Grain-size-dependent paleointensity results from very recent mid-oceanic ridge basalts

We report paleointensity analyses on a suite of four samples from two axial zero-age mid-oceanic ridge basalt flows from the East Pacific Rise. Paleointensity experiments have been performed on several profiles, each consisting of a batch of small (millimeter scale) subsamples going from the rapidly cooled glassy margin to the interior. The Coe version of the Thellier double-heating procedure was used with back checks performed after every other heating step. Most of the samples show very good behavior, i.e., constant ratio between the lost and acquired magnetization through a large temperature range (quality factor usually above 10) and positive checks, which lead to unambiguous paleointensity determinations. Paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of Earth's magnetic field intensity. However, results found within the crystalline part of the samples show values that are considerably (up to 50%) higher than expected. These variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern. An extensive study of magnetic properties allows us to link this incongruent behavior to (1) the presence of multidomain effects and (2) the cooling rate difference between laboratory experiments and in situ cooling

Étude des variations en éléments traces dans un carbonate archéologique de la ville antique d'Ostie : conséquences environnementales et archéologiques

International audienceHigh resolution laser ablation ICP-MS analyses are performed along the growth axis of an archaeological carbonate deposit from a water tower in the antique city of Ostia (Italy). Results show a composite structure with high frequency cycles, thought to be seasonal, and longer-term variations in Mg/Ca, Sr/Ca, Ba/Ca, Pb/Ca and U/Ca ratios. Changes in the broad trace element composition at the beginning of the deposit are interpreted as the hallmark of important modifications in the sources supplying the water to the city. Our study suggests that trace elements analysis in archaeological carbonates have the potential to provide high-resolution seasonal records, which could give insights into historic environment variability and past policy of natural resource administration.Les étapes de croissance d'un dépôt de carbonate provenant d'un château d'eau de la ville antique d'Ostie sont étudiées à l'aide d'un ICP-MS à ablation laser. Les résultats montrent une structure composite, avec des cycles hautes fréquences, probablement saisonniers, se superposant à un signal à plus long terme pour les rapports élémentaires suivants : Mg/Ca, Sr/Ca, Ba/Ca, Pb/Ca et U/Ca. Un changement de composition au début du dépôt est interprété comme la mémoire de modifications importantes dans la localisation de la source d'eau qui approvisionnait la ville. À l'issue de cette étude, on peut entrevoir les potentialités de l'analyse des éléments traces dans les carbonates archéologiques, afin d'aborder avec une haute résolution la question environnementale durant la période historique et, ainsi de mieux cerner les politiques passées de gestion des ressources naturelles

Timing of volcanism along the northern East Pacific Rise based on paleointensity experiments on basaltic glasses

Samples from two adjacent and contrasting ridge segments along the East Pacific Rise were measured for their magnetic paleointensity in order to further explore the possibilities of dating very young volcanic samples using secular variations in the Earth's magnetic field. The ridge segment north of the Orozco transform fault (15°22′-16°20′N) is the shallowest and broadest along more than 5000 km of the East Pacific Rise, whereas the adjacent segment to the north (16°16′-18°N) has a "typical" morphology for its intermediate spreading rate. Both ridge segments were densely sampled during the PANR01MV cruise and 36 samples of axial lava flows, consisting mainly of glasses from the rims of the flows and some fragments of lobate basalts, were selected from this collection for paleointensity experiments. The Coe version of the Thellier double-heating procedure (in air) was used. Twenty-seven units provide internally consistent paleointensity estimates leading to precise estimates of the paleofield, which range between 8 μT and 57 μT. Comparisons with reference paleointensity curves compiled from subaerial flows, archeomagnetic data and sedimentary records projected to the sampling site coordinates show that the measured values can be used to constrain the volcanic history of the ridge segments over the past few thousand years. A good agreement was found between apparent "freshness" of the glasses, the geochemistry of the lavas, and their magnetic paleointensity values. The inflated southern segment seems characterized by recent activities as indicated by numerous flows with paleointensities clustering around today's value (39 μT) or around the high values typical of 2000-3000 years ago (~55 μT). We interpret this distribution to indicate the flooding by effusive lava flows of the entire axial plateau some 2000-3000 years ago, followed by a volcanic phase producing smaller volume lava flows confined to the innermost 200 m of the ridge axis. The northern ridge segment is characterized by dispersed paleointensity values consistent with a series of small eruptions of diverse ages. Samples collected at the tips of both ridge segments across the 16°20′N axial discontinuity have the lowest paleointensities and are thus thought to be significantly older, consistent with models advocating reduced magmatism near ridge axis discontinuities. This study demonstrates the strong potential of paleointensity measurements as a tool to help constrain volcanic history at ridge axes

A negative test of orbital control of geomagnetic reversals and excursions

A ~41 Kyr periodic component has been reported in some sedimentary paleointensity records, allowing speculation that there may be some component of orbital control of geomagnetic field generation such as by obliquity modulation. However, no discernable tendency is found for astronomically-dated geomagnetic reversals in the Plio-Pleistocene (0 to 5.3 Ma) or excursions in the Brunhes (0 to 0.78 Ma) to occur at a consistent amplitude or phase of obliquity cyclicity, nor of orbital eccentricity. An implication is that paleointensity lows which are characteristically associated with these features are not distributed in a systematic way relative to obliquity and eccentricity, supporting the idea that orbital forcing does not power the geodynamo

