Changes In Geomagnetic Field Intensity Research Articles

Paleoenvironmental Controls on the Abundances of Magnetofossils in the Southwestern Iberian Margin

AbstractMagnetofossils are nanosized magnetic fossil remnants of magnetotactic bacteria (MTB) that are widely distributed in marine and freshwater sediments. Past studies have revealed that changes in the morphology and abundance of magnetofossils are linked with diverse paleoenvironmental changes, such as glacial‐interglacial variation, redox conditions associated with primary productivity and organic matter supply, chemical weathering and diagenesis. Besides, it has long been speculated that changes in geomagnetic field intensity could also affect the abundance of magnetofossils. However, it is not yet clear, how these environmental factors controlled past MTB populations in combination. To answer this question, we studied magnetofossils‐rich sediments from cores MD01‐2443 and 2444 off the Southwest Iberian Margin. The paleomagnetic records of our samples reveal geomagnetic excursions corresponding to the Laschamp, Blake, Iceland Basin, and Pringle Falls events. We estimated the relative abundance of magnetofossils in the samples based on isothermal remanent magnetization unmixing and found it to increase during warmer periods and decrease during colder periods, while not covary with changes in the geomagnetic field intensity. To further distinguish the role of different paleoenvironmental factors, we compared magnetofossil abundance with planktonic δ18O, δ13C of organic matter, K/Al and Si/Al, which indicate continental ice volume (paleotemperature), organic matter origin, run‐off intensity and productivity, respectively. The results indicate that less continental ice volume (higher paleotemperature), enhanced productivity and run‐off intensity are closely related and jointly promoted magnetofossil production and preservation. Our study offers a comprehensive approach to understanding how magnetofossil assemblages relate to paleoenvironmental changes off the SW Iberian Margin.

Refining geomagnetic field intensity changes in Europe between 200 CE and 1800 CE. New data from the Mediterranean region

Absolute past geomagnetic intensity values can mainly be recovered by fired archaeological materials and volcanic rocks. Here, we present 10 new archeointensities from the Mediterranean region that help to better constrain geomagnetic field intensity changes in Europe over the last two millennia. The new archeointensity results were obtained from the Thellier classical method including thermoremanent magnetization (pTRM) checks and both the TRM anisotropy and cooling rate corrections and were derived from at least three specimens. The new data presented, together with a selection of previous archeointensities satisfying a set of quality criteria, confirm the presence of several intensity maxima in Europe over the last 2000 years. In particular, the new archeointensities allow to better define the starting point of the double-oscillation feature that occurred in Europe during the second half of the first millennium CE, and reinforce the existence of a relative maximum at the end of the 14th century - beginning of the 15th century in Western Europe. From selected European archeointensities two new paleosecular variation curves are constructed for Western and Eastern Europe using temporal cubic b-splines in a bootstrap approach. The obtained curves suggest that the occurrence of the intensity maxima is characterized by a period of about 300 ± 50 years. In addition, our results suggest that the maxima do not occur simultaneously in Western and Eastern Europe, pointing out an intensity eastward drift with a mean lag-time of about 100 years.

Geomagnetic field intensity changes in the Central Mediterranean between 1500 BCE and 150 CE: Implications for the Levantine Iron Age Anomaly evolution

The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.

The yellow stingray (Urobatis jamaicensis) can discriminate the geomagnetic cues necessary for a bicoordinate magnetic map

