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The age and significance of in-situ sinter at the Te Kopia thermal area, Taupo Volcanic Zone, New Zealand
Geothermics 29 (2000) 367±375
The age and signi®cance of in-situ sinter at the Te Kopia thermal area, Taupo Volcanic Zone, New Zealand Rod Martin a, Dallas C. Mildenhall b, Patrick R.L. Browne a,c, Kerry A. Rodgers a,* a
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand b Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand c Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand Received 25 November 1997; accepted 5 October 1999
Abstract One hundred pollen grains and spores, recovered from a single sample of in situ silica sinter from the Te Kopia geothermal ®eld, include some from a podocarp forest that grew in a temperate, frost-free climate, unlike that of today, as indicated by the presence of Ascarina. Also present are pollen from taxa introduced within the last 100 years. Ascarina has been absent from the area since at least 1800 B.P. and its presence in the sinter indicates that alkali chloride waters discharged at the surface of Te Kopia before 1800 B.P., and possibly before 3500 B.P. Although palynology is a powerful tool to place age limits on ¯uctuations in the shallow hydrology of geothermal ®elds, interpretation must be moderated by considering the present ¯ora and also changes in both local and regional ¯ora, habitat and climate. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. Keywords: Palynology; Sinter; Ascarina; Geothermal; Te Kopia; Taupo; New Zealand
* Corresponding author. Tel.: +64-9-373-7509; fax: +64-9-373-7436. E-mail address: [email protected] (K.A. Rodgers). 0375-6505/00/$20.00 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 5 - 6 5 0 5 ( 0 0 ) 0 0 0 0 8 - 0
368
R. Martin et al. / Geothermics 29 (2000) 367±375
1. Introduction Surface alteration is a common expression of geothermal systems and is a persistent signature of the hydrological conditions prevailing at the time it occurred. This feature may be used to assess ®elds where no springs now discharge and thus standard chemical geothermometry techniques cannot be used. In these situations, knowledge of the distribution and types of surface alteration serve as useful substitutes. For example, the presence of sur®cial silica sinter implies that the alkali chloride waters from which it was deposited originated in a reservoir whose temperatures were hotter than 1808C. Some geothermal ®elds have lifespans of the order of 200,000 years (Browne, 1978) and a ®eld could still have an exploitable reservoir long after its surface thermal activity has ceased. In this case, it becomes necessary to determine the age of any surface alteration to decide whether or not the prospect is worthy of further investigation. The ages of some silica sinters can be determined by tephrochronology (e.g., Lloyd, 1972), dating wood or charcoal fragments that they contain (e.g., Bignall and Browne, 1994), or by palynology (e.g., Browne, 1981). The last named method oers promise despite the sometimes mistaken belief that pollen grains do not survive in hot water. We distinguish here, however, between silica sinter and silica residue. The latter is a product of protracted acid leaching that occurs in a corrosive environment where pollen and plant material are seldom preserved. Although palynology has been used to date sinters, and hence, past sur®cial thermal activity, at the Ngawha (Harper, 1980), Orakeikorako (Herdianita, 1997) and Ohakuri (Henneberger, 1983) ®elds, it has not been used routinely to do so as far as we know. Here we report the application of palynology to newly discovered in situ silica sinter at the Te Kopia geothermal ®eld. Only a single sample was analyzed and no systematic seasonal data were available on the modern pollen rain beyond that from general regional studies. This situation is similar to some geothermal exploration surveys constrained by time, costs and poor exposure. At Te Kopia, the pollen data proved sucient, when considered together with knowledge of the present ¯ora and changes that have occurred to it, the habitat and the climate, to help interpret the Holocene shallow evolution of the hydrology of this long-lived system (0120,000 years: Bignall and Browne, 1994). 2. Setting and general description The Te Kopia ®eld (Fig. 1) is located in the Taupo Volcanic Zone at latitude 388 24' S, longitude 1768 13' E and straddles the west-facing scarp of the upthrown eastern block of the active Paeroa fault (Bignall and Browne, 1994). Patches of altered and steaming ground extend for over 2.5 km along the fault scarp and within 500 m of it. Two deep wells, drilled in the ®eld to about 1400 m depth, showed that temperatures of up to 2408C occur within its reservoir. Today, the sur®cial activity is dominated by steam discharges but several areas
R. Martin et al. / Geothermics 29 (2000) 367±375
