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Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery

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A rock magnetic study was performed on sediment cores from four sites in the South Atlantic off the western coast of Africa, which were taken during the Ocean Drilling Program Leg 175 (Sites 1078, 1082, 1084, and 1085). The sites are within the Angola-Namibia upwelling system, and the sediments have a high total-organic-carbon content. Concentration of ferrimagnetic minerals at these sites is very low, and the magnetic susceptibility is dominated by paramagnetic and diamagnetic minerals. Severe and rapid loss of remanent magnetization occurred during storage of the cores, with less than 10% of the initial intensity remaining a few months after core recovery. The loss of magnetization may prevail in organic-rich sediments. Changes of magnetic properties with time were examined using samples that were kept frozen before the experiment. Hysteresis parameters and the ratio of ARM (anhysteretic remanent magnetization) to SIRM (saturation isothermal remanent magnetization) indicate increases in the average magnetic grain size with the decay of magnetization, which suggests preferential dissolution of finer magnetic minerals. Loss of low-coercivity magnetic minerals with time was estimated from the decrease of S ratios. Low-temperature magnetometry revealed the presence of magnetite in the sediments even after the completion of sulfate reduction. Magnetization attributable to magnetite decreased with the loss of magnetization. This suggests the transformation of magnetite into non-magnetic phases, which is consistent with the decrease of S ratios.


  1. Banerjee, S. K., J. King, and J. A. Marvin, Rapid method for magnetic granulometry with applications to environmental studies, Geophys. Res. Lett., 8, 333–336, 1981.

  2. Bloemendal, J., J. W. King, F. R. Hall, and S.-J. Doh, Rock magnetism of Late Neogene and Pleistocene deep-sea sediments: relationship to sediment source, diagenetic processes, and sediment lithology, J. Geophys. Res., 97, 4361–4375, 1992.

  3. Canfield, D. E. and R. A. Berner, Dissolution and pyritization of magnetite in anoxic sediments, Geochim. Cosmochim. Acta, 51, 645–649, 1987.

  4. Canfield, D. E., R. Raiswell, and S. Bottrell, The reactivity of sedimentary iron minerals toward sulfide, Amer. J. Science, 292, 659–683, 1992.

  5. Channell, J. E. T. and B. Lehman, The last two geomagnetic polarity reversals recorded in high-deposition-rate sediment drifts, Nature, 389, 712–715, 1997.

  6. Colin, C., C. Kissel, D. Blamart, and L. Turpin, Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon, Earth Planet. Sci. Lett, 160, 623–635, 1998.

  7. Day, R., M. Fuller, and V. A. Schmidt, Hysteresis properties of titanomagnetites: grain-size and compositional dependence, Phys. Earth Planet. Inter, 13, 260–267, 1977.

  8. Dunlop, D. J. and Ö. Özdemir, Rock Magnetism: Fundamentals and Frontiers, 573 pp., Cambridge University Press, Cambridge, 1997.

  9. Giraudeau, J., B. A. Christensen, O. Hermelin, C. B. Lange, I. Motoyama, and Shipboard Scientific Party, Biostratigraphic age models and sedimentation rates along the southwest African margin, in Proc. ODP Init. Repts., 175, edited by G. Wefer, W. H. Berger, and C. Richter et al., pp. 543–546, Ocean Drilling Program, College Station, TX, 1998.

  10. Guyodo, Y. and J.-P. Valet, Global changes in intensity of the Earth’s magnetic field during the past 800 kyr, Nature, 399, 249–252, 1999.

  11. Haese, R. R., H. Petermann, L. Dittert, and H. D. Schulz, The early diagenesis of iron in pelagic sediments: a multidisciplinary approach, Earth Planet. Sci. Lett., 157, 233–248, 1998.

  12. Hartl, P. and L. Tauxe, A precursor to the Matuyama/Brunhes transition-field instability as recorded in pelagic sediments, Earth Planet. Sci. Lett., 138, 121–135, 1996.

  13. Hilton, J., Greigite andthe magnetic propertiesofsediments, Limnol. Ocean - ogr., 35, 509–520, 1990.

  14. Hounslow, M. K. and B. A. Maher, Source of the climate signal recorded by magnetic susceptibility variations inIndian Ocean sediments, J. Geophys. Res., 104, 5047–5061, 1999.

  15. Hu, S., E. Appel, V. Hoffmann, W. W. Schmahl, and S. Wang, Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization, Geophys. J. Int., 134, 831–842, 1988.

  16. Karlin, R. and S. Levi, Diagenesis of magnetic minerals in Recent haemipelagic sediments, Nature, 303, 327–330, 1983.