Elasmobranch fishes (sharks, skates, and rays) are hypothesized to use environmental cues, such as the geomagnetic field (GMF), to navigate across the ocean. However, testing the sensory and navigation abilities of large highly migratory fishes in the field is challenging. This laboratory study tested whether the yellow stingray, Urobatis jamaicensis, could detect and distinguish between the GMF cues used by other magnetically sensitive species to actively determine their location. Stingrays were divided into two cohorts for initial behavioral conditioning: one was trained to associate a change in GMF intensity with an aversive stimulus, whereas the other was trained using a change in GMF inclination angle. Individuals from each cohort remained naive to the GMF conditioning stimulus used to condition the other cohort. The combined group learned the initial association within a mean (± SE) of 184.0 ± 34.8 trials. Next, stingrays from each cohort were randomly exposed to their original GMF conditioning stimulus and the novel GMF stimulus. The original magnetic stimulus continued to be reinforced, whereas the novel stimulus was not. The group demonstrated a significantly different response to the original (reinforced) and novel (non-reinforced) stimuli, which indicates that stingrays could distinguish between the intensity and inclination angle of a magnetic field. This experiment is the first to show that a batoid (skate or ray) can detect and distinguish between changes in GMF intensity and inclination angle, and supports the idea that elasmobranchs might use GMF cues to form a magnetically based cognitive map and derive a sense of location.

Geomagnetic field absence reduces adult body weight of a migratory insect by disrupting feeding behavior and appetite regulation

The geomagnetic field (GMF) is well documented for its essential role as a cue used in animal orientation or navigation. Recent evidence indicates that the absence of GMF (mimicked by the near-zero magnetic field, NZMF) can trigger stress-like responses such as reduced body weight, as we have previously shown in the brown planthopper, Nilaparvata lugens. In this study, we found that consistent with the significantly decreased body weight of newly emerged female (-14.67%) and male (-13.17%) adult N. lugens, the duration of the phloem ingestion feeding waveform was significantly reduced by 32.02% in 5th instar nymphs reared under the NZMF versus GMF. Interestingly, 5th instar nymphs that exhibited reduced feeding had significantly higher glucose levels (+16.98% and +20.05%; 24 h and 48 h after molting), which are associated with food aversion, and expression patterns of their appetite-related neuropeptide genes (neuropeptide F, down-regulated overall; short neuropeptide F, down-regulated overall; adipokinetic hormone, up-regulated overall; and adipokinetic hormone receptor, down-regulated overall) were also altered under the absence of GMF in a manner consistent with diminishing appetite. Moreover, the expressions of the potential magnetosensor cryptochromes (Crys) were found significantly altered under the absence of GMF, indicating the likely upstream signaling of the Cry-mediated magnetoreception mechanisms. These findings support the hypothesis that strong changes in GMF intensity can reduce adult body weight through affecting insect feeding behavior and underlying regulatory processes including appetite regulation. Our results highlight that GMF could be necessary for the maintenance of energy homeostasis in insects.

Change in geomagnetic field intensity alters migration-associated traits in a migratory insect.

Geomagnetic field (GMF) intensity can be used by some animals to determine their position during migration. However, its role, if any, in mediating other migration-related phenotypes remains largely unknown. Here, we simulated variation in GMF intensity between two locations along the migration route of a nocturnal insect migrant, the brown planthopper Nilaparvata lugens, that varied by approximately 5 µT in field intensity. After one generation of exposure, we tested for changes in key morphological, behavioural and physiological traits related to migratory performance, including wing dimorphism, flight capacity and positive phototaxis. Our results showed that all three morphological and behavioural phenotypes responded to a small difference in magnetic field intensity. Consistent magnetic responses in the expression of the phototaxis-related Drosophila-like cryptochrome 1 (Cry1) gene and levels of two primary energy substrates used during flight, triglyceride and trehalose, were also found. Our findings indicate changes in GMF intensity can alter the expression of phenotypes critical for insect migration and highlight the unique role of magnetoreception as a trait that may help migratory insects express potentially beneficial phenotypes in geographically variable environments.

