of the SLEB seems tohave been from the south rather than through northeastcatchment channels (Webb 2008). However,Meiolania,Pallimnarchus and large fish species probably reachedthe SLEB through the latter. A number of moderncrocodile species deeply penetrate continents (Ross &Magnusson 1989). The Nile crocodile (Crocodylus nilo-ticus) and the Mugger (Crocodylus pylustris) occupyareas well within Africa and subcontinent India, re-spectively, while caimans (Paleosuchus sp. and Caimancrocodylus) occupy upper catchments of major SouthAmerican rivers and salt water crocodiles move at least200 km up north Australian rivers. The movement ofcrocodiles inland as far as Lake Eyre against a back-drop of receptive environments, therefore, is not un-expected.

Discussion

Mega-Lake Eyre could have formed comparativelyquickly given high enough inflows. Its size was limitedby evaporation rates, which had to be �500–700mm/a,i.e. less than one-fifth of rates today. Increased pre-cipitation and catchment efficiency, local rainfall on thelake and high, constant inflow were also required. Thissuggests an MIS 5e climate system unlike anything inCentral Australia today. These may have includedstronger monsoons with deeper incursions allied withstronger tropical low influence from the WPWP. Com-bining these systems would have optimized Lake Eyreinflows and instigated mega-lake events that included

BOREAS Mega-Lake Eyre in late Quaternary Central Australia 321

Lake Frome. Environmental changes through thecatchment, as well as feedback mechanisms developedby the mega-lake system itself, probably amelioratedthe requirement for persistent catchment rainfall.

Lake Eyre is the only large Central Australian lakewith the range of fossil fauna from which any kind ofpalaeotrophic setting can be derived. Its distributionshows that it occupied the entire mega-lake system. Thelack of such remains around other lakes strongly in-dicates that the opportunity for colonization by terres-trial and aquatic species did not exist elsewhere inCentral Australia. Lake Eyre’s major catchment chan-nels made the difference by providing corridors fromadjacent catchments. Ease of colonization was not sur-prising, nor was it remarkable given enough time tobring animals in, beginning when regular catchmentchannel flow ensued. However, the arrival of largeraquatic species is not dependent on the time taken to fillthe lake, but on the time taken to assemble the requiredtrophic chains to support them. Also large animalshave slower rates of colonization than small passivedispersers, so large aquatic species and marsupialmegafauna probably took some time to arrive at themega-lake. Building biodiversity was a stepwise processthat gradually assembled trophic networks which couldsupport the variety of large colonizers arriving as soonas their energy base formed. If the lake grew over dec-ades then it is likely there would have been a substantiallag-time between the initial colonist-producers and thearrival of larger species.

Trophic building probably began in a similar mannerto that following the 1974 floods. Eighty species of wa-ter birds and 47 fish species have been recorded withinthe modern LEB (Morton et al. 1995). Readiness foralmost instant propagule colonization of the lake wasshown in the 1974 floods, with nine species each ofzooplankton and phytoplankton recorded (Glover1989). Pelicans (Pelecanus conspicillatus), cormorants(Phalacrocorax sp.), sea-birds (gulls and terns), stilts,plovers and dotterels rapidly moved in and small fish,such as hardyhead (Craterocephalus sp.), smelt (Retro-pinna sp.) and yellowbelly (Macquaria ambigua), werethe first aquatic propagules to arrive via river systems(Glover 1989). Similar colonists were probably the firstto arrive during MIS 5e, with birds initiating passivejump dispersal of seeds (zoochores) from high catch-ment areas and small fish populating the lake throughactive dispersal via catchment channels. Jump dispersalis one way of propagule colonization where large dis-tances can be crossed quickly by a carrier agent (birds)and disjunct populations (inoculation centres) set upbefore they grow then coalesce. Animals and plantsprobably spread from refuge areas occupied duringMIS 6, following the movement of permanent waterinland. Establishment, dispersal and growth of vegeta-tion within the catchment probably occurred at differ-ent rates.

Each propagule followed its own pattern of dispersaland population growth depending on carrying capacityand other environmental constraints. Both carryingcapacity and range expansion limit population growthfor animals and plants (MacDonald 2003). Initially,range expansion in an inoculation centre is slow be-cause of the small colonizing population. It then ex-pands as a response to lake size and carrying capacitygrowth. Exponential population growth would followgiven a burgeoning carrying capacity till that growthexceeded the resource base. Coalescence of inoculationcentres and jump dispersal from them would expandrange limits. Once diversity was in place, animal popu-lations could spread throughout the lake. The key tomega-lake biodiversity is the permanence of water andmaintenance of the resource base, i.e. not particularlylake size: a 50 km2 lake could have the same speciesrange as a 200 km2 lake, the latter would just have moreof each species. Endemic populations formed with lakepermanence, but this required aquatic and terrestrialvegetation mosaics that, for example, supported croco-dile breeding habitats. Aquatic fauna should not beseen in isolation from terrestrial species because theecosystems they require needed regular local rainfall toproduce savannah, gallery forest, back swamps, thick-ets, woodland areas and a wide array of intermixedeconiches (Webb 2008, 2009).