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contain perched, shallow steam-heated ponds ®lled with acid-sulphate waters that are mixtures of steam condensate and rainwater. The maximum concentration of chloride in these waters is 7 mg/kg, so little thermal water in them derives from the reservoir itself. In the central lakes area (Fig. 1) there are occasional remnants of silica sinter scattered along the foot of the scarp and incorporated into landslide debris. Wood encased in one ¯oat block of silica sinter gave a C14 age of 3026243 B.P. (Bignall and Browne, 1994). Not until 1996 was silica sinter located in place. The in-situ sinter occurs in bush near steaming ground some 30 m above the Te Kopia valley ¯oor in the central lakes region (Fig. 1). It outcrops as a near continuous buttress 19 m long, 2 m wide and from 3 to 5 m high. Its top is ¯at and the deposit has its longer axis oriented in a north-east±south-west direction i.e. parallel to that of the Paeroa fault itself. The buttress is composed of many layers of porous, spongy silica, locally stained orange and of varying thickness (up to 1.5 cm). These layers are
Fig. 1. Locality map, Te Kopia thermal area, showing location of in situ sinter buttress. TK1 refers to one of the two drill hole sites that tapped the reservoir waters.
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horizontal on the inner, eastern side of the buttress, but drape to become nearly vertical at the outer, western face. The silica is dominantly opal-A with minor opal-CT and rare quartz. No other phases were seen in thin section and wood, charcoal and tephra were not found. 3. Palynology 3.1. Method Sampling for palynological analysis was restricted by conservation provisions to a single specimen. This was a typical porous, friable sinter, 60 20 20 mm and weighing 024 g, taken some 600 mm above the ground at the northern end of the buttress: AU47311, New Zealand Fossil Record File Number U17/f15, grid reference U17/904 054 [sheet U17, 1:50,000 topographic map series INFOMAP 260]. Pollen and spores were extracted by standard palynological procedures. The specimen was ®rst washed in distilled water and then dissolved in hydro¯uoric acid to release its organic contents, which were ®ltered. The ®ner fraction containing spores and pollen was mounted on a glass slide for microscopic examination. Many of the spores and pollen are a distinct yellow rather than pale coloured or translucent, as modern pollen become, following extraction treatment. The darker colour, although slight, may result from either aging or, perhaps by protracted heating in hot spring water. Although spores and pollen are easy to extract from sinter, they are sparse, commonly requiring more than 5 g of material to be processed to obtain at least 100 grains. The Te Kopia sample was no exception. A count of only 100 grains was completed from three slides. Since spores and pollen are produced in abundance much pollen may have been ¯ushed from the site. Alternatively, their entrapment and preservation in the sinter may have been somewhat arbitrary. 3.2. Results The palynomorph assemblage (Table 1) consists of a mixture of taxa from: 1. species, including grass, Plantago and Pinus, introduced into the Te Kopia area within the last 100 years following European settlement; 2. native species, typical of a podocarp dominant forest, some now growing in but others now absent from the Te Kopia district. The poor preservation of some grains made them dicult to identify. Along with spores and pollen of higher plants, ®nely disseminated organic material, isolated vessels and ®bres, and rare fungal spores were also recovered. All the identi®ed palynomorphs are assumed to have been incorporated in the sinter during its deposition or else deposited later into cavities such as some of the grass grains that still have cell contents preserved. Laboratory contamination is
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unlikely as regular contamination checks show that no more than 1 to 2 grains could be accidentally introduced from the atmosphere during sample preparation, in the unlikely event of the material being exposed to the atmosphere for more than 10 min. There are some slight dierences in preservation of the grass pollen, Table 1 Pollen and spore assemblage extracted from Te Kopia sinter Spores and pollen from taxa likely to be in situ Mosses Bryophyta Sphagnum Ferns Pteridophyta Dicksonia squarrosa (tree fern) Cyathea species (tree fern) Undierentiated monolete spores Trees Podocarpaceae Phyllocladus (celery pine) Dacrydium cupressinum (rimu) Podocarpaceae (undierentiated) Nothofagaceae Nothofagus (Fuscospora ) (southern beech) Small trees and shrubs Chloranthaceae Ascarina lucida (hutu) Rubiaceae Coprosma Myrtaceae Metrosideros (rata) Malvaceae Plagianthus (ribbonwood) Rhamnaceae Pomaderris Asteraceae (daisy) Elaeocarpaceae Herbs Poaceae (grass) Pollen from taxa known to be introduced Trees Pinaceae Pinus (exotic pine) Herbs Poaceae (grass) Plantaginaceae Plantago lanceolata (plantain) Unidenti®ed very poorly preserved pollen
Raw counts 1 2 11 6 8 12 8 3 2 1 5 1 ±a 1 5 Part of 14
15 Part of 14 1 4
a A single Pomaderris grain was identi®ed after a count of 100 grains had been made from three slides.
