J. Geomag. Geoelectr., 37, 129-137, 1985

Status of the Geomagnetic Polarity Time Scale

Ian MCDOUGALL

ResearchSchool of Sciences,A ustralianNational University, Canberra, A.C.T. 2601,Australia

(ReceivedApril 2, 1984;Revised September 14, 1984)

During the last four the directly measured geomagnetic polarity time

scale covering the last s Ma of geological time has not been changed significant-

ly, but several short intervals of inverse polarity appear to have been authen-

ticated from measurements on volcanic rocks. The polarity time scale extending

to about the Middle (～165Ma)is based upon interpretation of marine

data. For the interval extending to ～80Ma ago several new

time scales have been published in recent years, with calibration by means of

the directly determined scale for the last 3.5Ma, and then by use of a number

of calibration points relating individual magnetic anomalies to the numerical time

scale, usually through and less commonly by direct dating of

volcanic sequences. Considerable improvements have been effected, but further

changes will occur in the future. From about 80Ma ago to the limit of the

marine magnetic anomaly data, calibration is far less well controlled. Only the

broad pattern of geomagnetic field reversals is known from the Middle Jurassic

to the Middle Paleozoic, with little information available for earlier times.

1. Introduction

The geomagnetic polarity time scale has assumed great importance in a wide range of geological and geophysical studies, and thus its accurate calibration is of continuing interest and relevance.

The polarity time scale is conveniently considered in terms of three overlap- ping time intervals, relating to how it is calibrated. For the time interval from the present back to about s Ma ago, the polarity time scale was developed by measurement of the age(using the K-Ar method)and magnetic polarity of subaerial volcanic rocks from diverse localities. This directly determined polarity time scale initially was limited to the last s Ma of geological time because of loss of resolution in the K-Ar age measurements with increasing age.

MCDOUGALL(1979)and MANKINEN aril DALRYMPLE(1979)gave compreheriSiVe summaries of this part of the polarity time scale, and in the present review com- ments will be confined to summarizing new information that has become available since then.

Reporter review paper for IAGA Division I, WG 5.

129 130 I. MCDoUCALL

Historically the directly determined polarity time scale, developed mainly between 1963 and 1966, was of great significance in that it facilitated the quan- titative interpretation of the marine magnetic anomaly profiles observed across mid ocean ridges(VINE and MATTHEWS, 1963), leading to the rapid acceptance of the hypothesis of (HESS, 1962), and to the development of plate tectonic models of earth behavior. A detailed account of this history recently was given by GLEN(1982). VINE(1966)grid HEIRTZLER et al.(1968)used the known polarity time scale for the Last 3.5Ma to obtain spreading rates at the mid ocean ridges; they then applied these rates to the uncalibrated parts of the marine magnetic anomaly patterns to derive predicted polarity time scales extending back to about 80Ma ago into the . This history of polarity of the geomagnetic field is recorded in quite remarkable fidelity in the marine magnetic anomaly record, which is now documented from the present back to Middle Jurassic times, about 165Ma ago. There has been considerable effort expended in recent years in improving the calibration of the polarity time scale based upon interpretation of marine magnetic anomaly data, and a brief sum- mary of this work will be given below. For times earlier than about 165Ma ago, the history of polarity of the

geomagnetic field is rather poorly known, with little definitive information prior to the Middle Paleozoic. This part of the polarity time scale always will be less well defined, as it has to be pieced together from a combination of biostratigraphic,

paleomagnetic and isotopic age data from stratigraphic sequences from different localities. We will not consider this earlier part of the polarity time scale further in this review; auseful brief summary as to its present status was given by HARLAND et al. (1982).

2. Directly Determined Polarity Time Scale

Since the reviews of MCDOUGALL(1979)and MANKINEN and DALRYMPLE

(1979),no major changes have been suggested for the polarity time scale for the last s Ma, as determined from direct age and paleomagnetic measurements

on subaerial lava flows. However, a development of significance is that there is an increasing amount of evidence from volcanic sequences for very short intervals(～0.01Ma)of opposite polarity to that expected in the particular time range. These discoveries provide support for earlier claims that at least some of the short intervals of opposite polarity found in sedimentary sequences are real, and indeed record inversions of the geomagnetic field. Within the Brunhes chron, there are at least two examples of this behavior

that appear to be well established. The reverse polarity Laschamp and Olby lava flows in the Chaine des Puys, France indicate a subchron of short duration

somewhere within the interval 30,000 to 50,000 years ago based upon extensive K-Ar, 40Ar/39Ar, thermoluminescence and 230Th/238U disequilibrium dating

