11 million years of Oligocene geomagnetic field behaviour.
Geophysical Journal International. 128:217-229. 10.1111/j.1365-246X.1997.tb04082.x Abstract
An II million year long record of the Oligocene geomagnetic field has been obtained from pelagic sediments of DSDP Hole 522. An average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr. The rock magnetics are remarkably consistent across the entire interval. Previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite. The natural remanent magnetism (NRM) carries an Oligocene polarity timescale that is in excellent agreement with the Oligocene reversal record as determined from marine magnetic anomalies (MMAs), including many of the so-called 'crypto-chrons'. Normalized NRM intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences. These features include consistent, major decreases in palaeointensity (DIPs) at reversal boundaries, and occasional DIPs between reversal boundaries that could correspond to lineated 'tiny wiggles' in the MMA records. The data set suggests that the overall field strength was 40 per cent higher in the first half of the Oligocene when the average reversal frequency was 1.6 Myr(-1) than in the second half when the reversal frequency was 4 Myr(-1). There is also a weak dependence of average field strength on length of polarity interval. Finally, in the three cores suited to spectral analysis (of coherent polarity and relative intensity independent of lithological contamination), there is a persistent ca. 30-50 ka periodicity in the variations of the relative intensity, suggesting that the geomagnetic held 'pulses' at about this frequency, not only during the Brunhes (as demonstrated by Tauxe & Shackleton 1994), but in the Oligocene as well.
Acquisition of viscous remanent magnetization.
Physics of the Earth and Planetary Interiors. 159:32-42. 10.1016/j.pepi.2006.05.002 Abstract
Viscous remanent magnetization (VRM) has been frequently suggested as a possible source of magnetic anomalies in ocean crust and as an important contributor of natural remanent magnetization in some volcanic and sedimentary rocks. Most references to VRM assume a linear dependence with log(t), yet long-term experimental data on VRM, while scarce often disagree with this simple notion. Here, we investigate the angular-dependence, grain-size dependence, initial-state dependence, and time (t) dependence of VRM acquisition. We observe a non-linear log(t) dependence of VRM acquisition over all time intervals (> 10(4) S) for all the samples regardless of various experimental conditions. Echoing earlier studies, we also find that VRM acquisition is strongly dependent on the initial magnetic state of the samples. Samples in a thermally demagnetized initial-state were most susceptible to VRM acquisition. When VRM is produced parallel to the initial thermal remanence, VRM acquisition is negligible. VRM acquisition is more prevalent for magnetites in single-domain and multidomain (MD) grains than in the pseudo-single-domain range. In MD magnetites, frequent removal and restoration of samples (for the purpose of zero-field remanence measurement) apparently increases the efficiency of VRM acquisition. (c) 2006 Elsevier B.V. All rights reserved.
AMSSpin: A LabVIEW program for measuring the anisotropy of magnetic susceptibility with the Kappabridge KLY-4S.
Geochemistry Geophysics Geosystems. 9 10.1029/2008gc001976 Abstract
Anisotropy of magnetic susceptibility (AMS) data are widely used as a petrofabric tool because the technique is rapid and nondestructive and because static measurement systems are capable of determining small degrees of anisotropy. The Kappabridge KLY-4S provides high resolution as a result of the large number of measurements acquired while rotating the sample about three orthogonal axes. Here we describe a graphical-based program called AMSSpin for acquiring AMS data with this instrument as well as a modified specimen holder that should further enhance the utility of this instrument. We also outline a method for analysis of the data (that differs in several ways from that of the software supplied with the instrument) and demonstrate that the measurement errors are suitable for using linear perturbation analysis to statistically characterize the results. Differences in the susceptibility tensors determined by our new program and the SUFAR program supplied with the instrument are small, typically less than or comparable to deviations between multiple measurements of the same specimen.
Analysis of 11 Myr of geomagnetic intensity variation.
