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Quantitative estimates of the effect of climate change on dust storm activity in Australia during the Last Glacial Maximum
GHHORPIHOGY ELSEVIER
Geomorphology 17 (1996) 263-271
Quantitative estimates of the effect of climate change on dust storm activity in Australia during the Last Glacial Maximum G.H. McTainsh *, A.W. Lynch Faculty of Environmental Sciences, Griffith University, Nathan, Brisbane, Queensland, 411, Australia Received 7 April 1994; revised 4 January 1995; accepted 17 February 1995
Abstract Quantitative estimates are made of the effect of climate change upon dust activity during the Last Glacial Maximum (LGM), which peaked 18,000 years ago, using present-day measurements of the occurrence of dust storms recalculated in the light of information on LGM climatic change. Dust activity was increased by 57% in northeast Australia and by 52% in the southeast, in relatioa to present day. Dust storm seasons were lengthened and dust paths were modified. Estimates based upon climate alone, without considering the increased supplies of sediment to dust source areas during the LGM, however, probably underestimate LGM dust activity.
1. Introduction Australia is a dry continent, but aridity is a relatively recent phenomenon in geological time. Aridity became widespread only in the Quaternary and most of the dunefields which today dominate the Australian outback, were formed in the last 700,000 years. (Wasson, 198"1). Recent studies of Quaternary dust processes (Bowler, 1976; McTainsh, 1989) indicate activity during recent geological time. Few reliable estimates exist, however, about how active past dust processes were. Such estimates can now be made using improved estimates of Quaternary climate (Wasson,
* Corresponding author.
1987) and recent advances in our knowledge of present-day dust processes in Australia. This paper will use a broad-scale climatic model of dust to examine dust activity in eastern Australia during the Last Glacial Maximum (LGM), which peaked 18,000 years ago.
2. Dust paths Our current understanding is that two major dust paths occurred in the Australian region during the Quaternary (Bowler, 1976). One passed to the northwest off the West Australian coast and another to the southeast out over the Tasman Sea heading towards New Zealand (Fig. 1). Recent studies of these pathways and associated dust processes indicate that, although they persist today, the position and intensity may be different from the past.
0169-555X/96/$15.00 C,opyright © 1996 Elsevier Science B.V. All rights reserved
SSDl O169-555X(95)O0106-9
264
G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271
3. Methods 3.1. Measurements o f dust processes
A number of studies on dust have been carried out over the past 10 years (summary in McTainsh and Leys, 1993) with the aim of developing a body of knowledge on present-day dust processes that can be extended back through the Quaternary to understand the role of dust in the evolution of the Australian landscape. Measurements of dust processes have been made using meteorological records of dust events. The Australia-wide pattern of dust events (Fig. 2) (McTainsh and Pitblado, 1987) provides initial support for the existence of the two dust paths, although evidence of the southeast path is patchy and reflects inadequacies of the meteorological data (McTainsh and Pitblado, 1987). An additional area of dust activity in Western Australia also exists. Direct measures of suspended dust by high volume air samplers and of deposited dust (by glass bead deposition traps) have been made at up to six
sites within the southeast dust path since 1985 (Fig. 3). These data provide initial estimates of dust loads and rates of deposition. Recent studies of individual dust storms in 1983 (Raupach et al., 1994) and in 1987 (Knight et al., 1995) and studies of the trajectories of dust plumes (Tapper and Wain, pers. commun., 1994) confirm the existence of the southeast dust path, and show that it covers most of eastern Australia.
4. Results and discussion 4.1. Australian dust in a world-wide context
Fundamental similarities exist between transport of dust in Australia and West Africa (McTainsh, 1985). Until recently, however, quantitative data were lacking in Australia to compare with the dust levels in Africa and the rest of the world. Levels of dust activity in Australia appear to be comparable with the USA, but significantly lower than in West Africa. Annual rates of dust deposition in eastern Australia
(. •
.
.
.
.
L : .'•'.I.?•U
- .i -i~ i
~ -
Longitudinal dune systems Dust
p a t h s
Fig. 1. Australian dust paths during the Quaternary(after Bowler, 1976).
265
G.H. McTainsh, A.W. Lynch / Geomorphology 17 (1996) 263-271
Fig. 2. Annual average frequency of dust events (1957-1984) (McTainsh and Pitblado, 1987).
-24"
NT ~J.
I I I I I I
141)"
146 "
152 "
OLD
3~"~
• Charteville
AusTRALIA ~I,,S ]
~
SOUTH
- - ~ ~ B R I S B A N E
SA
PACIFIC
O"S
• FowlersGap
/
OCEAN
• Gunnedah/ NSW
Iv'c "k
// i~"
,. Waoga-Wagga /
,, 152"
Fig. 3. Dust-monitoring sites in eastern Australia.
