Palaeomagnetic Record Research Articles

Holocene chronostratigraphy of northeastern Baffin Bay based on radiocarbon and palaeomagnetic data

The northeastern Baffin Bay continental margin, which experiences high sediment accumulation rates, is an excellent location to study Holocene sedimentary variations. However, it is often difficult to obtain reliable chronologies of the sediment archives using traditional methods (d 18 O and radiocarbon) due to specific oceanographic conditions (e.g. corrosive bottom waters). Here we propose a chronostratigraphy of three cores collected on the northwestern Greenland margin (AMD14-204, AMD14-210 and AMD14-Kane2B) based on a combination of radiocarbon dating and palaeomagnetic records. Geophysical properties of discrete samples were used to verify the reliability of the palaeomagnetic records. Palaeomagnetic analyses indicate a strong and stable natural remanent magnetization carried by low coercivity ferrimagnetic minerals (magnetite) in the pseudo-single domain grain size range. Correlation of the full palaeomagnetic vector (inclination, declination, and relative palaeointensity) was used to establish a reliable chronostratigraphical framework for two of the cores (AMD14-204 and AMD14-210) and to propose an original palaeomagnetic record for the previously 14 C-dated core AMD14-Kane2B that covers most of the Holocene. Overall, this new chronostratigraphy allowed improvement of the timing of the main palaeoenvi-ronmental changes that occurred in this area during the Holocene.

Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian times: Palaeomagnetism confirms prolonged quasi-integrity and absence of supercontinent cycles

Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period, the possibility of Lid Tectonics is eminently testable because it seeks accordance of the wider dataset over prolonged intervals of time; deficiencies and complexities in the data merely contribute to dispersion. Accordance of palaeomagnetic poles across a quasi-integral continental crust for time periods of up to thousands of millions of years, together with recognition of very long intervals characterised by minimal polar motions (∼2.6–2.0, ∼1.5–1.25 and ∼0.75–0.6 Ga) has been used to demonstrate that Lid Tectonics dominated this aeon. The new PALEOMAGIA database is used to refine a model for the Precambrian lid incorporating a large quasi-integral crescentric core running from South-Central Africa through Laurentia to Siberia with peripheral cratons subject to reorganisation at ∼2.1, ∼1.6 and ∼1.1 Ga. The model explains low levels of tidal friction, reduced heat balance, unique petrologic and isotopic signatures, and the prolonged crustal stability of Earth's “Middle Age”, whilst density concentrations of the palaeomagnetic poles show that the centre of the continental lid was persistently focussed near Earth's rotation axis from ∼2.8 to 0.6 Ga. The exception was the ∼2.7–2.2 Ga interval defined by ∼90° polar movements which translated the periphery of the lid to the rotation pole for this quasi-static period, a time characterised by glaciation and low levels of magmatic activity; the ∼2.7 Ga shift correlates with key interval of mid-Archaean crustal growth to some 60–70% of the present volume and REE signatures whilst the ∼2.2 Ga shift correlates with the Lomagundi δ13 C and Great Oxygenation events. The palaeomagnetic signature of breakup of the lid at ∼0.6 Ga is recorded by the world-wide Ediacaran development of passive margins and associated environmental signatures of new ocean basins. This event defined the end of a dominant Lid Tectonic phase in the history of Earth's continental lithosphere recorded by the quasi-integral Precambrian supercontinent Palaeopangaea and the beginning of the comprehensive Plate Tectonics which has characterised the Phanerozoic aeon. Peripheral modifications to the lid achieved a symmetrical and hemispheric shape in Neoproterozoic times comparable to the familiar short-lived supercontinent (Neo)Pangaea (∼350–150 Ma) and this appears to be the sole supercontinent cycle recorded by the palaeomagnetic record. Prolonged integrity of a large continental nucleus accompanied by periodic readjustments of peripheral shields can reconcile divergent tectonic analyses of Precambrian times which on the one hand propose multiple Wilson Cycles to explain some signatures of Plate Tectonics, and alternative interpretations which consider that Plate Tectonics did not commence until the end of the Neoproterozoic.

