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Home , Pangaea

Nature Vol. 299 16 September 1982 207

can lead to the syndrome in which scattered A Proterozoic Pangaea? data are examined and the odd points that are near to previous observations are from D. H. Tarling accepted for publication and the remainder appear, if at all, in the inaccessible IN a recent review of palaeomagnetic data erected. Even with a single craton, the appendix of a thesis. Despite Piper's ( planet. Sci. Lett. 59, 61; 1982), radiometric dating of any intrusive event is attempts to be objective, therefore, the Piper concludes that a single huge uncertain within 100-200 Myr, even data may well already have been subjec­ , Pangaea, existed throughout ignoring problems of open chemical tively selected to conform to previous data the Proterozoic period of the Earth's systems, .during which time major con­ or models. However, the only way in which history (2,600-570 Myr ago). Such a tinental motions could have occurred. It is such problems can be resolved is by the conclusion is consistent with many also worrying that much Precambrian data erection of specific models which can then geological features that indicate extensive are based on alternating magnetic field be tested by a variety of methods and by continental stability during this period, yet demagnetizations - a technique that is new data. Piper's analyses also require that this 'noisier' then thermal demagnetization - This is not Piper's first model and it Pangaean was in motion yet thermal demagnetization is often seems unlikely to be his last. However, the relative to the Earth's axis of rotation. apparently ineffective in determining fact that he can make a very plausible case Such motion must, presumably, be discrete components of remanence. Even for support of his model is, in itself, of associated with the same mantle convective when such components can be isolated, great importance. It demonstrates, for motion that currently results in continental there is doubt about their age. There is a example, that continental motions splitting and ocean-floor spreading. tendency to assume that the component occurred, thus indicating the action of Piper's conclusions are thus of with the highest stability is, in fact, the mantle convection currents. This is no fundamental importance in testing some of oldest - yet this may simply reflect the great surprise in view of the much higher the basic assumptions in both presence of haematite that may have radiogenic heat production in the past, but palaeomagnetism and . formed very much later than the rock itself. raises the problem of why such motions did Palaeomagnetic studies allow the deter­ Even when such components have been not cause continental splitting, as they do mination of the palaeolatitude and orien­ isolated and dated, it is rare for there to be now and also did in pre-Proterozoic times. tation (relative to the palaeomagnetic pole) adequate tectonic control of subsequent The implication is that the continental of the sampled area. Where sufficient data movements, although the Proterozoic lithosphere, in Proterozoic times, was too are available, they can be used to construct crust appears to have been little deformed strong or too thick to be split by such a palaeomagnetic polar wandering curve, as the overlying sediments tend to have motions. But how did such a thickness indicating the changing position of the pole only shallow dips. The uncertainties develop suddenly at the end of Archaean relative to the stable tectonic block from involved are thus immense and require times and what caused the change to which the samples were obtained. Such subjective evaluation. Unfortunately, this present-day continental splitting? 0 curves do not, in fact, mean that the pole is moving, as this is fixed relative to the ecliptic, but results from a movement of the sampled relative to the pole. The fact that polar wandering curves exist for the Proterozoic is thus of the utmost The sunflower seed importance as it clearly indicates that the continent, or , were in motion protection business and were presumably driven by mantle convection. from Peter D. Moore If polar wandering paths for the same period of time can be matched between The consumption of the seeds of plants by seed predation. The high population different tectonic units, then the relative invertebrate and vertebrate predators is a density of mice and voles, regarded by him positions of these tectonic units can be serious problem for the agronomist, and as major culprits, could in turn be related precisely defined. Thus the palaeomagnetic much effort has been devoted to finding to rearing of game birds and consequent data from the different cratonic blocks of ways of limiting the predators' numbers. destruction of predators, a view supported the can be used to determine the There is, however, a quite different way of and popularized by Tansley3 • existence, or not, of one or more con­ tackling the problem. In a recent paper', The possibility that the problem might tinental units during the Proterozoic. Piper field trials are reported of attempts to be overcome through providing an alter­ finds that he can define Pangaea protect economically important seeds by native food source was first approached throughout the Proterozoic as a cigar­ buying off the predators with an abundant experimentally by Sullivan4 who supplied shaped continent comprising , supply of another seed. deer mice (Peromyscus manlculatus, a China, and backing onto In many North American forests, small major seed predator of douglas fir) with western -, with have often been found responsi­ sunflower seed in order to distract it from Kazakhstan- against North Africa ble for seed predation and depression of the seed. He found that very large and bordering onto western woodland regeneration, in particular the quantities of sunflower seed had to be (, Greenland and ). douglas fir (Pseudotsuga menziesil) has provided (seven sunflower seeds for every Piper finds that such a continent is not only been much studied. That seed predators douglas fir seed) in oroer to reduce required by the palaeomagnetic data but is may limit the regeneration of plant species predation significantly. also consistent with the available geological has been known for some time. As long ago Sullivan and Sullivan5 have now evidence. as 1919 Watt's' records of the fate of acorns repeated these experiments with lodgepole It is only too easy to be sceptical about in oak woods showed that high mortality of pine (Pine contorta) in an area of British the data base on which any such model is acorns in oakwoods could be largely put Columbia from which the pine had been down to predation by birds and mammals. He also found that in the case of beech2 , invertebrates were involved and concluded D. H. Tarling is Reader in Palaeomagnetism in Peter D. Moore is Senior Lecturer in the the Department of Geophysics, The that the low level of regeneration found in Department of Plant Sciences at King's University, Newcastle upon Tyne NEI 7RU. British woodland was a consequence of Coll~ge, University of London.

0028.0836/82/ 370207·02$01.00 \1:> 1982 Macmillan Journals l!d