An autochthonous geological model for the eastern Andes of Ecuador

We describe a traverse across the Cordillera Real and sub-Andean Zone of Ecuador, poorly known areas with very little detailed mapping and very little age control. The spine of the Cordillera comprises deeply eroded Triassic and Jurassic plutons, the roots of a major arc, emplaced into probable Palaeozoic pelites and metamorphosed volcanic rocks. The W flank comprises a Jurassic (?) submarine basaltic–andesitic volcanic sequence, which grades up into mixed Jurassic/Cretaceous volcanic and sedimentary rocks of the Inter-Andean Valley. The sub-Andean Zone, on the E flank of the Cordillera, comprises a newly recognized Cretaceous basin of cleaved mudrocks, quartz arenites and limestones. East of the syndepositional Cosanga Fault, the Cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon-bearing Oriente Basin. The principal penetrative deformation of the Cordillera Real was probably latest Cretaceous/Palaeocene. It telescoped the magmatic belts, but shortening was largely partitioned into the pelites between plutons. The plutons suffered inhomogenous deformation; some portions completely escaped tectonism. The pelites conserve two foliations. The earliest comprises slaty cleavage formed under low- or sub-greenschist conditions. The later is a strong schistosity defined by new mica growth. It largely transposed and obliterated the first. Both foliations may have developed during a single progressive deformation. We find inappropriate recent terrane models for the Cordillera Real and sub-Andean Zone of Ecuador. Instead we find remarkable similarities from one side of the Cordillera to the other, including a common structural history. In place of sutures, we find mostly intrusive contacts between major plutons and pelites. Triassic to Cretaceous events occurred on the autochthonous western edge of the Archaean Guyana Shield. The latest Cretaceous–Paleocene deformation is interpreted as the progressive collision of an oceanic terrane(s) with the South American continent. Young fault movements have subsequently juxtaposed different structural levels through the Cordillera Real orogen.     

Field investigation into the biodegradation of TCE and BTEX at a former metal plating works

The paper is based on a recent programme of groundwater monitoring at an industrial site in west London. Redevelopment of the site in 1997 revealed high levels of soil and groundwater pollution by hydrocarbon fuels, trichloroethylene (TCE) and soluble metal salts (e.g. free cyanide, chromium VI and nickel). The pollution originated from a previous metal plating and galvanising works at the site. As part of the redevelopment works, the owners undertook limited excavation works and groundwater extraction to remove the pollutant. However, groundwater sampling has continued to show high levels of pollution. Following discussion with the environment regulator in late 1998, a groundwater monitoring programme was agreed to investigate the potential for co-degradation of the petroleum fuel and TCE. Groundwater samples have been taken from six existing boreholes (1C to 6C). The location of the monitoring boreholes relates to past pollution spillages and the layout of the new factory building. Chemical analyses of groundwater samples show elevated aqueous concentrations of chloroethenes with a classical reduction pathway for trichloroethylene (TCE) leading to an accumulation of vinyl chloride.     

B[a]P–DNA binding in early life-stages of Atlantic tomcod: population differences and chromium modulation

Atlantic tomcod (Microgadus tomcod) from the Hudson River (HR) are resistant at the molecular and organismic levels to the effects of exposure to dioxin-like aromatic hydrocarbon (AH) compounds, but much less so to benzo[a]pyrene (B[a]P). The aims of this study were to determine in early life-stages of tomcod exposed to B[a]P: (1) if DNA binding levels differed between fish from the HR and Miramichi River (MR), and (2) if co-exposure to chromium could modulate this genotoxic effect. After exposure to [³H]B[a]P alone, DNA-bound radioactivity was 5–10-fold higher in embryos and larvae of MR than HR descent. Co-exposure to chromium modulated DNA binding levels in offspring of both populations. In MR embryos, co-exposure to chromium inhibited B[a]P uptake. These results demonstrated resistance to the genotoxic effects of B[a]P in early life stages of HR tomcod at an ecologically important endpoint and suggest the ability of chromium to modulate AH-induced genotoxicity.     

