Post-depositional Remanent Magnetization Research Articles

Paleomagnetic investigation of the basal Maieberg Formation (Namibia) cap carbonate sequence (635 Ma): Implications for Snowball Earth postglacial dynamics

In this study, we investigate the paleomagnetism of the basal Maieberg Formation (Namibia) cap carbonate sequence to elucidate its magnetic properties and paleolatitude of deposition, establish global correlations, and contribute to the understanding of Snowball Earth postglacial dynamics. Two distinct magnetization components, C1 and C2, were identified. C1 is interpreted as a depositional or post-depositional remanent magnetization carried by detrital pseudo-single domain (PSD) magnetite, while the C2 component is a thermochemical remanent magnetization carried by fine authigenic single domain (SD)−PSD magnetite. The deposition paleolatitude provided by C1 is 33.3° ± 3.2°, which gives an initial quantitative approximation of the paleolatitude for the underlying Marinoan Ghaub diamictites. The thickness of the Keilberg Member cap dolostone is anomalously high for the paleolatitude calculated with C1, which suggests that other factors besides the influence of the paleolatitude on carbonate oversaturation may have influenced the sedimentary production of cap dolostones and the overall thickness of the flooding cap carbonate sequence. Possible explanations could include the influence of alkalinity input combined with local tectonic subsidence during a long glacial period with unusually low sedimentation rates, which appear to be in a favorable configuration for the substantial thickness of the Keilberg Member. Paleomagnetic field reversals at the Keilberg cap dolostone and analogous units globally suggest a longer duration of marine transgression than energy-balance deglaciation models and sedimentological-geochemical observations have constrained. Factors such as ocean warming, thermal expansion, and local glacio-isostatic adjustments imply extended marine transgressions beyond the deglaciation period. Still, magnetostratigraphic estimates for postglacial transgressive sequences require longer time scales by a factor of five or more. Thus, the conflict arising between estimates derived from paleomagnetic data and the constraints imposed by climate physics underscores uncertainties regarding an unconventional field state or a remanence acquisition mechanism within these cap carbonates that is not fully understood. Importantly, if such a phenomenon proves to be primary and global, the widespread occurrence of these stratigraphically compressed reversals would support the precise temporal correlation between Marinoan cap dolostones. The C2 pole correlates with Cambrian remagnetization poles observed in carbonates from West Gondwana, which now extend to the Congo craton. The remanence acquisition of C2 likely stems from diagenesis-related low-temperature authigenic magnetite formation after the conversion of iron-rich smectite to iron-poor illite. Cooling associated with the Kaoko orogen’s exhumation and tectonic uplift possibly locked the magnetic system at ca. 520 Ma, supported by the C2 pole position on the West Gondwana apparent polar wander path, although other explanations remain valid.

Impact of Upward Oxygen Diffusion From the Oceanic Crust on the Magnetostratigraphy and Iron Biomineralization of East Pacific Ridge-Flank Sediments

Recent measurements of pore-water oxygen profiles in ridge flank sediments of the East Pacific Rise revealed an upward-directed diffusive oxygen flux from the hydrothermally active crust into the overlying sediment. This double-sided oxygenation from above and below results in a dual redox transition from an oxic sedimentary environment near the seabed through suboxic conditions at sediment mid-depth back to oxic conditions in the deeper basal sediment. The potential impact of this redox reversal on the paleo- and rock magnetic record was analyzed for three sediment cores from the Clarion-Clipperton-Zone (low-latitude eastern North Pacific). We found that the upward-directed crustal oxygen flux does not impede high quality reversal-based and relative paleointensity-refined magnetostratigraphic dating. Despite low and variable sedimentation rates of 0.1–0.8 cm/kyr, robust magnetostratigraphic core chronologies comprising the past 3.4 resp. 5.2 million years could be established. These age-models support previous findings of significant local sedimentation rate variations that are probably related to the bottom current interactions with the topographic roughness of the young ridge flanks. However, we observed some obvious paleomagnetic irregularities localized at the lower oxic/suboxic redox boundaries of the investigated sediments. When analyzing these apparently remagnetized sections in detail, we found no evidence of physical disturbance or chemical alteration. A sharp increase in single-domain magnetite concentration just below the present lower oxic/suboxic redox boundary suggests secondary magnetite biomineralization by microaerophilic magnetotactic bacteria living as a separate community in the lower, upward oxygenated part of the sediment column. We therefore postulate a two-phased post-depositional remanent magnetization of ridge flank sediments, first by a shallow and later by a deep-living community of magnetotactic bacteria. These findings are the first evidence of a second, deep population of probably inversely oriented magnetotactic bacteria residing in the inverse oxygen gradient zone of ridge flank sediments.

