Margaret A. Harper

A long late-Quaternary record from Lake Poukawa, Hawke's Bay, New Zealand

Abstract The Lake Poukawa Basin is a large co-seismic depression located at 20 m above mean sea level in Hawke’s Bay in eastern North Island, New Zealand. We present a detailed environmental history of the basin for the last c. 60 ka based on analyses of the top 105 m of a 200-m core record. Dating control is provided by radiocarbon, optically stimulated luminescence (OSL) and U/Th disequilibrium ages. The chronology is supported by nine tephras of inferred age including marker tephras, Kawakawa (22 590±230 yr BP at −18.25 m core datum), Tahuna (c. 35–43 ka) at −33.1 m core datum, and Rotoehu (45–50 ka) at −39.1 m core datum. Disagreements between some of the older tephra ages and the numerical ages from the OSL and U/Th dating mean that more than one age model can be applied. Three major lithostratigraphic units are identified: a basal calcareous silt with lignitic peats between 105.28 and 98.58 m of marine oxygen isotope stage (MIS) 3 age; an extended sequence of detrital shelly sands and silts, between 98.58 and 8 m of MIS 3 and 2 age; and a Holocene peat unit (MIS 1) from 8 to 0 m. Alternatively, but less likely, the basal unit may represent stage MIS 5a and the detrital shelly sands would then contain an amalgam of MIS 4, 3, and 2 deposits. We propose a notably moist phase represented by the peat which our numerical dating model places near the start of isotope stage 3. This suggests the existence of mild conditions during an interstadial in central New Zealand at c. 55–50 ka when a podocarp–beech–broadleaf forest of near-interglacial affinity surrounded the basin. The interstadial is marked by both lake and peat formation in the basin. After 50 ka a thermal decline set in, though the climate remained moist initially. Under these conditions, the Poukawa Basin was rapidly infilled by alluvial fan deposits from the surrounding hills. The floor of the basin was occupied by grasses and sedges, responding to both the highly disturbed environment and swampy conditions in the basin. After the deposition of the Rotoehu Ash, effective precipitation declined markedly and woody shrubs expanded across the previously swampy basin floor. The data suggest an apparent thermal decline of c. 6–7°C for much of MIS 2 and the latter half of MIS 3. The Holocene was marked by the establishment of fen and lake environments on the basin floor. Prior to human disturbance, podocarp–broadleaf forest surrounded the basin.

Environmental and sea-level changes on Banks Peninsula (Canterbury, New Zealand) through three glaciation–interglaciation cycles

Abstract A greater than 200 ka record of marine transgressions and regressions is recorded from a 75 m core from Banks Peninsula, Canterbury, New Zealand. This record comprises thick suites of muddy sediments attributed to back barrier, lake and lagoonal environments alternating with thin soil and loess complexes. These deposits have been dated using radiocarbon and thermoluminescence (TL) techniques supported by proxy data (diatoms, phytoliths, pollen and sedimentology). The aqueous deposits are attributed to three interglacials and an interstadial (Marine Isotope Stages 1, 5a, 5c, 6, and 7). The loesses and paleosols date to the intervening stadials (Isotope Stages 2, 5d (or 6?) and probably 8). On the basis of transgressive beach facies, back barrier swamps and barrier-blocked lake deposits, a partial sea-level curve including data from Isotope Stage 5 is presented. Our data indicate that Banks Peninsula has been tectonically stable over that period and we provide sea-level points that support the existing isotope curve during Stages 5 and 6. Detailed diatom records are limited to Isotope Stage 1 and the latter part of Stage 5. Diatom histories recorded from these stages are remarkably consistent. Both indicate a progressive floral change from marine types through freshwater colonising species to freshwater planktonic assemblages. These reflect parallel histories of coastal evolution during the two interglacials. In both cases, marine transgression in the early part of the isotope phase was followed by lagoon development implying that a gravel spit extended across the embayment from the west. This was succeeded by lake development when the lagoon was cut off by the juncture of the spit with Banks Peninsula. This lake deepened as the coast rotated into swash alignment and the spit was converted into a gravel barrier. The vegetation history of the site indicates that mixed podocarp broadleaf forests, similar to the pre-European flora of Banks Peninsula, occupied the region during Isotope Stages 1 and 7. This contrasts with the palynological interpretation of a marine record (DSDP Site 594) from off the Canterbury coast which suggested that Isotope Stage 7 was markedly cooler than the Holocene. During glacial periods, forest was eliminated and replaced by a tall shrubland of mixed montane and coastal affinities.