Searching for geomagnetic spikes in numerical dynamo simulations

We use numerical dynamo simulations to investigate rapid changes in geomagnetic field intensity. The work is motivated by paleomagnetic observations of ‘geomagnetic spikes’, events where the field intensity rose and then fell by a factor of 2–3 over decadal timescales and a confined spatial region. No comparable events have been found in the historical record and so geomagnetic spikes may contain new and important information regarding the operation of the geodynamo. However, they are also controversial because uncertainties and resolution limitations in the available data hinder efforts to define their spatio-temporal characteristics. This has led to debate over whether such extreme events can originate in Earth's liquid core. Geodynamo simulations produce high spatio-temporal resolution intensity information, but must be interpreted with care since they cannot yet run at the conditions of Earth's liquid core. We employ reversing and non-reversing geodynamo simulations run at different physical conditions and consider various methods of scaling the results to allow comparison with Earth. In each simulation we search for ‘extremal events’, defined as the maximum intensity difference between consecutive time points, at each location on a 2∘ latitude–longitude grid at Earth's surface, thereby making no assumptions regarding the spatio-temporal character of the event. Extremal events display spike-shaped time-series in some simulations, though they can often be asymmetric about the peak intensity. Maximum rates of change reach 0.75 μTyr−1 in several simulations, the lower end of estimates for spikes, suggesting that such events can originate from the core. The fastest changes generally occur at latitudes >50∘, which could be used to guide future data acquisitions. Extremal events in the simulations arise from rapid intensification of flux patches as they migrate across the core surface, rather than emergence of flux from within the core. The prospect of observing more spikes in the paleomagnetic record appears contingent on finding samples at the right location and time to sample this particular phase of flux patch evolution.

Rock‐magnetic artifacts on long‐term relative paleointensity variations in sediments

Long‐term changes of geomagnetic field intensity, including possible dependence on lengths of polarity intervals, provide fundamentally important information for understanding the geodynamo. A positive correlation between paleointensity and polarity interval length was previously suggested from an Oligocene (ca. 23–34 Ma) relative paleointensity record at Deep Sea Drilling Program Site 522 in the Atlantic Ocean, which is the only continuous paleointensity data set published so far for this age interval. We have conducted a paleomagnetic study of Eocene to Oligocene sediments at three sites in the eastern equatorial Pacific Ocean. Our objectives include revisiting the issue of the paleointensity‐polarity length correlation. Magnetic properties of the sediments meet the frequently used criteria for reliable relative paleointensity estimation. Although short‐wavelength normalized remanence intensity fluctuations associated with polarity boundaries and possible geomagnetic excursions agree among the three sites, long‐term changes are inconsistent. Apparent positive correlation between normalized intensity and polarity length was observed, but the normalized intensity has an obvious anti‐correlation with the ratio of anhysteretic remanent magnetization (ARM) to isothermal remanent magnetization (IRM), which is mainly controlled by the relative abundance of biogenic and terrigenous magnetic minerals. Furthermore, the normalized intensity correlates with sedimentation rate. These facts indicate a lithological contamination on the normalized intensity records. The dependence on ARM/IRM and sedimentation rate is also evident at Site 522. It is inferred that variations in sedimentation rate and the relative abundance of biogenic magnetite on depositional remanent magnetization acquisition efficiency may not be well compensated by the normalization. It is therefore premature to conclude that stronger geomagnetic fields were recorded during longer polarity intervals from currently available normalized intensity records.

Improving our knowledge of rapid geomagnetic field intensity changes observed in Europe between 200 and 1400 AD

Available archaeomagnetic data indicate that during the past 2500yr there have been periods of rapid geomagnetic field intensity fluctuations interspersed with periods of almost constant field strength. Despite Europe being the most widely covered region in terms of archaeomagnetic data the occurrence and the behaviour of these rapid geomagnetic field intensity changes is under discussion and the challenge now is to precisely describe them. The aim of this study is to obtain an improved description of the sharp intensity change that took place in western Europe around 800 AD as well as to investigate if this peak is observed at the continental scale. For this purpose 13 precisely dated early medieval Spanish pottery fragments, four archaeological French kilns and three collections of bricks used for the construction of different French historical buildings with ages ranging between 335 and 1260 AD have been studied. Classical Thellier experiments performed on 164 specimens, and including anisotropy of thermoremanent magnetisation and cooling rate corrections, gave 119 reliable results. The 10 new high-quality mean archaeointensities obtained confirm the existence of an intensity maximum of ∼85μT (at the latitude of Paris) centred at ∼800 AD and suggest that a previous abrupt intensity change occurred around 600 AD. Together with previously published data from western Europe that we deem to be the most reliable, the new data also suggest the existence of two other abrupt geomagnetic field intensity variations during the 12th century and around the second half of the 13th century AD. High-quality archaeointensities available from eastern Europe indicate that very similar geomagnetic field intensity changes occurred in this region. European data indicate that very rapid intensity changes (of at least 20μT/century) took place in the recent history of the Earth's magnetic field. The results call for additional high-quality archaeointensities obtained from precisely dated samples and for a selection of the previously published data if a refined description of geomagnetic field intensity changes at regional scales is to be obtained.