Journal of Geophysical Research-Solid Earth. 103:17735-17748. 10.1029/98jb01519 Abstract
We have conducted a detailed exploratory analysis of an II million year long almost continuous record of relative geomagnetic paleointensity from a sediment core acquired on Deep Sea Drilling Project Leg 73, at Site 522 in the South Atlantic. We assess the quality of the paleointensity record using spectral methods and conclude that the relative intensity record is minimally influenced by climate variations. Isothermal remanence is shown to be the most effective normalizer for these data, although both susceptibility and anhysteretic remanence are also adequate. Statistical analysis shows that the paleointensity variations follow a gamma distribution, and are compatible with predictions from modified paleosecular variation models and global absolute paleointensity data. When subdivided by polarity interval, the variability in paleointensity is proportional to the average, and further, the average is weakly correlated with interval length. Spectral estimates for times from 28.77 until 22.74 Ma, when the reversal rate is about 4 Myr(-1), are compatible with a Poisson model in which the spectrum of intensity variations is dominated by the reversal process in the frequency range 1-50 Mgr(-1) In contrast, between 34.7 and 29.4 Ma, when the reversal rate is about 1.6 Myr(-1), the spectra indicate a different secular variation regime. The magnetic field is stronger, and more variable, and a strong peak in the spectrum occurs at about 8 Myr(-1). This peak magi be a reflection of the same signal as recorded by the small variations known as tiny wiggles seen in marine magnetic anomaly profiles.
Application of copper slag in geomagnetic archaeointensity research.
Journal of Geophysical Research-Solid Earth. 113 10.1029/2007jb005235 Abstract
Paleointensity and archaeointensity studies since the 1950s have produced numerous geomagnetic intensity data for the last seven millennia. As a consequence of different experiments and materials, there is a complex and internally inconsistent picture of the geomagnetic field behavior. In this study we present data using a recently developed experimental design on a heretofore unexploited recording medium: copper slag deposits. Our results, based on hundreds of specimens from various archaeometallurgical sites of the Southern Levant, demonstrate the applicability of copper slag material for archaeointensity studies. In addition to frequently exhibiting good experimental behavior, slag has further advantages such as dense multilayer deposits and in cases embedded charcoals, which open the door to data sets with excellent age control and resolution. The data presented here augment the high quality database from the Middle East and support previously observed periods of rapid change of the intensity of the geomagnetic field.
Ar-40/Ar-30 geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya.
Journal of Human Evolution. 42:117-140. 10.1006/jhev.2001.0521 Abstract
Ar-40/Ar-39 single-crystal laser-fusion dating, K-Ar dating, and paleo-magnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6(.)56-3(.)8 Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5(.)88-5(.)72 Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5(.)31 +/- 0(.)03 Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined Ar-40/Ar-39 and paleomagnetic data constrain the age of this sequence to 4(.)63-3(.)837 Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4(.)48-4(.)41 Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17 cm/ka over the 2(.)7 Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses. (C) 2002 Academic Press.
Ar-40/Ar-39 ages and paleomagnetism of Sao Miguel lavas, Azores.
Earth and Planetary Science Letters. 160:637-649. 10.1016/s0012-821x(98)00117-4 Abstract
We present new Ar-40/Ar-39 ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. Ar-40/Ar-39 age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The Ar-40/Ar-39 ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. (C) 1998 Elsevier Science B.V. All rights reserved.
Archaeointensity results spanning the past 6 kiloyears from eastern China and implications for extreme behaviors of the geomagnetic field.
Proceedings of the National Academy of Sciences. 114:39-44. 10.1073/pnas.1616976114 Abstract
Variations of the Earth’s geomagnetic field during the Holocene are important for understanding centennial to millennial-scale processes of the Earth’s deep interior and have enormous potential implications for chronological correlations (e.g., comparisons between different sedimentary recording sequences, archaeomagnetic dating). Here, we present 21 robust archaeointensity data points from eastern China spanning the past ∼6 kyr. These results add significantly to the published data both regionally and globally. Taking together, we establish an archaeointensity reference curve for Eastern Asia, which can be used for archaeomagnetic dating in this region. Virtual axial dipole moments (VADMs) of the data range from a Holocene-wide low of ∼27 to “spike” values of ∼166 ZAm2 (Z: 1021). The results, in conjunction with our recently published data, confirm the existence of a decrease in paleointensity (DIP) in China around ∼2200 BCE. These low intensities are the lowest ever found for the Holocene and have not been reported outside of China. We also report a spike intensity of 165.8 ± 6.0 ZAm2 at ∼1300 BCE (±300 y), which is either a prelude to or the same event (within age uncertainties) as spikes first reported in the Levant.
Archaeomagnetic Dating of Copper Smelting Site F2 in the Timna Valley (Israel) and Its Implications for the Modelling of Ancient Technological Developments.
