Late Holocene environmental record and geological history of the Lake Colenso area, north-western Ruahine Range, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Quaternary Science, Soil and Earth Sciences, Institute of Natural Resources, Massey University, Palmerston North, New Zealand

Sediment cores from a landslide-dammed lake, Lake Colenso (North Island, New Zealand), contain a decadal- to centennial-scale record of changing climate spanning the past 1800 years. A multi-proxy approach has been used to obtain a high-resolution record of variability from the Lake Colenso catchment, and tephra horizons combined with radiocarbon ages provide chronological constraints. Since the lake is located within a mountainous forested catchment of the northern Ruahine Range, it has remained pristine and isolated from human disturbance. Additionally, pollen analysis indicates minimal human influence in the lake catchment; hence the site offers a rare opportunity to investigate natural environmental change during a period in which anthropogenic impact has tended to obscure natural variability in many records from elsewhere in New Zealand (Wilmshurst et al., 1997). Sedimentology and elemental geochemistry reflect periods of rapid sediment influx into the lake, here interpreted as storm events which are preserved at an average of 1 every 150 years. This record, supported by stable isotope records from ostracods, shows distinct periods of increased storminess, and is related to the interaction between regional atmospheric circulation systems, El Niño-Southern Oscillation, Southern Annular Mode and the Pacific Decadal Oscillation. Furthermore, the association with other regional records of climate over the late Holocene highlights the effect of regional climatic forcing. A combination of findings is characterised by broad changes that correlate to the regionally distinctive Medieval Warm Period and Little Ice Age periods, providing further evidence for a climatically variable Holocene (Mayewski et al., 2004). The multi-proxy record presented here is a valuable contribution to existing paleoenvironmental knowledge of the late Holocene in New Zealand. The geology of the study area is characterised by alternating periods of subsidence and uplift throughout the Plio-Pleistocene which resulted in the deposition of Wanganui Basin sediments in the region. Historic earthquake records from nearby major faults are commensurate with ages obtained for landslides at Lake Colenso, which allow a further understanding into landscape evolution and the development of present-day Lake Colenso