Paleosecular Variation Models Research Articles

LAPOD, Latin-American paleomagnetic online database: An online interface to access paleomagnetic data from Latin America

We present the Latin-American Paleomagnetic Online Database (LAPOD), which includes an online tool for consulting and visualizing paleomagnetic data. This platform allows users to search, filter, and query over 2000 paleomagnetic data entries from locations across Latin America, including North America (Mexico and the Southwestern USA), Central America, and South America. Paleomagnetic data has been pre-validated for geographic locations and ages, enhancing its reliability for users.The platform features a robust database engine with up to 26 filters, allowing users to perform queries with high precision and avoiding data duplication, and to download data in various formats for personal use. It also offers graphical visualization of some data directly on screen. Color indicators are used to indicate data quality and magnetic polarity. All data included in LAPOD are properly referenced to their original authors, with links to the original published articles.LAPOD is proposed as a community-driven platform, allowing users to contribute their data and provide comments on their own and others' data. This fosters discussion about the reliability of paleomagnetic data and finally enhances our knowledge of the Earth's magnetic field in the region. This collaborative approach supports the development of new regional models of paleosecular variation with improved reliability. It may be repeated for other regions in the world, and ultimately lead to improved global field models. To access the platform, please visit: https://paleomagnetismo.org/pmagdb/

A Lake Record of Geomagnetic Secular Variations for the Last 23 ka From Lake Chala: Toward a Composite Directional Lake Record of the Earth's Magnetic Field for Equatorial East Africa

AbstractThe documentation and understanding of variations in the Earth's magnetic field through time is fundamental for several disciplines, but current geomagnetic models rely on datasets heavily biased toward the mid‐ and high northern latitudes. The African continent and surrounding islands and oceans are particularly underrepresented. Here, we present a new record of paleo‐secular variation (PSV) of the inclinations over the last 23 ka from Lake Chala, situated at 3°S near Mt Kilimanjaro in eastern equatorial Africa. This groundwater‐fed crater lake is characterized by a high sedimentation rate (ca. 1 cm/10 years) and a particularly well‐constrained age model based on 210Pb and 14C dating. The magnetic mineralogy of the sediments is tested with rock magnetic analyses. The Lake Chala inclination record shows four highs and lows over 20 ka and compares well with that of Lake Malawi (10°S) between 20 and 16.2 ka, and from 9.8 to 2.6 ka. This record is linked to PSV records at Lakes Victoria and Malawi using a sequence slotting technique to generate a composite PSV model for east Africa. Analyzed at best‐possible resolutions up to 200 years, the Lake Chala PSV record not only represents an important contribution to improve our understanding of local and global features of the Earth's magnetic field. It also expands the utility of paleomagnetism as a key tool for dating and correlation both for archeological sites throughout East Africa and the many volcanoes, active or dormant, of the East African Rift System.

Assessing the reliability of paleomagnetic datasets using the R package PmagDiR

Paleomagnetism is the most important source of information for determining the position of tectonic plates in the geological past. Over the last few decades significant advancements have been made in improving the reliability of paleomagnetic data, ranging from analytical methods to statistical assessments. Here I present the first version of PmagDiR, an R-based open-source package which displays, assesses the reliability, and, when possible, corrects given paleomagnetic directions distributions. The main functions of the package are to: plot paleomagnetic directions, their averages and confidence boundaries; apply different direction cut-offs to identify and filter outliers; perform a test for antipodality; compare the distribution shape with the one predicted by a widely accepted paleosecular variation model; correct flawed distributions for paleomagnetic inclination flattening; and correct for strain-derived paleomagnetic directions deviations when the strain fabric is known. Furthermore, directions can be converted in virtual geomagnetic poles and plotted on a spherical projection for comparison with reference apparent polar wander paths. All main functions of the PmagDiR package return results as comma separated value text files as well as vector graphic files (pdf), optimized for publication purpose with minimal manipulation.