Archaeometry. 52:1110-1121. 10.1111/j.1475-4754.2010.00528.x Abstract
Site F2 in the Timna Valley, Israel, is a small copper smelting site of 'primitive' technology, dated by its excavator to the Pottery Neolithic (sixth to fifth millennium bce). This early date challenges the common view of the beginning of smelting technology in the Levant and has been contested by various scholars since its publication. In this study, we present results of archaeointensity experiments conducted on slag fragments from the site. The slag yielded an excellent ancient geomagnetic value (64.1 +/- 1.1 mu T) that, when compared to the Levantine master curve, suggests an age not older than the second millennium and most probably between the 13th and 11th centuries bce. In addition to demonstrating the applicability of geomagnetic archaeointensity experiments to independent dating of slag, we discuss the implications of the current results for the socio-historical picture of the Timna Valley, and in particular for the way in which technological developments were previously modelled in the archaeometallurgical research on the region.
Archaeomagnetic intensity results from California and Ecuador: evaluation of regional data.
Earth and Planetary Science Letters. 203:967-981. 10.1016/s0012-821x(02)00927-5 Abstract
We present new archaeointensity data for southeastern California (similar to33degreesN, similar to115degreesW, 50-1500 yr BP) and northwestern South America (Ecuador, 2.4degreesS, 80.7degreesW, 4000-5000 yr BP). These results represent the only data from California, as well as the oldest archaeointensity data now available in northwestern South America. In comparing our results to previously published data for the southwestern United States and northwestern South America, we note that significant scatter in the existing data makes comparisons and interpretations difficult. We undertake an analysis of the sources of data scatter (including age uncertainty, experimental errors, cooling rate differences, magnetic anisotropy, and field distortion) and evaluate the effects of scatter and error on the smoothed archaeointensity record. By making corrections where possible and eliminating questionable data, scatter is significantly reduced, especially in South America, but is far from eliminated. However, we believe the long-period fluctuations in intensity can be resolved, and differences between the Southwestern and South American records can be identified. The Southwest data are distinguished from the South American data by much higher virtual axial dipole moment values from similar to 0-600 yr BP and by a broad low between similar to 1000-1500 yr BP. Comparisons to global paleofield models reveal disagreements between the models and the archaeointensity data in these two regions, underscoring the need for additional intensity data to constrain the models in much of the world. (C) 2002 Elsevier Science B.V. All rights reserved.
Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences.
Earth and Planetary Science Letters. 140:133-146. 10.1016/0012-821x(96)00030-1 Abstract
We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the delta(18)O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19. Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed. The mean of the Stage 19 age estimates for the MBB is 777.9 +/- 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 +/- 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 +/- 3.5 (N = 10) from a compilation of Ar-40/Ar-39 results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 +/- 1.7 (N = 28). The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface. Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.
Biostratigraphic synthesis: Leg 108, Eastern Equatorial Atlantic.
Proceedings of the Ocean Drilling Program Scientific Results. 108:455-461. 10.2973/odp.proc.sr.108.171.1989 Abstract
Leg 108 cored 12 sites in the eastern equatorial Atlantic and along the northwest African continental margin to investigate the late Neogene and Quaternary oceanographic and climatic history of these regions. Sediments recovered during Leg 108 provide in part a high-resolution stratigraphic record for the upper Pliocene through Holocene interval. The bio- and magnetostratigraphy are intercalibrated where possible and provide a useful chronostratigraphy for paleoceanographic studies.
The Bootstrap for Magnetic Susceptibility Tensors.
Journal of Geophysical Research-Solid Earth and Planets. 95:8383-8395. 10.1029/JB095iB06p08383 Abstract
In studies of the anisotropy of susceptibility or remanence of paleomagnetic samples it is conventional to specify the anisotropy in terms of the parameters of the anisotropy ellipsoids, namely the directions of the principal axes of the ellipsoid and their associated eigenvalues. Confidence intervals for these parameters have in the past often been estimated by using a linearization scheme to propagate the effect of small changes through the eigenvalue decomposition. The validity of these approximations is explored using a Monte-Carlo simulation from measurements that are presumed normally distributed, showing that there are circumstances in which the linearization scheme gives confidence intervals that are much too small. Q-Q plots indicate that the common assumption that the noise in the measurements is Gaussian does not always hold. Because of these shortcomings in the conventional technique we propose using a bootstrap resampling scheme to find empirically the distribution of uncertainties in the results. Confidence intervals for the eigenvalues are found directly from their empirical distributions. For the principal axes, approximate elliptical regions of confidence on the unit sphere are parameterized in terms of the Kent or FB5 distribution. The number of modes observed in the distribution of eigenvalues obtained by bootstrapping is used to classify the shape of the susceptibility ellipsoid as spherical, oblate, prolate or triaxial. The empirical nature of the bootstrap technique allows the extension of the analysis of uncertainties to parameters derived from the principal susceptibilities, such as percentage anisotropy or shape factor.
