We have conducted a paleomagnetic study of Holocene sediments from Lake Victoria in order to develop a high-resolution record of paleomagnetic secular variation (PSV). This study has recovered PSV records from two cores (V95-1P and V95-7P) in northern Lake Victoria (0.5°S). The PSV is recorded in fine-grained detrital magnetite/titanomagnetite grains, but the rock magnetic data suggest that significant magnetic mineral dissolution has occurred, which limits our paleomagnetic studies to the uppermost ~5 m of both cores. Detailed alternating field (af) demagnetization of the natural remanence (NRM) shows that a distinctive characteristic remanence (ChRM) is demagnetized from ~10 to 40 mT, which decreases simply toward the origin. The resulting directional PSV records for 1P and 7P are correlatable with 22 distinct inclination features and 19 declination features. Radiocarbon dating of the cores is based on eight radiocarbon dates from core 1P, which can be correlated into core 7P using both the PSV and rock magnetic/environmental measurements. The final PSV time series cover the last 11,000 years with an average sediment accumulation rate of ~40 cm/kyr. The Lake Victoria PSV records can be correlated with new PSV records from Lake Malawi. Comparison of the correlatable PSV feature ages between the two lakes indicates that the PSV records are not significantly different in age, although Lake Victoria PSV ages might average ~100 years younger. We think that the Lake Victoria and Lake Malawi PSV records, together, provide the most accurate, well-dated, and consistent record of Holocene PSV for Africa yet developed.
Quaternary lavas of the Stardalur Caldera, 20 km northeast of Reykjavik, Iceland, create a 27 300 nT magnetic anomaly visible in both ground and aeromagnetic surveys. Here, we provide a comprehensive mineralogical and rock magnetic data set to analyse NRM intensities and Koenigsberger ratios of 57 drill-core samples from the critical zone (CZ) of the anomaly high at depths between 41 and 131 m. This extends previous studies and verifies that the anomaly is due to an unusually high intensity of remanent magnetization carried by magnetite. The NRM of the CZ samples was acquired during the Olduvai subchron in a field of at most today’s strength. NRM intensities range from 20 to 128 A m–1 with a median of 55 A m–1, and an average of 61 A m–1, respectively, approximately 13–15 times higher than in typical Icelandic basalts (AIB) with an NRM intensity of 4 A m–1. Our new data set shows that the magnetite concentration throughout the CZ basalts is at most twofold higher than in AIB lavas. New data on domain state and TRM efficiency prove that these properties account for an additional factor of at most 2.3. Because magnetite is the most abundant remanence carrier in rocks on Earth, and its remanence acquisition is considered to be extremely well understood, we assert that the remaining discrepancy is a critical enigma in rock magnetism. Results from scanning electron microscopy show that a significant fraction of all CZ magnetite particles have dendritic shapes with grain sizes <1 μm, indicating rapid crystallization. Most large magnetite grains are heavily subdivided by very fine oxidation-exsolution lamellae of ilmenite, and subordinate amount of exsolved spinel as needles, blebs and blades. These common microstructures found throughout the CZ subdivide the initially homogeneous mineral into separate cubicles, here denoted as compartments. The magnetite compartments then have sizes below 1 μm. Hysteresis data, Preisach maps and FORC data consistently confirm that the coercivity distribution is dominated by values above 10 mT, such that multidomain behaviour is of little relevance in the CZ. Between 5 and 20 per cent of the IRM is carried by coercivities above 100 mT, which for magnetite indicates unusually high anisotropy effects in the individual particles. Based on the quantitative analysis of all magnetic contributions to the NRM, we can demonstrate that the average efficiency of NRM acquisition in the CZ Stardalur basalts must be at least a factor 3 higher than in typical basalts. We speculate that this is related to the observed focused compartment size distribution <1 μm, and indicates thermochemical remanence acquisition below the Curie temperature of magnetite. Yet, a detailed physical mechanism for the extreme overefficiency of NRM acquisition remains enigmatic.
more » « less- NSF-PAR ID:
- 10368956
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 231
- Issue:
- 2
- ISSN:
- 0956-540X
- Format(s):
- Medium: X Size: p. 835-855
- Size(s):
- ["p. 835-855"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
-
more » « less
SUMMARY International Ocean Drilling Program (IODP) Expedition 341 recovered sediments from the south Alaska continental slope that preserves a well resolved and dated inclination record over most of the past ∼43 000 yr. The Site U1419 chronology is among the highest resolution in the world, constrained by 173 radiocarbon dates, providing the ability to study Palaeomagnetic Secular Variation (PSV) on centennial to millennial timescales. This record has an exceptionally expanded late Pleistocene sedimentary record with sedimentation rates commonly exceeding 100 cm kyr–1, while also preserving a lower resolution Holocene PSV record at the top. Natural and laboratory-induced magnetic remanences of U1419 u-channels from the 112-m-long spliced record were studied using stepwise AF demagnetization. Hysteresis loops were obtained on 95 and IRM acquisition curves on 9 discrete samples to facilitate magnetic domain state, coercivity and magnetic mineralogical determinations. Due to complexities related to lithology, magnetic mineralogy, and depositional and post-depositional processes, Site U1419 sediments are not suitable for palaeointensity studies and declination could not be robustly reconstructed. Progressive (titano-)magnetite dissolution with depth results in decreasing NRM intensity and signal-to-noise that is exacerbated at higher demagnetization steps. As a result, inclination measured after the 20 mT AF demagnetization step provides the most reliable directional record. Inclination appears to be well resolved with removal of just a few intervals influenced by depositional and/or sampling and coring deformation. The shipboard inclination stack from nearby IODP Site U1418, on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC, verifies centennial to millennial scale variations in inclination observed in U1419. Comparisons with other independently dated records from the NE Pacific and western North America suggest that these sites likely capture regional geomagnetic variability. As such, this new high-resolution and well-dated inclination record, especially robust between 15 and 30 cal kyr BP, offers new geomagnetic insights and a regional correlation tool to explore this generally understudied part of the world.
