Tuatara: Volume 32, April 1993
Last Glaciation/Postglacial Pollen Record for Porirua, near Wellington, New Zealand
Last Glaciation/Postglacial Pollen Record for Porirua, near Wellington, New Zealand
Spores and pollen obtained from a condensed stratigraphic sequence of carbonaceous silts and muds from Porirua, near Wellington, indicate that the sequence straddles the Last Glaciation/Postglacial boundary. The lower part of the sequence contains dispersed rhyolitic glass shards identified as belonging to the Kawakawa Tephra. This, and a radiocarbon date on a basal Phyllocladus log of 21100 +/- 300 yrs BP, gives a maximum age for the base of the sequence. The spores and pollen indicate a change from subalpine Phyllocladus scrubland and/or grassland to lowland podocarp forest dominated by Podocarpus totara and Dacrydium cupressinum and a change from a cold, moist environment to a warm, humid one. The sharp junction between these two assemblages indicates an unconformity, but none is apparent in the monotonous sequence of silts and muds. Sequences straddling the Last Glaciation/Postglacial boundary without major and distinctly recognisable discontinuities have not been recognised from the Wellington are, but at Porirua it is suggested that much less time is missing.
Pollen analysis, pollen diagram, paleoenvironments, radiocarbon dates, Kawakawa Tephra, Last Glaciation, Postglacial, Porirua, Wellington.
In late 1988 an excavation, beneath what is now the Porirua K-Mart shopping complex, exposed a 1.7 m sequence of carbonaceous mud infilling a fossil gully. The site is about 50 m above sea level on the east side of the main road to Titahi Bay, Porirua, near Wellington, at grid reference R27/644 061 (grid reference for sheet R27 based on the national thousand metre grid of the 1:50 000 topographical map series INFOMAP 260). The fossil record number assigned is R27/f163 (New Zealand Fossil Record File number based on the metric INFOMAP260 series). A more general picture of the locality, which is about 1 km from the head of Porirua Harbour, is given by Mildenhall (1980) and Pillans et al. (figure 1; 1993). The sequence is a non-bedded, graded mud with secondary granules dispersed through it. The colour when wet is medium to dark grey.
A sequence of fifteen samples was collected in December 1988 by Brad Pillans and Brent Alloway (Victoria University of Wellington) for palynological analysis. Also noted at the time were pieces of wood, leaves and cladodes of Phyllocladus. Subsequently, a radiocarbon date on Phyllocladus wood from the base of the section, close to rhyolitic glass shards, which form about 5% of the fine sand fraction in the basal 40 cm and identified as belonging to the Kawakawa Tephra (Pillans et al. 1993), gave an age of 21 100 +/- 300 yrs BP (NZ7707). Since the glass shards are dispersed throughout much of the base of the sequence, and not in a discrete horizon, the tephra is almost certainly recycled.
The sequence overlies weathered Mesozoic greywacke basement, downthrown to the east of the Ohariu (Owhariu) Fault (Grant-Taylor 1976; Mildenhall 1980), and forming part of a depressed basin modified by a Last Glaciation river valley system which formed when sealevel was much lower (Stevens 1974). The subsequent rise in sealevel, which began about 14 000 years ago, was the catalyst allowing for Postglacial sedimentation to occur.
All fifteen palynological samples were examined and counted 500 grains were counted from each sample and the percentages of the various taxa calculated. In the summary pollen diagram (Fig. 1) the percentages of spores are based on the full count while those of the pollen grains are based on total pollen only. Only the most significant taxa (in terms of numbers) are presented, while the distribution of climatically and environmentally significant taxa are indicated. A brief summary of the palynological results, including the same pollen diagram, was originally published in Pillans et al. (1993).
The pollen percentages determined from the samples from 1.3–1.7 m are less reliable than the others, as they are completely dominated by Cyathea and smooth monolete spores (Blechnum?) leaving total pollen counts at less than 100 grains.