Detrital magnetization of laboratory-redeposited sediments

Abstract We conducted several redeposition experiments in laboratory using natural and artificial sediments in order to investigate the role of grain size and lithology on sedimentary remanence acquisition. The role of grain size was investigated by using sorted sediment from natural turbidites. Taking advantage of the magnetic grain size distribution within turbidites, we compared redeposition experiments performed with coarse magnetic grains taken from the bottom layers of a turbidite with fine grains from the upper layers of the same turbidite. In order to document the magnetization acquired for increasing sediment concentrations that is analogous to increasing depth in the sediment column, the samples were frozen at temperatures between −5 and −10 °C. Magnetization acquisition behaved similarly in both situations, so that little smearing of the palaeomagnetic signal should be linked to grain size variability within this context. Other series of experiments were aimed at investigating the influence of lithology. We used clay or carbonated sediments that were combined with magnetic separates from basaltic rocks or with single-domain biogenic magnetite. The experiments revealed that the magnetization responded differently with clay and carbonates. Clay rapidly inhibited alignment of magnetic grains at low concentrations and, therefore, significant magnetization lock-in occurred despite large water contents, perhaps even within the bioturbated layer. Extension of the process over a deeper interval contributes to smear the geomagnetic signal and therefore to alter the palaeomagnetic record. In carbonates, the magnetization was acquired within a narrow window of 45–50 per cent sediment concentration, therefore, little smearing of the geomagnetic signal can be expected. Finally, experiments on carbonate sediments and biogenic magnetite with increasing field intensities indicate that magnetization acquisition is linear with respect to field intensity. Altogether, the results suggest that sediments with dominant carbonate content should be favoured for records of geomagnetic field changes provided that the minor clay fraction does not vary excessively. They confirm the advantage of using cultures of magnetotactic bacteria for redeposition experiments.

The signature of inner-core nucleation on the geodynamo

Energy considerations indicate that the power delivered to the present-day geodynamo comes mainly from the growth of the solid inner core, through light element and latent heat releases. The nucleation of the inner core was, therefore, a major transition for the geodynamo. Here, we use numerical dynamo simulations linked by thermochemical evolution of the core to investigate the effects of inner-core nucleation (ICN) on the geodynamo, and identify possible ICN footprints in the palaeomagnetic field. Our results suggest that a strong-field dipole-dominated dynamo prior and after ICN (the uniformitarian scenario) is most likely. We predict little footprint of ICN on the surface field intensity, consistent with the observed lack of a long-term trend in paleointensity data. Instead, we find that the best proxy for inner-core growth is an increase in axial octupole strength with age, from present-day to ICN, which might be resolvable in the palaeomagnetic record. We confirm the existence of weak-field, multipolar dynamos prior to ICN, suggesting the dynamo might have transiently reached a weak-field state (the catastrophic scenario). The narrow stability domain of these weak-field dynamos implies that the catastrophic scenario is less probable than the uniformitarian scenario.

Geochronology and Palaeomagnetic Records of the Snaigupėlė Section in South Lithuania

The interglacial deposits in the South Lithuanian Snaigupėlė outcrop and borehole Snaigupėlė-705 are marked for different bedding conditions though the sections are in proximity of each other. In the borehole section, the interglacial deposits are thicker and bedding at a greater depth than the analogous deposits of the outcrop. In the Snaigupėlė outcrop, the highest compatibility of isochronic-correction dates, calculated using L/L (method of leaching) and TSD (method of total sample dissolution) models, was determined for combination of three samples. With the help of the f value (section of isochrones in the axis of ordinates), the contribution of the primary pollution with thorium was determined where, as based on corrected analytical data, isochronic-correction dating was performed: 127<sup>+18</sup><sub>-14</sub> ka years for the L/L model and 132<sup>+22</sup><sub>-16</sub> ka for the TSD model. Palaeomagnetic investigations showed that the section of lacustrine sediments in the lower part of Snaigupėlė outcrop were orientated by reversed magnetic excursion and in the upper part by normal magnetic polarity. Collation of the obtained data with the global geomagnetic scale showed that the palaeomagnetic in-version observed in the Snaigupėlė section was related with the Blake Event in the Eemian Interglacial.

