- Email: [email protected]
Geomagnetism of baked clays and recent sediments
Physics of the Earth and Planetary Interiors, 38 (1985) 81-82 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
81
Book Review Geomagnetism of Baked Clays and Recent Sediments. K.M. Creer, P. Tucholka and C.E. Barton (Editors). Elsevier, 1983, xx + 324 pp., US $53.25, ISBN O-444-42231-5. This volume edited by Creer, Tucholka and Barton is built around a selection of papers from the symposium on “Time Scales of Geomagnetic Secular Variations” held at the 4th Assembly of the International Association of Geomagnetism and Aeronomy at Edinburgh in August 1981. Such volumes tend to suffer from the inherent unevenness of treatment by the various authors and by being somewhat at the mercy of the contributors’ choices of topics to be discussed. However, this book has been carefully planned to minimize these problems. It is organized into four chaptersMagnetization Processes, Dating and Related Problems, Archaeomagnetism of Baked Clays, and Paleomagnetism of Lake Sediments. The chapters have their own set of references and read as coherent units. This strategy has involved considerably more editorial work than is usual for such volumes and has succeeded in giving a final product which reads as a book rather than a collection of unconnected papers. This, in conjunction with the opportune time for a survey of archaeomagnetic and lake sediment data makes the book a valuable contribution. The chapter on magnetization processes is pitched at about the right level; specialists can gain something, while the generalist is provided with a suitable overview. The discussion of single domain thermoremanent magnetization (TRM) is good and acknowledges the difficulties encountered in trying to apply this beautiful theory to the real world of particles in rocks and archaeomagnetic artifacts. Discussions of multidomain TRM are given in terms of the Schmidt model of NCel’s multidomain treatment on the grounds that much of the magnetization is carried by particles that are small but too large to be single domain. It is, however, this reviewer’s interpretation that the 0031-9201/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
TRM of ensembles of such grain sizes is dominated by particles which have not nucleated a domain wall even though they exceed the MD grain size, as suggested by Halgedahl. Depositional and post-depositional magnetization (DRM and PDRM) is central to the magnetization of lake sediments, as is TRM for archaeomagnetic samples and are discussed in appropriate detail. The recognition of the role of the dewatering of the sediments as a blocking mechanism is discussed in terms of Hamano’s model. To bring this chapter to a close there is a discussion of the curious shear remanent magnetization (SRM) acquired by adobe bricks as they are made. The analysis of this magnetization is reviewed by Games and is interpreted to be acquired as the clay is thrown into the former. As was noted at the outset, the chapter is well written and at a suitable level for such a book -neither too deep to bore the generalist, nor too shallow for the specialist to find anything of interest. Chapter 2 covers the various dating techniques used in archaeomagnetic and paleomagnetic studies of secular variation. Discussions of relative and absolute methods are given in considerable detail. This is entirely appropriate; given modern magnetometers and demagnetization equipment, the magnetic measurements are often relatively straightforward and frequently the determination of age is more tricky. The first area described is palynology and it is shown how relative dating combined with radiocarbon calibrated type sections provide a most powerful technique for sediments. 14C is recognized as the most important method for absolute dating of archaeomagnetic material, in addition to its role in calibration of relative methods for sediments. The extension of the 14C method to older times using accelerator techniques is discussed as are the various problems in relating 14C years to calendar years. Tree rings and varves are suggested as sources of well-dated ‘magnetic material. It is acknowledged that no good data have yet emerged
82 from measurements of the natural remanent magnetism (NRM) of trees. Perhaps with the next generation of SQUID magnetometers available, another try is in order. The situation with varves is quite different; some of the earliest reliable data used came from varves. The discussion of the relatively new 21°Pb method was sufficiently clear that your reviewer was able to convince himself that he understood the technique having little prior knowledge of the method. Similarly the discussion of thermoluminescence gave an excellent introduction to the method and an impression of its applicability. The archaeomagnetism of baked clays is covered in Chapter 3, beginning with an excellent review of techniques. The Thellier method is described with its variants as is the important alternative method of Shaw. At the risk of tilting at windmills, it seems appropriate to point out that the essence of what has become known as the Thellier method was described by Koenigsberger, whose work was cut short by World War II. It is sad that his contribution is not recognized. This can be achieved by calling the method, as Nagata has suggested, the Koenigsberger-Thellier-Thellier or K T T method. In all events, the analysis of the method in this volume by Walton is particularly penetrating and he is surely right in drawing