holocene geomagnetic secular variation records from...

Geophys. J. R. astr. Soc. (1985) 81, 103-120

Holocene geomagnetic secular variation records from north-eastern Australian lake sediments

c. G. Constable*and M. w. McElhinny? Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia

Accepted 1984 September 21. Received 1984 April 25

Summary. Secular variation records have been obtained from cores from Lakes Barrine and Eacham, two north-eastern Australian volcanic crater lakes. The results from several cores have been stratigraphically correlated and then stacked and smoothed. The chronology provided by radiocarbon dating indicates that the Lake Eacham sequence spans the last 5700 calendar years. The time-scale for the Lake Barrine record is less weil constrained but it appears to cover about 1600 to 16 200yr B P .

VGP paths for the sites show two periods of anticlockwise motion between about 5710 and 3980 B P and 10500 and 8800 B P . These times correspond to periods of anticlockwise motion in south-eastern Australian records (Barton & McElhinny) and Argentine records (Creer et al.), to within the uncertainties of the assigned time-scales.

Introduction

Under suitable circumstances fine grained material deposited in lake sediments can provide a record of the ambient geomagnetic field in its depositional or post-depositional remanent magnetization (DRM or PDRM). This record serves to extend knowledge about the geomagnetic field back beyond the age of the earliest historical records, which only span a few centuries in most parts of the world. The sedimentary record is continuous (unlike archaeomagnetic records), but much poorer in quality than that obtained from observatory instruments because of the smoothing inherent in the signal recording process. Slumping and disturbance of the sediment in situ or during coring will contribute to noise in the signal so that stacking and smoothing of data from a number of cores is necessary. Wherever possible it is desirable to substantiate the record by sampling coeval sequences from a number of sites.

Creer & Tucholka (1983) have reviewed the current state of lake sediment palaeomagnetic research. Records covering a substantial period of time are now available from a *Present address: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA. t Present address: Division of Geophysics, Bureau of Minerd Resources, PO Box 378, Canberra City, ACT 2601, Australia.

104 considerable number of sites in both Europe and North America (eg. Turner & Thompson 1981; Mothersill 1979, 1981; Lund & Banerjee 1979; Creer, Anderson, Lewis 1976; Creer et al. 1979, 1980; Creer, Readman & Papamarinopoulos 1981; Stober & Thompson 1977; Turner, Evans & Hussin 1982). The data from these sites typically show large amplitude well defined swings in the declination records and smaller less well defined inclination swings. Previous Australian work by Barton & McElhinny (1981) has provided records in which inclination records are substantially larger than the declination ones. Barton & McElhinny have compared the British and Australian secular variation paths over the past l 0000yr and concluded that there is no obvious correlation between them (and hence no common dipole wobble component). Creer & Tucholka (1982) have compared British and North American results and similarly concluded that there is no simple relationship between records from these two continents. Other southern hemisphere data come from Argentine lake sediments. This does not appear to correlate well with previous Australian data (Creer et al. 1983).

This paper reports results from two volcanic crater lakes situated on the Atherton Tableland of North Queensland, Australia.

C. G. Constable and M. W. McElhinny

Site description

Fig. 1 indicates the locations of Lakes Barrine (17'15'S, 145'37'E) and Eacham (17'17'S, 145"37'E), the two lakes sampled for this study. The Atherton Tableland is a basaltic lava flow with over 40 eruption points on it, varying in age from late Pleistocene to Holocene. The ages of the Barrine and Eacham craters are not known but they show more signs of weathering than Euramoo, a nearby maar whose basal organic sediments have been dated at around 10000yr B P (Timms 1976). Lynch's Crater, another site on the Tableland, is estimated to span at least 60 000 yr (Kershaw 1974). Eacham and Barrine are probably intermediate in age between Euramoo and Lynch's Crater.

Both lakes are approximately 65 m deep as the centre and surrounded by crater rims of pyroclastics with low outer and high inner slopes. The catchments extend only as far as the crater rims. Neither lake has any inlet stream, but Lake Barrine is drained by a creek on its south-eastern margin. These lakes could thus be expected to provide the ideal quiet sedimentary environment for the acquisition of a record of the geomagnetic signal.

Core descriptions

Seven 1 2 m Mackereth cores (Mackereth 1958) were obtained from Lake Barrine using the 6/12 m convertible corer described by Barton & Burden (1979). Four 6 m Mackereth cores from Lake Eacham were also studied.

