Bacterial Magnetite Research Articles

DNA extraction using modified bacterial magnetic particles in the presence of amino silane compound

Magnetic particles produced by magnetic bacteria have been used to carry out magnetic separation of DNA. Separation was achieved using magnetite coated with 3-aminopropyltriethoxysilane, N-(trimethoxy-silylpropyl) isothiouronium chloride or 3-[2-(2-aminoethyl)-ethylamino]-propyltrimethoxysilane (AEEA). The DNA binding efficiency increased with the number of amino groups present on the silane compounds and was 14 fold higher than with untreated magnetite. Addition of AEEA to aqueous solutions containing coated magnetite increased efficiency due to co-condensation of DNA. From 10 8 Escherichia coli cells, 7.1 μg of DNA was recovered using 100 μg of magnetite. E. coli DNA extracted with modified bacterial magnetite was suitable for restriction enzyme digestion and polymerase chain reaction (PCR). Ten replicate samples of E. coli cells were extracted using an automated magnetic robot and 9.5 μg of DNA was extracted using 100 μg of modified bacterial magnetite and possessed a 1.94 absorbance ratio (260:280 nm).

Discovery of nanocrystalline botanical magnetite

Based on transmission electron microscopy and electron diffraction observations, we report the discovery of nanocrystalline botanical magnetite in iron-rich extracts from disrupted grass cells. The majority of the magnetite nanocrystals extracted from the grass plants display cubo-octahedral shapes, with a minority of hexagonal prism morphologies. In addition to the constrained morphologies, each group has a narrow size distribution typical of intracellular-boundary organized biomineralization processes responsible for bacterial magnetite. The smallest cubo-octahedral botanical nanocrystals (4 ± 1 nm) are an order of magnitude smaller than their bacterial counterparts. These botanical nanocrystals are self-organized in ordered, micrometer-sized agglomerates, distinct from magnetite strings in magnetotactic bacteria and similar to some pedogenic magnetite currently attributed to inorganic processes. We discuss the implications of our findings on the search for magnetite records of extraterrestrial and ancient terrestrial life, the origin of the terrestrial topsoil magnetite, and the potential for bioremediation using botanical magnetite.

Magnetic microstructure of bacterial magnetite by electron holography

A brackish lagoon at Itaipu, Brazil, contains magnetotactic bacteria with unusually large magnetite magnetosomes (lengths 100–200 nm). The micromagnetic structures of the magnetosomes from two different coccoid organisms from the lagoon have been determined by electron holography. The results are consistent with single-magnetic-domain structure in the elongated magnetosomes from one organism and metastable, single-magnetic-domain structure in the larger, more equi-axed, magnetosomes from the other organism. The results are consistent with theoretical predictions of the transition dimension between stable and metastable single-domain structure in magnetite.

Low‐temperature magnetic properties of pelagic sediments (Ocean Drilling Program Site 805C): Tracers of maghemitization and magnetic mineral reduction

Magnetic properties of pelagic sediments from the western equatorial Pacific (Ocean Drilling Program (ODP) Site 805C) have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth. A saturation isothermal remanent magnetization (Mrs) was given to the samples by applying a strong magnetic field (2.5 T) at 20 K after cooling both in the absence (zero field cooling, ZFC) and presence of a 2.5 T magnetic field (field cooling, FC). The thermal demagnetization of both MrsZFC and MrsFC was measured from 20 to 300 K. No pronounced Verwey transition was observed in the data from samples shallower than the iron redox boundary. Below the boundary the Verwey transition is visible for all samples studied. We interpret this behavior as reflecting low‐temperature oxidation (maghemitization) of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary. The low‐temperature oxidation may occur both at the water‐sediment interface and deeper in the suboxic zone of sediments as a result of bacteria‐mediated processes, including the reduction of nitrate compounds. MrsZFC and MrsFC thermal demagnetization curves of all samples diverge for the entire temperature range measured. The divergence may be caused by the low‐temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system. Together, maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low‐temperature properties of the sediment studied. As a result, low‐temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments. The changes in effective oxidation state below the iron redox boundary detected by low‐temperature methods may be useful in the study of some paleoenvironmental processes.

