An official website of the United States government
Plain Language Summary Nearly synchronous global changes in geomagnetic polarity give both a detailed irregular pacing to geological time and provide a glimpse into heat transfer processes across the core—mantle boundary which drives the Earth's geodynamo. Although the Late Carboniferous is characterized by some well‐studied reversals, details of the tempo of polarity changes in the Early Carboniferous are unknown. This work addresses this by providing a detailed record of polarity changes over a ∼2 million year interval at around 334.5–332.5 million years ago‐from the Trowbarrow Quarry section in NW England. We demonstrate that these limestones likely preserve magnetization from close to their time of formation and record at least 31 polarity reversals. These observations support the idea that the Earth's dynamo was in a hyperactive reversing state similar to those sustained for tens of Myr in the Late Jurassic, parts of the Cambrian and the Late Ediacaran. It further corroborates a ∼200 Myr cyclicity in paleomagnetic field behavior since the Precambrian, potentially linked to variable core heat flow forced by mantle convection.
more »
« less
Late Cambrian geomagnetic instability after the onset of inner core nucleation
The Ediacaran Period marks a pivotal time in geodynamo evolution when the geomagnetic field is thought to approach the weak state where kinetic energy exceeds magnetic energy, as manifested by an extremely high frequency of polarity reversals, high secular variation, and an ultralow dipole field strength. However, how the geodynamo transitioned from this state into one with more stable field behavior is unknown. Here, we address this issue through a high-resolution magnetostratigraphic investigation of the ~494.5 million-year-old Jiangshanian Global Standard Stratotype and Point (GSSP) section in South China. Our paleomagnetic results document zones with rapid reversals, stable polarity and a ~80 thousand-year-long interval without a geocentric axial dipole field. From these changes, we suggest that for most of the Cambrian, the solid inner core had not yet grown to a size sufficiently large to stabilize the geodynamo. This unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander, supporting instead the rotational stability of the solid Earth during the great radiation of life in the Cambrian.
more »
« less
- Award ID(s):
- 1828817
- PAR ID:
- 10505297
- Publisher / Repository:
- Nature Communications
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
SUMMARY It is well known that the axial dipole part of Earth’s magnetic field reverses polarity, so that the magnetic North Pole becomes the South Pole and vice versa. The timing of reversals is well documented for the past 160 Myr, but the conditions that lead to a reversal are still not well understood. It is not known if there are reliable ‘precursors’ of reversals (events that indicate that a reversal is upcoming) or what they might be. We investigate if machine learning (ML) techniques can reliably identify precursors of reversals based on time-series of the axial magnetic dipole field. The basic idea is to train a classifier using segments of time-series of the axial magnetic dipole. This training step requires modification of standard ML techniques to account for the fact that we are interested in rare events—a reversal is unusual, while a non-reversing field is the norm. Without our tweak, the ML classifiers lead to useless predictions. Perhaps even more importantly, the usable observational record is limited to 0–2 Ma and contains only five reversals, necessitating that we determine if the data are even sufficient to reliably train and validate an ML algorithm. To answer these questions we use several ML classifiers (linear/non-linear support vector machines and long short-term memory networks), invoke a hierarchy of numerical models (from simplified models to 3-D geodynamo simulations), and two palaeomagnetic reconstructions (PADM2M and Sint-2000). The performance of the ML classifiers varies across the models and the observational record and we provide evidence that this is not an artefact of the numerics, but rather reflects how ‘predictable’ a model or observational record is. Studying models of Earth’s magnetic field via ML classifiers thus can help with identifying shortcomings or advantages of the various models. For Earth’s magnetic field, we conclude that the ability of ML to identify precursors of reversals is limited, largely due to the small amount and low frequency resolution of data, which makes training and subsequent validation nearly impossible. Put simply: the ML techniques we tried are not currently capable of reliably identifying an axial dipole moment (ADM) precursor for geomagnetic reversals. This does not necessarily imply that such a precursor does not exist, and improvements in temporal resolution and length of ADM records may well offer better prospects in the future.more » « less
-