372
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for example, which indicates that some may be younger but the variations are so minor that they may be due to dierences in degradation prior to or after deposition. 4. Interpretation 4.1. Present vegetation at Te Kopia Like much of the North Island of New Zealand, vegetation of the Te Kopia area has undergone major changes in the past several hundred years. Following the last glacial period, and prior to Maori occupation some 800 years ago, the region became covered by extensive podocarp forest (McKinnon, 1997). Over several hundred years, repeated burnings by the Maori helped destroy this forest (Vucetich and Wells, 1978). This process was continued by European occupiers who introduced grasses and other northern hemisphere plants, beginning in the late nineteenth century, followed by the planting of vast exotic pine forests throughout the Taupo Volcanic Zone from 1920 (McKinnon, 1997). In a survey of the present vegetation of the Te Kopia area, Burns (1997) recognised a distinctive ¯ora among which important forms are mosses, ferns, Kunzea ericoides var. micro¯ora (kanuka), and Weinmannia racemosa (kamahi). The dominant palynomorphs in the three slides made from the sinter dier from those of this modern vegetation. The presence of pollen of introduced plants (grass, Plantago and Pinus ), if they were deposited with the sinter, would indicate that alkali-chloride waters discharged <100 B.P. Hochstetter's (in Fleming, 1959) description of the thermal activity at Te Kopia, following his 1859 visit, fairly closely matches that of the present day and, although he noted a siliceous sinter in the area, he makes no mention of discharging chloride springs. Nor have later visitors reported sur®cial alkali chloride pools or springs. This suggests that the pollen grains of introduced plants must have become trapped in the cavities of the porous sinter, despite attempts to wash them out prior to processing. Kunzea ericoides produces abundant pollen but none from it nor Leptospermum scoparium (manuka), also now locally abundant, were identi®ed in the sample. However, pollen of Metrosideros (rata), another member of the family Myrtaceae, forms 5% of the total assemblage. Metrosideros is not listed by Burns (1997) as occurring locally. This suggests that this pollen in the sinter came either solely from a distant source or from vegetation now locally absent. The former is unlikely. Regional pollen samples contain a preponderance of Kunzea and Leptospermum (Macphail and McQueen, 1983). This is the case even where the ¯owering by these plants is intermittent (Burns, 1997). The absence of modern Kunzea and Leptospermum pollen in the cavities of the sinter is unexplained unless washing in distilled water prior to processing eectively eliminated these very small grains (8±18 mm: McIntyre, 1963) but not the larger grass, Plantago and Pinus grains.
