Depositional Remanent Magnetization Research Articles

Contrasting Recording Efficiency of Chemical Versus Depositional Remanent Magnetization in Sediments

AbstractHow and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols.

Holocene Paleosecular Variations Recorded by Relict Magnetic Minerals in the Anoxic Black Sea Sediments

Continuous paleosecular variations reconstructed from sedimentary archives have remarkably deepened our insights into the dynamics of the Earth's magnetic field as well as the chronological purpose. Nevertheless, to construct reliable sedimentary paleomagnetic records in diagenetic reducing sediments is challenging generally due to the pervasive magnetic mineral diagenesis. The relict magnetic minerals are residuals after diagenesis and probably able to record a depositional remanent magnetization, thus it is worthy to explore their paleomagnetic potentials. For this study, two Black Sea sediment cores covering the past 8 ka were subjected to mineralogical and paleo- and rock magnetic analyses. Paramagnetic pyrite framboids are pervasive in the studied sediments deposited under anoxic bottom water conditions in the Black Sea. In addition, relict magnetic minerals of ferrous hemoilmenite, Fe-Mn and Fe-Cr spinels, and magnetite inclusions are also present in the studied cores. Compared to the previous published paleomagnetic results from the same cores over the last glacial period (20-30 ka) which are dominated by detrital (titano-)magnetite particles, the studied relict magnetic mineral samples exhibit a similar behavior in recording the depositional remanent magnetization. Furthermore, the obtained paleosecular variations spanning the past 8 ka reproduce the high intensity patterns observed in the regional archeomagnetic and volcanic datasets. Thus, the successful reconstruction of paleomagnetic secular variations from the anoxic Black Sea sediments greatly extend the application of paleomagnetism in sediments deposited in water with a reducing sub-surface environments from where paleomagnetic data are generally sparse.

Detrital remanent magnetization of single-crystal silicates with magnetic inclusions: constraints from deposition experiments

SUMMARY Quasi-linear field-dependence of remanence provides the foundation for sedimentary relative palaeointensity studies that have been widely used to understand past geomagnetic field behaviour and to date sedimentary sequences. Flocculation models are often called upon to explain this field dependence and the lower palaeomagnetic recording efficiency of sediments. Several recent studies have demonstrated that magnetic-mineral inclusions embedded within larger non-magnetic host silicates are abundant in sedimentary records, and that they can potentially provide another simple explanation for the quasi-linear field dependence. In order to understand how magnetic inclusion-rich detrital particles acquire sedimentary remanence, we carried out depositional remanent magnetization (DRM) experiments on controlled magnetic inclusion-bearing silicate particles (10–50 μm in size) prepared from gabbro and mid-ocean ridge basalt samples. Deposition experiments confirm that the studied large silicate host particles with magnetic mineral inclusions can acquire a DRM with accurate recording of declination. We observe a silicate size-dependent inclination shallowing, whereby larger silicate grains exhibit less inclination shallowing. The studied sized silicate samples do not have distinct populations of spherical or platy particles, so the observed size-dependence inclination shallowing could be explained by a ‘rolling ball’ model whereby larger silicate particles rotate less after depositional settling. We also observe non-linear field-dependent DRM acquisition in Earth-like magnetic fields with DRM behaviour depending strongly on silicate particle size, which could be explained by variable magnetic moments and silicate sizes. Our results provide direct evidence for a potentially widespread mechanism that could contribute to the observed variable recording efficiency and inclination shallowing of sedimentary remanences.