100 I
=="
0
I
5(Xlkm
i m m ~ ~ SCIALE ' ' I
I I
G.H. McTainsh, A. W. Lynch / Geomorphology 17 (1996) 263-271
266
Table 1 Annual average rates of dust deposition and concentrations of dust in eastern Australia Station
Average annual rate of deposition ( t / k m 2)
Annual average concentration of dust (t.Lg/m 3)
Charleville Fowlers Gap Buronga Gunnedah
31.4 43.8 33.0
25.7 27.6 41.3 17.4
range from 31.4 to 43.8 t km -z yr - l (Table 1), compared with a USA range of 0.8 to 126 t krn -2 yr -1 (Pye, 1987). In West Africa annual rates are consistently higher. In northern Nigeria annual rates of deposition of 137 to 181 t km -2 yr -1 were recorded by McTainsh and Walker (1982) and in Mali (W.G. Nickling, pers. commun., 1994) annual rates could be even higher. Annual average concentrations of suspended dust at four stations in Australia range from 17 to 41 p , g / m 3 (Table 1) compared with 35 to 55 ~ g / m 3 in the USA (Thomas and Buseck, 1983). Annual average data for Africa are rare. Short-term concentrations of dust in eastern Australia (1986-1993) range from 2 to 936 i z g / m 3 at four stations. At two stations (Buronga and Charleville), concentrations of dust exceeded 100 i z g / m 3 on ten occasions in five years and on four of these occasions the 24-h environmental health standard of 260 p , g / m 3 was exceeded (NHMRC, 1985). In West Africa, short-term concentrations of dust are much higher. Twelve-hour measurements of the concentration of dust by Gillies et al. (1996) in Mali range from 575 p , g / m 3 for background dust, to an incredible 12,918 p , g / m 3 during a Harmattan dust plume event. 4.2. Dust processes during the Last Glacial Maximum (LGM) Although direct measures of dust activity during the LGM are not available, a variety of evidence indicates that environmental conditions were conducive to high levels of dust activity (Wasson, 1987). The effect of present-day environmental conditions (wind energy-soil moisture/vegetation relationships) upon dust activity (measured by the occur-
rence of dust storms) are described by the Et index of McTainsh and Lynch (1996). Climatic conditions during the LGM, described by Wasson (1987), are the estimates used in the Et index to quantify LGM dust activity. This analysis does not, however, describe the influence of the supply of sediment upon past dust processes, which according to McTainsh (1985, 1989) is an important aspect of Quaternary dust processes. The Australian arid zone is relatively well supplied with wind-blown sands, which cover 40% of desert lands, compared with 20% of the Sahara (Williams, 1984). Sand supply in North American deserts is likely to be much lower. The supply of fine sediments (silts and clays) may, however, be more limited in Australia because of the absence of largescale glaciations in Australia which results instead in longer periods of exposure of Australian soils and sediments to erosion processes. Alluvial sediments within the internally draining rivers of the Lake Eyre and Murray-Darling Basins may have been the main source of fines for the southeast dust path (McTainsh, 1989). Although the supply of these sediments would be affected by climatic change, this aspect is not quantitatively described here. 4.2.1. Measures of dust activity Direct measures of dust (concentration and deposition of dust) better describe dust processes than meteorological records of the occurrence of dust events. But direct measures are currently available for only a few years and at a few stations. By contrast, meteorological data on the occurrence of dust events (dust storms) are available for 41 stations in eastern Australia since 1960 and have, therefore, been used to calibrate broadscale dust models such as the Et index. The frequency of dust storms is the most commonly used meteorological measure of wind erosion because records of these large events are more reliable than for smaller events. Because smaller events are not used, however, the frequency of dust storms underestimates total rates of wind erosion. A relationship exists between meteorological records and direct measures (concentration of dust) for the limited period for which data are available. Fig. 4 shows a good relationship between average
G.H. McTainsh, A. W. Lynch/Geomorphology 17 (1996) 263-271 60
t
267
4.2.2. Et index of wind erosion The Et index, used here in a geological context,
500.75
0.5 30
0.25
)i o
10 .
.
.
.
.
z
0
Month
Fig. 4. Average monthly occurrence of dust haze in relation to the concentration of dust at Charleville, Queensland (1987-1991).
monthly occurrence of dust haze and concentrations of dust at Charlevilli;, Queensland over the period 1987-1991 ( r 2 = 0.87). In Fig. 5 the relationship with average monthly frequency of dust storms is not quite as good in stat:istical terms ( r 2 = 0.58) but a clear temporal trend exists. The closeness of these relationships between meteorological records and more direct measures of dust activity justifies our assumption that meteorological measures can be used to reconstruct dust activity in the geological past.
60 ,
3.0
))
2.5
[]
1.5
~
1.o ~ [
estimates how the changed environmental conditions during the LGM affected dust storm activity in two regions: northeast and southeast Australia. In the northeast region the dust storm season is in late spring and early summer, whereas in the southeast region dust storms are most prevalent in summer. Briefly, the Et index describes interactions between wind energy and effective soil moisture (also a surrogate for vegetation cover) and the influence upon monthly average frequencies of dust storms. The index has a modular structure, in which each module describes a particular set of soil-moisturewind-energy interrelationships that apply at certain times of the year. The basic module describes the interactions of wind and effective soil moisture as described by the Ew index of McTainsh et al. (1990), which is similar in structure to the climatic index of Chepil et al. (1963). Ew = W( P - E) -2
(1)
where W = mean annual wind run, and P - E = Thomthwaite's (1931) index. Antecedent conditions of soil moisture are described by two modules: an Ewt module which describes a wetting curve by factoring the potential wind erosion for the month by the relatively low soil moisture in the previous month, and an Ewd module which describes a drying curve, taking account of the relatively high soil moisture in the previous month. A threshold of wind energy is also used in some modules. Below the threshold the wind factor (W) is deleted, and either of the antecedent soil moisture effects (wetting and drying) are described, or if absolute soil moisture levels are low, the Ew module is used. Fig. 6 illustrates the modular structure of the Et index for northeastern Australia. This model is described in more detail by McTainsh and Lynch (1996). 4.2.3. LGM climate scenario
,
,
i
i
,
0.0
Month
Fig. 5. Averagemonthly occurrence of dust storms in relation to the concentration of dust at Charleville, Queensland ( 1987-1991).
An ongoing study of climatic changes in Australia during the Quaternary (CLIMANZ) (Wasson, 1987) estimates that mean annual rainfall was decreased by > 40%, windiness was increased by > 20% and temperatures were reduced by up to 8°C (Jouzel et al., 1987). The LGM climate change scenario used
268
G.H. McTainsh, A.W. Lynch/Geomorphology 17 (1996) 263-271
0.30
4-
' " ' N .....
