Toward constraining the long-term reversing behavior of the geodynamo: A new “Maya” superchron ∼1 billion years ago from the magnetostratigraphy of the Kartochka Formation (southwestern Siberia)
Résumé
We present new magnetostratigraphic data obtained from the Late Mesoproterozoic (∼1 Ga) Kartochka Formation in the East Angara terrane of the Yenisey Ridge region (southwestern Siberian platform). A ∼200 m-thick section encompassing the carbonate Kartochka Formation was densely sampled and another more limited outcrop several kilometers away was sampled in order to conduct a paleomagnetic fold test between the two sites. Paleomagnetic analyses revealed the existence of two magnetization components. A low unblocking temperature component, which likely has a viscous origin, was first isolated below 200 °C. A characteristic component, carried either by magnetite or by a mixture of magnetite and hematite, was then isolated in the medium to high temperature range (up to 565-585 °C or 680 °C). The hematite-bearing component has a shallower inclination than the magnetite component showing that magnetite was less sensitive to flattening. A positive paleomagnetic fold test was obtained between the two studied sections indicating that the characteristic magnetization was likely acquired during or very soon after sediment deposition. This primary origin was further verified by comparison between the paleomagnetic poles derived for the Kartochka Formation and other late Mesoproterozoic Siberian sections. All data from the Kartochka Formation contain a single magnetic polarity assumed to be normal. This long normal polarity interval is consistent with the polarity zonation previously determined from magnetostratigraphic and paleomagnetic data from the Siberian Uchur Maya region. The new data fortify evidence of a normal polarity superchron spanning the ∼1000 Ma old Mesoproterozoic-Neoproterozoic boundary, which we propose to call the Maya superchron. These data confirm the occurrence of sharp transitions between a frequently reversing regime and a non-reversing regime of the geodynamo. This behavior, also observed in other regions, may represent a consistent property of the Proterozoic geomagnetic field. Together with changes in the amount of time spent in the superchron regime, they may testify to a different field behavior during the late Precambrian than during the Phanerozoic. This difference could reflect a stronger influence of the heterogeneous heat flux patterns at the core-mantle boundary during the Precambrian, possibly as a consequence of the inner core not yet being nucleated at this time.