attention to difficulties which can be encountered with the thermally activated destruction of phases. This gives one pause for thought concerning the usual rather cookbook application of the method. His method involving high temperature measurements is particularly intriguing, and is one of the few original contributions to this area for some time. It is also worth drawing attention to the special virtue of the Shaw method when the spectrum of microscopic coercivities of the phases carrying the N R M is distributed, but the blocking temperatures are confined to a very small temperature range. The remainder of Chapter 3 provides a review of the data of archaeomagnetism, provided by many of the most prominent workers in the field. This review of the data is one of the important contributions of the book, coming at a time when
a major increase in the data base has recently taken place. Data are described from the Balkans, Egypt, Greece, the Near East, USSR, China, Japan, the American Southwest, Peru and Australia. Chapter 4 provides a thorough analysis of the Lake Sediment techniques and data. The various methods of coring are discussed briefly. In view of the development of the Mackereth corer for these studies, it seems a pity that a longer discussion was not given. Again, there is little mention of the long core method of measurement of cores. Since the method has been around for about 10 years, it is remarkable that more measurements have not been made this way. It is much more rapid, avoids disturbing the sediment, and with refinement the resolution could be made comparable to that of the standard measurements. Data are presented from Europe, North America, Argentina, and Australia. The last two sections of the chapter provide particularly welcome discussions of statistical analyses and of spectrum analysis of Lake Sediment data. In a brief epilogue, the various possible causes of secular variation are considered and simulated records constructed for particular models. One forms the impression that it is far from clear how much of secular variation is due to change in low order harmonics of the field and how much is due to very high order effects. There is no mention of the important recognition by Lin of the dipole contribution. In view of its publication in the Chinese literature (Seismology and Geology, v. 1, no. 2, 83-85, 1979), such omission is clearly forgivable. As the authors note in closing the Epilogue, there are plenty of problems remaining for solution. Their book is a most helpful discussion of the state of present studies, which will be invaluable to anyone starting out in the subject in this fascinating field. They are to be congratulated in providing a b a d l y n e e d e d s o u r c e b o o k for Archaeomagnetic and Lake Sediment studies of Secular Variation. M. F U L L E R (Santa Barbara, CA, U.S.A.)
81
Book Review Geomagnetism of Baked Clays and Recent Sediments. K.M. Creer, P. Tucholka and C.E. Barton (Editors). Elsevier, 1983, xx + 324 pp., US $53.25, ISBN O-444-42231-5. This volume edited by Creer, Tucholka and Barton is built around a selection of papers from the symposium on “Time Scales of Geomagnetic Secular Variations” held at the 4th Assembly of the International Association of Geomagnetism and Aeronomy at Edinburgh in August 1981. Such volumes tend to suffer from the inherent unevenness of treatment by the various authors and by being somewhat at the mercy of the contributors’ choices of topics to be discussed. However, this book has been carefully planned to minimize these problems. It is organized into four chaptersMagnetization Processes, Dating and Related Problems, Archaeomagnetism of Baked Clays, and Paleomagnetism of Lake Sediments. The chapters have their own set of references and read as coherent units. This strategy has involved considerably more editorial work than is usual for such volumes and has succeeded in giving a final product which reads as a book rather than a collection of unconnected papers. This, in conjunction with the opportune time for a survey of archaeomagnetic and lake sediment data makes the book a valuable contribution. The chapter on magnetization processes is pitched at about the right level; specialists can gain something, while the generalist is provided with a suitable overview. The discussion of single domain thermoremanent magnetization (TRM) is good and acknowledges the difficulties encountered in trying to apply this beautiful theory to the real world of particles in rocks and archaeomagnetic artifacts. Discussions of multidomain TRM are given in terms of the Schmidt model of NCel’s multidomain treatment on the grounds that much of the magnetization is carried by particles that are small but too large to be single domain. It is, however, this reviewer’s interpretation that the 0031-9201/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
