New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

New Perspectives on the Chronostratigraphy and Magmatic Evolution of the Auckland Volcanic Field, New Zealand

<p>Understanding the eruptive history of a volcanically active region is critical in assessing the hazard and risk posed by future eruptions. In regions where surface deposits are poorly preserved, and ambiguously sourced, tephrostratigraphy is a powerful tool to assess the characteristics of past eruptions. The city of Auckland, New Zealand’s largest urban centre and home to ca. 1.4 million people, is built on top of the active Auckland Volcanic Field (AVF). The AVF is an intraplate monogenetic basaltic volcanic field, with ca. 53 eruptive centres located in an area of ca. 360 km2. Little is known however, about the evolution of the field because the numerical and relative ages of the eruptions are only loosely constrained, and therefore the precise order of many eruptions is unknown. Here I apply tephrostratigraphic and geochemical techniques to investigate the chronology and magmatic evolution of the AVF eruptions. First, I present an improved methodology for in-situ analysis of lacustrine maar cores from the AVF by employing magnetic susceptibility and X-ray density scanning on intact cores. These techniques are coupled with geochemical microanalysis of the tephra-derived glass shards to reveal details of reworking within the cores. These details not only allow assessment of the deposit relationships within cores (e.g. primary vs. reworked horizons), but also to correlate tephra horizons between cores. Through the correlation of tephra units across cores from a variety of locations across the field, an improved regional tephrostratigraphic framework for the AVF deposits has been established. Following on from this, I detail the methods developed in this study to correlate tephra horizons within the maar cores back to their eruptive source. This technique uses geochemical fingerprinting to link the glass analyses from tephra samples to whole rock compositions. Such an approach has not been previously attempted due to the complications caused by fractional crystallisation, which affects concentrations of certain key elements in whole rock analyses. My method resolves these issues by using incompatible trace elements, which are preferentially retained in melt over crystals, and therefore retain comparable concentrations and concentration ratios between these two types of sample. Because of the primitive nature of the AVF magmas, their trace element signature is largely controlled by the involvement of several distinct mantle sources. This leads to significant variability between the volcanic centres that thus can be used for individually fingerprinting, and correlating tephra to whole rocks. Nevertheless, in some cases geochemistry cannot provide an unambiguous correlation, and a multifaceted approach is required to allow the correlation of the tephra horizons to source. The other criteria used to correlate tephra deposits to their source centre include, Ar-Ar ages of the centres, modelled and calculated ages of the tephra deposits, the scale of eruption, and the deposit locations and thicknesses. The results of this research outline the methodology for assessing occurrence and characteristics of basaltic tephra horizons within lacustrine maar cores, and the methodology for correlating these horizons to their eruptive source. In doing this the relative eruption order of the AVF is accurately determined for the first time. Temporal trends suggest acceleration of eruption repose periods to 21 ka followed by deceleration to present. Although no spatial evolution is observed, coupling of some centres is seen when spatial and temporal evolution are combined. The geochemical signature of the magmas appears to evolve in a cyclic manner with time, incorporating increasing amount of a shallow source. This evolution is seen both during a single eruption sequence and throughout the lifespan of the AVF. Finally, pre-eruptive processes are assessed as part of the study of the magmatic evolution of the AVF. The effects of contamination from the crust and lithosphere through which the magma ascends are evaluated using the Re-Os isotope system. The results show there are variable inputs from crustal sources, which have previously not been identified by traditional isotope systems (e.g. Pb-Sr-Nd isotopes). Two sources of contamination are identified based on their Os systematics relating to two terranes beneath the AVF: the metasedimentary crust and the Dun Mountain Ophiolite Belt. The identification of this process suggests there is interaction of ascending melt with the crust, contrary to what previous studies have concluded. This body of research has provided a detailed reconstruction of the chronostratigraphy and magmatic evolution of the AVF to aid accurate and detailed risk assessment of the threat posed by a future eruption from the Auckland Volcanic Field.</p>

Visitation Rate Analysis of Geoheritage Features from Earth Science Education Perspective Using Automated Landform Classification and Crowdsourcing: A Geoeducation Capacity Map of the Auckland Volcanic Field, New Zealand

The increase in geoheritage studies has secured recognition globally regarding the importance of abiotic natural features. Prominent in geoheritage screening practices follows a multicriteria assessment framework; however, the complexity of interest in values often causes decision making to overlook geoeducation, one of the primary facets of geosystem services. Auckland volcanic field in New Zealand stretches through the whole area of metropolitan Auckland, which helps preserve volcanic cones and their cultural heritage around its central business district (CBD). They are important sites for developing tourist activities. Geoeducation is becoming a significant factor for tourists and others visiting geomorphological features, but it cannot be achieved without sound planning. This paper investigates the use of big data (FlickR), Geopreservation Inventory, and Geographic Information System for identifying geoeducation capacity of tourist attractions. Through landform classification using the Topographic Position Index and integrated with geological and the inventory data, the underpromoted important geoeducation sites can be mapped and added to the spatial database Auckland Council uses for urban planning. The use of the Geoeducation Capacity Map can help resolve conflicts between the multiple objectives that a bicultural, metropolitan city council need to tackle in the planning of upgrading open spaces while battling of growing demand for land.