(HALL and YORK, 1978; CONDOMINES, 1978; GILLOT et al., 1979). Note, however, that HEELER and PETERSEN(1982)believe there is some evidence for self-reversal Status of the Geomagnetic Polarity Time Scale 131

effects, particularly in the Olby flow. Reverse polarity for welded tuff of age about 30,000 years at Shibutami in Japan(TANAKA and TACHIBANA,1981)may also record the same inversion of the geomagnetic field. Two reverse polarity lava flows in Idaho with estimated age of 0.46±0.05

Ma, based on K-Ar dating(CHAMPION et al.,1981), provide good evidence for the reality of the Emperor reversed polarity event(subchron)within the Brunhes normal chron. RYAN(1972)named this short reverse polarity interval on the basis of reverse polarity found in some deep sea cores at a level thought to be about O.45Ma old. There also is supporting evidence from the interpretation of marine magnetic anomaly profiles across the Galapagos spreading center(WILSON and HEY, 1981). MANKINEN et al. (1978)recorded evidence for an excursion toward normal

polarity within the rhyolite of Adler Creek, California, dated at 1.12±0.02Ma, and named this the Cobb Mountain normal polarity event(subchron)within the Matuyama reversed chron, somewhat older than the Jaramillo subchron. This interpretation was reinforced by MANKINEN and GROMME(1982)who found two basalt flows of normal polarity and similar age in the Coso Range; California. They referred to other supporting data from volcanic sequences elsewhere, and

provided an estimate of 1.10±0.02Ma for its age with a maximum duration in the order of 10,000 years, see also SUEISHI et al. (1979). Collectively these data from volcanic rocks are providing convincing evidence that very short intervals of inverse polarity are present in the geomagnetic field

polarity spectrum, as predicted by Cox(1968). No doubt further examples will be documented in the future.

3. Polarity Time Scale from Marine Magnetic Anomaly Data

It is now widely recognized that the marine magnetic anomaly data provide an excellent time-integrated history of polarity of the geomagnetic field. As noted above, this was realized at an early stage by VINE(1966)and HEIRTZLER et al.

(1968), who derived polarity time scales from the marine magnetic anomaly record extending to about 80 Ma ago, based upon extrapolations from the directly calibrated polarity time scale for the last 3.5Ma. Although very large extrapola- tions were involved, over a factor of 20 times the length of the baseline, the time scale proposed by HEIRTZLER et al.(1968)remains within 10 percent of current estimates. The particular magnetic anomaly profile from the South Atlantic USed by HEIRTZLER et al.(1968)in their derivation of the polarity time scale clearly was an enlightened choice, and indicates that indeed the rate of seafloor

spreading in this region must have been nearly constant over this 80Ma interval . Since that time, numerous attempts have been made to improve the polarity time scale based upon the marine magnetic anomaly record , especially during the last six years(LABRECQUE et al., 1977; NESS et al ., 1980; LOWRIE and ALVAREZ, 1981; HARLAND et al., 1982; BERGGREN et al ., 1984). BUt instead of looking for a particular marine magnetic anomaly profile from an oceanic region 132 I. MCDOUGALL thought to have undergone uniform spreading,a differentphilosophy has been adopted. Stacking techniqueshave enabled an overallmarine magnetic polarity record to be establishedusing data from many profiles.The approach normally used has been to establishone or more calibrationpoints between the marine magnetic record and the numerical time scale;apolarity time scale is then derived by linearinterpolation between the chosen calibrationpoints. Clearly if a sufficientnumber of accurate ties are established,then a precisepolarity time scale of high resolutionwill be obtained. As a by product, information as to uniformity or otherwise of seafloorspreading within various parts of the ocean basins can be derived. The major problem with this approach, and the reason why there is a proliferationof time scales,is that there are difficulties in providingprecise and accuratecalibration points for definitionof the polarity time scale. Unfortunately it has not usuallybeen possibleto measure the age of a par- titularmarine magnetic anomaly directly,because basaltsrecovered from by drillingalmost invariablyare unsuitablefor-isotopic dating. Nor can sediments immediately overlyinga particularsegment of oceanic crust be dated reliablyby isotopicmethods to provide a minimum age for the underlyingcrust containing a record of a particularmagnetic anomaly. Thus indirectmethods involving correlationsof various kinds, often through several steps,must be employed in derivingan age for a particularmagnetic anomaly. The procedures used in these correlationsvariously rely heavily upon , biostratigraphy,isotopic age determinationsand the numerical geologicaltime scale. An approach commonly used, and which has led to the recent upsurge in publicationof new time scales,is to determine the magnetostratigraphyof long sequences of marine sediments recovered during deep sea drilling(e.g.,PooxE et al.,1982)or now exposed on land. The latteris well illustratedby the detailed work on the Late Cretaceous-Paleogenesequences at Gubbio, Italy(ALVAREZ et al., 1977; LOWRIE et al., 1982; NAPOLEONE et al., 1983). With reasonably uniform sedimentation rates,the thicknessof the normal and rever-sepolarity zones gives the relativeduration of the polarityintervals, and this pattern can often be correlatedwith a high degree of confidence with the standard marine magnetic polarityrecord. The fossilassemblages in the marine sedimentary se- quences are of great importance, as they not only provide information as to when the sediments were depositedin terms of the relativegeological time scale, but also provide the means by which correlationcan be effectedto the numerical time scale.The numerical age control derivesfrom isotopicallydated samples of suitablerocks, preferablyvolcanic, from localitieswhere more or lessprecise biostratigraphiccontrol is available.Considerable effortover the last 30 years has been devoted to numericallycalibrating the relativegeological time scale by isotopicdating, and synthesesof these data have been provided in recent times by BERGGREN(1981), BERGGREN et al.(1984).and ODIN(1982). Thus in the Gubbio sequence,the Cretaceous-Tertiaryboundary is recogniz- Status of the Geomagnetic Polarity Time Scale 133

ed from the biostratigraphy to lie within the reverse polarity interval between normal polarity intervals correlated with marine anomalies 29 and 30. The Cretaceous-Tertiary boundary has an age of about 66±1 Ma. Using this as a major tie point, LA BRECQUE et al.(1977)derived their time scale by interpola- tion from the marine magnetic polarity recorcl. That the correlations between magnetostratigraphy, determined in a

stratigraphic sequence, and the marine magnetic anomaly record, using biostratigraphic data can be equivocal is illustrated by apparent inconsistencies that appear in results obtained in the Late Cretaceous to Eocene terrestrial se-

quences in North America(cf. LINDSAY et al., 1981; BUTLER et al., 1981; BUTLER and CONEY, 1981; BERGGREN et al., 1984). Interpretations of these data, for example, involve differences in the order of 3Ma or more in the assignment of age to anomaly 24 chron at about 55 Ma, and may in part be accentuated in terrestrial sequences owing to additional biostratigraphic correlations that have to be made between the non-marine and marine fossil record. Arelated approach that has been used with considerable success in lceland

is to determine the magnetostratigraphy of long sequences of subaerial lavas together with isotopic age measurements on selected lavas. In Iceland more or less continuous lava sequences are known that extend from the present back to about 15Ma ago. The magnetostratigraphy again can provide the tie to the marine magnetic polarity record through distinctiveness of the pattern of normal and reverse intervals. For example the long normal polarity interval known as anomaly s in the marine magnetic record, and dated at around g to 10Ma, has now been recognized and mapped in three long composite lava sequences ire different parts of Iceland(MCDOUGALL et al., 1976; SAEMUNDSSON et al., 1980; MCDOUGALL et al., 1984). The ability of directly measure K-Ar ages on these rocks circumvents the uncertainties associated with correlat iins to the numerical time scale through biostratigraphy. However, unfortunately, the Icelandic lavas are not always ideal for dating, as they are mainly low potassium basalts, often showing some alteration. Nevertheless very good results have been obtained, and the most recent data from northwest Iceland(MCDOUGALL et al., 1984)suggest

that anomaly 5 time extends from 9.6 to 11.1Ma ago. This indicates that current

polarity time scales based upon the marine magnetic record need to be modified toward an increase in age for this part of the time scale by up to 10 percent Table 1). Based on these types of correlations, as noted above, a number of polarity time scales have appeared in recent years. In Table l are listed the estimated

ages from these time scales for several marine magnetic anomalies , chosen rather arbitrarily to cover the interval from about 80Ma ago to the present . All the dime scales essentially use the present time and the age of the Gauss/Gilbert

chron boundary(=older limit of marine magnetic anomaly 2-3') at 3 .40Ma as talibration points, together with one or more additional tie points to control the older part of the time scale, with the exception of the original HEIRTZLER et al. (1968) scale. Note that all ages are calculated or, if necessary , recalculated 134 I. MCDOUGALL

Table 1. Estimated age of several marine magnetic anomalies according to various polarity time scales.

(y): younger boundary; (o): olderboundary. Note: Ages adjustedwhere necessaryto conform with40K decayconstants as recommend- ed in STEIGER and JAGER(1977). to conform with currently recommended decay constants for 40K as given by the IUGS Subcommission on Geochronology(STEIGER and JAGER, 1977). La BRECQuE et al. (1977)used one additional correlationor calibrationpoint derived from the tie established between the Cretaceous-Tertiary boundary and the marine magnetic reversal sequence at Gubbio, Italy.NESS et al.(1980)add- ed two additional calibration points, establishing ages for the older boundaries of marine anomalies s and 24. COWRIE and ALVAREZ(1981)employed 10 age calibrationpoints in addition to the youngest two common to allscales. HARLAND et al.(1982)chose to recalculatethe polarity time scale of COWRIE and ALVAREZ (1981)after eliminating two of the control points in the Eocene and , as they argued that the implied marked changes in seafloor spreading, required if these points were left in, were unrealistic.Thus we see here a return to the notion that overall rates of seafloor spreading on a global scale are likely to be fairly uniform. BERGGREN et al.(1984 have adopted a somewhat similar philosophy in deriving their time scale, but have used only five age calibration points in addition to the present and the Gauss/Gilbert chron boundary dated at 3.4Ma. These particular tie points were chosed because a good correlation between age and magnetozone had been established in most cases. BERGGREN et al.(1984)identify three straight line segments on a diagram of calibration age versus the marine magnetic polarity time scale of La BRECQUE et al. (1977), corresponding to three periods of somewhat different seafloor spreading rates on a global scale. L,OWRIE(1982)also briefly reviewed this question. It will be seen from Table l that over the last several years the polarity time scales appear to have converged, so that differences between them are now relativelysmall, generally less than 5 percent. This is indeed encouraging, par- ticularlyas the number of calbration points employed varies from scale to scale, and the concordancy may be interpreted to mean that the polarity time scale back to 80Ma ago is known with a fair degree of accuracy. However, it should be recognized that essentiallythe same data base is being employed by allworkers, with several of the criticalcalibration points common to most of the recent Status of the Geomagnetic Polarity Time Scale 135

time scales. When it is remembered that each tie point commonly entails one or more correlation steps, each step having possibilities of error, it is inevitable that further changes in the time scale will be necessary in the future as more and better calibration points are established. Nevertheless unless some of the presently accepted correlations and assigned numerical ages are found to be in considerable error, it is expected that changes in the polarity time scale are not likely to exceed perhaps 5 percent. But the possibility has been raised that the age of marine magnetic anomaly s may be greater than presently accepted by nearly 10 percent. The geomagnetic polarity time scale for the interval from the Early Cretaceous to the limit of the marine magnetic anomaly record in the Middle Jurassic(about 165Ma ago)is based mainly upon syntheses by CARSON and HILDE(1975)and

CANDE et al.(1978)using limited and not very precise biostratigraphic control. HARLAND et al.(1982)have provided a revised version of this part of the time scale, and CHANNELL et al.(1982)have summarized available data from magnetostratigraphic studies of Early Cretaceous and Jurassic sediments now exposed on land. As calibration points are few and not very well constrained, the polarity time scale for the Early Cretaceous and Jurassic is likely to change considerably as improved correlations and age assignments become available.

4. Conclusions

During the last four years little significant change has occurred in the calibra- tion of the directly determined geomagnetic polarity time scale, covering the last 5Ma of geological time. An important development, however, is that several short(～0.01Ma)inversions of the geomagnetic field seem to have been authen- ticated from studies of lavas'and more evidence of this kind is expected to be forthcoming in the future. Several polarity time scales extending back to more than 80Ma ago, and based upon extrapolations and interpolations using the marine magnetic reversal record, have been published in recent years. These time scales show a good degree of convergence, possibly in part because they rely upon many of the same calibra- tion points. But judging from past experience, further changes must be expected in the future as the data base improves. Quite large changes might well occur in the earlier part of the polarity time scale extending to the limit of the marine magnetic anomaly record, because of the few calibration points that are present- ly available. The polarity time scale for times earlier than Middle Jurassic is only known in broad outline back to the Middle Paleozoic, and is hardly defined at all for times earlier than this.

Ithank P.L. McFadden for helpful comments. 136 I. MCDOUGALL

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