Gravity and magnetic 3D inversion of Morro do Engenho complex, Central Brazil

In the Goiás Alkaline Province, in central Brazil, some mafic–ultramafic alkaline bodies crop out, such as Morro do Engenho Complex (ME), whereas other intrusive bodies are completely covered by Quaternary sediments, such as A2 body, and can only be defined by their magnetic and gravity responses. The bodies give rise to strong gravity and magnetic anomalies. We inverted the gravity and magnetic data to estimate 3D density and magnetic intensity contrasts. The gravity inversion reveals that ME and A2 intrusions, totally emplaced in the upper crust, have maximum depths of about 10 and 6 km, respectively. The estimated density contrast distribution indicates a NE–SW preferential direction for ME intrusive body. Since the remnant magnetization is very important in the area, we inverted the magnetic data, testing two hypotheses: only induced field and induced plus remnant magnetizations. The results of the inversion using only induced field could not explain the observed magnetic field, which was about 1000 nT larger than the calculated one. Induced plus remnant field inferred from wandering polar path fitted the observations. 3D magnetic inversions when remnant magnetization was included recover deeper and thinner bodies compared with gravity inversion. The inverted bodies have spherical shapes which are more consistent with the model of magmatic chambers instead of plug intrusions.     

Tracing the influence of sewage discharge on coastal bays of Southern Vancouver Island (BC,Canada) using sedimentary records of phytoplankton

The impact of sewage and stormwater effluents on phytoplankton is investigated by comparing organic-walled dinoflagellate cyst abundance and diversity from 38 surface sediment samples, flanking southern Vancouver Island. Site locations include those directly adjacent to wastewater outfall at Clover and Macaulay Points and Saanich Peninsula, as well as from a variety of near-shore environments with differing tidal flow influences. Excellently preserved dinoflagellate cyst assemblages have been recovered and 36 cyst taxa were identified. Local assemblages are characterized by a high relative proportion (average 56%) of cysts produced by heterotrophic dinoflagellates, which is typical for regions of high primary production. Relative proportional increases of cysts from heterotrophic species with particular increases of Polykrikos kofoidii/schwartzii and Dubridinium species, known to reflect areas affected by eutrophication, occur directly adjacent to all three sewage outfalls, as well as in the more stagnant waters of Esquimalt and Victoria Harbours and at the mouth of Cadboro Bay. Further effects of an anthropogenic effluent can be seen in the relatively higher concentrations of organic carbon and the diatom production proxy, biogenic opal. Results from this study clearly indicate a much larger impact zone than predicted by a sewage effluent plume model or trends found in monitored benthic biota and sediment chemistry that evidence primary outfall affects <800 m eastward of Macaulay Point and about 200 m eastward of the Clover Point. Enhanced production of cysts from potentially toxic Alexandrium species is also observed near locations of sewage outfalls.     

Volcanic threat in developing countries of the Asia�CPacific region: probabilistic hazard assessment, population risks, and information gaps

The importance of disaster reduction has gained increased awareness within the international development community and thereby highlighted a need for a preliminary assessment of natural hazard risk in developing countries of the Asia?CPacific, including that for volcanic eruption. In this paper, we present a key component of such an assessment, which involved qualifying the frequency and potential consequences of large??Volcanic Explosivity Index of four or more??volcanic eruptions. The frequencies of large eruptions from volcanoes grouped by region were determined from frequency?Cmagnitude plots using data provided by the Smithsonian Institution??s Global Volcanism Program. However, calculated frequencies represent only minimum values due to an incomplete eruption record. Unfortunately, limited data precluded the calculation of eruption frequencies for the Solomon Islands, Fiji and Samoa. A first-order analysis of the populations potentially impacted by large volcanic eruptions suggest that (1) volcanic disasters affecting populations of >100,000 can be expected at least every decade in Indonesia and once every few decades in the Philippines and (2) a volcanic disaster impacting >1% of the population can be expected twice a century in Vanuatu, twice a millennium for Indonesia and the Philippines, and around every millennium in Papua New Guinea and Tonga.