SUMMARY Paleomagnetic records provide us with information about the extreme geomagnetic events known as excursions and reversals, but the sparsity of available data limits detailed knowledge of the process and timing. To date there are no agreed on criteria for categorizing such events in terms of severity or longevity. In an analogy to categorizing storms in weather systems, we invoke the magnitude of the global (modified) paleosecular variation index $$P_{i_D}$$ to define the severity of the magnetic field state, ranging in level from 0 to 3, and defined by instantaneous values of $$P_{i_D}$$ with level 0 being normal ($$P_{i_D}\lt 0.5$$) to extreme ($$P_{i_D}\ge 15$$). We denote the time of entry to an excursional (or reversal) event by when $$P_{i_D}$$ first exceeds 0.5, and evaluate its duration by the time at which $$P_{i_D}$$ first returns below its median value, termed the end of event threshold. We categorize each excursional event according to the peak level of $$P_{i_D}$$ during the entire event, with a range from Category-1 (Cat-1) to Cat-3. We explore an extended numerical dynamo simulation containing more than 1200 events and find that Cat-1 events are the most frequent (72 per cent), but only rarely lead to actual field reversals where the axial dipole, $$g_1^0$$, has reversed sign at the end of the event. Cat-2 account for about 20 per cent of events, with 34 per cent of those leading to actual reversals, while Cat-3 events arise about 8 per cent of the time but are more likely to produce reversals (43 per cent). Higher category events take as much as 10 times longer than Cat-1 events. Two paleomagnetic field models separately cover the Laschamp excursion and Matuyama–Brunhes (M-B) reversal which are Cat-2 events with respective durations of 3.6 and 27.4 kyr. It seems likely that Cat-2 may be an underestimate for M-B due to limitations in the paleomagnetic records. Our overall results suggest no distinction between excursions and reversals other than a reversal having the ending polarity state opposite to that at the start.more » « less
-
There has been longstanding controversy about whether the influence of lateral variations in core-mantle boundary heat flow can be detected in paleomagnetic records of geomagnetic field behavior. Their signature is commonly sought in globally distributed records of virtual geomagnetic pole (VGP) paths that have been claimed to exhibit specific longitudinal preferences during polarity transitions and excursions. These preferences have often been linked to thermal effects from large low seismic velocity areas (LLVPs) in the lowermost mantle, but the results have been contested because of potential sensitivity to sparse temporal and spatial sampling. Recently developed time varying global paleofield models spanning various time intervals in 1–100 ka, three of which include excursions, allow us to complement assessments of spatial distributions of transitional VGP paths with distributions of minimum field intensity. Robustness of the results is evaluated using similar products from four distinct numerical dynamo simulations with and without variable thermal boundary conditions and including stable geomagnetic polarity, excursions and reversals. We determine that VGP distributions are less useful than minimum field intensity in linking the influences of thermal CMB structure to geographical variations in actual paleofield observables, because VGP correlations depend strongly on good spatial sampling of a sufficient number of relatively rare events. These results provide a basis for evaluating comparable observations from four paleofield models. The distribution of VGP locations provide unreliable results given the restricted time span and available data locations. Rough correlations of global distributions of minimum intensity with areas outside the LLVPs give some indications of mantle control during excursions, although the results for the eastern hemisphere are complex, perhaps highlighting uncertainties about the hemispheric balance between thermal and compositional variations in the lowermost mantle. However, access to other geomagnetic properties (such as intensity and radial field at the CMB) provides a strong argument for using extended and improved global paleofield models to resolve the question of mantle influence on the geodynamo from the observational side.more » « less
-
Abstract Continuous sedimentary records of paleomagnetic directional variability and relative paleointensity (RPI) provide valuable information on the evolution of the geodynamo while also facilitating stratigraphic correlation and age control. While the Quaternary RPI record has received much attention, Pliocene records are relatively rare. Here, a u‐channel paleomagnetic study from Integrated Ocean Drilling Program (IODP) Site U1396 in the Caribbean Sea refines the shipboard‐derived polarity stratigraphy and generates an RPI and directional record extending back 4.5 Ma. Rock magnetic data reveal changes in magnetic coercivity around 2.1 Ma that influences the quality of the paleomagnetic record; while the older record is well‐resolved and passes RPI quality criteria, much of the younger section does not. To facilitate the development of the RPI record, spike noise associated with discrete intervals of volcanogenic sediments are filtered from the data set. The resulting record passes RPI reliability criteria between 0–0.6 Ma and 2.1–4.5 Ma and represents the highest resolution RPI record extending into the early Pliocene. We use this record to refine the existing benthic δ18O chronology and open the door to high‐resolution RPI chronostratigraphies during the Pliocene. Although we find no evidence for a previously observed increase in magnetic field intensity after ∼4 million years ago, we do observe an asymmetrical form to RPI in the normal polarity intervals of the Gauss chron. We also find a polarity bias in inclination that cannot be simply explained by a drill string overprint, suggesting polarity‐driven field asymmetries that are particularly pronounced during the Pliocene.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41467-023-40309-7
