Palaeoclimate

Palaeoclimate conditions not only play an important role in the formation and evolution of the landscape on which tephra is deposited (peat and lake environments and their response to climate fluctuations over time) but also influence tephra distribution and deposition (e.g., wind directions, fluvial influx, rates of erosion and weathering). Understanding past climate conditions is essential for interpreting lacustrine records and any enclosed tephra beds. Therefore, a brief review of the late Quaternary climate in New Zealand with a focus on the North Island and the Taranaki region is appropriate.

A detailed climate-event stratigraphy for New Zealand spanning the last 30,000 years has been presented by Alloway et al. (2007), Lowe et al. (2013), and Barrell et al. (2013) as part of the NZ-INTIMATE (INTegration of Ice-core, Marine and Terrestrial records) project (Fig. 1.7). Studies of records from the North Island (i.e., Auckland maars, Kaipo and Otamangakau wetlands, Taranaki region, eastern offshore) and South Island (i.e., Okarito wetland, Twin Forks Cave, Galway Tarn, Mount Cook National Park) are mainly based on various climate-proxies including pollen, diatoms, oxygen isotopes in speleothem and marine sediments, total carbon and carbon isotopes in lacustrine sediments, as well as glacial and fluvial deposits, loess and loess-like (aeolian quartz) deposits. Age control derives from radiocarbon dating and stratigraphically well-constrained tephra marker beds (Lowe et al., 2008, 2013). Four major climate periods have been distinguished:

(1) Preceding Interstadial – older than c. 29 cal ka; mild climate conditions.

McGlone et al. (1984) describes cool and extremely wet climate conditions between 40- 30 ka, recorded in pollen assemblages in the Inaha Formation, South Taranaki coast. The vegetation was mainly dominated by shrub- and grassland.

(2) Last Glacial Coldest Period (LGCP) – c. 29 to 18 cal ka; mainly cold climate conditions interrupted by two interstadial periods characterised by cool climate conditions (Fig. 1.7).

The LGCP encompasses the Last Glacial Maximum (LGM) sensu stricto, defined as the

last major glaciation period (c. 21±3 cal ka) with global ice sheets reaching their maximum integrated volume (Mix et al., 2001; Clark and Mix, 2002) and global sea level lowstand of 135 m below present levels (Yokoyama et al., 2000). The onset of the LGM sensu stricto has been later redefined by Newnham et al. (2007) to c. 29 cal ka for

the New Zealand palaeoclimate record, based on glacial evidence from the South Island and pollen evidence from the North Island, hence, the term ‘extended Last Glacial Maximum’ – eLGM has been introduced.

The climate in the Taranaki region during the LGCP was mainly cold and dry, which is shown by widespread fluvial and aeolian depositions, which therefore reflect increased erosion across the North Island (Neall, 1975; Pillans et al., 1993). Alloway et al., (1992a) describes an increased quartz influx during two significant periods (at 27 and 21 cal ka) recorded in andesitic soil deposits (andisols) in the Taranaki region. The source of quartz-rich dust is assumed to be the continental shelf of the southern North Island, which was exposed by the low global sea level during the LGCP. The latest study of Tinkler (2013) used aerosolic quartz dust from peat-material from the Eltham Swamp, Taranaki, to reconstruct palaeowind intensities and directions. Major dust peaks at c. 31.4, 21.3 and 18.9 cal ka can be readily correlated with the Vostok ice core (Jouzel at al. 1993), the marine core P69 (Stewart and Neall, 1984) and Onaero/Waitui sample sites in Taranaki (Alloway et al., 1992a).