Diagenetic Dissolution of Biogenic Magnetite in Surface Sediments of the Benguela Upwelling System

Intense primary biologic productivity in the surface waters of the Benguela upwelling system provides a high supply of organic matter to the sea floor at the continental slope off Namibia and sustains extreme concentrations of magnetite producing bacteria in the top sediment layers. Biogenic magnetite is thus by far the dominant carrier of the magnetic signal in these deposits also because of a very minor input of terrigenous ferrimagnetic minerals. Reducing conditions in the sediment column cause a selective dissolution of the bacterial magnetite fraction just a few centimeters below the main mineralization horizon. This diagenetic process is documented in detail by high-resolution rock magnetic analyses and transmission electron microscopy. Concentration dependent and grain-size sensitive magnetic parameters, such as susceptibility, laboratory imparted remanences, and hysteresis data, reveal a significant drop in ferrimagnetic mineral content within the upper 10 cm of the sediments accompanied by a gradual downward coarsening of the ferrimagnetic mineral assemblage from primarily magnetic single-domain particles in the top centimeters to multi-domain grains in deeper strata. Electron microscope observations enable both an unequivocal identification of bacterial magnetite on the basis of shape and grain-size and to trace dissolution effects on the biogenic magnetic mineral component to depth.

Problems of core correlation, sediment source ascription and yield estimation in Ponsonby Tarn, West Cumbria, UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from 210Pb and 134Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a−1 and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a−1. These represent yields of 7·0 t km−2 a−1 and 39·8 t km−2 a−1, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.

Problems of core correlation, sediment source ascription and yield estimation in Ponsonby Tarn, West Cumbria, UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from 210Pb and 134Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a−1 and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a−1. These represent yields of 7·0 t km−2 a−1 and 39·8 t km−2 a−1, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.

Possible evidence for dissimilatory bacterial magnetite dominating the magnetic properties of recent lake sediments

Rock magnetic measurements, including low temperature susceptibility and Curie Temperature experiments, show that the most recent sediments from the Bay of Vidy, close to Lausanne on the north shore of Lake Geneva, Switzerland, are rich in superparamagnetic magnetite. The 137Cs dated increase in magnetite concentration coincides with the beginning of sewage de-phosphatization by the addition of iron chloride. Extracts of fine-grained magnetite, imaged by transmission electron microscopy, form amorphous masses visually comparable to those detected as the product of extracellular magnetite production by bacteria. This is the first indication that extracellular, dissimilatory magnetite may survive and make a dominant contribution to the magnetic properties of sediments. Final confirmation of the bacterial origin of the magnetite will depend on rRNA sequencing.

Oxygen and Iron Isotope Studies of Magnetite Produced by Magnetotactic Bacteria

A series of carefully controlled laboratory studies was carried out to investigate oxygen and iron isotope fractionation during the intracellular production of magnetite (Fe(3)O(4)) by two different species of magnetotactic bacteria at temperatures between 4 degrees and 35 degrees C under microaerobic and anaerobic conditions. No detectable fractionation of iron isotopes in the bacterial magnetites was observed. However, oxygen isotope measurements indicated a temperature-dependent fractionation for Fe(3)O(4) and water that is consistent with that observed for Fe(3)O(4) produced extracellularly by thermophilic Fe(3+)-reducing bacteria. These results contrast with established fractionation curves estimated from either high-temperature experiments or theoretical calculations. With the fractionation curve established in this report, oxygen-18 isotope values of bacterial Fe(3)O(4) may be useful in paleoenvironmental studies for determining the oxygen-18 isotope values of formation waters and for inferring paleotemperatures.