Remagnetization of Red Beds on the Tibetan Plateau: Mechanism and Diagnosis

AbstractRed beds are important targets for paleomagnetic studies, yet discriminating secondary chemical remanent magnetization (CRM) from primary depositional remanent magnetization (DRM) in them remains challenging. Here we reanalyze the thermal demagnetization data of and conduct comprehensive rock magnetic, Mössbauer spectroscopic and petrographic studies on red beds from four Cenozoic basins on the eastern Tibetan Plateau (China): the Gongjue, Nangqian, Shanglaxiu, and Jinggu basins. The red beds in the latter two basins are remagnetized, as are most Nangqian red beds. The Gongjue red beds and some Nangqian red beds retain a DRM. Our results reveal that detrital (titano)magnetite and hematite are well preserved in red beds retaining the DRM, whereas remagnetized red beds contain large amounts of authigenic hematite and goethite with detrital Fe‐bearing minerals strongly altered. Postdepositional diagenetic alteration induced by heating and/or fluid circulation related to magmatism and/or crustal deformation is probably the main reason for the remagnetization. Hematite carrying the CRM in remagnetized red beds has wide distribution of grain size and unblocking temperature spectra, while hematite carrying the DRM is usually coarse and has confined unblocking temperature spectrum. This can be used as a criterion for diagnosing remagnetization. Nanoscale goethite appears to occur only in remagnetized red beds: another sensitive criterion for discriminating CRM from DRM. These property‐based criteria constrain the origin of the NRM in red beds better than the classic paleomagnetic field tests. Our research emphasizes that integrated rock magnetic, Mössbauer spectroscopic and petrographic studies provide robust tools to diagnose remagnetization in red beds.

Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts

AbstractThe magnetization of hematite‐bearing sedimentary rocks provides critical records of geomagnetic reversals and paleogeography. However, the timing of hematite remanent magnetization acquisition is typically difficult to constrain. While detrital hematite in sediment can lead to a primary depositional remanent magnetization, alteration of minerals through interaction with oxygen can lead to the postdepositional formation of hematite. In this study, we use exceptionally preserved fluvial sediments within the 1.1‐billion‐year‐old Freda Formation to gain insight into the timing of hematite remanence acquisition and its magnetic properties. This deposit contains siltstone intraclasts that were eroded from a coexisting lithofacies and redeposited within channel sandstone. Thermal demagnetization, petrography, and rock magnetic experiments on these clasts reveal two generations of hematite. One population of hematite demagnetized at the highest unblocking temperatures and records directions that rotated along with the clasts. This component is a primary detrital remanent magnetization. The other component is removed at lower unblocking temperatures and has a consistent direction throughout the intraclasts. This component is held by finer‐grained hematite that grew and acquired a chemical remanent magnetization following deposition resulting in a population that includes superparamagnetic nanoparticles in addition to remanence‐carrying grains. The data support the interpretation that magnetizations of hematite‐bearing sedimentary rocks held by >400‐nm grains that unblock close to the Néel temperature are more likely to record magnetization from the time of deposition. This primary magnetization can be successfully isolated from cooccurring authigenic hematite through high‐resolution thermal demagnetization.

Widespread occurrence of silicate‐hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording

AbstractMagnetic mineral inclusions occur commonly within other larger mineral phases in igneous rocks and have been demonstrated to preserve important paleomagnetic signals. While the usefulness of magnetic inclusions in igneous rocks has been explored extensively, their presence in sediments has only been speculated upon. The contribution of magnetic inclusions to the magnetization of sediments, therefore, has been elusive. In this study, we use transmission electron microscope (TEM) and magnetic methods to demonstrate the widespread preservation of silicate‐hosted magnetic inclusions in marine sedimentary settings. TEM analysis reveals detailed information about the microstructure, chemical composition, grain size, and spatial arrangement of nanoscale magnetic mineral inclusions within larger silicate particles. Our results confirm the expectation that silicate minerals can protect magnetic mineral inclusions from sulfate‐reducing diagenesis and increase significantly the preservation potential of iron oxides in inclusions. Magnetic inclusions should, therefore, be considered as a potentially important source of fine‐grained magnetic mineral assemblages and represent a missing link in a wide range of sedimentary paleomagnetic and environmental magnetic studies. In addition, we present depositional remanent magnetization (DRM) modeling results to assess the paleomagnetic recording capability of magnetic inclusions. Our simulation demonstrates that deposition of larger silicate particles with magnetic inclusions will be controlled by gravitational and hydrodynamic forces rather than by geomagnetic torques. Thus, even though these large silicates may contain ideal single‐domain particles, they cannot contribute meaningfully to paleomagnetic recording. However, smaller (e.g., silt‐ and clay‐sized) silicates with unidirectionally magnetized magnetic inclusions can potentially record a reliable DRM.