The end of the LGCP, known as the Termination I (19-18 cal ka), is characterised by an abrupt climate amelioration towards warmer and wetter conditions accompanied by the replacement of pre-existing shrub-grassland taxa with tall podocarp forest communities (e.g., Newnham et al., 2003; Vandergoes et al., 2005; Hajdas et al., 2006). Further, increased fluvial activity due to much higher precipitation rates during Termination I led to increased incision into fluvial terraces in the North Island (Berryman et al., 2000; Litchfield and Berryman, 2006). The transition from last-glacial to post-glacial climate

conditions is marked by the prominent rhyolitic Rerewhakaaitu Tephra (17,900±200 cal yr BP) from the Okataina Volcanic Centre (Newnham et al., 2003; Lowe et al., 2008).

(3) Last Glacial-Interglacial Transition (LGIT) – c. 18 to 12 cal ka; transitional climate conditions characterised by progressive warming conditions with a short cooling period, which is known as the late-glacial cool episode or Late-glacial Reversal (LGR) around 13.8 – 12.5 cal ka (Lowe et al., 2013) (Fig. 1.7).

The general warming trend and rapid expansion of forest during the LGIT reached its maximum between 15.5 – 14 cal ka (e.g., Taranaki region: Durham Road – Alloway et al., 1992b; Eltham Swamp – McGlone and Neall, 1994). The cooling of the LGR is characterised by a decrease in forest vegetation and concomitant expansion of shrub communities. This trend is seen in most pollen records including Taranaki (e.g., Alloway et al., 1992b; McGlone and Neall, 1994; Alloway et al., 2007). Further evidence for a short-time cooling interval in the main deglaciation period is recognised in the total organic carbon (TOC) and isotope curves from marine and lacustrine records from the North Island (e.g., Newnham and Lowe, 2000; Carter et al., 2002; Shane and Hoverd, 2002), as well as glacial sequences from the Southern Alps (Denton and Hendy, 1994). The LGR in New Zealand began slightly later than the start of the Antarctic Cold Reversal and continued through to the end of Younger Dryas time (Alloway et al., 2007).

(4) Holocene Interglacial – c. 11.8 cal ka to present; warm climate conditions with two phases of greatest warmth (early Holocene and mid Holocene).

In general, variations of the Holocene climate are poorly understood, but significant climate fluctuations during this period are recorded (Mayewski et al., 2004; Schaefer et al., 2009; Striewski et al., 2013). Palynological studies (McGlone et al., 1993; Newnham and Lowe, 2000), isotopic sea surface temperature estimates (Weaver et al., 1998), and isotopic analyses on speleothems (Hellstrom et al., 1998; Williams et al., 2004, 2010) indicate a clear, unusually warm, period around 11.5 cal ka, with temperatures between 1.5 and 3.0 ˚C above those recorded at present. After this early Holocene thermal optimum, a drop in temperature occurred between 9 – 7 cal ka

followed by another warm period in mid-Holocene c. 7 – 6.5 cal ka (Williams et al., 2010). The late-Holocene is characterised by even stronger climate fluctuations, which are shown in several studies based on high-precision recording of Holocene glacier activity in New Zealand, particular in the Southern Alps (c. 5 cal ka; Schaefer et al., 2009). Recent studies of Brook et al. (2011) documented a similar trend of glacier expansion in the North Island by investigating glacial moraines on the southern slopes of Mt. Taranaki. Although palynological studies in the Taranaki region are rare, McGlone et al. (1984, 1988) and McGlone and Neall (1994) suggest that climate cooled and became drier, and eventually windier between 3 – 2.5 cal ka and 1.4 – 0.45 cal ka, based on the abundance of Libocedrus bidwillii pollen at Mt. Taranaki sites. The arrival

of humans at around 650 cal years BP resulted in obscured palynological records since this time (Wilmshurst et al., 1999).

Figure 1. 7 The NZ-INTIMATE climate event stratigraphy (after Barrell et al., 2013 and Alloway et al., 2007).

