The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

Abstract

We firstly present the results of a detailed palaeointensity study performed on 54 samples from 9 volcanic units of late Archaean age (2724-2772 Ma) from the Pilbara Craton, Western Australia. These results were severely affected by magnetomineralogical alteration occurring during the laboratory heating process necessitating the application of a correction procedure. The correction allowed results from three lavas to pass strict selection criteria but we deem that only one of these exhibits sufficient internal consistency to be considered moderately reliable. It yields a virtual dipole moment of 47±6 ZAm 2 which is 60% of the present-day value. We combine this determination with a filtered dataset from the updated IAGA (International Association of Geomagnetism and Aeronomy) palaeointensity database, PINT08. Directional secular variation has recently been shown to have changed fundamentally since the Archaean, probably as a consequence of inner core growth since that time. However, here we argue that it is still unclear whether this evolution was accompanied by a single long timescale change in average poloidal field intensity. While the distribution of Precambrian palaeointensity determinations as a whole is significantly lower than that for the last 300 Myr, we show that this finding largely reflects data from the Proterozoic aeon. The distribution of more ancient measurements from the late Archaean-earliest Proterozoic is indistinguishable from that of the last 300 Myr which might suggest that a 'Proterozoic dipole low' period existed between two periods of higher field intensity. Were this pattern of long-term geomagnetic intensity variation to be supported by the addition of new data in the future, then it could indicate a related three-stage evolution in core dynamics, namely: vigorous thermal convection caused by high core-mantle heat flux early in the Earth's history, weaker thermal convection later as the heat flux fell, and finally, strong compositional convection since the inner core nucleated.

Paleointensity Database Updated and Upgraded

“…The database, fi rst compiled by Tanaka and Kono [1994], has received numerous updates [Perrin and Shcherbakov, 1997;Perrin et al, 1998;Perrin and Schnepp, 2004;Tauxe and Yamazaki, 2007;Biggin et al, 2009]. It now contains 3764 paleointensity measurements from 291 references published between 1938 and 2009.…”

Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations?

“…3.2.1 -Estimation of the cosmogenic paleo-production rate (Biggin et al, 2009) induced cosmogenic production variations of less than 5% over the considered time period (last 10 Myr) (Dunai, 2001). Therefore, we assume that the paleoproduction rate of 10 Be was similar to the modern rate.…”

Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic

“…Our understanding of the geomagnetic field is limited by the 43 quality of the palaeointensity (ancient geomagnetic field intensity) database, which is incomplete both 44 spatially and temporally. The PINT08 database contains all published absolute palaeointensity data older 45 than 50 ka (Biggin et al, 2009). Over recent years the palaeomagnetic community has made a great effort 46 to populate this database, which now contain in excess of 4000 records.…”