摘要:

Sediment palcomagnetism has made important contributions to magnetostratigraphy, tectonic reconstructions, and studies of geomagnetic field behavior. Interpretations of the sedimentary palcomagnetic record depend critically on when and how the natural remanent magnetization (NRM) is acquired and what postdepositional modifications have occurred. This special section of theJournal of Geophysical Researchis a collection of 16 papers that address recent advances in our understanding of the physical, chemical, and biological processes which affect the acquisition and alteration of the NRM in sediments. The issue is an outgrowth of a special symposium on magnetization processes in sediments that was convened at the 1987 Spring AGU Meeting in Baltimore, Maryland.

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04353

年代:1990

数据来源: WILEY

摘要:

Biogenic magnetites are produced through the reduction of ferric iron by both biologically induced (extracellular) and biologically controlled (intracellular) processes. With few exceptions, all are ultra‐fine‐grained, single‐domain magnetite. Biogenic magnetites formed by magnetotactic bacteria (biologically controlled) have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones, hemipelagic and deep‐sea marine sediments. The input into sediments of ultra‐fine‐grained magnetite produced by dissimilatory iron reducing bacteria (biologically induced) has yet to be firmly established but may be even more significant. Whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation, reduction by substitution, and dissolution. Unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques. It is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual, and in some cases unique, morphologies, size range, and composition of the magnetite crystals. Magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range, but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites. The presence of dissimilatory iron reducing bacteria, however, can be detected using microbiological techniques and sediment geochemistry. Biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments, both modern

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04355

年代:1990

数据来源: WILEY

摘要:

Magnetic separates and rock magnetic tests from Holocene carbonate surface sediments of the Bahamas indicate that fine‐grained magnetite/maghemite is common at or near the sediment/water interface and forms a major portion of the magnetic fraction. Magnetite crystals range from 400 to 1000 Å in diameter as observed under the transmission electron microscope (TEM) and often occur as elongate chains or multi‐grain clusters. Crystals are very similar in dimension and grain habit to those of known biogenic origin, mainly bacteria. All measured grains are within the single‐domain stability field for magnetite based on crystal dimensions. Coercivity distributions are fairly broad (5–100 mT or greater), likely the result of partial oxidation of the magnetic minerals. No large multidomain grains were observed in the TEM study. Individual grains occur in several shapes in the microscope image, mainly hexagonal and cuboidal/prismatic. Widespread occurrence of single‐domain magnetite in this isolated carbonate setting suggests that biogenic precipitation is a major contributor of early in situ depositional magnetization. A contribution of magnetite from authigenic precipitation during burial has not been documented in these near‐surface

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04363

年代:1990

数据来源: WILEY

摘要:

Unfossiliferous pelagic clay (red clay) cores from the Pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface. The origin of this instability has been considered to be viscous remanent magnetization (VRM). We conducted a rock‐magnetic study of a pelagic clay core from the South Pacific, which has the unstable‐to‐stable transition at about 1.5 m below the surface. Using the suspension method of Yoshida and Katsura (1985), we determined downcore variation of complete alignment magnetization (CAM) and magnetic moment distribution (geometric mean moment, mG, and log standard deviation, loga) assuming a lognormal distribution. Mean magnetic grain diameter was calculated from the mGusing the saturation magnetization of magnetite, 4.8×105A M−1, on the assumption that all the magnetic grains are single‐domain magnetites of spherical shape. This assumption is supported by thermomagnetic analyses, isothermal remanent magnetization (IRM) acquisition experiments, and the ratio of CAM to saturation IRM The grain diameter ranges from about 0.02 to 0.15 μm and decreases with depth below about 1.5 m. The derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite. The frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains. On the other hand, the magnetic viscosity acquisition coefficient normalized by the CAM increases with depth. We conclude that magnetic grain size can control the magnitude of the secondary magnetization, VRM, of pelagic clay. It has been proposed that the major source of pelagic clay is atmospherically transported dust. Intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component, which can cause the unstable‐to‐stable transition of the remanen

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04373

年代:1990

数据来源: WILEY

摘要:

We studied the detrital remanent magnetization of laboratory redeposited lake sediments, previously shown to be excellent recorders of the paleomagnetic field. The redeposited and original/natural sediments have indistinguishable stabilities to alternating fields (AF) demagnetization and remanence intensities, indicating that their remanences reside in the same magnetic particles. Thermomagnetic behavior, hysteresis parameters, and relatively high AF stabilities of the detrital and anhysteretic remanences are essentially identical throughout the sediment column, suggesting uniform distributions of submicron magnetite grains. The declination is accurately recorded, but inclination shallowing averaging 18° is observed for routinely hand‐stirred, rapidly redeposited sediments. However, sonically disaggregated sediments settle more slowly with anomalous shallowing averaging 7°, decreasing upward in the redeposited column. These results indicate that thorough disaggregation of the sediment reduces the magnitude of the inclination shallowing probably by more efficiently isolating the magnetic grains from the matrix, thus diminishing the dominance of the gravitational over geomagnetic aligning torques during deposition. The progressive decrease of the inclination shallowing upward in the sediment column might be due to a parallel upward increase in the fraction of isolated magnetic particles, which undergo Brownian motion and alignment by the geomagnetic field. The dominance of gravitational over geomagnetic aligning torques during deposition might be a cause for inclination shallowing in some coarser sediments. In contrast, inclination shallowing would be minimized for slow deposition of isolated submicron magnetite grains during natural sedimentat