Overall, the environment where tephra is deposited is greatly influenced by climate fluctuations. During moist and warm climate conditions, large forest expanses can be formed and thick peat beds accumulate. Mild/warm and dry conditions result in a change in soil-forming processes and enhanced weathering, which can lead to the development of allophane-rich andisols. Cold and dry climate conditions favour the development of loess-rich, poorly developed soils and the influx of aerosolic quartz dust. Deglaciation periods, or simply wetter climate periods, produce higher fluvial activity and erosion, which can lead to greater sediment influx into lake and peat environments. An increased precipitation rate, or the formation of large volumes of meltwater, can also produce debris avalanches, flash floods and lahars, which can travel great distances from their sources (Zernack et al., 2011a). Collapses from the volcano triggered/or not triggered by climate conditions can deposit large amounts of material across the surrounding ring plain, affectedly changing the pre-existing landscape (e.g., damming streams and rivers, re-routing drainage), and hence, leading to the formation of new environmental settings. The state of the environment is crucial to deposition and preservation of tephra deposits, since andesitic tephra is easily prone to erosion and weathering (Lowe et al., 1988b).

1.5 Study sites

Lakes and peatlands are relatively common on the lower portions of the volcanic ring- plain of Mt. Taranaki, where large debris avalanches and lahar deposits lap up against the deeply dissected hill country to the east (Zernack et al., 2011a) (Fig. 1.8) and have altered or blocked existing drainage patterns, initiating the formation of peats and lakes. One lake and three peatlands located within a 20 to 30 km radius of Mt. Taranaki’s summit were selected according to their potential for preserving deep sequences of organic deposits and their positions within the main dispersal axis of mapped tephra falls (Alloway et al., 1995). From north to south, the study sites are Lake Richmond (LRi), Tariki Swamp (TS), Ngaere Swamp (NS), and Eltham Swamp (ES) (Fig. 1.8). To derive the most complete and detailed tephrostratigraphic record I added data from cores at Lake Umutekai (LU) and Lake Rotokare (LR), obtained by Turner (2008), and re-examined in the present study.

Lake Umutekai lies c. 5 km SE of New Plymouth and about 25 km NNE of Mt.

Taranaki’s summit. It is a 0.02 km2, shallow (<2 m) closed depression on a gently undulating volcaniclastic surface that is ≥170 ka old (Neall, 1979). The surrounding landscape was densely forested throughout the Holocene (McGlone and Neall, 1994). Lake Umutekai has been cored for a pollen-based vegetation and climate survey (Newnham, 1990) and more recently by Turner et al. (2008a, 2009) who identified 104 tephra layers between c. 1.5 to 10.5 cal ka BP.

Lake Richmond is located c. 5 km NE of Inglewood and about 25 km NE of Mt.

Taranaki’s summit. The lake is interpreted to be formed by landslide-damming and lies in an N-S oriented valley. It is drained to the north-east by the Mangaonaia Stream and fed by two unnamed streams from the south. The 0.03 km2 lake has a maximum water depth of 4 m.

Tariki Swamp is located c. 2.5 km SW of Ratapiko and about 22 km ENE of Mt.

Taranaki’s summit. It is bordered by the Manganui River to the west and the man-made Lake Ratapiko to the east. This small, peat-filled basin is abutted to the north by a massive debris avalanche deposit (Okawa Formation, 105 ka BP; Neall and Alloway,

2004). Alteration of drainage by the emplacement of this deposit is interpreted to have formed the peat bog.

Midhirst Swamp, located along the Salisbury Rd c. 3 km north-east of the village

Midhirst, is a small flat peat-filled area of 285m elevation. It has been first studied by Lees and Neall (1993), who collected two cores for pollen analysis from this area. This site is now mainly pasture but a small remnant of lowland forest lies about 1 km to the northeast. Midhirst Swamp is situated on top of a debris avalanche, named Ngaere Formation (c. 23,000 yr BP; Neall and Alloway, 2004; Fig. 1.3).