-
SUMMARY Anisotropy of remanent magnetization and magnetic susceptibility are highly sensitive and important indicators of geological processes which are largely controlled by mineralogical parameters of the ferrimagnetic fraction in rocks. To provide new physical insight into the complex interaction between magnetization structure, shape, and crystallographic relations, we here analyse ‘slice-and-view’ focused-ion-beam (FIB) nano-tomography data with micromagnetic modelling and single crystal hysteresis measurements. The data sets consist of 68 magnetite inclusions in orthopyroxene (Mg60) and 234 magnetite inclusions in plagioclase (An63) were obtained on mineral separates from the Rustenburg Layered Suite of the Bushveld Intrusive Complex, South Africa. Electron backscatter diffraction was used to determine the orientation of the magnetite inclusions relative to the crystallographic directions of their silicate hosts. Hysteresis loops were calculated using the finite-element micromagnetics code MERRILL for each particle in 20 equidistributed field directions and compared with corresponding hysteresis loops measured using a vibrating sample magnetometer (VSM) on silicate mineral separates from the same samples. In plagioclase the ratio of remanent magnetization to saturation magnetization (Mrs/Ms) for both model and measurement agree within 1.0 per cent, whereas the coercivity (Hc) of the average modelled curve is 20 mT lower than the measured value of 60 mT indicating the presence of additional sources of high coercivity in the bulk sample. The VSM hysteresis measurements of the orthopyroxene were dominated by multidomain (MD) magnetite, whereas the FIB location was chosen to avoid MD particles and thus contains only particles with diameters <500 nm that are considered to be the most important carriers of palaeomagnetic remanence. To correct for this sampling bias, measured MD hysteresis loops from synthetic and natural magnetites were combined with the average hysteresis loop from the MERRILL models of the FIB region. The result shows that while the modelled small-particle fraction only explains 6 per cent of the best fit to the measured VSM hysteresis loop, it contributes 28 per cent of the remanent magnetization. The modelled direction of maximal Mrs/Ms in plagioclase is subparallel to [001]plag, whereas Hc does not show a strong orientation dependence. The easy axis of magnetic remanence is in the direction of the magnetite population normal to (150)plag and the maximum calculated susceptibility (χ*) is parallel to [010]plag. For orthopyroxene, the maximum Mrs/Ms, maximum χ* and the easy axis of remanence is strongly correlated to the elongation axes of magnetite in the [001]opx direction. The maximum Hc is oriented along [100]opx and parallel to the minimum χ*, which reflects larger vortex nucleation fields when the applied field direction approaches the short axis. The maximum Hc is therefore orthogonal to the maximum Mrs/Ms, controlled by axis-aligned metastable single-domain states at zero field. The results emphasize that the nature of anisotropy in natural magnetite does not just depend on the particle orientations, but on the presence of different stable and metastable domain states, and the mechanism of magnetic switching between them. Magnetic modelling of natural magnetic particles is therefore a vital method to extract and process anisotropic hysteresis parameters directly from the primary remanence carriers.more » « less
-
more » « less
We have carried out a paleomagnetic study of three piston cores collected from Lake Turkana. The goal is to recover a Holocene paleomagnetic secular variation (PSV) record for this lake and to correlate it with other Holocene PSV records from the East Africa Rift Valley (EARV). All three cores were sampled with u-channels and magnetic measurements of magnetic susceptibility, the natural remanence (NRM), and two artificial remanences, anhysteretic remanence (ARM) and saturation isothermal remanence (SIRM), were made on them. The remanences were routinely step-wise demagnetized and measured at 10 mT steps up to 60 mT. The NRMS had a simple pattern of demagnetization with a characteristic direction (ChRM) removed between 10 and 60 mT. ARM and SIRM demagnetization indicated that the magnetic grains were relatively soft with median destructive fields (MDF) less than 30 mT. We interpret the magnetic grains to be multi-domain (silt-sized) magnetite/titanomagnetite. The resulting magnetic records of all three cores could be correlated. A chronology for these cores was determined from four radiocarbon dates on core 4P. We also estimated the sediment ages by correlating the PSV to two other well-dated PSV records from the same region, Lakes Malawi and Victoria. PSV age estimation indicates that the radiocarbon dates are about 500 year too old; a correction for that offset causes three of the four radiocarbon ages to become consistent with the PSV age estimates. The final composite Lake Turkana PSV record for the last ~4000 years is the highest resolution directional record of PSV (~200 cm/ky) ever to be recovered from Africa.
-
more » « less
Abstract Gyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite.
