Phil A. Shane

A 6000-year palaeoenvironmental record from Harataonga, Great Barrier Island, New Zealand

Abstract A pollen, sediment, and tephra record from a drained swamp at Harataonga contains a history of the local coastal environment from the Mid Holocene. This commences c. 6000 cal yr BP in a freshwater environment with swamp forest composed mainly of Laurelia, Leptospermum, Ascarina, and Cyathea spp. Dodonaea and Cyperaceae grew on margins of this forest. Forest on the hills surrounding the wetland comprised mainly Metrosideros, with emergent Dacrydium and Libocedrus. Ascarina, Rhopalostylis, and Cyathea dealbata type were a significant part of the understorey of the hillside forest. Around the time of deposition of the 5550 cal yr BP Whakatane tephra, a freshwater lake developed at the site. Extensive Cyperaceae swamp developed on the fringes of the lake. Shortly after c. 2900 cal yr BP, Dacrycarpus briefly invaded swamp forest, possibly as a result of storm disturbance, and the site made the final transition to swamp. Myrsine and then Hebe shrubs invaded fringes of the swamp as the water table fell, possibly as a result of a change to drier conditions in the Late Holocene. Polynesian deforestation, as indicated by the presence of abundant charcoal and Pteridium spores, is recorded in this core as occurring shortly after deposition of the c. 600 cal yr BP Kaharoa tephra.

A palaeoenvironmental record of natural and human change from the Auckland Isthmus, New Zealand, during the late Holocene

Abstract A multi‐proxy analysis of a sediment core from Waiatarua, Auckland Isthmus, adds to an environmental history from the local wetland spanning the Late Glacial to modern times. Several distal tephra were recorded in the core: 8.5 ka Rotoma (reworked), 6.1 ka Tuhua (primary and reworked), most likely the 1.8 ka Taupo (the latter is previously unreported for the Auckland Isthmus), and one unidentified, possibly 665 yr BP Kaharoa. Pollen and diatom analyses of the core show that during the period c. 6000‐c. 4800 yr BP, the site was a lake fringed with Cyperaceae/Leptospermum swamp. The lake became progressively shallower after c. 4800 yr BP, probably due to hydroseral infilling. Surrounding the lake was forest dominated by Dacrydium, Prumnopitys, Metrosideros, and Nestegis. Transition to the Polynesian era appears unclear because the site probably endured a hiatus due to destruction of peat by burning in European times.

A palynological study of Polynesian and European effects on vegetation in Coromandel, New Zealand, showing the variability between four records from a single swamp

Abstract Seven cores were extracted from a river terrace swamp in the forested Kauaeranga valley, Coromandel Peninsula, New Zealand. High‐resolution (c. 36–73 yr interval) pollen records were obtained from four of the cores and aged by radiocarbon dating and with stratigraphic reference to the 665 ± 15 14C yr BP Kaharoa Tephra. The records span the last c. 1800 yr and show that the vegetation consisted of lowland podocarp‐hardwood forest before deforestation by burning occurred. The pattern of deforestation at Kauaeranga, indicated by the abrupt dominance of Pteridium with concurrent increased charcoal, is typical of pollen records associated with early Polynesian settlement in New Zealand. Peaks of Pteridium and charcoal were also found in sediments deposited after European settlement. Different cores show marked palynological and stratigraphic differences relative to the Kaharoa Tephra, most importantly with regard to the timing of deforestation. Deforestation occurred close to the Kaharoa, at a calculated age of c. 750 BP in one core but well above the Kaharoa (c. 480 BP) in another. The stratigraphic unconformities between cores are attributed to variable fluvial processes causing an uneven deposition of sediments within the swamp.