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

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

Paleomagnetic and paleoenvironmental implications of magnetofossil occurrences in late Miocene marine sediments from the Guadalquivir Basin, SW Spain

Although recent studies have revealed more widespread occurrences of magnetofossils in pre-Quaternary sediments than have been previously reported, their significance for paleomagnetic and paleoenvironmental studies is not fully understood. We present a paleo- and rock-magnetic study of late Miocene marine sediments recovered from the Guadalquivir Basin (SW Spain). Well-defined paleomagnetic directions provide a robust magnetostratigraphic chronology for the two studied sediment cores. Rock magnetic results indicate the dominance of intact magnetosome chains throughout the studied sediments. These results provide a link between the highest-quality paleomagnetic directions and higher magnetofossil abundances. We interpret that bacterial magnetite formed in the surface sediment mixed layer and that these magnetic particles gave rise to a paleomagnetic signal in the same way as detrital grains. They, therefore, carry a magnetization that is essentially identical to a post-depositional remanent magnetization, which we term a bio-depositional remanent magnetization. Some studied polarity reversals record paleomagnetic directions with an apparent 60–70 kyr recording delay. Magnetofossils in these cases are interpreted to carry a biogeochemical remanent magnetization that is locked in at greater depth in the sediment column. A sharp decrease in magnetofossil abundance toward the middle of the studied boreholes coincides broadly with a major rise in sediment accumulation rates near the onset of the Messinian salinity crisis (MSC), an event caused by interruption of the connection between the Mediterranean Sea and the Atlantic Ocean. This correlation appears to have resulted from dilution of magnetofossils by enhanced terrigenous inputs that were driven, in turn, by sedimentary changes triggered in the basin at the onset of the MSC. Our results highlight the importance of magnetofossils as carriers of high-quality paleomagnetic and paleoenvironmental signals even in dominantly terrigenous sediments.

An estimate of post-depositional remanent magnetization lock-in depth in organic rich varved lake sediments

We studied the paleomagnetic properties of relatively organic rich, annually laminated (varved) sediments of Holocene age in Gyltigesjön, which is a lake in southern Sweden. An age–depth model was based on a regional lead pollution isochron and Bayesian modelling of radiocarbon ages of bulk sediments and terrestrial macrofossils, which included a radiocarbon wiggle-matched series of 873 varves that accumulated between 3000 and 2000 Cal a BP (Mellström et al., 2013). Mineral magnetic data and first order reversal curves suggest that the natural remanent magnetization is carried by stable single-domain grains of magnetite, probably of magnetosomal origin. Discrete samples taken from overlapping piston cores were used to produce smoothed paleomagnetic secular variation (inclination and declination) and relative paleointensity data sets. Alternative temporal trends in the paleomagnetic data were obtained by correcting for paleomagnetic lock-in depths between 0 and 70cm and taking into account changes in sediment accumulation rate. These temporal trends were regressed against reference curves for the same region (FENNOSTACK and FENNORPIS; Snowball et al., 2007). The best statistical matches to the reference curves are obtained when we apply lock-in depths of 21–34cm to the Gyltigesjön paleomagnetic data, although these are most likely minimum estimates. Our study suggests that a significant paleomagnetic lock-in depth can affect the acquisition of post-depositional remanent magnetization even where bioturbation is absent and no mixed sediment surface layer exists.

Paleomagnetism of the Neoproterozoic diamictites of the Qiaoenbrak formation in the Aksu area, NW China: Constraints on the paleogeographic position of the Tarim Block