Fig. 2 shows a typical core log from Lake Barrine alongside the horizontal intensity of magnetization. The sediment may be divided into four main zones as described in the f!gure. The cause of the laminations in the uppermost section is as yet unknown, but it is possible that they are seasonable in origin (D. Walker, private communication).

In contrast to the material from Lake Barrine the top section of the Lake Eacham cores is unlaminated and consists of almost featureless brown mud. This is somewhat surprising in view of the general similarity of the environments and close proximity of the two lakes. Below about 1 m from the surface occasional fine clay bands start to appear and by a depth of 2 m there are strongly laminated sections interspersed with unlaminated material. This section is broadly similar in character to the material found in the 3-7.5 m section of the Lake Barrine cores. Grain sizes of the sediments generally lie in the clay-fine silt range.

Holocene records from Australian lakes 105

Figure 1. Location of Lakes Barrine and Eacham.

Intra-lake core matching

The stacking of directional data from a number of cores from the same lake requires a reliable method of identifying equivalent horizons. Susceptibility and magnetic remanence profiles provide a convenient means of doing this. Fig. 3 shows the individual specimen susceptibility measurements which were used as a basis for the matching of the Lake Barrine cores. The Lake Eacham cores were matched using their sedimentary stratigraphy which was in complete agreement with the magnetic stratigraphy. Generally the same levels could be identified to within a few centimetres in each core. The most notable exception to this was at depths beyond about 8 m in the Lake Barrine cores, where the correlations between cores were somewhat obscure. Depth scales for each core were transposed to those of a lake master core using the correlatable horizons. Linear interpolation was used to reset depths between tie points.

106 C. G. Constable and M. W. McElhinny

Corm B1

flnely laminoted claye noterial, dork brawn and khaki, interspersed w i t h dork brown .mlaminated section (-)

gbscurely bonded, moat

dork brown, slightly sllty clay.

sectioning of core tube

reddish broun mud - m fine laminae.

lighter broun banded I laminated clays

graduol dorkening in colau

almost blach I metrotifled mud

HOR I NTENS I T Y ( Y A / U ) 0

0.8 I I 0 103 1030 HOR I N T E N S I T Y ( Y A / M )

91 WC

Figure 2. Log of typical Lake Barrine 12 m core. Note the correlation between changes in the sediment and intensity of magnetization.

Chronology

The establishment of an absolute time-scale for the acquisition of magnetic remanence is crucial if secular variation data from different lakes and regions are to be compared. Age control in lake sediment studies is usually achieved by radiocarbon dating of the organic fraction of the sediment. However, there is no guarantee that the organic carbon in the sediment was contemporaneous at the time of deposition. Systematic incorporation of ancient carbon in the sediment will result in a radiocarbon age which is depressed compared with the true age. This effect has been reported by a number of authors (e.g. Davis 1969;


( a ) BI SUSC

Li

( b ) 83 SUSC ( c ) 84 SUSC ( d ) 85 SUSC ( e ) 87 susc

Figure 3. Individual specimen susceptibilities for the five Lake Barrine cores which were subsampled. Lines join pairs of data at the same stratigraphic level in the core. Units are mu ~ m - ~ .

Kendall 1969; Barton & Barbetti 1982). It can be detected if the apparent surface age of the sediment is depressed or (in cases where reworking of old material has occurred) if age fails to increase monotonically with depth in the sediments. A further source of error in secular variation age control arises because of uncertainty about how long the sediment will take to acquire a PDRM. Tucker (1979, 1980, 1981) has suggested that this will depend on the relative grain sizes of the remanence carriers and the sedimentary matrix, as well as other factors such as water content and the effects of slumping or bioturbation.

It is now generally accepted that conventional radiocarbon ages have to be calibrated in order to compensate for fluctuations in the concentration of 14C in the atmosphere in the course of time. Also, the dates are normally calculated using a half-life for I4C of 5568yr instead of the currently accepted value of 5730 yr. This correction is included in the calibration. Calibration has been carried out here using Clark's (1975) scheme which only extends back to 6500 radiocarbon yr B P . More recent calibration tables (Klein et al. 1982) range from 10 to 7240 radiocarbon yr BP. However, this range is still not sufficiently long for the Lake Barrine records.