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation.

The Earth's inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth's core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

The Pleistocene palaeoenvironment and stratigraphy of Flatey island (northern Iceland): a preliminary assessment based on palynomorphs

For the first time, palynological data (pollen, spores and dinoflagellate cysts) from Pleistocene outcrop and core deposits from Flatey, off northern Iceland, are presented. The study provides a reconnaissance stratigraphical assessment of the deposits, the correlation between the Pleistocene of Flatey and the well‐known Tjörnes section on the mainland, and the reconstruction of the Pleistocene palaeoenvironment. The preservation of the assemblages is poor to moderate, whereas the diversity and richness are moderate to high. The pollen and spores indicate vegetation composed mainly of sedges and heath, comparable to the vegetation in the Tjörnes area during deposition of the Pleistocene Breidavík Group. Analysis of dinoflagellate cyst assemblages from an outcrop sample allowed their assignment to the Lower Pleistocene dinoflagellate cyst zone DAZ5, previously defined in the Tjörnes section. A core sample yielded a dinoflagellate cyst assemblage unknown from the Tjörnes area. The dinoflagellate cyst data combined with the palaeomagnetic record, the radiometric dating of lavas, and the position of the diamictites and tuff layers allowed refinement of the previously proposed stratigraphical correlations between the Tjörnes area and the Flatey core. The deposition of the sequence from the Flatey core took place during the Matuyama, i.e. between 2.59 and 0.78 Ma, and several hiatuses are postulated. The Lower Pleistocene dinoflagellate cyst assemblages from the outcrop sample bear similarities to those recovered from fully marine subsurface samples on the northern shelf of Iceland within the Polar Front realm. Similar oceanographical conditions influenced by the Irminger Current and the East Icelandic Current can be inferred for Early Pleistocene times.

Long-lived magnetism from solidification-driven convection on the pallasite parent body.

Palaeomagnetic measurements of meteorites suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields. Convection on these bodies is currently thought to have been thermally driven, implying that magnetic activity would have been short-lived. Here we report a time-series palaeomagnetic record derived from nanomagnetic imaging of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core is consistent with our measured magnetic field characteristics and cooling rates. Solidification-driven convection was probably common among small body cores, and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System.

Post-depositional remanent magnetization lock-in depth in precisely dated varved sediments assessed by archaeomagnetic field models

Accurate and precise chronologies are needed to evaluate the existence and effect of a post-depositional remanent magnetization lock-in process on sedimentary palaeomagnetic records. Here we present lock-in modelling results of two palaeomagnetic records from varved lake sediments (lakes Kälksjön and Gyltigesjön) in Sweden by using model predictions based on archaeomagnetic data. We used the 14C wiggle-match dating technique to improve the precision of the Kälksjön varve chronology in the period between 3000 and 2000 cal BP, which is characterized by pronounced palaeomagnetic secular variation. This method allowed us to infer an age model with uncertainties of ±20 years (95.4% probability range). Furthermore, we compared the palaeomagnetic record of Kälksjön to Gyltigesjön, which has a corresponding 14C wiggle-matched chronology. The ages of palaeomagnetic features derived from the wiggle-matched varve chronologies are older than those predicted by the archaeomagnetic models. Lock-in modelling was performed with different filter functions to explain the temporal offset and the amplitude of the lake sediment palaeomagnetic data. The analyses suggest that a linear lock-in function with lock-in depths (the depth below which no more natural magnetic remanence is acquired) that range between 30 and 80 cm in Kälksjön and 50 and 160 cm in Gyltigesjön are most appropriate to explain the data. These relatively deep lock-in depths in sediments without a bioturbated ‘mixed-zone’ can be attributed to the relatively high organic contents and low density of the lake sediments, which contribute to a thick unconsolidated upper zone of the sediment sequence in which re-alignment of magnetic particles can take place.