Quantifying Inclination Shallowing and Representing Flattening Uncertainty in Sedimentary Paleomagnetic Poles

AbstractInclination is the angle of a magnetization vector from horizontal. Clastic sedimentary rocks often experience inclination shallowing whereby syn‐ to post‐depositional processes result in flattened detrital remanent magnetizations relative to local geomagnetic field inclinations. The deviation of recorded inclinations from true values presents challenges for reconstructing paleolatitudes. A widespread approach for estimating flattening factors (f) compares the shape of an assemblage of magnetization vectors to that derived from a paleosecular variation model (the elongation/inclination [E/I] method). Few studies exist that compare the results of this statistical approach with empirically determined flattening factors and none in the Proterozoic Eon. In this study, we evaluate inclination shallowing within 1.1 billion‐year‐old, hematite‐bearing red beds of the Cut Face Creek Sandstone that is bounded by lava flows of known inclination. Taking this inclination from the volcanics as the expected direction, we found that detrital hematite remanence is flattened with whereas the pigmentary hematite magnetization shares a common mean with the volcanics. Using the pigmentary hematite direction as the expected inclination results in . These flattening factors are consistent with those estimated through the E/I method supporting its application in deep time. However, all methods have significant uncertainty associated with determining the flattening factor. This uncertainty can be incorporated into paleomagnetic poles with the resulting ellipse approximated with a Kent distribution. Rather than seeking to find “the flattening factor,” or assuming a single value, the inherent uncertainty in flattening factors should be recognized and incorporated into paleomagnetic syntheses.

Paleomagnetism and paleolatitude of igneous drill core samples from the Izu-Bonin forearc and implications for Philippine Sea plate motion

The Philippine Sea plate is an integral part of the east Asian plate mosaic but its past motions can only be reconstructed from paleomagnetic data. Moreover, because the plate is largely submarine, few fully-oriented paleomagnetic data are available. International Ocean Discovery Program (IODP) Expedition 352 cored at four sites on the Izu-Bonin forearc, penetrating ∼1170 m of forearc igneous crust radiometrically dated at ∼51 Ma. Paleomagnetic measurements on 361 samples produced paleo-inclination data that were used to estimate paleolatitude. Twenty five independent magnetic units give a paleocolatitude of 90.1° ±4.4° (2σ uncertainty), implying a paleolatitude exactly on the equator. Unit variance agrees with global paleosecular variation models, implying that these variations have been properly sampled and averaged. The paleolatitude implies northward drift of ∼28°, which agrees with other Izu-Bonin forearc sites of similar age. A slight difference in northward drift with southern Philippine Sea plate sites is consistent with plate rotation. Certain parts of the cored sections yield samples that were remagnetized to the present geocentric axial dipole inclination after ∼30 Ma, implying a tectonic event of unclear origin.

Paleosecular variation record from Pleistocene-Holocene lava flows in southern Colombia

Improvements in the spatial and temporal coverage of paleomagnetic data are essential to better evaluate paleofield behaviour over the past 10 Myr, especially due to data scarcity at low latitudes in the South American region. Here, we provide new Pleistocene-Holocene (0–2 Ma age interval) paleodirectional data from three volcanic systems (Doña Juana Volcanic Complex, Galeras Volcanic Complex and Morasurco Volcano) in southwestern Colombia between latitudes 1.2 and 1.4°N. A total of 38 paleodirectional sites were studied using progressive alternating field and thermal demagnetization treatments. After excluding transitional data, we obtain thirty site-mean directions for analysis of paleosecular variation (PSV) and the time-averaged field (TAF) in the study area. The mean direction (Dec = 351.2°, Inc = −3.4°, α95 = 6.2°, k = 20.0) and the paleomagnetic pole (Plat = 80.7°N, Plon = 173.1°E, A95 = 5.2°, K = 29.1) of these sites are not statistically compatible with the expected geocentric axial dipole (GAD) field direction and geographic north pole, respectively. Virtual geomagnetic pole dispersion (SB) for our filtered dataset (SB(2Ma) = 15.212.017.6°) and the Brunhes chron (SB(Bru) = 16.011.619.1°) are consistent at the 95% confidence level with South American studies at equatorial latitudes and recent PSV models for the 0–10 Ma and Brunhes intervals. Likewise, the corresponding inclination anomaly (ΔI) for two age groups ΔI2Ma = − 5.9−12.10.3° and ΔIBru = − 5.3−13.73.1° suggests large deviations relative to the GAD model, in accordance with predictions from zonal TAF models. The high VGP dispersion could be linked to strong longitudinal variability of the magnetic equator position over South America. This feature reflects the presence of significant non-dipole field components in this region that have been detected in geomagnetic field models for the most recent centuries and millennia, probably associated with the presence of the South Atlantic Magnetic Anomaly in the South American region.

