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Analysis of a database of Precambrian palaeomagnetic intensity measurements reveals a clear transition in the Earth’s magnetic field that is probably the signature of the inner core first forming, suggesting a modest value of core thermal conductivity and supporting a simple thermal evolution model for the Earth. Tracking Earth's thermal evolution The record of the intensity and orientation of Earth's magnetic field provides a constraint on the thermal evolution of Earth through its influence on the geodynamo. Here Andrew Biggin et al . analyse a database of palaeomagnetic intensity measurements and confirm that the time-averaged Precambrian magnetic field was — as is commonly assumed — dominantly dipolar. They also find evidence for long-term variations in geomagnetic field strength, with an increase in both average field strength and variability observed to occur between 1 and 1.5 billion years ago. They conclude that this increase could be explained by nucleation of the inner core during this interval, the timing of which would support a simple thermal evolution model for the Earth. 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 1 , 2 , 3 , 4 , 5 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 6 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 7 selected using a new set of reliability criteria 8 . 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 9 ; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

Absolute paleointensity (API) of the geomagnetic field can be estimated from volcanic rocks by comparing the natural remanent magnetization (NRM) to a laboratory‐induced thermoremanent magnetization (Lab‐TRM). Plots of NRM unblocking versus Lab‐TRM blocking from API experiments often exhibit nonideal curvature, which can result in biased estimates. Previous work showed that curvature can increase with age; however, selection criteria designed to eliminate such behavior yielded accurate estimates for two‐year‐aged specimens (70.3 ± 3.8 μT; N = 96 specimens out of 120 experiments). API can also be estimated in coercivity space. Here, we use the Tsunakawa‐Shaw (TS) method applied to 20 specimens aged in the laboratory field of 70.0 μT for 4 years, after acquisition of zero‐age (fresh) Lab‐TRM in the same field. Selection criteria for the TS experiment also yielded accurate results (68.5 ± 4.5 μT; N = 17 specimens). In thermal API experiments, curvature is related to internal structure with more single domain‐like behavior having the least curvature. Here we show that the fraction of anhysteretic remanent magnetization demagnetized by low‐temperature treatment was larger for samples with larger thermal curvatures suggesting a magnetocrystalline anisotropy source. We also tested experimental remedies that have been proposed to improve the accuracy of paleointensity estimates. In particular, we test the efficacy of the multi‐specimen approach and a strategy pretreating specimens with low field alternating field demagnetization prior to the paleointensity experiment. Neither yielded accurate results. Plain Language Summary The strength of the geomagnetic field in the past is a fundamental property of the Earth and volcanic rocks are magnetized as they cool, potentially retaining a record of the ancient field. However, because natural materials were not \"designed\" to retain records of the field, there are many difficulties in obtaining reliable estimates. Consequently, there are many experimental approaches and methods of analyzing the data. Here, we explore several very different experimental approaches on replicate specimens from a set of samples chosen to represent the range of behaviors observed in lava flows. Specimens were given a fresh magnetization and some were allowed to \"age\" in a laboratory field for up to 5 years. We find that two methods work quite well while other approaches fail to provide accurate estimates of the field. Key Points Both the Tsunakawa‐Shaw and the IZZI paleointensity methods give accurate results when applied to aged laboratory thermal remanences Introducing alternating‐field demagnetization steps into the IZZI method resulted in inaccurate results, as did the multi‐specimen protocol Low temperature demagnetization (targeting grains with magnetocrystalline anisotropy) is strongly correlated with “fragile” curvature