/
~2
f200 i
1~o~
t
0.30 t 0.25
/ I
~
9/ \
-
"~"
\
•
t ro2o
~
0.15
! Windrun
/,.... Ol0
//
[oo'
L./
/
o
Ew ., ~wt '. Ewa'.~7,~:~7 , , I , , , I , I-', ; ', 140
0.25
0.00 / i i
i
[ i
Month
i
i i
i
i i
i
0.00
Month
Fig. 6. Modular structure of the Et index in northeast Australia (after McTainsh and Lynch, 1996).
Fig. 7. Average monthly frequencies of dust storms for northeast Australia during the Last Glacial Maximum and at present.
here, conservatively assumes a rainfall decrease of 40%, an increase in windiness of 20% and a temperature decrease of 8°C. We would expect that the decrease in rainfall and increase in wind energy would have increased dust activity. But the decreased temperatures would have had the opposite effect by decreasing evaporation rates, increasing soil moisture and reducing dust activity. The interactions of these effects are described here by the Et index.
increased by 52% during the LGM. The seasonality of dust storms was also affected (Fig. 8). During the main dust storm season (December to March), dust storms were 15% higher than at present, whereas during the low dust storm season (May to August) an increase of 473% in dust storm frequency occurred. This increase, like that in the northeast region, is in response to increased windiness which raised these months above the threshold of wind energy of the Et index. These changes produce a less pronounced dust storm season than in the northeast. Dust concentrations. The changes in dust activity
4.2.4. L G M dust processes Dust storm activity. The effects of Wasson's (1987) climate change scenario upon dust activity are examined separately in northeast and southeast Australia. In the northeast dust storm region, the Et index predicts that the annual average frequencies of dust storms would have been 57% higher during the LGM. The seasonal occurrence of dust storms would also have been different (Fig. 7). During the main dust storm season (September to January), dust storms were 38% higher than at present, whereas during the low dust storm season (March to June), an 199% increase in dust storm activity (albeit from low absolute levels) was predicted. The latter increase occurs because of increased windiness during the LGM which raised the low dust storm months above the threshold of wind energy of the Et index. In the southeast dust storm region, the annual average frequencies of dust storms would have been
tE
r.
0.20
0.20
0.15
~ 0.10
0.05
•" \
0.00
,
i
A'56'
I °~5
! i / ½"
V, lO
"
005
i , , ~.
\
i
i
• , ,o-000
Month
Fig. 8. Average monthly frequencies of dust storms for southeast Australia during the Last Glacial Maximum and at present.
G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271
measured in terms of the frequency of dust storms provide only a relative measure of dust activity during the LGM. Absolute levels of dust are more difficult to determine because the frequency of dust storms underestimate,; the total amount of wind erosion. Dust activity can be approximated in terms of concentration of dust (a more widely accepted measure) using the limited data available on the relationship between the frequency of dust storms and concentration of dust at Charleville (Fig. 5). Applying that relationship to the northeast region, the annual average concentrations of dust would be increased by 32% to 29.3 ~ g / m 3. Allowing for the conservative nature of these estimates, dust activity in Australia during the LGM was probably lower than current dust levels in West Africa. 4.3. Dust paths during the LGM
The increased dust activity throughout eastern Australia would have significantly increased the dust loads within the southeast dust path (Fig. 1). The northeast dust storm region would also have increased the dust loads within the northwest path. 4.3.1. Dust deposition, soils and ocean deposits If we assume that rates of dust deposition on land were increased by the same proportion as the frequencies of dust storms during the LGM, maximum rates of deposition could have increased to 82 t / k m 2. Although these LGM rates are still low by present-day dust standards in West Africa, they may well have been high enough to make significant contributions to soils on the margins of the arid zone. The role of the deposition of dust in soil formation is not widely acknowledged in Australia because, with the exception of the parna soils on the Riverine Plain of southern New South Wales (Butler, 1956), the soil evidence is difficult to interpret. Fried (1993) estimated that the rate ot parna soil deposition on part of the Riverine Plain during the LGM was 34 t / k m 2. We would expect this amount of soil deposition to be lower than the 82 t/krn 2 estimated rate of dust deposition, because not all deposited dust would be stabilised as soil. Yaaion and Dan (1974) estimate that 50% of dust deposited onto Israeli soils has not been remobilised locally. Therefore, taking 50% of the rate of dust deposition gives a rate of soil deposi-
269
tion of 41 t//km 2, which is close to Fried's rate of 34 t / k m 2. Chartres et al. (1988) estimated that the dust contribution to soils in the highlands to the east of the Riverine Plain was in the range of 20-70% of total soil material. The role of dust contributions to the ocean is similarly not well documented in the Australian region. McTainsh (1989) proposed that Australian dust has made significant contributions to the ocean sediments in the Australasian region, based upon spatial correlations between the distribution of clays within the ocean and the dust paths. This hypothesis receives support from the present study, plus a recent study by Hesse (1993), which identified dust within Tasman Sea sediments and concludes that rates of deposition during the LGM were significantly higher than at present. 4.4. Sediment supply to southeast dust path
During the LGM, the rates of sediment transport within the internally draining river systems of the Lake Eyre and Murray-Darling Basins, which feed the source areas of the southeast dust path, may have been significantly higher than sediment discharges from present-day well vegetated upper catchments. In addition, supplies of alluvial sediment to the southeast dust path would have been enhanced by a sediment feedback relationship between aeolian and alluvial systems in which dust deposition over the eastern headwaters of the Murray-Darling Basin supplied fine sediment to the rivers which in turn discharged westwards to resupply dust source areas (McTainsh, 1989). According to Fried (1993), contribution of dust to LGM rivers in southeast Australia is the major factor in explaining the change in river morphology from straight bedload to sinuous suspended load rivers during the LGM. Reconstructions of the southeast dust path during the LGM, based upon present-day sediment supplylimited conditions, can, therefore, significantly underestimate actual dust activity. The northwest dust path would have been less affected by changes in sediment supply because internally draining rivers play a less significant role in sediment supply. 4.5. Dust and future global change?
