Toni Veikkolainen

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

PALEOMAGIA: A PHP/MYSQL database of the Precambrian paleomagnetic data

Most paleomagnetic applications require a precise, rationally organized and up-todate catalogue or database of paleomagnetic results worldwide. These include reconstructions of continents, calculations of the Apparent Polar Wander Paths (APWPs) or paleolatitude drift curves, testing the Geocentric Axial Dipole (GAD) model, studies of geomagnetic paleosecular variation or reversal asymmetries, comparison of coeval results obtained from different types of rocks, estimation of inclination shallowing in sedimentary rocks and understanding the delay in remanence acquisition caused by slow cooling in large intrusions. For this purpose, various databases, such as the Global Paleomagnetic Database (GPMDB), and the Magnetics Information Consortium Database (MagIC) have been generated. This paper presents a new relational database (PALEOMAGIA) where 3278 entries of Precambrian data have been split geographically, sorted according to age and rock types and ranked using a revised version of the Van der Voo grading scheme. The latest geochronologic information is included wherever available. Significant effort has been put to the retrieval and archiving of data published in the last decade, which are virtually nonexistent in GPMDB. Here we present the database and its browser-based user interface from a scientific and a technical point of view.

Advancing Precambrian palaeomagnetism with the PALEOMAGIA and PINT(QPI) databases.

State-of-the-art measurements of the direction and intensity of Earth’s ancient magnetic field have made important contributions to our understanding of the geology and palaeogeography of Precambrian Earth. The PALEOMAGIA and PINT(QPI) databases provide thorough public collections of important palaeomagnetic data of this kind. They comprise more than 4,100 observations in total and have been essential in supporting our international collaborative efforts to understand Earths magnetic history on a timescale far longer than that of the present Phanerozoic Eon. Here, we provide an overview of the technical structure and applications of both databases, paying particular attention to recent improvements and discoveries.

Mesoproterozoic Nuna Supercontinent and the Geomagnetic Field in Light of Recent Paleomagnetic Data from Diabase Dykes of Finland

Mesoproterozoic supercontinent Nuna (e.g. Columbia, Hudsonland) has increased in recent years enabling more reliable global continental reconstructions (e.g Hoffman 1997; Evans and Mitchell 2011; Zhang et al. 2012; Pisarevsky et al. 2014). Supercontinent Nuna included Baltica, Laurentia, Siberia, proto-Australia and Antarctica, Amazonia and West Africa, Congo-São Francisco, North China, Kalahari and India cratons. Baltica and Laurentia are thought to represent two of the most important building blocks of this supercontinent in a single geologically valid NENA (North EuropeNorth America) juxtaposition between ca. 1.75-1.27 Ga forming the core of Nuna with Siberia (e.g. Gower et al. 1990; Evans and Mitchell 2011). Recent high quality, precisely dated Mesoproterozoic paleomagnetic poles of Baltica support the NENA connection. These include the pole from Åland (1575.9 ± 3.0 Ma; U-Pb) diabase dykes (Salminen et al. 2015) and coeval pole from Satakunta diabase dykes (Salminen et al. 2014) in Finland; a pole for the Mesoproterozoic Satakunta sandstones in Finland (Klein et al. 2014); and poles for Lake Ladoga basalts and intrusives (1459 ± 3, 1457 ± 2 Ma; U-Pb) in Russia (Salminen and Pesonen 2007; Lubnina et al. 2010). One striking feature of the 1.576 Ga high quality paleomagnetic data for Åland and Satakunta is the asymmetry of polarity, i.e. the mean directions of normal (N) and reversed (R) polarities are not antiparallel at 95% confidence level and do not pass the reversal test (McFadden and McElhinny 1990). One possible reason for such an asymmetry could be an unusual behavior of the geomagnetic field at the Mesoproterozoic, which would hamper the paleomagnetic reconstructions. Antipodality of N and R directions is expected in the case where the geomagnetic field is represented by the geocentric axial dipole (GAD), whereas steepening or shallowing of inclinations can result from the contamination of GAD by zonal multipolar fields. We used 26 global dual-polarity paleomagnetic results from PALEOMAGIA database (Veikkolainen et al. 2014a) to detect possible deviations from the GAD hypothesis (Hospers 1954) applying the quantity called inclination asymmetry (Veikkolainen et al. 2014b). The asymmetry tests indicate that GAD is a relatively good fit at the Mesoproterozoic (1.7-1.4 Ga) and therefore zonal multipolar fields do not explain the observed asymmetry. One other possible reason for asymmetry is an unremoved secondary component, which could explain the asymmetry for Åland and Satakunta data. Additional support for component mixing comes from the secondary component distribution, which is streaked in part toward the N-polarity direction. A third reason can be a small but significant age difference between N and R magnetized dykes which could explain the asymmetry. However, the actual age span for the Mesoproterozoic dykes for Baltica awaits further precise age dating. In addition to results from Åland, Satakunta and Lake Ladoga we present here new high quality Mesoproterozoic paleomagnetic and geochronological results from the Häme dykes (1642 ± 2 Ma, 1647 ± 14 Ma; U-Pb) in Finland that do not show asymmetry. These results also support the NENA connection placing Baltica on equatorial latitudes at 1.64 Ga.