L A K E E A C H A M

A total of 11 samples were dated from Lake Eacham sediment, most of which were 20 cm sections of core. These were transposed to equivalent depths in the lake master core as described in the previous section. A least squares unweighted linear regression was used to provide an age-depth correlation and is shown in Fig. 4 along with both the uncalibrated (a) and calibrated (b) radiocarbon ages. Visually the line provides a good fit to the data and

108 C. G. Constable and M. W. McElhinny (a) L. Eaeharn, uncal., unwcighted

5

h

4 l i

8 0

8 3 x

Y

2

1

0 I 0 1 2 3 4 5 6

6

5

4 n' d m k 3 x Y

2

tb) L. Eachan, cal., unweighted 1 t t

1

I /+ 0 4 , ' : 5 I

0 1 2 3 4 6 Depth in core (m)

Figure 4. (a) Uncalibrated and (b) calibrated radiocarbon dates for Lake Eacham plotted as a function of equivalent depth in the lake master core. Straight lines show the least squares unweighted linear regressions to the data.

a more sophisticated model is not warranted because of the small number of dates involved. The fitted age of 122 f 173 calendar yr at the top of the master core implies that little material is missing between the top of the 6 m Mackereth cores and the water-sediment interface.

L A K E B A R R I N E

A much larger suite of dates was obtained from the Lake Barrine cores. Altogether 45 samples were taken from four cores. The calibrated dates for each core are shown as a function of depth in the master core in Fig. 5 . The calibration procedure used is discussed below. It may be seen from these that equivalent depths in B5 appear systematically older than in the other cores, particularly in the top part of the core. B5 showed some indication of reworking of the sediment having taken place, which could have resulted in systematic incorporation of ancient carbon into the sediment. It was therefore excluded from the assignment of a time-scale to the sedimentation process.

Holoceae records from Australian lakes 109

14 '1 B7 /

Figure 5. Calibrated radiocarbon dates as a function of depth in the master core for Lake Barrine cores. Vertical bars indicate 95 per cent confidence limits on the ages obtained from Clark's (1975) calibration scheme. The linear regressions are unweighted.

It is also apparent from Fig. 5 that in all of the cores there is an enormous increase in scatter in the measured ages beyond about 6 m in depth in the master core, strongly suggesting that mixing of varying age carbon is occurring in the sediment. Obviously, in determining an age depth relationship it is desirable to assign less weight to these scattered data. Experi- mentation with various types of weight functions suggested that a step decrease in weight for the dates deeper than about 5.8m was most efficient at removing structure from the residuals of the fit to the data. Hence for depths less than 5.8 m in the master core the dates were assigned a standard error of slo = 637 and beyond this time a standard error of shi = I 6 9 6 yr, these being the standard deviations from the mean of the residuals on either side of the step. These errors are typically 5-10 times the standard errors quoted for the measured radiocarbon ages of the material (Constable 1982). This is the result of mixing of carbon of varying ages during the sedimentation process.

Fig. 6 shows the final least squares linear fit obtained for the calibrated dates, when the above weighting procedure is applied. A linear fit has been used because the data really do not warrant a more sophisticated model. The resulting time-scale for the Lake Barrine sequence is rather poorly constrained, mostly because of the high scatter in the radiocarbon

110 C. G. Constable and M. W. McElhinny

18

16

14

12

10

A

h8 F?

$ 6 (0

x Y

4

2

0 2 4 6 0 10 12 Depth in core (m)

Figure 6. Final fit to the calibrated Lake Barrhe dates.

ages deep in the cores. Reworking of material will result in an overestimate of the actual age as ancient carbon is incorporated into the sediment. Hence the ages assigned using this time-scale are likely to be maximum possible ages for the sediment.

The Lake Barrine results extend beyond the maximum age for the presently available calibration tables, which complicates attempts to assign a calendric time-scale to the sequence. The approach adopted here has been to use calendar ages where the calibration table is available and to taper the calendric time-scale into the uncalibrated section by assuming that the two coincide again at 10000yr BP. This is a somewhat crude calibration procedure, and it might be argued that it would be better to work entirely within the radiocarbon time-scale. However, fluctuations in the I4C concentration will result in apparent changes in the sedimentation rate even if it is actually constant. A constant sedimentation rate has been assumed here in deriving the Lake Barrine time-scale, and a better fit to the straight.line was obtained using the above calibration scheme than using the uncalibrated dates. In practical terms the effect of using the calendric instead of the radiocarbon time- scale was to shift the intercept for the regression from 930 2 290 back to 1360 f 290 yr BP and to alter the slope shown in Fig. 6 from 13.39k 0.73 to 13.41 k 0.73yrcm-*. The positive age intercept for both time-scales is due to a combination of the facts that material is missing between the top of the core and the water-sediment interface, and that the surface age is depressed by the incorporation of old carbon into the sediment. The time-scale derived using the above scheme is referred to as the hybrid l4C/calendric time-scale.