Intensity of the Earth’s magnetic field in Greece during the last five millennia: New data from Greek pottery

New archaeointensity results have been obtained from the study of four ceramic collections coming from four different archaeological sites in Greece. The ages of the ceramic fragments, based on archaeological constrains and radiocarbon analysis, range from 2200 BC to 565 AD. Low-field magnetic susceptibility versus temperature reveals a good thermal stability for most of the samples. However, for some samples the thermomagnetic curves are not reversible indicating mineralogical changes during heating. Isothermal remanent magnetisation (IRM) acquisition curves and thermal demagnetisation of three orthogonal IRM components have also been performed. The rock magnetic results identify magnetite and/ or Ti-magnetite as the main magnetic carriers in the studied samples. Classical Thellier experiments with regular partial thermoremanent magnetisation (pTRM) checks have been conducted on 125 specimens belonging to 34 independent ceramic fragments. Only 61 archaeointensity determinations (at specimen level) that correspond to linear NRM–TRM plots were used for the calculation of the site mean archaeointensities. The effect of the anisotropy of the thermoremanent magnetisation (TRM) and cooling rate upon TRM intensity acquisition have been investigated in all specimens. The maximum difference between the TRM anisotropy corrected and uncorrected intensities is around 30% at specimen level confirming that the TRM effect can be very important in ceramic samples. Cooling rate correction factors determined per specimen are up to 10% with only one exception that reaches 35%. Despite the moderate success rate of archaeointensity determination (around 50%) reliable mean site intensities have been obtained, with in situ intensities ranging from 53.6±4.1 to 69.3±3.9μT, corresponding to virtual axial dipole moments from 9.2±0.7 to 11.9±0.7×1022Am2. The new data are reasonably consistent with other available data for the studied region as well as with the SV reference curves for Greece and the South Balkan Peninsula, and the regional and global geomagnetic field models. Combined with previously published data from the area, they confirm that important changes of the Earth’s magnetic field intensity occurred in Greece during the last five millennia. For some periods, the available archaeointensity data for the Balkan area show a large dispersion, even for data corresponding to high quality intensity standards, whereas for other periods their limited number prevents an accurate description of geomagnetic field intensity changes. This evidences the need of new reliable and well dated archaeointensity data in order to obtain a robust description of geomagnetic field intensity changes during the last five millennia in this area.

氷床コアから復元される地磁気イベント時の宇宙線変動

Cosmogenic Radio Nuclides (CRNs) are produced by nuclear reactions induced by cosmic rays in the Earth's atmosphere. The production rate of CRNs varies because changes in geomagnetic field intensity and solar activity strongly influence galactic cosmic rays, especially the lower part of energy. CRNs are produced in the upper atmosphere and are deposited on ice sheets in both Greenland and Antarctica, hence, ice cores in the polar regions provide excellent records of past histories of cosmic rays because of their continuous and relatively simple depositional processes. In recent studies, high-resolution records of 10Be flux from ice cores have been reported, and these revealed the continuity of 205-year solar cycles over the last glacial period. In particular, the changes induced in the production rate of CRNs by solar modulations increased during periods of low geomagnetic field intensity, such as the Laschamp geomagnetic excursion. The results obtained in this study suggest that it may be possible to obtain a better understanding of long-term solar activity through a high-resolution 36Cl analysis of the polar ice sheet.