Time scales of intra‐oceanic arc magmatism from combined U‐Th and (U‐Th)/He zircon geochronology of Dominica, Lesser Antilles

The island of Dominica, located in the intra-oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U-Th)/He eruption ages from centers across the island range from ∼3 to ∼770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U-Th ages from individual centers range from near-eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40 km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (<50 ka) with narrow crystallization peaks consistent with the construction of a new magma reservoir. The younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward, arc-parallel over time. Overall, our data support geochemical models for the ongoing construction of a silicic intrusive complex, consisting of varying amounts of crystal mush, beneath the island. U-Pb zircon ages <1–2 Ma indicate that accumulation of this complex is entirely Quaternary in age. Together zircon U-Th and U-Pb ages for Dominica suggest that the magmatic processes and time scales operating in intra-oceanic arcs are similar to those documented for continental arcs.

A Late Quaternary Record of Natural Change and Human Impact from Rangihoua Bay, Bay of Islands, Northern New Zealand

Abstract A multiproxy analysis of back-dune sediment cores from Rangihoua Bay, northern New Zealand, provides an environmental history of the Late Quaternary, placing human impacts on the site into a geomorphological and ecological context. The inferred paleoecological significance of the trends is generally coeval between proxies. The history commences with a Late Pleistocene deposit that formed part of a river terrace during lower sea level. The dryland vegetation at that time was dominated by Fuscospora forest. The record recommences at ca. 7400 YBP, by which time Fuscospora had been replaced by podocarp-hardwood forest, comprising mainly Dacrydium and Prumnopitys taxifolia emerging through a Metrosideros canopy. One of the core sites was a lagoon fringed with mainly Cyperaceae, Leptospermum, and Dodonaea. Redox-sensitive elements reflect phases of anoxia related to variation in lagoon depth. Transition from lagoon to peat swamp, due to natural infilling and/or climate change, occurred after ca. 5500 YBP. Human impacts were of high magnitude and include deforestation of the catchment and drainage of the wetland by early Polynesians. Errors in the radiocarbon and tephra chronologies preclude an accurate date for this. Microfossils of introduced Polynesian plants and a thick gravel bed in one of the cores suggest that parts of the wetland were used for prehistoric horticulture.

Palaeoenvironment and human impact in modifying vegetation at Mt St John, Auckland Isthmus, New Zealand

Abstract A 2.34 m sediment profile from the base of the crater of Mt St John volcano (a small basaltic cone on Auckland Isthmus) provides a partial environmental record of the Late Quaternary. The record highlights potential age control problems with sediment cores taken directly from archaeological sites. Two distal tephras were recorded: 9.5 ka Rotoma and 7 ka Tuhua. A date of 16 309 ± 90 14C yr BP from the basal scoria of the profile provides a minimum date for the eruption of Mt St John. Pollen was present only in the upper 0.33 m of the profile, in a layer of peat and soil which caps highly weathered silts and clays eroded from the crater walls. In early Polynesian times (most likely after c. 800 14C yr BP), vegetation of the crater swamp was dominated by Cyperaceae sedges and Paesia ground fern. Dacrycarpus trees were also present. Podocarp‐hardwood forest, dominated by Metrosideros, grew on the rim and inner slopes of the crater. Elaeocarpus, Griselinia, and Cyathea were also present. A decline in Dacrycarpus pollen and an accelerated erosion rate mark Polynesian forest clearance within the crater. Typha became a major component of the swamp vegetation during the Late Polynesian‐European era.

Field survey, sedimentology and plant microfossil analysis of sediment cores from possible cultivation sites at Tolaga Bay, eastern North Island, New Zealand

Abstract Presented here are results of field survey, sedimentology and pollen and starch analysis of sediment cores from two different environmental settings in the Tolaga Bay catchment, New Zealand. One of the sites is an inland floodplain paddock with ditch‐like features (which appear to be mainly horse‐drawn plough lines), the other a back‐dune wetland adjacent to the beach. The former is capped with a thick alluvial layer deposited during Cyclone Bola (1988) and has a layer of reworked 1.7 ka Taupo Tephra. Both sites show evidence of large‐scale prehistoric deforestation, and reveal starch grains similar in appearance to Polynesian‐introduced sweet potato (Ipomoea batatas) and European‐introduced potato (Solanum tuberosum), suggesting cultivation of these crops.