Studies of post-LGM colonization, as well as morerecent examples, show that the rate of plant and animalcolonization is extremely variable. Sixty years after theKrakatau eruption, 300 plant and 30 bird species wereestablished on the island, while palynological evidencesuggests that trees spread during North America’s earlyHolocene at 80–400m/a or 1000 km in 2500–12 500years (MacDonald 2003). Doubling times range from31 to 1100 years depending on the genera, but evenwithin genera there are vast differences, e.g. pines (Pi-nus sp.). Trees took several thousand years to re-popu-late their previous habitats following glacial retreat.North American oaks re-colonized at between 380 and500m/a (Leveque & Mounolou 2003), while Europeanoaks (Quercus sp.), elms (Ulmus sp.), hazels (Carylussp.) and lindens (Tilia sp.) took almost 6000 years tocompletely re-colonize Europe, thus demonstratingconsiderable complexity in the colonizing process (An-derson et al. 2007).

Development of a full mega-lake trophic systemprobably took centuries (?200–400 years) rather thandecades to complete, although some elements may havebeen in place comparatively quickly. The varying abil-ity of different plant and tree species to colonize, as wellas competition between some, probably took completevegetation mosaics a long time to establish. Formationof the initial trophic system (primary producers, het-erotrophic organisms and invertebrates) depended onhow quickly they were introduced and on the substratesupon which they could build. If the process had a head

322 Steve Webb BOREAS

start, with some plant species already in place similar tothe 1974 event, then it may have begun quickly. Alter-natively, if it required a bootstrap process on thescoured landscape remaining after the aridMIS 6 stage,then the primary base would have taken much longer toestablish. Whatever the exact situation, at its peak at�125 ka the mega-lake and tributary river systems musthave achieved a biotic complexity akin to the rivers andswamps of the wet-tropics. Away from these, savannahtook over with its own abundance of terrestrial fauna,but the relationship between these will be addressed infuture work.

Conclusions

The filling of mega-Lake Eyre during MIS 5e may havebeen an isolated event in terms of other Central Aus-tralian lake highstands. Staggered floods occurredthroughout Australia, but their interrelationship andthe exact climatic conditions and flood sequence of eachremains unclear. The possibility of variable combina-tions of SM and WPWP episodes probably played apart in these floods. Filling and maintaining mega-LakeEyre, however, required lower evaporation rates andhigher local precipitation and regular inflows from up-per catchment weather systems. The actual filling timerequired may have been comparatively short in dura-tion, but constant inflow to the lake was required for anumber of years to build the largest lakes. This suggeststhat there was a very different climatic forcing duringMIS 5e which developed complex vegetation in theSLEB with regular over-lake precipitation.

It is suggested that construction of basic palaeo-trophic systems during the filling of the lake may nothave taken long, but that full completion of those sys-tems to a level that supported large complex animalpopulations with supportive vegetation cover certainlywould. The presence of crocodiles, large fish and turtlesacross the system indicates a fully developed aquatictrophic system but also a terrestrial environment sup-porting successful breeding habitats for crocodiles andturtles. The biodiversity and distribution of both ter-restrial megafauna and aquatic species throughout theSLEB also supports the presence of fully developed la-custrine and terrestrial environments. These environ-ments and the food chains required to support suchbiodiversity are suggestive of those found today in thewetland areas of the tropics. For those of us familiarwith the present-day environment of Lake Eyre, how-ever, this presents an environmental juxtaposition al-most too incredulous to contemplate.

Further examination of the environmental status andrelationship between the aquatic and terrestrial faunawill produce a broader ecological picture of the climateand biodiversity that existed in the SLEB. Future workwill focus on these relationships and the wider wetland-

savannah. We are approaching a synthesis of the lateQuaternary bioenvironmental and climatic status ofmega-Lake Eyre as well as other Central Australianlakes. The environmental vision described here stronglycontrasts with that of the area today, thus highlightingthe enormous climatic changes that have taken place.

Acknowledgements. – This research was made possible through BondUniversity Research Grants and transport provided by Bond Uni-versity. My fieldwork colleagues Giff Miller, Steve DeVogel and JohnWischusen made the Lake Mackay results possible. I have benefitedfrom a long research association with Gerald Nanson and extensivediscussions with him and his colleague Tim Cohen.

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