  17. Karlin, R., M. Lyle, and G. R. Heath, Authigenic magnetite formation in suboxic marine sediments, Nature, 326, 490–493, 1987.

  18. Kostka, J. E. and K. H. Nealson, Dissolution and reduction of magnetite by bacteria, Environ. Sci. Technol., 29, 2535–2540, 1995.

  19. Leslie, B. W., S. P. Lund, and D. E. Hammond, Rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the California continental borderland, J. Geophys. Res., 95, 4437–4452, 1990.

  20. Murray, R. W., R. Wigley, and Shipboard Scientific Party, Interstitial water chemistry ofdeeply buried sediments from the southwest African margin: a preliminary synthesisof results from Leg 175, in Proc. ODP Init. Repts., 175, edited by G. Wefer, W. H. Berger, and C. Richter et al., pp. 547–553, Ocean Drilling Program, College Station, TX, 1998.

  21. Oldfield, F., I. Darnley, G. Yates, D. E. France, and J. Hilton, Storage diagenesis versus sulfide authigenesis: possible implications in environmental magnetism, J. Paleolimnol., 7, 179–189, 1992.

  22. Pufahl, P. K., M. A. Maslin, L. Anderson, V. Brüchert, F. Jansen, H. Lin, M. Perez, L. Vidal, and Shipboard Scientific Party, Lithostratigraphic summary for Leg 175: Angola-Benguela upwelling system, in Proc. ODP Init. Repts., 175, edited by G. Wefer, W. H. Berger, and C. Richter et al., pp. 533–542, Ocean Drilling Program, College Station, TX, 1998.

  23. Richter, C., A. Hayashida, Y. Guyodo, J.-P. Valet, and K. L. Verosub, Magnetic intensity loss and core diagenesis in long-core samples from the East Cortez Basin and the San Nicolas Basin (California Borderland), Earth Planets Space, 51, 329–336, 1999.

  24. Roberts, A. P., J. S. Stoner, and C. Richter, Diagenetic magnetic enhancement of sapropels from the eastern Mediterranean Sea, Mar. Geol., 153, 103–116, 1999.

  25. Rochette, P., G. Fillion, J.-L. Mattei, and M. J. Dekkers, Magnetic transition at 30–34 Kelvin in pyrrhotite: insight into a widespread occurrence of this mineral in rocks, Earth Planet. Sci. Lett., 98, 319–328, 1990.

  26. Schmidt, A. M., T. von Dobeneck, and U. Bleil, Magnetic characterization of Holocene sedimentation in the South Atlantic, Paleoceanography, 14, 465–481, 1999.

  27. Snowball, I. F., Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden, Geophys. J. Int., 129, 624–636, 1997a.

  28. Snowball, I. F., The detection of single-domain greigite (Fe3S4) using rotational remanent magnetization (RRM) and the effective gyro field (Bg): mineral magnetic and palaeomagnetic application, Geophys. J. Int., 130, 704–716, 1997b.

  29. Torii, M., Low-temperature oxidation and subsequent downcore dissolution of magnetite in deep-sea sediments, ODP Leg 161 (Western Mediterranean), J. Geomag. Geoelectr., 49, 1233–1245, 1997.

  30. Torii, M., K. Fukuma, C.-S. Horng, and T.-Q. Lee, Magnetic discrimination of pyrrhotite- and greigite bearing sediment samples, Geophys. Res. Lett., 23, 1813–1816, 1996.

  31. Verosub, K. L. and A. P. Roberts, Environmental magnetism: Past, present, and future, J. Geophys. Res., 100, 2175–2192, 1995.

  32. Verwey, E. J. W., Electronic conduction of magnetite (Fe3O4) and its transition point at low temperature, Nature, 144, 327–328, 1939.

  33. Wefer, G., W. H. Berger, and C. Richter, et al., Proc. ODP Init. Repts., 175 (Part 1), 577 pp., Ocean Drilling Program, College Station, TX, 1998.

  34. Yamazaki, T., Relative paleointensity of the geomagnetic field during Brunhes Chron recorded in North Pacific deep-sea sediment cores: orbital influence?, Earth Planet. Sci. Lett., 169, 23–35, 1999.

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Correspondence to Toshitsugu Yamazaki.

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Yamazaki, T., Solheid, P.A. & Frost, G.M. Rock magnetism of sediments in the Angola-Namibia upwelling system with special reference to loss of magnetization after core recovery. Earth Planet Sp 52, 329–336 (2000) doi:10.1186/BF03351644

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  • Magnetite
  • Remanent Magnetization
  • Magnetic Mineral
  • Total Organic Carbon Content
  • Ocean Drill Program