Absence of caesium-137 from recent sediments in eastern Australia — Indications of catchment processes?

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Cremlingen 1991

ABSENCE OF CAESIUM-137 F R O M RECENT S E D I M E N T S IN EASTERN AUSTRALIA I N D I C A T I O N S OF C A T C H M E N T PROCESSES? E Bishop, Clayton, B. Campbell, Lucas Heights, C. McFadden, Sydney Summary

1

Sediment and soil containing the isotope caesium-137 (137Cs) are generally understood to have been sub-aerially exposed within the last 35 to 40 years. The horizon of first appearance of 137Cs in sedimentary deposits in Australia is equated with the mid-1950s, the time of first appearance of the isotope in Australia. Some sediments, however, which are known to have been deposited in alluvial cutoffs since the mid-t950s do not contain the isotope. This is interpreted as resulting from the high magnitude of the events which entrained and deposited the sediment. The sediment was eroded from sub-surface sites, and deposited and buried rapidly, thereby preventing the adsorption of the isotope. The data indicate that 137Cs should be used very cautiously as a dating tool in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available.

Caesium-137 (137Cs), an artificial radioactive isotope with a half life of 30 years, is a fallout product of atmospheric nuclear weapon testing. It falls out in rainfall and by gravitational settling and was first detected in Australia in 1955/56 (McCALLAN 1982). Total fallout in the southern hemisphere has been generally 10 or more times less than that in the northern hemisphere and so the temporal variation of 137Cs fallout in Australia "is represented by a relatively flat curve with minor peaks of similar amplitude in 1958, 1964 and 1971" (McCALLAN 1982, 311). The isotope is strongly adsorbed to clay, silt and organic matter in the nearsurface layers of soil and sediment. In depositional settings, sediment without 137Cs is usually interpreted as not having been within the near-surface zone of 137Cs adsorption since fallout started; conversely, it is generally understood that all sediment labelled with 137Cs must have been at or close to a subaerial surface since the mid-1950s (cf. L O U G H R A N et al. 1982, P O P P e t al. 1988). However, McCALLAN (1982) and DAVIS et al. (1984) have re-

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Bishop, Campbell & McFadden

62

ported 137Cs in sediments whose depo- ing periods of lower and less frequent sition quite clearly pre-dates the advent of nuclear weapons, and they attributed this to a range of mixing and molecular diffusion processes in the sediment column. McCALLAN (1982) suggested that these processes were partly dependent on the acid and saline conditions of the t w o lakes that she investigated. To our knowledge, the lack of 137Cs in post-1955 sediments has received little, if any, attention. Here we report on a reconnaissance survey of the 137Cs content of recent alluvial sediments in the Hunter valley, N.S.W., Australia. In some settings described here, sediments known to have been deposited since the mid-1950s lack 137Cs; we attribute this, in part, to the nature and magnitude of the catchment processes associated with the entrainment and subsequent deposition of the sediments.

2 2.1

Setting and methods Study area

The study area lies between Maitland and Morpeth on the floodplain of the lower Hunter valley, central eastern N.S.W. (fig. 1). The mean annual rainfall at Maitland is 760 mm. This figure is only a partial guide to annual rainfall, however, because the study area, along with much of eastern Australia over the last 150 years, has experienced alternating periods of relatively wet and relatively dry climates. W A R N E R (1987) and ERSKINE & W A R N E R (1988) have called the former, flood-dominated regimes (FDRs). These FDRs (18571900 and 1949 to the present) are characterised by more frequent and higher floods, and major channel change; they contrast with the preceding and interven-

floods, and channel stability (droughtdominated regimes - - DDRs). These regime changes are controlled by secular rainfall variations (PICKUP 1976, BELL & ERSKINE 1981, ERSKINE & BELL 1982, ERSKINE & WARNER 1988). The Hunter valley has been extensively cleared since it was first settled by Europeans in 1812. Considerable catchment degradation accompanied agricultural activities such as forest cutting, pasture clearing, and overstocking. Infestation by pests such as rabbits and prickly pear aggravated this degradation but eradication of the pests and, since 1949, the widespread implementation of soil conservation measures have meant that the yield of sediment from extra-channel sources is currently relatively low (GEARY & ERSKINE 1984, ERSKINE et al., in prep.). The current F D R has been marked, nonetheless, by massive influxes of sandy sediment into the lower Hunter study area, causing widespread sand deposition on the floodplain and marked aggradation of the bed of the lower Hunter (HOLMES & L O U G H R A N 1976, ERSKINE et al., in prep.). This sediment was derived from major, and locally catastrophic, erosion of the banks and bed of the upper Hunter and its tributaries, and was deposited rapidly in a series of large floods during the 1950s (GEARY & ERSKINE 1984, ERSKINE & MELVILLE 1984). On the lower Goulburn River, for example, the 1955 flood caused up to 40% enlargement of the channel by bench destruction and the erosion of 4.3M m 3 of terrace sediment (Water Resources Commission figures quoted by ERSKINE & MELVILLE 1984). When first settled by Europeans in the