A high-resolution Early Holocene-late MIS 3 environmental rock- and palaeomagnetic record from Lake Sf. Ana, Carpathian Mts, Romania

Lacustrine sediments are excellent sources of palaeoenvironmental and palaeoclimatic information because they usually provide continuous and high-resolution records. In centraleastern Europe however lacustrine records that extend beyond the Holocene are rather sparse. Palaeomagnetic records from this region are also insufficiently explored, and usually associated with terrestrial deposits such as loess. In this context, the lacustrine record of Lake Sf. Ana, a volcanic crater lake in the East Carpathians, Romania, provides an important archive for reconstructing past paleomagnetic secular variation in the region from early Holocene to late Marine Isotope Stage (MIS) 3.

Early and Middle Pleistocene fluvial series in northern foreland of the Carpathians (Poland and Ukraine) and their relation to Dnistr River terraces

This paper discusses buried fluvial series in northern foreland of the Carpathians and their possible correlation with the Upper Dnistr River terraces. These fluvial series have been deposited mostly by the rivers from the Carpathians, with their runoff either westwards or eastwards. The whole river system has been considerably transformed during Early and early Middle Pleistocene glaciations when a streamway in the so-called sub-Carpathian channel drained this area through the Dnistr valley to the Black Sea. Re-interpretation and compilation of the previously published material from key sites with examined interglacial pollen sequences and in some of them, with the palaeomagnetic record of the Brunhes/Matuyama boundary, distinguished the first appearance of the Scandinavian material in fluvial series, as well as the directions and chronology of river discharges. Several fluvial series were identified and correlated with river terraces in the Dnistr valley. The Ruda Series in the interfluves of Odra and Vistula, and the Majdan Series in the Upper Vistula drainage basin were deposited during the preglacial part of the Early Pleistocene and corresponded with the development of Dnistr terraces X–VIII. The Kończyce Series in the interfluves of Oder and Vistula and the Rakszawa Series in the eastern part of the upper Vistula drainage basin were deposited during the early Podlasian Interglacial (Cromerian I–III) and they corresponded with development of Dnistr terrace VII. The Torhanovychi Series in the interfluves of San and Dnistr was deposited mainly during the late Podlasian Interglacial, and it corresponded with development of Dnistr terrace VI. The Krukienichi Series in the interfluves of San and Dnistr and in the Upper Dnistr valley was deposited during the Ferdynandovian Interglacial (Cromerian IV) and corresponded with development of Dnistr terrace V.

The Precambrian supercontinent Palaeopangaea: two billion years of quasi-integrity and an appraisal of geological evidence

The Protopangaea-Palaeopangaea model for the Precambrian continental crust predicts quasi-integrity reflecting a dominant Lid Tectonics defined by a palaeomagnetic record showing prolonged near-static polar behaviour during very long time intervals (~2.7–2.2, 1.5–1.2, and 0.75–0.6 Ga). Intervening times show polar loops radiating from the geometric centre of the crust explaining the anomalous Precambrian magnetic inclination bias. The crustal lid was a symmetrical crescent-shaped body confined to a single hemisphere on the globe comparable in form to the Phanerozoic supercontinent (Neo)Pangaea. There were two major transitions in the tectonic regime when prolonged near-static motion was succeeded by widespread tectonic-magmatic activity, and each coincided with the major isotopic/geochemical signatures in the Precambrian record. The first comprised a ~90° reconfiguration of crust and mantle at ~2.2 Ga terminating the long 2.7–2.2 Ga static interval; the second was the largest continental break-up event in geological history and is constrained to the Ediacaran Period at ~0.6 Ga by multiple isotopic and geochemical signatures and the subsidence history of marine passive margins. Break-up of the lid at ~0.6 Ga defines a transition from dominant Lid Tectonics to dominant Plate Tectonics and is the primary cause of contrasts between the Precambrian and Phanerozoic aeons of geological times. The long interval of minimal continental motion in the mid-Proterozoic correlates with extensive emplacement of anorogenic anorthosite-rapakivi plutons unique to these times, and high-level emplacement was probably caused by blanketing of the mantle and comprehensive thermal weakening of the crust. Continental velocities were low during the two Proterozoic intervals characterized by profound glaciation (~2.4–2.2 and ~0.75–0.6 Ga) when partial or complete magmatic shutdown is likely to have reduced volcanic greenhouse gas production. Specific implications of Protopangaea-Palaeopangaea include: (i) support for recent evidence that 60–70% of the present continental crust had accreted by ~2.5 Ga; (ii) recognition from spatially constrained mineral provinces that sub-crustal lithosphere was already chemically differentiated by mid-Archaean times; (iii) strong axial alignment of younger greenstone belts, major Palaeoproterozoic shear zones, and later Meso–Neoproterozoic mobile/orogenic belts; (iv) concentration of anorogenic magmatism and progressive contraction of activity towards the orogenic margin subsequently to become the focus of Grenville (~1.1 Ga) orogenesis; and (v) late Neoproterozoic arc magmatism/tectonics at the instep margin of the continental crescent persisting until the Ediacaran continental break-up.

