Environmental magnetic signature of the Waipaoa system, offshore Hawkes Bay, over the last few millenia

Alissa N. Quinn (Alissa Nicole), 1983-

U-channel samples from seven cores collected from the Poverty Continental Shelf, East Coast, New Zealand, have been used in an environmental magnetic study of the Waipaoa sedimentary dispersal system, as part of the MARGINS Source to Sink (S2S) research programme. Magnetic measurements of K, NRM intensity, AF demagnetisation, ARM and IRM were performed on the u-channels and the results used to identify changes in magnetic mineralogy, magnetic concentration and magnetic grain size that occurred in the cores. Discrete parameters were identified from parametric ratios, biplots and King plots. Three sediment samples from the Waipaoa River were collected to provide a link between the magnetic signals observed in the sediment cores on the Poverty Shelf to the magnetic signature of sediment that is being discharged from land. Age control for the cores is provided by using tephra horizons and 210Pb accumulation rates. Tephra identified in the \ - CCA ~ ~A.c(a.r cores are the Taupo (1.718 ka), Mapara (2.10 ka) and Waimihia (3.47 ka) events. 210Pb ~,.e""'"-s accumulation rates vary widely over the Poverty Shelf area ranging 0.27-0.77 crnly. The dominant magnetic mineralogy of the cores is SD-PSD magnetite, with one core containing greigite, which formed during the quieter periods after three large storm events in the Waipaoa Catchment over the last ~350 years. Three outer shelf cores (K32, K54 and MD06- 3005) exhibit an increase in magnetic grain size and a decrease in magnetic concentration downcore at around 1.00 m depth. These magnetic variations are attributed to anthropogenic influence from deforestation during Polynesian settlement in the Waipaoa Catchment ~800- 900 years ago which resulted in increased erosion rates affecting the magnetic concentration and grain size. IRM variations in K47 at 0.60 m, indicating a slightly differing mineralogy, are attributed to a change in the dominant erosion style in the Waipaoa Catchment (landsliding versus gully erosion) as observed in MD97-2122. The magnetic stratigraphy of a core from the Hawke's Bay Slope (JPC56) is comparable with pollen records from nearby marine and lake cores suggesting the appearance and disappearance of certain taxa allows for varying erosion rates, erosion of different lithologies and weathering of soils. The magnetic signature of the Waipaoa River samples is similar to that of the Poverty Shelf cores. This indicates that the Waipaoa Catchment is a major source of the sediment deposited on the Poverty Shelf.

134 leaves : ill., maps ; 30 cm. + 1 CD-ROM..

2007Quinn

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