Geomagnetic Polarity Research Articles

The Potential Impacts of the Erratic Motion of Dip Equator and Magnetic Poles

AbstractThis paper examined the secular displacement of the dip equator and the geomagnetic poles as well as the variations of the global magnetic inclination and declination angles using magnetometer measurements onboard different low‐Earth orbit (LEO) satellite. The secular variation of the dip equator and geomagnetic poles has different impacts on different applications—from affecting the long‐term characterization of the low‐latitude ionosphere to degrading the precision of geomagnetic navigation. The strong displacement of the dip equator can result in a systematic error in the determination of the long‐term equatorial electric field variations and hence in the characterization of ionospheric density structure, especially in the region, where the displacement of dip equator is large enough (more than 20 km or 0.2°/year) within the time scale of a solar cycle or less. Similarly, the slowly moving locations of magnetic poles, estimated from magnetometer observations onboard LEO satellite, exemplify noticeable discrepancy with that of world magnetic model (WMM) and International Geomagnetic Reference Field (IGRF) values, indicating inevitable possible impact on the precise geomagnetic navigation for commercial and military applications. Thus, accurately estimated locations of the dip equator and magnetic poles, as well as declination angles, are critically important.

Covariant Giant Gaussian Process Models With Improved Reproduction of Palaeosecular Variation

AbstractA commonly used family of statistical magnetic field models is based on a giant Gaussian process (GGP), which assumes each Gauss coefficient can be realized from an independent normal distribution. GGP models are capable of generating suites of plausible Gauss coefficients, allowing for palaeomagnetic data to be tested against the expected distribution arising from a time‐averaged geomagnetic field. However, existing GGP models do not simultaneously reproduce the distribution of field strength and palaeosecular variation estimates reported for the past 10 million years and tend to underpredict virtual geomagnetic pole (VGP) dispersion at high latitudes unless trade‐offs are made to the fit at lower latitudes. Here we introduce a new family of GGP models, BB18 and BB18.Z3 (the latter includes non‐zero‐mean zonal terms for spherical harmonic degrees 2 and 3). Our models are distinct from prior GGP models by simultaneously treating the axial dipole variance separately from higher degree terms, applying an odd‐even variance structure, and incorporating a covariance between certain Gauss coefficients. Covariance between Gauss coefficients, a property both expected from dynamo theory and observed in numerical dynamo simulations, has not previously been included in GGP models. Introducing covariance between certain Gauss coefficients inferred from an ensemble of “Earth‐like” dynamo simulations and predicted by theory yields a reduced misfit to VGP dispersion, allowing for GGP models which generate improved reproductions of the distribution of field strengths and palaeosecular variation observed for the last 10 million years.

A Late Cretaceous‐Eocene Geomagnetic Polarity Timescale (MQSD20) That Steadies Spreading Rates on Multiple Mid‐Ocean Ridge Flanks

AbstractMagnetic anomalies over mid‐ocean ridge flanks record the history of geomagnetic field reversals, and the width of magnetized crustal blocks can be combined with absolute dates to generate a Geomagnetic Polarity Timescale (GPTS). We update here the current GPTS for the Late Cretaceous‐Eocene (chrons C33–C13, ~84–33 Ma) by extending to several spreading centers the analysis that originally assumed smoothly varying spreading rates in the South Atlantic. We assembled magnetic anomaly tracks from the southern Pacific (23 ship tracks), the northern Pacific (35), the southern Atlantic (45), and the Indian Ocean (51). Tracks were projected onto plate tectonic flow lines, and distances to magnetic polarity block boundaries were estimated by fitting measured magnetic anomalies with a Monte Carlo algorithm that iteratively changed block model distances and anomaly skewness angles. Distance data from each track were then assembled in summary sets of block model distances over 13 ridge flank regions. We obtained a final MQSD20 GPTS with another Monte Carlo algorithm that iteratively perturbed ages of polarity chron boundaries to minimize the variability of spreading rates over all ridge flanks and fit an up‐to‐date set of radioisotopic dates. The MQSD20 GPTS highlights a major plate motion change at 50–45 Ma, when spreading rates decreased in the Indian Ocean as India collided with Eurasia while spreading rates increased in the South Atlantic and Northern Pacific and the Hawaii‐Emperor seamount chain changed its orientation.