Twenty‐two sites, subjected to an IZZI‐modified Thellier‐Thellier experiment and strict selection criteria, recover a paleomagnetic axial dipole moment (PADM) of 62.2 ± 30.6 ZAm2 in Northern Israel over the Pleistocene (0.012–2.58 Ma). Pleistocene data from comparable studies from Antarctica, Iceland, and Hawaii, re‐analyzed using the same criteria and age range, show that the Northern Israeli data are on average slightly higher than those from Iceland (PADM = 53.8 ± 23 ZAm2, n = 51 sites) and even higher than the Antarctica average (PADM = 40.3 ± 17.3 ZAm2, n = 42 sites). Also, the data from the Hawaiian drill core, HSDP2, spanning the last half million years (PADM = 76.7 ± 21.3 ZAm2, n = 59 sites) are higher than those from Northern Israel. These results, when compared to Pleistocene results filtered from the PINT database (www.pintdb.org) suggest that data from the Northern hemisphere mid‐latitudes are on average higher than those from the southern hemisphere and than those from latitudes higher than 60°N. The weaker intensities found at high (northern and southern) latitudes therefore, cannot be attributed to inadequate spatiotemporal sampling of a time‐varying dipole moment or low quality data. The high fields in mid‐latitude northern hemisphere could result from long‐lived non‐axial dipole terms in the geomagnetic field with episodes of high field intensities occurring at different times in different longitudes. This hypothesis is supported by an asymmetry predicted from the Holocene, 100 kyr, and 5 million year time‐averaged geomagnetic field models. Plain Language Summary According to the Geocentric Axial Dipole hypothesis, the geomagnetic field may be approximated by a dipole that is aligned with the spin axis and positioned in the center of Earth. Such a field would produce field strengths that vary with respect to latitude with high latitudes associated with high intensities, or, converted to equivalent “virtual” dipole moments, would be essentially independent of latitude. It has long been suggested that high latitudes have had lower field strengths than predicted by such a model, when compared to data from mid‐latitudes, but these claims have always been accompanied by caveats regarding differences in temporal coverage or methodological approaches. Here, we present new data from Pleistocene aged rapidly cooled cinder cones and lava flow tops from Israel. We compare these data to other recent data sets obtained from rapidly cooled materials collected in Hawaii, Iceland, and Antarctica. These confirm that virtual dipole moments from mid northern hemisphere latitudes are higher than those from high latitudes and from the southern hemisphere. Global compilations spanning the Pleistocene, when filtered for quality also shows this behavior as do time averaged field models. Therefore, field strengths over even millions of years can have persistent non‐dipole field contributions. Key Points We present 26 40Ar/39Ar ages from volcanic rocks from Northern Israel (90 ka to 3.3 Ma) Twenty‐two Pleistocene intensity estimates have a mean paleomagnetic dipole moment of 62.2 ± 30.6 ZAm2 The northern hemisphere had persistently higher fields than the southern during the Pleistocene

The Pliocene and Early Pleistocene, between 5.3 and 0.8 million years ago, span a transition from a global climate state that was 2–3 °C warmer than present with limited ice sheets in the Northern Hemisphere to one that was characterized by continental-scale glaciations at both poles. Growth and decay of these ice sheets was paced by variations in the Earth’s orbit around the Sun. However, the nature of the influence of orbital forcing on the ice sheets is unclear, particularly in light of the absence of a strong 20,000-year precession signal in geologic records of global ice volume and sea level. Here we present a record of the rate of accumulation of iceberg-rafted debris offshore from the East Antarctic ice sheet, adjacent to the Wilkes Subglacial Basin, between 4.3 and 2.2 million years ago. We infer that maximum iceberg debris accumulation is associated with the enhanced calving of icebergs during ice-sheet margin retreat. In the warmer part of the record, between 4.3 and 3.5 million years ago, spectral analyses show a dominant periodicity of about 40,000 years. Subsequently, the powers of the 100,000-year and 20,000-year signals strengthen. We suggest that, as the Southern Ocean cooled between 3.5 and 2.5 million years ago, the development of a perennial sea-ice field limited the oceanic forcing of the ice sheet. After this threshold was crossed, substantial retreat of the East Antarctic ice sheet occurred only during austral summer insolation maxima, as controlled by the precession cycle. The volume of the East Antarctic ice sheet is influenced by changes in the Earth’s orbit. Ice-rafted debris accumulation between 4.3 and 2.2 million years ago suggests precession affected the extent of the marine margins of the ice sheet.