The middle to late Neoproterozoic records the transition from the dispersal of supercontinent Rodinia to the subsequent assembly of supercontinent Gondwana. Determining paleogeographic positions of the major cratons during this transition is crucial to understand the displacement histories of these blocks. However, high-quality paleomagnetic results from well-dated rocks during this period remain sparse for the cratons. In this paper, we have carried out a paleomagnetic study of the succession of the Cryogenian Qiaoenbrak formation (Fm) in the Aksu area, NW China, which contains diamictites that are of probable glacial origin. The age of the studied section is constrained at around 730Ma based on the stratigraphy and recent zircon U–Pb chronologic data. Stepwise alternating field (AF) and thermal demagnetizations generally reveal two-component magnetizations. The low temperature (coercivity) component was removed by ∼300°C (∼30mT) and the high temperature (coercivity) component decayed toward the origin, defining the characteristic remanent magnetization (ChRM). Rock magnetic data indicate that magnetite is the major remanence carrier. A negative fold test for an isoclinal soft-sediment fold, the presence of both normal and reversed polarities, and positive conglomerate tests in conjunction with the presence of pristine sedimentary features by microscopic inspections of thin sections suggest that the remanence was acquired shortly after the deposition, i.e. post-depositional remanent magnetization (pDRM), which is very close to the magnetic field at the time of deposition. Fourteen out of 17 sites yield stable ChRMs with a mean direction of Ds=246.1°, Is=30.4°, ks=14.1, α95=11.0° in stratigraphic coordinates, corresponding to a paleopole (Q=6) at λ=6.3°S, φ=17.5°E, A95=9.1°. Together with geologic constraints, the new paleomagnetic results place the Tarim Block at 16.3±5.6°N adjacent to either the west or east of Australia (WOA or EOA) at around 730Ma, but the WOA position is preferred based on the overall relatively favorable evidence. In addition, since the studied diamictites of the Qiaoenbrak Fm that are of probable glaciogenic origin yield a low paleolatitude, this study may lend support to the “Snowball Earth” hypothesis during the Neoproterozoic times.

Post-depositional remanent magnetization lock-in for marine sediments deduced from Be-10 and paleomagnetic records through the Matuyama - Brunhes boundary

Post-depositional remanent magnetization lock-in for marine sediments deduced from Be-10 and paleomagnetic records through the Matuyama - Brunhes boundary

In situ remagnetization experiments of loess on the Chinese Loess Plateau: Evidence for localized post-depositional remanent magnetization

[1] Inconsistent stratigraphic positions of reversal boundaries and spurious paleomagnetic variations associated with polarity reversals recorded by loess/paleosols on the Chinese Loess Plateau (CLP) have in general been attributed to inferred delayed acquisition of magnetization and high-frequency variations of the geomagnetic field, respectively. These conclusions are partly based on an incomplete knowledge of the magnetization acquisition process of loess. Recent laboratory experiments, which demonstrate that dry-deposited loess acquires a stable remanent magnetization after its first wetting (precipitation), resulted in significant inclination errors that are not generally encountered in Chinese loess. Laboratory experiments only remotely mimic natural conditions, and we have therefore performed in situ experiments at two sites on the CLP separated by some 650 km to elucidate whether variations in natural conditions can impose secondary remanent magnetization components in loess (L1). The experiments were performed by relocating blocks (volume ∼5·10−3 m3) of last interglacial loess after being turned upside down inside loess exposures. Inverted blocks were retrieved and analyzed after 8 and 17 months respectively. Thermal demagnetization reveals that partial remagnetization has occurred within different parts of the loess blocks. This secondary magnetization, which is characterized by blocking temperatures that overlap with those of the primary component to above 450°C, cannot be attributed to a viscous remanent magnetization and is therefore proposed to represent local and partial realignment of magnetic grains. We tentatively propose that physical ‘unlocking’ and realignment of magnetic grains may be assisted by pressure release caused by differential thermal expansion of the different mineral grains in loess. Localized remagnetization may account for a number of reported spurious paleomagnetic results from CLP.

Post-depositional remanent magnetization lock-in for marine sediments deduced from 10Be and paleomagnetic records through the Matuyama–Brunhes boundary

Geomagnetic field intensity records from marine sediments have contributed to improved understanding of variations in the Earth's magnetic field, and have helped to establish age models for marine sediments. However, lock-in of the geomagnetic signal below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization of marine sedimentary records. Although quantitative models enable assessment of delays in remanence acquisition associated with PDRM processes, the nature of the filter function and the PDRM lock-in zone thickness remain topics of debate. We performed both forward numerical simulations and inverse parameter estimation to assess the best-fit filter function and PDRM lock-in zone thickness in marine sediments based on a comparison of 10Be flux and relative paleointensity records. Our simulations reveal that the rate of PDRM lock-in increases in the middle of the lock-in zone and that a Gaussian function with a 17 cm lock-in zone thickness provides a good approximation to the PDRM lock-in within the studied core. With this function, PDRM lock-in is delayed, but with relatively little distortion of the geomagnetic signal. Our results also imply that a PDRM is not simply locked due to progressive marine sediment consolidation and dewatering, and that the arbitrary functions (linear, cubic, and exponential) that are often used to model PDRM lock-in starting from the base of the surface mixed layer cannot explain fully the observed paleomagnetic signal.