Towards radiocarbon calibration beyond 28 ka using speleothems from the Bahamas

We present a new speleothem record of atmospheric Δ 14C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS 14C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ 14C was calculated by correcting 14C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite 14C ages with the IntCal04 ( Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ 14C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology ( Svensson et al., 2008). These data are however in conflict with a previously published 14C data set for a stalagmite record from the Bahamas — GB-89-24-1 ( Beck et al., 2001), which likely suffered from 14C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ∆ 14C data do not show the extreme shifts between 44 and 40 ka reported in the previous study ( Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ 14C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ∆ 14C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.

Analysis of earth's magnetic field changes by correlation of its parameters

Changes of Earth magnetic field parameters: field intensity and its components along axes of rectangular coordinates are analyzed in space and time. Data from magnetic observatories Belsk and Hel in Poland were used. Correlation analysis of magnetic field components was applied. Influence of respective field components on changes of geomagnetic field intensity was determined. Shift of space and time factors was used to assess the influence of covariations of geomagnetic field components on changes of covariations of field intensity. Article in English Žemės magnetosferos pokyčių, taikant jos parametrų koreliaciją, analizė Straipsnyje analizuojama Žemės magnetosferos parametrų: lauko jėgos intensyvumo ir jos komponenčių pagal stačiakampių koordinačių ašis reikšmių kaita erdvėje bei laike. Tyrimams naudota Lenkijos Belsk ir Hel magnetinių observatorijų metraščių duomenys, taikyta magnetosferos lauko jėgos komponenčių koreliacinė analizė. Nustatyta atitinkamų geomagnetinio lauko jėgos komponenčių pokyčių įtaka geomagnetinio lauko jėgos intensyvumo pokyčiams. Įvertinant geomagnetinio lauko jėgos komponenčių kovariacijų įtaką lauko jėgos intensyvumo kovariacijų pokyčiams atsižvelgta į erdvės ir laiko faktorių kaitą.&nbsp

Geomagnetic field intensity during the last 60,000 years based on 10Be and 36Cl from the Summit ice cores and 14C

Based on 10Be, 36Cl and 14C records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates. Subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic. The geomagnetic field derived from the 10Be-flux to Summit in Central Greenland agrees well with independent geomagnetic field intensity reconstructions. However, compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20,000 to 30,000 years BP. 36Cl from the GRIP ice core agrees well with the 10Be record for the last ice age and hence also the reconstructed geomagnetic field. Based on the tree-ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the Holocene. A comparison of radiocarbon-based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10,000 years BP.

Geomagnetic field intensity changes in Sweden between 9000 and 450 cal BP: extending the record of “archaeomagnetic jerks” by means of lake sediments and the pseudo-Thellier technique

The pseudo-Thellier technique was applied to recover relative palaeointensity (PT-RPI) estimates from four piston cores retrieved from a Holocene lake sediment sequence in the province of Småland, central southern Sweden. A time–depth model between 11,000 and 450 cal BP was based on a combination of calibrated radiocarbon ages and the ages of established palaeomagnetic secular variation (PSV) features that were transferred from a varve-dated regional master curve. In Fennoscandia, significant peaks in geomagnetic field intensity occurred at ∼8400, ∼6400, 3900 and 2800 cal BP. The maximum field intensity at 2800 cal BP was associated with the most rapid change in the direction of the geomagnetic vector. While the high-resolution sedimentary data show that significant century scale increases and decreases in relative field intensity between 4000 and 2000 cal BP were associated with abrupt changes (jerks) in the direction of the geomagnetic vector, archaeomagnetic jerks younger than 2000 years were not reproduced in this multiple sediment core study. On the other hand, the PT-RPI reconstructions display century to millennial scale trends between 5000 and 450 cal BP that are coherent with archaeomagnetic data sets from western Europe and Central Asia. This agreement indicates that palaeointensity may be used as a relative dating technique across this region. Fennoscandian PSV and RPI records for the earlier Holocene do not show the same rapid style of vector movement, which suggests that northern Europe has been affected by a more turbulent geodynamo since ∼4000 cal BP.