Magnetostratigraphy and Authigenic 10Be/9Be Dating of Plio‐Pleistocene Abyssal Surficial Sediments on the Southern Slope of Mariana Trench and Sedimentary Processes During the Mid‐Pleistocene Transition

AbstractThe Mariana Trench is the deepest part of Earth's surface, with a maximum depth of close to 11,000 m in the Challenger Deep. Great progress has been made in studying the trench in the past years; however, several basic issues regarding the sedimentology and geochronology of the abyssal surficial sediments in the trench remain unclear. In this study, two gravity cores of Plio‐Pleistocene sediments were retrieved from the southern slope of the Mariana Trench. They were used for an integrated investigation of magnetostratigraphy, authigenic 10Be/9Be dating, and element manganese content. The principal results are as follows. (1) Ten magnetozones are recognized for Core A25, which can be correlated to the geomagnetic polarity time scale from Chrons C1n to C3r. (2) The paleomagnetic and 10Be/9Be‐based ages agree well for both of the two cores, yielding a continuous depositional hiatus since the middle Pleistocene (0.63–0.74 Ma). (3) The sediment accumulation rates prior to ~1.3 Ma are 35–83 cm/Myr and then increased substantially to 167–200 cm/Myr. Based on these results, a sedimentary transition at ~1.3–1.2 Ma is confirmed, which can be potentially correlated to climatic, oceanic, and tectonic changes in the Mid‐Pleistocene transition. In addition, an elemental Mn proxy is derived to reflect changes in bottom‐water ventilation, which is consistent with Lower Circumpolar Deep Water and North Atlantic Deep Water on glacial‐interglacial time scales. We conclude that the sedimentary record of the Mariana Trench provides a unique window for investigating the integrated effects of long‐term climatic, oceanic, and tectonic processes.

Development of scanning SQUID microscope system and its applications on geological samples: A case study on marine ferromanganese crust

We present developments and applications of a high resolution scanning superconducting quantum interference device (SQUID) microscope for imaging magnetic field of geological samples at room temperature. A directly coupled low-temperature SQUID with a 200 μm × 200 μm pickup loop was mounted on a sapphire rod and separated from room temperature by a sapphire window. The environmental noise of the SQUID was successfully reduced by subtracting the signal of an additional reference SQUID placed inside a cryostat. The resulting system noise level was estimated to be about 50 pT. A geological thin section could be placed on a non-magnetic sample holder with an XYZ stage for scanning in an area of 100 mm × 100 mm. The minimum achievable distance from the SQUID to the sample is measured as ∼200 µm. An application of the SSM to a marine ferromanganese crust successfully provided beautiful stripe patterns in the magnetic images. The patterns could be correlated to the history of geomagnetic field reversals. The boundaries of the magnetic polarity domains were useful guides for the estimation of the deposition age by correlation with the standard geomagnetic polarity timescale. The established age model gave an average growth rate of ∼2.7 mm/Ma, which is consistent with that obtained by radiometric dating using 10Be (∼2.6 mm/Ma).

Developing the Cambrian and Ordovician Magnetic Polarity Time Scale: Current Data and Attempt of Synthesis

