Flood hazard map of La Trinidad (NW Nicaragua). Method and results

During the Mitch Hurricane event (October 1998), severe floods occurred in the village of La Trinidad (Departamento de Estelí, NW Nicaragua), which spreads at the margin of La Trinidad river. As a consequence, the need for hazard assessment and land use planning to reduce the effects of these natural processes arose. Nicaragua is a developing country, which means that there is a scarcity of good quality data on which to base these hazard assessments (i.e., lack of detailed topographic maps, lack of meteorological and discharge data series). Therefore, the main objective of the present work was to generate a flood hazard map of La Trinidad by means of a simple method, with a resulting map easy to understand and to use by the municipality for land use planning. There is no topographic map of the area at a more detailed scale than 1:50,000. So the main document that supports all the data and on which the final hazard map was based is the orthophotograph at 1:5,000 scale (generated from vertical aerial photographs taken in 2000). The method used was based on classical interpretation of vertical aerial photographs (pre Mitch and a post Mitch event), detailed field work, inquiries among the population and analysis of the main pattern of storms occurring in the area. All these data allowed the reconstruction of different extensions and water levels corresponding to events of different frequency and magnitude, and the qualitative association of them to three hazard levels by means of energy and frequency. The use of orthophotographs of 1:5,000 proved to be very useful both for the development of the work and for the presentation of the final map, because they are very easily understandable for people not trained in the interpretation of topographic maps.     

Spatial and Temporal Variability of Benthic Respiration in a Scottish Sea Loch Impacted by Fish Farming: A Combination of In Situ Techniques

The effects of fish farm activities on sediment biogeochemistry were investigated in Loch Creran (Western Scotland) from March to October 2006. Sediment oxygen uptake rates (SOU) were estimated along an organic matter gradient generated from an Atlantic salmon farm using a combination of in situ techniques: microelectrodes, planar optode and benthic chamber incubations. Sulphide (H₂S) and pH distributions in sediment porewater were also measured using in situ microelectrodes, and dissolved inorganic carbon (DIC) fluxes were measured in situ using benthic chambers. Relationships between benthic fluxes, vertical distribution of oxidants and reduced compounds in the sediment were examined as well as bacterial abundance and biomass. Seasonal variations in SOU were relatively low and mainly driven by seasonal temperature variations. The effect of the fish farm on sediment oxygen uptake rate was clearly identified by higher total and diffusive oxygen uptake rates (TOU and DOU, respectively) on impacted stations (TOU: 70 ± 25 mmol O₂ m^?2 day^?1; DOU: 70 ± 32 mmol O₂ m^?2 day^?1 recalculated at the summer temperature), compared with the reference station (TOU: 28.3 ± 5.5 mmol O₂ m^?2 day^?1; DOU: 21.5 ± 4.5 mmol O₂ m^?2 day^?1). At the impacted stations, planar optode images displayed high centimetre scale heterogeneity in oxygen distribution underlining the control of oxygen dynamics by small-scale processes. The organic carbon enrichment led to enhanced sulphate reduction as demonstrated by large vertical H₂S concentration gradients in the porewater (from 0 to 1,000 μM in the top 3 cm) at the most impacted site. The impact on ecosystem functions such as bioirrigation was evidenced by a decreasing TOU/DOU ratio, from 1.7 in the non-impacted sediments to 1 in the impacted zone. This trend was related to a shift in the macrofaunal assemblage and an increase in sediment bacterial population. The turnover time of the organic load of the sediment was estimated to be over 6 years.     

Mercury in blood and eggs of the sea turtle Lepidochelys olivacea from a nesting colony in Oaxaca, Mexico

Mercury concentrations were assessed in the sea turtle Lepidochelys olivacea from a nesting colony of Oaxaca, Mexico; 25 female turtles were sampled, a total of 250 eggs were collected during the season 2005-2006. Higher concentrations were found in yolk fraction, while in blood and albumen mean levels were below of 0.0010 μg g⁻¹ dry wt. On the basis of one nesting season, the maternal transfer of Hg via eggs-laying was estimated in 2.0 ± 1.1%. According to international norms, the health of this population and its habitats is acceptable for Hg and corresponds to baseline levels of a nearly pristine environment.     