This article is composed of three independent commentaries about the state of Integrated, Coordinated, Open, Networked (ICON) principles (Goldman et al., 2021, https://doi.org/10.1002/essoar.10508554.1) in the Geomagnetism, Paleomagnetism, and Electromagnetism (GPE) section and discussion on the opportunities and challenges of adopting them. Each commentary focuses on a different topic: Global collaboration, reproducibility, data sharing and infrastructure; Inclusive equitable, and accessible science: Involvement, challenges, and support of early career, BIPOC, women, LGBTQIA+, and/or disabled researchers; Community engagement, citizen science, education, and stakeholder involvement. Data sharing practices and open repository use still varies strongly between GPE communities. Some have a long tradition of data sharing; others are only starting it. Globally, GPE leadership is strongly dominated by white males and diversity may increase through the creation of Science Equality Commissions. Improved global stakeholder involvement can increase research impacts and help fight inequalities. In all investigated topics we see promising beginnings but also recognize obstacles that include a lack of funding, a lack of understanding of diversity, and prioritizing short‐term gain over long‐term benefit. Nonetheless, we are hopeful that our community will embrace ICON science. Key Points Data accessibility is not consistent across Geomagnetism, Paleomagnetism, and Electromagnetism (GPE) disciplines. Some have a long tradition; others are still developing capabilities A global survey of GPE labs shows that leaders are predominantly white males. A network of Science Equality Commissions may increase equality Earth and planetary magnetism researchers can increase global stakeholder involvement through Coordinated, Open, and Networked investment

In contrast to our detailed knowledge of the directional behaviour of the Earth's magnetic field during geological and historical times 1 , 2 , data constraining the past intensity of the field remain relatively scarce. This is mainly due to the difficulty in obtaining reliable palaeointensity measurements, a problem that is intrinsic to the geological materials which record the Earth's magnetic field. Although the palaeointensity database has grown modestly over recent years 3 , 4 , 5 these data are restricted to a few geographical locations and more than one-third of the data record the field over only the past 5?Myr—the most recent database 5 covering the time interval from 5 to 160?Myr contains only about 100 palaeointensity measurements. Here we present 21 new data points from the interval 5–160?Myr obtained from submarine basalt glasses collected from locations throughout the world's oceans. Whereas previous estimates for the average dipole moment were comparable to that of the Earth's present field 6 , the new data suggest an average dipole moment of (4.2 ± 2.3) × 10 22 ?A?m 2 , or approximately half the present magnetic-field intensity. This lower average value should provide an important constraint for future efforts to model the convective processes in the Earth's core which have been responsible for generating the magnetic field.

A count-based method using technetium-99m sestamibi electrocardiography-gated myocardial perfusion single photon emission computed tomography imaging has been developed to extract the left ventricular (LV) regional phase of contraction (onset of mechanical contraction [OMC]) throughout the cardiac cycle. This study was performed to develop OMC normal databases and dynamic OMC displays for assessment of cardiac mechanic dyssynchrony. LV regional phases were extracted from 90 enrolled normal subjects (45 men and 45 women) by use of the Emory Cardiac Toolbox and then submitted to statistical analysis to generate the normal databases. The LV OMC wave was dynamically propagated over the perfusion polar map by blackening either sequential phase bins or all past phases. The developed OMC normal databases consisted of peak phase (134.5° ± 14.3° for men and 140.2° ± 14.9° for women), phase SD (14.2° ± 5.1° for men and 11.8° ± 5.2° for women), and phase histogram bandwidth (38.7° ± 11.8° for men and 30.6° ± 9.6° for women), skewness (4.19 ± 0.68 for men and 4.60 ± 0.72 for women), and kurtosis (19.72 ± 7.68 for men and 23.21 ± 8.16 for women). Both statistical analysis and dynamic OMC displays were incorporated into a user interface as a diagnostic tool. The OMC normal databases and dynamic OMC displays should help clinicians evaluate cardiac mechanic dyssynchrony. Prospective clinical trials are needed to validate whether this tool can be used to select patients with severe heart failure symptoms who might benefit from cardiac resynchronization therapy.

We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of [approximately]0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today.

We compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass. The basaltic glass data more than double the number of reliable (Thellier method with pTRM checks) palaeointensity estimates available. Although the new data dramatically improve both spatial and temporal coverage, there is still a strong bias toward the most recent past. The last 0.3 Ma claim over half of the data in our combined database. We therefore divide the data into two groups, the densely sampled last 0.3 Myr and the more sparsely sampled period of time comprising roughly half of the data from 0.3 to 300 Ma. Separating them in this way, it is clear that the dipole moment of the Earth over the past 0.3 Myr (