Correlation of rhyolitic pyroclastic eruptive units from the Taupo volcanic zone by Fe–Ti oxide compositional data

Grain-specific analyses of Fe–Ti oxides and estimates of eruption temperature (T) and oxygen fugacity (fO₂) have been used to fingerprint rhyolitic fall and flow deposits that are important for tephrostratigraphic studies in and around the Taupo volcanic zone of North Island, New Zealand. The analysed Fe–Ti oxides commonly occur in the rims of orthopyroxene crystals and appear to reflect equilibrium immediately prior to eruption because of geochemical correlation with the co-existing glass phase. The composition of the spinel phase is particularly diagnostic of eruptive centre for post-65 ka events and can be used to distinguish many tephra beds from the same volcano. The 29 different units examined were erupted over a wide range in T (690–990°C) and Δ log fO₂ (–0.1 to 2.0). These parameters are closely related to the mafic mineral assemblage, with hydrous mineral-bearing units displaying higher fO₂. Such trends are superimposed on larger differences in fO₂ that are related to eruptive centre. At any given temperature, all post-65 ka Okataina centre tephra have higher fO₂ values than post-65 ka Taupo centre tephra. This provides a useful criterion for identifying the volcanic source. There are no temporal T and fO₂ trends in the tephra record; over intervals >20 ka, however, tephra sequences from Taupo centre form characteristic T-fO₂ buffer trends mirroring the glass chemistry. Individual eruptive events display uniform spinel and rhombohedral phase compositions and thus narrow ranges in T (± <20°C) and log fO₂ (± <0.5), allowing these features to identify individual magma batches. These criteria can help distinguish tephra deposits of similar bulk or glass composition that originated from the same volcano. Distal fall deposits record the same T-fO₂ conditions as the proximal ignimbrite and enable distal–proximal correlation. Lateral and vertical compositional and T-fO₂ variability displayed in large volume (>100 km³) ignimbrites, such as the Oruanui, Rotoiti and Ongatiti, is similar to that found in a single pumice clast and thus mainly reflects analytical error; however, thermal gradients of ca. 50°C may occur in some units. Received: 6 April 1998 / Accepted: 16 June 1998     

Pure acoustic wave propagation in transversely isotropic media by the pseudospectral method

Seismic wave propagation in transversely isotropic (TI) media is commonly described by a set of coupled partial differential equations, derived from the acoustic approximation. These equations produce pure P‐wave responses in elliptically anisotropic media but generate undesired shear‐wave components for more general TI anisotropy. Furthermore, these equations suffer from instabilities when the anisotropy parameter ε is less than δ. One solution to both problems is to use pure acoustic anisotropic wave equations, which can produce pure P‐waves without any shear‐wave contaminations in both elliptical and anelliptical TI media. In this paper, we propose a new pure acoustic transversely isotropic wave equation, which can be conveniently solved using the pseudospectral method. Like most other pure acoustic anisotropic wave equations, our equation involves complicated pseudo‐differential operators in space which are difficult to handle using the finite difference method. The advantage of our equation is that all of its model parameters are separable from the spatial differential and pseudo‐differential operators; therefore, the pseudospectral method can be directly applied. We use phase velocity analysis to show that our equation, expressed in a summation form, can be properly truncated to achieve the desired accuracy according to anisotropy strength. This flexibility allows us to save computational time by choosing the right number of summation terms for a given model. We use numerical examples to demonstrate that this new pure acoustic wave equation can produce highly accurate results, completely free from shear‐wave artefacts. This equation can be straightforwardly generalized to tilted TI media.     

Bar-top hollows: A new element in the architecture of sandy braided rivers

Discrete hollows in the bar tops of the South Saskatchewan River are described that form a newly-recognized morphological element of sandy braided rivers. These bar-top hollows, which are up to 1.7 m deep and may extend for 10–30 m down and across flow, have a circular to ovoid planform and are shown, through use of ground penetrating radar, to be filled by a series of distinct, often angle-of-repose, foresets. The hollows form by both erosion and bar-top deposition and may be generated by bar-tail accretion, cross-bar channel cutoff and subsequent fill or lateral accretion at the bar-head. Bar-top hollows occur in the upper part of the bar depositional sequence and may thus prove useful indicators for braid bar reconstruction in ancient sediments, and should not be confused with channel scour.     

