Paleointensity and Geodynamo

These averaged palaeointensity data, though unevenly spanning some 40+ Myr of field behavior just prior to the onset of the CNS display, appear to follow a coherent trend: one in which the average field intensity increases as the CNS is approached. Put another way, this compilation suggests that an inverse relationship between reversal rate and palaeointensity may exist, supporting the contention of Cox (1968) that reversal rate and geomagnetic strength are coupled. More recently, a similar conclusion was proposed by Tarduno et al. (2001) based on a high geomagnetic intensity during the CNS.  The consistency of the pre-CNS data with Tarduno et al.'s CNS results suggests an underlying geodynamic cause that couples rate with strength. Nevertheless, it should be noted that with confidence couples exist from 165 to 120 Ma in which the density of data is high, whilst conclusion inside the CNS remain controversial until more reliable palaeointensity data are made available. [PDF]

The calculated mean VDM value of (4.2±0.1)´10²²Am² of this study is remarkably consistent with previously published data from the Zhuanchengzi and Sihetun sections dated to 120.93±0.88 Myr and 133-124 Myr. The findings suggest extension of the relative low nature of the pre-CNS magnetic field strength into the very beginning of the CNS. A relationship between reversal rate and paleointensity remains a contentious topic in the study of the geodynamo. In fact, two main interpretations have been advanced for the evolution of the geomagnetic reversal rate since 160 Myr. [PDF]

It should be noted, however, that when a time-averaged VDM of 12.5±1.4´10²² Am² covering time interval from 116 Myr to 113 Myr is considered, the VDM behavior seems to gradually increase from its value of about 2.34±0.2´10²² Am² at about 162 Myr to 4.2±0.1´10²² Am² at about 116.8 Myr (ISEA?), and then displays a rapid increase between 116 Myr and 113 Myr. This suggests that state of the geodynamo experienced a sudden change at about the time of the ISEA.[PDF]

Our study, employing Thellier-Thellier absolute palaeointensity method, provides a reliable determination of geomagnetic field strength for the interval of 105.5-102.2 Ma. We now attempt an analysis of palaeofield strength within the CNS, although there are relatively few palaeointensity estimates involved. Indeed, there are no criteria presently available that could provide an assurance in the absence of observatory geomagnetic field measurements. Therefore, in order to analyze the palaeointensity database, we temporally used the pTRM-check criterion.

When combined these selected data with our palaeointensity result, a mean VDM of (5.17±0.8)´10²² Am² is obtained within the CNS, a value that is approximately half of that of the present-day field. If we now remove the data obtained from SBG, it then gives a mean value of (5.37±0.82)´10²² Am², which is very similar to (5.17±0.8)´10²² Am². Hence, SBG data have been included though reliability of SBG study is now questioned.

The strength of the Earth magnetic field within the CNS is an important subject, not only to discuss the relationship between polarity reversal frequency and palaeointensity, but also to consider the geodynamo within the Earth. It is, therefore, valuable to compare mean palaeointensity within the CNS with that out of it, i.e., the entire 65-7 Ma interval. The mean dipole moment of (5.17±0.8)´10²² Am² within the CNS is close to the mean value of (5.26±2.89)´10²² Am² for the entire 65-7 Ma interval (Shi et al., 2002), suggesting that the mean strength of the Earth magnetic field within the CNS is comparable to that out of the CNS though some high paleointensities using the plagioclase crystals have been reported (Tarduno et al., 2001, 2002). Further, we noted that the paleointensity values within the CNS exhibit a very highly variable nature. It may be associated with the dynamo processes during the CNS, if those palaeointenisity estimations are correct. Clearly, more data are needed to better understand relationship between this long-term variation of palaeointensities and geodynamo process.

Is there a precursor to the Cretaceous normal superchron?

Although the geomagnetic reversal record for the past 160 Myr is relatively well documented, whether a precursor exists before the onset of the Cretaceous normal superchron (CNS) remains a contentious topic in the study of the evolution of geomagnetic reversals. In the present study we have tried to find out if new paleointensity data can provide more information about this topic. A paleointensity, geochronologic and paleomagnetic investigation has been conducted on four andesitic basalt lava flows from Hulahada in Liaoning Province, northeastern China. Radiometric ⁴⁰Ar/³⁹Ar dating experiments indicate the volcanism occurred at about 122.7±0.6 Myr, corresponding to marine anomaly M1n. A mean virtual dipole moment of (4.8±0.2)´10²² Am² was obtained about half the strength of the modern-day field. Our paleointensity results in conjunction with previous data suggest that there is no precursor to the CNS.