Microbially assisted recording of the Earth's magnetic field in sediment.

Sediments continuously record variations of the Earth's magnetic field and thus provide an important archive for studying the geodynamo. The recording process occurs as magnetic grains partially align with the geomagnetic field during and after sediment deposition, generating a depositional remanent magnetization (DRM) or post-DRM (PDRM). (P)DRM acquisition mechanisms have been investigated for over 50 years, yet many aspects remain unclear. A key issue concerns the controversial role of bioturbation, that is, the mechanical disturbance of sediment by benthic organisms, during PDRM acquisition. A recent theory on bioturbation-driven PDRM appears to solve many inconsistencies between laboratory experiments and palaeomagnetic records, yet it lacks experimental proof. Here we fill this gap by documenting the important role of bioturbation-induced rotational diffusion for (P)DRM acquisition, including the control exerted on the recorded inclination and intensity, as determined by the equilibrium between aligning and perturbing torques acting on magnetic particles.

Acquisition of chemical remanent magnetization during experimental ferrihydrite–hematite conversion in Earth-like magnetic field—implications for paleomagnetic studies of red beds

Hematite-bearing red beds are renowned for their chemical remanent magnetization (CRM). If the CRM was acquired substantially later than the sediment was formed, this severely compromises paleomagnetic records. To improve our interpretation of the natural remanent magnetization, the intricacies of the CRM acquisition process must be understood. Here, we contribute to this issue by synthesizing hematite under controlled ‘Earth-like’ field conditions (≲100 μT). CRM was imparted in 90 oriented samples with varying inclinations. The final synthesis product appeared to be dominated by hematite with traces of ferrimagnetic iron oxides. When the magnetic field intensity is ≳40 μT, the CRM records the field direction faithfully. However, for field intensities ≲40 μT, the CRM direction may deviate considerably from that of the applied field during synthesis. The CRM intensity normalized by the isothermal remanent magnetization (CRM/IRM@2.5 T) increases linearly with the intensity of growth field, implying that CRM could potentially be useful for relative paleointensity studies if hematite particles of chemical origins have consistent properties. CRM in hematite has a distributed unblocking temperature spectrum from ∼200 to ∼650 °C, while hematite with a depositional remanent magnetization (DRM) has a more confined spectrum from ∼600to680°C because it is usually coarser-grained and more stoichiometric. Therefore, the thermal decay curves of CRM with their concave shape are notably different from their DRM counterparts which are convex. These differences together are suggested to be a potential discriminator of CRM from DRM carried by hematite in natural red beds, and of significance for the interpretation of paleomagnetic studies on red beds.

An investigation of the magnetic carriers and demagnetization characteristics of the Gulang loess section, northwestern Chinese Loess Plateau

We report the results of a rock magnetic and paleomagnetic investigation of unit L9 of the Gulang (GL) loess section, in the northwestern Chinese Loess Plateau (CLP). We demonstrate that the dominant magnetic remanence carriers are coarse-grained pseudo single-domain/multidomain magnetite particles, and hematite. The application of either thermal (THD) or alternating-field demagnetization (AFD) yielded several normal polarity intervals interbedded with intervals of reversed polarity, but which cannot be correlated with standard geomagnetic polarity time scales. A greater number of normal polarity intervals, however, were evident after THD than after AFD. We propose that the normal polarities are either the result of overprinting of a viscous remanent magnetization (VRM) carried by coarse magnetite particles on the original depositional remanent magnetization (DRM); or that they represent genuine geomagnetic excursions (such as the Kamikatsura and Santa Rosa excursions) which are difficult to identify. In addition, we propose that in the case of the GL loess section, the effect of thermal demagnetization tails of a VRM is a possible reason why THD treatment yielded a greater number of normal polarity intervals than does AFD. However, reversed polarities obtained by either THD or AFD may be credible for sedimentary intervals with a relatively high content of hematite particles which preserve the primary DRM. Although numerous studies have proposed that THD provides more reliable paleomagnetic directions for loess sediments from the CLP, our results suggest that careful attention needs to be paid to the type of demagnetization method chosen, especially in the case of coarse-grained loess deposits.

