P A L Y N O L O G Y OF A P E A T L A Y E R I N T E R B E D D E D W I T H R H Y O L I T I C T E P H R A L A Y E R S A T B U C K L A N D S B E A C H , A U C K L A N D :

A P R E L I M I N A R Y I N V E S T I G A T I O N

by R . M . Newnham 1 and J .A . Grant-Mackie 2

'Department of Geography, University of Otago, P.O. Box 56, Dunedin 'Department of Geology, University of Auckland, Private Bag 92019, Auckland

S U M M A R Y

Pollens from a peat layer sandwiched between two >1 m thick rhyolitic tephra layers at Bucklands Beach, Auckland, include the extinct palynomorph Proteacidites franktonensis Couper and indicate a probable Hautawan age (c. 2 Ma). This determination has important implications for the age and source of widespread rhyolitic tephra layers in the Auckland region, suggesting that a Coromandel Volcanic Zone source should be seriously considered. The palynofloras also indicate that the peat, which occurs at sea-level today, accumulated at a level close to the contemporaneous coastline. Significant vegetation and climate changes occurred within the time encompassed by peat accumulation, with the uppermost peat accumulating more slowly and under a slightly cooler and possibly drier climate than the lowermost peat.

I N T R O D U C T I O N

The presence around Auckland of peats and associated whitish muddy to sandy pumiceous deposits has long been noted. Hochstetter (1864) correctly identified the latter and suggested they had originated from eruption in the Taupo region. Most recently, Moore (1989) documented a number of sections around Auckland, showing from glass chemistry that the tephra layers could have come from centres in the Taupo Volcanic Zone, which has been active over the last 2 M a (Cole 1986). Moore undertook no work with the peats nor has she or any other worker obtained any numerical ages from the rhyolitic tephras within the region, nor any precise geochemical correlations with Taupo or Coromandel rhyolitic centres.

In April 1992, M r Geoff Fairfield of Bucklands Beach, Auckland, drew the attention of one of us (JAG-M) to a local section of interbedded peat and tephra layers. With him, Grant-Mackie visited the property of M r A.R. Croucher at 67 The Parade, Bucklands Beach, and sampled the peat temporarily exposed in a 2m high 4-5m wide site being prepared for garage construction. We report here the

133 Tane VoL 34, 1993

results of a preliminary palynological investigation of the peat unit and discuss the implications for the age and depositional environment of the peat/tephra sequence. We include recommendations for future work.

P R E V I O U S W O R K

There has been little detailed stratigraphic work undertaken on the interbedded peat and rhyolitic tephra layers mantling the Miocene Waitemata surface in the Auckland region (see Moore, 1989). Nevertheless, there is considerable potential for a combination of palynology of peats, geochemical fingerprinting of tephra layers, their depositional nature, numerical dating (e.g., 14C, K/Ar , fission track), and sea-level comparisons to provide data for a much more detailed knowledge of the Quaternary history of the region.

The vegetation history of the Auckland Isthmus is sketchy, with most known about the late Quaternary. Palynological profiles from Little Barrier Island (Harris, 1961) and from Pollen Island, in the west of Waitemata Harbour (Wright, 1951), dated as >20,000 years old (Chapman & Ronaldson, 1958), show few changes from present indigenous vegetation remnants. Lateglacial and postglacial vegetation changes on the Isthmus are described by Pocknall et al. (1989), Newnham (1990), and Newnham & Lowe (1991).

S T R A T I G R A P H Y

The excavation at 67 The Parade (Fig. 1) exposed a sequence a couple of metres thick, consisting of 1.2 m of very fine-grained peat (perhaps more correctly termed gyttja, mud with a very high content of fine organic detritus), with its base 2 m above local high tide mark, sandwiched between two whitish tephra layers of fine sand grainsize. The top of the upper tephra layer and bottom of the

Fig. 1. Location of Bucklands Beach site in east l 0 W e f w e r e

Auckland at 67 The Parade (arrow tip).