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Absence of Caesium-137 from Recent Sediments

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Fig. 1: Study area showing the locations and ages of alluvial cutoffs, and the locations of auger holes. early nineteenth century, the study reach of the Hunter River was highly sinuous (sinuousity = 3.84). Since the 1970s, all of the meanders between Maitland and Morpeth have been cut off, resulting in a relatively straight channel of low sinuosity (1.38) (HOLMES & LOUGHRAN 1976, ERSKINE et al., in prep.) (fig. 1).

BRAY 1982). The relative fineness and well-sorted nature of this flood sediment mean that it is readily distinguishable from the underlying channel sediment dating from the time of formation of the cutoff (ERSKINE & MELVILLE 1982, ERSKINE et al., in prep.).

The cutting-off of meanders in the lower Hunter occurred in the late nineteenth and mid-twentieth centuries, dur2.2 Alluvial cutoffs ing the two FDRs recorded since EuCutoffs form when a river shortens its ropean settlement. Meanders were length by abandoning all or part of either breached in response to the higher and an individual bend, a meander loop or more frequent floods of the FDRs. The multiple loops. The ends of the cutoff are ages of these cutoffs and the known first plugged more or less rapidly by stream appearance of 137Cs in eastern Australia bedload, forming the classic ox-bow lake in the mid-1950s apparently make the or billabong, and the cutoff subsequently isotope ideal for determining rates of inreceives sediment only by surcharge of fill sedimentation. This was the original flood waters (ALLEN 1965, ERSKINE aim of the programme of measuring the & MELVILLE 1982, PAGE & MOW- isotope. CATENA--An Interdisciplinary Journal of SOIL SCIENCE--HYDROLOGY--(}EOMORPHOLOGY

I

Bishop, Campbell & McFadden

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2.3

Field and laboratory methods

One cutoff (King Island, 1890), which formed during the late nineteenth century FDR, and three cutoffs from the F D R of this century (Pitnacree, 1950; Narrowgut, 1952; Goulburn Grove, 1956) were selected for this survey (fig. 1). Only cutoffs that had closed rapidly after cutting off and had experienced minimal post-cutoff disturbance were investigated. Investigation of King Island cutoff was restricted to its upstream half because of the presence of a creek conveying local drainage water in the downstream section of the cutoff. Minimal work was carried out in the Narrowgut cutoff because it covers the very short time period between the Pitnacree and Goulburn Grove cutoffs. Seventeen auger holes were drilled by hand to the former channel bed in the four cutoffs, with only one hole being located at Narrowgut (fig. 1). Auger drilling results in virtually complete disruption of the fine structures of the sediment but sedimentary structures larger than about 2 cm are readily distinguishable. The auger holes were logged in the field immediately after drilling. The grain-size characteristics of the sediments were determined using field hand texturing (bolus manipulation) and described using the soil texture grades of N O R T H COTE (1971). Tab. 1 gives approximate clay content for the range of field texture grades encountered in this study. N O R T H C O T E (1971) has stressed that field texture is controlled by clay content but he lists a range of other important, but subsidiary, factors. The reconnaissance nature of this study, and particularly the relatively coarse scale of the sampling intervals in the auger samples, did not warrant greater detail on

the grain-size characteristics of the sediments. Field texture Approximate grade clay content (%) Sand Sandy loam Loam Clay loam Light clay Medium clay Heavy clay

Always <10, commonly <5 10 to 15 About 25 30 to 35 35 to 40 40 to 45 50 or greater

1 : Approximate clay content for the main field texture grades used in this study (after NORTHCOTE 1971, 26-28).

Tab.