Paleointensity studies on Holocene–Pleistocene sediments from lake Escondido, Argentina

Relative changes in geomagnetic field intensity over the last 16,000 years BP were recovered from the study of four cores obtained from Lake Escondido (South Argentina). Rock magnetic analysis shows that the main magnetic mineral is magnetite, the concentrations being mainly between 0.01 and 0.1%, and the grain size between 1 and 8 μm. In order to fulfil the criteria for assessing the reliability of paleointensity data derived from sediments [Rev. Geophys. 31 (1993) 319], the samples whose concentrations and size are beyond this range were rejected. The remanent magnetisation at 20 mT (NRM 20 mT ) was normalised using the anhysteric remanent magnetisation (ARM 100 mT ), the saturation of the isothermal remanent magnetisation (SIRM) and the low field magnetic susceptibility ( k). Coherence function analysis indicates that the records are not significantly affected by local environmental conditions. This suggests that the variations in normalised remanence are mostly likely due to geomagnetic paleointensity fluctuations. ARM 100 mT is the best normaliser for the Lake Escondido sediments. The record of relative paleointensity (NRM 20 mT /ARM 100 mT ) shows peaks and troughs whose amplitudes are similar to those in the St. Lawrence Estuary, Lake Baikal, Lake Pepin and Larsen-A Ice shelf data set and a stack of archeomagnetic data given as virtual axial dipole moment (VADM).

Geomagnetic field intensity, North Atlantic Deep Water circulation and atmospheric Δ 14C during the last 50 kyr

We present simulated records of past changes in the atmospheric Δ 14C for the last 50 kyr due to changes in geomagnetic field intensity and in the strength of the North Atlantic Deep Water (NADW). A new geomagnetic record was used, largely based on the NAPIS-75 record [Laj et al., Phil. Trans. R. Soc. London A 358 (2000) 1009–1025] which has been extended for the 0–20 kyr interval using archeomagnetic and volcanic data. Past changes of the NADW were derived from a mineral magnetic study of the cores used in the construction of NAPIS-75. Two box models of different complexity (4 and 17 boxes) were used to simulate the carbon cycle. Calculated records of Δ 14C are consistent with experimental determinations for the last 24 kyr. For older ages, the records calculated with variable oceanic circulation conditions reach values as high as 600‰ (with an average of 500‰) between 20 and 40 kyr with maxima around 21, 30 and 38 kyr (GISP2 age model), while low values are observed prior to 42 kyr. Although large inconsistencies in experimental data preclude precise comparison, the average record simulated with the 17-box model is overall consistent with the Icelandic Sea record [Voelker et al., Radiocarbon 40 (1998) 517–534; 42 (2000) 437–452], except for the extremely high peak observed in this record at 40.5 kyr. On the other hand, the results recently reported from a stalagmite recovered from a submerged cave in the Bahamas [Beck et al., Science 292 (2001) 2453–2458] are inconsistent with all our model simulations. In the 20–45 kyr interval, the improved geomagnetic record combined with the new NADW profile allows us to give a modeled evaluation of the relative contribution of these factors to changes in atmospheric Δ 14C. The average simulation provides a first order modeled correction for conventional radiocarbon ages older than 25 kyr for which no calibration curve is available as yet.

NRM intensity of altered oceanic basalts across the MAR (21°N, 0-1.5 Ma): a record of geomagnetic palaeointensity variations?

