Combining long- and short-term probabilistic volcanic hazard assessment with cost-benefit analysis to support decision making in a volcanic crisis from the Auckland Volcanic Field,New Zealand

By using BET_VH, we propose a quantitative probabilistic hazard assessment for base surge impact in Auckland, New Zealand. Base surges resulting from phreatomagmatic eruptions are among the most dangerous phenomena likely to be associated with the initial phase of a future eruption in the Auckland Volcanic Field. The assessment is done both in the long-term and in a specific short-term case study, i.e. the simulated pre-eruptive unrest episode during Exercise Ruaumoko, a national civil defence exercise. The most important factors to account for are the uncertainties in the vent location (expected for a volcanic field) and in the run-out distance of base surges. Here, we propose a statistical model of base surge run-out distance based on deposits from past eruptions in Auckland and in analogous volcanoes. We then combine our hazard assessment with an analysis of the costs and benefits of evacuating people (on a 1 × 1-km cell grid). In addition to stressing the practical importance of a cost-benefit analysis in creating a bridge between volcanologists and decision makers, our study highlights some important points. First, in the Exercise Ruaumoko application, the evacuation call seems to be required as soon as the unrest phase is clear; additionally, the evacuation area is much larger than what is recommended in the current contingency plan. Secondly, the evacuation area changes in size with time, due to a reduction in the uncertainty in the vent location and increase in the probability of eruption. It is the tradeoff between these two factors that dictates which cells must be evacuated, and when, thus determining the ultimate size and shape of the area to be evacuated.     

The practice of DEM stream burning revisited

Stream burning is a common flow enforcement technique used to correct surface drainage patterns derived from digital elevation models (DEM). The technique involves adjusting the elevations of grid cells that are coincident with the features of a vector hydrography layer. This paper focuses on the problematic issues with common stream burning practices, particularly the topological errors resulting from the mismatched scales of the hydrography and DEM data sets. A novel alternative stream burning method is described and tested using five DEMs of varying resolutions (1 to 30 arc‐seconds) for an extensive area of southwestern Ontario, Canada. This TopologicalBreachBurn method uses total upstream channel length (TUCL) to prune the vector hydrography layer to a level of detail that matches the raster DEM grid resolution. Network pruning reduces the occurrence of erroneous stream piracy caused by the rasterization of multiple stream links to the same DEM grid cell. The algorithm also restricts flow within individual stream reaches, further reducing erroneous stream piracy. In situations where two vector stream features occupy the same grid cell, the new tool ensures that the larger stream, designated by higher TUCL, is given priority. TUCL‐based priority minimizes the impact of the topological errors that occur during the stream rasterization process on modeled regional drainage patterns. The test data demonstrated that TopologicalBreachBurn produces highly accurate and scale‐insensitive drainage patterns and watershed boundaries. The drainage divides of four large watersheds within the study region that were delineated from the TopologicalBreachBurn‐processed DEMs were found to be highly accurate when compared with the official watershed boundaries, even at the coarsest grid resolutions, with Kappa index of agreement values ranging from 0.952 to 0.921. The corresponding Kappa coefficient values for a traditional stream burning method (FillBurn) ranged from 0.953 to 0.490, demonstrating a significant decrease in mapping accuracy at coarser DEM grid resolutions. Copyright © 2015 John Wiley & Sons, Ltd.     

Measuring the significance of a divide to local drainage patterns

This article presents a framework for estimating a new topographic attribute derived from digital elevation models (DEMs) called maximum branch length (B _max). Branch length is defined as the distance travelled along a flow path initiated at one grid cell to the confluence with the flow path passing through a second cell. B _max is the longest branch length measured for a grid cell and its eight neighbours. The index provides a physically meaningful method for assessing the relative significance of drainage divides to the dispersion of materials and energy across a landscape, that is, it is a measure of ‘divide size’. B _max is particularly useful for studying divide network structure, for mapping drainage divides, and in landform classification applications. Sensitivity analyses were performed to evaluate the robustness of estimates of B _max to the algorithm used to estimate flow lengths and the prevalence of edge effects resulting from inadequate DEM extent. The findings suggest that the index is insensitive to the specific flow algorithm used but that edge effects can result in significant underestimation along major divides. Edge contamination can, however, be avoided by using an appropriately extensive DEM.     

