Paleomagnetic Signal Research Articles

Precipitation of multiple light elements to power Earth's early dynamo

Earth has had a global magnetic field for at least 3.5 billion years, but if the iron-alloy in the core has high conductivity, it is difficult to explain this duration with energy from cooling and inner-core growth alone. Precipitation of light elements (e.g., magnesium, silicon, and oxygen) from Earth's core is a potential alternative energy source to power the dynamo. We develop a new framework of coupled thermo-chemical evolution of the Earth to consider precipitation of multiple light components from the core and their interaction with the overlying mantle layer. The precipitated material accumulates in a layer at the base of the mantle which is then continuously eroded by mantle convection. We allow the precipitation of three species (MgO, FeO, and SiO2), including their interactions not considered by most previous studies. We find that MgO, SiO2, and FeO precipitation may each dominate entropy production depending on the choice of equilibrium constants and initial model states and that the three species together can explain the duration of Earth's magnetic field across a range of plausible scenarios. Over the Earth's history, we find that the core can lose ∼1–2 wt% silicon and oxygen suggesting that light precipitation is potentially an important process for the core compositional evolution and core-mantle chemical exchange. Additionally, our results show that precipitation does not, in most cases, have a systematic influence on the timing of inner-core nucleation or magnitude of the resulting paleomagnetic signal with inner-core nucleation typically around 550 Mya. However, the onset of precipitation of individual species could produce additional sharp increases in paleomagnetic intensity at various points through Earth's history besides the inner-core nucleation event.

A Mediterranean perspective on 10Be, sedimentation and climate around the Matuyama/Brunhes boundary: les liaisons dangereuses?

The 10Be/9Be ratio is commonly employed as a tool for establishing the stratigraphic position of paleomagnetic excursions and reversals whenever the traditional paleomagnetic approach fails to provide conclusive results. In particular, it is held that 10Be production rates in the atmosphere depend on the strength of the Earth’s magnetic field, and the fallout and deposition of cosmogenic beryllium at the surface happen on a very short time scale. However, investigations performed on terrestrial and marine successions demonstrate that the 10Be record and the paleomagnetic signal are often asynchronous. Mechanisms that control the conveyance and deposition of cosmogenic 10Be to the seafloor are still ambiguous and poorly documented. Here, we discuss the dynamics of 10Be in a central Mediterranean marginal marine depositional scenario characterized by a pervasive terrigenous influx. Our data show that a very close correlation exists between 10Be concentrations and the local proxy of rainfall rates and regimes (pollen), indicating that a considerable 10Be transport from the mainland may occur in response to the remobilization of terrestrial reservoirs during periods of increased runoff. Superimposed is a dynamic oceanographic setting that further controls the preservation potential of 10Be at the bottom, in terms of changing water chemistry and/or composition of the sedimentary flux to the seafloor. Results of our investigation suggest that, in particular environmental and depositional settings, the interplay between climate, terrigenous yield and oceanography may jeopardize the sedimentary depiction of the meteoric 10Be contribution, thus challenging the use of 10Be for tracking the stratigraphic position of geomagnetic reversals.

Paleomagnetic Recording Efficiency of Sedimentary Magnetic Mineral Inclusions: Implications for Relative Paleointensity Determinations

AbstractSedimentary relative paleointensity (RPI) records are often carried by complex magnetic mineral mixtures, including detrital and biogenic magnetic minerals. Recent studies have demonstrated that magnetic inclusions within larger detrital silicate particles can make significant contributions to sedimentary paleomagnetic records. However, little is known about the role such inclusions play in sedimentary paleomagnetic signal recording. We analyzed paleomagnetic and mineral magnetic data for marine sediment core MD01‐2421 from the North Pacific Ocean, offshore of central Japan, to assess how magnetic inclusions and other detrital magnetic minerals record sedimentary paleomagnetic signals. Stratigraphic intervals in which abundant magnetic inclusions dominate the magnetic signal are compared with other intervals to assess quantitatively their contribution to sedimentary RPI signals. The normalized remanence record from core MD01‐2421 does not correlate clearly with global RPI stacks, which we attribute to a demonstrated lower paleomagnetic recording efficiency of magnetic inclusions compared to other detrital magnetic minerals. We also carried out the first laboratory redeposition experiments under controlled Earth‐like magnetic fields for particles with magnetic inclusions using material from core MD01‐2421. Our results confirm that such particles can be aligned by ambient magnetic fields but with a lower magnetic recording efficiency compared to other detrital magnetic minerals, which is consistent with normalized remanence data from core MD01‐2421. Our demonstration of the role of sedimentary magnetic inclusions should have wide applicability for understanding sedimentary paleomagnetic recording.