134 Tane, VoL 34, 1993

unexposed, but both layers were at least 1 m thick. Two samples were collected for pollen analysis, one (Rl l / f l83) from the base of the peat, the other (Rl l / f l82) from the top. These numbers, fl82 and f 183 denote localities entered in the archival New Zealand Fossil Record File of the Geological Society of New Zealand in 1:50,000 metre sheet district R l l . The sequence below the lower tephra layer is unexposed, but on the adjacent shore, c. 50 m to the north, mere is exposed at high tide level and below c. 0.5 m of pumiceous sand underlain by coarse, woody peat. It is assumed from other sections in the vicinity (see, e.g., those from Point England, Pakuranga and Highland Park in Moore, 1989) that these two sequences are in normal stratigraphic continuity.

P O L L E N R E C O V E R Y A N D P R E S E R V A T I O N

Standard extraction methods were employed (Faegri & Iversen, 1989) giving reasonable pollen yield and preservation quality for both samples. Two slides per sample were prepared and are lodged with the fossil collection of the Department of Geology, University of Auckland. It may be possible to improve pollen yield using more exhaustive methods. Pollen and spore counts and percentages based on a sum of dryland pollen are listed in Table 1. The estimated pollen concentrations are based on the method of adding exotic marker spores of known concentration to the samples.

DISCUSSION

Paleoenvironments

Both samples are dominated by pollen and spores indicative of restiad peat swamps: R l l / f l 8 3 contains abundant Empodisma (wire rush), Gleichenia (umbrella fern) and Cyperaceae (sedge) while R l l / f l 8 2 is dominated by Leptocarpus and to a lesser extent Empodisma and Cyperaceae (Table 1). These changes in local wetland vegetation may indicate that the site was comparatively closer to the coastline when the uppermost peats were deposited, as Leptocarpus is today characteristic of coastal and estuarine swamps. The dryland vegetation inferred from R l l / f l 8 3 , near the base of the peat unit, also indicates a coastal location, but perhaps further above sea level.

An unidentified species of Caryophyllaceae occurred quite close to the site during the time of R l 1/f 183, either in the restiad swamp or at its margins, where the swamp vegetation passed into shrub communities dominated by Leptospermum Kunzea (manuka/kanuka), Dodonaea (akeake), Pseudopanax and Pteridium (bracken). The inferred dryland vegetation of the Auckland region at

135

R l l / fl82 fl83

TALL GYMNOSPERM TREES: Agathis Dacrycarpus Dacrydium Halocarpus Libocedrus Phyllocladus podocarpoid Podocarpus Prumnopitys ferruginea Prumnopitys taxifolia Prumnopitys undiff.

(0) (0)

(5.7) (5.7) (0.8) (5.7) (3.3) (1.6) (2.5) (2.5)

(0)

(3.8) (5.7) (3.1) (1.3) (0.6)

15 (9.4) 8 (5.0)

15 (9.4) 11 (6.9) 4 (2.5) 2 (1.3)

TALL ANGIOSPERM TREES: Metrosideros Nestegis Nothofagus fusca type Weinmannia

1 (0.8) 1 (0.8)

18 (14.8) 1 (0.8)

24 (15.1) 1 (0.6) 4 (2.5) 4 (2.5)

SMALL ANGIOSPERM TREES & Aristotelia Ascarina Asteraceae Coprosma Dodonaea Griselinia Leptospermum type Leucopogon fasciculatus cf. Melicope Myrsine Pittosporum Pseudopanax Quintinia Scrophulariaceae

DRYLAND HERBS: Poaceae Haloragis

WETLAND PLANTS: Caryophyllaceae Cyperaceae

SHRUBS: 0 (0) 0 (0) 7 (5.7)

40 (32.8)

0 22

(0) (1.6) (2.5) (0.8) (0.8) d-6) (1.6) (3.3) (0.8)

(0)

1 3 0 1

11 1

(0.6) (1.9)

(0) (0.6) (6.9) (0.6)

18 (11.3) (0) (0)

(0.6) (0.6) (4.4)

(0) (0.6)

37 229

136

Empodisma 95 525 Gleichenia 0 238 cf. Leptocarpus 234 1

TREE FERNS: Cyathea dealbata type 1 17 Cyathea smithii type 1 6 Dicksonia squarrosa 0 6

FERNS: Hymenophyllum 1 0 monolete fern spores 2 0 Phymatosorus 0 1 Pteridium 0 6

EXTINCT: Proteacidites franktonensis 0 5

Pollen & Spore Concentration 272,963 136,321 (grains/cm3)

Table 1. The Parade pollen counts. Numbers in brackets are percentages for dryland taxa.

this time is mixed podocarp-kauri-angiosperm forest, resembling modern indigenous forest remnants in the area. Several species of tree fern were common but Nothofagus (beech) trees were probably only a minor component of the vegetation (Fig. 2). The presence of Ascarina (hutu) and Dodonaea and the overall inferred dryland vegetation composition suggests a moist, mild climate similar to the present. Dodonaea is today indicative of coastal forest.

