Environmental and Climatic Responses to Two Large Explosive Eruptions from Taupō Volcano, New Zealand

Two caldera forming ignimbrite eruptions of Taupō volcano (New Zealand) are among the most violent, powerful, and complex known Late Quaternary volcanic eruptions on Earth. The >530 km3 (dense rock equivalent [DRE]) ~25.5 ka Ōruanui, and 35 km3 (DRE) 232 ± 10 CE Taupō eruptions are world class examples of large volume explosive volcanism. Preservation of diagnostic pyroclastic material in Southern Hemisphere terrestrial, marine, and glacial palaeoarchives enable high resolution investigation of environmental and climatic proxy records to contextualise and assess the regional to global scale effects of these events. In this thesis, millimetre scale palynological records of three North Island lacustrine sediment cores were developed, and complemented by an investigation of Antarctic ice core records, to assess the environmental and climatic impacts of the Ōruanui and Taupō eruptions over short timescales of years to decades.

Deposits of the Ōruanui eruption (known as the Kawakawa Ōruanui Tephra [KOT]) represent an important time stratigraphic marker for the Last Glacial Maximum (LGM) within terrestrial and marine sedimentary archives across New Zealand. However, the top contact of such deposits, and therefore the immediate post eruptive effects, are generally missing near the eruption centre, and even where preserved, previous studies were unable to detect and quantify any palaeoecological responses to this event. An intact 3 cm thick layer of the KOT was preserved in the continuous, high resolution lacustrine sedimentary sequence of Onepoto maar (Auckland), located ~240 km upwind from Taupō volcano. Targeted palynological and geochemical analyses of sediments bracketing the KOT at 1 mm intervals resolved a temporary (<10 years) ashfall induced partial defoliation of tall trees in the forest canopy and margins (predominantly Fuscospora group), and a longer term (~60 years) expansion of wetland and littoral habitats surrounding the palaeolake. Short lived volcanogenic cooling inferred from Antarctic ice core records (where glass from the Ōruanui eruption has been found) likely had compounding ecological effects but could not be distinguished from the direct impacts of tephra deposition. These findings highlight that supereruptions may only drive short term changes in climate, similar in terms of timescales to those observed for smaller historical events.

The 1.8 ka Taupō eruption is the largest of the post Ōruanui eruptive sequence and generated additional caldera collapse. However, despite the large size and relatively young age of this event, volcanogenic glaciochemical signals attributed to the eruption in ice core records have not yet been verified. In this thesis, volcanic glass from the Taupō eruption was located in an interval of the Roosevelt Island (RICE) ice core of West Antarctica, with a modelled RICE17 age of 230 ± 19 CE (95 % confidence) that independently confirms the date and timing of this event. Six shards of a composition identical within analytical uncertainty to the geochemistry of proximal Taupō glass were accompanied by a single shard identical in composition within uncertainty to that of the Ōruanui glass, also from Taupō volcano. This double fingerprint uniquely identifies the source as Taupō volcano, with transport and deposition of the glass shards likely related to the final catastrophic phase of the Taupō eruption. Identification of a coeval Mt. Melbourne (Antarctica) cryptotephra in the RICE ice core not only provides a valuable time stratigraphic marker for the developing Antarctic tephrochronological framework but establishes a link to the circum Antarctic marine sediment record, which may offer further insight into atmospheric conditions at the time of these eruptions.

Occurring in late summer to early autumn, the six stage Taupō eruption devastated a large portion of the central North Island through heavy ashfall and the emplacement of a rapidly moving pyroclastic flow. However, largely due to sampling resolution, questions remain about the magnitude and duration of short term palaeoecological disturbance from volcanic phenomena produced by this event, particularly at distal localities. To resolve distal impacts, continuous millimetre scale charcoal and palynological analyses were carried out on organic rich lacustrine sediments from Lake Rotokauri (Waikato) bracketing a 6 mm thick deposit of the Taupō Tephra, related to cross wind fallout during the final phase of the eruption. The data provided new fine scaled insights into compounding ashfall and fire induced forest disturbance of the Waikato lowlands, with tephra deposition and lightning ignited fires inferred to be responsible for opening up the forest canopy and margins. Under inferred arid, fire prone climatic conditions, the ecological effects of the eruption were prolonged, with recovery of distal vegetation communities taking up to 70 years, highlighting the variable impacts of large explosive eruptions. At Lake Tūtira (Hawke’s Bay), located ~90 km southeast (downwind) from Taupō volcano, a 6 cm thick layer of Taupō Tephra was identified in a laminated sequence (LT24) that preserves a complex event stratigraphy of volcanic, seismic, and climatic disturbances. Aided by hyperspectral scanning, millimetre scale palynological analyses demonstrated that storm and earthquake derived sediment lithotypes preserve degraded, likely mixed age assemblages of palynomorphs, which if not accounted for, can distort and obscure contemporary palaeoecological signals. Informed by a combined litho and palyno facies approach, discriminative high resolution sampling allowed an undistorted palynological record of LT24 to be developed that resolved major disturbance of vegetation in the Tūtira catchment, as well as the near complete destruction of beech forests within range of ignimbrite emplacement. Identification of reworked pyroclastic materials and subsequent storm derived pulses of terrestrial sediment and vegetative debris above the primary tephra deposits reflected considerable damage to the catchment vegetation and destabilisation of the surrounding landscape in the months to decades after the eruption. The impacts of the Taupō eruption on the forests surrounding Tūtira were inferred to be greater and more prolonged than those recorded at Rotokauri. However, further investigation is required to temporally constrain the extent of forest recovery at Tūtira, which through lithologically informed palynological analysis can be distinguished from the influence of competing environmental processes in this highly active and complex catchment.

The high resolution, dual proxy approach developed in this thesis provides new information about the timing and environmental and climatic impacts of the Ōruanui and Taupō eruptions. The techniques developed in this thesis could offer valuable insights into other large scale explosive eruptions, and non volcanic disturbance events preserved in the geological record.