Sources of paleomagnetic signal in iron-bearing marine sedimentary deposits

This paper presents the results of interpreting the change in the magnetic signal of ferruginous marine sedimentary rocks using the example of the Cretaceous-Paleogene Bakchar deposit (Western Siberia). Anomalous magnetic susceptibility (MS) values are determined by two factors: (a) a diagenetic process accompanied by the formation of pyrrhotite, greigite and siderite in ironstones, (b) an increase detrital input with deposition of magnetite and ilmenite in the siltstones on the background of climate moistening and intense weathering. It is shown that increased MS values in iron-bearing marine sedimentary deposits can be associated with the formation of ferrimagnetic sulfides, which indicates to the methane emission through ancient marine bottom sediments.

Sources of the Paleomagnetic Signal in Iron-Rich Marine Sedimentary Rocks

In this work, the results of interpreting the variation of the magnetic signal of iron-rich marine sedimentary rocks were presented by the example of the Cretaceous–Paleogene section of the Bakchar iron ore deposit (Western Siberia). The anomalous values of the magnetic susceptibility are determined by two factors: first, a superimposed diagenetic process accompanied by the formation of pyrrhotite, greigite, and siderite among ooidal ironstones; and second, an increase in the continental runoff, causing the accumulation of terrigenous magnetite and ilmenite in the siltstone as the climate becomes increasingly humid and the weathering intensifies. It was shown that the elevated values of the magnetic susceptibility in iron-rich deposits can be due to the formation of ferrimagnetic iron sulfides, which is indicative of methane mobilization through ancient bottom sediments.

Paleomagnetic determination of the age and properties of the 1780–1800 AD dome effusion/collapse episode of Mt. Taranaki, New Zealand

Understanding the timing and nature of effusion of lava in the summit regions of stratovolcanoes is important, because these volcanoes generate deadly and destructive block-and-ash flows (BAFs). The most recent eruption of Mt. Taranaki in New Zealand left a portion of a lava dome which now forms its 2518 m summit. In this study we demonstrate the value of applying paleomagnetic techniques to understand recent effusive episodes. By matching the direction of remanent magnetization of the dome lava with the regional paleomagnetic secular variation curve NZPSV1k, we estimate that this eruption occurred between AD1780 and 1800. The paleomagnetic signal and geochemistry show that the summit dome was part of a large lava coulee that spilled from a NW-breach in the crater, with other parts of this structure still remaining down slope (the “Turtle”). Paleomagnetic analyses were also used to reconstruct the thermal history of deposits formed by the dome/coulee collapse. Emplacement temperature estimates and groundmass textural data from the BAF deposit clasts show that the lava dome collapsed in multiple stages. A small, cooler portion of lower dome coulee collapsed first, before the progressively thicker and hotter parts of the central dome. The collapses generated BAFs which travelled up to 5 km from the source and maintained temperatures > 500 °C more than 2 km from the source.

Revisiting the Paleomagnetic Reversal Test: A Bayesian Hypothesis Testing Framework for a Common Mean Direction

AbstractFor over 60 years a suite of field tests has provided paleomagnetists with a means to evaluate the timing of magnetic remanence acquisition and subsequent remanence stability. These tests are crucial for detecting potential overprinting by secondary remanences and for assessing the fidelity of the geological information carried by paleomagnetic signals. The reversal test was developed to detect secondary remanent magnetizations that could potentially bias paleomagnetic reconstructions. More recently, the reversal test has been applied to a broader set of problems, which require statistical assessment of common or antipodal paleomagnetic directions. From a statistical standpoint, the reversal test must distinguish whether two sets of paleomagnetic directions originate from populations with a common mean. However, earlier work has demonstrated that the reversal test may be ambiguous for small numbers of observations because insufficient information is available to reject the null hypothesis of a common mean direction. Here we develop a Bayesian framework to estimate directly the probability that two Fisher‐distributed sets of directions originate from populations with a common mean. This framework can be used to consider data sets with common or different precisions and, thus, provides a fully probabilistic version of the parametric reversal test. Additionally, adoption of a Bayesian framework means that ambiguity associated with the lack of information provided by small numbers of observations is incorporated into the final probability estimate in a natural way. Our new Bayesian test is demonstrated with numerical examples and case studies.

