New Zealand Journal of Geology and Geophysics abstracts
Environmental magnetism and magnetic correlation of high resolution lake
sediment records from Northern Hawke's Bay, New Zealand
GILLIAN M. TURNER
Institute of Geophysics
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
Abstract Magnetic measurements provide a rapid and flexible
means of studying a wide range of environmental processes. Here they are
applied to a suite of lacustrine sediment cores from Lakes Tutira and
Waikopiro, Northern Hawke's Bay. The cores span the period from just before the
settlement of Europeans in the area to the present day. They document the
impact of vegetation clearance and land use practices in a region of steep hill
country that is prone to recurring major rainstorms. Before European
settlement, the lake catchment was covered in bracken fern, which stabilised
the soil. The major changes in the magnetic properties of the sediments reflect
fluctuations in the concentration of magnetic minerals, rather than composition
or grain-size variations. Since the European arrival, however, each major storm
has produced an identifiable pulse of minerogenic sediment, which gives rise to
a peak in magnetic susceptibility. These peaks are readily recognised and can
be laterally correlated on downcore logs. Comparison of hysteresis parameters,
high temperature behaviour, and other magnetic measurements also indicates that
the magnetic minerals in the storm and inter-storm sediments differ
significantly. Whereas the storm sediments contain titanomagnetite, derived
from the greywacke bedrock and disseminated tephra from the Taupo Volcanic
Zone, which is probably enhanced during burnings, and minor amounts of
pedogenically formed maghemite, the inter-storm gyttja also shows evidence for
the presence of the iron sulphide, greigite. Greigite forms in
sulphate-reducing sedimentary environments, which must have developed during
the relatively quiescent periods between storms. Magnetic correlation of cores
from along the axis of Lake Tutira is used to illustrate major variations in
sedimentation rate, and to demonstrate the rapid infilling of some basins
during storms.
Keywords environmental magnetism; lake sediments; storm
sediments; paleomagnetism; magnetic minerals
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 287-298
0028-8306/97/4003-0287 $7.00/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (1146K); (scanned from paper original: notes about this process)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