Paleovegetation of marine isotope stages 4 and 3 in northern new zealand and the age of the widespread rotoehu tephra

Paleolake sediment, constrained by tephrochronology, from Onepoto basin volcanic crater in Auckland, Northern New Zealand (36° 48′S), provides one of the few uninterrupted records of paleovegetation for marine oxygen isotope stages (MIS) 4 and 3 (76,000–26,000 yr B.P.) in the region. This period was characterized by cool temperate conifer-hardwood forest that lacked some of the warmer taxa typical of the Holocene. The period 64,400–60,500 yr B.P. was marked by opening of forest canopy and expansion of small trees and shrubs, and correlates to the thermal minima of MIS 4. However, the landscape was never as open as the forest-shrubland mosaic of the MIS 2. The beginning of MIS 3 (60,500–50,500 yr B.P.) was marked by the dramatic expansion and then decline of conifer-hardwood forest dominated by Dacrydium cupressinum, a species that prefers wetter conditions. This forest was succeeded by the typically montane Nothofagus at 50,500 yr B.P., corresponding to a thermal decline. Thus, MIS 3 began with an abrupt change to moist cool conditions that lasted about 5000 yr, followed by gradual cooling and dryer conditions. This supports some interpretations from other parts of the southwest Pacific region, that MIS 3 was a period of increased precipitation. The widespread and stratigraphically important Rotoehu tephra, erupted from Okataina Volcanic Centre, has been variously dated at 45,000–65,000 yr B.P. At Onepoto, sedimentation rate and paleovegetation reconstruction imply an age of c. 44,300 yr B.P. The tephra provides a correlation horizon in the marine and terrestrial realms during a period (MIS 3) difficult to date by radiometric methods.     

Dissolved organic phosphorus speciation in the waters of the Tamar estuary (SW England)

The speciation of dissolved organic phosphorus (DOP) in the temperate Tamar estuary of SW England is described. Eight stations from the riverine to marine end-members were sampled during four seasonal campaigns in 2007 and the DOP pool in the water column and sediment porewater was characterized and quantified using a flow injection manifold after sequential enzymatic hydrolysis. This enabled the enzymatically hydrolysable phosphorus (EHP) fraction and its component labile monoester phosphates, diester phosphates and a phytase-hydrolysable fraction that includes myo-inositol hexakisphosphate (phytic acid), to be determined and compared with the total DOP, dissolved reactive phosphorus (DRP) and total dissolved phosphorus (TDP) pools. The results showed that the DOP pool in the water column varied temporally and spatially within the estuary (1.1-22 μg L⁻¹) and constituted 6-40% of TDP. The EHP fraction of DOP ranged from 1.1-15 μg L⁻¹ and represented a significant and potentially bioavailable phosphorus fraction. Furthermore the spatial profiles of the three components of the EHP pool generally showed non-conservative behavior along the salinity gradient, with apparent internal estuarine sources. Porewater profiles followed broadly similar trends but were notably higher at the marine station throughout the year. In contrast to soil organic phosphorus profiles, the labile monoester phosphate fraction was the largest component, with diester phosphates also prevalent. Phytic acid concentrations were higher in the lower estuary, possibly due to salinity induced desorption processes. The EHP fraction is not commonly determined in aquatic systems due to the lack of a suitable measurement technique and the Tamar results reported here have important implications for phosphorus biogeochemistry, estuarine ecology and the development of efficient strategies for limiting the effects of phosphorus on water quality.     

Distal record of multi-sourced tephra in Onepoto Basin,Auckland, New Zealand: implications for volcanic chronology,frequency and hazards

We have documented 80 tephra beds dating from ca. 9.5 to >50 ka, contained within continuously deposited palaeolake sediments from Onepoto Basin, a volcanic explosion crater in Auckland, New Zealand. The known sources for distal (>190 km from vent) tephra include the rhyolitic Taupo Volcanic Centre (4) and Okataina Volcanic Centre (14), and the andesitic Taranaki volcano (40) and Tongariro Volcanic Centre (3). The record provides evidence for four new events between ca. 50 and 28 ka (Mangaone Subgroup) suggesting Okataina was more active than previously known. The tephra record also greatly extends the known northern dispersal of other Mangaone Subgroup tephra. Ten rhyolitic tephra pre-date the Rotoehu eruption (>ca. 50 ka), and some are chemically dissimilar to post-50 ka rhyolites. Some of these older tephra were produced by large-magnitude events; however, their source remains uncertain. Eight tephra from the local basaltic Auckland Volcanic Field (AVF) are also identified. Interpolation of sedimentation rates allow us to estimate the timing of 12 major explosive eruptions from Taranaki volcano in the 27.5-9.5-ka period. In addition, 28 older events are recognised. The tephra are trachytic to rhyolitic in composition. All have high K₂O contents (>3 wt%), and there are no temporal trends. This contrasts with the proximal lava record that shows a trend of increasing K₂O with time. By combining the Onepoto tephra record with that of the previously documented Pukaki crater, 15 AVF basaltic fall events are constrained at: 34.6, 30.9, 29.6, 29.6, 25.7, 25.2, 24.2, 23.8, 19.4, 19.4, 15.8 and 14.5 ka, and three pre-50 ka events. This provides some of the best age constraints for the AVF, and the only reliable data for hazard recurrence calculations. The minimum event frequency of both distal and local fall events can be estimated, and demonstrates the Auckland City region is frequently impacted by ash fall from many volcanoes.     

