USING GROUND PENETRATING RADAR TO LINK SUBFOSSIL KAURI AND PALAEOCLIMATE RECORDS IN PEAT, NORTHLAND NEW ZEALAND

Peat deposits are excellent environmental change recorders. In northern New Zealand (34.5-38°S) the endemic giant conifer Agathis australis (kauri) have been preserved in peat sequences. Previous work has demonstrated that subfossil kauri older than 60,000 years age are common, and subfossil Holocene wood has been used to reconstruct a >4000-year record that has potential for exploration of past ENSO activity. In many cases, anoxic conditions in Northland peat bogs create an environment that preserves wood, macrofossils (cones, stems, and leaves) and pollen for tens of thousands of years. Studies throughout the world rely on coring peat deposits to recover sample for various analysis, including palynology, 14C dating, sedimentology, macrofossil identification and charcoal counts. Coring is typically conducted on transects perpendicular to the axial line of the peat deposit. This method has produced excellent results and some of the world’s best records of environmental change.

Locating buried kauri is difficult at best, and recovering cores from peat bogs with kauri is even more problematic. Current methods for finding and extracting wood include use of large 30-ton excavators to trench the landscape in hope of intersecting a buried tree. In addition, the large woody debris from relic forests exist in scattered horizons within the peat deposits at different depths. The density and random nature of the woody debris makes coring the peat deposits to extract environmental records challenging.

In early 2015, a series of ground penetrating radar surveys were conducted at Doubtless and Rangaunu bays in the Far North district and at Ruakaka and Omaha in Northland, New Zealand. The goals of the GPR surveys were to: 1) assess the feasibility of GPR for mapping isolated woody debris and horizons of woody debris; 2) determine appropriate survey methodology for mapping the peatlands and buried wood; and 3) collect a series of cores based on GPR mapping. The team utilized the MALA Geosciences GTX HDR system with 160 and 480 MHz antennas. The GPR data was collected in conjunction with RTK-GPS for high-resolution spatial mapping and processed in GPR Slice, allowing integration with other spatial data in ArcGIS. A series of field lines and interpretations from initial surveys are discussed in the context of evaluating project goals 1 and 2.