The sequence (Fig. 1; Zone A) records the drying and silting up of an acid, woody swamp, dominated by Phyllocladus, probably P. alpinus, and Halocarpus, through which fresh water flowed, and occurring in a moist, cool, subalpine grassland/shrubland environment. In the basal 1.2 m, pollen from Dracophyllum, Asteraceae and Poaceae is also common. The acidity of the depositional environment is indicated by the presence of numerous Gleichenia circinata spores. The lack of pollen from beeches and podocarps, other than Phyllocladus and Halocarpus, indicate that no substantial forests, or possibly even refugia, existed in the area at this time. The existence of fresh water is indicated by the presence of colonies of the shallow fresh water, lacustrine, alga Botryococcus. Since this alga requires some degree of warmth to exist, it suggests that the summers, at least, were warm enough to allow continual survival of this taxon.
This swamp changed into a full forest environment (Fig. 1; Zone B), dominated by tree podocarps, initially Podocarpus totara and Prumnopitys taxifolia, and subsequently Dacrydium cupressinum. The magnitude of the change from cool and relatively moist to warm, but more humid conditions, means that the sequence probably represents an abandoned stream bed and/or gully gradually silting up during a period of climate change rather than a seral change in the vegetation, although both effects are almost certainly involved. This change is not dated, but comparison with similar changes elsewhere in the Wellington area, at Pauatahanui, Petone, and Evans Bay (Mildenhall 1980; Mildenhall & Moore 1983; Lewis & Mildenhall 1985, respectively), would indicate that it is older than 9 500 yrs BP, since Dacrydium cupressinum dominates at the commencement of Postglacial sedimentation at each of these sites. This recommencement of sedimentation is almost certainly connected with the Postglacial rise in sealevel and is everywhere uncomformable on greywacke basement or unfossiliferous Last Glaciation gravel.
Thus, the top of the sequence at about 1.6 m above basement, where Dacrydium cupressinum first dominates, would be no younger than 9 500 yrs BP. If the exposed sequence represents sedimentation from the time immediately after the deposition of the Kawakawa Tephra, at 22 590 +/- 230 yrs BP (Wilson et al. 1988), to approximately 9 500 yrs BP, then 1.7 m of sediment in 13 100 years gives a very low sedimentation rate of about 1 mm every 8 years.
Therefore the section is either condensed or the disconformity represented by the sudden change in the vegetation represents more time than the sediments themselves. A gap in sedimentation after deposition of the Kawakawa Tephra, representing much of the late Last Glaciation and a small part of the early Postglacial, is widespread in the Wellington area (Lewis & Mildenhall 1985; Mildenhall 1992a; 1992b), and elsewhere (McGlone et al. 1984), where it is recorded as being the result of landscape instability marking the onset of the maximum of the Last Glaciation.page 24
The sequence also shows the following palynological features:
1. Unlike several other sequences of the same age in the Wellington area, for example Lindale, Paraparaumu (McIntyre 1970) and Wallaceville, near Upper Hutt (Harris & Mildenhall 1980; 1984), Nothofagus menziesii and to a lesser extent N. fusca type (pollen of Nothofagus fusca, N. solandri and N. truncata are indistinguishable) were not prominent forest trees in the area at any stage during the deposition of these sediments.
2. Most of the samples are dominated by spores which mask the character of the surrounding vegetation, especially in the upper part of the sequence. It is quite clear that the spores are local and come from plants growing in a damp valley or stream-side environment because they are generally well preserved, while the pollen grains are often poorly preserved and show signs of having been transported to the site of deposition by water.
3. The dominance of Phyllocladus (Zone A; Fig. 1) at the base of the sequence may prove to be useful in correlating other undated Phyllocladus dominant sequences in the Wellington area. However, Phyllocladus is not known from dated sequences to be as dominant at this time, although it is known to occur at some localities within this age range (Harris & Mildenhall 1980; 1984; Mildenhall 1987; 1992a), as well as much older localities (e.g. Mildenhall 1983). Other taxa, notably Nothofagus menziesii and N. fusca, are the more prominent pollen types from woody vegetation (Mildenhall 1992b).
4. The presence of some taxa gives additional information on the environment.
Epilobium grows in open spaces; its presence in the base of the sequence in tetrads shows that the genus grew at the site of deposition.
The presence of abundant Botryococcus, Haloragis, Myriophyllum, Plantago and Potamogeton in the basal 1.2 m of the sequence all indicate flowing, probably shallow, fresh water. The presence of numerous liverwort and hornwort spores (Anthoceros, Megaceros, Phaeoceros and the family Ricciaceae) suggests sheltered, moist, stream-side environments, with bare ground constantly available for these colonising plants. The frequency and abundance of Sphagnum spores indicates that much of the sedimentation occurred in a sphagnum swamp.