10Be evidence for delayed acquisition of remanent magnetization in marine sediments: Implication for a new age for the Matuyama–Brunhes boundary

Fluxes of the meteoric cosmogenic radionuclide 10Be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations. The variability in the 10Be flux can be used to synchronize ice cores, as well as marine sediments, by comparison with the relative paleointensity variations of the geomagnetic field. However, lock-in of the paleomagnetic signal at some depth below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization. Despite the long history of such studies, the magnitude of the PDRM lock-in depth remains controversial. In this article, we present clear evidence for a downward offset of the paleointensity minimum relative to the 10Be flux anomaly at the Matuyama–Brunhes (M–B) geomagnetic polarity boundary, which we interpret to result from a ∼ 15 cm PDRM lock in depth. This lock-in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments, and the age of the M–B boundary should be revised to ca. 10 kyr younger, which is consistent with a younger ice core derived age of 770 ± 6 ka (2 σ). This cosmogenic age tuning strategy will contribute to refining paleomagnetic-based age models for marine sediments and identifying of lead-lag relationships for global abrupt environmental changes.

How does Chinese loess become magnetized?

Despite nearly three decades of paleomagnetic research on the extensive Chinese loess deposits, a convincing explanation has yet to be developed for how Chinese loess becomes magnetized. To address this problem, we conducted re-deposition experiments using weakly consolidated Holocene Chinese loess that was disaggregated in the laboratory, and compare our results with published paleomagnetic data. We simulated a depositional remanent magnetization (DRM) associated with dry deposition of eolian sediments, and a post-depositional remanent magnetization (PDRM) in which the sediment was water-saturated after deposition. The simulated DRM faithfully records the declination of the applied field, but with systematic inclination flattening. Addition of minor water slightly improves recording of the applied field, but inclination flattening persists. Reliable recording of the applied field occurs for PDRM simulation in water-saturated sediment. Our synthesis of paleomagnetic data from Chinese loess indicates that time-averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole (GAD) field, but in many cases inclinations are shallower than for a GAD field. We conclude that the Chinese loess is magnetized by a combination of DRM and PDRM mechanisms, with water content providing the dominant control on which mechanism aligns the detrital mineral fraction. Where pedogenesis causes neoformation of magnetic minerals, an additional chemical remanent magnetization (CRM) will occur. The magnetization of Chinese loess therefore appears to be controlled by a complex time-varying combination of DRM, PDRM and CRM mechanisms.

Post-depositional remanent magnetization lock-in and the location of the Matuyama–Brunhes geomagnetic reversal boundary in marine and Chinese loess sequences

Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama–Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤ 20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences.

Terrestrial Records of the Latest Gauss Cooling Event in Japan and North Central China

Stratigraphic studies of magnetic polarity, paleobotany, and paleoenvironment using Plio-Pleisotocene terrestrial sediments in Japan and north central China were reviewed to investigate climate changes around the Gauss-Matuyama magnetic polarity boundary (GMB). Pollen and plant macrofossil data from Osaka, Aizu, and Tokachi in Japan show an extensive floral change associated with cooling in the latest Gauss chron, which is the largest in scale during the last 3-5 million years. The floral change occurred within a zone just below the GMB, where warmer elements disappeared stepwise and new cooler elements emerged also stepwise. In Tokachi, northern Japan, a long-term warmer climate that began at about 3.7Ma terminated just before the GMB, followed by recovery of a boreal climate. In the Chinese Loess Plateau, the GMB occurs around the loess/red clay boundary (LRB). However, the GMB lies at various depths ranging from 5m to -4m above the LRB, which is too wide to be explained by downward displacements due to delayed acquisition of post-depositional remanent magnetization. Local climates may have originated regional variability in lithology. The strong cooling event predating both the GMB and the LRB is consistent with that observed in Japan. Magnetic susceptibility of the red clay suggests occurrence of a long-term warm-moist climate from 3.4Ma to 2.75Ma, which may be related to that in Tokachi.