Reduktive Dehalogenierung von Chlorkohlenwasserstoffen während der anaeroben Stabilisierung von Hausmüll

Reductive Dehalogenation of Chlorinated Hydrocarbons during Anaerobic Stabilization of Municipal Wastes During sequential anaerobic digestion of municipal wastes, distinct biogeochemical phases exist which show different capabilities to transform halogenated hydrocarbons. Chlorophenols, tetrachloroethylene, and chloroanilines codisposed together with organic-rich waste substrates are reductively dehalogenated during methanogenic conditions. Lindane is degraded during acidogenesis as well as during methanogenesis. However, degradation in methanogenic leachates is faster by a factor of 10. The poor transformation potential during acidogenesis compared to subsequent transient methanogenic and stabile methanogenic phases cannot be explained by inadequate acclimation of prevailing microorganisms to the codisposed organochlorines. Thus, observed transformation capabilities are a pertinent feature of methanogenic leachates, probably due to prevailing low redox potential and/or presence of suitable microbial activities (not necessarily methanogenis). Dehalogenation of 2,3,4,6-tetrachlorophenol as a model compound is hampered in methanogenic leachate by addition of a surplus of sulfate and is completely suppressed by addition of molybdate which selectively inhibits sulfate reducing microorganisms. Competition for common electron donators (e.g. H₂) is discussed as an explanation of these results. The results point to sulfate reducing microorganisms being involved in reductive dehalogenation of chlorophenols.     

Model analysis of the measured concentration of organic gases in the Norwegian Arctic

A 2-D meridional model for the chemistry and transport in the troposphere is used to study the seasonal variation of the concentration of organic gases like C₂H₂, C₂H₆, C₃H₈, C₆H₆, C₇H₈. CHCl₃ and C₂Cl₄ at high latitudes. The anthropogenic sources for these species were estimated, and the temporal and latitudinal distribution of OH and O₃ was calculated using a complex photochemical reaction system. There is fair agreement between the calculated annual variation and the measured concentrations for C₂H₂, C₂H₆, C₃H₈, C₇H₈ and C₂Cl₄ at Spitsbergen during July 1982 and March/April 1983, with a distinct late winter maximum and summer minimum. For CHCl₃, the direct anthropogenic source is minor compared to indirect anthropogenic or natural sources. For benzene, emission in car exhaust is important, but other anthropogenic sources are required for the calculations to agree with the measurements. Measured C₂H₄ and C₃H₆ concentrations are much higher than the calculated ones based on anthropogenic emissions, and show opposite seasonal trends. This indicates biogenic sources for these compounds.A buildup of PAN (300 pptv) is calculated at high latitudes during winter. This makes it the dominant source for NO_x as the temperature increases in the spring. NO_x is found to be a limiting factor for O₃ production at high latitudes during spring.     

Concave-bank benches and associated floodplain formation

This paper describes the morphology, sequential development and general sedimentology of concave-bank benches on the Murrumbidgee River of southeastern Australia, and also notes their important role in floodplain formation on certain meandering rivers in western Canada. Benches form against the concave bank (cut-bank) of abruptly curving bends immediately upstream of the point of maximum curvature. As a result of flow deflection against the upstream limb of the convex bank, the channel widens here and produces a zone of expanded flow facilitating flow separation near the upstream limb of the concave bank. Sedimentation within this zone starts with a longitudinal-shaped bar of medium sand forming a platform isolated even at low flow by a narrow secondary channel against the concave bank. Aggradation of the longitudinal-shaped bar with fine sand, mud and organic matter permits the establishment of trees. Further sedimentation, particularly around the young trees, results in the formation of a fully developed bench isolated by the secondary channel from the remainder of the floodplain only during high flows. Observations on confined meandering rivers in western Canada provide evidence of substantial floodplain formation by concave-bank bench accretion, a process distinctly different in character to the more familiar mechanism of lateral point-bar accretion. Furthermore, the preservation of abundant organic debris means that extensive bench deposits may be a source of locally useful natural gas from within floodplain sediments.     

Hydrocarbons in the atmosphere

Trace concentrations of highly reactive hydrocarbons of biogenic origin have been proposed for some time as being important in aerosol formation processes in the atmosphere. More recently, assessments of potential photochemical reactions in the troposphere have proposed a role in the atmospheric ozone cycle for hydrocarbons, even for compounds such as methane that had previously been considered nonreactive. An assessment of the atmospheric hydrocarbon reaction system has been limited by a lack of observational information on the nature of conditions in the remote or non-urban atmosphere. Recent data on terpene concentrations and other biogenic hydrocarbon compounds are presented. Data on ethane and acetylene from aircraft samples taken over the north and south Pacific Ocean show concentrations in the 0.5 to 1 /m³ range for ethane and in the 0.05 to 0.3 g/m³ range for acetylene. A concentration gradient is present for these compounds between the northern and southern hemisphere. A rudimentary global concentration pattern for these C₂ compounds has been developed on the basis of recent data.     

