Magnetization of Greenland ice and its relationship with dust content
Journal of Geophysical Research, Vol. 109, D09104, doi:10.1029/2003JD004433, 2004L. Lanci
Istituto di Dinamica Ambientale, Università di Urbino, Urbino, Italy and Department of Geological Sciences, Rutgers University, Piscataway, New Jersey, USA.
D.V. Kent
Department of Geological Sciences, Rutgers University, Piscataway, New Jersey, USA and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA
P.E. Biscaye
Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA
J.P. Steffensen
Ice and Climate, The Niels Bohr Institute, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
ABSTRACT.
We estimate the concentration of fine magnetic particles in ice samples
from the North Greenland Ice Core Project core from the central
Greenland ice sheet, using
low-temperature (77K) isothermal remanent magnetization (IRM) analysis
and compare it with the mass concentration of aerosol dust. Samples
were taken from six climatic
intervals, spanning the time from the Holocene (Preboreal) back to the
Last Glacial Dansgaard/Oeschger cycle 5. The mean IRM intensity of the
ice varies by a factor
of 3 from glacial to interglacial stages, being lower during
interglacials. The IRM acquisition curves of the ice do not quite
saturate at the maximum available field
of 0.8 T and show a relatively broad coercivity, which is compatible
with a mixture of maghemite or magnetite and hematite. Comparison of
the IRM intensity and total
dust mass shows a remarkably good correlation but also reveals a large
background magnetization, which may be essentially constant over the
different climatic stages.
IRM suggests that the dust properties are independent of the background
signal and that the dust aerosol has a magnetization within about 30%
of pristine loess from
the Chinese Loess Plateau, which is considered to have the same source
in the same east Asian deserts as dust in Greenland ice. Ice
contamination and the flux of
extraterrestrial dust particles were considered in order to explain the
origin of the background magnetization. Nevertheless, we could not find
a convincing explanation
for this signal, which represents a considerable part of the IRM signal
and is the dominant component during interglacial intervals, without
invoking the presence of
undetected dust mass. The alternative hypothesis of a varying
magnetization of the ice dust at different climatic periods would
suggest that different sources of aerosol
are active during different climatic periods. This, however, has not
proven to be the case so far for studies of the provenance of dust in
Greenland ice.