On the origins of GEMS grains

From their birth as condensates in the outflows of oxygen-rich evolved stars, processing in interstellar space, and incorporation into disks around new stars, amorphous silicates predominate in most astrophysical environments. Amorphous silicates were a major building block of our Solar System and are prominent in infrared spectra of comets. Anhydrous interplanetary dust particles (IDPs) thought to derive from comets contain abundant amorphous silicates known as GEMS (glass with embedded metal and sulfides) grains. GEMS grains have been proposed to be isotopically and chemically homogenized interstellar amorphous silicate dust. We evaluated this hypothesis through coordinated chemical and isotopic analyses of GEMS grains in a suite of IDPs to constrain their origins. GEMS grains show order of magnitude variations in Mg, Fe, Ca, and S abundances. GEMS grains do not match the average element abundances inferred for ISM dust containing on average, too little Mg, Fe, and Ca, and too much S. GEMS grains have complementary compositions to the crystalline components in IDPs suggesting that they formed from the same reservoir. We did not observe any unequivocal microstructural or chemical evidence that GEMS grains experienced prolonged exposure to radiation.We identified four GEMS grains having O isotopic compositions that point to origins in red giant branch or asymptotic giant branch stars and supernovae. Based on their O isotopic compositions, we estimate that 1-6% of GEMS grains are surviving circumstellar grains. The remaining 94-99% of GEMS grains have O isotopic compositions that are indistinguishable from terrestrial materials and carbonaceous chondrites. These isotopically solar GEMS grains either formed in the Solar System or were completely homogenized in the interstellar medium (ISM). However, the chemical compositions of GEMS grains are extremely heterogeneous and seem to rule out this possibility. Based on their solar isotopic compositions and their non-solar elemental compositions we propose that most GEMS grains formed in the nebula as late-stage non-equilibrium condensates.     

Cues,Not an Endogenous Rhythm,Control the Dusk Peak in Water-Column Entry by Benthic Copepods

Individuals of some benthic species swim out of or away from the sediment surface into the water column, i.e., they emerge. Individuals of both emergent and nonemergent benthic species can be entrained by near-bottom flows. Both emergence and entrainment are of interest, e.g., for their roles in benthopelagic coupling, but the controlling factors are poorly understood. Our experiments with benthic copepods from contrasting environments showed that a factor (or factors) associated with the onset of darkness, rather than an endogenous rhythm, controls their dusk emergence. In addition, we argue that entrainment and emergence can interact in at least two ways: (1) Light-induced changes in oxygenation of the sediment pore water may affect the entrainment flux of benthic copepods and (2) if large numbers of individuals are entrained in the time leading up to sunset, few will remain in the sediment to be part of the dusk peak in emergence.     

Origin of Tertiary to Recent EM- and subduction-like chemical and isotopic signatures in Auca Mahuida region (37°–38°S) and other Patagonian plateau lavas

The alkaline volcanic rocks of the 1.8–0.9 Ma Auca Mahuida and post-mid-Pliocene Rio Colorado backarc volcanic fields east of the Andean Southern Volcanic Zone at ~37°–38°S have pronounced intraplate-like chemical signatures with some striking similarities to oceanic DM-EM1-like lavas of the south Atlantic Tristan da Cunha type. These backarc lavas are considered to have formed as a series of mantle batches typified by 4–7 % melting, with decompression melting initiating in a garnet-bearing mantle above a steepening subduction zone, and final equilibration occurring near the base of a ~65- to 70-km-thick lithosphere at temperatures of ~1,350–1,380 °C. Evolved Auca Mahuida mugearite to trachytic samples are best explained by crystal fractionation with limited mixing of partial melts of recently underplated basalts, in line with isotopic signatures that preclude significant radiogenic contamination in a preexisting refractory crust. Higher Ba/La and subtly higher La/Ta ratios than in nearby ~24–20 Ma primitive basalts or oceanic (OIB) lavas are attributed to the residual effects of slab fluids introduced during a shallow subduction episode recorded in the arc-like chemistry of the adjacent 7–4 Ma Chachahuén volcanic complex. Positive Sr, K and Ba spikes on mantle-normalized patterns of both primitive Auca Mahuida and ~24–20 Ma basalts, like those in EM-like OIB basalts, are attributed to mixing of continental lithosphere into the asthenosphere. In Patagonia, this mixing is suggested to have peaked as the South America continent accommodated to major late Oligocene plate convergence changes, as similar Sr, K and Ba spikes and DM-EM1 signatures are absent in ~50–30 Ma backarc lavas north of 51°S, and all of those south of 51°S. Introduction of an EM1-like component associated with lateral mantle flow of a Tristan da Cunha source is largely precluded by its Cretaceous age and distance to Patagonia.     