These results provide a first quantitative estimate of how past climatic changes affected dust activity.
270
G.H. McTainsh, A. W. Lynch / Geomorphology 17 (1996) 263-271
When the reliability of future predictions of climatic change for the Australian region is improved, the future impacts of climatic change upon dust activity could be predicted using the methodology described here. Current scenarios of global wanning for the year 2030 (CSIRO, 1994) in eastern Australia give a temperature increase of 1.5-4.5°C with a summer rainfall increase of about 20% and in the southeast a winter rainfall decrease of about 10%. Based upon LGM scenario calculations, the increase in evaporation rates from global warming are likely to increase dust activity, but only slightly. The summer rainfall increase would, however, significantly reduce dust activity during the main dust storm season. The winter rainfall decrease would increase the potential for wind erosion, but if wind energy remained below the threshold during this period, the actual increases in wind erosion would only be slight. On balance, therefore, according to this scenario of global warming, dust activity is more likely to decrease than increase in the year 2030. When the reliability and spatial resolution of the predictions of global warming improve, future predictions of these effects upon dust activity appear feasible. This would give validity to the notion of Doe (1983) that "the past is the key to the future".
5. Conclusions Present-day suspended concentrations of dust and rates of deposition in Australia are similar to the USA, but lower than West Africa. Levels of dust activity during the Last Glacial Maximum are estimated, using LGM climate scenarios along with the Et index of present-day wind erosion. Dust activity during the LGM was increased by 57% in northeast Australia and by 52% in the southeast, in relation to present day. Also, the dust storm seasons were longer and dust paths were modified. These estimates, which do not take into account supplies of sediment to wind erosion, probably significantly underestimate LGM dust activity because past supplies of sediment to dust source areas may have been more prolific than at present. These quantitative indications of increased LGM dust activity provide support for the hypothesis that dust deposition was a significant
factor in soil formation in subhumid areas of eastern Australia, and that dust contributions to ocean deposits were substantial. This methodology has the potential for predicting the effects of future climatic change upon dust activity.
Acknowledgements This research has been supported by funds from the Land and Water Resources Research and Development Corporation (LWRRDC) and the Australian Research Council (ARC).
References Bowler, J.M., 1976. Aridity in Australia: age, origins and expression in aeolian landforms and sediments. Earth Sci. Rev., 12: 279-310. Butler, B.E., 1956. Parna--an aeolian clay. Aust. J. Soil Res., 18: 145-151. Chartres, C.J., Chivas, A.R. and Walker, P.H., 1988. The affect of aeolian accessions on soil development on granitic rocks in south-eastern Australia, II. Oxygen-isotope, mineralogical and geochemical evidence for aeolian deposition. Aust. J. Soil Res., 26: 17-31. Chepil, W. S., Siddoway, F. H. and Armburst, D.V., 1963. Climatic index of wind erosion conditions in the Great Plains. Soil Soc. Am., Proc., 27(4): 449-452. CSIRO, 1994. Climate change scenarios for the Australian Region --issued November 1992. Climate Impact Group, CSIRO Division of Atmospheric Research, Paper presented at the Australia-New Zealand Climate Fornmr Toowoomba, February 1994. Doe, B.R., 1983. The past is the key to the present. Geochim. Cosmochim. Acta, 47: 1341-1354. Fried, A.W., 1993. Late Pleistocene river morphological change, southeastern Australia: the conundrum of sinuous channels during the Last Glacial Maximum. Palaeoecology, 101: 305316. Gillies, J.A., Nickling, W.G. and McTainsh, G.H., 1996. Sedimentological characteristics of an intense dust haze event: Inland Delta region of Mali, West Africa. Atmos. Environ., Vol. 30(7): 1081-1090. Hesse, P.P., 1993. A Quaternary record from aeolian dust in Tasman Sea sediments. PhD thesis, ANU, Canberra, 361 pp. Jouzel, J., Lorius, C., Petit, J.-R., Genthon, C., Barkov, N., Kotlyakov, V. and Petrov V., 1987. Vostok ice core: a continuous isotope temperature record over the last climatic cycle (160000 years). Nature, 329: 403-407. Knight, A.W., McTainsh, G.H: and Simpson, R.W., 1995. Sediment loads in an Australian dust storm: implications for present and past dust processes. Catena, 24: 195-213.
G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271 McTainsh, G.H., 1985. Dust processes in Australia and West Africa: a comparison. Search, 16(3-4): 104-106. McTainsh, G.H., 1989. Quaternary aeolian dust processes and sediments in the Australian region. Quat. Sci. Rev., 8: 235253. McTainsh, G.H. and Leys, J.F., 1993. Soil erosion by wind. In: G.H. McTainsh and W.C. Boughton (Editors), Land Degradation Processes in Auslralia. Longman-Cheshire, Melbourne, 383 pp. McTainsh, G.H. and Lynch, A.W., 1996. Climatic controls upon wind erosion in eastern Australia. J. Arid Environ. (in press). McTainsh, G.H. and Pitblado, J.R., 1987. Dust storms and related phenomena measured from meteorological records in Australia. Earth Surface Processes Landforms, 12(4): 415-424. McTainsh, G.H. and Walker, P.H., 1982. Nature and distribution of Harmattan dust. Z. Geomorphol., 26(4): 417-435. McTainsh, G.H., Lynch, A.W. and Burgess, R.C., 1990. Wind erosion in Eastern Australia. Aust. J. Soil Res., 28(2): 323-339. National Health and Medical Research Council (NHMRC), 1985. National Guidelines for the Control of Emissions of Air Pollutants from Stationary Sources. NH&MRC Australia.