Palaeomagnetic study of the Karacadağ Volcanic Complex, SE Turkey: Monitoring Neogene anticlockwise rotation of the Arabian Plate

Abstract The Karacadag Volcanic Centre in south east Turkey is a major basaltic complex sited at the northern margin of the Arabian Plate and emplaced in several pulses between ~ 11 Ma and Late Quaternary times. We have sampled 71 sites in lavas of this complex to constrain the palaeomagnetic record and hence the chronology of magmatic activity and regional rotation. Palaeomagnetic study at sixty two sites yields significant component definition with mixed normal and reversed polarities. From age dating and morphologic criteria we identify three major episodes of lava emplacement and site mean directions of magnetisation for these divisions resolve migration of the palaeofield direction for the northern sector of the Arabian Plate since mid-Miocene times. Successive (reversed polarity) group mean directions are: D/I = 175°/− 50.5° (N = 37, R = 33.13, α95 = 4.1°) for the oldest (Siverek) division (mean age estimate 11.1–6.7 Ma), D/I = 173.4°/− 46.0° (N = 16, R = 15.67, α95 = 5.5°) for the middle (Karacadag) division (mean age 3.3 Ma) and D/I = 167.7°/− 47.6° (N = 6, R = 5.93, α95 = 7.9°) for the youngest (Ovabag) division (~ 1.9 Ma — present). The first two results merit a tectonic interpretation and consistent anticlockwise rotation of ~ 9° is recognised in the Karacadag Volcanic Centre between ~ 11 and 3.3 Ma. The mean directions conform to palaeomagnetic results from other undeformed Neogene igneous complexes of comparable age range further to the west along the northern perimeter of the Arabian Plate. The amount of tectonic rotation observed in Late Pliocene and older volcanic units from this plate is found to be statistically-constant. Tectonism responsible for rotation therefore appears to have been temporally-confined to the last ~ 2 Myr which is long after collision with the Anatolides and closure of the Bitlis suture. This conclusion conforms to the young distributed block rotations recognised in the Anatolides and Aegean and correlates with the regime of tectonic escape of terranes to the west.

A planetary perspective on Earth evolution: Lid Tectonics before Plate Tectonics

Plate Tectonics requires a specific range of thermal, fluid and compositional conditions before it will operate to mobilise planetary lithospheres. The response to interior heat dispersion ranges from mobile lids in constant motion able to generate zones of subduction and spreading (Plate Tectonics), through styles of Lid Tectonics expressed by stagnant lids punctured by volcanism, to lids alternating between static and mobile. The palaeomagnetic record through Earth history provides a test for tectonic style because a mobile Earth of multiple continents is recorded by diverse apparent polar wander paths, whilst Lid Tectonics is recorded by conformity to a single position. The former is difficult to isolate without extreme selection whereas the latter is a demanding requirement and easily recognised. In the event, the Precambrian palaeomagnetic database closely conforms to this latter property over very long periods of time (~2.7–2.2Ga, 1.5–1.3Ga and 0.75–0.6Ga); intervening intervals are characterised by focussed loops compatible with episodes of true polar wander stimulated by disturbances to the planetary figure. Because of this singular property, the Precambrian palaeomagnetic record is highly effective in showing that a dominant Lid Tectonics operated throughout most of Earth history. A continental lid comprising at least 60% of the present continental area and volume had achieved quasi-integrity by 2.7Ga. Reconfiguration of mantle and continental lid at ~2.2Ga correlates with isotopic signatures and the Great Oxygenation Event and is the closest analogy in Earth history to the resurfacing of Venus. Change from Lid Tectonics to Plate Tectonics is transitional and the geological record identifies incipient development of Plate Tectonics on an orogenic scale especially after 1.1Ga, but only following break-up of the continental lid (Palaeopangaea) in Ediacaran times beginning at ~0.6Ga has it become comprehensive in the style evident during the Phanerozoic Eon (<0.54Ga).