The dryland vegetation inferred for R l 1/f 182 is rather different. Coprosma species, and to a lesser extent those of the Asteraceae (daisies), were probably common in the restiad mire and may also have featured in the dryland vegetation. The dominant tree pollen taxon, Nothofagus fusca type, indicates a change to beech (probably hard beech) forest, with some podocarps present, although generally at lower levels. Agathis (kauri) pollen was not recorded in this sample, nor were Ascarina or Dodonaea pollen. Overall, the inferred vegetation suggests a slightly cooler and possibly drier climate than for the previous sample. The higher estimated pollen concentrations (grains/cm3 of sediment; see Table 1) for R l 1/f 182 are interpreted as indicating a slower rate of sedimentation, rather man greater pollen deposition, as the vegetation communities inferred for R l 1/f 183 would probably have had greater biomass and higher pollen production.

137

Rll/fl82 Rll/fl83

I 100

*o

80

60

40

20 -

0 - 1

Herbs

S m a l l Angiosperm Trees k Shrubs

Other Ta l l Angiosperm Trees

Nothofagus

Tal l Gymnosperm Trees

Fig. 2. The Parade dryland pollen percentages for various ecological groups.

Age determinations

Most palynomorphs can be ascribed to extant taxa, which together with the fresh appearance of the sediment indicates at least a late Cainozoic age. The occurrence of Dodonaea pollen in R l 1/f 183 indicates both samples are no older than Pliocene (Mildenhall, 1980). Sample R l l / f l 8 3 also contains several specimens of the only extinct taxon recorded, Proteacidites franktonensis Couper, which has an age range of Tongaporutan to Hautawan (10 - 1.8 million years) and last appears in Northland in the Restionaceae Zone of the (Hautawan) Ohuka Formation (D.C. Mildenhall, pers comm., 1992). This sample is therefore likely to be either early Quaternary, or Hautawan. The apparent absence of other characteristic Hautawan pollen types, Cranwellia striata, Nothofagidites cranwelliae, Podosporites parvus, Cupanieidites, Beaupreaidites elegansiformis, and Polycolpites reticulatus, may reflect an inadequate pollen recovery rather than true absence. Future investigations involving this material should aim for a greater pollen recovery.

Reworking of the five P. franktonensis grains recorded is assumed not to have occurred as the palynomorphs are in an excellent state of preservation,

138

unlike that which would be expected for reworked grains. However, we cannot discount this possibility and it should be further considered in future investigations.

Sample R l l /f l82, from 1 metre above sample R l 1/f 183, contains only extant pollen types, and is therefore younger, by an unknown amount. On the one hand, the inferred changes in local wetland as well as regional dryland vegetation suggest a significant passage of time separated the two samples. Quaternary palynostratigraphic investigations in Northland indicate that the dominant regional vegetation cover may have alternated between kauri/podocarp forest and beech forest, possibly related to interglacial/glacial cycles (Harris, 1953; Murray & Grant-Mackie, 1989; Newnham, 1992). Abundant Nothofagus pollen has been reported from at least one site on the Auckland Isthmus, the Aotea Centre excavations (Newnham, 1990), suggesting that this Northland pattern may at times have extended to the Auckland region also. If this interpretation is correct, sample R l 1/f 182 may indicate a change to a glacial environment with an inferred time gap of about 104 - 105 years following the time of R l 1/f 183.

On the other hand, the development of Leptocarpus ?coastal/estuarine swamp suggests a higher sea-level which is inconsistent with an inferred change to a glacial environment. It may be that sea level was not higher at the time of R l 1/f 182 but that an estuarine environment extended some distance inland as occurs at several points on the Isthmus today. This contention requires further investigation. Regardless, however, it is likely that the entire peat unit accumulated in a position near the contemporaneous coast. As the peat unit occurs today near sea level, this suggests that, in the Auckland region, at times during the Hautawan, sea level was close to that at present.