Bootstrap Statistics for Paleomagnetic Data.
Journal of Geophysical Research-Solid Earth and Planets. 96:11723-11740. 10.1029/91jb00572 Abstract
The power and utility of paleomagnetic analyses stem largely from the ability to quantify such parameters as the degree of rotation of a rock body, or the orientation of an anisotropy axis. Until recently, estimates for uncertainty in these paleomagnetically determined parameters derived from assumptions concerning the underlying parametric distribution functions of the data. In many geologically important situations, the commonly used parametric distribution functions fail to model the data adequately and the uncertainty estimates so obtained are unreliable. Such essentials as the test for common mean require data sets consistent with a spherically symmetric underlying distribution; their application in inappropriate circumstances can result in flawed interpretations. Moreover, the almost universally used approximation for a cone of 95% confidence for the mean of a sample drawn from a Fisher distribution is quite biased even for moderate dispersions (kappa = 25). The availability of inexpensive, powerful computers makes possible the empirical estimation of confidence regions by means of data resampling techniques such as the bootstrap. These resampling schemes replace analytical solutions with repeated simple calculations. We describe a bootstrap approach for the calculation of uncertainties for means or principal directions of paleomagnetic data. The method is tested on means of simulated Fisher distributions with known parameters and is found to be reliable for data sets with more than about 25 elements. Because a Fisher distribution is not assumed, the approach is applicable to a wide range of paleomagnetic data and can be used equally well on directions or associated virtual poles. We also illustrate bootstrap techniques for the discrimination of directions and for the fold test which enable the use of these powerful tests on the wider range of data sets commonly obtained in paleomagnetic investigations.
Characteristics of Magnetite in Submarine Basaltic Glass.
Geophysical Journal International. 119:116-128. 10.1111/j.1365-246X.1994.tb00917.x Abstract
We have compared the rock-magnetic characteristics of Cretaceous submarine basaltic glass (SBG) from DSDP/ODP Holes 417D, 418A, 807C and 543A to data from Holocene glasses from the East Pacific Rise. Both groups are very similar in that they contain single-domain, low-Ti magnetite as the carrier of a strong and stable remanent magnetization. The behaviour of magnetic hysteresis in SBG is, however, dominated by superparamagnetic grain-size populations. Transmission electron microscopy revealed spherically shaped grains of 10-20 nm diameter with crystal lattice properties consistent with magnetite. These grains could be responsible for the magnetic behaviour of SBG.
Chemical Remanent Magnetization in Synthetic Magnetite.
Journal of Geophysical Research-Solid Earth and Planets. 96:9925-9936. 10.1029/91jb00706 Abstract
As a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field, its moment becomes blocked and it acquires a chemical remanent magnetization, CRM. Despite its importance to paleomagnetism, the properties of CRM and the controls on its behavior are still poorly understood. We have modified techniques developed by Taylor et al.  to synthesize magnetite at room temperature and atmospheric pressure, whereby magnetite is produced by alteration of "green rust" under reducing conditions. The period of synthesis is twenty-four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mT. The average direction of the CRM parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field. The intensity is linearly related to the applied field up to approximately 1 mT, consistent with existing CRM theory developed by analogy to the acquisition of thermal remanent magnetization (TRM). Above about 1 mT, however, the CRM intensity approaches saturation at a rate substantially lower than that predicted by the TRM analog theory. Furthermore, the ratio of the CRM to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field. Analogy to TRM predicts this ratio to be independent of growth field. Both, the principal eigenvector of the anisotropy of isothermal remanence (AIR) matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases. The above observations suggest that the model of CRM acquisition based on analogy to TRM acquisition is incomplete. We therefore propose a revised model which incorporates the increased alignment of easy axes with growth field. Finally, the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish CRM from TRM in natural remanence.
Constraining early to middle Eocene climate evolution of the southwest Pacific and Southern Ocean.