Fuchsia, found in the top 0.5 m of the sequence, is often found growing along banks of streams; it needs semi-shade for germination and seedling development. The presence of Pseudowintera often indicates disturbance in marginal forests; it also needs light for germination and seedling development.
The relative scarcity of Prumnopitys taxifolia and Dacrycarpus dacrydioides and the relative abundance of Podocarpus totara and Dacrydium cupressinum indicates an overall lack of drought conditions, although it is possible that soil conditions rather than climate determined the types of forest plants.
The presence of pollen from the root parasite Dactylanthus taylori, towards the base of the sequence, is interesting for several reasons. Firstly, it is restricted to the North Island, where it is most abundant on the Volcanic Plateau (Macphail & Mildenhall 1980) and occurs no further south than Kaitoke, near Wellington (Aston 1909). Secondly, it is more tropical or sub-tropical in origin, and, finally, it is not common in the fossil record. Its occurrence at the base of the sequence in sediments deposited during a period of cool to cold climate to the south of its current range is unexpected. It may be recycled.
Most of the other taxa identified are wide-ranging in ecological tolerance.
There are several other sedimentary sequences in the Wellington region, containing the Kawakawa Tephra, that show similar characteristics.
At Wainuiomata the Kawakawa Tephra occurs near the top of a 61.6 m sequence (Begg & Brown 1991) which goes from fusca beech at the base, through menziesii beech into Phyllocladus. The topmost Last Glaciation samples are dominated by Poaceae (Mildenhall 1992b).
Similarly, at Taita Phyllocladus, with Poaceae and Nothofagus menziesii, occurs immediately below the Kawakawa Tephra with N. menziesii and Podocarpus and Prumnopitys species dominant above (Harris & Mildenhall 1980; 1984). The sharp changes in the vegetation above and below the Kawakawa Tephra indicate an unconformity and that considerable time may be missing from the condensed 2 m section at this locality.
At the site of the proposed new Pacific Cultural Centre and National Museum, Lambton Harbour, Wellington, 28 m of blue-grey to brown-grey silt occurs below 12 m of fill. At about 16–17 m glass shards, presumed to be from the Kawakawa Tephra, were identified in the field by R.D. Beetham (J.G. Begg pers. comm. August 1992). Here the entire silt sequence consists of grass-dominant assemblages, with Phyllocladus most common between 18–16 m (Mildenhall 1987).
Clearly, therefore, parts of the Wellington region, at the peak and towards the end of the Last Glaciation, contained a montane grassland/shrubland with sparse, probably stunted Phyllocladus as the dominant tree species (Mildenhall 1992b). Conditions were cold, constantly damp and probably windy, with temperatures the equivalent of the upper part of the Tararuas at the present day. The occasional pollen of more temperate taxa, like Dacrydium cupressinum, Dacrycarpus dacrydioides and Nothofagus probably arrived via long distance transport (along with the occasional Australian taxon Casuarina, which occurs in many of the sequences), or from trees that existed locally in sheltered valleys.
Brad Pillans and Brent Alloway, Victoria University of Wellington, are thanked for collecting the samples and for the use of some of their site information. Geoff Gregory, Institute of Geological and Nuclear Sciences Limited, and Bruce Sampson. Victoria University of Wellington, reviewed the manuscript and made useful suggestions for improvement. The pollen diagram was drafted at Victoria University under the guidance of Brad Pillans, who also reviewed a draft of the manuscript. The samples were processed by Roger Tremain to whom thanks are also extended.
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Fig 1. Summary pollen diagram and stratigraphic column (with heights in metres above base) from Porirua, near Wellington. x = less than 1%. The diagonal lines at the base of the stratigraphic column represent weathered greywacke basement. The following spores and pollen are also present:- Ac = Acaena; B = Botryococcus; D = Dacrycarpus dacrydioides; Dt = Dactylanthus taylori; E = Epilobium; F = Fuchsia; G = Geranium; L = Leptospermum; Ps = Pseudowintera; R = liverworts/hornworts; S = Sphagnum; T = Typha. NZ-7707 = 14C date on wood of 21 100 +/- 300 yrs BP. From Pillans et al. (1993).