Submarine silicic volcanism of the Healy caldera,southern Kermadec arc (SW Pacific): I – volcanology and eruption mechanisms

The submarine Healy volcano (southern Kermadec arc), with a 2-2.5 km wide caldera, is pervasively mantled with highly vesicular silicic pumice within a water depth of 1,150-1,800 m. Pumices comprise type 1 white-light grey pumice with ⢾ mm vesicles and weak-moderate foliation, type 2 grey pumice with millimetre-scale laminae, flow banded foliation, including stretched vesicles ⣗ mm in length, and a minor finely vesicular type 3 pumice. All types are sparsely porphyritic, with undevitrified glassy groundmass (68-70% SiO₂), which is microlite and lithic free. Coexisting pyroxenes yield magma temperatures of ~950 °C. Pumice density is А.5 g cm^-3 and vesicularity is 78-83%. Vesicle size distributions for types 1 and 2 pumice, range from ~20 µm to >20 mm, with a strong power-law relation (with d=-2.5ǂ.4) for vesicles <1-2 mm. Larger vesicles have variable size modes. The vesicle size distribution and packing indicates rapid magma decompression and ascent. Consideration of the pressure dependent, solubility of H₂O at a magma temperature of 𙧶 °C and water content of Ж wt%, with pumice petrography and vesicle granulometry, strongly suggests a pyroclastic eruption. Reconstructions of the submarine edifice between water depths of 1,000 and 550 m constrain the ambient hydrostatic pressure to ~6-9 MPa. Pressures >~9 MPa will limit vesicularity to less than the observed 78-83%, whereas pressure <~6 MPa require a more shallower reconstruction of the edifice and larger-volume syn-eruptive collapse. Uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts. Most pyroclasts cool, condensing and ingesting water into steam-inflated vesicles, and then sink. Progression into pyroclastic mode would expand the eruption column, displace ambient water, reduce the hydrostatic load, and further promote vesiculation and fragmentation. Pyroclasts within the column would quench at these reduced pressures. We argue that Healy eruptions deeper than ~1,000 m cannot be pyroclastic. Volumes for the lower and upper bounds of edifice size are 2.36 and 3.58 km³, respectively, but do not account for intra-caldera pumice fill. These volumes are considered to be predominantly primary eruption output, as shown by a dearth of accessory lithics in all pumice, yielding (at an average 81% vesicularity) eruptive pumice volumes of between 10 and 15 km³. Some pyroclasts may have risen to the sea surface and be a correlative of the sea-rafted Loisels pumice; the latter occurs in some New Zealand Holocene beach sequences and has a estimated age of 590ᇤ calendar years.     

Effect of different growth parameters on endoglucanase enzyme activity by bacteria isolated from coir retting effluents of estuarine environment

The cellulolytic enzyme-endoglucanase activity against coir fibre, a major biowaste by bacteria such as Cellulomonas, Bacillus and Micrococcus spp. isolated from coir retting effluents of estuarine environment was studied. The enzyme assay was carried out by using various concentrations (0.5 ? 2%) of substrate of coir powder as a carbohydrate in different pH (5–9) and temperature (20–50 °C). The enzyme activity was minimum in 0.5% substrate concentration at lower pH 5 (0.0087, 0.0143 and 0.0071 U/mL) and at 20 °C temperature (0.0151, 0.0154 and 0.0122 U/mL) by the bacterial strains such as Cellulomonas, Bacillus and Micrococcus spp respectively. Then this level was increased and reached maximum at the neutral pH (0.0172, 0.0165 and 0.0121 U/mL) and at 40 °C (0.0336, 0.0196 and 0.0152 U/mL) by the selected bacterial species. Further increase of pH and temperature, the enzyme activity reduced considerably to 0.0083, 0.0143 and 0.0037 U/mL at pH 9 and 0.0154, 0.0197 and 0.0121 U/mL at 50 °C by the tested bacterial strains. The same trend was also obtained in oth er substrate concentrations such as 1.0, 1.5 and 2.0 %. With in the four substrate concentrations, the endoglucanase enzyme activity was more in 1.5% concentration at the tested pH and temperatures. From the over all result, it was observed that, among the three bacterial strains, the enzyme activity was more in Cellulomonas sp, followed by Bacillus and Micrococcus spp. in varying pH and temperature.