Earth and Planetary Science Letters. 433:380-392. 10.1016/j.epsl.2015.11.010 Abstract
Studies of early Paleogene climate suffer from the scarcity of well-dated sedimentary records from the southern Pacific Ocean, the largest ocean basin during this time. We present a new magnetostratigraphic record from marine sediments that outcrop along the mid-Waipara River, South Island, New Zealand. Fully oriented samples for paleomagnetic analyses were collected along 45 m of stratigraphic section, which encompasses magnetic polarity Chrons from C23n to C21n (similar to 51.5-47 Ma). These results are integrated with foraminiferal, calcareous nannofossil, and dinoflagellate cyst (dinocyst) biostratigraphy from samples collected in three different expeditions along a total of similar to 80 m of section. Biostratigraphic data indicates relatively continuous sedimentation from the lower Waipawan to the upper Heretaungan New Zealand stages (i.e., lower Ypresian to lower Lutetian, 55.5 to 46 Ma). We provide the first magnetostratigraphically-calibrated age of 48.88 Ma for the base of the Heretaungan New Zealand stage (latest early Eocene). To improve the correlation of the climate record in this section with other Southern Ocean records, we reviewed the magnetostratigraphy of Ocean Drilling Program (ODP) Site 1172 (East Tasman Plateau) and Integrated Ocean Drilling Program (IODP) Site 131356 (Wilkes Land Margin, Antarctica). A paleomagnetic study of discrete samples could not confirm any reliable magnetic polarity reversals in the early Eocene at Site 1172. We use the robust magneto-biochronology of a succession of dinocyst bioevents that are common to mid-Waipara, Site 1172, and Site U1356 to assist correlation between the three records. A new integrated chronology offers new insights into the nature and completeness of the southern high-latitude climate histories derived from these sites. (C) 2015 Elsevier B.V. All rights reserved.
Decadal-scale variations in geomagnetic field intensity from ancient Cypriot slag mounds.
Geochemistry, Geophysics, Geosystems. 10.1002/2014GC005455 Abstract
Geomagnetic models based on direct observations since the 1830s show that the averaged relative change in field intensity on Earth's surface over the past 170 years is less than 4.8% per decade. It is unknown if these rates represent the typical behavior of secular variations due to insufficient temporal resolution of archaeomagnetic records from earlier periods. To address this question we investigate two ancient slag mounds in Cyprus - Skouriotissa Vouppes (SU1, 4th - 5th centuries CE, 21 meter in height), and Mitsero Kokkinoyia (MK1, 7th - 5th BCE, 8 meter in height). The mounds are multi-layered sequences of slag and charcoals that accumulated near ancient copper production sites. We modeled the age-height relation of the mounds using radiocarbon dates, and estimated paleointensities using Thellier-type IZZI experiments with additional anisotropy, cooling rate, and non-linear TRM assessments. To screen out ambiguous paleointensity interpretations we applied strict selection criteria at the specimen/sample levels. To ensure objectivity, consistency, and robust error estimation we employed an automatic interpretation technique and put the data available in the MagIC database. The analyses yielded two independent sub-century scale paleointensity time series. The MK1 data indicate relatively stable field at the time the mound accumulated. In contrast, the SU1 data demonstrate changes that are comparable in magnitude to the fastest changes inferred from geomagnetic models. We suggest that fast changes observed in the published archaeomagnetic data from the Levant are driven by two longitudinally-paired regions, the Middle East and South Africa, that show unusual activity in geomagnetic models. This article is protected by copyright. All rights reserved.
Depositional remanent magnetization: Toward an improved theoretical and experimental foundation.
Earth and Planetary Science Letters. 244:515-529. 10.1016/j.cpsl.2006.02.003 Abstract
The first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field, yet redeposition experiments showed a strong (and quasi-linear) dependence on the external field. This empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades. We present redeposition experiments which suggest instead that the relationship between depositional remanence (DRM) and applied field may frequently be curved for magnetic fields in the range of the Earth's. Numerical simulations using a flocculation model can explain the redeposition data and suggest that DRM will be significantly non-linear when the floes are small (several microns). There is a strong dependence of floe size on salinity particularly in low salinity environments. Floe size has a profound influence on the efficiency of DRM, hence low salinity environment may give results with poor reproducibility. The size of the floe in which magnetic particles are embedded is not accounted for in current methods of normalization, yet is the most important parameter. On the bright side, however, it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floe size distribution and settling times can be constrained. (c) 2006 Elsevier B.V All fights reserved.
Detecting compaction disequilibrium with anisotropy of magnetic susceptibility.
