Environmental magnetism and magnetic correlation of high resolution lake sediment records from northern Hawke's 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 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.