Paleomagnetic Biases Inferred From Numerical Dynamos and the Search for Geodynamo Evolution

The paleomagnetic record is central to our understanding of the history of the Earth. The orientation and intensity of magnetic minerals preserved in ancient rocks indicate the geodynamo has been alive since at least the Archean and possibly the Hadean. A paleomagnetic signature of the solidification of the inner core, arguably the singular most important event in core history, however, has remained elusive. In pursuit of this signature we investigate a suite of numerical dynamo simulations from a paleomagnetic perspective to explore how long the field should be time-averaged to obtain stable paleomagnetic pole directions and intensities. We find that running averages over $20-40$ kyr are needed to obtain stable paleomagnetic poles with $\alpha_{95}<10^\circ$, and over $40-120$ kyr for $\alpha_{95}<5^\circ$, depending on the variability of the field. We find that models with higher heat flux and more frequent polarity reversals require longer time averages, and that obtaining stable intensities requires longer time averaging than obtaining stable directions. Running averages of local field intensity and inclination produce reliable estimates of the underlying dipole moment when reversal frequency is low. However, when heat flux and reversal frequency are increased we find that local observations tend to underestimate virtual dipole moment (VDM) by up to $50\%$ and overestimate virtual axial dipole moment (VADM) by up to $150\%$. A latitudinal dependence is found where VDM underestimates the true dipole moment more at low latitudes, while VADM overestimates the true axial dipole moment more at high latitudes. The cause for these observed intensity biases appears to be a contamination of the time averaged field by non-GAD terms, which grows with reversal frequency. We derive a scaling law connecting reversal frequency and site paleolatitude to paleointensity bias (ratio of observed to the true value). Finally we apply this adjustment to the PINT paleointensity record. These biases produce little change to the overall trend of a relatively flat but scattered intensity over the last 3.5 Ga. A more careful intensity adjustment applied during periods when the reversal frequency is known could reveal previously obscured features in the paleointensity record.

Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First‐Order Reversal Curves

AbstractDiagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first‐order reversal curve unmixing method on well‐studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate‐methane transition. Below the sulfate‐methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.

Microstructural and paleomagnetic insight into the cooling history of the IAB parent body

The IABs represent one of only two groups of iron meteorites that did not form by fractional crystallization of liquid Fe-Ni in the core of a differentiated planetesimal. Instead, they are believed to originate from a partially differentiated body that was severely disrupted by one or more impacts during its early history. We present a detailed microstructural and paleomagnetic study of the Odessa and Toluca IAB meteorites, with a view to further constraining the complex history of the IAB parent body. X-ray photoemission electron microscopy and energy dispersive spectroscopy were used to generate high-resolution Ni/Fe maps. The crystallographic architecture of Odessa was analysed using electron backscatter diffraction. Paleomagnetic signals and the magnetic properties of several microstructures were also assessed using X-ray magnetic circular dichroism. Odessa exhibits a complex series of microstructures, requiring an unusual evolution during slow cooling. A conventional Widmanstätten microstructure, consisting of multiple generations of kamacite lamellae surrounded by M-shaped diffusion profiles, developed via continuous precipitation to temperatures below ∼400 °C. Multiple generations of pearlitic plessite nucleated from kamacite/taenite (T > 400 °C) and tetrataenite rim/taenite interfaces (T < 400 °C), via a process of discontinuous precipitation. Rounded rafts of Ni-rich taenite, observed within some regions of pearlitic plessite, are shown to have the same crystallographic orientation as the parental taenite, and a non-standard orientation relationship with the enclosing kamacite. Contrary to current theories, these rafts cannot have formed by coarsening of pre-existing pearlitic plessite. A new bowing mechanism is proposed, whereby rafts of Ni-enriched taenite form between advancing lobes of an irregular reaction front during discontinuous precipitation. Subsequent coarsening leads to the growth of the taenite rafts, and the partial or complete removal of pearlite lamellae, resulting in spheroidised plessite with a crystallographic architecture matching the experimental observations. We find no evidence for a strong magnetic field on the IAB parent body, suggesting it did not have an active core dynamo at the time of cloudy zone formation. This supports the prediction that the IAB parent body was unable to form a significant core due to the redistribution of metal during an earlier impact event.