271
Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London, 329 pp. Raupach, M.R., McTainsh, G.H. and Leys, J.F., 1994. Estimates of dust mass in recent major Australian dust storms. Aust. J. Soil Water Conserv., 7(3): 20-24. Thomas, E. and Buseck, P.R., 1983. Characterisation of a rural aerosol from eastern Arizona. Atmos. Environ., 17: 22992301. Wasson, R.J., 1987. Desert dune building dust raising and palaeoclimate in the Southern Hemisphere during the last 280 000 years. In: T.H. Donnelly and R.J. Wasson (Editors), Proceedings of the 3rd Symposium on Late Quaternary Climatic History of Australasia, Melbourne, pp. 123-137. Williams, M.A.J., 1984. Paleoclimates and paleoenvironments (A) Quaternary Environments. In: J.J. Veevers (Editor), Phanerozoic Earth History of Australia. Clarendon Press, Oxford, pp. 42-47. Yaalon, D.H. and Dan, J., 1974. Accumulation and distribution of loess-derived deposits in the semi-desert and desert fringe areas of Israel. Z. Geomorphol. N.F. Suppl., 20: 91-105.
Geomorphology 17 (1996) 263-271
Quantitative estimates of the effect of climate change on dust storm activity in Australia during the Last Glacial Maximum G.H. McTainsh *, A.W. Lynch Faculty of Environmental Sciences, Griffith University, Nathan, Brisbane, Queensland, 411, Australia Received 7 April 1994; revised 4 January 1995; accepted 17 February 1995
Abstract Quantitative estimates are made of the effect of climate change upon dust activity during the Last Glacial Maximum (LGM), which peaked 18,000 years ago, using present-day measurements of the occurrence of dust storms recalculated in the light of information on LGM climatic change. Dust activity was increased by 57% in northeast Australia and by 52% in the southeast, in relatioa to present day. Dust storm seasons were lengthened and dust paths were modified. Estimates based upon climate alone, without considering the increased supplies of sediment to dust source areas during the LGM, however, probably underestimate LGM dust activity.
1. Introduction Australia is a dry continent, but aridity is a relatively recent phenomenon in geological time. Aridity became widespread only in the Quaternary and most of the dunefields which today dominate the Australian outback, were formed in the last 700,000 years. (Wasson, 198"1). Recent studies of Quaternary dust processes (Bowler, 1976; McTainsh, 1989) indicate activity during recent geological time. Few reliable estimates exist, however, about how active past dust processes were. Such estimates can now be made using improved estimates of Quaternary climate (Wasson,
* Corresponding author.
1987) and recent advances in our knowledge of present-day dust processes in Australia. This paper will use a broad-scale climatic model of dust to examine dust activity in eastern Australia during the Last Glacial Maximum (LGM), which peaked 18,000 years ago.
2. Dust paths Our current understanding is that two major dust paths occurred in the Australian region during the Quaternary (Bowler, 1976). One passed to the northwest off the West Australian coast and another to the southeast out over the Tasman Sea heading towards New Zealand (Fig. 1). Recent studies of these pathways and associated dust processes indicate that, although they persist today, the position and intensity may be different from the past.
0169-555X/96/$15.00 C,opyright © 1996 Elsevier Science B.V. All rights reserved
SSDl O169-555X(95)O0106-9
264
G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271
3. Methods 3.1. Measurements o f dust processes
A number of studies on dust have been carried out over the past 10 years (summary in McTainsh and Leys, 1993) with the aim of developing a body of knowledge on present-day dust processes that can be extended back through the Quaternary to understand the role of dust in the evolution of the Australian landscape. Measurements of dust processes have been made using meteorological records of dust events. The Australia-wide pattern of dust events (Fig. 2) (McTainsh and Pitblado, 1987) provides initial support for the existence of the two dust paths, although evidence of the southeast path is patchy and reflects inadequacies of the meteorological data (McTainsh and Pitblado, 1987). An additional area of dust activity in Western Australia also exists. Direct measures of suspended dust by high volume air samplers and of deposited dust (by glass bead deposition traps) have been made at up to six
sites within the southeast dust path since 1985 (Fig. 3). These data provide initial estimates of dust loads and rates of deposition. Recent studies of individual dust storms in 1983 (Raupach et al., 1994) and in 1987 (Knight et al., 1995) and studies of the trajectories of dust plumes (Tapper and Wain, pers. commun., 1994) confirm the existence of the southeast dust path, and show that it covers most of eastern Australia.
4. Results and discussion 4.1. Australian dust in a world-wide context
Fundamental similarities exist between transport of dust in Australia and West Africa (McTainsh, 1985). Until recently, however, quantitative data were lacking in Australia to compare with the dust levels in Africa and the rest of the world. Levels of dust activity in Australia appear to be comparable with the USA, but significantly lower than in West Africa. Annual rates of dust deposition in eastern Australia
(. •
.
.
.
.
L : .'•'.I.?•U
- .i -i~ i
~ -
Longitudinal dune systems Dust
p a t h s
Fig. 1. Australian dust paths during the Quaternary(after Bowler, 1976).
265
G.H. McTainsh, A.W. Lynch / Geomorphology 17 (1996) 263-271
Fig. 2. Annual average frequency of dust events (1957-1984) (McTainsh and Pitblado, 1987).
-24"
NT ~J.
I I I I I I
141)"
146 "
152 "
OLD
3~"~
• Charteville
AusTRALIA ~I,,S ]
~
SOUTH
- - ~ ~ B R I S B A N E
SA
PACIFIC
O"S
• FowlersGap
/
OCEAN
• Gunnedah/ NSW
Iv'c "k
// i~"
,. Waoga-Wagga /
,, 152"
Fig. 3. Dust-monitoring sites in eastern Australia.
