New paleointensity results from rapidly cooled Icelandic lavas: Implications for Arctic geomagnetic field strength

Abstract

The Earth's magnetic field is assumed to be a geocentric axial dipole (GAD) when averaged over sufficient time (10 5 -10 6 years). Recent investigations of global paleosecular variation and time-averaged field behavior on million year timescales generally support a predominantly dipole field in the Northern Hemisphere, but unique field structures at high southern latitudes suggest the presence of a substantialḡ 0 2 quadrupolar component. Average paleointensity results from Antarctica are approximately half the value predicted by a GAD field; this behavior has not been sufficiently investigated because there is a paucity of absolute paleointensity data from the high latitudes of the Arctic and Antarctic, so no adequate comparisons have been made between the two regions. We collected glassy volcanic material from 129 subaerial and subglacial volcanic units in Iceland in order to provide a suitable intensity data set at high northern latitudes. Forty-four sites met our very strict specimen and site level selection criteria. Four Holocene sites have a median intensity value of 55.8 ± 15.6 μT (virtual axial dipole moment = 78.1 ± 22.0 ZAm 2 ), consistent with the present-day field. Thirty-seven sites are between 11 ka and 3.35 Ma with a median intensity of 33.1 ± 8.3 μT (47.0 ± 11.6 ZAm 2 ). This median intensity is indistinguishable from some long-term global field strength estimates. Reevaluation of existing high-latitude data suggests a general agreement with our Iceland results, but there are still too few Antarctic sites to adequately compare Arctic and Antarctic field behaviors.

Paleointensity From Subaerial Basaltic Glasses From the Second Hawaii Scientific Drilling Project (HSDP2) Core and Implications for Possible Bias in Data From Lava Flow Interiors

“…If the time‐averaged geomagnetic field is well approximated by a GAD model (as strongly suggested by directional data, for example, Opdyke & Henry, ), then the paleointensity of the field should follow the relation of (1 + 3cos 2 θ m ) 1/2 (where θ m is colatitude), which is twice as strong at the poles than at the equator. However, the time‐averaged field calculated from published paleointensity data from either the past 0.78 Ma (Figure ) or 5 Ma (Cromwell, Tauxe, & Halldórsson, ; Lawrence et al, ) is poorly fit by a GAD model, with the paleointensities at midlatitudes similar to those at the poles.…”

Holocene Paleointensity of the Island of Hawai `i From Glassy Volcanics

“…;Hill & Shaw, 2000;Laj et al, 2002;Teanby et al, 2002;Chauvin et al, 2005;Gratton et al, 2005;Oishi et al, 2005;Pressling et al, 2006;Pressling et al, 2007;de Groot et al, 2013;Cromwell, Tauxe, & Halldorsson, 2015). These published…”

Considerations for Latitudinal Time-Averaged-Field Palaeointensity Analysis of the Last Five Million Years

“…In studies that compare intensity vs. latitude (e.g., Lawrence et al, 2009;Cromwell et al, 2015;Wang et al, 2015;Døssing et al, 2016), it is the geographical latitude, not the palaeomagnetic latitude, that is used; this removes the circular logic of using GAD theory to calculate the palaeomagnetic latitude in a study of the robustness of the GAD hypothesis. Only palaeointensity data from the last 5 Myr are usually considered, because, as a first-order approximation, this removes the need to include any tectonic corrections to the sample locations' geographic latitudes.…”

“…However, in the last 20 years the palaeointensity database has changed significantly, with the inclusion of far more high-latitude data points. Additionally, the selection criteria for drawing samples from the database are far more stringent now than it was in the late 1990s (e.g., Lawrence et al, 2009;Cromwell et al, 2015;Wang et al, 2015;Døssing et al, 2016). As we will TABLE 1 | Parameters for secular variation models considered in this paper.…”

“…In the analysis of palaeointensity data as a function of latitude, it is common to plot palaeointensity data as a function of latitude for latitudinally binned "reliable" absolute palaeointensity data from the last 5 Myr using the PINT database (Biggin et al, 2009) available at http://earth.liv.ac.uk/pint; when binning the data sometimes the average of each bin is taken (e.g., Lawrence et al, 2009;Wang et al, 2015) and sometimes the median (e.g., Cromwell et al, 2015;Døssing et al, 2016). The definition of "reliable" varies between studies, but the same trend is observed: the measured intensity data deviates most from the GAD intensity model B D at high latitudes, where B D varies with the (magnetic) co-latitude θ by.…”