Abstract—The time scale of the geomagnetic polarity is the quintessence of our knowledge about the changes in the polarity of the Earth’s magnetic field in the geological past. The pattern of the Earth’s magnetic field polarity reversals is relatively well known for the Cenozoic, Mesozoic, and Late Paleozoic. However, our understanding of the behavior of geomagnetic polarity in the more ancient epochs is non-systematic and fragmentary. Obtaining the reliable high-quality information for the Lower and Middle Paleozoic should become the next important step.This paper compiles the main data yielded by recent studies on magnetic stratigraphy of the Lower Paleozoic (Cambrian and Ordovician) of the Siberian Platform and other regions conducted by our team and our colleagues. The synthesis of these data allows us to propose a working model of the Early Paleozoic magnetic polarity time scale which embodies the current knowledge about the character of changes in the geomagnetic field polarity on this interval of the geological time. The process of geomagnetic field polarity reversals in the Early Paleozoic is described by the following most important characteristics: (a) the existence of a superchron of reversed polarity during a significant part of the Early and Middle Ordovician (the third Phanerozoic superchron Moyero); (b) a high (probably peaking in the Phanerozoic) frequency of the geomagnetic reversals in the Middle Cambrian; (c) a decrease in the frequency of geomagnetic reversals in Late Cambrian and Tremadocian with the approach of the superchron; (d) the presence of anomalous period in the behavior of the geomagnetic field close to the Phanerozoic/Precambrian boundary; and (e) the absence of a double superchron in the Ordovician (Algeo, 1996).

The Potential of Marine Ferromanganese Nodules From Eastern Pacific as Recorders of Earth's Magnetic Field Changes During the Past 4.7 Myr: A Geochronological Study by Magnetic Scanning and Authigenic 10Be/9Be Dating

AbstractThe Clarion and Clipperton Fracture Zones in the Eastern Pacific are well known as a manganese nodule belt, which developed since the late Oligocene. The slow growth rate of these marine ferromanganese nodules implies that they have potentially recorded long‐term environmental changes. To produce environmental records from these nodules, their chronology needs to be established first. In order to achieve this objective, many dating methods have been applied; however, due to relatively low‐resolution temporal constraint, high‐resolution environmental signals recorded in marine ferromanganese nodules have not been well documented yet. In this paper, we present a geochronological study based jointly on magnetic scanning and analyses of the authigenic beryllium isotopes (authigenic 10Be/9Be) and the cobalt (Co) flux of a marine ferromanganese nodule. Results lead to the following observations: (1) The growth of the studied nodule resulted from a combination of hydrogenetic and diagenetic processes, and (2) a total of 24 magnetozones is recognized for the studied nodule. Correlation to the geomagnetic polarity timescale suggests that chrons from C3n.2r to C1n were recorded in the nodule, implying that the growth of this nodule initiated ~4.70 Ma, which agrees well with the authigenic 10Be/9Be and Co chronometers. Furthermore, a consistent pattern of broad paleomagnetic field variations was observed between the nodule's magnetic signal and the ocean‐floor magnetic fields of the South Atlantic, confirming the validity of our growth model. Overall, our study highlights the potential of marine ferromanganese nodules from the Eastern Pacific as recorders of changes in Earth's magnetic field.

Cataclysmic Geomagnetic Field Collapse: Global Security Concerns

In 2015, Tyler J. Williams authored “Cataclysmic Polarity Shift: Is U. S. National Security Prepared for the Next Geomagnetic Pole Reversal?” That document provides an extremely cogent and thorough description of some of the risks to national security and infrastructure expected to result from a geomagnetic polarity reversal. However, it describes geomagnetic field generation solely as currently promoted by the geophysics community which is based upon old ideas, circa 1940s-1960s, that are taken to be factual without any attempt to understand their limitations or to evaluate their validity in light of subsequent scientific developments. Moreover, the security concerns Williams described are relevant to humanity globally. Here I have reviewed the historical development of those old ideas, pointed out their problematic nature, and reviewed subsequent published advances that overcome their inherent problems and lead to a better understanding of the geophysics related to geomagnetic polarity reversals, geomagnetic excursions, and, at some yet unknown time, the permanent demise of the geomagnetic field. Mechanisms of rapid geomagnetic field collapse, both natural and potentially human-induced, are described. The present state of nuclear georeactor activity, whether geomagnetic field collapse leads to increased georeactor output, and whether it is likely to trigger earthquakes and volcano eruptions are yet unknown matters of seriously troubling human security concerns. Global security preparedness, even though addressed by sovereign nations, should be predicated upon the latest and most correct scientific understanding. In some areas that may be the case, but in the scientific areas described here there are clearly problems. The inherent problems, I submit, do not result from inadequate funding, but from inadequate methodologies, expectations and responsibilities of scientists, their national and parent institutions, publishers, and respective funding-agencies.

