Browsing by Author "Banerjee, Subir"
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Item Contributions of fine‐particle magnetism to reading the global paleoclimate record (invited)(Journal of Applied Physics (American Institute of Physics), 1994) Banerjee, SubirPaleoclimate changes are recorded by proxy as variations in concentration, composition, and grain size of magnetic minerals, principally magnetite (Fe3O4), in the sediments deposited in lakes,oceans, and continental eolian deposits. Cross‐validated multiple‐parameter magnetic measurements of such sediment cores provide global change data of high temporal resolution, useful for constructing a base‐line record against which anthropogenic modifications may be discerned. Theories of superparamagnetism and magnetic domains are used to explain the physical basis of magnetic proxy recording. Examples of applications to validation of Milankovitch theory of climate change and delineation of the glacial and interglacial stages of the last 1 000 000 years are provided.Item Effect of hydrostatic pressure on viscous remanent magnetization in magnetite-bearing specimens(Geophyical Research Letters (American Geophysical Union), 1995) Kelso, Paul; Banerjee, SubirRocks at depth in the crust acquire a viscous (i.e., time-dependent) magnetization under the pressure-temperature conditions at which they reside. There have been numerous studies on the effect of temperature on viscous magnetization but little work has been performed on the effect of hydrostatic pressure. We have studied viscous remanent magnetization at 22°C in a 0.1 mT field at 0.1 and 100 MPa for natural and synthetic multidomain magnetite. The viscous remanent magnetization was found to increase nearly linearly with log(time) at both pressures. There was little change in the rate of viscous acquisition between 0.1 and 100 MPa over the multidomain grain size range studied. Thus for rocks buried at depth in the earth the enhancement of magnetic viscosity by thermal fluctuations will dominate over effects due to hydrostatic pressure.Item The effect of low‐temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites: A case for acquisition of chemical remanent magnetization(Journal of Geophysical Research (American Geophysical Union), 1991) Kelso, Paul; Banerjee, Subir; Worm, HorstItem Experimental deformation of synthetic magnetite-bearing calcite sandstones: effects on remanence, bulk magnetic properties, and magnetic anisotropy(Journal of Geophysical Research (American Geophysical Union), 1993) Jackson, Mike; Borradaile, Graham; Hudleston, Peter; Banerjee, SubirItem Grain sizes of susceptibility and anhysteretic remanent magnetization carriers in Chinese loess/paleosol sequences(American Geophysical Union, 2004) Liu, Qingsong; Banerjee, Subir; Jackson, Mike; Maher, Barbara; Pan, Yongxin; Zhu, Rixiang; Deng, Chenglong; Chen, FahuItem IRM Newsletter, Volume 01, Number 1 (Spring 1991). Cover article: IRM Established(University of Minnesota. Institute for Rock Magnetism, 1991) Banerjee, SubirItem IRM Quarterly, Volume 03, Number 1 (Spring 1993). Cover article: A Rock-Magnetic Journey through Asia(University of Minnesota. Institute for Rock Magnetism, 1993) Banerjee, SubirItem IRM Quarterly, Volume 05, Number 1 (Spring 1995). Cover article: IRM is Fulfilling its Educational Mission(University of Minnesota. Institute for Rock Magnetism, 1995) Banerjee, SubirItem IRM Quarterly, Volume 14, Number 1 (Spring 2004). Cover article: Santa Fe VI(University of Minnesota. Institute for Rock Magnetism, 2004) Jackson, Mike; Banerjee, SubirItem IRM Quarterly, Volume 16, Number 2 (Summer 2006). Cover article: Santa Fe VII(University of Minnesota. Institute for Rock Magnetism, 2006) Banerjee, Subir; Moskowitz, BruceItem IRM Quarterly, Volume 20, Number 1 (Spring 2010). Cover article: Interpretation of Low-Temperature Data Part II: The Hematite Morin Transition(University of Minnesota. Institute for Rock Magnetism, 2010) Bowles, Julie; Jackson, Mike; Banerjee, SubirItem IRM Quarterly, Volume 21, Number 4 (Winter 2011). Cover article: Interpretation of Low-Temperature Data Part V: The Magnetite Verwey Transition (Part B): Field-Cooling Effects on Stoichiometric Magnetite Below TV(University of Minnesota. Institute