PSVM: A Global Database for the Miocene Indicating Elevated Paleosecular Variation Relative to the Last 10 Myrs

AbstractStatistical studies of paleosecular variation (PSV) are used to infer the structure and behavior of the geomagnetic field. This study presents a new database, paleosecular variation of the Miocene (PSVM), of high‐quality directional data from the Miocene Epoch (5.3–23 Ma), compiled from 1,454 sites from 44 different localities. This database is used to model the latitude dependence of paleosecular variation with varying selection criteria using a quadratic form based on Model G. Our fitted model parameter for latitude‐invariant PSV (Model G a) is 15.6° and the latitude dependent PSV term (Model G b) is 0.23. The latitude invariant term is substantially higher than previously observed for the past 10 Myrs or any other studied ages. We also present a new stochastic model of the time‐average field, BB‐M22, using a covariant giant Gaussian process (GGP) which is constrained using data from PSVM, PINT and Earth‐like geodynamo numerical simulations. BB‐M22 improves the fit to PSVM data relative to prior GGP models, as it reproduces the higher virtual geomagnetic pole (VGP) dispersion observed during the Miocene. Our findings suggest a more variable magnetic field and more active geodynamo in the Miocene Epoch than the past 10 Myrs, perhaps linked to elevated core‐mantle heat flow. Our results suggest that the average axial dipole dominance of the time‐instantaneous field was lower than in more recent times. We note however, that the inclination anomaly estimates, suggest that it cannot be ruled out that the Miocene time averaged field resembles a geocentric axial dipole.

Four‐Dimensional Paleomagnetic Dataset: Plio‐Pleistocene Paleodirection and Paleointensity Results From the Erebus Volcanic Province, Antarctica

AbstractA fundamental assumption in paleomagnetism is that a geocentric axial dipole (GAD) geomagnetic field structure extends to the ancient field. Global paleodirectional compilations that span 0–5‐million year support a GAD dominated field structure with minor non‐GAD contributions, however, the paleointensity data over the same period do not. In a GAD field, higher latitudes should preserve higher intensity, but the current database suggests that intensities are independent of latitude. To determine whether the seemingly “low” intensities from Antarctica reflect the ancient field, rather than low‐quality data or inadequate temporal sampling, we have conducted a new study of the paleomagnetic field in Antarctica. This study focuses on the paleomagnetic field structure over the Plio‐Pleistocene. We combine and reanalyze new and published paleodirectional and paleointensity results from the Erebus volcanic province to recover paleodirections from 98 sites that were both thermally and alternating field demagnetized and then subjected to a set of strict selection criteria and paleointensities from 26 sites from the Plio‐Pleistocene that underwent the IZZI modified Thellier‐Thellier experiment and were also subjected to a strict set of selection criteria. The paleopole (201.85°, 87.65°) and α95 (5.51°) recovered from our paleodirectional study supports the GAD hypothesis and the scatter of the virtual geomagnetic poles falls within the uncertainty of that predicted by TK03 paleosecular variation model. Our time‐averaged field strength estimate, 33.57 ± 2.71 μT, is significantly weaker than that expected from a GAD field estimated by the present field.

Comment on “New Late Pennsylvanian Paleomagnetic Results From Paraná Basin (Southern Brazil): Is the Recent Giant Gaussian Process Model Valid for the Kiaman Superchron?” by Brandt et al.

AbstractThe late Pennsylvanian glacial rhythmites of the Mafra Formation (Itararé Group) from the Paraná Basin of Southern Brazil, have a complex rock‐magnetic signature. Rock‐magnetic tests imply that both hematite and magnetite in varying grain sizes are responsible for the magnetic remanence. Thermal and alternating field (AF) demagnetization of the natural remanence reveal different behaviors that are attributed to a mixture of remanence‐carrying magnetic minerals and grain sizes, implying a series of magnetic overprints. A great circle analysis of the distribution characteristic directions reported suggests that these rocks may not be entirely immune from magnetic overprints acquired during the Jurassic‐Cretaceous in the context of widespread remagnetizations in South America. The nature of the magnetization recorded by these rocks warrants caution in their use for the evaluation of paleosecular variation models.

Reply to Comment by Dario Bilardello on “New late Pennsylvanian Paleomagnetic Results From Paraná Basin (southern Brazil): Is the Recent Giant Gaussian Process Model Valid for the Kiaman Superchron?”