Thirty three sediment samples from the auger holes were analysed for 137Cs activity. Sample selection was guided by the age of the cutoff, the stratigraphic (vertical) position within the cutoff infill, and the location of apparent hiatuses (time breaks) in this infill. Samples were collected from the material at and immediately underlying a stratigraphic (time) break, the presence of the break being generally interpreted from an abrupt sedimentary grain size change. Sample selection was generally restricted to the finer-grained (muddy) or organic sediments, given the known preferential adsorption of 137Cs by these materials. The method for determining caesium137 presence by gamma spectrometry has been described by L O U G H R A N et al. (1990). In summary, samples were ovendried at 100°C overnight. They were then crushed to pass a 2 mm sieve, thereby giving a grain-size distribution and packing geometry in the Marinelli beaker (part of the gamma-ray counting apparatus) similar to that used in the calibration of the counter. The gamma-ray

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Absence o f Caesium-137 from Recent Sediments

activity was detected using a hyperpure germanium crystal detector and samples were counted for up to 16 hours; the minimum activity of 137Cs detectable by the counting system is 0.2 millibecquerels per gram. It should be noted that the low levels of fallout of caesium in Australia mean that levels of the isotope in Australian soils and sediments would be expected to be quite low, even in ideal settings. The objective of the counting, however, was simply to check for the presence of the gamma-ray spectral peak associated with 137Cs activity and thereby to determine only the presence or absence of the isotope. The coarse scale of sampling and the disturbance of the fine scale sedimentation of the samples by augering did not justify the measurement of the precise level of 137Csactivity.

65

which is thickest over the pools in the old channel and at greatest distance from the present channel (e.g. auger hole 2, figs 1 and 2). In the entrance reach of the cutoff (auger holes 12, 13 and 1), this clay is abruptly overlain by loamy and more sandy sediments at about mid-depth in the infill. These sediments thin along the cutoff and, as in the younger cutoffs, represent flood sediments deposited during the current FDR. The lower 3 to 4 m of clay in the King Island infill, therefore, were deposited between 1890 and the early to mid-1950s, and the upper 1 to 4 m since about the mid 1950s, as a result of the arrival of generally sandy flood sediments in the study area.

4

137Cs c o n t e n t o f the infill

sediments 3

Cutoff infill stratigraphy

The auger hole data and generatised cutoff infilts are shown in fig. 2. Immediately after abandonment, deposition in all three cutoffs (to a lesser extent in Goulburn Grove) consisted of discontinuous peat or organic-rich sediment of varying grain size, indicative of the oxbow lake sedimentary environment formed by the cutting off. In the recent cutoffs (Pitnacree and Goulburn Grove), this initial organic-rich layer was succeeded immediately by extensive sandy deposits, particularly within the entrance reaches of the cutoffs. These are flood deposits, reflecting the influx of sandy sediment during the floods of the 1950s (HOLMES & LOUGHRAN 1976, ERSKINE et al., in prep.). In the King Island cutoff (1890), the basal organic-rich, muddy layers are thicker and are overlain by an extensive and relatively thick (up to 4 m) clay layer

Fig. 2 indicates the presence/absence of 137Cs in the cutoff infill sediments. There is no completely clear pattern in the distribution of the isotope in the sediments, unlike the findings of POPP et al. (1988). In Goulburn Grove, the isotope is not distributed in the expected way, given the age of the cutoff. In particular, it is lacking from the deepest and finestgrained sediments of auger hole 3 which were deposited in the oxbow lake immediately after cutting off in 1956. Subsequent sediments, however, contain 137Cs in a regular way. Several horizons in the Pitnacree and Narrowgut cutoffs both contain 137Cs but the pattern of its distribution is again unclear. The presence of the isotope in auger holes 7 and 8 is consistent with expectation from the age of the cutoff. The lack of t37Cs in the correlative horizons in auger hole 6, therefore, is striking, as is perhaps its absence from the shallow-

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66

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2: Infill stratigraphy of the alluvial cutoffs examined in this study (elevations to Australian Height Datum). The presence of more than one grain-size symbol in an horizon indicates that the more abundant symbol is being qualified by the less abundant (e.g. sandy loam). Fig.

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Absence o f Caesium-137 from Recent Sediments

est sample in auger hole 7. The absence of 137Cs in any of the sediment sampled from auger hole 6, however, may be interpreted as a grain-size effect, reflecting the poor adsorption of the isotope by sand fractions. This interpretation is borne out by auger hole 4 at Narrowgut (1952), where 137Cs is lacking from medium-depth sandy horizons but is present in shallower, organic-rich, sandy sediments. The King Island cutoff is noteworthy for its complete lack of 137Cs in all samples measured, be the samples finegrained or coarser (sandy). The samples from auger holes 1 and 12 that were measured for presence of the isotope were chosen so as to bridge the transition from the previous D D R to the current F D R in the early 1950s. Given the widespread influx of sandy sediment into the study area in the floods of the early 1950s (HOLMES & L O U G H R A N 1976, ERSKINE & W A R N E R 1988), the onset of the sandier sedimentation at about mid-depth in auger holes 12, 13 and 1 is taken to mark this transition. Yet none of the finer-grained sediments deposited during this F D R contains 137Cs. Auger hole 12 is striking in this regard.

