ENVIRONMENTAL MAGNETISM AND MAGNETIC CORRELATION OF HIGH-RESOLUTION LAKE SEDIMENT RECORDS FROM NORTHERN HAWKES BAY, NEW-ZEALAND

Magnetic measurements provide a rapid and flexible means of studying a wide range of environmental processes. Here they are applied to a sui te of lacustrine sediment cores from Lakes Tutira and Waikopiro, North ern Hawke's Bay. The cores span the period from just before the settle ment of Europeans in the area to the present day. They document the im pact of vegetation clearance and land use practices in a region of ste ep hill country that is prone to recurring major rainstorms. Before Eu ropean settlement, the lake catchment was covered in bracken fern, whi ch stabilised the soil. The major changes in the magnetic properties o f 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 identifiab le pulse of minerogenic sediment, which gives rise to a peak in magnet ic susceptibility. These peaks are readily recognised and can be later ally correlated on downcore logs. Comparison of hysteresis parameters, high temperature behaviour, and other magnetic measurements also indi cates that the magnetic minerals in the storm and inter-storm sediment s differ significantly. Whereas the storm sediments contain titanomagn etite, 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 g yttja also shows evidence for the presence of the iron sulphide, greig ite. Greigite forms in sulphate-reducing sedimentary environments, whi ch must have developed during the relatively quiescent periods between storms. Magnetic correlation of cores from along the axis of Lake Tut ira is used to illustrate major variations in sedimentation rate, and to demonstrate the rapid infilling of some basins during storms.