Richard K. Bono

A Hadean to Paleoarchean geodynamo recorded by single zircon crystals

Unlocking Earths ancient magnetic past The magnetic field protects Earths surface from deadly cosmic radiation and provides clues about the planets interior. Tarduno et al. found that some of the oldest minerals on Earth, Jack Hills zircons, preserved a record of a magnetic field over 4 billion years ago (see the Perspective by Aubert). Earths magnetic field appears to have been fully operational a mere few hundred million years after the planet formed. This suggests an early start for plate tectonics and an ancient cosmic radiation shield that was important for habitability Science, this issue p. 521; see also p. Paleomagnetic measurements on Jack Hills zircons suggest that the magnetic field may be at least 4.2 billion years old. [Also see Perspective by Aubert] Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.

A stable Ediacaran Earth recorded by single silicate crystals of the ca. 565 Ma Sept-Îles intrusion

The archetypical example of inertial interchange true polar wander (IITPW), the rapid rotation of the entire solid Earth by 90°, is based on two nearly orthogonal directions seen in paleomagnetic studies of the Sept-Iles (ca. 565 Ma) intrusion (Quebec, Canada). This motion has also been proposed as the driving force for the early Cambrian explosion of life. Others have challenged these interpretations, linking the data instead to flips between an axial and equatorial dipole field configuration. We examine this enigma using single silicate crystal paleomagnetic analyses. We conclude that only one of the previously reported magnetic directions is carried by single domain magnetic grains and can be considered primary. Thus, the Ediacaran diversification occurred on a rotationally stable Earth driven by biotic and longer term abiotic forcing. Whether IITPW ever occurred in the past on Earth remains uncertain.

A Large Ornithurine Bird ( Tingmiatornis arctica ) from the Turonian High Arctic: Climatic and Evolutionary Implications

Bird fossils from Turonian (ca. 90 Ma) sediments of Axel Heiberg Island (High Canadian Arctic) are among the earliest North American records. The morphology of a large well-preserved humerus supports identification of a new volant, possibly diving, ornithurine species (Tingmiatornis arctica). The new bird fossils are part of a freshwater vertebrate fossil assemblage that documents a period of extreme climatic warmth without seasonal ice, with minimum mean annual temperatures of 14 °C. The extreme warmth allowed species expansion and establishment of an ecosystem more easily able to support large birds, especially in fresh water bodies such as those present in the Turonian High Arctic. Review of the high latitude distribution of Northern Hemisphere Mesozoic birds shows only ornithurine birds are known to have occupied these regions. We propose physiological differences in ornithurines such as growth rate may explain their latitudinal distribution especially as temperatures decline later in the Cretaceous. Distribution and physiology merit consideration as factors in their preferential survival of parts of one ornithurine lineage, Aves, through the K/Pg boundary.

New Archeomagnetic Directional Records From Iron Age Southern Africa (ca. 425-1550 CE) and Implications for the South Atlantic Anomaly

The paucity of Southern Hemisphere archeomagnetic data limits the resolution of paleosecular variation models. At the same time, important changes in the modern and historical field, including the recent dipole decay, appear to originate in this region. Here a new directional record from southern Africa is presented from analysis of Iron Age (ca. 425–1550 CE) archeological materials, which extends the regional secular variation curve back to the first millennium. Previous studies have identified a period of rapid directional change between 1225 and ∼1550 CE. The new data allow us to identify an earlier period of relatively rapid change between the sixth and seventh centuries CE. Implications for models of recurrent flux expulsion at the core-mantle boundary are discussed. In addition, we identify a possible relationship of changes recorded in these African data with archeomagnetic jerks. Plain Language Summary Earth’s dipole magnetic field is presently undergoing a rapid decay, best expressed by a deepening area of low field called the South Atlantic Anomaly (SAA). This apparent collapse of the geomagnetic field, and speculation about a future field reversal, has captured the public imagination. But we know little about the history of the SAA, limiting our ability to place current changes within a long-term context. Here we present a new magnetic record from sites of southern Africa. The new record supports our prior inferences that the SAA is just the most recent manifestation of a recurring phenomenon in the core beneath Africa—called flux expulsion—that is having a profound impact on the expression of the geomagnetic field.