Paleomagnetic and geochemical record from cores from the Sea of Marmara, Turkey: Age constraints and implications of sapropelic deposition on early diagenesis

We present results of a multi-proxy analysis of two sediment cores from the Marmara Sea. The cores were ana- lyzed using paleomagnetic and geochemical measurements. Two sapropels are documented in the last 11 kyr and are recorded in several locations across the Marmara Sea. These two sapropels have contrasting magnetic prop- erties. The magnetic record is affected by intense early diagenesis; the most recent upper sapropelic layer has low remanence and susceptibility values. A record of paleomagnetic inclinations could still be isolated above the dia- genesis front and is compared with secular variation models. The lower sapropel is identified in the deep part of the oldest studied core (Klg07) and has distinct magnetic properties characterized by high remanence and sus- ceptibility values. Using the magnetic properties it is possible to constrain bottom water ventilation and recon- nection episodes between the Marmara Sea and the Black Sea following the sea level rise during the last glacial to inter-glacial transition.Seismic History of Istanbu

Oligo-Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia.

Magnetostratigraphy of 222 remanent directions together with late Oligocene to early Miocene mammal and charophyte paleontology suggest that 2179 m of the Xishuigou section (Subei, Gansu Province, China) were deposited from ~26 to ~19 Ma. Stratigraphic patterns of bulk susceptibility, anisotropy of magnetic susceptibility parameters, and natural and anhysteretic remanent magnetization intensities demonstrate that (1) faulting does not significantly affect the record, (2) sediment deposition was relatively continuous, (3) sediment source changed around 23 Ma, and (4) rapid uplift near Subei occurred at 21 Ma. Subei rotated 27° ± 5° counterclockwise with respect to the 20 Ma pole from the Eurasian synthetic apparent polar wander path. Folding and rotation of the section took place after 19 Ma. The paleolatitude of Subei is 14° less than at present and 19° ± 3° less than predicted from the reference pole. Both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing, a phenomenon which has likely affected other paleomagnetic data from central Asia

Source of oceanic magnetic anomalies and the geomagnetic polarity time scale

Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160 Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30 ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the ocean crust. Indeed, many short wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies. The extrusive layer, with an average remanence of ∼ 5 A m−1, is the largest contributor to magnetic anomalies. However, enhanced sampling of oceanic gabbros (average remanence ∼ 1 A m−1) and, to a lesser extent, dikes (average remanence ∼ 2 A m−1) reveals that these deeper (and thicker) layers likely generate anomalies comparable to those from the lavas. Lava accumulation at intermediate- and fast-spreading ridges typically occurs over a narrow (1–3 km) region and dike emplacement is even more narrowly confined, resulting in a relatively high fidelity record of geomagnetic field behavior. The slow cooling of the gabbroic layer, however, results in gently dipping polarity boundaries that significantly affect the skewness of the resulting anomalies, which is also a sensitive measure of net rotations of the source layer(s). The magnetizations of the dikes and gabbros are characterized by high stability and are not expected to significantly change with time, although there are insufficient data to confirm this. The lavas, however, typically show evidence of low-temperature alteration, which has been long regarded as a process that progressively reduces the magnetization (and degrades the geomagnetic signal) in the extrusive layer and reduces the amplitude of magnetic anomalies. Sufficient data have become available to examine this conventional wisdom. There is a substantial (∼ 4x) reduction in magnetization from on-axis samples to immediately off-axis drillsites (∼ 0.5 My), but little further change in half-dozen or so deep crustal sites to ∼ 160 Ma. High paleointensity that characterizes the last few thousand years may contribute significantly to the high on-axis magnetization. The task of evaluating changes in remanence of the extrusive layer is made more difficult by substantial cooling-rate-dependent changes in magnetic properties and the systematic variation in remanence with iron content (magnetic telechemistry). The commonly cited magnetic anomaly amplitude envelope is in fact not systematically observed – the Central Anomaly is elevated at slow-spreading ridges but is not as prominent at faster spreading rates. Nonetheless, magnetic anomaly amplitudes are consistent with magnetization change is poorly constrained. Direct determinations of the degree of low-temperature oxidation reveal the presence of highly oxidized titanomagnetite in samples less than 1 My old, suggesting a short (∼ 105 years) time constant though the effects of low-temperature oxidation are quite heterogeneous. While low-temperature oxidation does have some affect on lava magnetization and anomaly amplitudes, there is increasing evidence that marine magnetic anomalies are capable of recording a broad spectrum of geomagnetic field behavior, ranging from millennial-scale paleointensity variations to polarity reversals to apparent polar wander to, more speculatively, long-term changes in average field strength. Several emerging tools and approaches – autonomous vehicles, oriented samples, absolute paleointensity of near-ridge lavas, and measurements of the vector anomalous field – are therefore likely to significantly advance our understanding of the geomagnetic signal recorded in the oceanic crust, as well as our ability to utilize this information in addressing outstanding problems in crustal accretion processes

A geomagnetic record over the last 3.5 million years from deep-tow magnetic anomaly profiles across the Central Indian Ridge

International audienceHigh-resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short-lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep-tow (-500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well-dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short-wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific-Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small-scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near-bottom results support the existence of three geo-magnetic features between the Gauss-Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. I n subchron within the Gauss chron. This study emphasizes the potential of deep-tow magnetic surveys in detecting fluctuations in geo-magnetic field intensity and, in particular, short-lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum