Spatial controls on the distribution and dynamics of a marginal snowpack in the Australian Alps

Seasonal snowpacks in marginal snow environments are typically warm and nearly isothermal, exhibiting high inter‐ and intra‐annual variability. Measurements of snow depth and snow water equivalent were made across a small subalpine catchment in the Australian Alps over two snow seasons in order to investigate the extent and implications of snowpack spatial variability in this marginal setting. The distribution and dynamics of the snowpack were found to be influenced by upwind terrain, vegetation, solar radiation, and slope. The role of upwind vegetation was quantified using a novel parameter based on gridded vegetation height. The elevation range of the catchment was relatively modest (185 m), and elevation impacted distribution but not dynamics. Two characteristic features of marginal snowpack behaviour are presented. Firstly, the evolution of the snowpack is described in terms of a relatively unstable accumulation state and a highly stable ablation state, as revealed by temporal variations in the mean and standard deviation of snow water equivalent. Secondly, the validity of partitioning the snow season into distinct accumulation and ablation phases is shown to be compromised in such a setting. Snow at the most marginal locations may undergo complete melt several times during a season and, even where snow cover is more persistent, ablation processes begin to have an effect on the distribution of the snowpack early in the season. Our results are consistent with previous research showing that individual point measurements are unable to fully represent the variability in the snowpack across a catchment, and we show that recognising and addressing this variability are particularly important for studies in marginal snow environments.     

Cultural implications of architectural mortar selection at Mesa Verde National Park,Colorado

Architectural mortar from two ancestral Pueblo sites (Spruce Tree House and Nordenskiöld's Ruin 12) located in Mesa Verde National Park was investigated using visual, mineralogical, and geochemical techniques. Results indicate ancestral Pueblo people had a preference for mortars composed of sand and clay contents that produce a USDA textural class of sandy clay loam to clay loam. A temporal trajectory of soil selection is observed at Spruce Tree House, with mesa‐top soils being preferred during the early period of occupation, but with soils below cliff dwellings preferred during later periods. Mortar geochemical composition is found to differ between cliff dwellings, and sometimes between households within a cliff dwelling, due to local soil differences and/or potential amendment additions. Results from Spruce Tree House indicate that contemporaneous households shared access to mortar sources. The prevalence and possible origins of gypsum found in mortar are discussed. Finally, this research examines the possibility that land tenure rights may have extended beyond those lands used exclusively for agricultural purposes. © 2011 Wiley Periodicals, Inc.     

Modeling Alternative Sequences of Events in Dynamic Geographic Domains

A model that emphasizes possible alternative sequences of events that occur over time is presented in this article. Representing alternative or branching events captures additional semantics unrealized by linear or non‐branching approaches. Two basic elements of branching, divergence and convergence are discussed. From these elements, many complex branching models can be built capturing a perspective of events that will take place in the future or have occurred in the past. This produces likely sequences of events that a user may compare and analyze using spatial or temporal criteria. The branching events model is especially useful for spatiotemporal decision support systems, as decision‐makers are able to identify alternative locations and times of events and, depending on the context, also identify regions of multiple possible events. A branching events viewer application is presented illustrating a case study based on a tornado response.     

A Simple Index of Trophic Status in Estuaries and Coastal Bays Based on Measurements of pH and Dissolved Oxygen

Measurements of pH and dissolved oxygen saturation in summer from 90 Irish estuaries and coastal bays were used to develop a simple index of trophic state. The index is based on the assumption that large fluctuations in both these variables are likely to be a characteristic feature of eutrophication. In this paper, we use a simple index to capture the relative variation in both pH and dissolved oxygen saturation in each water body during a period of 3 years. We show that the index is in good agreement with other trophic status schemes and strongly correlated with parameters indicative of trophic state such as chlorophyll a and biochemical oxygen demand. We conclude that the index could be used as a simple screening tool to group individual water bodies into broad categories that reflect their trophic state.     

Quantifying volcanic ash fall hazard to electricity infrastructure

Quantifying the potential ash fall hazards from re-awakening volcanoes is a topic of great interest. While methods for calculating the probability of eruptions, and for numerical simulation of tephra dispersal and fallout exist, event records at most volcanoes that re-awaken sporadically on decadal to millennial cycles are inadequate to develop rigorous forecasts of occurrence, much less eruptive volume. Here we demonstrate a method by which eruption records from radiocarbon-dated sediment cores can be used to derive forecasting models for ash fall impacts on electrical infrastructure. Our method is illustrated by an example from the Taranaki region of New Zealand. Radiocarbon dates, expressed as years before present (B.P.), are used to define an age-depth model, classifying eruption ages (with associated errors) for a circa 1500–10 500 year B.P. record at Mt. Taranaki (New Zealand). In addition, data describing the youngest 1500 years of eruption activity is obtained from directly dated proximal deposits. Absence of trend and apparent independence in eruption intervals is consistent with a renewal model using a mix of Weibulls distributions, which was used to generate probabilistic forecasts of eruption recurrence. After establishing that interval length and tephra thickness were independent in the record, a thickness–volume relationship (from [Rhoades, D.A., Dowrick, D.J., Wilson, C.J.N., 2002. Volcanic hazard in New Zealand: Scaling and attenuation relations for tephra fall deposits from Taupo volcano. Nat. Hazards, 26:147–174]) was inverted to provide a frequency–volume relationship for eruptions. Monte Carlo simulation of the thickness–volume relationship was then used to produce probable ash fall thicknesses at any chosen site. Several critical electrical infrastructure sites in the Taranaki Region were analysed. This region, being the only gas and condensate-producing area in New Zealand, is of national economic importance, with activities in and around the area depending on uninterrupted power supplies. Forecasts of critical ash thicknesses (1 mm wet and 2 mm dry) that may cause short-circuiting, surges or power shutdowns in substations show that the annual probabilities of serious impact are between ~ 0.5% and 27% over a 50 year period. It was also found that while large eruptions with high ash plumes tend to affect “expected” areas in relation to prevailing winds, the direction impacts of small ash falls are far less predictable. In the Taranaki case study, areas out of normal downwind directions, but close to the volcano, have probabilities of impact for critical thicknesses of 1–2 mm of around half to 60% of those in downwind directions and therefore should not be overlooked in hazard analysis. Through this method we are able to definitively show that the potential ash fall hazard to electrical infrastructure in this area is low in comparison to other natural threats, and provide a quantitative measure for use in risk analysis and budget prioritisation for hazard mitigation measures.     

Eruption episodes and magma recharge events in andesitic systems: Mt Taranaki,New Zealand

Eruption episodes, where a series of eruption events are generically related, can include the eruption of a wide spectrum of volcanic activity over decadal periods. This paper concentrates on the opening phases of an eruption episode which occurred approximately 1800 yrs BP from Mt Taranaki, New Zealand. These events spanned the eruption of differing bulk compositions and styles from two distinct vent locations; an andesitic sub-plinian eruption from the summit vent and a scoria cone-building eruption of basaltic magma from a satellite vent. Compositional profiles and zoning textures of plagioclase, amphibole and clinopyroxene phenocrysts from the opening andesitic event show evidence of magma mixing and subsequent crystallisation just prior to the initiation of the eruption episode. Titanomagnetite grain morphology and Ti variation suggest that the magma mixing event occurred within a few days to weeks before the eruption acting as a trigger for it. We present a magmatic model which is constrained by the petrological observations and eruptions of the episode. In this model magma differentiation at depth causes its rise and recharging of a mid-crustal magma storage area at 5–7 km. Although the recharging magma differed slightly in oxygen fugacity and temperature, it was compositionally and physically similar enough to the residing andesitic magma to allow efficient mixing. The petrological characteristics described here can be readily observed and enable identification of mixing events in other recent eruption episodes.     

Merging eruption datasets: building an integrated Holocene eruptive record for Mt Taranaki,New Zealand

Acquiring detailed eruption frequency datasets for a volcano system is essential for realistic eruption forecasts. However, accurate datasets are inherently difficult to compile, even if one or more well-dated eruption records are available. A single record typically under-represents the eruption frequency, while combining two or more records may result in an overrepresentation. Although glass compositions have proven to be successful in tephrochronological studies of dominantly rhyolitic tephras; microlitic growth and thin glass shards inhibit their application to andesitic tephras. A method consisting of a combination of two techniques for correlating syn-eruptive deposits is demonstrated on data from the typical andesitic stratovolcano of Mt. Taranaki, New Zealand. Firstly, tentative matches are identified using the radiocarbon age and associated error of each event. Secondly, the compositions of titanomagnetite micro-phenocrysts are used as an independent check, and shown to be a useful correlation tool where age data is available. Using two lake-core records containing tephra layers in an overlapping time-frame, the radiocarbon age-correlation procedure suggested 31 tephra matches. Geochemistry data were available for 15 of these pairs. In three of these cases, the titanomagnetite compositions did not match. Hence, these “paired” tephras were from compositionally distinct magmas and therefore likely represent separate events. An additional three matches were reassigned within the temporal uncertainty limits of the dating procedure, based on better geochemical pairing. The final combined dataset suggests that there have been at least 138 separate ash fall-producing eruptions between 96 and 10 150 years B.P. from Taranaki. Using the combined dataset the mixture of Weibulls renewal model forecasts a probability of 0.52 for an eruption occurring in the next 50 years at this volcano. The present annual eruption probability is estimated at 1.6%. This likelihood is almost double that obtained when relying on a single stratigraphic record.     

Phreatomagmatic volcanic hazards where rift-systems meet the sea, a study from Ambae Island, Vanuatu

Ambae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE–SW rift-controlled elongated shape. Several hundred scoria cones and fissure-fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10,000 inhabitants. Its activity remained focused at the central (crater-lake filled) vent and this is where hazard-assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred < 500 and < 300 yr B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-continuous cliff section exposes an up to 25 m thick succession of near-vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-grained tephra with undulatory bedding and cross-lamination as well as megaripple bedforms. These surge and fall beds are more common in the uppermost part of the succession and form a few-m-thick pile. An older tephra succession of similar character occurs below, and buried trees in growth position, as well as those flattened within base surge beds. This implies that the centre of this eruption was very near the coastline. The processes implied by these deposits are amongst the most violent forms of volcanism on this island. In addition, the lowland and coastal areas affected by these events are the most heavily populated. This circumstance is mirrored on many similar volcanic islands, including the nearby SW Pacific examples of Taveuni (Fiji), Upolu and Savai'i (Samoa), and Ambrym (Vanuatu). These locations are paradoxically often considered safe areas during summit/central-vent eruptions, simply because they are farthest from the central sources of ash-fall and lahar hazard. The observations presented here necessitate a revision of this view.