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4.2. Present-day ambient temperatures in the Te Kopia area The present climate at Te Kopia is characterised by frequent heavy frosts; there are over 110 frost days per year at Wairakei, 10 km to the south and at 60 m lower elevation (New Zealand Meteorological Service, 1980). There are occasional snow falls at Te Kopia but snow does not remain long on the ground. The area is very de®nitely not frost-free. 4.3. The podocarp assemblage: the signi®cance of Ascarina and the age of the sinter The podocarp assemblage is consistent with species that grew in the Te Kopia region until climatic change and man-induced deforestation eliminated them. The presence of Ascarina pollen in the sinter indicates that the podocarpdominant forest grew in a frost-free climate. Ascarina lucida was well established in the central North Island 10,000 years ago but declined sharply in abundance some 5000 years B.P., becoming very sparse about 3500 B.P., to ®nally die out in this region about 1800 B.P. (McGlone, 1983; cf. McGlone and Neall, 1994). McGlone (1983) suggested that Ascarina had been eliminated from the Rotorua catchment, 20 km north of Te Kopia, by ca. 4000 B.P. The colder climate in Te Kopia would have caused it to die out there even earlier and there is no evidence of a relict community surviving in the area. Today, the species is largely con®ned to scattered coastal drought- and frost-free localities in areas of high rainfall (McGlone and Moar, 1977). The closest location where it now grows is over 100 km to the northwest. The dominance of Dacrydium cupressinum and the low abundance of Nothofagus pollen indicate that the sinter is at least older than 600 B.P. (cf. McGlone, 1983), predating deforestation. However, the Ascarina pollen point to an age no younger than 1800 B.P., and one that may well be older than 3500 B.P. Such an interpretation means that the change from podocarp forest to a local Weinmannia/Leptospermum/Kunzea association occurred at Te Kopia since the sinter was deposited. Bignall and Browne (1994) reported a C14 age of 3026 2 43 B.P. from wood in sinter ¯oat some 200 m to the north and at a slightly higher elevation. This ¯oat did not come from the in situ sinter but its age provides evidence of the contemporary discharge of alkali chloride waters elsewhere at Te Kopia.
5. Conclusions Despite the limitations of a data set of only 100 grains, we conclude: 1. The presence of silica sinter at the Te Kopia thermal area shows that alkali chloride waters discharged here as geysers and/or hot pools perhaps as recently as 1800 B.P. but possibly no later than 3500 B.P. 2. Alkali chloride waters do not discharge at Te Kopia now but the mere presence
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of the sinter points to the existence of a geothermal reservoir whose temperatures exceeded 1808C at the time sinter formed. Given the longevity of geothermal ®elds, on this evidence alone, the Te Kopia thermal area would oer a worthwhile drilling target as con®rmed by the two drill holes. 3. The pollen trapped within the sinter record a ¯ora growing in the Te Kopia area, prior to human occupation, that indicates warm, frost-free conditions prevailed Ð unlike those of today. 4. Palynology is a powerful technique to assign age limits to silica sinter that provides evidence about the sur®cial evolution of a geothermal ®eld. 5. Optimum interpretation of palynological results requires an understanding of the present ¯ora and for this to be balanced against a knowledge of past botanical, habitat and climate changes.
Acknowledgements The Department of Conservation, along with Mike and Delwyn Murphy, generously allowed access to the ®eld area and also granted approval to remove the specimen for analysis. The pollen sample was prepared by Roger Tremain. Four referees (Professor A. Traverse, Drs. P. Wigand and J. Ward and one anonymous) greatly improved the rigor of the paper.
References Bignall, G., Browne, P.R.L., 1994. Surface hydrothermal alteration and evolution of the Te Kopia thermal area, New Zealand. Geothermics 26, 645±658. Browne, P.R.L., 1978. Hydrothermal alteration in active geothermal ®elds. Annual Reviews, Earth and Planetary Science, 6, 229±250. Browne, P.R.L., 1981. Hydrothermal alteration at Ngawha. Department of Scienti®c and Industrial Research Geothermal Report 7, 87±94. Burns, B., 1997. Vegetation change along a geothermal stress gradient at the Te Kopia steam®eld. Journal of the Royal Society of New Zealand 27, 279±294. Fleming, C.A. translator, 1959. Hochstetter, F. von (1864) Geology of New Zealand. Contributions to the Geology of the Provinces of Auckland and Nelson. Government Printer, Wellington. 320 pp. Harper, R.T., 1980. Geochemical study of the Omapere Soda Springs silica sinter deposit (Ngawha Geothermal Field), Northland, New Zealand. Unpublished Project Report, Geothermal Institute, University of Auckland, 80.13. Henneberger, R., 1983. Petrology and evolution of the Ohakuri hydrothermal system, Taupo Volcanic Zone, New Zealand. Unpublished M.Sc. Thesis, University of Auckland. Herdianita, N.R., 1997. Characteristics of silica sinter deposited from thermal waters. Unpublished M.Sc. Thesis, University of Auckland. Lloyd, E.F., 1972. Geology and hot springs of Orakeikorako. New Zealand Geological Survey Bulletin 85, 164. Macphail, M.K., McQueen, D.R., 1983. The value of New Zealand pollen and spores as indicators of Cenozoic vegetation and climates. Tuatara 26, 37±59.