5

Discussion

The absence of the isotope in any sediments measured for its presence in the King Island cutoff, irrespective of grainsize, cannot be explained by grain-size effects. Fig. 2 shows quite clearly that 137Cs is absent from clay and loam sediments in auger holes 12, 13 and 1. Nor can it be argued that no sediment has been deposited in the King Island cutoff since 1955. Flood mitigation works constructed since the 1955 flood have been designed so that the King Island cut-

67

off is the route for both initial floodflow from the Hunter in the early stages of a flood event and principal floodflow during a major flood event (CONNOR & H U L C O M E 1972). Records show that it has certainly operated as both since' levee construction during the present F D R (cf. SINCLAIR K N I G H T & PARTNERS 1981). Rather, the absence may be interpreted to result from the processes involved in the erosion, transport and deposition of the sediment during the current FDR. That is, the extensive erosion of bed and banks during the middle 1950s and subsequently (ERSKINE & MELVILLE 1984, GEARY & ERSKINE 1984), has yielded large amounts of sediment that do not contain 137Cs, either because the sediment was entrained and deposited before 1955, or because the sediment entrained after 1955 has largely been derived from sub-surface settings in the bed and banks. Erosion of these settings would largely yield Csfree sediment, given the tendency for the isotope to be strongly bonded to nearsurface sediment. Both of these explanations for the observed lack of 137Cs in post-1955 sediments also demand that, soon after deposition, the Cs-free sediment was itself rapidly covered by subsequent sedimentation. If it had not been rapidly sealed by subsequent deposition, it would have received 137Cs in the normal way. This interpretation is supported by the data from Goulburn Grove (1956) where the lack of 137Cs in the post-1956 sediments immediately overlying the old channel bed may be accounted for by the rapid burial of these initial post cutoff sediments by the overlying loamy sand deposit. The absence of the isotope in the most recent sediments, in the shallowest

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horizons of the infills, is presumably related to its very low fallout levels in eastern Australia since the mid-1970s (McC A L L A N 1982, fig. 1). Thus, the 137Cs content of the post1955 sediments can be interpreted in terms of the nature of the catchment processes in the study area. The lack of the isotope in the sediments cannot be interpreted either as indicative of deposition prior to 1955 or solely as a grain-size effect. The lack of 137Cs in post-1955 sediment sampled from the King Island cutoff is most likely the result of erosion from sub-surface sites, followed by rapid transport, deposition and burial of the sediment. It is also important to remember the low total amounts of the isotope that have fallen out over Australia since its first appearance. Low fallout amounts combined with high rates of sediment deposition will result in very low concentrations of the isotope. That is, the absence of a signature of the isotope may be a strong indicator of the catchment processes, as well as the low amounts of fallout. It is therefore clear that caution must be exercised in interpreting the 137Cs content of sediments that may have been entrained, transported and deposited under high energy conditions. It is also implicit in this interpretation that other recent sedimentary horizons in this study that lack the isotope may do so because of erosional and depositional processes, rather than because of deposition prior to 1955. That is, the horizon of first appearance of 137Cs, even in a sedimentary column that exhibits an 'orderly' or 'expected' content of the isotope, but which is composed partly of sediments derived and deposited under high energy conditions, cannot necessarily be equated with the mid-1950s.

6

Conclusion

Whereas the use of 137Cs as a dating tool is firmly-based in low energy depositional settings in Australia, such as reservoirs ( C A M P B E L L 1983), lakes (McC A L L A N 1982) or grassed or cultivated hillslopes ( L O U G H R A N et al. 1982), its use in high energy settings has received less attention. The investigation reported here partly redresses this imbalance. The lack of the isotope in some of the post-1955 sediments described here prompts caution with the use of the isotope in settings of high erosional, transportational and depositional energy. More generally, in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available, 137Cs should be used very cautiously as a dating tool. References BELL, F.C. & ERSKINE, W.D. (1981): Effectsof

recent increases in rainfall on floods and runoff in the upper Hunter Valley. Search 12, 82-83. CAMPBELL, B. (1983): Applications of environmental Caesium-137for the determination of sedimentation rates in reservoirs and lakes and related catchment studies in developing countries. In: Radioisotopes in sediment studies. International Atomic Energy Agency Technical Document 298, 7-30. CONNOR, L.J. & HULCOME, W.E. (1972): Flood control works on the lower Hunter River, New South Wales. Institute of Engineers, Australia. Civil Engineering Transactions CE14, 69-75. DAVIS, R.B., HESS, C.T., NORTON, S.A.,

