Sediment oxygen profiles in a super-oxygenated antarctic lake

Perennially ice-covered lakes are found in the McMurdo Dry Valleys of southern Victoria Land, Antarctica. In contrast to temperate lakes that have diurnal photic periods, antarctic (and arctic) lakes have a yearly photic period. An unusual feature of the antarctic lakes is the occurrence of O2 at supersaturated levels in certain portions of the water column. Here we report the first sediment O2 profiles obtained using a microelectrode from a perennially ice-covered antarctic lake. Sediment cores collected in January and October 1987 from Lake Hoare in Taylor Valley show oxygenation down to 15, and in some cases, 25 cm. The oxygenation of sediments several centimeters below the sediment-water interface is atypical for lake sediments and may be characteristic of perennially ice-covered lakes. There is a significant difference between the observed January and October sediment O2 profiles. Several explanations may account for the difference, including seasonality. A time-dependent model is presented which tests the feasibility of a seasonal cycle resulting from the long photoperiod and benthic primary production in sediments overlain by a highly oxygenated water column.     

Evolution of tuff ring-dome complex: the case study of Cerro Pinto, eastern Trans-Mexican Volcanic Belt

Cerro Pinto is a Pleistocene rhyolite tuff ring-dome complex located in the eastern Trans-Mexican Volcanic Belt. The complex is composed of four tuff rings and four domes that were emplaced in three eruptive stages marked by changes in vent location and eruptive character. During Stage I, vent clearing produced a 1.5-km-diameter tuff ring that was then followed by emplacement of two domes of approximately 0.2 km³ each. With no apparent hiatus in activity, Stage II began with the explosive formation of a tuff ring ~2 km in diameter adjacent to and north of the earlier ring. Subsequent Stage II eruptions produced two smaller tuff rings within the northern tuff ring as well as a small dome that was mostly destroyed by explosions during its growth. Stage III involved the emplacement of a 0.04 km³ dome within the southern tuff ring. Cerro Pinto’s eruptive history includes sequences that follow simple rhyolite-dome models, in which a pyroclastic phase is followed immediately by effusive dome emplacement. Some aspects of the eruption, however, such as the explosive reactivation of the system and explosive dome destruction, are more complex. These events are commonly associated with polygenetic structures, such as stratovolcanoes or calderas, in which multiple pulses of magma initiate reactivation. A comparison of major and trace element geochemistry with nearby Pleistocene silicic centers does not show indication of any co-genetic relationship, suggesting that Cerro Pinto was produced by a small, isolated magma chamber. The compositional variation of the erupted material at Cerro Pinto is minimal, suggesting that there were not multiple pulses of magma responsible for the complex behavior of the volcano and that the volcanic system was formed in a short time period. The variety of eruptive style observed at Cerro Pinto reflects the influence of quickly exhaustible water sources on a short-lived eruption. The rising magma encountered small amounts of groundwater that initiated eruption phases. Once a critical magma:water ratio was exceeded, the eruptions became dry and sub-plinian to plinian. The primary characteristic of Cerro Pinto is the predominance of fall deposits, suggesting that the level at which rising magma encountered water was deep enough to allow substantial fragmentation after the water source was exhausted. Isolated rhyolite domes are rare and are not currently viewed as prominent volcanic hazards, but the evolution of Cerro Pinto demonstrates that individual domes may have complex cycles, and such complexity must be taken into account when making hazard risk assessments.     

Ambient Changes in Tracer Concentrations from a Multilevel Monitoring System in Basalt

Starting in 2008, a 4‐year tracer study was conducted to evaluate ambient changes in groundwater concentrations of a 1,3,6‐naphthalene trisulfonate tracer that was added to drill water. Samples were collected under open borehole conditions and after installing a multilevel groundwater monitoring system completed with 11 discrete monitoring zones within dense and fractured basalt and sediment layers in the eastern Snake River aquifer. The study was done in cooperation with the U.S. Department of Energy to test whether ambient fracture flow conditions were sufficient to remove the effects of injected drill water prior to sample collection. Results from thief samples indicated that the tracer was present in minor concentrations 28 days after coring, but was not present 6 months after coring or 7 days after reaming the borehole. Results from sampling the multilevel monitoring system indicated that small concentrations of the tracer remained in 5 of 10 zones during some period after installation. All concentrations were several orders of magnitude lower than the initial concentrations in the drill water. The ports that had remnant concentrations of the tracer were either located near sediment layers or were located in dense basalt, which suggests limited groundwater flow near these ports. The ports completed in well‐fractured and vesicular basalt had no detectable concentrations.     

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the M_s = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 N, 22 ° 12.0 E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment M_o = 3.4 10¹⁸ N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake – 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.     