On the directional geomagnetic signature of the Pringle Falls excursion recorded at Pringle Falls, Oregon, USA

Abstract We studied the detailed characteristics of the Pringle Falls excursion from samples at the original site recovered from four profiles drilled along the Deschutes River, Oregon. We drilled 827 samples spaced along 5 km for their detailed directional study. The profiles registered a high-resolution (&gt;10 cm/ka) palaeomagnetic record of the excursion ( c. 211±13 ka) recorded by diatomaceous lacustrine sediments. We conducted palaeomagnetic and rock magnetic studies to investigate the reproducibility of the signal throughout the profiles. We performed low-field susceptibility v. temperature analysis that indicated that the main magnetic carrier is pure magnetite (Curie point 575 °C). The magnetic grain size also indicated single domain–multi domain (SD–MD) magnetite. The demagnetization was performed by alternating field experiments and the mean directions were determined by principal component analyses. The detailed behaviour of the palaeosignal is highly consistent since they are rapidly deposited sediments providing a detailed representation of the palaeofield. The dissected virtual geomagnetic pole paths in three different phases are highly internally consistent and are defined by clockwise and anticlockwise loops travelling from high northern latitudes over eastern North America and the North Atlantic to South America and then to high southern latitudes; then they return to high northern latitudes through the Pacific and over to Kamchatka.

Is there a normal magnetic-polarity event during the Palaeocene-Eocene thermal maximum (∼55 Ma)? Insights from the palaeomagnetic record of the Belluno Basin (Italy)

SUMMARY In the lowermost Eocene sedimentary record of Ocean Drilling Program Site 1262 (Leg 208, Walvis Ridge, South Atlantic Ocean), the presence of a ∼53-kyr-long normal polarity event has been recorded within the ∼2.55-Myr-long reverse polarity Chron C24r (∼53.3–55.9 Ma) and termed Palaeocene–Eocene magnetic reversal (PEMR). The origin of the PEMR has been speculatively related to a change in the Earth’s rotation rate that was in turn influenced by an abrupt overturning of the ocean-atmosphere circulation that occurred during the Palaeocene–Eocene thermal maximum (PETM) at ∼55 Ma. Such provocative genesis for a magnetic-polarity reversal demands the PEMR to be confirmed (or refuted) in additional PETM sections. Here, we present detailed palaeomagnetic and rock-magnetic data from the Forada and Cicogna sections of the Belluno Basin in NE Italy, which contain an expanded and continuous record of the PETM termed clay marl unit (CMU). Our data indicate that these sediments were deposited during a continuous interval of reverse geomagnetic field polarity. We therefore conclude that no magnetic-polarity reversals occurred throughout the PETM. In addition, we studied the origin of the high degree of flattening affecting the characteristic magnetic component directions of the sediments, which we interpret as due to a combination of depositional inclination shallowing typical of detrital haematite, and post-depositional compaction of clays, particularly abundant in the carbonate-depleted CMU.