Age and source of the tephra layers

The presence of an extinct pollen species and an Hautawan age allocation for f 183 (accepting that P. franktonensis is not reworked in fl83) are very significant in considerations of sources for the associated tephra layers and also others that are thick and widespread around the Auckland region. An Hautawan or older Pliocene age for the two local tephras below fl83 probably lies beyond the age range of the Taupo Volcanic Zone rhyolites (Cole, 1986) but within that of the Coromandel Volcanic Zone centres (9 - 1.5 Ma: Skinner, 1986). Moore (1989) considered the latter too old to be sources for the Auckland tephra layers, which she found to be geochemically compatible with those of the Taupo and Okataina volcanic centres. However, this compatibility was in general terms only and does not deny the possibility of derivation from a Coromandel source. Not enough geochemical fingerprinting has yet been done to confirm or refute such a possibility, let alone relate any of the Auckland tephras to any one eruptive unit.

139

Clearly, however, this Hautawan correlation demands that a Coromandel Volcanic Zone source for at least the older Auckland tephras be seriously) considered.

A C K N O W L E D G E M E N T S

Thanks to D.C. Mildenhall who kindly agreed to check the palynologic identifications and whose comments assisted with the age determinations. Dr D.J. Lowe kindly reviewed the manuscript. We also thank Mr Geoff Fairfield for alerting us to the site and for encouraging this investigation. Louise Cotterill draughted Figure 1.

R E F E R E N C E S

Chapman, V . J . & Ronaldson, J.W. 1958: The mangrove and saltmarsh flats of the Auckland Isthmus. N.Z.D.S.1.R Bulletin 135

Cole, J.W. 1986: Distribution and tectonic setting of late Cenozoic volcanism in New Zealand. Royal Society of New Zealand Bulletin 23: 7-20

Faegri, K . & Iversen, J. 1989: Textbook of Pollen Analysis, 4th edn. John Wiley & Sons, Chichester. 328 pp.

Harris, W.F . 1953: Palynology as a key to the history of New Zealand vegetation. Proceedings of the 7th Pacific Science Congress 5: 164-178.

Harris, W.F . 1961: Peat samples. In W . M . Hamilton (ed.): Little Barrier Island (Hauturu). N.ZD.S.I.R Research Bulletin 137: 78-86.

Hochstetter, F. von, 1864: Reise der Osterreischen Fregatte Novara urn die Erde in den Jahren 1857, 1858, 1859. Geol. Theil. 1. Vienna, Govt. Printing Office (in English translation by C .A. Fleming, 1959, Govt. Printer, Wellington).

Mildenhall, D .C. 1980: New Zealand late Cretaceous and Cenozoic plant biogeography: A contribution. Palaeogeography, Palaeoclimatology, Palaeoecology 31: 197-233.

Moore, C . L . 1989: Rhyolitic tephra in the Auckland region. Unpubl. thesis, University of Auckland: 53 pp.

Murray, D.F. & Grant-Mackie, J. A . 1989: Lower Pleistocene pollen profile, Rototuna Formation, North Kaipara Barrier, New Zealand. In K . B . Sporli and D. Kear (eds): Geology of New Zealand: accretion, allochthons and arcs at the edge of the New Zealand microcontinent. Royal Society of New Zealand Bulletin 26: 225-235.

Newnham, R . M . 1990: Late Quaternary palynological investigations into the history of vegetation and climate in northern New Zealand. Unpublished PhD thesis, University of Auckland: 359 pp.

Newnham, R . M . 1992: in press (Journal of Biogeography): A 30,000 year pollen, vegetation, and climate record from Otakairangi, Northland, New Zealand.

Newnham, R . M . & Lowe, D.J. 1991: Holocene vegetation and volcanic activity, Auckland Isthmus, New Zealand. Journal of Quaternary Science 6: 177-193.

Pocknall, D.T., Gregory, M.R . & Greig, D . A . 1989: Palynology of core 80/20 and its implications to understanding Holocene seal level changes in the Firth of Thames, New Zealand. Journal of the Royal Society of N.Z. 19: 171-179.

Skinner, D .N .B . 1986: Neogene volcanism of the Hauraki Volcanic Region. Royal Society of New Zealand Bulletin 23: 21-47.

Wright, N . B . 1951: A preliminary investigation of some peat deposits near Kaitaia and Auckland, New Zealand. Unpublished M S c thesis, University of Auckland.

140