TRM of ensembles of such grain sizes is dominated by particles which have not nucleated a domain wall even though they exceed the MD grain size, as suggested by Halgedahl. Depositional and post-depositional magnetization (DRM and PDRM) is central to the magnetization of lake sediments, as is TRM for archaeomagnetic samples and are discussed in appropriate detail. The recognition of the role of the dewatering of the sediments as a blocking mechanism is discussed in terms of Hamano’s model. To bring this chapter to a close there is a discussion of the curious shear remanent magnetization (SRM) acquired by adobe bricks as they are made. The analysis of this magnetization is reviewed by Games and is interpreted to be acquired as the clay is thrown into the former. As was noted at the outset, the chapter is well written and at a suitable level for such a book -neither too deep to bore the generalist, nor too shallow for the specialist to find anything of interest. Chapter 2 covers the various dating techniques used in archaeomagnetic and paleomagnetic studies of secular variation. Discussions of relative and absolute methods are given in considerable detail. This is entirely appropriate; given modern magnetometers and demagnetization equipment, the magnetic measurements are often relatively straightforward and frequently the determination of age is more tricky. The first area described is palynology and it is shown how relative dating combined with radiocarbon calibrated type sections provide a most powerful technique for sediments. 14C is recognized as the most important method for absolute dating of archaeomagnetic material, in addition to its role in calibration of relative methods for sediments. The extension of the 14C method to older times using accelerator techniques is discussed as are the various problems in relating 14C years to calendar years. Tree rings and varves are suggested as sources of well-dated ‘magnetic material. It is acknowledged that no good data have yet emerged
82 from measurements of the natural remanent magnetism (NRM) of trees. Perhaps with the next generation of SQUID magnetometers available, another try is in order. The situation with varves is quite different; some of the earliest reliable data used came from varves. The discussion of the relatively new 21°Pb method was sufficiently clear that your reviewer was able to convince himself that he understood the technique having little prior knowledge of the method. Similarly the discussion of thermoluminescence gave an excellent introduction to the method and an impression of its applicability. The archaeomagnetism of baked clays is covered in Chapter 3, beginning with an excellent review of techniques. The Thellier method is described with its variants as is the important alternative method of Shaw. At the risk of tilting at windmills, it seems appropriate to point out that the essence of what has become known as the Thellier method was described by Koenigsberger, whose work was cut short by World War II. It is sad that his contribution is not recognized. This can be achieved by calling the method, as Nagata has suggested, the Koenigsberger-Thellier-Thellier or K T T method. In all events, the analysis of the method in this volume by Walton is particularly penetrating and he is surely right in drawing attention to difficulties which can be encountered with the thermally activated destruction of phases. This gives one pause for thought concerning the usual rather cookbook application of the method. His method involving high temperature measurements is particularly intriguing, and is one of the few original contributions to this area for some time. It is also worth drawing attention to the special virtue of the Shaw method when the spectrum of microscopic coercivities of the phases carrying the N R M is distributed, but the blocking temperatures are confined to a very small temperature range. The remainder of Chapter 3 provides a review of the data of archaeomagnetism, provided by many of the most prominent workers in the field. This review of the data is one of the important contributions of the book, coming at a time when
a major increase in the data base has recently taken place. Data are described from the Balkans, Egypt, Greece, the Near East, USSR, China, Japan, the American Southwest, Peru and Australia. Chapter 4 provides a thorough analysis of the Lake Sediment techniques and data. The various methods of coring are discussed briefly. In view of the development of the Mackereth corer for these studies, it seems a pity that a longer discussion was not given. Again, there is little mention of the long core method of measurement of cores. Since the method has been around for about 10 years, it is remarkable that more measurements have not been made this way. It is much more rapid, avoids disturbing the sediment, and with refinement the resolution could be made comparable to that of the standard measurements. Data are presented from Europe, North America, Argentina, and Australia. The last two sections of the chapter provide particularly welcome discussions of statistical analyses and of spectrum analysis of Lake Sediment data. In a brief epilogue, the various possible causes of secular variation are considered and simulated records constructed for particular models. One forms the impression that it is far from clear how much of secular variation is due to change in low order harmonics of the field and how much is due to very high order effects. There is no mention of the important recognition by Lin of the dipole contribution. In view of its publication in the Chinese literature (Seismology and Geology, v. 1, no. 2, 83-85, 1979), such omission is clearly forgivable. As the authors note in closing the Epilogue, there are plenty of problems remaining for solution. Their book is a most helpful discussion of the state of present studies, which will be invaluable to anyone starting out in the subject in this fascinating field. They are to be congratulated in providing a b a d l y n e e d e d s o u r c e b o o k for Archaeomagnetic and Lake Sediment studies of Secular Variation. M. F U L L E R (Santa Barbara, CA, U.S.A.)