Letteratura scientifica selezionata sul tema "Oruanui Tephra"

AbstractThe Oruanui eruption was the largest known outburst of Taupo volcano, New Zealand, and is among the larger Quaternary eruptions documented. The eruption deposits are variously known as the Oruanui, Wairakei, Kawakawa Tephra, or Aokautere Ash formations, and represent a bulk volume probably exceeding 500 km3. Four new 14C age determinations on carbonized vegetation in the non-welded Oruanui ignimbrite are combined to give a conventional age of 22590±230 yr b.p. Compared with the previously accepted figure of 20000 yr b.p., this new age resolves the anomaly of apparently older 14C ages being obtained from a demonstrably younger New Zealand deposit, and strengthens correlation of this eruption with an Antarctic ice-core acid anomaly. The trace of this eruption has great potential as a time-plane marker in the Antarctic just prior to the last glacial maximum. The close similarity in ages between the Oruanui and a comparable sized eruption (Ito/Aira-Tn) in Japan suggests that this period of activity may represent the best chance of resolving any linkages between large-scale explosive silicic volcanism and climate changes.

Many authors have reported significant soil erosion resulting from the grazing of sheep, rabbit plagues and invasion of the exotic Hawkweed (Hieracium sp.) in the sub humid alpine region of Mackenzie Basin, South Island, New Zealand. In the present study, we investigated the soil redistribution of four study plots with varying vegetation depletion over historic (54 years) and long (25 ka) time scales. Historic soil loss, quantified by bomb fallout 137Cs, under plots of depleted short tussock and herbfield vegetation was no more than the adjacent undisturbed reference plot of red tussock (Chionochloa rubra). This indicates the present landscape characterised by soil and vegetation degradation is not due to erosion since 1953. There is no evidence from the present study to suggest that establishment and rapid invasion of Hieracium sp. and major periodic rabbit plagues have accelerated soil erosion over the past 54 years. By contrast, low topsoil thickness under Hieracium sp. indicates that Hieracium sp. is colonising bare ground and may have, at least in the short-term, a stabilising effect. Long-term soil loss was quantified by the profile distribution of volcanic glass originating from Kawakawa-Oruanui tephra (KOT). The peak concentration, and hence the tephra’s 25.4 ka isochron, occurred at a depth of 70–85 cm at the reference plot. The degraded plots showed significant decreases in glass concentration and depth to peak concentration with progressively shallower soils and vegetation depletion. This equated to a minimum erosion rate averaged over the past ~25.4 k years of 0.020 mm year–1 in the most eroded plot. The extent of bare ground and topsoil thickness were poor indicators of soil erosion status. The tephra results show a potentially long history of soil erosion that has predisposed soil and vegetation degradation within the European era.

Taupo volcano is the southerly of two dormant caldera volcanoes in the rhyolite-dominated central portion of the Taupo Volcanic Zone in the North Island of New Zealand. Taupo has an average magma output rate of 0.2 m 3 s -1 over the past 65 000 years, and is one of the most frequently active and productive rhyolite volcanoes known. The structure of the modern ‘inverse’ volcano was formed largely by caldera collapse associated with the voluminous 22 600 14 C years BP Oruanui eruption, and has been little modified since except for collapse following the 1850 14 C years BP eruption. The products of 28 eruptions (labelled T, f2, A, ..., Z), all of which post-date the Oruanui eruption, are defined and described here. Twenty-seven of these eruptions are represented by pyroclastic deposits (of which three were accompanied by a mappable lava extrusion), and one eruption (Z) solely by evidence for a lava extrusion. The deposits of seven eruptions (B, C, E, S, V, X and Y) largely correspond to previously defined tephra formations (Karapiti, Poronui, Opepe, Waimihia, Whakaipo, Mapara and Taupo, respectively). The previously defined Motutere and Hinemaiaia Tephras are reinterpreted to represent the products of 12 eruptions (G to R), while the remaining eight deposits and one eruption are newly recognized. Eruption T occurred at ca . 