Deglaciation history and age estimate of the Younger Dryas end moraines in the Kalevala region, NW Russia

Three lake basins were cored in the Kalevala area, northwestern Russia in order to determine the Weichselian deglaciation history of the eastern flank of the Scandinavian Ice Sheet and to date the Kalevala and Pääjärvi end moraines adjacent to these basins. Two of the lake basins, Ala-Kuittijärvi and Keski-Kuittijärvi, are situated on the proximal side of the Kalevala end moraine while the third lake basin, Tuoppajärvi, is located on the distal side of the Pääjärvi end moraine. One site from each lake basin was chosen for sedimentological and chronological study. The chronology presented in this paper is based on palaeomagnetic measurements and counting of varved clays and the results are compared to the Finnish palaeomagnetic master curve. The results indicate that the deglaciation sediments in the Ala-Kuittijärvi and Keski-Kuittijärvi lakes were deposited mainly by extra-marginal rivers, while those in Tuoppajärvi were deposited in an ice-contact setting. After the glacial meltwater input ceased, typical large lake gyttja clay/clay gyttja sediment accumulated in all three basins. The palaeomagnetic record obtained from the Ala-Kuittijärvi sediment sequence extends back to 10,900 cal. BP, while that of Keski-Kuittijärvi and Tuoppajärvi dates back to 9800 cal. BP. The palaeomagnetic record, together with 450 counted varves in Ala-Kuittijärvi, indicates that the basin was deglaciated at around 11,350 ± 300 cal. BP. Based on palaeomagnetic results, geomorphological considerations and ice retreat rates, it is estimated that the Kalevala end moraine was formed prior to 11,450 ± 350 cal. BP, while the Pääjärvi end moraine, west of Tuoppajärvi, was formed at prior to 11,000 ± 250 cal. BP.

Nonlinear feedback and coupling in self-exciting dynamos

Systems of steadily-forced self-exciting Faraday-disk homopolar dynamos that incorporate series motors and the autonomous sets of nonlinear ordinary differential equations (ODEs) governing their behaviour are of intrinsic engineering and mathematical interest. And by providing instructive “low-dimensional” models of self-exciting fluid dynamos in stars and planets they elucidate nonlinear magnetohydrodynamic (MHD) feedback and coupling processes involving the spatial redistribution of kinetic energy by the action of Lorentz ponderomotive forces. The time (t)-dependence of the persistent dynamo current I(t) depends on the control parameters of the system, especially A/B defined by the relationship between I and Lorentz torque, T, that drives the armature of the series motor into rotation relative to the ambient magnetic field, M (amb.), namely T = AI + BI 2 where A and B depend on the design characteristics of the motor [Hide, R., Nonlinear quenching of current fluctuations in a self-exciting homopolar dynamo. Nonlinear Process. Geophys. 1997a, 4, 201–205; Hide, R., The nonlinear equations governing a hierarchy of self-exciting coupled Faraday-disk homopolar dynamos. Phys. Earth Planet. Interiors 1997b, 103, 281–291]. A is non-zero when “external” sources of M (amb.) are present, such as permanent magnets and/or other self-exciting dynamo “sub-units”, whereas B is non-zero when the motor includes a stationary “field winding” through which dynamo current is diverted. Numerical analysis of the governing ODEs using digital and (electronic circuit) analogue computers and guided by Hopf-bifurcation instability theory indicate (i) that when A is non-zero there are regions of multi-dimensional “control-parameter” space where I(t) exhibits large-amplitude chaotic fluctuations reminiscent of polarity reversals and excursions of the main geomagnetic field, and (ii) that such regions shrink in volume as A/B decreases in magnitude and they disappear altogether in the special (but physically realistic) case when A/B = 0, with the sign (direction) of I determined by the initial conditions and its magnitude by the other control parameters. This process whereby nonlinear Lorentz forces attenuate persistent fluctuations or even quench them altogether offers a theoretical basis for interpreting the highly irregular geomagnetic polarity timeseries in the palaeomagnetic record [Hide, R., Generic nonlinear processes in self-exciting dynamos and the long-term behaviour of the geomagnetic field, including polarity superchrons. Phil. Trans. Roy. Soc. Lond. A 2000, 358, 943–955]. In the absence of permanent magnets, the governing ODEs exhibit the same “magnetic symmetry” property characteristic of the governing partial differential equations (PDEs) of unbiased MHD-dynamos, namely that for every solution (u, B) there is a corresponding solution (u, −B) (where u is the instantaneous Eulerian flow velocity and B the magnetic field at a general point P).