A centennial-resolution terrestrial climatostratigraphy and Matuyama\u2013Brunhes transition record from a loess sequence in China

Terrestrial records of the last geomagnetic reversal often have few age constraints. Chronostratigraphy using suborbital-scale paleoceanic events during marine isotope stage 19 may contribute to solving this problem. We applied the method to an 8 m long, high-resolution paleomagnetic record from a loess sequence in China and revealed millennial-to-sub-centennial scale features of the Matuyama–Brunhes (MB) transition. All samples were subjected to progressive thermal demagnetization with 14–15 steps up to 650–680 °C. As a result, 96% of the samples yielded a high-quality remanent magnetization. The MB transition terminated with a 75 cm thick zone with nine polarity flips. The polarity flip zone, dated at about 779–777 ka, began between the warm events “I” and “J” and terminated at the end of the cooling event coincident with the lowest axial-dipole strength interval. Most polarity flips occurred within 70 years. The virtual geomagnetic poles (VGPs) in the upper polarity flip zone clustered in the SW Pacific region, where the MB transitional VGPs from lavas of the Hawaiian and Canary Islands and lacustrine deposits of Java also clustered. These sites were probably dominated by dipolar fields. The absence of transitional fields across polarity flips implies a short time span for averaging fields due to a thin loess-magnetization lock-in zone. The reverse-to-normal polarity reversal dated at about 778 ka in Lingtai occurred at the end of the SW Pacific VGP zone, an important key bed for MB transition stratigraphy. The reversal is a good candidate for the main MB boundary. We found an excursion at about 766 ka spanning about 1 ka.

Coniacian-Campanian magnetostratigraphy of the Marambio Group: The Santonian-Campanian boundary in the Antarctic Peninsula and the complete Upper Cretaceous – Lowermost Paleogene chronostratigraphical framework for the James Ross Basin

Recent magnetostratigraphic works from different areas of the James Ross Basin have expanded on chronostratigraphic studies previously based on ammonite, palynomorph and nanoplankton biostratigraphy, and strontium isotope stratigraphy. Here we present a new magnetostratigraphy of Coniacian through Campanian marine sedimentary rocks from Hidden Lake, Santa Marta and Snow Hill Island Formations, on northwest James Ross Island. A total of 189 paleomagnetic directions were obtained along more than 1500 m of stratigraphic thickness from Brandy Bay to Santa Marta Cove areas, identifying three polarity chrons of the global polarity time scale. The local magnetostratigraphic column starts in the upper part of the Cretaceous Normal Superchron C34N (Coniacian) and ends in Chron C33N (middle Campanian). The correlation between the magnetostratigraphy and the age framework given by ammonite biostratigraphy allowed the assignment of precise ages to particular horizons of the Santa Marta Formation. The newly identified geomagnetic polarity reversals are the earliest identified in the James Ross Basin and include: a) C34N/C33R (84.2 Ma, late Santonian – early Campanian) in the Alpha Member of the Santa Marta Formation and b) C33R/C33N (79.9 Ma, middle Campanian) in the upper Beta Member (Santa Marta Formation). By integrating this new data with previous work, we present a complete Upper Cretaceous – lowermost Paleogene chronostratigraphical framework for the basin, spanning both proximal to distal sedimentary facies of the Marambio Group.