Magnetostratigraphy of sediments from Lake El'gygytgyn ICDP Site 5011-1: paleomagnetic age constraints for the longest paleoclimate record from the continental Arctic

Abstract. Paleomagnetic measurements were performed on sediments drilled from ICDP Site 5011-1 in Lake El'gygytgyn (67°30' N, 172°05' E) located in Far East Russian Arctic. The lake partly fills a crater formed by a meteorite impact 3.58 ± 0.04 Ma ago. Sediments from three parallel cores (5011-1A, 5011-1B and 5011-1C), recovered from the middle part of the lake, yield a total of 355 m of sediment. Sediments are characterized by a variable lithology, where intervals of homogenous and laminated sediments alternate, and mass movement deposits occur frequently along the sediment profile. Mineral magnetic investigation made on sediments enclosed in core catchers suggests that magnetic carrier in these sediments is partly maghemitized Ti-rich pseudo-single domain magnetite. Its detrital origin can be shown by mineral magnetic measurements and SEM-EDS analyses performed on mini-sized cylindrical rock samples, polished rock sections and creek sediments. The intensity of the natural remanent magnetization in the sediments is high with a range from about 1 to 1000 mA m−1. Most of the sediments carry a stable magnetization interpreted as primary depositional remanent magnetization. Characteristic inclination data show alternating intervals of steep positive and negative inclinations that are used to assign magnetic polarity to the lake sediment profile. This is a rather straightforward procedure owing to the mainly high quality of data. The Matuyama/Gauss (M/G) (2.608 Ma) and Brunhes/Matuyama (B/M) (0.780 Ma) reversals were recognized in the sediments. The Mammoth and Kaena reversed subchrons were identified during the Gauss chron, and the Olduvai and Jaramillo normal subchrons as well as the Réunion and Cobb Mountain cryptochrons were identified during the Matuyama chron. Sediments also provide a record of the Olduvai precursor and Intra-Jaramillo geomagnetic excursions. Sediment deposition rate is highest at the base of the sequence laid down in the early Gauss chron, when the deposition rate is approximately 44 cm kyr−1. Sediment deposition decelerates upcore and it is an order of magnitude lower during the Brunhes chron in comparison with the early Gauss chron. Decrease in sediment deposition in the late Pliocene probably relates to atmospheric and oceanic reorganization heralding the onset of Quaternary climate change. The high-quality magnetostratigraphy reconstructed from Lake El'gygytgyn sediments provides 12 first-order tie points to pin down the age of the longest paleoclimate record from the continental Arctic.

The origin of magnetic remanence in stalagmites: Observations from electron microscopy and rock magnetism

Speleothems, especially stalagmites, hold great potential as recorders of the Earth's ancient magnetic field. However, our understanding of the magnetic mineral assemblages within speleothems is not well developed. We present tandem rock magnetic characterization of bulk samples and electron microscopy of magnetic extracts from five North American stalagmites. Magnetite and goethite are present in all samples, albeit in varying abundances and morphologies. Magnetite grains are likely detrital, showing evidence of transport and weathering, including plumose markings and etch pits. These grains are unlikely to have precipitated abiotically in the cave environment. Titanomagnetite and exsolved intergrowths of Fe‐Ti oxides indicate that a significant portion of the magnetic mineral assemblage is allochthonous and was transported into the caves. These Ti‐rich grains display a range of morphologies from euhedral to well rounded, indicating that earlier morphological models for speleothem magnetization do not apply in all geologic settings. Goethite was observed in three morphologies: isolated needles, needle aggregates, and polycrystalline aggregates of randomly oriented nanoscale grains. The magnetite and titanomagnetite, as well as their partially oxidized equivalents, likely hold a depositional remanent magnetization (DRM), whereas if goethite grains were precipitated in situ on the stalagmite drip surface, they are more likely to hold a chemical remanent magnetization (CRM) because of their small size and low saturation magnetization. Until the remanence of goethite can be shown to be paleomagnetically meaningful, we propose that paleomagnetic studies of speleothems include a 150°C thermal demagnetization step to remove any goethite remanence.