100 I
=="
0
I
5(Xlkm
i m m ~ ~ SCIALE ' ' I
I I
G.H. McTainsh, A. W. Lynch / Geomorphology 17 (1996) 263-271
266
Table 1 Annual average rates of dust deposition and concentrations of dust in eastern Australia Station
Average annual rate of deposition ( t / k m 2)
Annual average concentration of dust (t.Lg/m 3)
Charleville Fowlers Gap Buronga Gunnedah
31.4 43.8 33.0
25.7 27.6 41.3 17.4
range from 31.4 to 43.8 t km -z yr - l (Table 1), compared with a USA range of 0.8 to 126 t krn -2 yr -1 (Pye, 1987). In West Africa annual rates are consistently higher. In northern Nigeria annual rates of deposition of 137 to 181 t km -2 yr -1 were recorded by McTainsh and Walker (1982) and in Mali (W.G. Nickling, pers. commun., 1994) annual rates could be even higher. Annual average concentrations of suspended dust at four stations in Australia range from 17 to 41 p , g / m 3 (Table 1) compared with 35 to 55 ~ g / m 3 in the USA (Thomas and Buseck, 1983). Annual average data for Africa are rare. Short-term concentrations of dust in eastern Australia (1986-1993) range from 2 to 936 i z g / m 3 at four stations. At two stations (Buronga and Charleville), concentrations of dust exceeded 100 i z g / m 3 on ten occasions in five years and on four of these occasions the 24-h environmental health standard of 260 p , g / m 3 was exceeded (NHMRC, 1985). In West Africa, short-term concentrations of dust are much higher. Twelve-hour measurements of the concentration of dust by Gillies et al. (1996) in Mali range from 575 p , g / m 3 for background dust, to an incredible 12,918 p , g / m 3 during a Harmattan dust plume event. 4.2. Dust processes during the Last Glacial Maximum (LGM) Although direct measures of dust activity during the LGM are not available, a variety of evidence indicates that environmental conditions were conducive to high levels of dust activity (Wasson, 1987). The effect of present-day environmental conditions (wind energy-soil moisture/vegetation relationships) upon dust activity (measured by the occur-
rence of dust storms) are described by the Et index of McTainsh and Lynch (1996). Climatic conditions during the LGM, described by Wasson (1987), are the estimates used in the Et index to quantify LGM dust activity. This analysis does not, however, describe the influence of the supply of sediment upon past dust processes, which according to McTainsh (1985, 1989) is an important aspect of Quaternary dust processes. The Australian arid zone is relatively well supplied with wind-blown sands, which cover 40% of desert lands, compared with 20% of the Sahara (Williams, 1984). Sand supply in North American deserts is likely to be much lower. The supply of fine sediments (silts and clays) may, however, be more limited in Australia because of the absence of largescale glaciations in Australia which results instead in longer periods of exposure of Australian soils and sediments to erosion processes. Alluvial sediments within the internally draining rivers of the Lake Eyre and Murray-Darling Basins may have been the main source of fines for the southeast dust path (McTainsh, 1989). Although the supply of these sediments would be affected by climatic change, this aspect is not quantitatively described here. 4.2.1. Measures of dust activity Direct measures of dust (concentration and deposition of dust) better describe dust processes than meteorological records of the occurrence of dust events. But direct measures are currently available for only a few years and at a few stations. By contrast, meteorological data on the occurrence of dust events (dust storms) are available for 41 stations in eastern Australia since 1960 and have, therefore, been used to calibrate broadscale dust models such as the Et index. The frequency of dust storms is the most commonly used meteorological measure of wind erosion because records of these large events are more reliable than for smaller events. Because smaller events are not used, however, the frequency of dust storms underestimates total rates of wind erosion. A relationship exists between meteorological records and direct measures (concentration of dust) for the limited period for which data are available. Fig. 4 shows a good relationship between average
G.H. McTainsh, A. W. Lynch/Geomorphology 17 (1996) 263-271 60
t
267
4.2.2. Et index of wind erosion The Et index, used here in a geological context,
500.75
0.5 30
0.25
)i o
10 .
.
.
.
.
z
0
Month
Fig. 4. Average monthly occurrence of dust haze in relation to the concentration of dust at Charleville, Queensland (1987-1991).
monthly occurrence of dust haze and concentrations of dust at Charlevilli;, Queensland over the period 1987-1991 ( r 2 = 0.87). In Fig. 5 the relationship with average monthly frequency of dust storms is not quite as good in stat:istical terms ( r 2 = 0.58) but a clear temporal trend exists. The closeness of these relationships between meteorological records and more direct measures of dust activity justifies our assumption that meteorological measures can be used to reconstruct dust activity in the geological past.
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estimates how the changed environmental conditions during the LGM affected dust storm activity in two regions: northeast and southeast Australia. In the northeast region the dust storm season is in late spring and early summer, whereas in the southeast region dust storms are most prevalent in summer. Briefly, the Et index describes interactions between wind energy and effective soil moisture (also a surrogate for vegetation cover) and the influence upon monthly average frequencies of dust storms. The index has a modular structure, in which each module describes a particular set of soil-moisturewind-energy interrelationships that apply at certain times of the year. The basic module describes the interactions of wind and effective soil moisture as described by the Ew index of McTainsh et al. (1990), which is similar in structure to the climatic index of Chepil et al. (1963). Ew = W( P - E) -2
(1)
where W = mean annual wind run, and P - E = Thomthwaite's (1931) index. Antecedent conditions of soil moisture are described by two modules: an Ewt module which describes a wetting curve by factoring the potential wind erosion for the month by the relatively low soil moisture in the previous month, and an Ewd module which describes a drying curve, taking account of the relatively high soil moisture in the previous month. A threshold of wind energy is also used in some modules. Below the threshold the wind factor (W) is deleted, and either of the antecedent soil moisture effects (wetting and drying) are described, or if absolute soil moisture levels are low, the Ew module is used. Fig. 6 illustrates the modular structure of the Et index for northeastern Australia. This model is described in more detail by McTainsh and Lynch (1996). 4.2.3. LGM climate scenario
,
,
i
i
,
0.0
Month
Fig. 5. Averagemonthly occurrence of dust storms in relation to the concentration of dust at Charleville, Queensland ( 1987-1991).
