Long-term variations in palaeointensity

Abstract

We compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass. The basaltic glass data more than double the number of reliable (Thellier method with pTRM checks) palaeointensity estimates available. Although the new data dramatically improve both spatial and temporal coverage, there is still a strong bias toward the most recent past. The last 0.3 Ma claim over half of the data in our combined database. We therefore divide the data into two groups, the densely sampled last 0.3 Myr and the more sparsely sampled period of time comprising roughly half of the data from 0.3 to 300 Ma. Separating them in this way, it is clear that the dipole moment of the Earth over the past 0.3 Myr (ca. 8 10 22 A m 2 ) is dramatically higher than the average dipole moment over the preceding 300 Myr (ca. 5 10 22 A m 2 ). Inclusion of poor-quality results leads to an overestimate of the average dipole moment. Interestingly, no other signi cant changes in the distribution of dipole moments are evident over the 300 million year span of the data.

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

“…This points to the necessity of using experimental designs that avoid or correct for pervasive MD behaviors with sufficient independent cooling units to obtain reliable time-averaged paleointensities. The consistency of our high-fidelity MD-corrected Galapagos lava paleointensity results and the reported SD-behaved McMurdo (and now also Iceland) results with GAD predictions allows us to confirm with more confidence that the Pliocene-Pleistocene average paleointensity is indeed much weaker than today's geomagnetic field, as already suspected from results from targeted SD recorders like submarine basaltic glass (19). This has several significant implications.…”

“…Foremost, these results show that the past several-millionyear-long time-averaged geomagnetic field is predominantly that of a GAD in direction as well as intensity, validating, for example, the calculation of VADMs and various total field models [e.g., TK03 (20)]. Second, our average dipolar paleointensity estimation for the past ∼5 My is consistent with average paleointensities estimated for the most SD material for the past 140 My [VADM = ∼4.2 × 10 22 A·m 2 (21)] and even the past 300 My [VADM = ∼4.6 × 10 22 A·m 2 (19)], values that are also only about 50-60% of the present-day magnitude. Third, a lower average paleointensity over at least the past few million years results in a shorter average steady-state magnetopause standoff distance of only ∼9 Earth radii (R E ) compared with ∼11 R E today (22).…”

Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C

“…SCAT is meant to replace the various statistics designed to test for sample alteration, pTRM tails and excessive noise in the NRM lost versus pTRM gained (Arai plots of Nagata et al [1963]). The slope of the best-fit line through the data in the Arai plot is allowed to vary within the bounds defined by threshold value of the ''scatter'' statistic [Coe et al, 1978, renamed by Selkin andTauxe, 2000] which is the ratio of the standard error of the slope b to the absolute value of the slope (jbj). The threshold value of , threshold , can be used to define two bounding lines (Figure 2).…”

“…In an attempt to compare data of similar quality, Selkin and Tauxe [2000] had excluded all data sets that were not done by double heating methods including pTRM checks (a widely used test for alteration). In fact, the exclusion of studies with no pTRM checks does indeed change the debate concerning the Jurassic field as none of the Armenian data had been subjected to the complete paleointensity experiment including pTRM checks.…”

The Fluid Mechanics of Astrophysics and Geophysics

“…To date no long term correlation has been found between average absolute intensity of the paleomagnetic field and long term reversal rate (Selkin and Tauxe, 2000), although one was suspected for some time (see Prévot et al, 1990). It should be emphasized, however, that there remain insufficient data to characterize absolute paleointensity very well over the past 300 Ma, and in the CNS a period which is of great interest because of the absence of documented reversals there is only a handful of reliable absolute paleointensity data available despite continuing efforts to acquire such observations (see e.g.…”