for Rock Magnetism, 2011) Jackson, Mike; Bowles, Julie; Banerjee, SubirItem IRM Quarterly, Volume 22, Number 2 (Summer 2012). Cover article: Interpretation of Low-Temperature Data Part VI: The MagnetiteVerwey Transition (Part C): Low-Temperature Demagnetization of Stoichiometric Magnetite(University of Minnesota. Institute for Rock Magnetism, 2012) Bowles, Julie; Jackson, Mike; Banerjee, SubirItem IRM Quarterly, Volume 24, Number 4 (Winter 2014-2015). Cover article: A new basis for the SI system of units: occasion to reconsider the presentation and teaching of magnetism(University of Minnesota. Institute for Rock Magnetism, 2014) Stacey, Frank; Moskowitz, Bruce; Jackson, Mike; Dunlop, David; Ӧzdemir, Ӧzden; Banerjee, Subir;Item IRM Quarterly, Volume 25, Number 3 (Fall 2015). Cover article: The IRM at 25: A Quarter Century of Community-Based Research and Education(University of Minnesota. Institute for Rock Magnetism, 2016-03) Moskowitz, Bruce; Banerjee, Subir; Feinberg, Joshua; Jackson, Mike; Solheid, Peat; Bilardello, DarioItem Low-temperature magnetic properties of siderite and magnetite in marine sediments(Geophysical Research Letters (American Geophysical Union), 1996) Housen, Bernard; Banerjee, Subir; Moskowitz, BruceLow temperature magnetic techniques provide useful tools to detect the presence of magnetite and pyrrhotite in sediments through identification of their low temperature transitions, to determine the amount of ultrafine-grained (superparamagnetic) material in sediments, and can potentially detect the presence of certain types of magnetotactic bacteria. Application of these types of experiments to nannofossil chalks from beneath the Barbados accretionary prism led to some unusual results, which are attributed to the presence of siderite. Thermal demagnetization of low-temperature remanence after cooling in zero field and in a 2.5 T field both displayed large remanence losses from 20 K to 40 K. Below 40 K, the magnetization of the chalks was much higher in the field-cooled experiments than in the zero-field-cooled experiments. Low temperature hysteresis experiments, made after cooling in a 2.5 T field, displayed offsets in magnetization parallel to the direction of the initial applied field, when measured below 40 K. The offset loops can be due to either an exchange anisotropy between siderite and magnetite phases in the sediments, a defect moment in the siderites, or a canted moment in the siderites. Apparent similarity between the low-temperature thermal demagnetization results from these siderite-bearing sediments, pure siderite, and pure rhodochrosite samples and the well-known 34 K transition in pyrrhotite should lead to caution in identification of pyrrhotite in marine sediments based on low-temperature remanence studies alone.Item Magnetic stratigraphy of Chinese loess as a record of natural fires(Geophysical Research letters (American Geophysical Union), 1995) Kletetschka, Gunther; Banerjee, SubirMagnetic susceptibility records of paleosols and loess show high correlation with oxygen-isotope stratigraphy of ocean sediments [Kukla, 1987], providing a global paleoclimatic record. Different models have been put forth to explain the nature and cause of susceptibility variations, but consensus has not yet been achieved. Our low-temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher Verwey transition (115K) than that for the magnetite (100 K) in unaltered loess. The same shift in the Verwey transition can be achieved by heating and cooling loess samples. This is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past. Natural fire intensity is sensitive to the amount of annual precipitation, so that increased fire-induced susceptibilities should reflect an increase in the humidity of regional climate.Item Magnetite dissolution in deep sediments and its hydrologic implication: a detailed study of sediments from site 808, leg 131(Journal of Geophysical Research (American Geophysical Union), 1994) Lu, Ran; Banerjee, SubirOur previous study of deep sea sediments from site 808, leg 131 of the Ocean Drilling Program has demonstrated that the two anomalously low-intensity zones of natural remanent magnetization (NRM), 675 to 925 and 1080 to 1243 m below seafloor, were caused