AbstractIn this response, we address the comments by D. Bilardello on the paper by Brandt et al. (2019, https://doi.org/10.1029/2018JB016968). His main point is that the Mafra rhythmites are remagnetized as are supposed to be all Paleozoic sediments in the Paraná Basin in the context of a widespread Jurassic‐Cretaceous remagnetization in South America. In this way, the obtained results are not suitable for investigations of the paleosecular variation models. The authors do not agree with his conclusions and present counterevidence to support the approach and the conclusions of Brandt et al. (2019, https://doi.org/10.1029/2018JB016968).

Paleomagnetism and Paleosecular Variations From the Plio‐Pleistocene Golan Heights Volcanic Plateau, Israel

AbstractStatistical analysis of geomagnetic paleosecular variation (PSV) and time‐averaged field has been largely based on global compilations of paleomagnetic data from lava flows. These show different trends in the averaged inclination anomaly (ΔI) between the two hemispheres, with small positive (<2°) anomalies in midsouthern latitudes and large negative (> −5°) anomalies in midnorthern latitudes. To inspect the large ΔI between 20°N and 40°N we augment the global data with a new paleomagnetic data set from the Golan‐Heights (GH), a Plio‐Pleistocene volcanic plateau in northeast Israel, located at 32–33°N. The GH data set consists of 91 lava flows sites: 40 sites obtained in the 1990s and 51 obtained in this study. The chronology of the flows is constrained by 57 40Ar/39Ar ages: 39 from previous studies and 18 from this study, which together cover most of the GH plateau. We show that the 1990s data set might be affected by block rotations and does not fully sample PSV. The Plio‐Pleistocene pole (86.3°N, 120.8°E, N = 44, k = 25, α95 = 4.4°), calculated after applying selection criteria with Fisher precision parameter (k) ≥ 100 and number of specimens per site (n) ≥ 5 is consistent with a geocentric axial dipole field and shows smaller inclination anomaly (ΔI = −0.4°) than predicted by global compilations and PSV models. Reexamination of the inclination anomaly in the global compilation using different calculation methods and selection criteria suggests that inclination anomaly values are affected by (1) inclusion of poor quality data, (2) averaging data by latitude bins, and (3) the way the inclination anomaly is calculated.

A comprehensive paleomagnetic study from the last Plinian eruptions of Popocatepetl volcano: absolute chronology of lavas and estimation of emplacement temperatures of PDCs

Three lava flows (Buenavista, Xalitzintla and Nealtican) and pyroclastic density currents (Lorenzo and Pink Pumice) from two Popocatepetl Plinian eruptions were sampled for paleomagnetic dating. A detailed rock-magnetic characterization of the lavas, scoria clasts and pottery shards intercalated between the volcanic deposits was also carried out. Reliable results, both in direction and in intensity, were obtained for the Nealtican lava flow, which enabled its full-vector paleomagnetic dating using the archaeo_dating tool together with the global paleosecular variation model SHA.DIF.14 k, obtaining an age interval between 1040 AD and 1140 AD (95% probability confidence level), in good agreement with its associated 14C age. The well-grouped paleomagnetic direction of the seven specimens from two different scoria clasts of the Lorenzo Pumice pyroclastic density current suggests that clasts were emplaced hot, at a temperature that seems to have almost completely erased the original remanent magnetization of the clasts. This fact is supported by the reheating of the underlying pottery shards, evidenced as a clear secondary low-temperature range (room temperature to 350 °C) component at the orthogonal vector plots. Similarly, the three mean clusters directions obtained for site PO-2 (Pink Pumice)—roughly concentrated around the present geomagnetic field—suggest also a high emplacement temperature. Also, the first archeointensity dating of a pottery shard within the pyroclastic density current is reported. Finally, results of the rock-magnetic and paleomagnetic dating of the last Plinian eruptions from the Popocatepetl volcano, applied to different volcanic materials (lava and pyroclastic density currents), show the usefulness of these nonconventional and alternative techniques in the study of the eruptive activity of volcanoes.Graphic abstract.

Speleothems as Magnetic Archives: Paleosecular Variation and a Relative Paleointensity Record From a Portuguese Speleothem

AbstractWe provide a high‐resolution and complete paleomagnetic study from a middle‐Holocene (~4,100–3,300 years Before Common Era, BCE) dome‐shaped speleothem (SPAIV) from Algarve, Portugal. Our results show that the SPAIV speleothem carries a primary and stable remanent magnetization, for which directions are similar to other speleothems from the western Alps. Magnetic declination and inclination curves are also comparable to current paleosecular variation models (SHA.DIF.14k, CALS10k.1b, and pfm9k.1a), one of them (SHA.DIF.14k) fitting better with the present data set. Relative paleointensity was estimated using two different methods: conventional normalization of natural remanent magnetization by anhysteretic and isothermal, and the pseudo‐Thellier method, which is being tested here for the first time in a speleothem. Both methods show similar results, with a minimum intensity peak at ~3,850 years BCE. This low relative intensity is observed in all samples pertaining to the same respective calcite laminae. These results suggest that high growth rate speleothems are good high‐resolution recorders of the direction and intensity of the Earth's magnetic field.