Provenance variations in the Late Paleozoic accretionary complex of central Chile as indicated by detrital zircons

We present detrital zircon UPb SHRIMP age patterns for the central segment (34–42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34–42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3 Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34–38°30′S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39–42°S). The youngest igneous detrital zircons in both the northern Western (307 Ma) and Eastern Series (345 Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and LuHf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253–295 Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times.     

An assessment of hydrothermal alteration in the Santiaguito lava dome complex, Guatemala: implications for dome collapse hazards

A combination of field mapping, geochemistry, and remote sensing methods has been employed to determine the extent of hydrothermal alteration and assess the potential for failure at the Santiaguito lava dome complex, Guatemala. The 90-year-old complex of four lava domes has only experienced relatively small and infrequent dome collapses in the past, which were associated with lava extrusion. However, existing evidence of an active hydrothermal system coupled with intense seasonal precipitation also presents ideal conditions for instability related to weakened clay-rich edifice rocks. Mapping of the Santiaguito dome complex identified structural features related to dome growth dynamics, potential areas of weakness related to erosion, and locations of fumarole fields. X-ray diffraction and backscattered electron images taken with scanning electron microscopy of dacite and ash samples collected from around fumaroles revealed only minor clay films, and little evidence of alteration. Mineral mapping using ASTER and Hyperion satellite images, however, suggest low-temperature (<150 °C) silicic alteration on erosional surfaces of the domes, but not the type of pervasive acid-sulfate alteration implicated in collapses of other altered edifices. To evaluate the possibility of internal alteration, we re-examined existing aqueous geochemical data from dome-fed hot springs. The data indicate significant water–rock interaction, but the Na–Mg–K geoindicator suggests only a short water residence time, and δ¹⁸O/δD ratios show only minor shifts from the meteoric water line with little precipitation of secondary (alteration) minerals. Based on available data, hydrothermal alteration on the dome complex appears to be restricted to surficial deposits of hydrous silica, but the study has highlighted, importantly, that the 1902 eruption crater headwall of Santa María does show more advanced argillic alteration. We also cannot rule out the possibility of advanced alteration within the dome complex interior that is not accessible to the methods used here. It may therefore be prudent to employ geophysical methods to make further assessments in the future.     

Tracer hydrology of the data-scarce and heterogeneous Central American Isthmus

Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial–temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface water–groundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the mid-summer drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong orographic separation into the Caribbean and Pacific domains, mainly induced by the governing moisture transport from the Caribbean Sea, complex rainfall producing systems across the N-S mountain range, and the subsequent mixing with local evapotranspiration, and, to a lesser degree, the eastern Pacific Ocean fluxes. Groundwater recharge was characterized by (a) depleted recharge in highland areas (72.3%), (b) rapid recharge via preferential flow paths (13.1%), and enriched recharge due to near-surface secondary fractionation (14.6%). Median recharge elevation ranged from 1,104 to 1,979 m a.s.l. These results are intended to enhance forest conservation practices, inform water protection regulations, and facilitate water security and sustainability planning in the Central American Isthmus.     

The Westphalian D fossil lepidodendrid forest at Table Head, Sydney Basin, Nova Scotia: Sedimentology, paleoecology and floral response to changing edaphic conditions