The role of magnetostatic interactions in sediment suspensions

SUMMARY The processes that influence a detrital remanent magnetization as well as the physical microscale factors that control formation of a stable post-depositional remanent magnetization are still not fully understood. Previous laboratory studies and statistical numerical approaches have shown the possibility that sediment suspensions can display complex magnetization phenomena. Such behaviour has been attributed to the effect of magnetostatic interactions in the suspension, which could provide one explanation for spurious magnetizations observed in marine sediment cores. In laboratory experiments we investigated magnetization decay as a function of time in sediment suspensions produced with varying lithologies and particle concentrations. A companion model takes into account the physics of magnetic particle‐ particle interactions, Brownian motion and hydrodynamic forces to investigate numerically the magnetization behaviour of sediment suspensions. When combined, the experiments and the numerical models reveal a weak effect of magnetostatic interactions in the natural sediment suspensions, which is expressed as an increase in the magnetization decay rate. In addition, a calculation of effective particle size based on the response of each suspension to Brownian motion indicates that the majority of the sedimentary magnetic particles are attached to larger clay particles.

Constraints on the acquisition of remanent magnetization in fine-grained sediments imposed by redeposition experiments

The magnetization of sediments is acquired through complex processes involving a large number of physical, mineralogical and magnetic parameters. Despite many attempts, the degree to which these processes distort the record of the geomagnetic field as it is archived as a natural remanent magnetization (NRM) remains poorly documented. Among many other parameters, it is important to evaluate the amount of smoothing inherent to the signal, its relation with the field intensity and its variability in the sediment column. In order to address these problems, we performed new redeposition experiments using carbonate-rich, Ocean Drilling Program (ODP) Site 851, and clay-rich, ODP Site 854, sediments. We used a dilute solution of gelatin, which gels below 20 °C, thereby allowing mechanical blocking of the magnetic grains. We observed two critical results: (1) The efficiency of detrital remanent magnetization (DRM) decreases with increasing sediment concentration for a given slurry. Sediment concentration is defined as: c = m s / ( m s + m H2O), where m s and m H2O are the sediment and water mass, respectively. Higher c would then reflect greater compaction, lower water content and, presumably, greater depth in the sediment column. This effect reduces DRM efficiency nearly to zero for c > ∼ 50%. (2) Post-depositional remanent magnetization (pDRM) is important for c < ∼ 50%. pDRM is carried by grains covering the entire coercivity spectrum. By comparing the mean value of NRM divided by anhysteretic remanent magnetization from the previous magnetostratigraphic study at Site 851 with the relevant ratio derived from our redeposition experiments, we were able to estimate that pDRM was significant within the depth interval where ∼ 44% < c < ∼ 56%. If the sediment concentration profile for the uppermost sediment was known at Site 851, we could define the transfer function for the deconvolution of the field variations. Finally, the dependence of DRM efficiency on c suggests that changes in the thickness of the surface mixed layer would change DRM efficiency. Thus, fluctuations in maximum bioturbation depth could possibly cause DRM intensity changes, regardless of changes in earth's field.

Timing of remanent magnetization acquisition in red beds: A case study from a syn-folding sedimentary basin

Red beds have been recognized as a useful source of ancient geomagnetic field. However, the timing of remanence acquisition in red beds is still incompletely understood. Fold tests may be used for an estimation of the timing of red bed remanence acquisition, but an essential ability of this kind of test is to conclude whether magnetizations are acquired before or after folding. Even a positive fold test does not necessarily represent a primary nature of magnetization. Because the relationship between the timing of fold growth and sedimentation is well understood for a small, syn-folding sedimentary basin, we select such a sedimentary basin as a focus of study. A case study example examining the timing of red bed remanence acquisition based on fold tests is described in this paper. It shows that the observed remanence directions carried by hematite are given a syn-folding nature. This syn-folding nature is more observable in the lower part of the sedimentary sequence that was under the more intense folding during its sedimentation than the upper part. A pure depositional remanent magnetization when beds were horizontal is not applicable to this case, but a post-depositional remanent magnetization, with possible existence of an initial slope of the deposition surface, is considered to have been imparted prior to complete lithification. Microscopic observations showing the presence of hematite as sedimentary particles can be used for an independent estimation supporting the detrital origin. Chemical compositions of the hematite grains agree well with the maximum laboratory unblocking temperatures of the samples, and we draw the conclusion that these grains are the carrier of the detrital magnetization.