Magnetic measurements

Automated long core spinner magnetometer measurements of declination and horizontal intensity of remanence (Molyneux et al. 1972; Barton 1978) were made and used to select cores with smooth declination and correlatable intensity profiles for subsampling. The cores were then split lengthwise and pairs of 5.3cm3 approximately cubic specimens taken at 2.5 cm intervals along their length.

NRMs were measured on a 2-axis Super Conducting Technology cryogenic magnetometer (Goree & Fuller 1976). A group of specimens from each lake was then subjected to an AF pilot study in order to determine an optimum cleaning field. Fig. 7 shows some typical results from the pilot study. Median demagnetizing fields ranged from about 13 to 23 mT and were generally slightly higher for Eacham than for Barrine. An AF field of 10mT was selected for cleaning the bulk of the specimens. In general the effects of cleaning were a slight re'duction in the scatter of the data and a shift away from the present-day field direction towards the axial dipole field direction for the site.

Susceptibility measurements were performed on a Digico bulk susceptibility bridge. ARMS were imparted to specimens from one core in each lake and used to obtain relative geomagnetic field intensity estimates. Those results will be reported elsewhere.

-0.5

-0.5

Data analysis and results

Subsample measurements were obtained for five cores from Lake Barrine and four from Lake Eacham. Prior to stacking the data from different cores a certain amount of processing

7-

E2A175.0 , H.X : \ -2

E2A275.0

112

was required. This included adjusting the depth scales to that of the master core, 'sieving' to remove inconsistent pairs of data, and compensation for non-vertical entry of the core and twisting of the core tube during the coring process.

The 'sieving' procedure suggested by Barton (1978, p. 17) was used to reject stratigraphic pairs of data with angular differences greater than 20". This resulted in a culling of slightly less than 10 per cent of the data for most cores. The vector mean was then taken of each pair of data. Many of the cores showed a trend in declination along their length, indicating that twisting of the PVC tube had occurred during coring. A least squares linear regression was used to estimate this trend for each core and the resulting linear trend removed. The trends removed averaged to zero when summed over all the cores suggesting that there is no linear trend in the real geomagnetic declination and no bias in the direction of twisting of the corer.

The orientation of the banding and laminations in the sediment indicated that many of the cores (particularly the 12 m Lake Barrine ones) had penetrated the sediment at an angle to the vertical. This was also reflected in the average inclinations for these cores which deviated substantially from the axial dipole value for the site (by up to 30" in one case). Before stacking data from different cores it is evidently desirable to remove this tilt from the directional data.

Denham (1981) has devised a 'palaeomagnetic pattern matching' technique for finding the best correlation between two unit vector time series. This technique employs a simple rotational adjustment on the unit sphere by finding iteratively the pole and angle of rotation which brings one set of vectors ai (i = 1 , 2 , . . . n) into the closest possible proximity with the other set bi (i = 1 ,2 , . , . n). This is done by maximizing the pairwise scalar product sum

C G. Constable and M. W. McElhinny

R = Z r a i - b i

where r is the unknown pure rotation operator required on the ai.

INCLINATION

(a 1

-I DECLINAT

) -20 20 -I DECLIN 1 -20 _t

?. .

. K.. ..E. . .

5

. :.,. . h . r' ;'5 :

.i

. .

ON I I 0

Figure 8. Directional data from cores B1 (squares) and B3 (dashes). (a) and (b) are before and (c) and (d) after using the pattern matching technique.


McFadden (1982) has suggested a method for improving the estimate used for the pole of rotation and also a test to determine whether there is actually any significant rotation detectable between the two time series. He also proposes a method of detecting whether there are different trends superimposed on the variations in the two cores.