17200 14 C or 20500 calibrated years BP and eruption Z about 1740 calibrated years BP. Eruption volumes vary by more than three orders of magnitude between 0.01 and more than 44 km 3 , and repose periods by more than two orders of magnitude from ca . 20 to 6000 years. The eruption deposits reflect great variations in parameters such as volume, the dispersal characteristics of the fall deposits, the presence or absence of intraeruptive time breaks, the formation of pyroclastic flows, the degree of magmawater interaction, the vesiculation state of the magma on fragmentation and the relative proportions of juvenile obsidian versus foreign lithologies in the lithic fractions. All but seven fall deposits are plinian in dispersal; two (Y1 and probably W) are sub-plinian, one (Y5) has been termed ‘ultraplinian’, while 4/ and A are too poorly preserved for their dispersal to be assessed. The lengths of repose periods in the post-Oruanui sequence range are not randomly distributed but show self-similar properties (fractal dimensionality); repose intervals ( r , in years) of not more than 350 years follow n = 53.5r-0'21, and those of not less than 350 years follow n = 2096 r -0-83 , where n is the number of eruptions. The shorter repose periods may reflect triggering processes, such as regional extension, affecting magma bodies during their viable lifetimes, while longer repose intervals (i.e. not less than 350 years) may reflect an episodicity of major rifting events or the production of magma bodies below the volcano. Bulk volumes ( v , in km 3 ) of the eruption products also show self-similar properties (fractal dimensionality), with n = 6.17 v -0.46 . However, there are then apparently random relationships between eruption volumes and the preceding or succeeding repose period such that prediction of the time and size of the next eruption is impossible. The post-Oruanui activity at Taupo represents ‘noise’ superimposed on the more uniform, longer term activity in the central Taupo Volcanic Zone, where large (greater than 100 km 3 ) eruptions, such as the Oruanui, occur at more evenly spaced intervals of one per 40-60000 years.

Charwell Basin is a 6 km-wide structural depression situated at the boundary between the axial ranges and faulted and folded Marlborough Fault Zone of north-eastern South Island, New Zealand. The basin contains the piedmont reach of the Charwell River, and a series of late Quaternary loess-mantled alluvial terraces and terrace remnants that have been uplifted and translocated from their sediment source due to strike-slip motion along the Hope Fault which bounds the basin to its immediate north. The aim of this study was to provide an interdisciplinary, integrated and holistic analysis of late Quaternary landscape evolution and environmental change in Charwell Basin using terrain analysis, loess stratigraphy, soil chemistry and paleoecological data. The study contributes new understanding of New Zealand landscape and ecosystem responses to regional and global climatic change extending to Marine Isotope Stage (MIS) 6, and shows that climatically-forced shifts in biogeomorphic processes play a significant role in lowland landscape evolution. Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. Paleotopographic analysis of a >150 ka terrace mantled by up to three loess sheets revealed multiple episodes of alluvial aggradation and degradation and, subsequent to river abandonment, gully incision prior to and coeval with loess accumulation. Spatial heterogeneity in loess sheet preservation showed a complex history of loess accumulation and erosion. A critical profile curvature range of -0.005 to -0.014 (d2z/dx2, m-1) for loess erosion derived from a model parameterised in different ways successfully predicted loess occurrence on adjacent slope elements, but incorrectly predicted loess occurrence on an older terrace remnant from which all loess has been eroded. Future analyses incorporating planform curvature, regolith erosivity and other landform parameters may improve identification of thresholds controlling loess occurrence in Charwell Basin and in other South Island landscapes. A loess chronostratigraphic framework was developed for, and pedogenic phases identified in, the three loess sheets mantling the >150 ka terrace. Except for one age, infrared-stimulated luminescence dates from both an upbuilding interfluve loess exposure and colluvial gully infill underestimated loess age with respect to the widespread Kawakawa/Oruanui Tephra (KOT; 27,097 ± 957 cal. yr BP), highlighting the need for improvements in the methodology. Onset of loess sheet 1 accumulation started at ca. 50 ka, with a break at ca. 27 ka corresponding to the extended Last Glacial Maximum (eLGM) interstadial identified elsewhere in New Zealand. Loess accumulation through MIS 3 indicates a regional loess flux, and that glaciation was not a necessary condition for loess generation in South Island. Loess accumulation and local alluvial aggradation are decoupled: the youngest aggradation event only covers ~12 kyr of the period of loess sheet 1 accumulation. Older local aggradation episodes could not be the source because their associated terraces are mantled by loess sheet 1. In the absence of numerical ages, the timing of L2 and L3 accumulation is inferred on the basis of an offshore clastic sediment record. The upbuilding phase of loess sheet 2 occurred in late MIS 5a/MIS 4, and loess sheet 3 accumulated in two phases in MIS 5b and late MIS 6. Biogenic silica data were used to reconstruct broad shifts in vegetation and changes in gully soil saturation status. During interglacial/interstadial periods (MIS 1, early MIS 3, MIS 5) Nothofagus¬-dominated forest covered the area in association with Microlaena spp grasses. Lowering of treeline altitude during glacial/stadial periods (MIS 2, MIS 3, MIS 5b, late MIS 6) led to reduction in forest cover and a mosaic of shrubs and Chionochloa spp, Festuca spp and Poa spp tussock grasses. Comparison of interfluve and gully records showed spatial heterogeneity in vegetation cover possibly related to environmental gradients of exposure or soil moisture. A post-KOT peak in gully tree phytoliths corresponds to the eLGM interstadial, and a shift to grass-dominated vegetation occurred during the LGM sensu stricto. Diatoms indicated the site became considerably wetter from ca. 36 ka, with peak wetness at ca. 30, 25 and 21 ka, possibly due to reduced evapotranspiration and/or increased precipitation from a combination of strengthened westerly winds and increased cloudiness, or strengthened southerly flow and increased precipitation. Human influence after ca. 750 yr BP led to re-establishment of grassland in the area, which deposited phytoliths mixed to 30 cm depth in the soil. A coupled gully colluvial infilling/vegetation record showed that sediment flux during the late Pleistocene was ~0.0019 m3 m-1 yr-1 under a shrubland/grassland mosaic, and Holocene sediment flux was ~0.0034 m3 m-1 yr-1 under forest. This increase of 60% through the last glacial-interglacial transition resulted from increased bioturbation and down-slope soil transport via root growth and treethrow, which formed a biomantle as evidenced by slope redistribution of the KOT. These results contrast with sediment transport rates and processes hypothesised to occur contemporaneously in adjacent mountain catchments. This suggests that intraregional biogeomorphic processes can differ significantly depending on topography and geological substrate, with different landscapes responding in unique ways to the same climate shifts. Analysis of Quaternary terrestrial landscape evolution in non-glaciated mountainous and lowland areas must therefore consider spatial and temporal heterogeneity in sediment fluxes and underlying transport processes.