An ongoing study of climatic changes in Australia during the Quaternary (CLIMANZ) (Wasson, 1987) estimates that mean annual rainfall was decreased by > 40%, windiness was increased by > 20% and temperatures were reduced by up to 8°C (Jouzel et al., 1987). The LGM climate change scenario used
268
G.H. McTainsh, A.W. Lynch/Geomorphology 17 (1996) 263-271
0.30
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/ I
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-
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! Windrun
/,.... Ol0
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i
[ i
Month
i
i i
i
i i
i
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Fig. 6. Modular structure of the Et index in northeast Australia (after McTainsh and Lynch, 1996).
Fig. 7. Average monthly frequencies of dust storms for northeast Australia during the Last Glacial Maximum and at present.
here, conservatively assumes a rainfall decrease of 40%, an increase in windiness of 20% and a temperature decrease of 8°C. We would expect that the decrease in rainfall and increase in wind energy would have increased dust activity. But the decreased temperatures would have had the opposite effect by decreasing evaporation rates, increasing soil moisture and reducing dust activity. The interactions of these effects are described here by the Et index.
increased by 52% during the LGM. The seasonality of dust storms was also affected (Fig. 8). During the main dust storm season (December to March), dust storms were 15% higher than at present, whereas during the low dust storm season (May to August) an increase of 473% in dust storm frequency occurred. This increase, like that in the northeast region, is in response to increased windiness which raised these months above the threshold of wind energy of the Et index. These changes produce a less pronounced dust storm season than in the northeast. Dust concentrations. The changes in dust activity
4.2.4. L G M dust processes Dust storm activity. The effects of Wasson's (1987) climate change scenario upon dust activity are examined separately in northeast and southeast Australia. In the northeast dust storm region, the Et index predicts that the annual average frequencies of dust storms would have been 57% higher during the LGM. The seasonal occurrence of dust storms would also have been different (Fig. 7). During the main dust storm season (September to January), dust storms were 38% higher than at present, whereas during the low dust storm season (March to June), an 199% increase in dust storm activity (albeit from low absolute levels) was predicted. The latter increase occurs because of increased windiness during the LGM which raised the low dust storm months above the threshold of wind energy of the Et index. In the southeast dust storm region, the annual average frequencies of dust storms would have been
tE
r.
0.20
0.20
0.15
~ 0.10
0.05
•" \
0.00
,
i
A'56'
I °~5
! i / ½"
V, lO
"
005
i , , ~.
\
i
i
• , ,o-000
Month
Fig. 8. Average monthly frequencies of dust storms for southeast Australia during the Last Glacial Maximum and at present.
G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271
measured in terms of the frequency of dust storms provide only a relative measure of dust activity during the LGM. Absolute levels of dust are more difficult to determine because the frequency of dust storms underestimate,; the total amount of wind erosion. Dust activity can be approximated in terms of concentration of dust (a more widely accepted measure) using the limited data available on the relationship between the frequency of dust storms and concentration of dust at Charleville (Fig. 5). Applying that relationship to the northeast region, the annual average concentrations of dust would be increased by 32% to 29.3 ~ g / m 3. Allowing for the conservative nature of these estimates, dust activity in Australia during the LGM was probably lower than current dust levels in West Africa. 4.3. Dust paths during the LGM
The increased dust activity throughout eastern Australia would have significantly increased the dust loads within the southeast dust path (Fig. 1). The northeast dust storm region would also have increased the dust loads within the northwest path. 4.3.1. Dust deposition, soils and ocean deposits If we assume that rates of dust deposition on land were increased by the same proportion as the frequencies of dust storms during the LGM, maximum rates of deposition could have increased to 82 t / k m 2. Although these LGM rates are still low by present-day dust standards in West Africa, they may well have been high enough to make significant contributions to soils on the margins of the arid zone. The role of the deposition of dust in soil formation is not widely acknowledged in Australia because, with the exception of the parna soils on the Riverine Plain of southern New South Wales (Butler, 1956), the soil evidence is difficult to interpret. Fried (1993) estimated that the rate ot parna soil deposition on part of the Riverine Plain during the LGM was 34 t / k m 2. We would expect this amount of soil deposition to be lower than the 82 t/krn 2 estimated rate of dust deposition, because not all deposited dust would be stabilised as soil. Yaaion and Dan (1974) estimate that 50% of dust deposited onto Israeli soils has not been remobilised locally. Therefore, taking 50% of the rate of dust deposition gives a rate of soil deposi-
269
tion of 41 t//km 2, which is close to Fried's rate of 34 t / k m 2. Chartres et al. (1988) estimated that the dust contribution to soils in the highlands to the east of the Riverine Plain was in the range of 20-70% of total soil material. The role of dust contributions to the ocean is similarly not well documented in the Australian region. McTainsh (1989) proposed that Australian dust has made significant contributions to the ocean sediments in the Australasian region, based upon spatial correlations between the distribution of clays within the ocean and the dust paths. This hypothesis receives support from the present study, plus a recent study by Hesse (1993), which identified dust within Tasman Sea sediments and concludes that rates of deposition during the LGM were significantly higher than at present. 4.4. Sediment supply to southeast dust path
During the LGM, the rates of sediment transport within the internally draining river systems of the Lake Eyre and Murray-Darling Basins, which feed the source areas of the southeast dust path, may have been significantly higher than sediment discharges from present-day well vegetated upper catchments. In addition, supplies of alluvial sediment to the southeast dust path would have been enhanced by a sediment feedback relationship between aeolian and alluvial systems in which dust deposition over the eastern headwaters of the Murray-Darling Basin supplied fine sediment to the rivers which in turn discharged westwards to resupply dust source areas (McTainsh, 1989). According to Fried (1993), contribution of dust to LGM rivers in southeast Australia is the major factor in explaining the change in river morphology from straight bedload to sinuous suspended load rivers during the LGM. Reconstructions of the southeast dust path during the LGM, based upon present-day sediment supplylimited conditions, can, therefore, significantly underestimate actual dust activity. The northwest dust path would have been less affected by changes in sediment supply because internally draining rivers play a less significant role in sediment supply. 4.5. Dust and future global change?