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McGlone, M.S., 1983. Holocene pollen diagrams, Lake Rotorua, North Island, New Zealand. Journal of the Royal Society of New Zealand 13, 53±65. McGlone, M.S., Moar, N.T., 1977. The Ascarina decline and post-glacial climatic change in New Zealand. New Zealand Journal of Botany 15, 485±489. McGlone, M.S., Neall, V.E., 1994. The late Pleistocene and Holocene vegetation history of Taranaki, North Island, New Zealand. New Zealand Journal of Botany 32, 251±269. McIntyre, D.J., 1963. Pollen morphology of New Zealand species of Myrtaceae. Journal of the Royal Society of New Zealand, Botany 2 (7), 83±107. McKinnon, M. (Ed.), 1997. New Zealand Historical Atlas. Ko papatuanuku e takoto nei. David Bateman, Auckland, p. 290 pp. New Zealand Meteorological Service, 1980. Summaries of Climatological Observations to 1980, vol. 177. New Zealand Meteorological Service Miscellaneous Publication, 172 pp. Vucetich, C.G., Wells, N., 1978. Soils, agriculture and forestry of Waiotapu region, central North Island, New Zealand. New Zealand Soil Bureau Bulletin 31, 100.
The age and signi®cance of in-situ sinter at the Te Kopia thermal area, Taupo Volcanic Zone, New Zealand Rod Martin a, Dallas C. Mildenhall b, Patrick R.L. Browne a,c, Kerry A. Rodgers a,* a
Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand b Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand c Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand Received 25 November 1997; accepted 5 October 1999
Abstract One hundred pollen grains and spores, recovered from a single sample of in situ silica sinter from the Te Kopia geothermal ®eld, include some from a podocarp forest that grew in a temperate, frost-free climate, unlike that of today, as indicated by the presence of Ascarina. Also present are pollen from taxa introduced within the last 100 years. Ascarina has been absent from the area since at least 1800 B.P. and its presence in the sinter indicates that alkali chloride waters discharged at the surface of Te Kopia before 1800 B.P., and possibly before 3500 B.P. Although palynology is a powerful tool to place age limits on ¯uctuations in the shallow hydrology of geothermal ®elds, interpretation must be moderated by considering the present ¯ora and also changes in both local and regional ¯ora, habitat and climate. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. Keywords: Palynology; Sinter; Ascarina; Geothermal; Te Kopia; Taupo; New Zealand
* Corresponding author. Tel.: +64-9-373-7509; fax: +64-9-373-7436. E-mail address: [email protected] (K.A. Rodgers). 0375-6505/00/$20.00 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 5 - 6 5 0 5 ( 0 0 ) 0 0 0 0 8 - 0
368
R. Martin et al. / Geothermics 29 (2000) 367±375
1. Introduction Surface alteration is a common expression of geothermal systems and is a persistent signature of the hydrological conditions prevailing at the time it occurred. This feature may be used to assess ®elds where no springs now discharge and thus standard chemical geothermometry techniques cannot be used. In these situations, knowledge of the distribution and types of surface alteration serve as useful substitutes. For example, the presence of sur®cial silica sinter implies that the alkali chloride waters from which it was deposited originated in a reservoir whose temperatures were hotter than 1808C. Some geothermal ®elds have lifespans of the order of 200,000 years (Browne, 1978) and a ®eld could still have an exploitable reservoir long after its surface thermal activity has ceased. In this case, it becomes necessary to determine the age of any surface alteration to decide whether or not the prospect is worthy of further investigation. The ages of some silica sinters can be determined by tephrochronology (e.g., Lloyd, 1972), dating wood or charcoal fragments that they contain (e.g., Bignall and Browne, 1994), or by palynology (e.g., Browne, 1981). The last named method oers promise despite the sometimes mistaken belief that pollen grains do not survive in hot water. We distinguish here, however, between silica sinter and silica residue. The latter is a product of protracted acid leaching that occurs in a corrosive environment where pollen and plant material are seldom preserved. Although palynology has been used to date sinters, and hence, past sur®cial thermal activity, at the Ngawha (Harper, 1980), Orakeikorako (Herdianita, 1997) and Ohakuri (Henneberger, 1983) ®elds, it has not been used routinely to do so as far as we know. Here we report the application of palynology to newly discovered in situ silica sinter at the Te Kopia geothermal ®eld. Only a single sample was analyzed and no systematic seasonal data were available on the modern pollen rain beyond that from general regional studies. This situation is similar to some geothermal exploration surveys constrained by time, costs and poor exposure. At Te Kopia, the pollen data proved sucient, when considered together with knowledge of the present ¯ora and changes that have occurred to it, the habitat and the climate, to help interpret the Holocene shallow evolution of the hydrology of this long-lived system (0120,000 years: Bignall and Browne, 1994). 