High-resolution composite stratigraphy and multi-method dating of Auckland maar sediment records spanning the Last Glacial Interval

Abstract

Palaeoclimatic reconstructions are key to understanding and quantifying nature, rates, and drivers of palaeoclimatic changes and fundamental to predictions of future climate change. A holistic picture of past changes and their drivers requires wide spatial and temporal coverage of palaeo-climate archives but few suitable mid-latitude terrestrial records from the SW Pacific exist. In this context, the Orakei maar sediment sequence (Auckland, New Zealand) provides a continuous and high-resolution archive of palaeoclimatic and -environmental changes in the region. Nevertheless, without a robust composite stratigraphy and chronology, multi-proxy changes recorded in the sediment sequence cannot be interpreted in a regional, let alone global, palaeo-climate context. This thesis aims to develop a composite stratigraphy, robust chronology and detailed chemo-stratigraphy with which to assess palaeo-environmental change from Orakei maar. As part of this study, the Orakei eruption age has been constrained to >139 200 ± 820 yr and thereby constrains the sedimentation onset in Orakei maar to marine isotope stage (MIS) 6. The sequence contains a continuous lake sediment archive until ca. 9750 cal. yrBP when post-glacial sea-level rise breached the crater rim. For the first time, this study demonstrated that μ-XRF core scanning allows rapid tephra identification in sediment cores. However, an extensive reference μ-XRF database is necessary for future applications using this approach to identify tephra in sediment cores. Tephrochronology, radiocarbon dating and correlation of relative palaeointensity variations of the Earth’s magnetic field were successful for chronology development whereas luminescence dating and 10Be radionuclide variations could not improve the Orakei chronology due to rapid influx of detrital sediment. The chronology allows the placement of the Orakei palaeoclimatic change in a robust temporal framework. Based on μ-XRF elemental variability, no MIS 5 sub-stages could be differentiated, MIS 4 is an environmentally homogeneous, cool interval, whereas MIS 3 displayed considerable geochemical variability signifying increased climatic instability. This study produced the first long, high-resolution and complete lake sediment record from the Southern Hemisphere mid-latitudes anchored in a robust chronology extending beyond the radiocarbon dating limit and offers unique possibilities for detailed palaeoclimatic reconstructions based on the μ-XRF-inferred evolution of Orakei maar.