Giant Gaussian process models of geomagnetic palaeosecular variation: a directional outlook

SUMMARY Assessment of long-term palaeosecular variation (PSV) of the geomagnetic field is frequently based on simplified versions of a class of statistical models known as giant Gaussian processes (GGP) used to represent temporal variations in spherical harmonic descriptions of the field. Here we propose a new type of analysis to assess the shape and dispersion of the directional distributions caused by PSV. The quantities analysed in this study are equal-area coordinates of rotated distributions of palaeomagnetic directions, ${x_E}$ (east−west) and ${x_N}\ $(north−south) and their standard deviations (${\sigma _E}$ and ${\sigma _N}$). These are easy to determine, and can readily be numerically predicted for any GGP model, avoiding the need for the numerous simulations generally used to determine the scatter and/or elongation of directional distributions. Mean predictions of $\overline {{x_N}} $ for a simplified GGP model are different from the expected geocentric axial dipole (GAD) directions, in agreement with inclination differences noted in previous studies. The best estimates for palaeomagnetic inclination are the expected directions from the mean of virtual geomagnetic poles (VGPs) calculated using an iterative angular cut-off process. Predictions of ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$ vary with latitude and are symmetric about the Equator. The N–S direction (${\sigma _{\rm N}}$) is always larger than E–W (${\sigma _{\rm E}}$), but the difference decreases from a maximum at the Equator to the poles, where ${\sigma _{\rm N}} = \ {\sigma _{\rm E}}$. A simplified GGP model is used to show that the parameter α (affecting variances in all Gauss coefficients) is positively correlated with ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}\ $ while the β parameter, the ratio of dipole to quadrupole family standard deviations, modifies the latitudinal dependence of ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$. Experimental error in ${\sigma _{\rm E}}$ and ${\sigma _{\rm N}}$ can be accommodated using the common statistical parameters found in palaeomagnetic data sets, as ${\alpha _{95}}$ from site-mean directions. Predictions of simplified GGP models are compared with both numerical simulations and real data spanning the last 10 Ma. The latitudinal dependence of the proposed measures of PSV (${\sigma _{\rm E}}$ and ${\sigma _N}$) provide useful diagnostics for testing the validity of a GGP model. For the past 10 Ma the best-fitting GGP model with a mean GAD field set to $g_1^0 = \ - 18\ \mu T$ has α = 6.7 µT and β = 4.2. These new directional diagnostics will be used to investigate changes in overall geomagnetic field behaviour over other geological time intervals.

Volcanic Record of the Last Geomagnetic Reversal in a Lava Flow Sequence From the Azores

We present paleomagnetic and geochronological data of twenty-five stacked lava flows from São Miguel Island (Azores) that were emplaced during the last geomagnetic reversal (Matuyama-Brunhes, M-B). The characteristic direction of natural remanent magnetization was isolated through alternating field and/or thermal demagnetization. From bottom to top, the directional changes display a first phase with reverse, transitional and normal directions which is followed by a second phase with transitional and normal directions. Dating by Potassium-Argon (K-Ar) of fresh separated groundmass provide ages of 768 ± 5 ka and 765 ± 5 ka (1σ) for the first and second transitional episodes, respectively. We infer that this sequence recorded the last reversal and reveal the presence of a rebound that occurred soon after the transition. The age of the transition is 5-15 kyr younger than at other volcanic localities, but in agreement with astronomical ages that were recently proposed for the transitional interval. The reversal is characterized by weak field intensity during the transition, a recovery phase and a subsequent decrease during the rebound. The two transitional virtual geomagnetic poles (VGPs) that characterize the transition are found above the South Atlantic and the North Pacific. When compared to other M-B records, no VGP is found above preferred locations.

Paleomagnetism, paleointensity and geochronology of a Proterozoic dolerite dyke from southern West Greenland

