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 BP. VGP paths for the sites show two periods of anticlockwise motion between about 5710 and 3980 BP and 10500 and 8800 BP. 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 palaeo- magnetic 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 C. G. Constable and M. W.McElhinny 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 l0000yr 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. 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 BP (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 sedi- mentary environment for the acquisition of a record of the geomagnetic signal. Core descriptions Seven 12m 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 1m 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 8m 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 HOR I NTENS I TY (YA/U) 0 flnely laminoted claye noterial, dork brawn and khaki, interspersed with 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 0.8 I I0 103 1030 HOR INTENSITY (YA/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; Holocene records from Australian lakes 107 (a) BI SUSC (b) 83 SUSC (c) 84 SUSC (d) 85 SUSC (e) 87 susc Li 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 calibra- tion. Calibration has been carried out here using Clark's (1975) scheme which only extends back to 6500 radiocarbon yr BP. 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.