Strata of Westphalian D age on the western coast of the Sydney Basin expose a fossil forest of approximately 30 lepidodendrid trees within one of several clastic splits of the Harbour Seam. A multidisciplinary approach was employed to interpret the origins of the coat bed, the depositional history of the site and the response of the fossil forest to changing edaphic conditions. The megaspore and miospore records indicate that the mire vegetation was dominated by arboreous lycopsids, especially Paralycopodites, with subdominant tree ferns. Petrographic, palynological and geochemical evidence suggest that the Harbour coal bed at Table Head originated as a rheotrophic (cf. planar) mire (eutric histosol). The mire forest is interpreted to have been engulfed by prograding distributary-channel sediments; sparse protist assemblages are suggestive of a freshwater delta-plain lake environment occasionally in contact with brackish waters. Lepidodendrids persisted as site colonizers of clastic substrates even after burial of the rheotrophic peatland and influenced the morphology of deposited sediment, but apparently were unable to colonize distributary channels. Equivocal taxonomic data (compression fossils) show the fossil forest to have been composed of both monocarpic (Lepidodendron) and polycarpic (Diaphorodendron, Paralycopodites, ?Sigillaria) lycopsids, genera recorded in the palynology of the uppermost ply of the underlying coal bed. Comparatively rare within the clastic beds of the fossil forest, however, is the stem compression of Paralycopodites, whose dispersed megapores and miospores dominate the underlying coal bed. Tree diameter data recorded equivalent to breast height indicate a forest of mixed age. These data would appear to suggest that some lepidodendrids employing a polycarpic reproductive strategy were better able to cross the ecological barrier imposed between peat and clastic substrates. Foliar compressions indicate that an understory or stand of Psaronius type tree ferns co-existed with the lepidodendrids on clastic substrates, which developed as incipient gleysol soils. The entombment of the forest can be ascribed to its distributary coastal setting, local subsidence and a seasonal climate that fostered wildfire and increased sedimentation.     

The solubility of medium molecular weight aromatic hydrocarbons and the effects of hydrocarbon co-solutes and salinity

The solubilities of several medium molecular weight aromatic hydrocarbons were determined at 25°C in aqueous binary, ternary, and quaternary systems. The binary systems consisted of distilled water plus one each of the following: naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, biphenyl, 2-ethylnaphthalene, 1,5-dimethylnaphthalene, 2,3-dimethylnaphthalene, 2,6-dimethylnaphthalene, acenaphthene and phenanthrene. The ternary systems consisted of distilled water plus one of the following hydrocarbon pairs: naphthalene/biphenyl, naphthalene/acenaphthene, naphthalene/phenanthrene, biphenyl/phenanthrene, biphenyl/2-methylnaphthalene, acenaphthene/phenanthrene and 2-methylnaph-thalene/phenanthrene. Quaternary systems consisted of distilled water and one of the following hydrocarbon mixtures: naphthalene/biphenyl/phenanthrene, 2-methylnaphthalene/biphenyl/phenanthrene and naphthalene/acenaphthene/phenanthrene. In binary systems at S%. = 0, solubilities ranged from 31.3 ppm for naphthalene to 1.07 ppm for phenanthrene. The alkyl naphthalenes exhibited solubilities which were a function of molar volume and substituent size and position. In ternary and quaternary systems, solubilities often deviated from values obtained in binary systems presumably due to solute-solute interactions and/or formation of solid solutions. Aromatic hydrocarbons are ‘salted out’ by increasing concentrations of inorganic salts. At S%. = 35, the solubility of naphthalene is 22.0 ppm and of phenanthrene, 0.71 ppm. The empirical salting parameters are identical in both binary and quaternary systems.     

Early diagenesis of fatty acids and hydrocarbons in a salt marsh environment

In a salt marsh environment, plant-produced fatty acids and aliphatic hydrocarbons undergo significant modification upon being deposited in sediment. The major changes include alteration of the distribution pattern of straight chain components, increase in the concentration of branched components and decrease in the concentration of unsaturated components. Changes are similar in both oxidizing and reducing environments. Carbon isotope measurements indicate that there is little horizontal movement of sediment organics. Spartina alterniflora has a δC¹³ = ? 12.5% (vs NBS 20) and Juncus romerianus has a δC¹³ = ?23.2%. Sediment organic matter taken from cores up to 60 cm in depth retains a δC¹³ value similar to the plant species growing at the core site. Microbial metabolism appears to be responsible for the observed in situ changes in sediment lipids. The in situ changes did not result in significant alteration of δC¹³ values.