Magnetostratigraphy of the Northern Tian Shan foreland, Taxi He section, China

TheTian Shan range formed in the late Cenozoic in response to the northward propagation of deformation related to the India^Eurasia continental collision. Precise timing of theTian Shan uplift is required to understand possible mechanisms of continental lithosphere deformation and interactions between climate, tectonism and erosion.Here, we provide magnetostratigraphic age control on the northern ChineseTian Shan foreland successions. A thorough rock magnetic analysis identifies haematite- and magnetite-bearing alluvial fan deposits in the upper portion of the sampled strata as more reliable palaeomagnetic recorders than magnetite-bearing £uvial and lacustrine deposits that are often maghaemitized in the lower part of the record. As a result, a robust correlation to the geomagnetic polarity time scale is obtained from6 to 2Ma while a tentative correlation is proposed from6 to 16Ma. Sediment accumulation rates increase from155 to 260mMyr-1 at 3.9 ± 0.3Ma.This change coincides with a gradual lithologic transition from £uvial (sandstonedominated) to alluvial fan (conglomerate-dominated) deposits that likely records an approaching erosional source related to tectonically increased subsidence rather than diierential compaction. Clear evidence for growth strata starting at an estimated age of ~2Ma provides a minimum age for folding.These results are compared with previous magneotstratigraphic studies from the same and other sections of the northernTian Shan foreland basin fill, thus enabling a critical assessment of the reliability of magnetostratigraphic dating and the significance of sediment accumulation rate variations with respect to facies variations and growth strata. Our results in theTaxiHe section provide a sequence of events that is consistentwith enhanced tectonic forcing starting at ~4Ma, although a climatic contribution must be considered given the close relationship of these ages with the Pliocene climate deterioration.

Palaeomagnetism of Archaean rocks of the Onverwacht Group, Barberton Greenstone Belt (southern Africa): Evidence for a stable and potentially reversing geomagnetic field at ca. 3.5 Ga

Palaeomagnetic data from the Palaeoarchaean Era (3.2–3.6 Ga) have the potential to provide us with a great deal of information about early conditions within, and processes affecting, the Earth's core, mantle, and surface environment. Here we present new data obtained from some of the oldest palaeomagnetic recorders in the world: igneous and sedimentary rocks from the Onverwacht Group of the Barberton Greenstone Belt (Kaapvaal Craton, southern Africa). Our palaeomagnetic measurements strengthen a recently published positive conglomerate test (Y. Usui, J.A. Tarduno, M. Watkeys, A. Hofmann and R.D. Cottrell, 2009) and our new U–Pb date constrains the conglomerate to older than 3455 ± 8 Ma. The new palaeomagnetic data from other units are nontrivial to interpret and are of uncertain reliability when taken individually; similar, we argue, to all other published palaeomagnetic data of this age. Nonetheless, four poles (two new, two derived from published data) produced from high temperature components of magnetisation recorded in the Komati, Noisy, and Hooggenoeg formations exhibit considerably improved clustering when their directions are corrected for differences in attitude resulting from a large fold structure dated at 3.23 Ga. On the basis of this enhanced consistency in stratigraphic coordinates, the positive conglomerate test, and the absence of any clear indications of their remagnetisation from comparison with younger poles, we argue that these are the most trustworthy palaeomagnetic results yet produced from any rocks of Palaeoarchaean age. When taken in conjunction with published data, the new results present the most compelling evidence to date that the Earth had a stable geomagnetic field at ca. 3.5 Ga in addition to presenting tentative evidence that it was undergoing polarity reversals. The data do not appear to support a claim, made previously from Palaeoarchean palaeomagnetic data from the Pilbara Craton (Y. Suganuma, Y. Hamano, S. Niitsuma, M. Hoashi, T. Hisamitsu, N. Niitsuma, K. Kodama and M. Nedachi, 2006), of extremely rapid latitudinal plate motion during this period. Finally, when compared with similarly aged data from the Pilbara Craton (Western Australia), the new data do not rule out the hypothesis that the two cratons were conjoined at this point in their history in the supercraton Vaalbara.