2. Setting and general description The Te Kopia ®eld (Fig. 1) is located in the Taupo Volcanic Zone at latitude 388 24' S, longitude 1768 13' E and straddles the west-facing scarp of the upthrown eastern block of the active Paeroa fault (Bignall and Browne, 1994). Patches of altered and steaming ground extend for over 2.5 km along the fault scarp and within 500 m of it. Two deep wells, drilled in the ®eld to about 1400 m depth, showed that temperatures of up to 2408C occur within its reservoir. Today, the sur®cial activity is dominated by steam discharges but several areas
R. Martin et al. / Geothermics 29 (2000) 367±375
369
contain perched, shallow steam-heated ponds ®lled with acid-sulphate waters that are mixtures of steam condensate and rainwater. The maximum concentration of chloride in these waters is 7 mg/kg, so little thermal water in them derives from the reservoir itself. In the central lakes area (Fig. 1) there are occasional remnants of silica sinter scattered along the foot of the scarp and incorporated into landslide debris. Wood encased in one ¯oat block of silica sinter gave a C14 age of 3026243 B.P. (Bignall and Browne, 1994). Not until 1996 was silica sinter located in place. The in-situ sinter occurs in bush near steaming ground some 30 m above the Te Kopia valley ¯oor in the central lakes region (Fig. 1). It outcrops as a near continuous buttress 19 m long, 2 m wide and from 3 to 5 m high. Its top is ¯at and the deposit has its longer axis oriented in a north-east±south-west direction i.e. parallel to that of the Paeroa fault itself. The buttress is composed of many layers of porous, spongy silica, locally stained orange and of varying thickness (up to 1.5 cm). These layers are
Fig. 1. Locality map, Te Kopia thermal area, showing location of in situ sinter buttress. TK1 refers to one of the two drill hole sites that tapped the reservoir waters.
370
R. Martin et al. / Geothermics 29 (2000) 367±375
horizontal on the inner, eastern side of the buttress, but drape to become nearly vertical at the outer, western face. The silica is dominantly opal-A with minor opal-CT and rare quartz. No other phases were seen in thin section and wood, charcoal and tephra were not found. 3. Palynology 3.1. Method Sampling for palynological analysis was restricted by conservation provisions to a single specimen. This was a typical porous, friable sinter, 60 20 20 mm and weighing 024 g, taken some 600 mm above the ground at the northern end of the buttress: AU47311, New Zealand Fossil Record File Number U17/f15, grid reference U17/904 054 [sheet U17, 1:50,000 topographic map series INFOMAP 260]. Pollen and spores were extracted by standard palynological procedures. The specimen was ®rst washed in distilled water and then dissolved in hydro¯uoric acid to release its organic contents, which were ®ltered. The ®ner fraction containing spores and pollen was mounted on a glass slide for microscopic examination. Many of the spores and pollen are a distinct yellow rather than pale coloured or translucent, as modern pollen become, following extraction treatment. The darker colour, although slight, may result from either aging or, perhaps by protracted heating in hot spring water. Although spores and pollen are easy to extract from sinter, they are sparse, commonly requiring more than 5 g of material to be processed to obtain at least 100 grains. The Te Kopia sample was no exception. A count of only 100 grains was completed from three slides. Since spores and pollen are produced in abundance much pollen may have been ¯ushed from the site. Alternatively, their entrapment and preservation in the sinter may have been somewhat arbitrary. 3.2. Results The palynomorph assemblage (Table 1) consists of a mixture of taxa from: 1. species, including grass, Plantago and Pinus, introduced into the Te Kopia area within the last 100 years following European settlement; 2. native species, typical of a podocarp dominant forest, some now growing in but others now absent from the Te Kopia district. The poor preservation of some grains made them dicult to identify. Along with spores and pollen of higher plants, ®nely disseminated organic material, isolated vessels and ®bres, and rare fungal spores were also recovered. All the identi®ed palynomorphs are assumed to have been incorporated in the sinter during its deposition or else deposited later into cavities such as some of the grass grains that still have cell contents preserved. Laboratory contamination is
