Reconstruction of aspects of Northern New Zealand paleoclimate over the last 200 ka using laminated Auckland maar lake sequences

Abstract

Continuous lake sediment records spanning two glacial cycles are rarely available from the Southern Hemisphere mid-latitudes, let alone with the requisite resolution and robust chronological control required to optimise paleoenvironmental reconstructions. In this context Onepoto maar paleolake in the Auckland Volcanic Field is important as it contains a high-resolution record of continuous sedimentation in the lake from its formative phreatomagmatic eruption ≤190 ka until the post-glacial sea-level rise breached the crater tuff ring ca. 9 cal. ka BP. This thesis presents the first continuous record of sedimentary palaeoenvironment from northern New Zealand to date nearly spanning the last two glacial cycles. Using revised ages of 12 rhyolitic marker tephra layers combined with revised ages for seven basaltic and two andesitic tephra, in addition to bulk carbon and pollen-derived radiocarbon ages, a new age model for the Onepoto Maar lake sediment sequence was established. In particular, the refinement of ages for tephra that predate the Rotoehu eruption has enabled significant improvement of the age model for the pre-45 ka part of the Onepoto record. Furthermore, correlation of the Onepoto tephra sequence to the recently refined Ōrākei tephra sequence and its multi-method age model supports the robustness of the new Onepoto chronology. Improved age control for the interpretation of paleoclimate signals and variability as recorded in the lacustrine sequence is exemplified by the reassessment of available pollen data that span the upper ~ 9.4 m of the Onepoto Maar record. The new On18 age model enabled the interpretation of proxy data from Onepoto maar lake and the correlation of paleoclimate signals to Late Quaternary climate phases such as the global Marine Isotope Stages (MIS) as well as identification of short-lived climate events (e.g., Antarctic Cold Reversal) with increased reliability and precision. In addition, this thesis presents a reconstruction of the depositional history of Onepoto maar lake using μ-XRF core scanning, dry bulk density, loss-on-ignition measurements, and visual lithostratigraphic descriptions. As another aspect of this work, a multi-method high-resolution study of deep Ōrākei maar lake sediment micro-facies represents a first of its type from Australasia and underlines the potential of this approach to reconstruction of lake depositional dynamics and aspects of the associated regional paleoclimate. The results of this work assist verification and expansion of a tentative climate reconstruction using micro-XRF scanning inferred elemental variability encountered in the Ōrākei maar sediment cores. Moreover, the newly established diatom record also correlates to other palaeoecological records from the Auckland region and from across New Zealand that span early MIS 5 (ca. 123 ka) to the early Holocene (ca. 10 ka). The combination of light microscope analysis of thin-sections, aided by high-resolution μ- XRF elemental scanning, FE-EMPA-based elemental mapping (resolution: down to 1 μm) and RADIUS analysis has allowed reconstruction of: (i) shifts in trophic state of the lake; (ii) water salinity (marine influence, windiness); (iii) phases of thermal-stratification; (iv) water column mixing; (v) oxygen availability. Storm related influx of terrestrial sediments (aeolian), as well as phases of regular clastic sediment in-wash from the catchment and sub-aqueous slope collapses (turbidite development) were identified. This may have significant implications for identifying the recurrence of storm events as well as aid in establishing the timing and frequency of seismic events (earthquakes) in the Auckland region through the last glacial cycle. Consequently, the latter will also likely allow for improved regional earthquake hazard assessment.