by unusually low magnetite content in the sediment sections and that this was not related to the variations in sediment sources, calcite dilution, or magnetite destruction occurring in the top sediment layer during early diagenesis. For an explanation, we now suggest that these low INRM intensity zones were produced by catagenesis at great depth by organic matter decomposition in sediments, which in turn causes magnetite dissolution and hence a lower magnetite content. A similar process also applies to manganese ions; as a consequence, iron and manganese concentration, grain size and content of magnetite, and NRM intensity all decrease, whereas sulfur content increases in these sediments. Our interpretation is also supported by other studies of organic geochemistry and sedimentology of these same sediments. We suggest, therefore, that detailed rock magnetic and geochemical tests should be carried out before geomagnetic field variations are studied using ocean sediments from such great depth. Additionally, we suspect that catagenesis may have been retarded and magnetic degradation was prevented near a décollement due to cold water percolation.Item Millennial-scale climatic change during the last interglacial period: Superparamagnetic sediment proxy from paleosol S1, western Chinese Loess Plateau.(Geophysical Research Letters (American Geophysical Union), 1999) Fang, Xiaomin; Li, J.-J.; Banerjee, Subir; Jackson, Mike; Oches, E.A.; Van der Voo, RobDetailed magnetic analyses of samples from the Jiuzhoutai well section on the western Chinese Loess Plateau reveal high-resolution signals of summer monsoon change. The last interglacial period is represented here by the 8-meter thick S1 paleosol complex. We have used low-temperature remanence studies to analyze variations in the concentration of Superparamagnetic (SP) grains, which previous studies have shown to be largely of pedogenic origin. The SP concentration, interpreted as a proxy for the extent of pedogenesis, shows millennial scale variations within S1. We conclude that the last interglacial period in Asia was characterized by rapid climate fluctuations, with at least one brief return to near-glacial conditions in the middle of oxygen-isotope substage 5e.Item A new high-resolution geomagnetic paleointensity record for the North American Holocene: A comparison of sedimentary and absolute intensity data(Journal of Geophysical Research (American Geophysical Union), 2000) Brachfeld, Stefanie; Banerjee, SubirA new high-resolution paleointensity record for North America has been constructed using Holocene sediments from Lake Pepin, Minnesota. Lake Pepin sediments yield the same Holocene paleosecular variation curve as nearby Lake St. Croix and satisfy all of the criteria recommended for paleointensity studies. Absolute paleointensity data for North America recorded by Holocene volcanic and archeomagnetic samples provide an independent record of geomagnetic field paleointensity against which the relative intensity records from Lake Pepin and Lake St. Croix can be compared. Since the absolute field paleointensity is known a priori, the effects of the magnetic recording assemblage can be isolated. Anhysteretic remanent magnetization (ARM) is the best choice among normalization parameters for the Lake Pepin sediments as the natural remanent magnetization (NRM) normalized by ARM shows no coherence with magnetic grain size proxies and yields a record of relative paleointensity peaks and troughs whose amplitudes are very similar to those in the archeomagnetic (ARCMAG) and Lake St. Croix data sets. Features with a wavelength of 1000 years are correlative between the three paleointensity records. NRM normalized by saturation isothermal remanent magnetization (SIRM) and by susceptibility (χ) shows grain-size dependences that cause errors in the amplitudes of paleointensity features. NRM/SIRM and NRM/χ are not coherent with their normalizers but are both strongly coherent with independent grain size proxies such as the median destructive field of the NRM and HCR. We successfully removed the grain size dependences from NRM/SIRM by applying a correction function based on the linear relationship between normalized intensity and the median destructive field of the NRM.