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04383

年代:1990

数据来源: WILEY

摘要:

It is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone. However, anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains; this in turn may reflect the development of the magnetic fabric of the sediment over time, so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence. In this work, we make a study of the anisotropy of magnetic susceptibility (AMS) in Siwalik red beds of Miocene age from northern Pakistan. Comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it, a coherent magnetization. Furthermore, it should be possible to use AMS fabric information to quantify the degree of pedogenesis in these Miocene soils. We attempted to determine the anisotropy of isothermal remanence (AIR) but found AMS to be the technique of choice because of apparent changes in coercivity during AIR experiments. We interpret the AIR data as resulting from metastable domains in hematite grains which change domain state during the AIR experiment.

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04391

年代:1990

数据来源: WILEY

摘要:

The magnetochemistry of sediments from the Oregon continental margin is examined to determine the effects of iron‐sulfur diagenesis on the paleomagnetic record. Magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites. These processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities, systematic changes in other rock magnetic properties, and increases in solid phase sulfur concentrations with depth. Hysteresis measurements are used to evaluate downcore changes in magnetite concentration and grain size. Magnetite abundances decrease downcore from initial values of about 0.1%, and nominal grain diameters lie within a narrow pseudosingle domain range of 0.08 to 0.6 μm. A first‐order surface area reaction model,dA/dt= ‐kA, is proposed to explain the magnetite dissolution mechanism, whereAis the total magnetite surface area andkis the rate constant. The solution of this equation predicts that the surface area and concentration decrease exponentially, and the concentration, in addition, depends on grain size. Application of this model in two cores where grain size varies with depth successfully explains the downcore profiles of both concentration and surface area. Despite extensive magnetite destruction, magnetic directions in such sediments appear to reliably record long‐wavelength trends of the geomagnet

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04405

年代:1990

数据来源: WILEY

摘要:

The effects of iron diagenesis on paleomagnetism and rock magnetism were examined in suboxic sediments from Bettis Site W‐N in the NE Pacific Ocean. As part of the oxidative decomposition of organic matter, ultrafine‐grained authigenic magnetites are produced immediately above the iron reduction zone. This zone is commonly observed in deep‐sea sediments as a prominent color change from brown to greenish grey. The magnetite authigenesis is evidenced by systematic increases in natural magnetic intensities and stabilities and by comparable shifts in other rock magnetic properties. The magnetites are composed of relatively pure Fe3O4and fall within a restricted size range of 0.06–0.12 μm, which is nominally single domain. Some of these finest‐grained magnetites are rapidly destroyed during burial upon entering the zone of iron reduction, leaving a relatively coarser population. Magnetite dissolution appears to continue with depth according to a first‐order surface area reaction process when at steady state. Iron reduction is independent of sulfide formation in these sediments, because solid Sulfur profiles show no increase at depth and most of the sulfur occurs as barite. Despite a well‐constrained carbonate stratigraphy, comparisons of paleomagnetic directions between nearby cores show little agreement, suggesting that the directions are composed of a complex mixture of remanences acquired at different depths. Subsurface magnetic peaks may represent paleoredox markers preserved by non‐steady state redox conditions caused by climatically induced variations in organic matter flux and sed

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04421

年代:1990

数据来源: WILEY

摘要:

The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest‐grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyr

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04437

年代:1990

数据来源: WILEY

摘要:

Solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical, magnetic, and X‐ray diffraction techniques in three California continental borderland basins. The observed profiles have been fit by simple reaction‐diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur. More than 95% of the solid phase reduced sulfur is pyrite, and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth. Net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent. Depth integrated sulfate reduction rates for modern sediments in San Pedro, Santa Catalina, and San Nicolas Basins are, 11.4, 6.3, and 6.3–8.8 (μmol cm‐2yr‐1), respectively. Measurements of solid phase iron species indicate that surficial sediments are enriched in easily‐reducible ferric oxyhydroxides. The enrichment is maintained by a combination of oxidation of Fe2+diffusing upward from underlying anoxic sediments, as well as input of fresh sediment enriched in ferric oxyhydroxides. The three primary sources for iron converted to pyrite and the sequence in which they are utilized are: ferric oxyhydroxides, magnetite and other crystalline oxides, and “exchangeable” iron in phyllosilicates. The majority (50–80%) of the iron converted to pyrite is from the silicates, and budgetary calculations indicate the amount of iron released from San Pedro basin silicates agrees within 35% with the amount of magnesium removed from pore water to solid phases. Fe2+is enriched in near‐surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation. At depth, pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates. Sulfide produced at depth diffuses upward until it reaches sediments with available iron, causing a step‐like increase in solid phase sulfur concentration. Over 90% of the magnetite present in surficial sediments is dissolved at depth due to reaction with H2S. A model is developed to predict the depth at which magnetite dissolution should occur, based on sulfate reduction rates and the flux of ferric oxyhydroxides. The results of this model predict the onset of dissolution at depths of 5–40 cm in different basins and agree well with the observed depths

ISSN:0148-0227    

DOI:10.1029/JB095iB04p04453

年代:1990

数据来源: WILEY