Why are geomagnetic excursions not always recorded in sediments? Constraints from post-depositional remanent magnetization lock-in modelling

Many geomagnetic excursions have been documented during the Brunhes Chron. However, high-resolution paleomagnetic studies of sediments often provide evidence of few (if any) excursions. We have investigated the reasons for this observation by modelling the post-depositional remanent magnetization (PDRM) lock-in process. A cubic lock-in function produces more rapid lock-in compared to linear and exponential functions proposed in the literature. We therefore used a cubic function to model “best-case” scenarios for the quality of the paleomagnetic record when a high-frequency geomagnetic input signal is convolved with the sediment lock-in function for a wide range of sedimentation rates. Even for a lock-in depth of 10 cm, where 95% of the PDRM is locked in 5 cm below the surface mixed layer, an input signal containing abundant excursions (with 1-kyr duration) is smoothed so drastically at low sedimentation rates (1–3 cm/kyr) that no excursions are recorded. Excursions are further attenuated by increasing the lock-in depth, as well as by sampling at discrete stratigraphic intervals. The PDRM process acts as a low-pass filter for high-frequency secular variation signals even at moderate sedimentation rates (>10 cm/kyr). We have also modelled the effects of modulation of sedimentation rate by a climatic forcing function (e.g., insolation). Even though the average sedimentation rate might be substantial, PDRM recording becomes much less reliable in intervals where sedimentation rates are lowest. In these cases, some excursions are reliably recorded in detail, while others may not be recorded at all. Our modelling suggests that in order to consistently detect the presence of geomagnetic excursions, it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm/kyr. If the normal spectrum of geomagnetic field behaviour contains abundant excursions, failure to document excursions in many apparently high-resolution analyses probably results from PDRM lock-in within relatively slowly deposited sediments, or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates.

Does lithology influence relative paleointensity records? a statistical analysis on South Atlantic pelagic sediments

The relative paleointensity (RPI) method assumes that the intensity of post depositional remanent magnetization (PDRM) depends exclusively on the magnetic field strength and the concentration of the magnetic carriers. Sedimentary remanence is regarded as an equilibrium state between aligning geomagnetic and randomizing interparticle forces. Just how strong these mechanical and electrostatic forces are, depends on many petrophysical factors related to mineralogy, particle size and shape of the matrix constituents. We therefore test the hypothesis that variations in sediment lithology modulate RPI records. For 90 selected Late Quaternary sediment samples from the subtropical and subantarctic South Atlantic Ocean a combined paleomagnetic and sedimentological dataset was established. Misleading alterations of the magnetic mineral fraction were detected by a routine Fe/κ test (Funk, J., von Dobeneck, T., Reitz, A., 2004. Integrated rock magnetic and geochemical quantification of redoxomorphic iron mineral diagenesis in Late Quaternary sediments from the Equatorial Atlantic. In: Wefer, G., Mulitza, S., Ratmeyer, V. (Eds.), The South Atlantic in the Late Quaternary: reconstruction of material budgets and current systems. Springer-Verlag, Berlin/Heidelberg/New York/Tokyo, pp. 239–262). Samples with any indication of suboxic magnetite dissolution were excluded from the dataset. The parameters under study include carbonate, opal and terrigenous content, grain size distribution and clay mineral composition. Their bi- and multivariate correlations with the RPI signal were statistically investigated using standard techniques and criteria. While several of the parameters did not yield significant results, clay grain size and chlorite correlate weakly and opal, illite and kaolinite correlate moderately to the NRM/ARM signal used here as a RPI measure. The most influential single sedimentological factor is the kaolinite/illite ratio with a Pearson's coefficient of 0.51 and 99.9% significance. A three-member regression model suggests that matrix effects can make up over 50% of the observed RPI dynamics.

A reassessment of post-depositional remanent magnetism: preliminary experiments with natural sediments

Realignment of magnetic grains below the sediment–water interface is thought to impart a post-depositional remanent magnetization (pDRM). However, there is little convincing evidence in the published literature that pDRM is the dominant mechanism by which sediments become magnetized. We report here preliminary results from two kinds of laboratory experiments designed to investigate whether post-depositional reorientation of magnetic particles is likely to occur in nature. In the first experiment, we monitored changes in the magnetization of natural sediments in response to changing laboratory fields. Our results are inconsistent with post-depositional reorientation of magnetic particles. In a second experiment, we put live worms in a multi-core tube with the original sediment–water interface intact. Remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface. These pellets had been suspended by a worm, and redeposited in the laboratory field. The other samples, which were not resuspended, but nonetheless bioturbated by the worms, showed no change in magnetization. Our preliminary results do not support the hypothesis that post-depositional reorientation occurs in natural, undisturbed sediments below the sediment–water interface.

Studies of post-depositional remanent magnetization and their relevance to the palaeomagnetic record

Studies of post-depositional remanent magnetization and their relevance to the palaeomagnetic record