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Keywords:

  • Argentina;
  • magnetic susceptibility;
  • magnetite;
  • palaeoclimate;
  • Quaternary;
  • rock magnetism;
  • sediments

SUMMARY

Magnetic susceptibility has proved to be a very useful tool for understanding Northern Hemisphere loess/palaeosol sequences, both for intersite correlation and for broad climatic interpretation. However, there are two competing magnetoclimatological models that predict completely opposite patterns of variability—the so-called wind-vigour and pedogenic models. In the former, stronger winds during glacial intervals entrain larger quantities of dense magnetic particles than during interglacials. Glacial loess is therefore more magnetic than interglacial palaeosol (e.g. Kurtak, Siberia). In the pedogenic model, biological and/or chemical processes lead to the neo-formation of magnetic material so that palaeosols are more magnetic than pristine parent loess (e.g. the Chinese Loess Plateau). We are currently extending our investigation of loess/palaeosol sequences to the Southern Hemisphere starting with a new 13 m section in the El Cristo quarry in the city of La Plata, Buenos Aires Province, Argentina. The magnetic susceptibility profile shows a clear sequence of maxima and minima that correspond to lithological variations, but interpretation of these correlations is not straightforward. In the top 4 m of the section, parent material (loess) has high susceptibility whereas palaeosols have low values, consistent with the wind-vigour model. The same pattern is seen for most of the remaining 9 m of section, but there are intervals where gleying has occurred, particularly between depths of 8 and 10 m. Under the waterlogged conditions implied, iron oxides are likely to be removed and any original magnetoclimatological signal is lost (or at least strongly modified). A further complication arises from the measured values of the frequency dependence of susceptibility, which is usually taken to indicate the presence of ultrafine magnetic particles produced during pedogenesis. The El Cristo values fall squarely in between those observed in Siberia and China. An important consideration in attempting to understand the Pampean loess is the fact that it differs markedly in mineralogical composition from its Northern Hemisphere counterparts, being dominated by volcanic material. We have therefore determined the appropriate magnetic properties of samples taken from an ash layer embedded in a typical Argentinean loess sequence. Thermomagnetic analysis shows the ash to be magnetically indistinguishable from the El Cristo deposits. However, its measured frequency dependence of susceptibility cannot explain the intermediate values found there. The overall outcome is that neither of the existing magnetoclimatological models can adequately account for the complexities of the Pampean loess.