Archean to Paleoproterozoic rocks potentially record the evolution of the geodynamo and the tectonic mode of the early Earth. The paleomagnetic intensity and direction data provide important information on the Earth’s core–mantle revolution. Herein, we report the results of paleomagnetic and geochronological studies of a Proterozoic dolerite dyke from southern West Greenland. Clinopyroxene grains from the dyke yielded Ar–Ar plateau ages from 1808 to 1887 Ma (1816.0 ± 14.6 Ma; 2σ). The paleomagnetic direction of the dyke (D = 243.6°, I = 66.3°, α95 = 3.9°) yielded a virtual geomagnetic pole (VGP) of 33.5 °N and 96.4 °W. This 1.8 Ga pole falls in a limited area where Paleoproterozoic poles between 2.5 Ga and 1.7 Ga for southern West Greenland are distributed. Comparison of the Paleoproterozoic poles of southern West Greenland with those of North America suggests that the North Atlantic Craton of southern Greenland could have been an independent stagnant tectonic block, different from the drifting Superior and Slave Cratons in the Early Proterozoic. Thellier experiments on 13 specimens yielded a mean paleointensity value of 14.8 ± 2.3 μT, indicating a virtual dipole moment of 2.88 ± 0.46 × 1022 Am2. This value is approximately one-third of the present-day Earth's field intensity, and is consistent with the value for the period between 1400 Ma and 2400 Ma. This small paleointensity value of the Proterozoic rocks is due to a gradual dipole moment change over a long period (∼1 Gyrs) since 4000 Ma.

Uncertainty Propagation in Hierarchical Paleomagnetic Reconstructions

AbstractEstimation of paleomagnetic directions plays a crucial role in magnetostratigaphy, paleogeographic reconstruction, and constraining past geomagnetic field behavior. While analysis and aggregation of paleomagnetic directional data are performed in a hierarchical fashion, the standard statistical framework employed by paleomagnetists does not consider uncertainty propagation through each level of the hierarchy. With this limitation, inferences drawn from paleomagnetic data will be affected by underestimated uncertainties. We provide here an approximate directional uncertainty propagation scheme that applies to Fisher distributions and, thus, to a number of paleomagnetic data processing tasks. The scheme is a straightforward addition to the existing paleomagnetic statistical framework and is demonstrated using case studies to show how uncertainties can be propagated through different stages of the paleomagnetic data processing chain. Furthermore, we discuss situations, such as the transformation into virtual geomagnetic poles, where this simple form of uncertainty propagation cannot be employed because the data are not Fisher distributed.

Probing the hydrothermal system of the Chicxulub impact crater.

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth's crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 105 km3 of Earth's crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years.

Magnetochronology applied to assess tempo of turbidite deposition: A case study of ponded sheet-like turbidites from the lower Miocene of the northern Apennines (Italy)

This paper investigates the magnetostratigraphy of the ~750 m-thick Costa Grande Member (lower Miocene) from the Castagnola Basin of NW Italy, which represents the turbidite fill of a structurally confined basin where flow ponding resulted in a complete record of deposition from diverse sediment gravity flow types. The magnetostratigraphic profile of the Costa Grande Mb was correlated to the Geomagnetic Polarity Timescale using a well-established statistical method devised to be applied to sedimentary successions with steady accumulation rate. The results of the correlation exercise, validated with the available biostratigraphy, indicate an early Miocene age (from Chron C6AAr.3r to C6Bn.2n) between 21.7 and 22.3 Ma. The obtained age model was then used to calculate accumulation rates and frequencies of small volume and low-density vs. large volume and high-density gravity flows over an estimated 650 kyr time span. Results show that low-density flows contributed at a constant pace of 545 m Myr−1 to sediment accumulation, depositing relatively thin-bedded turbidites with a minimum recurrence time of 1.7 kyr, whereas the high-density flows, which are at least four times less frequent, are clustered in the stratigraphy, and become more abundant up-section. We also show that the minimum sediment volumes discharged by high-density flows suggest triggering by submarine failures whose recurrence and magnitude were probably not random. Lastly, we propose that the statistical method used in this study to correlate the Costa Grande Mb magnetostratigraphy to the GPTS provides best results when the products of episodic but voluminous depositional events are carefully identified and removed.

