DEVELOPING METHODS FOR IDENTIFYING CRYPTIC TEPHRA IN PEAT CORES FROM THE WAIKATO REGION, NORTH ISLAND, NEW ZEALAND

Peat cores collected from two ombrogenous bogs in the Waikato region of the North Island of New Zealand are subjected to a range of analytical methods that are capable of detecting the cryptic ('hidden') tephra content in the cores. The peat sections of interest are bracketed by the Taupo (-1750 cal BP) and the Tuhua tephras (-7000 cal BP) in two cores from Kopouatai Bog (37°26'S, 175°34'E) and by the Taupo and the Mamaku tephras (-8000 cal BP) in one core from Moanatuatua Bog (37°58'S, 175°72'E). Non-destructive methods used include photospectromeiry and magnetic susceptibility. Results from Total Organic Carbon (TOC) analyses were made available from an ongoing study (Hazell in prep ). Concentrations of tephra shards are determined by ‘ashing' the peat and by counting under a light microscope tephra shards mounted onto microslides. More detailed counts in one core are used to assess the vertical spread of the cryptic component of the visible Taupo Tephra and to determine its sedimentological characteristics. Images from Scanning Electron Microscopy and low-power light microscopy reveal characteristic micromorphological features of selected tephra shards. An appraisal of the various detection methodologies indicates that problems are associated with all. Magnetic susceptibility measured by the Bartington MS2 only detects tephra high in ferro-magnesium content and does not yield satisfactory results for tephras embedded in peat. Reflectance measurements using a Perkin Elmer Luminescence Spectrometer give reasonable results for high-quality (undisturbed and smooth) cores, but peaks may also be produced by material other than tephra. TOC analysis is best capable of detecting cryptic tephra, but the method is partially destructive and time-consuming. The tephra attenuation study shows that fine shards of the Taupo Tephra were detected at 30 cm below and at 11 cm above the visible component of the layer It is likely that bioturbation by micro-organisms, transport along root canals and re-deposition by wind are processes that may redistribute cryptic tephra shards through the peat. Micromorphological analyses of the cryptic tephra shards are useful to characterise potentially distinguishing features of cryptic tephra (e.g.. rholytic vs. andesitic appearance, chemical weathering, etc.). These characteristics are used in combination with radiocarbon ages and chronostratigraphy of visible tephras to link discreet cryptic tephra horizons to possible eruptive events, but additional geochemical analyses are necessary to ‘fingerprint' the tephras more conclusively. It is concluded that detection of cryptic tephra holds promising potential to enhance the region's tephrostratigraphic framework