Paleomagnetism on the Quaternary marine sediment at the DH-1 long-core site in the Korean continental margin of the East Sea

A long core of 23.6 m was acquired in the Korean continental margin of the western East Sea. The core site of the DH-1 is located in the offshore of the Donghae City and the water depth is 357.8 m deep. In this area, the paleomagnetism and magnetostratigraphy were firstly reported using 420 samples collected from the long-core sediments. Based on the inclination distribution of the depositional remanent magnetization, the DH-1 core could be divided into two upper and lower units at the boundary of 1750 cm below seafloor. The upper unit is characterized by a positive polarity, whereas the lower unit by a negative polarity. The boundary of the upper and lower units was interpreted as the Brunhes-Matuyama boundary (778 ka). The chemical components of tephra layer at 2014 cm below seafloor belong to alkaline series, plotted between the tephra components of the Mount Baekdu and Ulleung Island.

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

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

Detrital magnetizations from redeposition experiments of different natural sediments

We carried out several experiments with carbonate-rich and clay-rich sediments that were redeposited in plastic cubes or in 1-meter-long cylindrical tubes with the aim of investigating the sensitivity of the depositional remanent magnetization (DRM) to various environmental and physical parameters. In contrast to previous studies, we did not observe any difference in the DRM acquired with or without saline water for either kind of sediment. Taking advantage of a gelatin that fixes the position of particles in suspension, we were able to measure the magnetization of suspended sediments within water-filled columns and thus to test the degree of alignment of magnetic grains. The magnetization of clay-rich sediments was not far from saturation but the magnetization was considerably reduced after deflocculation because the large flocs that favored alignment of magnetic grains were destroyed by the deflocculant. Similarly, the large flocs found at the bottom of the deposition tubes also reveal an efficient magnetic alignment. In contrast, the weak magnetization of the carbonate-rich sediments with small flocs yielded accurate and well-grouped magnetization directions that were not sensitive to deflocculation. Only a fraction of magnetic grains was mechanically oriented by the field in this case because of the weak net magnetic moments of the grains embedded within small flocs. The DRM was constrained by redeposition in plastic cubes performed without gelatin and with different field intensities. The DRM intensity of the carbonate-rich sediments is linearly related to field strength, which attests to their suitability for studies of relative paleointensity. Tests performed with mud from several stratigraphic levels in different marine sediment cores failed to reveal a significant influence of carbonate content on DRM, which suggests a relatively minor role of lithological changes on records of relative paleointensity.

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.

On the mechanism of formation of depositional remanent magnetization

Coagulation of particles into aggregates during their settling in an aqueous solution is numerically simulated with regard to Brownian motion, Van der Waals and Stokes's forces, gravitation, and magnetostatic interactions. Clusters obtained have a fractal structure with the average fractal dimension d = 1.83. Magnetic grains do not group until their concentration exceeds at least a few percent. The deposition process obeys a scaling principle: the sizes of clusters arriving at the bottom of a basin do not change if the product of the basin depth H and the concentration of initial material c0 is constant. Attempts at numerical simulations of laboratory redeposition experiments are made. Good agreement between numerical simulations and experimental results by van Vreumingen (1993) demonstrates that the modeling algorithm is based on reasonable physical assumptions. The magnetization of a flocculating suspension is defined by at least seven parameters, which characterize magnetic and nonmagnetic particles, as well as the aqueous medium. This multiparametric dependence hinders estimations of paleofield intensity by the redeposition method because it is practically impossible to reproduce natural conditions in the laboratory. Flocculation influences the magnetization intensity of the settling suspension at concentrations c0 typical for redeposition experiments or natural sedimentation in lakes and shallow seas. Flocculation is of minor importance for deep oceanic regions because of their extremely low sedimentation rate. However, factors like small‐scale turbulence and biotic processes are not taken into account by the model and may require modification of these conclusions. Also, a simple model of pDRM acquisition based on elastic and plastic properties of sediment slurry is proposed.