Detailed Jaramillo field reversals recorded in lake sediments from Armenia – Lower mantle influence on the magnetic field revisited

While it is well established that the Earth's magnetic field is generated by a self sustaining dynamo that reversed its polarity at irregular intervals in the geological past, the very mechanisms causing field reversals remain obscure. Paleomagnetic reconstructions of polarity transitions have been essential for physically constraining the underlying mechanisms in terms of time scale, but thus far remain ambiguous with regard to the transitional field geometry. Here we present new paleomagnetic records from a rapidly deposited lacustrine sediment sequence with extraordinarily stable paleomagnetic signals, which has captured in unprecedented detail the bottom (reverse to normal: R–N) and top (normal to reverse: N–R) transitions of the Jaramillo subchron (at 1.072 Ma and at 0.988 Ma). The obtained virtual geomagnetic pole (VGP) path indicates an oscillatory transitional field behavior with four abrupt transequatorial precursory jumps across the Pacific.The distribution of VGP positions indicates regions of preferred occurrence. Our results are in agreement with previously proposed bands of transitional VGP occurrence over the Americas and Australia/northwest Pacific. Additionally, our VGP positions seem to avoid large low shear velocity provinces (LLSVPs) above the core mantle boundary (CMB). Thus, our data supports the idea that the transitional field geometry is controlled by heat flux heterogeneities at the CMB linked to LLSVPs.

Resolving the Origin of Pseudo‐Single Domain Magnetic Behavior

AbstractThe term “pseudo‐single domain” (PSD) has been used to describe the transitional state in rock magnetism that spans the particle size range between the single domain (SD) and multidomain (MD) states. The particle size range for the stable SD state in the most commonly occurring terrestrial magnetic mineral, magnetite, is so narrow (~20–75 nm) that it is widely considered that much of the paleomagnetic record of interest is carried by PSD rather than stable SD particles. The PSD concept has, thus, become the dominant explanation for the magnetization associated with a major fraction of particles that record paleomagnetic signals throughout geological time. In this paper, we argue that in contrast to the SD and MD states, the term PSD does not describe the relevant physical processes, which have been documented extensively using three‐dimensional micromagnetic modeling and by parallel research in material science and solid‐state physics. We also argue that features attributed to PSD behavior can be explained by nucleation of a single magnetic vortex immediately above the maximum stable SD transition size. With increasing particle size, multiple vortices, antivortices, and domain walls can nucleate, which produce variable cancellation of magnetic moments and a gradual transition into the MD state. Thus, while the term PSD describes a well‐known transitional state, it fails to describe adequately the physics of the relevant processes. We recommend that use of this term should be discontinued in favor of “vortex state,” which spans a range of behaviors associated with magnetic vortices.

Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales

Interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of paleomagnetic signals in rocks are usually larger, nonuniformly magnetized particles, for which there is no empirically complete, thermally activated model. This study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years.

An integrated stratigraphic study across the Santonian/Campanian boundary at Bocieniec, southern Poland: A new boundary stratotype candidate

An integrated biostratigraphic (foraminifera, calcareous nannofossils, crinoids), chemostratigraphic (stable carbon isotopes) and magnetostratigraphic study of the Bocieniec section (southern Poland) is presented here. The section presents a continuous and lithologically monotonous sedimentary record across the Santonian–Campanian boundary transition. A large number of macrofossil, foraminiferal and calcareous nannofossil bioevents along with several well-identified carbon-isotope excursions of the upper Santonian and lowermost Campanian are documented. The base of the Campanian is well-constrained by the last occurrence (LO) of the crinoid Marsupites testudinarius, and correlates to the onset of the first δ13C positive peak of the Santonian–Campanian Boundary Event (SCBE peak a). A presumable primary Cretaceous paleomagnetic signal highlights the potential presence of the C34N/C33R magnetic reversal although its exact position remains uncertain between peaks a and b of the SCBE. The planktic foraminifer Dicarinella asymetrica is very rare at Bocieniec but a potential LO of this important marker may be recorded in coincidence with peak b of the SCBE. The first occurrence (FO) of calcareous nannofossil Broinsonia parca parca coincides with the lower part of chron C33R and with the early Campanian pilula zone event. A large set of additional nannofossil events and benthic foraminifer events further constrain the stratigraphy of the section and along with the carbon isotopes, allows for correlation with other important sections of the Boreal realm. Although the Bocieniec section is relatively thin and condensed (5.5 m), the successive order of events and presence of all past proposed stratigraphic criteria for the Santonian-Campanian boundary makes it the most complete reference section for this interval at the European and at the global scale. Moreover, this section allows for a precise correlation of the Tethyan and Boreal domains. The Bocieniec section fulfils the geological criteria to be a potential boundary stratotype candidate for the base of the Campanian Stage.