The Mount Feather Diamicton of the Sirius Group: an accumulation of indicators of Neogene Antarctic glacial and climatic history

Abstract A paucity of data from the Antarctic continent has resulted in conflicting interpretations of Neogene Antarctic glacial history. Much of the debate centres on interpretations of the glacigene Sirius Group strata that crop out as discrete deposits along the length of the Transantarctic Mountains and in particular on its age and the origin of the siliceous microfossils it encloses. Pliocene marine diatoms enclosed within Sirius Group strata are inferred to indicate a dynamic East Antarctic ice sheet that was much reduced, compared with today, in the early–middle Pliocene and then expanded again in the late Pliocene. However, the geomorphology of the Dry Valleys region is interpreted to represent a relatively long-lived (middle Miocene–recent) and stable polar climatic regime similar to that of today. The Mount Feather Diamicton infills a palaeovalley at ca. 2500 m on the NE flank of Mount Feather in the Dry Valleys region and has been included within the Sirius Group. We obtained four shallow cores (COMRAC 8, 9, 10 and 11) from beneath the permafrost boundary in the Mount Feather Diamicton in order to understand its origin and relationship with the surrounding landscape. Detailed studies of these cores (stratigraphy, sedimentology, palaeontology, micromorphology, petrography and fabric) have yielded new data that demonstrate a much more complex climatic and glacial history for the Mount Feather Diamicton than in previous interpretations. The data indicate that the Mount Feather Diamicton was deposited beneath a wet based glacier fed from a larger ice sheet behind the Transantarctic Mountains. It is, however, unlikely that this ice sheet overtopped Mount Feather (2985 m). A near-in situ non-marine diatom assemblage was recovered from 90 cm depth in COMRAC 10 and indicates a maximum depositional age of Late Miocene for the Mount Feather Diamicton. A subsequent glacial episode has distributed a boulder blanket across the surface of the diamicton. Other post-depositional processes include drying, infilling of surface layers with aeolian sediment, and the development of melt-water runnels. We interpret these combined data to indicate the persistence of more temperate climatic and glacial conditions in the vicinity of Mount Feather until at least the Late Miocene.

High-flying diatoms: Widespread dispersal of microorganisms in an explosive volcanic eruption

Explosive eruptions create a transient bridge between the solid Earth and atmosphere, frequently injecting volcanic aerosols to stratospheric levels. Although known to disrupt terrestrial and aquatic ecosystems at the surface, the role of explosive volcanism in airborne transport of microscopic organisms has never been characterized. This study documents abundant freshwater diatoms (microskeletons of siliceous algae) in widespread tephra from the 25.4 ka Oruanui eruption of Taupo volcano, New Zealand. By matching the tephra-hosted species assemblages to those in coerupted clasts of lacustrine sediment, we demonstrate that ~0.6 km 3 of diatom remains were incorporated during magma-water interaction with a lake system overlying the vents, and were dispersed along with fi ne ash particles hundreds of kilometers downwind. One of the dominant species, Cyclostephanos novaezeelandiae, is endemic to New Zealand’s North Island and serves as a unique identifi er of the eruptive source region. Our results suggest that dispersal of microorganisms may be an overlooked feature of a number of ancient and modern eruptions, and indicate a novel pathway of microbe transport in airborne volcanic plumes. We conclude that the biogenic signatures contained within distal tephras have potential application in the characterization of eruption dynamics, location, and environmental settings of volcanic source areas.

A paleoenvironmental study of subsurface Quaternary sediments at Wainuiomata, Wellington, New Zealand, and tectonic implications

Abstract A stratigraphic drillhole (WS‐1 ) sited on the floor of the Wainuiomata Valley near Lower Hutt, revealed a 61.6 m thick Quaternary sequence overlying Torlesse Supergroup greywacke sandstone and argillite. The Quaternary sediments consist of three sequences separated by dis‐conformities. The lower sequence, 10.7 m thick (61.6–50.9 m), consists of fluvial sediments of probable early Quaternary age. The middle sequence, about 48.3 m in thickness (50.9‐c. 2.6 m), spans most of the Last Glaciation. Fluvial/overbank (50.9–42.0 m), floodplain/swamp (42.0–34.5 m), and fluvial (34.5–31.3 m) sediments overlie the disconformity at 50.9 m. Conformably overlying these sediments are swamp and lacustrine deposits between 31.3 and 4.1 m. Diatoms and algal spores and coenobia show the existence of an extensive lake during much of this sequence, from 25.6 to 4.0 m. At the peak of its development, at a drillhole depth of c. 23 m, the lake was >10 m deep and had a high algal biomass. Kawakawa Tephra (22 600 yr B.P.)...

TWO UNUSUAL DIATOMS FROM NEW ZEALAND: TABULARIA VARIOSTRIATA A NEW SPECIES AND EUNOPHORA BERGGRENII

Several new diatom taxa have been described from nearly pristine habitats in New Zealand, but the new species Tabularia variostriata, described here, was abundant in a Wellington City suburban stream. It has typical features of Tabularia species: internally its pores are sunk down well below prominent short ribs, and externally its pores are occluded by cribra. It differs from other Tabularia species in having uniseriate striae with variable numbers of pores, the striae forming irregular edges to a wide sternum. The innermost pores on the longer striae appear enlarged as spacing of pores in striae increases near the edge of the sternum. The Australasian freshwater diatom Eunophora berggrenii, formerly known as Amphora berggrenii Cleve or ‘Eunophora sp. 1’, was studied and a lectotype was designated. Tasmanian specimens differed subtly from the type material in valve dimensions and may represent a different but closely related species.