Eltham Swamp and Ngaere Swamp are large broad peat areas located east of Eltham and

separated from each other by a low, east-west trending ridge of Tertiary mudstone. The peatlands lie about 28 km ESE of Mt. Taranaki’s summit. At 220m, Ngaere Swamp is 10 m higher than Eltham Swamp. Both are peat-filled basins about 6 km long and 2 km wide. The Eltham Swamp is drained by the Mangimangi Stream to the south and by the Mangawhero Stream to the west. The Ngaere Swamp is drained to the north by the Ngaere Stream, a tributary of the Patea River. These peat bogs formed in response to a series of lahar and debris avalanche deposits emplaced between 23-50 ka BP (Neall and Alloway, 2004; Zernack et al., 2011a). These deposits altered the local topography, provided shallow aquitards, and partially blocked the tributary valleys of the eastern hill country, leading to peat accumulation. The Eltham Swamp has been previously investigated by McGlone and Neall (1994) and Tinkler (2013), who undertook pollen- based palaeovegetation investigations. Pollen analyses of peat-cores from the bog indicate that at ~13 ka BP, towards the end of the Last Glacial Maximum, the region was mainly open grassland. Shortly after (~12.5 ka BP) there was a rapid transition to a forest-dominated landscape. Today, these peatlands are cleared of their original forest, drained and developed into pasture, which has caused the peatland to subside at least 3 m (McGlone and Neall, 1994).

Lake Rotokare lies several kilometres further downwind, approximately 34 km ESE of

Mt. Taranaki’s summit. This Y-shaped lake was formed by damming of its drainage system following a Holocene landslide (Taranaki Catchment Commission, 1980). It has

an area of 0.25 km2 and a maximum water depth of 12 m. Turner et al. (2009) identified 42 tephra layers dated between c. 0.5 to 7 cal ka B.P within sediments of this lake.

Figure 1. 8 Location map of the study sites. Peatlands are highlighted by the orange fields; whereas stars represent specific coring location. Lakes are also indicated by stars, whereas green stars indicate previous (pv) coring sites of Turner et al. (2008a, 2009). Note the change in physiographic textures, which is the contrast between the smooth volcanic ring-plain to the west and the dissected mudstone hill country to the east.

1.6 Thesis outline

This thesis comprises primarily published or submitted manuscripts so there is some degree of repetition between chapters, particularly of the study site description and the methods used.

Chapter 1 provides a broad overview of the background geology and geography of the Taranaki region and outlines previous tephrostratigraphic studies, as well as identifying the aims and objectives of this study. Chapter 2 describes the methods used in this study and potential limitation that may come with them. The development of a robust stratigraphic framework of the eruption record at Mt. Taranaki is contained within Chapter 3 and 4. Chapter 3 considers distal tephra, whereby several lake and peat sediment tephra records are integrated to a composite high-resolution record by using physical and mineral-chemical characteristics, in conjunction with precise age determination of the tephra layers. Chapter 4 applies the techniques of Chapter 3 to on- cone flank and ring-plain proximal deposits and correlates previously defined tephra formations into the composite record to derive an overall revised tephrostratigraphy at Mt. Taranaki. Once this robust eruption record was established, it was used in Chapter 5 to investigate the eruption frequency of Mt. Taranaki, and, in Chapter 6, to model the underlying magmatic system(s) of the volcano. As a result, Chapter 5 focuses on probabilistic estimations, which are realised to be highly sensitive to incomplete and/or biased data sets and provides a revised and more accurate probabilistic eruption and hazard forecast for Mt. Taranaki. Chapter 6 synthesises all the chemical data and provides a detailed interpretation of the magmatic evolution of Mt. Taranaki over the past ~30 cal ka BP. A model is proposed that explains the compositional diversity of analysed tephra layers, e.g., in particular titanomagnetite compositions, in relation to the temporal aspect of magma evolution and eruption history. Chapter 7 closes this thesis with conclusions and recommendations for further work.

The entire data set compiled during this research is presented in the printed and electronic appendices, whereby geochemical analyses are presented as averages (±S.D.) in the printed version and as complete data sets within the electronic version.

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