PySCu: A new python code for analyzing remagnetizations directions by means of small circle utilities

The Small Circle (SC) methods are founded upon two main starting hypotheses: (i) the analyzed sites were remagnetized contemporarily, acquiring the same paleomagnetic direction. (ii) The deviation of the acquired paleomagnetic signal from its original direction is only due to tilting around the bedding strike and therefore the remagnetization direction must be located on a small circle (SC) whose axis is the strike of bedding and contains the in situ paleomagnetic direction. Therefore, if we analyze several sites (with different bedding strikes) their SCs will intersect in the remagnetization direction.The SC methods have two applications: (1) the Small Circle Intersection (SCI) method is capable of providing adequate approximations to the expected paleomagnetic direction when dealing with synfolding remagnetizations. By comparing the SCI direction with that predicted from an apparent polar wander path, the (re)magnetization can be dated. (2) Once the remagnetization direction is known, the attitude of the beds (at each site) can be restored to the moment of the acquisition of the remagnetization, showing a palinspastic reconstructuion of the structure. Some caveats are necessary under more complex tectonic scenarios, in which SC-based methods can lead to erroneous interpretations. However, the graphical output of the methods tries to avoid ‘black-box’ effects and can minimize misleading interpretations or even help, for example, to identify local or regional vertical axis rotations. In any case, the methods must be used with caution and always considering the knowledge of the tectonic frame.In this paper, some utilities for SCs analysis are automatized by means of a new Python code and a new technique for defining the uncertainty of the solution is proposed. With pySCu the SCs methods can be easily and quickly applied, obtaining firstly a set of text files containing all calculated information and subsequently generating a graphical output on the fly.

Remanence acquisition efficiency in biogenic and detrital magnetite and recording of geomagnetic paleointensity

AbstractRelative paleointensity (RPI) variations of Earth's magnetic field are widely used to understand geomagnetic field behavior and to develop age models for sedimentary sequences. RPI estimation is based on a series of assumptions. One key assumption that is rarely considered is that all magnetic particles in the sediment acquired a magnetization in an identical manner. In this paper, we test this assumption for sediments from the eastern equatorial Pacific Ocean that record well‐documented global RPI variations over the last ∼780 kyr. The magnetization is carried by two stable single domain magnetic components, which we identify as magnetite magnetofossils and titanomagnetite nanoparticle inclusions within larger silicate particles. By analyzing signals carried by the two components separately, we determine for the first time that magnetic nanoparticle inclusions can cause their host particles to record reliable but inefficient sedimentary paleomagnetic signals. The magnetization carried by biogenic magnetite is acquired more efficiently than that carried by the nanoparticle inclusions. Variations in the concentration of both components are modulated climatically so that they record nearly identical RPI signals. In many sediment types, there is no correlation between the concentrations of different magnetic components so that variable remanence acquisition efficiency will complicate RPI recording. Our work demonstrates that detailed assessment of paleomagnetic recording by each constituent magnetic component needs to become a routine part of sedimentary RPI analysis.

Widespread occurrence of silicate‐hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording

AbstractMagnetic mineral inclusions occur commonly within other larger mineral phases in igneous rocks and have been demonstrated to preserve important paleomagnetic signals. While the usefulness of magnetic inclusions in igneous rocks has been explored extensively, their presence in sediments has only been speculated upon. The contribution of magnetic inclusions to the magnetization of sediments, therefore, has been elusive. In this study, we use transmission electron microscope (TEM) and magnetic methods to demonstrate the widespread preservation of silicate‐hosted magnetic inclusions in marine sedimentary settings. TEM analysis reveals detailed information about the microstructure, chemical composition, grain size, and spatial arrangement of nanoscale magnetic mineral inclusions within larger silicate particles. Our results confirm the expectation that silicate minerals can protect magnetic mineral inclusions from sulfate‐reducing diagenesis and increase significantly the preservation potential of iron oxides in inclusions. Magnetic inclusions should, therefore, be considered as a potentially important source of fine‐grained magnetic mineral assemblages and represent a missing link in a wide range of sedimentary paleomagnetic and environmental magnetic studies. In addition, we present depositional remanent magnetization (DRM) modeling results to assess the paleomagnetic recording capability of magnetic inclusions. Our simulation demonstrates that deposition of larger silicate particles with magnetic inclusions will be controlled by gravitational and hydrodynamic forces rather than by geomagnetic torques. Thus, even though these large silicates may contain ideal single‐domain particles, they cannot contribute meaningfully to paleomagnetic recording. However, smaller (e.g., silt‐ and clay‐sized) silicates with unidirectionally magnetized magnetic inclusions can potentially record a reliable DRM.

A full-vector paleomagnetic secular variation record (PSV) from Pyramid Lake (Nevada) from 47–17 ka: Evidence for the successive Mono Lake and Laschamp Excursions

We have carried out a paleomagnetic study of late-Pleistocene Pyramid Lake core PLC08-1 (1680 cm). Our goals were to develop a full-vector paleomagnetic secular variation (PSV) record for the core, establish a paleomagnetic chronostratigraphy for the lake based on correlation of the PSV record to other dated PSV records in the region, compare that chronostratigraphy with previously developed radiocarbon and ash chronologies, and search for evidence of the Mono Lake Excursion and Laschamp Excursion. We have recovered a full-vector PSV record (inclination, declination, relative paleointensity) for the interval 47 ka to 17 ka. Twenty radiocarbon dates and four dated ashes provided a chronostratigraphic framework for this record. We have also used the link between our PSV and other dated PSV records to develop an independent PSV chronostratigraphy for the core. The PSV chronostratigraphy is not significantly different from that estimated by the radiocarbon and ash chronologies. We note the existence of two intervals of anomalous paleomagnetic directions. The younger interval, centered at 34.1±0.4 ka, has the characteristic vector component features of the Mono Lake Excursion. The older interval, centered at 40.9±0.5 ka, has the characteristic paleomagnetic signature of the Laschamp Excursion. This is the first time both intervals of excursional behavior have been found in the same sediment record from the western USA. Our new PSV record also corroborates previous estimates of the Mono Lake Excursion directional field behavior (Liddicoat and Coe, 1979) and age (Benson et al., 2003).

Pallasite paleomagnetism: Quiescence of a core dynamo

Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth.

A paleolatitude reconstruction of the South Armenian Block (Lesser Caucasus) for the Late Cretaceous: Constraints on the Tethyan realm

The continental South Armenian Block – part of the Anatolide–Tauride South Armenian microplate – of Gondwana origin rifted from the African margin after the Triassic and collided with the Eurasian margin after the Late Cretaceous. During the Late Cretaceous, two northward dipping subduction zones were simultaneously active in the northern Neo-Tethys between the South Armenian Block in the south and the Eurasian margin in the north: oceanic subduction took place below the continental Eurasian margin and intra-oceanic subduction resulted in ophiolite obduction onto the South Armenian Block in the Late Cretaceous.The paleolatitude position of the South Armenian Block before its collision with Eurasia within paleogeographic reconstructions is poorly determined and limited to one study. This earlier study places the South Armenian Block at the African margin in the Early Jurassic. To reconstruct the paleolatitude history of the South Armenian Block, we sampled Upper Devonian–Permian and Cretaceous sedimentary rocks in Armenia. The sampled Paleozoic rocks have likely been remagnetized. Results from two out of three sites sampled in Upper Cretaceous strata pass fold tests and probably all three carry a primary paleomagnetic signal. The sampled sedimentary rocks were potentially affected by inclination shallowing. Therefore, two sites that consist of a large number of samples (>100) were corrected for inclination shallowing using the elongation/inclination method.These are the first paleomagnetic data that quantify the South Armenian Block's position in the Tethys ocean between post-Triassic rifting from the African margin and post-Cretaceous collision with Eurasia. A locality sampled in Lower Campanian Eurasian margin sedimentary rocks and corrected for inclination shallowing, confirms that the corresponding paleolatitude falls on the Eurasian paleolatitude curve. The north–south distance between the South Armenian Block and the Eurasian margin just after Coniacian–Santonian ophiolite obduction was at most 1000km.