Temporal variation of magnetotactic bacterial communities in two freshwater sediment microcosms

Magnetotactic bacteria (MTB), which can mineralize nanosized magnetite or greigite crystals within cells, play important roles in biogeochemical processes, for example iron and sulfur cycling, and depositional remanent magnetization acquisitions. Despite decades of research, the knowledge of MTB distribution and ecology is still limited. In the present study, we investigated the temporal variation of MTB communities in freshwater sediment microcosms based on 16S rRNA genes and unifrac analyses. Two microcosms (MY8 and MY11) collected from two separate sites in Lake Miyun (Beijing, China) were analyzed. The majority of retrieved sequences belonged to alphaproteobacterial magnetotactic cocci in both microcosms (representing 64.29% of clones from MY8 and 100% of clones from MY11), whereas so-called 'Magnetobacterium bavaricum'-like MTB affiliated within Nitrospira phylum were exclusively found in microcosm MY8. Over a 3-month period, the temporal variation of MTB communities was evident in both microcosms. In addition, the phylogenetic discrepancy of MTB communities between two microcosms is more prominent than that of the same microcosm at different times, implying adaptation of MTB phylogenetic lineages to specific microenvironments. Among the physical-chemical parameters measured, a strong correlation was shown between nitrate and the main genetic variability of MTB communities, indicating that nitrate may influence the occurrence of MTB phylogenetic lineages in natural environments.

Are hydrodynamic shape effects important when modelling the formation of depositional remanent magnetization?

Recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column. Magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached, making remanence acquisition an inefficient process. Alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel, reducing the overall magnetic torque. Existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other. In the case of non-spherical flocs, the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column. Calculations will be presented that show the torque exerted on a non-spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque. Non-spherical flocs will, therefore, align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence.

Characterization of magnetic particles and magnetostratigraphic dating of shallow‐water carbonates in the Ryukyu Islands, northwestern Pacific

Abstract Characterization of magnetic particles, and magnetostratigraphic dating, of Holocene and Pleistocene shallow‐water carbonates (the Ryukyu Group) in the Ryukyu Islands was carried out to infer the age of reef formation. Magnetic particles from these strata are dominated by fine‐grained, single‐domain magnetite/maghemite (40–140 nm in length). The magnetite crystals exhibit the size and morphologies characteristic of the magnetite formed by magnetotactic bacteria. No large multidomain grains were seen in transmission electron microscopy observations. The widespread presence of single‐domain magnetite in the Ryukyu Islands suggests that bacterial magnetites carry depositional remanent magnetization, which is stable enough for magnetostratigraphic dating and thereby for elucidating the evolution of shallow‐water carbonates. The polarity‐reversal sequence seen in a core taken from the Ryukyu Group correlates with the timescale of the Matuyama chron, including the Jaramillo subchron, Kamikatsura or Santa Rosa Excursions, through the Brunhes chron. These magnetostratigraphic results imply that the time of reef initiation in the Ryukyu Islands of the northwestern Pacific was earlier than in the Great Barrier Reef of the southwestern Pacific, where it started after the Brunhes–Matuyama boundary.

Depositional remanent magnetization: Toward an improved theoretical and experimental foundation

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 flocs are small (several microns). There is a strong dependence of floc size on salinity particularly in low salinity environments. Floc size has a profound influence on the efficiency of DRM, hence low salinity environment may give results with poor reproducibility. The size of the floc 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 floc size distribution and settling times can be constrained.