Magnetite from magnetotactic bacteria; size distributions and twinning

Other| December 01, 1998 Magnetite from magnetotactic bacteria; size distributions and twinning Bertrand Devouard; Bertrand Devouard Arizona State University, Departments of Geology and Chemistry/Biochemistry, Tempe, AZ, United States Search for other works by this author on: GSW Google Scholar Mihaly Posfai; Mihaly Posfai Search for other works by this author on: GSW Google Scholar Xin Hua; Xin Hua Search for other works by this author on: GSW Google Scholar Dennis A. Bazylinski; Dennis A. Bazylinski Search for other works by this author on: GSW Google Scholar Richard B. Frankel; Richard B. Frankel Search for other works by this author on: GSW Google Scholar Peter R. Buseck Peter R. Buseck Search for other works by this author on: GSW Google Scholar Author and Article Information Bertrand Devouard Arizona State University, Departments of Geology and Chemistry/Biochemistry, Tempe, AZ, United States Mihaly Posfai Xin Hua Dennis A. Bazylinski Richard B. Frankel Peter R. Buseck Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online ISSN: 1945-3027 Print ISSN: 0003-004X GeoRef, Copyright 2004, American Geological Institute. American Mineralogist (1998) 83 (11-12_Part_2): 1387–1398. https://doi.org/10.2138/am-1998-11-1228 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bertrand Devouard, Mihaly Posfai, Xin Hua, Dennis A. Bazylinski, Richard B. Frankel, Peter R. Buseck; Magnetite from magnetotactic bacteria; size distributions and twinning. American Mineralogist 1998;; 83 (11-12_Part_2): 1387–1398. doi: https://doi.org/10.2138/am-1998-11-1228 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract We studied intracellular magnetite particles produced by several morphological types of magnetotactic bacteria including the spirillar (helical) freshwater species, Magnetospirillum magnetotacticum, and four incompletely characterized marine strains: MV-1, a curved rod-shaped bacterium; MC-1 and MC-2, two coccoid (spherical) microorganisms; and MV-4, a spirillum. Particle morphologies, size distributions, and structural features were examined using conventional and high-resolution transmission electron microscopy. The various strains produce crystals with characteristic shapes. All habits can be derived from various combinations of the isometric {111}, {110}, and {100} forms. We compared the size and shape distributions of crystals from magnetotactic bacteria with those of synthetic magnetite grains of similar size and found the biogenic and synthetic distributions to be statistically distinguishable. In particular, the size distributions of the bacterial magnetite crystals are narrower and have a distribution asymmetry that is the opposite of the nonbiogenic sample. The only deviation from ideal structure in the bacterial magnetite seems to be the occurrence of spinel-law twins. Sparse multiple twins were also observed. Because the synthetic magnetite crystals contain twins similar to those in bacteria, in the absence of characteristic chains of crystals, only the size and shape distributions seem to be useful for distinguishing bacterial from nonbiogenic magnetite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Glacial to interglacial mineral magnetic and palaeoceanographic changes at Chatham Rise, SW Pacific Ocean

Core CHAT 1K from 3556 m depth east of New Zealand shows marked changes in magnetic susceptibility, correlating very well with δ 18O of foraminifera over the last 150 ka. Susceptibility is high during interglacial stages 1 and 5, and substages 5a to 5e are well marked. The susceptibility is shown to be enhanced during interglacials and is neither an artefact of glacial carbonate dilution nor due to an interglacial increase of terrigenous material. Mineral magnetic property measurements (ARM, SIRM, S ratio) also show the same well-defined glacial–interglacial variability, interpreted as an interglacial increase of single domain magnetite grains less than 0.1 μm in size. Examination by TEM of magnetic extracts reveals abundant cubic to octahedral grains of 0.02–0.2 μm size, often arranged in long chains characteristic of bacterial magnetite. This material is much more abundant in interglacial than glacial samples. It is suggested that during interglacials, with lower productivity (and organic carbon input to the bed), the oxic zone in the sediments which can be colonised by magnetotactic bacteria is thicker and therefore is colonised for longer by magnetite producers. This yields a climate-controlled stratigraphy which leads that of infaunal benthic foraminifera by no more than a few centimetres.

The magnetostratigraphy of Coniacian-Late Campanian chalk sequences in southern England

Magnetostratigraphic data are reported from the Coniacian to early Late Campanian chalk sequences at Culver Cliff and Scratchell's Bay on the Isle of Wight and Seaford Head in East Sussex, UK. Mean natural remanent magnetisation (NRM) intensity values for individual sample sites range from 0.0013 to 1.6008 mA/m, with an overall mean of 0.0165 mA/m. Reliable determination of the remanence of the very weakly magnetic units was achieved by carrying out replicate palaeomagnetic measurements on large volume samples, using a 2-G ‘wholecore’ cryogenic magnetometer. Palaeomagnetic measurements on other, standard-sized samples were made with a CCL ‘discrete sample’ cryogenic magnetometer. Incremental thermal and AF demagnetisation treatment was used to remove magnetic overprints and to isolate the characteristic remanent magnetisation (ChRM). A reliability classification scheme has been developed and applied in this study, to provide an objective assessment of the quality of the palaeomagnetic results. IRM acquisition experiments suggest that the dominant magnetic mineral in these sediments is magnetite, but mixtures of hematite and magnetite occur also. SEM and TEM studies of magnetic mineral extracts indicate the presence of magnetite and hematite, preserved as inclusions within silicate grains, together with abundant bacterial magnetite preserved as individual grains and chains in some samples. Samples containing the greatest proportions of bacterial magnetite generally have the highest NRM intensity. The composite magnetic polarity sequence, derived by combining the data from all three sections, is characterised by three principal magnetic polarity zones which can be correlated with geomagnetic Chrons C34n, C33r and C33n, respectively. The positions of the chron boundaries have been tied to macrofossil and nannofossil biostratigraphic zones in these sections. A series of previously undocumented short magnetic polarity sub-chrons has been identified within the longer magnetochrons. These have potential value for high-resolution stratigraphic correlations and significance for theories of geodynamo behaviour.