HANSON, D.W., HOAGLAND, K.D. & ANDERSON, D.S. (1984): 137Csand 21°pb dating of sediments from soft-water lakes in New England (U.S.A.) and Scandinavia, a failure of 137Cs dating. Chemical Geology 44, 151-185. ERSKINE, W. & BELL, F.C. (1982): Rainfall, floods and river channel changes in the upper Hunter. Australian Geographical Studies 20, 183-196.

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ERSKINE, W. & MELVILLE, M.D. (1982): Cutoff and oxbow lake. (a) On a straightsimulating river. Australian Geographer 15, 174-177. ERSKINE, W. & MELVILLE, M.D. (1984): Sediment transport, supply and storage in sandbed channels of the northern Sydney Basin. In: RJ. Loughran (Compiler), Drainage basin erosion and sedimentation. University of Newcastle, N.S.W., and Soil Conservation Service of N.S,W, 59-70. ERSKINE, W. & WARNER, R.F. (1988): Geomorphic effects of alternating flood- and drought-dominated regimes on NSW coastal rivers. In: R.E Warner (ed.), Fluvial geomorphology of Australia. Academic Press, Sydney, 223-244. ERSKINE, W., McFADDEN, C. & BISHOP, P. (in prep.): Alluvial cutoffs as indicators of former channel conditions. Earth Surface Processes and Landforms. GEARY, P.M. & ERSKINE, W. (1984): Sediment transport and channel changes in the Hunter River since European settlement. In: R.J. Loughran (Compiler), Drainage basin erosion and sedimentation. University of Newcastle, N.S.W., and Soil Conservation Service of N.S.W., 51-58. HOLMES, J.tL & LOUGHRAN, R. (1976): Man's impact on a river system: the Hunter Valley. In: J.H. Holmes (ed.), Man and environment: regional perspectives. Longman, Hawthorn, 96-114. LOUGHRAN, R.J., CAMPBELL, B.L. & ELLIOTT, G.L. (1982): The identification and quantification of sediment sources using t37Cs. In: Recent developments in the explanation and prediction of erosion and sediment yield. Proceedings Exeter Symposium, July 1982. IAHS Publication 137, 361-369. LOUGHRAN, R.J., CAMPBELL, B.L., ELLIOTT, G.L. & SHELLY, D.J. (1990): Determination of the rate of sheet erosion on grazing land using caesium-137. Applied Geography 10, 125-133.

McCALLAN, M.E. (1982): The Caesium-137 dating technique and associated applications in Australia - - A review. In: W. Ambrose & P. Duerden (eds.), Archaeometry: an Australian perspective. Department of Prehistory, Australian National University, Canberra, 310-321. NORTHCOTE, K.H. (1971): A factual key for the recognition of Australian soils. 3rd edition.

69

Rellim Technical Publications, Adelaide, South Australia. PAGE, K.J. & MOWBRAY, P.D. (1982): Cutoff and oxbow lake. (b) On a meandering river. Australian Geographer 15, 177-180. PICKUP, G. (1976): Geomorphic effects of changes in river runoff, Cumberland Basin, N.S.W. Australian Geographer 13, 188-193. POPP, C.J., HAWLEY, J.W., LOVE, D.W. & DEHN, M. (1988): Use of radiometric (Cs-137, Pb-210), geomorphic, and stratigraphic techniques to date recent oxbow sediments in the Rio Puerco drainage Grants uranium region, New Mexico. Environmental Geology and Water Sciences 11, 253-269. SINCLAIR KNIGHT & PARTNERS (1981): Hunter Valley, New South Wales Coastal Rivers Flood Plain Management Studies, 123 p. WARNER, R.F. (1987): Spatial adjustments to temporal variations in flood regime in some Australian rivers. In: K. Richards (ed.), River channels, environments and processes. Blackwell, London.

Addresses of authors: Paul Bishop Department of Geography University of Sydney N.S.W. 2006 Australia

Present address: Department of Geography & Environmental Science Monash University Clayton VIC. 3168 Australia

Bryan Campbell Australian Nuclear Science and Technology Organisation Private Mail Bag 1 Lucas Heights N.S.W. 2234 Australia

Christopher McFadden Department of Geography University of Sydney N.S.W. 2006 Australia

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