These results provide a first quantitative estimate of how past climatic changes affected dust activity.
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G.H. McTainsh, A. W. Lynch / Geomorphology 17 (1996) 263-271
When the reliability of future predictions of climatic change for the Australian region is improved, the future impacts of climatic change upon dust activity could be predicted using the methodology described here. Current scenarios of global wanning for the year 2030 (CSIRO, 1994) in eastern Australia give a temperature increase of 1.5-4.5°C with a summer rainfall increase of about 20% and in the southeast a winter rainfall decrease of about 10%. Based upon LGM scenario calculations, the increase in evaporation rates from global warming are likely to increase dust activity, but only slightly. The summer rainfall increase would, however, significantly reduce dust activity during the main dust storm season. The winter rainfall decrease would increase the potential for wind erosion, but if wind energy remained below the threshold during this period, the actual increases in wind erosion would only be slight. On balance, therefore, according to this scenario of global warming, dust activity is more likely to decrease than increase in the year 2030. When the reliability and spatial resolution of the predictions of global warming improve, future predictions of these effects upon dust activity appear feasible. This would give validity to the notion of Doe (1983) that "the past is the key to the future".
5. Conclusions Present-day suspended concentrations of dust and rates of deposition in Australia are similar to the USA, but lower than West Africa. Levels of dust activity during the Last Glacial Maximum are estimated, using LGM climate scenarios along with the Et index of present-day wind erosion. Dust activity during the LGM was increased by 57% in northeast Australia and by 52% in the southeast, in relation to present day. Also, the dust storm seasons were longer and dust paths were modified. These estimates, which do not take into account supplies of sediment to wind erosion, probably significantly underestimate LGM dust activity because past supplies of sediment to dust source areas may have been more prolific than at present. These quantitative indications of increased LGM dust activity provide support for the hypothesis that dust deposition was a significant
factor in soil formation in subhumid areas of eastern Australia, and that dust contributions to ocean deposits were substantial. This methodology has the potential for predicting the effects of future climatic change upon dust activity.
Acknowledgements This research has been supported by funds from the Land and Water Resources Research and Development Corporation (LWRRDC) and the Australian Research Council (ARC).
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G.H. McTainsh, A.W. Lynch/ Geomorphology 17 (1996) 263-271 McTainsh, G.H., 1985. Dust processes in Australia and West Africa: a comparison. Search, 16(3-4): 104-106. McTainsh, G.H., 1989. Quaternary aeolian dust processes and sediments in the Australian region. Quat. Sci. Rev., 8: 235253. McTainsh, G.H. and Leys, J.F., 1993. Soil erosion by wind. In: G.H. McTainsh and W.C. Boughton (Editors), Land Degradation Processes in Auslralia. Longman-Cheshire, Melbourne, 383 pp. McTainsh, G.H. and Lynch, A.W., 1996. Climatic controls upon wind erosion in eastern Australia. J. Arid Environ. (in press). McTainsh, G.H. and Pitblado, J.R., 1987. Dust storms and related phenomena measured from meteorological records in Australia. Earth Surface Processes Landforms, 12(4): 415-424. McTainsh, G.H. and Walker, P.H., 1982. Nature and distribution of Harmattan dust. Z. Geomorphol., 26(4): 417-435. McTainsh, G.H., Lynch, A.W. and Burgess, R.C., 1990. Wind erosion in Eastern Australia. Aust. J. Soil Res., 28(2): 323-339. National Health and Medical Research Council (NHMRC), 1985. National Guidelines for the Control of Emissions of Air Pollutants from Stationary Sources. NH&MRC Australia.
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Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, London, 329 pp. Raupach, M.R., McTainsh, G.H. and Leys, J.F., 1994. Estimates of dust mass in recent major Australian dust storms. Aust. J. Soil Water Conserv., 7(3): 20-24. Thomas, E. and Buseck, P.R., 1983. Characterisation of a rural aerosol from eastern Arizona. Atmos. Environ., 17: 22992301. Wasson, R.J., 1987. Desert dune building dust raising and palaeoclimate in the Southern Hemisphere during the last 280 000 years. In: T.H. Donnelly and R.J. Wasson (Editors), Proceedings of the 3rd Symposium on Late Quaternary Climatic History of Australasia, Melbourne, pp. 123-137. Williams, M.A.J., 1984. Paleoclimates and paleoenvironments (A) Quaternary Environments. In: J.J. Veevers (Editor), Phanerozoic Earth History of Australia. Clarendon Press, Oxford, pp. 42-47. Yaalon, D.H. and Dan, J., 1974. Accumulation and distribution of loess-derived deposits in the semi-desert and desert fringe areas of Israel. Z. Geomorphol. N.F. Suppl., 20: 91-105.