Organic carbon flux controls the morphology of magnetofossils in marine sediments

Magnetotactic bacteria produce chains of magnetite crystals within a cell. Bacterial magnetites have characteristic morphologies and sizes that are strictly biologically controlled. We examined morphologies of fossil bacterial magnetites (magnetofossils) preserved in Pacific deep-sea sediments and their relations to organic carbon fluxes. Isotropic crystals dominate magnetofossils in sediments in relatively oxidized conditions, and anisotropic crystals predominate in more reduced conditions. Our finding has important implications for biomineralization processes and demonstrates the potential of magnetofossil morphology as a paleoenvironmental indicator.

Early sediment diagenesis on the Blake/Bahama Outer Ridge, North Atlantic Ocean, and its effects on sediment magnetism

Sediment magnetic and geochemical studies of a suite of deep‐sea sediment cores from the Blake/Bahama Outer Ridge (BBOR), North Atlantic Ocean, have identified two current redox boundaries in surficial (Holocene), carbonate‐rich sediments over much of the BBOR. The upper Mn+4/Mn+2 redox boundary is associated with a spike in the concentration of solid‐phase Mn (as MnO2); the lower Fe+3/Fe+2 redox boundary is associated with a spike in the concentration of solid‐phase Fe (as goethite, αFeOOH). Over much of the BBOR, high sediment magnetic intensities occur in surficial, carbonate‐rich sediments associated with these redox boundaries and lower intensities occur in deeper (late Pleistocene) carbonate‐poor sediments. This relationship is opposite to that expected if sediment magnetism simply reflects the clastic (noncarbonate) sediment fraction. The surficial, high sediment magnetic intensities are due primarily to the following two factors: (1) magnetic mineral authigenesis associated with early diagenesis and (2) the presence of abundant <0.1 μm magnetite crystals interpreted to be bacterial magnetosomes. Magnetosomes are almost absent in the late Pleistocene low‐carbonate sediments owing, most likely, to local Pleistocene environmental conditions (high clastic flux, low organic flux) which did not favor their growth. The sediment natural remanent magnetization is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and the presence of abundant bacterial magnetite. If this is typical, bacterial magnetite may be more abundant in nature but less important to sediment paleomagnetic records, than previously thought.

Observations under electron microscopy of magnetic minerals extracted from speleothems

Magnetic minerals have been extracted from a selection of British speleothems and studied by electron microscopy and energy dispersive X-ray analysis (EDXA). Observations of the surface morphology, size and elemental composition of these grains has assisted in determining their origin. Thereby, direct evidence as to the carriers of the Natural Remanent Magnetization (NRM) of speleothems is provided. Three groups of grain morphology were identified: (1) detrital grains ranging in size from 0.01 μm to ⪢ 10 μm, composed of magnetite, hematite and titanomagnetite; (2) unabraded grains ranging in size from 0.01 μm to 0.1 μm, composed of magnetite, these grains were further subdivided into those having an affinity with bacterial magnetites and those having an affinity with inorganic authigenic magnetites; (3) needle-like grains, less than 2 μm in length, probably composed of goethite. Preliminary laboratory experiments suggest that authigenic magnetite and goethite may precipitate under conditions likely to prevail during speleothem growth. The presence of detrital magnetic grains in all samples studied is strong evidence for the presence of a Detrital Remanent Magnetization (DRM) in all speleothems. The presence of authigenic magnetite in some samples is the first direct evidence for the presence of a Chemical Remanent Magnetization (CRM) in some speleothems.