Charwell Basin is a 6 km-wide structural depression situated at the boundary between the axial ranges and faulted and folded Marlborough Fault Zone of north-eastern South Island, New Zealand. The basin contains the piedmont reach of the Charwell River, and a series of late Quaternary loess-mantled alluvial terraces and terrace remnants that have been uplifted and translocated from their sediment source due to strike-slip motion along the Hope Fault which bounds the basin to its immediate north. The aim of this study was to provide an interdisciplinary, integrated and holistic analysis of late Quaternary landscape evolution and environmental change in Charwell Basin using terrain analysis, loess stratigraphy, soil chemistry and paleoecological data. The study contributes new understanding of New Zealand landscape and ecosystem responses to regional and global climatic change extending to Marine Isotope Stage (MIS) 6, and shows that climatically-forced shifts in biogeomorphic processes play a significant role in lowland landscape evolution. Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. Paleotopographic analysis of a >150 ka terrace mantled by up to three loess sheets revealed multiple episodes of alluvial aggradation and degradation and, subsequent to river abandonment, gully incision prior to and coeval with loess accumulation. Spatial heterogeneity in loess sheet preservation showed a complex history of loess accumulation and erosion. A critical profile curvature range of -0.005 to -0.014 (d²z/dx², m⁻¹) for loess erosion derived from a model parameterised in different ways successfully predicted loess occurrence on adjacent slope elements, but incorrectly predicted loess occurrence on an older terrace remnant from which all loess has been eroded. Future analyses incorporating planform curvature, regolith erosivity and other landform parameters may improve identification of thresholds controlling loess occurrence in Charwell Basin and in other South Island landscapes. A loess chronostratigraphic framework was developed for, and pedogenic phases identified in, the three loess sheets mantling the >150 ka terrace. Except for one age, infrared-stimulated luminescence dates from both an upbuilding interfluve loess exposure and colluvial gully infill underestimated loess age with respect to the widespread Kawakawa/Oruanui Tephra (KOT; 27,097 ± 957 cal. yr BP), highlighting the need for improvements in the methodology. Onset of loess sheet 1 accumulation started at ca. 50 ka, with a break at ca. 27 ka corresponding to the extended Last Glacial Maximum (eLGM) interstadial identified elsewhere in New Zealand. Loess accumulation through MIS 3 indicates a regional loess flux, and that glaciation was not a necessary condition for loess generation in South Island. Loess accumulation and local alluvial aggradation are decoupled: the youngest aggradation event only covers ~12 kyr of the period of loess sheet 1 accumulation. Older local aggradation episodes could not be the source because their associated terraces are mantled by loess sheet 1. In the absence of numerical ages, the timing of L2 and L3 accumulation is inferred on the basis of an offshore clastic sediment record. The upbuilding phase of loess sheet 2 occurred in late MIS 5a/MIS 4, and loess sheet 3 accumulated in two phases in MIS 5b and late MIS 6. Biogenic silica data were used to reconstruct broad shifts in vegetation and changes in gully soil saturation status. During interglacial/interstadial periods (MIS 1, early MIS 3, MIS 5) Nothofagus-dominated forest covered the area in association with Microlaena spp grasses. Lowering of treeline altitude during glacial/stadial periods (MIS 2, MIS 3, MIS 5b, late MIS 6) led to reduction in forest cover and a mosaic of shrubs and Chionochloa spp, Festuca spp and Poa spp tussock grasses. Comparison of interfluve and gully records showed spatial heterogeneity in vegetation cover possibly related to environmental gradients of exposure or soil moisture. A post-KOT peak in gully tree phytoliths corresponds to the eLGM interstadial, and a shift to grass-dominated vegetation occurred during the LGM sensu stricto. Diatoms indicated the site became considerably wetter from ca. 36 ka, with peak wetness at ca. 30, 25 and 21 ka, possibly due to reduced evapotranspiration and/or increased precipitation from a combination of strengthened westerly winds and increased cloudiness, or strengthened southerly flow and increased precipitation. Human influence after ca. 750 yr BP led to re-establishment of grassland in the area, which deposited phytoliths mixed to 30 cm depth in the soil. A coupled gully colluvial infilling/vegetation record showed that sediment flux during the late Pleistocene was ~0.0019 m³ m⁻¹ yr⁻¹ under a shrubland/grassland mosaic, and Holocene sediment flux was ~0.0034 m³ m⁻¹ yr⁻¹ under forest. This increase of 60% through the last glacial-interglacial transition resulted from increased bioturbation and down-slope soil transport via root growth and treethrow, which formed a biomantle as evidenced by slope redistribution of the KOT. These results contrast with sediment transport rates and processes hypothesised to occur contemporaneously in adjacent mountain catchments. This suggests that intraregional biogeomorphic processes can differ significantly depending on topography and geological substrate, with different landscapes responding in unique ways to the same climate shifts. Analysis of Quaternary terrestrial landscape evolution in non-glaciated mountainous and lowland areas must therefore consider spatial and temporal heterogeneity in sediment fluxes and underlying transport processes.