Huapei Wang

Solar nebula magnetic fields recorded in the Semarkona meteorite

Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there have been no experimental constraints on the intensity of these fields. Here we show that dusty olivine-bearing chondrules from the Semarkona meteorite were magnetized in a nebular field of 54 ± 21 microteslas. This intensity supports chondrule formation by nebular shocks or planetesimal collisions rather than by electric currents, the x-wind, or other mechanisms near the Sun. This implies that background magnetic fields in the terrestrial planet-forming region were likely 5 to 54 microteslas, which is sufficient to account for measured rates of mass and angular momentum transport in protoplanetary disks. Magnetic field strength in the early solar system is recorded in chondrules within a meteorite born of the asteroid Vesta. Magnetic moments in planetary history To know the magnetic history of the solar nebula in the age of planet formation, researchers turn to the most primitive meteorites. Samples such as the Semarkona chondrite are composed partly of chondrules, which reflect the strength of the ambient magnetic field when this material was last molten. Fu et al. used a SQUID microscope to measure the remnant magnetization in a section of Semarkona. The findings reveal secrets about what goes on inside protoplanetary disks. Science, this issue p. 1089

Evidence for abundant isolated magnetic nanoparticles at the Paleocene-Eocene boundary

New rock magnetic results (thermal fluctuation tomography, high-resolution first-order reversal curves and low temperature measurements) for samples from the Paleocene–Eocene thermal maximum and carbon isotope excursion in cored sections at Ancora and Wilson Lake on the Atlantic Coastal Plain of New Jersey indicate the presence of predominantly isolated, near-equidimensional single-domain magnetic particles rather than the chain patterns observed in a cultured magnetotactic bacteria sample or magnetofossils in extracts. The various published results can be reconciled with the recognition that chain magnetosomes tend to be preferentially extracted in the magnetic separation process but, as we show, may represent only a small fraction of the overall magnetic assemblage that accounts for the greatly enhanced magnetization of the carbon isotope excursion sediment but whose origin is thus unclear.

Lifetime of the solar nebula constrained by meteorite paleomagnetism

Meteorite magnetism in the early solar system The young solar system contained a disc of gas and dust within which planet formation occurred. The disc eventually dissipated after the Sun ignited and the planets formed, but exactly when that happened has been difficult to determine. Wang et al. measured tiny magnetic fields preserved in angrites, an ancient type of meteorite. They interpret a drop in magnetic field strength about 4 million years after the solar system formed as a sign that the gas had cleared—along with the magnetic field that it carried. The results will enhance our understanding of planet formation, both in our solar system and around other Sun-like stars. Science, this issue p. 623 Magnetic fields in meteorites show how long it took for the gas in the protosolar disk to clear. A key stage in planet formation is the evolution of a gaseous and magnetized solar nebula. However, the lifetime of the nebular magnetic field and nebula are poorly constrained. We present paleomagnetic analyses of volcanic angrites demonstrating that they formed in a near-zero magnetic field (<0.6 microtesla) at 4563.5 ± 0.1 million years ago, ~3.8 million years after solar system formation. This indicates that the solar nebula field, and likely the nebular gas, had dispersed by this time. This sets the time scale for formation of the gas giants and planet migration. Furthermore, it supports formation of chondrules after 4563.5 million years ago by non-nebular processes like planetesimal collisions. The core dynamo on the angrite parent body did not initiate until about 4 to 11 million years after solar system formation.

A two-billion-year history for the lunar dynamo

Paleomagnetic evidence suggests the lunar dynamo persisted beyond 2.5 Ga, requiring an exceptionally long-lived power source. Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history.

Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Significance Our multidomain-corrected paleointensity results from near-equatorial lavas from the Galapagos give a mean intensity only about one-half of that obtained from the only robust published result from near-polar lavas from Antarctica. This new evidence is consistent with the factor-of-2 equator-to-pole paleointensity signature of a geocentric axial dipole field and also indicates that the time-averaged field is considerably weaker than the present-day field. The resulting dipole moment provides a new calibration standard for cosmogenic isotope production rates and suggests that the present decrease in geomagnetic field intensity may simply be a return to a more average magnitude rather than a harbinger of a polarity reversal. The geomagnetic field is predominantly dipolar today, and high-fidelity paleomagnetic mean directions from all over the globe strongly support the geocentric axial dipole (GAD) hypothesis for the past few million years. However, the bulk of paleointensity data fails to coincide with the axial dipole prediction of a factor-of-2 equator-to-pole increase in mean field strength, leaving the core dynamo process an enigma. Here, we obtain a multidomain-corrected Pliocene–Pleistocene average paleointensity of 21.6 ± 11.0 µT recorded by 27 lava flows from the Galapagos Archipelago near the Equator. Our new result in conjunction with a published comprehensive study of single-domain–behaved paleointensities from Antarctica (33.4 ± 13.9 µT) that also correspond to GAD directions suggests that the overall average paleomagnetic field over the past few million years has indeed been dominantly dipolar in intensity yet only ∼60% of the present-day field strength, with a long-term average virtual axial dipole magnetic moment of the Earth of only 4.9 ± 2.4 × 1022 A⋅m2.

Quantified abundance of magnetofossils at the Paleocene–Eocene boundary from synchrotron-based transmission X-ray microscopy

Significance The Paleocene–Eocene thermal maximum (PETM) is an abrupt global warming event that occurred at about 55.8 Ma and is closely linked to a large carbon isotope excursion. What caused the PETM is unresolved. An unusual abundance of single-domain magnetite particles in PETM sediments on the Atlantic Coastal Plain might represent condensates from a comet impact. Alternatively, the magnetic nanoparticles may be of biogenic origin. We are now able to quantify the concentration of those magnetic grains that are distinctly of biogenic origin using synchrotron-based transmission X-ray microscopy. These and related findings allow us to exclude magnetofossils as a significant source of magnetization of the PETM sediments and point to an impact condensate origin of the magnetite particles. The Paleocene–Eocene boundary (∼55.8 million years ago) is marked by an abrupt negative carbon isotope excursion (CIE) that coincides with an oxygen isotope decrease interpreted as the Paleocene–Eocene thermal maximum. Biogenic magnetite (Fe3O4) in the form of giant (micron-sized) spearhead-like and spindle-like magnetofossils, as well as nano-sized magnetotactic bacteria magnetosome chains, have been reported in clay-rich sediments in the New Jersey Atlantic Coastal Plain and were thought to account for the distinctive single-domain magnetic properties of these sediments. Uncalibrated strong field magnet extraction techniques have been typically used to provide material for scanning and transmission electron microscopic imaging of these magnetic particles, whose concentration in the natural sediment is thus difficult to quantify. In this study, we use a recently developed ultrahigh-resolution, synchrotron-based, full-field transmission X-ray microscope to study the iron-rich minerals within the clay sediment in their bulk state. We are able to estimate the total magnetization concentration of the giant biogenic magnetofossils to be only ∼10% of whole sediment. Along with previous rock magnetic studies on the CIE clay, we suggest that most of the magnetite in the clay occurs as isolated, near-equidimensional nanoparticles, a suggestion that points to a nonbiogenic origin, such as comet impact plume condensates in what may be very rapidly deposited CIE clays.