Geochemistry Geophysics Geosystems. 7 10.1029/2006gc001378 Abstract
In clay-rich sediment, microstructures and macrostructures influence how sediments deform when under stress. When lithology is fairly constant, anisotropy of magnetic susceptibility (AMS) can be a simple technique for measuring the relative consolidation state of sediment, which reflects the sediment burial history. AMS can reveal areas of high water content and apparent overconsolidation associated with unconformities where sediment overburden has been removed. Many other methods for testing consolidation and water content are destructive and invasive, whereas AMS provides a nondestructive means to focus on areas for additional geotechnical study. In zones where the magnetic minerals are undergoing diagenesis, AMS should not be used for detecting compaction state. By utilizing AMS in the Santa Barbara Basin, we were able to identify one clear unconformity and eight zones of high water content in three cores. With the addition of susceptibility, anhysteretic remanent magnetization, and isothermal remanent magnetization rock magnetic techniques, we excluded 3 out of 11 zones from being compaction disequilibria. The AMS signals for these three zones are the result of diagenesis, coring deformation, and burrows. In addition, using AMS eigenvectors, we are able to accurately show the direction of maximum compression for the accumulation zone of the Gaviota Slide.
Detecting uniaxial single domain grains with a modified IRM technique.
Geophysical Journal International. 187:1250-1258. 10.1111/j.1365-246X.2011.05224.x Abstract
Mid-ocean ridge basalt (MORB) specimens have often been found to have high ratios of saturation remanence to saturation magnetization (M(rs)/M(s)). This has been attributed either to dominant cubic anisotropy or to insufficient saturating field leading to overestimation of M(rs)/M(s) of a dominantly uniaxial single domain (USD) assemblage. To resolve this debate, we develop an independent technique to detect USD assemblages. The experimental protocol involves subjecting the specimen to bidirectional impulse fields at each step. The experiment is similar to the conventional isothermal remanent magnetization (IRM) acquisition experiment but the field is applied twice, in antiparallel directions. We define a new parameter, IRAT, as the ratio of the remanences at each field step and show it to have characteristic behaviour for the two assemblages; IRAT similar to 1 at all field steps for USD and <1 with a strong field dependence for multi-axial single domain (MSD) grains. We verified the theoretical predictions experimentally with representative USD and MSD specimens. Experiments with MORBs gave low IRATs for specimens having high M(rs)/M(s). This argues for a dominant MSD assemblage in the MORBs, possibly cubic in nature. Although undersaturation of the samples can indeed be a contributing factor to the exceptionally high M(rs)/M(s), this study shows that the nature of the assemblage cannot be dominantly USD.
Dike surface lineations as magma flow indicators within the sheeted dike complex of the Troodos Ophiolite, Cyprus.
Journal of Geophysical Research-Solid Earth. 103:5241-5256. 10.1029/97jb02717 Abstract
Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma now through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite peudostratigraphy and ridge-and-groove structures termed hot slickenlines found throughout the complex. A unique now direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or backflow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (epsilon(1)) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain-scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near-vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.
DSDP Leg-73: Contributions to Paleogene Stratigraphy in Nomenclature, Chronology and Sedimentation-Rates.
Palaeogeography Palaeoclimatology Palaeoecology. 42:91-&. 10.1016/0031-0182(83)90040-8 Abstract
DSDP Leg 73 was successful in determining magnetostratigraphic—biostratigraphic correlations throughout much of the Paleogene. This paper treats three aspects of the data analysis.The first section treats the development of a chron nomenclature which facilitates the precise correlation of arbitrary events with respect to the geomagnetic polarity history.The second section analyzes the accuracy of radiometric dates for the Paleogene. The conclusion is that despite the recent advances in radiochronology ‘South Atlantic Standard’ remains the most convenient and probably the most reliable chronological standard.The final section studies the correlation in sedimentation rates between the Umbrian and South Atlantic sites. The conclusion is that sedimentation rate changes determined from magnetostratigraphy provide a high-resolution source of paleoenvironmental information. Strong correlations are noted between sites and with respect to other paleo-environmental studies involving oxygen isotope ratios, biogeography and CCD fluctuations within the Paleogene marine sediments.
Dynamic behaviour of the East Antarctic ice sheet during Pliocene warmth.