Quantitative extraction and analysis of carriers of magnetization in sediments

We have applied an integrated procedure for quantitative magnetic mineral extraction, based on the separation method of Petersen, von Dobeneck & Vali (1986), to a range of sediment types, to examine the efficiency and representative nature of the extraction process. Carriers of magnetization have been identified by rock magnetic measurements, microscopy and X-ray diffraction. Quantification of the extraction efficiencies is achieved by before- and after-extraction rock magnetic measurements (susceptibility, anhysteretic and isothermal remanences). These magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types (e.g. over 75 per cent for magnetite-dominated sediments). The extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains. Susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin. The amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction. In terms of the mineralogies extracted, we identify a diverse and complex range of mineral assemblages. All sizes of discrete grains of magnetite are extracted (including single-domain and superparamagnetic grains, and chains of bacterial magnetite). Other commonly extracted iron and titanium oxides are haematite and ilmenite. Ferrimagnetic chromites and sulphides were also obtained from some samples. Considerable amounts of quartz and feldspar are extracted, due to the presence of magnetic inclusions within these diamagnetic host grains. In the deep-sea sediments we examined, feldspars constitute a large proportion of the extracts, but are significantly less abundant in other sediments, where quartz is dominant. A wide variety of paramagnetic minerals was identified in the extracts, including pyroxenes, amphiboles, chlorites, micas, Mg–Cr-spinels, garnets, Ti-oxides, apatites, tourmaline and zircon, many of which contain ferrimagnetic inclusions, possibly less than 0.1 μm in grain size. Dissolution of ultrafine grains of magnetite during pre-extraction carbonate dissolution, as suggested by Sun & Jackson (1994), does not occur in our samples.

Application of bacterial magnetic particles as novel DNA carriers for ballistic transformation of a marine cyanobacterium

Bacterial magnetite particles (BMPs) of 50 to 100nm diam were used as DNA carriers for the ballistic transformation of the marine cyanobacteriumSynechococcus. BMPs were bombarded into the cyanobacterial cells at several bombardment velocities using a particle gun. Successful transformation and gene expression were confirmed by Southern hybridization and CAT assay, respectively. The BMPs were also observed in the cyanobacterial cells by transmission electron microscopy. These results suggested that BMPs can be used as carriers for introducing DNA into bacterial cells.

Magnetic properties of biogenic soil greigite (Fe3S4)

Magnetic measurements were performed on soil samples, which contained bacterial greigite. The greigite‐containing soil horizon is characterized by SIRM/χ > 70 kA m−1 and by Q‐factors >5. Alternating field (AF) demagnetization curves of the NRM gave stable directions. According to IRM acquisition and alternating field demagnetization of the IRM and ARM, the greigite in bulk soil samples shows single domain behaviour. This SD behaviour is also confirmed by hysteresis and by frequency‐dependent susceptibility measurements. The thermomagnetic behaviour was checked on extracted greigite. The magnetic parameters of soil greigite are compared with parameters of sedimentary greigite and of bacterial magnetite.

Bacterial magnetite and the magnetic properties of sediments in a Swedish lake

A comparison of the magnetic properties of lake sediments and catchment materials from Pajep Njakajaure (north Sweden) demonstrates that high concentrations of single-domain (SD) magnetite in the upper 70 cm of gyttja in the deepest part of the basin are not of detrital origin. TEM and light microscope examination revealed the existence of bacterial cells and magnetosomes in the upper sediments which are responsible for the presence of the SD magnetite. Most of the fossil magnetosomes appear to have been dissolved through reductive processes at lower depths, although some are preserved in the lower sediments and carry a post-depositional remanent magnetization (PDRM). The results highlight the complex effects of post-depositional processes on mineral magnetic assemblages and natural remanent magnetization aquisition, and demonstrate how mineral magnetic measurements may be used to identify magnetic grain-size changes brought about by sediment diagenesis.

Magnetic Enhancement and Palaeoclimate: Study of A Loess/Palaeosol Couplet Across the Loess Plateau of China

SUMMARY The origin of the Milankovitch palaeoclimatic signal so dramatically recorded in the Chinese loess remains uncertain. At least five hypotheses have been put forward to explain the well-documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter. We report here a magnetic study of a prominent loess-soil couplet (L4-S3) at three widely separated sites where present-day mean annual rainfall and temperature range from ∼350mm to ∼700mm and from ∼6°C to ∼13°C. the properties determined are laboratory remanences (isothermal, IRM; anhysteretic, ARM; viscous, VRM, including viscous decay), room-temperature susceptibility, and thermomagnetic behaviour. At fields beyond 100 mT, the IRM spectra are all essentially identical, both in shape and in absolute magnitude, suggesting a uniform magnetic component widespread across the Plateau. Subtracting the signal of this component isolates a second component, the amount of which varies by an order of magnitude between sites. the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils. Viscous decay of VRM takes place in two distinct stages, one of which falls to zero in < 10min. We attribute magnetic enhancement-which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite, which we characterize in our interpretation as two separate forms of bacterial magnetofossils. the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material.