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unlikely as regular contamination checks show that no more than 1 to 2 grains could be accidentally introduced from the atmosphere during sample preparation, in the unlikely event of the material being exposed to the atmosphere for more than 10 min. There are some slight dierences in preservation of the grass pollen, Table 1 Pollen and spore assemblage extracted from Te Kopia sinter Spores and pollen from taxa likely to be in situ Mosses Bryophyta Sphagnum Ferns Pteridophyta Dicksonia squarrosa (tree fern) Cyathea species (tree fern) Undierentiated monolete spores Trees Podocarpaceae Phyllocladus (celery pine) Dacrydium cupressinum (rimu) Podocarpaceae (undierentiated) Nothofagaceae Nothofagus (Fuscospora ) (southern beech) Small trees and shrubs Chloranthaceae Ascarina lucida (hutu) Rubiaceae Coprosma Myrtaceae Metrosideros (rata) Malvaceae Plagianthus (ribbonwood) Rhamnaceae Pomaderris Asteraceae (daisy) Elaeocarpaceae Herbs Poaceae (grass) Pollen from taxa known to be introduced Trees Pinaceae Pinus (exotic pine) Herbs Poaceae (grass) Plantaginaceae Plantago lanceolata (plantain) Unidenti®ed very poorly preserved pollen
Raw counts 1 2 11 6 8 12 8 3 2 1 5 1 ±a 1 5 Part of 14
15 Part of 14 1 4
a A single Pomaderris grain was identi®ed after a count of 100 grains had been made from three slides.
372
R. Martin et al. / Geothermics 29 (2000) 367±375
for example, which indicates that some may be younger but the variations are so minor that they may be due to dierences in degradation prior to or after deposition. 4. Interpretation 4.1. Present vegetation at Te Kopia Like much of the North Island of New Zealand, vegetation of the Te Kopia area has undergone major changes in the past several hundred years. Following the last glacial period, and prior to Maori occupation some 800 years ago, the region became covered by extensive podocarp forest (McKinnon, 1997). Over several hundred years, repeated burnings by the Maori helped destroy this forest (Vucetich and Wells, 1978). This process was continued by European occupiers who introduced grasses and other northern hemisphere plants, beginning in the late nineteenth century, followed by the planting of vast exotic pine forests throughout the Taupo Volcanic Zone from 1920 (McKinnon, 1997). In a survey of the present vegetation of the Te Kopia area, Burns (1997) recognised a distinctive ¯ora among which important forms are mosses, ferns, Kunzea ericoides var. micro¯ora (kanuka), and Weinmannia racemosa (kamahi). The dominant palynomorphs in the three slides made from the sinter dier from those of this modern vegetation. The presence of pollen of introduced plants (grass, Plantago and Pinus ), if they were deposited with the sinter, would indicate that alkali-chloride waters discharged <100 B.P. Hochstetter's (in Fleming, 1959) description of the thermal activity at Te Kopia, following his 1859 visit, fairly closely matches that of the present day and, although he noted a siliceous sinter in the area, he makes no mention of discharging chloride springs. Nor have later visitors reported sur®cial alkali chloride pools or springs. This suggests that the pollen grains of introduced plants must have become trapped in the cavities of the porous sinter, despite attempts to wash them out prior to processing. Kunzea ericoides produces abundant pollen but none from it nor Leptospermum scoparium (manuka), also now locally abundant, were identi®ed in the sample. However, pollen of Metrosideros (rata), another member of the family Myrtaceae, forms 5% of the total assemblage. Metrosideros is not listed by Burns (1997) as occurring locally. This suggests that this pollen in the sinter came either solely from a distant source or from vegetation now locally absent. The former is unlikely. Regional pollen samples contain a preponderance of Kunzea and Leptospermum (Macphail and McQueen, 1983). This is the case even where the ¯owering by these plants is intermittent (Burns, 1997). The absence of modern Kunzea and Leptospermum pollen in the cavities of the sinter is unexplained unless washing in distilled water prior to processing eectively eliminated these very small grains (8±18 mm: McIntyre, 1963) but not the larger grass, Plantago and Pinus grains.