Denham's (198 1) algorithm, along with the modifications and tests described by McFadden (1982), were used to provide a matching routine for all the cores collected. Although it was only really necessary for the Lake Barrine data a considerable improvement in resolution was attained when it was applied to the Lake Eacham data before stacking. One core from each lake was chosen as a master core using the criteria that the record should be as smooth as possible and that the mean inclination should be close to the expected axial dipole value (SE Australian records covering the past l 0 0 0 0 y r average to within 1" of the axial dipole value for the site, Barton & McElhinny 1981).

Linear interpolation was then performed on the declination and inclination pairs separately in order to provide data on an equally spaced grid for matching pairs of cores. The interpolated records were truncated to the length of the shorter one. Then an optimum fit was found using the pattern matching algorithm. The effective pole and angular rotation were calculated and these applied to the original data. Fig. 8 shows the effect of applying the pattern matching technique to cores B1 and B3 from Lake Barrine.

After the corrections described above had been applied all the data within each lake were combined to yield a stack of the results. These results were transformed from a depth to a time-scale using the fits derived from the radiocarbon dates. Fig. 9(a, b) show the Lake Eacham results on radiocarbon and calendar age time-scales respectively, while Fig. 9(c) shows the Lake Barrine stack. Lines join pairs of data at equivalent stratigraphic horizons in these figures. It is evident that despite the sieving procedure used to discard inconsistent data the records are still extremely scattered. A robust smoothing procedure is necessary to delineate trends in the secular variation more clearly. The procedure adopted here has been to smooth the declination and inclination records independently by taking block medians. A l 00yr interval has been used for Barrine, 60yr for the calendar age Eacham and 50yr for the radiocarbon Eacham record. The smoothed curves are shown in Fig. 10. There is little difference between using the median value per block and the mean of the median 50 per cent of the data per block. The median has been used here because it resulted in slightly less scatter near the ends of the records.

It is notable that the amplitudes of the swings in the Eacham records are substantially greater than in the Barrine one. This is to be expected from the much higher sedimentation rate which might well result in less smoothing of the geomagnetic signal during the acquisition of a PDRM. There is reasonable agreement between the two records.

Present-day inclinations in Australia are appreciably higher than the axial dipole values as a consequence of the current close proximity of the southern magnetic pole. Archaeo- magnetic data from SE Australia date the swing towards present-day inclinations at about 500yr BP (Barbetti 1977). The trend towards steeper inclinations at the top of the Eacham records (although grossly exaggerated) is consistent with this. However, because of the high scatter near the end of the record not too much should be read into this.

If the unreliable sections near the ends of the records are ignored, one sees swings of about 10" amplitude in both declination and inclination. Neither record shows the increasing amplitude attenuation with sediment age which has been reported at some other sites, e.g. Lake Windermere, Mackereth 1971; SE Australian sites, Barton & McElhinny 1981. This suggests that the attenuation results from long time-scale remagnetization processes in the sediment rather than representing a real feature of the secular variation. The cores cannot be correlated with those from SE Australia on a swing by swing basis but are generally very similar in character.

114 EACHAM INCLlh

1 -m rlON 3 o t -MI -I

EACHAM

-I

DECLINATION -P 20 I I

DECLINATION -20 20 I

Figure 9. Stacks of directional data. (a) Lake Eacham on the radiocarbon time-scale, (b) Lake Eacham on the calendric time-scale, ( c ) Lake Barrine on the hybrid I4C calendric time-scale.

Holocene records from Australian lakes 115 B A R R I N E

INCLINA -90 -60 -?lo

Figure 9

-I - I

continued

DECLINATION -20 20

VGP paths and comparison with other sites

Comparison with data from other sites is facilitated by plotting VGP paths from the .acular variation records. The sense of looping of the VGP paths for the Queensland site is predominantly clockwise, two notable exceptions being between about 57 10 and 3980yr BP and 10500 and 8800yr B P (Fig. 11). These two periods of anticlockwise motion are also observed in the SE Australian record (Barton & McElhinny 1981), although the older one is definitely suppressed there in comparison with the Lake Barrine record. This may be a consequence of the increasing amplitude attenuation with age noted in the SE Australian records. Clockwise motion of the VGP paths is usually assumed to be generated by westward drift of the non-dipole sources causing the secular variation, and anticlockwise motion by eastward -drift (Runcorn 1959; Skiles 1970; Dodson 1979). However, this interpretation is non-unique and Dodson (1 979) has shown that for certain source configurations anticlockwise loops may be caused by westward drifting sources and clockwise loops by eastward drifting sources. It is of interest to note that the two periods of anticlockwise motion are also observed in the Argentine records of Creer et al. (1983) at approximately the same times (after converting our calibrated dates back to the radiocarbon time-scale which they have used). A closer comparison of the two records would appear to be warranted, although the poorly constrained nature of the Barrine time-scale may mean that it is impossible to

116 C. G. Constable and M. W. McElhinny EACHAM

INCLINATION

INCLINATION

L

T-

BARRINE

DECLINATION

DE -2 -

Figure 10. Smoothed directional data:


Figure 11. VGP paths for the Lake Barrine data. Data points every 100 yr.

determine whether features at the two sites are contemporaneous or are due to westward drift of non-dipole sources.