Rethinking the chronology of early Paleogene sediments in the western North Atlantic using diatom biostratigraphy

Ocean Drilling Program (ODP) Site 1051 (Blake Nose, western North Atlantic) is of crucial importance for reconstructing diatom evolution and biosiliceous sedimentation patterns through the early Cenozoic period of extreme greenhouse warmth followed by the progressive global cooling. The magnetostratigraphy in Hole 1051A, however, has been subject to divergent interpretations resulting in multi-million-year age control uncertainties, especially for events surrounding the early to middle Eocene transition. To resolve these uncertainties, we compare the stratigraphy of Hole 1051A to the neighboring Holes 1050A, C. We compile the published biomagnetostratigraphic data for both sites and identify three possible magnetostratigraphic solutions for Hole 1051A, the difference being the number of hiati and their duration. In order to identify the most plausible magnetostratigraphic solution for Hole 1051A, we employ the graphic correlation method, in which we compare the depth of individual magnetic reversals in both study sites against an independent proxy, i.e., 49 diatom evolutionary events identified in Holes 1050A,C and 1051A. The distribution of diatom bioevents lends strong support to the presence of two major hiati in both study sites: the upper hiatus juxtaposes magnetozone C21n on C22n, and the lower hiatus juxtaposes magnetozone C23n on C24n, eliminating the record of the initial part of the Early Eocene Climatic Optimum. Diatom, calcareous nannofossil and foraminiferal biostratigraphic markers also indicate that Hole 1051A terminated within magnetozone C28n rather than C27n. This age interpretation is strongly supported by the alignment of high-resolution weight percent biogenic SiO2 records from both study sites. The revised age models developed here for Holes 1050A,C and 1051A have profound consequences for interpretations of western North Atlantic paleoceanographic and paleoclimatic history through the early Paleogene. We propose a revised labelling of the early Eocene carbon cycle perturbations identified to date in Hole 1051A, and show how published Blake Nose δ30Si records change when data from Sites 1050 and 1051 are rescaled to the age models proposed herein. We emphasize that these refinements to the stratigraphy of Sites 1050 and 1051 are based on a study of diatom bioevents, which are here identified and calibrated to the Geomagnetic Polarity Timescale. This successful application of diatom biostratigraphy is a substantial advance toward their future utility in providing high-resolution age control for Paleogene deep-sea sites.

Long‐eccentricity regulated climate control on fluvial incision and aggradation in the Palaeocene of north‐eastern Montana (USA)

AbstractAggradation and fluvial incision controlled by downstream base‐level changes at timescales of 10 to 500 kyr is incorporated in classic sequence stratigraphic models. However, upstream climate control on sediment supply and discharge variability causes fluvial incision and aggradation as well. Orbital forcing often regulates climate change at 10 to 500 kyr timescales while tectonic processes such as flexural (un)loading exert a dominant control at timescales longer than 500 kyr. It remains challenging to attribute fluvial incision and aggradation to upstream or downstream processes or disentangle allogenic from autogenic forcing, because time control is mostly limited in fluvial successions. The Palaeocene outcrops of the fluvial Lebo Shale Member in north‐eastern Montana (Williston Basin, USA) constitute an exception. This study uses a distinctive tephra layer and two geomagnetic polarity reversals to create a 15 km long chronostratigraphic framework based on the correlation of twelve sections. Three aggradation–incision sequences are identified with durations of approximately 400 kyr, suggesting a relation with long‐eccentricity. This age control further reveals that incision occurred during the approach of – or during – a 405 kyr long‐eccentricity minimum. A long‐term relaxation of the hydrological cycle related to such an orbital phasing potentially exerts an upstream climate control on river incision. Upstream, an expanding vegetation cover is expected because of an increasingly constant moisture supply to source areas. Entrapping by vegetation led to a significantly reduced sediment supply relative to discharge, especially at times of low evapotranspiration. Hence, high discharges resulted in incision. This study assesses the long‐eccentricity regulated climate control on fluvial aggradation and incision in a new aggradation–incision sequence model.