Nature Geoscience. 6:765-769. 10.1038/ngeo1889 Abstract
Warm intervals within the Pliocene epoch (5.33-2.58 million years ago) were characterized by global temperatures comparable to those predicted for the end of this century(1) and atmospheric CO2 concentrations similar to today(2-4). Estimates for global sea level highstands during these times(5) imply possible retreat of the East Antarctic ice sheet, but ice-proximal evidence from the Antarctic margin is scarce. Here we present new data from Pliocene marine sediments recovered offshore of Adelie Land, East Antarctica, that reveal dynamic behaviour of the East Antarctic ice sheet in the vicinity of the low-lying Wilkes Subglacial Basin during times of past climatic warmth. Sedimentary sequences deposited between 5.3 and 3.3 million years ago indicate increases in Southern Ocean surface water productivity, associated with elevated circum-Antarctic temperatures. The geochemical provenance of detrital material deposited during these warm intervals suggests active erosion of continental bedrock from within the Wilkes Subglacial Basin, an area today buried beneath the East Antarctic ice sheet. We interpret this erosion to be associated with retreat of the ice sheet margin several hundreds of kilometres inland and conclude that the East Antarctic ice sheet was sensitive to climatic warmth during the Pliocene.
E/I corrected paleolatitudes for the sedimentary rocks of the Baja British Columbia hypothesis.
Earth and Planetary Science Letters. 242:205-216. 10.1016/j.epsl.2005.11.052 Abstract
Paleomagnetic inclinations from sediments of the western terranes of Canada are consistently too shallow for their reconstructed paleogeographic positions. Two contradicting explanations for these discrepancies are: (1) terranes have been displaced northward with respect to the stable American craton by several thousands of kilometres between the Late Cretaceous (similar to 75 Ma) and the Eocene (similar to 50 Ma) and (2) sedimentary inclination error has caused a shallow bias in the paleomagnetic directions. Here, we apply the elongation/inclination (E/I) method to paleomagnetie data sets from sedimentary rocks of supposedly allochtonous terranes of Nvestem North America to correct for inclination flattening. Our results indicate that the paleomagnetic directions from the continental Silverquick sediments (95-92 Ma) of southern British Colombia are not seriously affected by inclination error, because the magnetic signal most likely concerns a chemical remanent magnetisation (CRM). In contrast, the marine sediments of the Nanaimo Group (84-72 Ma) of Vancouver Island region appear seriously affected by inclination flattening (f=0.7) and the EA corrected mean inclinations are about 9 degrees steeper than the original data. We arrive at corrected inclinations/paleolatitudes of I** = 57 degrees/lambda = 38 degrees N for the Silverquick and I** = 55 degrees/lambda = 36 degrees N for the Nanaimo sediments. Our corrected paleolatitudes indicate that the Canadian terranes were indeed located adjacent to the Baja Californian margin during the Late Cretaceous, thus supporting the Baja BC hypothesis. (c) 2005 Elsevier B.V. All rights reserved.
Earliest Oligocene Increase in South-Atlantic Productivity as Interpreted from Rock Magnetics at Deep-Sea Drilling Project Site-522.
Paleoceanography. 10:311-325. 10.1029/94pa03150 Abstract
The magnetic properties of the sediments (''rock magnetics'') at DSDP Site 522 in the South Atlantic exhibit clear differences between the latest Eocene and earliest Oligocene. Based on low temperature behavior of saturation remanence and hysteresis loops, we attribute the difference to a slightly greater proportion of the finest grained, so-called ''superparamagnetic'' magnetite in the Eocene sediments. We believe that the lower proportion of very fine-grained magnetite in the Oligocene sediments is a result of incipient reduction diagenesis caused by increased productivity and hence increased labile organic carbon transport to the sediments due to an early Oligocene increase in thermohaline circulation. The Eocene-to-Oligocene transition at Site 522 is also expressed by changes in microfossil assemblages, increased carbonate content, decreased insoluble residue, and decreased foraminiferal shell fragmentation. The increase in carbonate is synchronous with and parallels a change in the ratio of two of the rock magnetic parameters, a ratio that tracks the decrease in the very fine-grained magnetite component. Also parallel to these is a trend toward heavier delta(13)C values in foraminiferal tests. The increase in organic carbon transport to the sediments led to chemical dissolution of the finest grain-size fraction of magnetite in the Oligocene sediments, hence a reduction in the superparamagnetic component and the change in the rock magnetic ratio. In this way, rock magnetics can be sensitive indicators of environmental changes, such as fluctuations in organic carbon transport, which may leave little other trace in the sedimentary record.