Dilemma of Geoconservation of Monogenetic Volcanic Sites under Fast Urbanization and Infrastructure Developments with Special Relevance to the Auckland Volcanic Field, New Zealand

Geoheritage is an important aspect in developing workable strategies for natural hazard resilience. This is reflected in the UNESCO IGCP Project (# 692. Geoheritage for Geohazard Resilience) that continues to successfully develop global awareness of the multifaced aspects of geoheritage research. Geohazards form a great variety of natural phenomena that should be properly identified, and their importance communicated to all levels of society. This is especially the case in urban areas such as Auckland. The largest socio-economic urban center in New Zealand, Auckland faces potential volcanic hazards as it sits on an active Quaternary monogenetic volcanic field. Individual volcanic geosites of young eruptive products are considered to form the foundation of community outreach demonstrating causes and consequences of volcanism associated volcanism. However, in recent decades, rapid urban development has increased demand for raw materials and encroached on natural sites which would be ideal for such outreach. The dramatic loss of volcanic geoheritage of Auckland is alarming. Here we demonstrate that abandoned quarry sites (e.g., Wiri Mountain) could be used as key locations to serve these goals. We contrast the reality that Auckland sites are underutilized and fast diminishing, with positive examples known from similar but older volcanic regions, such as the Mio/Pliocene Bakony–Balaton UNESCO Global Geopark in Hungary.

Composite development and stratigraphy of the Onepoto maar lake sediment sequence (Auckland Volcanic Field, New Zealand)

The accurate and precise reconstruction of Quaternary climate as well as the events that punctuate it is an important driver of the study of lake sediment archives. However, until recently lake sediment-based palaeoclimate reconstructions have largely concentrated on Northern Hemisphere lake sequences due to a scarcity of continuous and high-resolution lake sediment sequences from the Southern Hemisphere, especially from the southern mid-latitudes. In this context, the deep maar lakes of the Auckland Volcanic Field of northern New Zealand are significant as several contain continuous and well-laminated sediment sequences. Onepoto Basin potentially contains the longest temporal lake sediment record from the Auckland Volcanic Field (AVF), spanning from Marine Isotope Stage 6e (MIS 6e) to the early Holocene when lacustrine sedimentation was terminated by marine breach of the south-western crater tuff ring associated with post-glacial sea-level rise. The Onepoto record consists of two new, overlapping cores spanning ca. 73 m combined with archive material in a complete composite stratigraphy. Tephrochronology and 14C dating provide the fundamental chronological framework for the core, with magnetic relative palaeo-intensity variability downcore, and meteoric 10Be influx into the palaeolake to refine the chronology. The µ-XRF (micro X-ray fluorescence) downcore variability for the entirety of the lake sediment sequence has been established with measurement of a range of proxies for climate currently underway. This work will produce the first continuous record of the last 200 kyr of palaeoclimate from northern New Zealand to date.

Seismic monitoring of the Auckland Volcanic Field during New Zealand's COVID-19 lockdown

The city of Auckland, New Zealand (Tāmaki Makaurau, Aotearoa), sits on top of an active volcanic field. Seismic stations in and around the city monitor activity of the Auckland Volcanic Field (AVF) and provide data to image its subsurface. The seismic sensors – some positioned at the surface and others in boreholes – are generally noisier during the day than during nighttime. For most stations, weekdays are noisier than weekends, proving human activity contributes to recordings of seismic noise, even on seismographs as deep as 384 m below the surface and as far as 15 km from Auckland's Central Business District. Lockdown measures in New Zealand to battle the spread of COVID-19 allow us to separate sources of seismic energy and evaluate both the quality of the monitoring network and the level of local seismicity. A matched-filtering scheme based on template matching with known earthquakes improved the existing catalogue of five known local earthquakes to 35 for the period between 1 November 2019 and 15 June 2020. However, the Level-4 lockdown from 25 March to 27 April – with its drop in anthropogenic seismic noise above 1 Hz – did not mark an enhanced detection level. Nevertheless, it may be that wind and ocean swell mask the presence of weak local seismicity, particularly near surface-mounted seismographs in the Hauraki Gulf that show much higher levels of noise than the rest of the local network.

Development of a multi-method chronology spanning the Last Glacial Interval from Orakei maar lake, Auckland, New Zealand

Northern New Zealand is an important location for understanding Last Glacial Interval (LGI) palaeoclimate dynamics, since it is influenced by both tropical and polar climate systems which have varied in relative strength and timing. Sediments from the Auckland Volcanic Field maar lakes preserve records of such large-scale climatic influences on regional palaeo-environment changes, as well as past volcanic eruptions. The sediment sequence infilling Orakei maar lake is continuous, laminated, and rapidly deposited, and it provides a high-resolution (sedimentation rate above ∼ 1 m kyr−1) archive from which to investigate the dynamic nature of the northern New Zealand climate system over the LGI. Here we present the chronological framework for the Orakei maar sediment sequence. Our chronology was developed using Bayesian age modelling of combined radiocarbon ages, tephrochronology of known-age rhyolitic tephra marker layers, 40Ar∕39Ar-dated eruption age of a local basaltic volcano, luminescence dating (using post-infrared–infrared stimulated luminescence, or pIR-IRSL), and the timing of the Laschamp palaeomagnetic excursion. We have integrated our absolute chronology with tuning of the relative palaeo-intensity record of the Earth's magnetic field to a global reference curve (PISO-1500). The maar-forming phreatomagmatic eruption of the Orakei maar is now dated to > 132 305 years (95 % confidence range: 131 430 to 133 180 years). Our new chronology facilitates high-resolution palaeo-environmental reconstruction for northern New Zealand spanning the last ca. 130 000 years for the first time as most NZ records that span all or parts of the LGI are fragmentary, low-resolution, and poorly dated. Providing this chronological framework for LGI climate events inferred from the Orakei sequence is of paramount importance in the context of identification of leads and lags in different components of the Southern Hemisphere climate system as well as identification of Northern Hemisphere climate signals.