Paleogeographic and tectonic evolution of the Pampia Terrane in the Cambrian: New paleomagnetic constraints

Paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the late Middle to early Late Cambrian Campanario Formation exposed in NW Argentina. The study also presents preliminary results from the Early Cambrian metasediments of the Puncoviscana Formation and from Mojotoro intrusive. A new paleomagnetic pole, C2, was computed for the Campanario Formation (23.6°N, 346.5°E, A95, 7.0°, N: 11 sites) from the localities of Tilcara (T), Terma de Reyes (R) and El Perchel (EP), including previous data from Santa Victoria Oeste (M), in northernmost Argentina. A positive fold test was obtained for Tilcara locality while a positive regional tilt test was obtained for mean site directions for the four localities. This pole can be considered as the representative pole position for the Pampia terrane for the late Middle to Late Cambrian. Previous paleomagnetic data for the same formation from the Iruya-Matancillas locality show an in situ clockwise rotation of about 30° around a vertical axis when compared with the mean Pampia pole. The same is observed when comparing the early Ordovician pole for the Santa Rosita Formation at those localities with the recently obtained coeval pole of the La Pedrera Formation, indicating that local tectonic rotations, affected that area. These results suggest that previous paleomagnetic interpretations for these Late Cambrian and Early Ordovician rocks in the region as recording displacement of the Pampia terrane in Cambrian times are probably incorrect. Additionally, two virtual geomagnetic poles were computed for the Puncoviscana Formation (TP1: 34.5°N, 52.3°E, A95: 5.0°, n: 22 specimens and TP2: 38.3°N, 33.6°E, A95: 5.7°, n: 14 specimens) and one for the Mojotoro Intrusive (MO: 9.1°N, 345.6°E, A95: 16.4°, n: 11 specimens). These and previous Early Cambrian paleomagnetic data from the Pampia terrane differ from Gondwana coeval reference directions in inclination and/or declination, lacking a simple pattern.

High-precision U-Pb zircon age constraints on the Guadalupian in West Texas, USA

The Guadalupian Epoch was characterized by major changes in paleogeography, paleoclimate, and biodiversity. Yet, the paucity of precise and accurate radioisotopic dates from the Guadalupian stages in their type area, Guadalupe Mountains National Park in West Texas has rendered their calibration inadequate. In this study, we report high-precision U-Pb zircon geochronology by the CA-ID-TIMS method from three ash beds (2σ internal errors only) in the Rader Member of the Bell Canyon Formation at the Back Ridge Section (262.127 ± 0.097 Ma, MSWD = 0.89, n = 3), the lower Pinery Member of the Bell Canyon Formation at the Frijole Section (264.23 ± 0.13 Ma, MSWD = 0.89, n = 8) and the basal South Wells Member of the Cherry Canyon Formation at the Monolith Canyon Section (266.525 ± 0.078 Ma, MSWD = 0.62, n = 5). The Bayesian interpolation statistics method is used to establish an age-stratigraphy model that estimates the base of the Capitanian to be 264.28 ± 0.16 Ma, serving as the best age estimate for the Capitanian Stage at present. In addition, we review the existing geochronology from the Guadalupian Series in West Texas and seek to propose more precise temporal estimates of Guadalupian geological and biological events. These data constrain the high-frequency sequences of the Cherry Canyon and Bell Canyon formations in the Guadalupe Mountains National Park area. Accordingly, the base of the Wordian is estimated at 266.9 ± 0.4 Ma and the Illawarra geomagnetic polarity reversal in West Texas at 267.4 ± 0.4 Ma to 266.5 ± 0.3 Ma. The global end-Guadalupian extinction began in the conodont zone of Jinogondolella altudaensis above the base of the Reef Trail Member of the Bell Canyon Formation and might continue to the Clarkina postbitteri postbitteri Zone in the earliest Wuchiapingian. The conodonts display a rapid evolutionary rate during this interval. This constrains the biotic crisis from ca. 260 Ma to 259 Ma based on our conodont age estimation. The emplacement of the Emeishan Large Igneous Province (ELIP) in South China has been constrained to ca. 260 Ma to 257.4 Ma based on zircon U-Pb geochronology by the CA-ID-TIMS method, overlapping with the end-Guadalupian extinction, which provides support for the temporal relationship between them. Additionally, the ELIP persisted into the early Wuchiapingian and may have hampered ecosystem restoration during the post-extinction interval.