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

AbstractThe 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.

The effect of speleothem surface slope on the remanent magnetic inclination

AbstractSpeleothems are of interest for high‐resolution reconstruction of the Earth's magnetic field. However, little is known about the influence of speleothem morphologies on their natural remanent magnetization (NRM) record. Here we report on a high‐resolution paleomagnetic study of a dome‐shaped speleothem of middle Holocene age from southern Portugal, with special attention to the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). To assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization, we compare magnetic directions and AMS and AARM fabrics from subhorizontal to gradually subvertical calcite growth layers collected in a transversal cross section of the speleothem. A linear correlation is observed between magnetic inclinations, calcite laminae slope, and AARM k1 inclination. The AMS fabric is mostly controlled by calcite crystals, with direction of the minimum axes (k3) perpendicular to laminae growth. Magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation (PSV) model. After extrapolating magnetic inclinations to the horizontal, the corrected data better fit the PSV model but are still lower than the predicted magnetic inclinations, suggesting that inclination shallowing affects the entire speleothem. We suggest that speleothem morphology exerts a critical role on the magnetic inclination recording, which is controlled by the Earth's magnetic field but also influenced by particle rolling along the sloping surfaces. These observations open new avenues for reconstructing high‐resolution paleomagnetic secular variation records from speleothems and provide new insights into their NRM acquisition mechanisms.

Paleomagnetic study of El Metate shield volcano (Michoacán, Mexico) confirms its monogenetic nature and young age (~ 1250 CE)

In a recent study, Chevrel et al. (2016a, b) radiocarbon-dated the oldest lava flow of the voluminous (~9.2km3) El Metate shield volcano (Michoacán, Mexico) at cal 1250–1260 CE and proposed that its eruption was monogenetic in origin, with twelve younger lava flows emplaced during a short period of only ~35years, but certainly <275years. In order to test this hypothesis, we undertook a detailed paleomagnetic study of five lava flows from El Metate to check the consistency of their paleomagnetic directions. Additionally, a group of representative specimens was treated with the double-heating Thellier experiment using the IZZI protocol for paleointensity determination. Flow mean paleomagnetic directions obtained for four of the flows are indistinguishable, and discordant directions were obtained from the site of the 5th flow measured, probably due to the tilting of the sampled block after remanence acquisition. Mean paleodirections and intensities were used for paleomagnetic dating applying the global paleosecular variation model SHA.DIF.14k. The resulting age range for the eruption is 1150–1290 CE, which overlaps with the range previously determined by the 14C method by Chevrel et al. (2016a). Accepting the 14C age of the oldest flow as the maximum age, the age range would be reduced to 1250–1290 CE, which strongly supports the hypothesis of a monogenetic nature of the El Metate eruption.

Paleomagnetism and 40Ar/39Ar geochronology of the Plio-Pleistocene Boring Volcanic Field: Implications for the geomagnetic polarity time scale and paleosecular variation

Paleomagnetic directions and 40Ar/39Ar ages have been determined for samples of lava flows from the same outcrops, where possible, for 84 eruptive units ranging in age from 3200ka to 60ka within the Boring Volcanic Field (BVF) of the Pacific Northwest, USA. This study expands upon our previous results for the BVF, and compares the combined results with the current geomagnetic polarity time scale (GPTS). Lava flows with transitional directions were found within the BVF at the Matuyama-Brunhes and Jaramillo-Matuyama polarity boundaries, and replicate ages corresponding to these and other boundaries have been newly ascertained. Although the BVF data generally agree with GPTS chronozone boundaries, they indicate that onset of the Gauss-Matuyama transition and Olduvai subchron occurred significantly earlier than given in the current time scale calibration. Additional comparisons show that the BVF results are consistent with recent statistical models of geomagnetic paleosecular variation.