Summary During cruise Tammar (May 1996) of R/V Nadir and submersible Nautile, 66 basaltic rock samples were collected along two long cross-sections encompassing the oceanic crust between 0 and 1.5 Myr at the centre of the mid-Atlantic ridge segment located at 21°N. This dense sampling provides a means to investigate natural remanent magnetization (NRM) variability with age using rock magnetic studies (high-field and k–T experiments, palaeofield intensity determinations by the original Thellier method) together with the good geophysical and geological knowledge of this peculiar segment. NRM intensities range from 1.3 to 25.4 A m−1 but do not display any rapid exponential decay with age, as expected. Despite the scatter, they seem to present short-wavelength variations consistent on both flanks of the two lines. Because of the relative uniformity in grain size (i.e. single domain, SD), ulvospinel content (i.e. x = 0.6) and amount of magnetic minerals in Tammar samples, the observed across-axis NRM intensity variations may be due either to oxidation degree variations or to geomagnetic field intensity changes. Curie temperatures display no increase towards the flanks nor a clear relationship with NRM intensities, suggesting that oxidation degree is not the major process controlling the NRM variations. Half of the collected samples, either fresh or highly altered, provide good-quality palaeointensity determinations. Depending on the alteration degree of samples, two major types of NRM/TRM diagrams are observed. For relatively fresh materials, only one line segment and the major part of NRM is used for field intensity calculations. For moderately or highly altered specimens, two line segments are observed with the first most probably corresponding to thermal demagnetization of the original remanence, and the second to the product resulting from chemical modification (i.e. inversion). Despite the difference in the amount of NRM that can be thermally demagnetized, samples of similar age but of different non-stoichiometric degree give coincident palaeofield strengths. Palaeofield values, from 15 to 62 µT (i.e. VADM from 3.4 to 13.6 × 1022 A m2), are in good agreement with the global palaeointensity database, and display coherent short-wavelength undulations with age, consistent (assuming a 1-km-wide neo-volcanic zone and steady, symmetrical spreading) with the relative palaeointensity record deduced from sediment core analysis (Guyodo & Valet 1999). Comparison between NRM and field intensity variations with age reveals a remarkable coincidence between both signals, suggesting that the NRM intensities are more sensitive to the field intensity than to the alteration degree. These observations suggest that magnetic particles are titanomaghemites, generated during the initial magma cooling and storing a durable record of the geomagnetic field intensity. The type of remanence is either (1) a chemical remanent magnetization that closely mimics the properties of the initial NRM or (2) a thermoremanent magnetization (the maghemitization precedes the TRM acquisition) or (3) a mixture of both.

Geomagnetic intensity and 14C abundance in the atmosphere and ocean during the past 50 kyr

We have used a new sedimentary record of geomagnetic field intensity, combined with published volcanic data, to examine the extent to which geomagnetic changes have affected 14C production in the past. The production record was then converted into an atmospheric Δ14C signal using a 4‐box model of past oceanic circulation. The results indicate that changes in geomagnetic field intensity account for at least 80% of the Δ14C shift documented by published U‐Th data, which are of the order of 500 per mil with respect to a constant production hypothesis. Model simulations show that changes in past oceanic circulation account for only ±100 per mil of Δ14C variation, which is within the uncertainty of the geomagnetic correction. In terms of dating, these results show that the radiocarbon ages have to be shifted by 2 to 3.5 kyr towards older ages during the 20–40 ka interval.

Relative geomagnetic field intensity and reversals from Upper Miocene sections in Crete

Two Late Miocene sections at Potamida and Kotsiana in western Crete have been resampled at about 10 cm intervals (288 and 130 horizons, respectively). The two sections record the same two reversals bounding a normal polarity zone just below the Tortonian-Messinian boundary. Mineral-magnetic analysis identifies pseudo single-domain magnetite as the carrier of the main fraction of magnetization in both sections. Normalizations with ARM, SIRM and χ, on different specimens of the same cores, yield very similar results. After checking that it is not correlated to bulk magnetic parameters, the ARM-normalized record was used as a proxy for geomagnetic field intensity changes. In the lower part of the reverse polarity zone, sampled at Potamida, the field intensity appears to increase slightly then to decrease progressively to low values, prior to the R/N polarity transition. There is no evidence for a fast recovery of field intensity across the two transitions: the intensity remains very low after the R/N reversal and only progressively builds to a maximum in the middle of the following normal chron. It then progressively decreases until the N/R transition. No sudden recovery of the field intensity is observed across this reversal either. These results are thus inconsistent with the ‘saw-tooth’ record obtained by Valet and Meynadier for the last 4 million years. Either the behaviour of the field is different for different intervals of time or the hypothesis of sedimentary artefacts in the original saw-tooth record should be considered.