Revised geomagnetic polarity time scale for the interval 0–5 m.y. B.P.

Abstract

A change in the constants used in K-Ar dating and a significant increase in new data have made a recompilation and recomputation of data used to define the Late Cenozoic K-Ar'polarity time scale highly desirable at this time. All available data in the range 0-5 m.y. have been recalculated using the refined constants, with 354 data points in this time interval now meeting the minimum criteria for acceptability. Recalculation of the major polarity epoch boundaries has yielded ages of 0.73 m.y. for the Brunhes-Matuyama, 2.48 m.y. for the Matuyama-Gauss, and 3.40 m.y. for the Gauss-Gilbert boundaries. A revised polarity time scale has been constructed based on available K-Ar data and information obtained from marine magnetic anomalies and deep-sea sedimentary cores.Over the past few years there have appeared new data on the atomic abundance and decay activities of 4øK that suggest that a change in the constants used in K-Ar dating is appropriate. Table 1). The abundance ratios for the argon isotopes remain unchanged. Many laboratories have already adopted these improved constants, and it is anticipated that the rest will do so shortly. In August 1977 the International Union of Geological Sciences (IUGS) Subcommission on Geochronology recommended the adoption of new decay and abundance constants [Steiger and Jitter, 1977] (The adoption of new constants is always troublesome, but it is especially critical in time-scale applications and when comparing new data with old. Since the Late Cenozoic K-Ar geomagnetic reversal time scale was developed in the middle and late 1960's (see reviews by Dalrymple [1972] and Watkins [1972]), it has been used extensively as a correlation tool on both the continents and the sea floor. Although accurately defined by K-Ar dating only over the interval from about 0.01 to 5 m.y. ago, this land-based reversal time scale has been extended by extrapolation into the Mesozoic using the sea floor magnetic anomaly pattern [Heirtzler et al., 1968] and is now a primary dating tool for the ocean basins. The purpose of this paper is to update, through late 1977, the compilation of data used to define the K-Ar polarity time scale and to reconcile all the data with the new K-Ar constants. In addition, since the last compilation [Dalrymple, 1972] the number of data in the interval 0.01-5 m.y. has nearly doubled, making a recalculation of the principal polarity epoch boundaries worthwhile. Because of the logarithmic term in the K-Ar age equation the effect of the change in constants is nonlinear. K-Ar ages calculated with the new constants are 2.68% older than those calculated with the old constants at 1 m.y. but 1.73% younger at 4500 m.y.; there is no difference at 1900 m.y. (Figure 1). Over the period of the K-Ar reversal time scale, i.e., 0.01-5 m.y., the relationship between new and old ages is both constant and linear to within 0.01%. Between 0.01 and 100 m.y. (the 'sea floor spreading' time scale) the difference changes by 0.19% and becomes noticeably nonlinear above about 10 m.y. instances a simple ...

Integrated magnetostratigraphy and biostratigraphy of the upper Pliocene-lower Pleistocene from the Monte Singa and Crotone areas in Calabria, Italy

“…Linear interpolation between the Gauss/Matuyama and the Matuyama/Brunhes boundary in deep-sea cores yields an average duration of 150 ka (1.91-1.76 Ma; [30,33]), whereas analysis of near-bottom marine magnetic anomalies yields a duration of 220 ka (1.88-1.66 Ma; [31,34,35]). The second value is closely approached by the duration of 200 ka (1.87-1.67 Ma) in the polarity time scale of Mankinen and Dalrymple [36], who apparently placed the boundaries at the oldest and youngest K/Ar ages of absolutely dated normal polarity observations. A duration of 160 ka for the Olduvai subchron agrees well with other astronomically based estimates of 170 ka (1.82-1.65 Ma; [25]) and 180 ka (1.95-1.77 Ma; [32]).…”

“…Furthermore, several Indian Ocean deep-sea cores have revealed the presence of a single site with reversed polarity just below the upper boundary of the Olduvai subzone [39]. Nevertheless, Mankinen and Dalrymple [36] conclude that the lack of sufficient confirmation in other cores prevents the unambiguous recognition of reversed polarities below the upper boundary of the Olduvai subzone (or a short normal polarity event following the Olduvai subchron). The lack of sufficient confirmation, however, might rather be due to a lack of sufficiently detailed sampling.…”

Geophysical evidence for widespread reversely magnetised pyroclastics in the western Taupo Volcanic Zone (New Zealand)

“…Although the K-Ar age of 1.06 ± 0.09 Ma of this flow appears to be older than that indicated by its paleomagnetic normal polarity, we can reconcile both results by assuming that this flow was deposited during the Jaramillo geomagnetic epoch (i.e. between 0.90 and 0.97 Ma ago; Mankinen & Dalrymple 1979). Since the K-Ar age of the hornblende minerals dates crystallisation (Cliff 1985), whereas magnetic polarity relates to the date of extrusion, we believe that the small difference between the two dates is not significant.…”

Fission-track ages of some volcanic rocks in the Yaeyama area, Kagoshima Prefecture, Japan.

“…The ages of the pyroclastic flow 3 were 1.2 m. y. and 1.0 m. y. The errors of these data have not been stated, but in their previous report on the fission-track ages of some pyroclastic flows in southern * MANKINEN and DALRYMPLE (1979) ** NISHIMURA et al , (1973,1976) Fig Kyushu (Nishimura and Miyachi, 1973), errors of about 20 percent are estimated. Assuming that the three ages of pyroclastic flow deposits also have about 20 percent errors, the fission-track ages of those pyro clastic flow deposits match the stratigraphic sequence.…”