VGP paths are conventionally plotted on a stereographic projection. When dealing with recent lake sediment secular variation records this generally produces something resembling a bowl of spaghetti, because of the small amplitude quasi-periodic motions involved (Fig. 11). Most authors attempt to overcome this problem by presenting VGP plots in a piecemeal fashion, with sections spanning one or two thousand years on each plot. This renders the comparison of paths from different sites difficult, especially if there are large uncertainties in the time-scales. An algorithm has been developed to include a third dimension (namely time) in the conventional VGP plot, thereby allowing the whole record to be viewed at once (Fig. 12). The standard stereographic projection used for a VGP path is tilted sideways and slightly downwards as in a perspective view, and the third axis shows the progression of time from left to right.

In a normal stereographic projection about the North Pole

cos 8 sin h

1 + sin 10 I -cos 8 cos X

y(t) = 1 + sin I 8 I

x(t) =

z(t) = 0

where 8(t) is the latitude and X(t) the longitude of the VGP position at time t. In this case two extra parameters are introduced, a, the angular rotation about the y-axis, and 0, the rotation about the z-axis and

cos 8 sin X cos a cos 0 1 +sin I0 I

x'(t) =

Y ' ( t ) =

z'(t) = t,

-cose[cosX+sinXsinP]

1 t s i n I 8 I

It was found that values of a = 45" and 0 = +lo" provided a satisfactory perspective for the Australian secular variation records. To some extent (Y and 0 will depend on the data

118 C. G. Constable and M. W. McElhinny 9

BARRINE l 0 0 y calendar age VGPe

h e - k y to 18 ky B.P.

KEILAMBETE lWy oalmdar aga VCPm

7

~- tine * 0 k y to 18 k y B.P.

Figure 12. Perspective VGP plots for Lakes Barrine and Keilambete data. See text for explanation.

being presented, e.g. it is often difficult to see detailed structure when there is low amplitude motion of the VGP path very close to the North Pole in wllich case it may be desirable to alter 0.

The disadvantage of this type of plot is that it often makes it difficult to interpret the sense of looping of the VGP path because it is stretched out along the time axis. However, its utility becomes apparent when records from different sites are compared. Fig. 12 shows the Lake Barrine data alongside the Lake Keilambete, SE Australian data published by Barton & McElhinny (1981). Dotted lines joining the outline of the path to its time point indicate pole longitudes lying between 0" and 180"E and solid lines those between 180" and 360"E. Prior to calculating the VGP positions the declination and inclination data were rotated so that the vector means corresponded to the axial dipole values for the site, and a seven point linear filter was used to smooth the data. Tie marks indicate the features which we regard as the same in the two records. The somewhat neurotic motions (e.g. around 7500yr B P in Barrine) which occasionally occur in one lake and not the other may be attri- buted to differing degrees of smoothng in acquisition of the signal, as well as to the differences whch undoubtedly arise in sedimentation rates with time, thereby affecting the assigned radiocarbon time-scales.

Conclusions Secular variation records have been obtained which nominally span the last 16000yr. Eacham has a good chronology associated with it but that for Barrine is less well defined.


The records are similar in type to those from SE Australia a!though correlations between the declination and inclination records from the sites cannot be made on a swing by swing basis.

VGP paths for the sites show two periods of anticlockwise motion of the geomagnetic vector, between 5710 and 3980yr B P and about 10500 and 8800yr BP. These times correspond to periods of anticlockwise motion in both the south-eastern Australian records and Argentine records.

Acknowledgments

We thank H. A. Polach and J . Head of the Australian National University Radiocarbon Laboratory for radiocarbon measurements, and D. Walker of the Research School of Pacific Studies for providing the 6 m Lake Eacham cores. This work was supported by the Australian National University.

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