The distribution of light nonmethane hydrocarbons over the mid-Atlantic: Results of the Polarstern cruise ANT VII/1

During the cruise ANT VII/1 (September/October 1988) of the German research vessel Polarstern the latitudinal distributions of several nonmethane hydrocarbons were measured over the Atlantic between 45°N and 30°S by in-situ gas chromatography.On the average, the highest mixing ratios of ethane, propane, i- and n-butane, ethene and acetylene were observed in the Northern Hemisphere around 40° N and just north of the intertropical convergence zone, respectively. South of the equator, a bulge in the mixing ratios of ethane and acetylene was observed indicating aged biomass burning emissions. This observation coincided with enhanced tropospheric ozone found in this region at this season. On the average ethane and acetylene mixing ratios were around 500 and 100 ppt, respectively, whereas the levels of the other NMHC were in the range of some ppt up to 100 ppt.compared with the results of the cruise ANT V/5 (March/April, 1987), the ethane mixing ratios in September/October proved to be a factor of 3 lower in the Northern Hemisphere and a factor of 2 higher in the Southern Hemisphere, probably due to seasonal effects. Possible causes are the higher OH radical concentrations in summer, which result in a faster removal of ethane or stronger emission from biomass burning which also peaks in the dry season.The relative pattern of the hydrocarbons just north of the ITCZ was very similar for both measurement series. In this region, the NMHC were advected by long-range transport from the continent, whereas generally the ocean itself acts as a major NMHC source. This is supported by the results of a balance calculation between oceanic emissions and atmospheric removal rates.     

Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere

A one-dimensional photochemical model was used to explore the role of chlorine atoms in oxidizing methane and other nonmethane hydrocarbons (NMHCs) in the marine troposphere and lower stratosphere. Where appropriate, the model predictions were compared with available measurements. Cl atoms are predicted to be present in the marine troposphere at concentrations of approximately 10³ cm^-3, mostly as a consequence of the reaction of OH with HCl released from sea spray. Despite this low abundance, our results indicate that 20 to 40% of NMHC oxidation in the troposphere (0–10 km) and 40 to 90% of NMHC oxidation in the lower stratosphere (10–20 km) is caused by Cl atoms. At 15 km, NMHC-Cl reactions account for nearly 80% of the PAN produced.The model was also used to test the longstanding hypothesis that NOCl is an intermediate to HCl formation from sea salt aerosols. It was found that the NOCl concentration required (10 ppt) would be incompatible with field observations of reactive nitrogen and ozone abundance. Chlorine nitrate (ClONO₂) and methyl nitrate (CH₃ONO₂) were shown to be minor components of the total NO _y abundance. Heterogeneous reactions that might enhance photolysis of halocarbons or convert ClONO₂ to HOCl or Cl₂ were determined to be relatively unimportant sources of Cl atoms. Specific and reliable measurements of HCl and other reactive chlorine species are needed to better assess their role in tropospheric chemistry.     

Do mature hydrocarbons have an influence on acid rock drainage generation?

The generation of acid rock drainage (ARD) is a biogeochemical process that causes severe ecological impacts, threatening human health worldwide. Microbes involved in acid drainage reactions are generally considered autotrophic but heterotrophic and mixotrophic microorganisms have often been identified at ARD sites. This raises questions about the role of organic matter naturally present at these sites, such as mature hydrocarbons, in promoting the microbial processes underpinning ARD generation. To investigate this, aerobic microcosm experiments were carried out using ARD samples collected at a well-characterised site in northern England (Mam Tor, Derbyshire). Organic analyses indicated the presence of substantial amounts of mature, petroleum-derived hydrocarbons and microbial analyses indicated that the sediment hosts acidophilic bacteria with the capability of degrading petroleum-derived compounds. However, the aerobic microcosm experiments indicated that these petroleum-derived hydrocarbons were not used by the bacterial community and, therefore, are not involved in the reactions that ultimately lead to ARD generation. These observations support a primary role for autotrophs in ARD generation.