Plio‐Pleistocene paleomagnetic secular variation and time‐averaged field: Ruiz‐Tolima volcanic chain, Colombia

AbstractPaleomagnetic results obtained from 47 Plio‐Pleistocene volcanic flows from the Ruiz‐Tolima Volcanic Chain (Colombia) are presented. The mean direction of magnetization among these flows, which comprise normal (n = 43) and reversed (n = 4) polarities, is Dec = 1.8°, Inc = 3.2°, α95 = 5.0°, and κ = 18.4. This direction of magnetization coincides with GAD plus a small persistent axial quadrupolar component (around 5%) at the site‐average latitude (4.93°). This agreement is robust after applying several selection criteria (α95 &lt; 10º; α95 &lt; 5.5º; polarities: normal, reversed, and tentatively transitional). The data are in agreement with Model G proposed by McElhinny and McFadden (1997) and the fit is improved when sites tentatively identified as transitional (two that otherwise have normal polarity) are excluded from the calculations. Compliance observed with the above mentioned time‐averaged field and paleosecular variation models, is also observed for many recent similar studies from low latitudes, with the exception of results from Galapagos Islands that coincide with GAD and tend to be near sided.

Paleomagnetic secular variation study of Ar–Ar dated lavas flows from Tacambaro area (Central Mexico): Possible evidence of Intra-Jaramillo geomagnetic excursion in volcanic rocks

More than 350 oriented paleomagnetic cores were obtained for rock-magnetic and paleomagnetic analysis from radiometrically dated (40Ar–39Ar) magmatic rocks occurring in the southern segment (Jorullo and Tacámbaro areas) of the Michoacán-Guanajuato Volcanic Field in the Trans-Mexican Volcanic Belt. Most of the lavas (37) stem from monogenetic volcanoes dated at less than 4Ma. Two additional sites were sampled from the plutonic basement dated at 33–30Ma. Primary remanences carried by low-Ti titanomagnetites allowed to determining 34 reliable site-mean directions of mostly normal (27) but also reversed (7) polarities. The mean directions of these two populations are antipodal, and suggest neither major vertical-axis rotations with respect to the North America craton nor tilting in the region for the last 4Ma (rotation and flattening of the inclination parameters being less than −5.9±3.8 and 0.1±3.9, respectively). The corresponding paleomagnetic pole obtained for Pliocene–Pleistocene times is PLAT=83.4°, PLON=2.4° (N=32, A95=2.7°). Virtual geomagnetic poles also contribute to the time averaged field global database and to the paleosecular variation (PSV) investigations at low latitudes from lavas for the last 5Ma, showing a geomagnetic dispersion value that is in agreement with available PSV models. When comparing the magnetic polarities and corresponding radiometric ages of the studied sites with the Cenozoic geomagnetic polarity time scale (GPTS), a good correlation is observable. This finding underscores the suitability of data obtained on lavas in Central Mexico for contributing to the GPTS. Furthermore, the detection of short-lived geomagnetic features seems possible, since the possible evidence of Intra-Jaramillo geomagnetic excursion could be documented for the first time in these volcanic rocks.

Revised and updated paleomagnetic results from Costa Rica

Paleomagnetic results from globally distributed lava flows have been collected and analyzed under the time‐averaged field initiative (TAFI), a multi‐institutional collaboration started in 1996 and designed to improve the geographic and temporal coverage of the 0–5 Ma paleomagnetic database for studying both the time‐averaged field and its very long‐term secular variations. Paleomagnetic samples were collected from 35 volcanic units, either lava flows or ignimbrites, in Costa Rica in December 1998 and February 2000 from the Cordilleras Central and Guanacaste, the underlying Canas, Liberia and Bagaces formations and from Volcano Arenal. Age estimates range from approximately 40 ka to slightly over 6 Ma. Although initial results from these sites were used in a global synthesis of TAFI data by Johnson et al. (2008), a full description of methodology was not presented. This paper documents the definitive collection of results comprising 28 paleomagnetic directions (24 normal, 4 reversed), with enhanced precision and new geological interpretations, adding two paleointensity estimates and 19 correlated 40Ar/39Ar radiometric ages. The average field direction is consistent with that of a geocentric axial dipole and dispersion of virtual geomagnetic poles (17.3 ± 4.6°) is in general agreement with predictions from several statistical paleosecular variation models. Paleointensity estimates from two sites give an average field strength of 26.3 μT and a virtual axial dipole moment of 65 ZAm2. The definitive results provide a useful augmentation of the global database for the longer term goal of developing new statistical descriptions of paleomagnetic field behavior.