Stratigraphic Architecture of a Former Lowland Kauri Swamp in Ruakaka, North Island, New Zealand.

Abstract:

The long-lived Kauri (Agathis australis) is an endemic conifer that presently exists within 4% of their pre-human contact range in northern New Zealand. Kauri preserve well in anoxic swamp and wetland environments in the lowlands across Northland and buried, subfossil samples of this species are commonly termed ‘swamp kauri’. Subfossil kauri have been recently employed as a proxy to reconstruct past climate and establish a long dendrochronological records that have direct use for building the radiocarbon calibration curve. One component of work related to interpreting ancient kauri tree ring records is improving the understanding of the stratigraphic and geomorphic history of former lowland kauri environments to outline the role of environmental change in preserving this ancient wood resource.

This study contributes to improving general understanding the subsurface stratigraphy of former lowland swamp kauri sites. A combination of ground penetrating radar, sediment cores, probing transects and trench exposures, provide details for the stratigraphic relationships for one type of swamp kauri site –relic coastal dune sequences- that will form a basis for future sediment and geochemistry work. Based on GPR and trenches, the stratigraphy includes several units –Peat, woody debris, dune sand, and coastal sand. Thickness of the peat, which usually contains the subfossil kauri, varies from thin veneers over antecedent coastal deposits 10-50cm to > 2.2m, with the thickest peat accumulation located between relic foredune ridges.

Prior work at locations nearby have shown Rotoehu Tephra (>45ka) has been observed as a 40 cm thick deposit. Early sedimentary analysis suggests the Rotoehu is present at our study site, but it is possibly disseminated and bioturbation may have mixed the tephra into the underlying peat sediments. OSL samples and peat sediment samples were recovered for future chemical and chronological analysis from selected locations based on GPR and trench sections.