Heavitree Quartzite,a Neoproterozoic (ca 800–760 Ma), high‐energy,tidally influenced,ramp association,Amadeus Basin,central Australia

The Neoproterozoic Heavitree Quartzite is widespread in the Amadeus Basin and has correlatives in all of the major central Australian intracratonic basins. The origin of the formation is enigmatic, not only because of its widespread sheet‐like distribution and uniformity of composition, but also because intense silicification makes facies studies difficult. Recently discovered exposures at the eastern end of the basin are relatively free of diagenetic quartz allowing a detailed study of sedimentary structures and an understanding of the depositional architecture of the formation. The formation, which consists largely of pale‐tan or white quartzose sandstone interbedded with rare laminated mudstone and conglomerate intervals, was deposited in at least four depositional sequences. The sheet‐like nature of the sandstone results from an abundant supply of sediments deposited in a high‐energy, open, shelf‐like environment on a regionally subsiding, low‐gradient ramp. Environmental settings switched both laterally and temporally between sand waves deposited by reversing tidal flow and higher velocity unidirectional currents involving dunes and plane beds. In the early stages of deposition, mud‐dominated, tidal‐flat environments alternated with higher energy, sand‐dominated, tidally influenced settings. However, in the later stages of deposition a major eustatic sea‐level fall moved base‐level basinwards, earlier sediments were reworked by streams to form a ravinement surface, gravel was carried well into the basin and fines largely disappeared from the environment. Gravel deposition was followed by a return to high‐energy, tidally influenced deposits involving large sand waves or dunes. Towards the top of the formation sand waves deposited by reversing tidal currents gradually decline and are eventually replaced by dunes deposited by unidirectional current flow. The transition to the shallow‐marine, anoxic rocks of the Bitter Springs Formation is gradational in response to increased accommodation in a ramp setting which lacked a clearly defined shelf break. The Heavitree Quartzite was probably deposited as a direct response to the events surrounding the assembly and breakup of Rodinia, in particular peneplanation during regional uplift in response to a rising mantle plume followed by broad regional subsidence as the plume decayed prior to the breakup of the supercontinent. The large supply of quartz sand resulted from peneplanation associated with the rising plume and the lack of soil‐stabilising vascular plants, an environmental setting with no modern analogue. The ultimate disposition of fines is not known but, given the environment of deposition, it is likely that they were removed during peneplanation and bypassed the sag basin completely.     

Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism

The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low ⁸⁷Sr/⁸⁶Sr (0.70294 to 0.70342) and high ɛ_Nd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low ⁸⁷Sr/⁸⁶Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO₂-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000     

Climate Change and the Disappearance of Malaria from England

As the world's climate continues to change there is concern that this may cause malaria to spread to new areas. Here we examine whether past changes in temperature, in addition to social changes, may have affected malaria in England. Our analysis indicates that cold summers experienced during the 1800s may have contributed to the disappearance of this disease from the country. As our summers become progressively warner it is unlikely that malaria will become firmly re-established on these shores, although the possibility of future outbreaks should not be ignored.     

Fault zone hydrogeology in arid environments: The origin of cold springs in the Wadi Araba Basin,Egypt

The role of faults in controlling groundwater flow in the Sahara and most of the hyper-arid deserts is poorly understood due to scarcity of hydrological data. The Wadi Araba Basin (WAB), in the Eastern Sahara, is highly affected by folds and faults associated with Senonian tectonics and Paleogene rifting. Using the WAB as a test site, satellite imagery, aeromagnetic maps, field observations, isotopic and geochemical data were examined to unravel the structural control on groundwater flow dynamics in the Sahara. Analysis of satellite imagery indicated that springs occur along structurally controlled scarps. Isotopic data suggested that cold springs in the WAB showed a striking similarity with the Sinai Nubian aquifer system (NAS) water and the thermal springs along the Gulf of Suez (e.g., δ¹⁸O = −8.01‰ to −5.24‰ and δD = −53.09‰ to −31.12‰) demonstrating similar recharge sources. The findings advocated that cold springs in the WAB represent a natural discharge from a previously undefined aquifer in the Eastern Desert of Egypt rather than infiltrated precipitation over the plateaus surrounding the WAB or through hydrologic windows from deep crystalline basement flow. A complex role of the geological structures was inferred including: (1) channelling of the groundwater flow along low-angle faults, (2) compartmentalization of the groundwater flow upslope from high-angle faults, and (3) reduction of the depth to the main aquifer in a breached anticline setting, which resulted in cold spring discharge temperatures (13–22°C). Our findings emphasize on the complex role of faults and folds in controlling groundwater flow, which should be taken into consideration in future examination of aquifer response to climate variability in the Sahara and similar deserts worldwide.