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4.2. Present-day ambient temperatures in the Te Kopia area The present climate at Te Kopia is characterised by frequent heavy frosts; there are over 110 frost days per year at Wairakei, 10 km to the south and at 60 m lower elevation (New Zealand Meteorological Service, 1980). There are occasional snow falls at Te Kopia but snow does not remain long on the ground. The area is very de®nitely not frost-free. 4.3. The podocarp assemblage: the signi®cance of Ascarina and the age of the sinter The podocarp assemblage is consistent with species that grew in the Te Kopia region until climatic change and man-induced deforestation eliminated them. The presence of Ascarina pollen in the sinter indicates that the podocarpdominant forest grew in a frost-free climate. Ascarina lucida was well established in the central North Island 10,000 years ago but declined sharply in abundance some 5000 years B.P., becoming very sparse about 3500 B.P., to ®nally die out in this region about 1800 B.P. (McGlone, 1983; cf. McGlone and Neall, 1994). McGlone (1983) suggested that Ascarina had been eliminated from the Rotorua catchment, 20 km north of Te Kopia, by ca. 4000 B.P. The colder climate in Te Kopia would have caused it to die out there even earlier and there is no evidence of a relict community surviving in the area. Today, the species is largely con®ned to scattered coastal drought- and frost-free localities in areas of high rainfall (McGlone and Moar, 1977). The closest location where it now grows is over 100 km to the northwest. The dominance of Dacrydium cupressinum and the low abundance of Nothofagus pollen indicate that the sinter is at least older than 600 B.P. (cf. McGlone, 1983), predating deforestation. However, the Ascarina pollen point to an age no younger than 1800 B.P., and one that may well be older than 3500 B.P. Such an interpretation means that the change from podocarp forest to a local Weinmannia/Leptospermum/Kunzea association occurred at Te Kopia since the sinter was deposited. Bignall and Browne (1994) reported a C14 age of 3026 2 43 B.P. from wood in sinter ¯oat some 200 m to the north and at a slightly higher elevation. This ¯oat did not come from the in situ sinter but its age provides evidence of the contemporary discharge of alkali chloride waters elsewhere at Te Kopia.
5. Conclusions Despite the limitations of a data set of only 100 grains, we conclude: 1. The presence of silica sinter at the Te Kopia thermal area shows that alkali chloride waters discharged here as geysers and/or hot pools perhaps as recently as 1800 B.P. but possibly no later than 3500 B.P. 2. Alkali chloride waters do not discharge at Te Kopia now but the mere presence
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of the sinter points to the existence of a geothermal reservoir whose temperatures exceeded 1808C at the time sinter formed. Given the longevity of geothermal ®elds, on this evidence alone, the Te Kopia thermal area would oer a worthwhile drilling target as con®rmed by the two drill holes. 3. The pollen trapped within the sinter record a ¯ora growing in the Te Kopia area, prior to human occupation, that indicates warm, frost-free conditions prevailed Ð unlike those of today. 4. Palynology is a powerful technique to assign age limits to silica sinter that provides evidence about the sur®cial evolution of a geothermal ®eld. 5. Optimum interpretation of palynological results requires an understanding of the present ¯ora and for this to be balanced against a knowledge of past botanical, habitat and climate changes.
Acknowledgements The Department of Conservation, along with Mike and Delwyn Murphy, generously allowed access to the ®eld area and also granted approval to remove the specimen for analysis. The pollen sample was prepared by Roger Tremain. Four referees (Professor A. Traverse, Drs. P. Wigand and J. Ward and one anonymous) greatly improved the rigor of the paper.
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