Soluble iron sulfide species in natural waters: Reappraisal of their stoichiometry and stability constants

A wide range of stoichiometries has been previously proposed for soluble iron sulfide species and there is no general agreement on their importance in natural waters. The solubility of Fe(II) in 0.1 M NaClO₄ equilibrated at 20 - 0.1°C with various partial pressures of H₂S (0.1, 0.001, 0.0001, 0.00001 MPa) was measured in the pH range 3.1-7.9. Equilibrium was established within 1-6 h when amorphous FeS was the solid phase. The results could all be fitted using values for the solubility product constant (I = 0) of p*K_s = 3.00 - 0.12 and of the stability constant for a soluble Fe(HS) ₂ species (I = 0) of p#₂ = -6.45 - 0.12 where *K_s = aFe²⁺ · aHS-/aH⁺ and #₂ = aFe(HS) ₂/aFe²⁺ · (aHS^-)². Any soluble species of the form Fe_x (HS) _2x where x = &gif1; would fit the data equally well. Measurements at different partial pressures are inconsistent with labile species of the form Fe_xS_x. There was no evidence for a Fe(HS) ⁺ species. When a solution is saturated with respect to amorphous FeS, Fe(HS) ₂ will only be a significant proportion of Fe(II) when S(-II) is higher than 0.2mmoll^-1. The constants for Fe(HS) ₂ or Fe_x (HS) _2x (x S 2) are consistent with all freshwater data where constant values of measured ion activity products provide no evidence for soluble complex formation. For marine waters with high sulfide concentrations (S 6mmol l^-1), measured concentrations of Fe(II) are consistent with there being negligible soluble iron sulfide. The data are better fitted if the dissolved species are polymeric as predicted concentrations of the monomer Fe(HS) ₂ are significant. These findings suggest that rather than the dissolved species being Fe(HS) ₂, it is probably polymeric, that is Fe_x (HS) _2x (x S 2).     

The restricted buffer capacity of a south Baltic estuary — The oder estuary

In the river Oder high-waters of the river with high nutrient loads and low biological activity mainly occur during winter. Thus, a remarkable portion of the annual load passes the estuary untransformed. During summer high level of biological activity is observed in the whole estuary, but while more than 10 mol/l dissolved inorganic nitrogen (DIN) is found in the Großes Haff, where the theoretical water-residence time is only 1 month, in the western part of the estuary, where the water-residence time is approximately 3 months, all DIN is transformed into organic matter.The transformed nutrients settle partly as biological products, but they are released again in these shallow waters due to biological and physical degradation, and transported into the Baltic Sea.This behaviour may explain why in the sediments of the Oderhaff with a sedimentation rate of 1 mm/year only 1% of the annual nutrient load of the Oder river can be found.     

The BEAM-project: prediction and assessment of mixture toxicities in the aquatic environment

Freshwater and marine ecosystems are exposed to various multi-component mixtures of pollutants. Nevertheless, most ecotoxicological research and chemicals regulation focus on hazard and exposure assessment of individual substances only, the problem of chemical mixtures in the environment is ignored to a large extent. In contrast, the assessment of combination effects has a long tradition in pharmacology, where mixtures of chemicals are specifically designed to develop new products, e.g. human and veterinary drugs or agricultural and non-agricultural pesticides. In this area, two concepts are frequently used and are thought to describe fundamental relationships between single substance and mixture effects: Independent Action (Response Addition) and Concentration Addition. The question, to what extent these concepts may also be applied in an ecotoxicological and regulatory context may be considered a research topic of major importance, as the concepts would allow to make use of already existing single substance toxicity data for the predictive assessment of mixture toxicities. Two critical knowledge gaps are identified: (a) There is a lack of environmental realism, as a huge part of our current knowledge about the applicability of the concepts is restricted to artificial situations with respect to mixture composition or biological effect assessment. (b) The knowledge on what exactly is needed for using the concepts as tools for the predictive mixture toxicity assessment is insufficient. Both gaps seriously hamper the necessary, scientifically sound consideration of mixture toxicities in a regulatory context.In this paper, the two concepts will be briefly introduced, the necessity of considering the toxicities of chemical mixtures in the environment will be demonstrated and the applicability of Independent Action and Concentration Addition as tools for the prediction and assessment of mixture toxicities will be discussed. An overview of the specific aims and approaches of the BEAM project to fill in the identified knowledge gaps is given and first results are outlined.     

Testing a statistical curve‐fitting procedure for quantifying sediment populations within multi‐modal particle‐size distributions

This paper describes a series of tests designed to evaluate the capacity of a personal computer (PC) based statistical curve‐fitting program called MIX to quantify composite populations within multi‐modal particle‐size distributions. Three natural soil samples were analysed by a Coulter Multisizer, and their particle‐size distributions analysed using MIX software to identify the modes, standard deviations and proportions of their composite populations. The particle‐size distributions of the three natural soil samples were then numerically combined in equal proportions using a spreadsheet program to create synthetic particle‐size distributions of known populations. MIX was then tested on the synthetic particle‐size distributions to see if the modes and proportions it identified were similar to those modes and proportions known to characterize the synthetic particle‐size distributions. The main outcome is that MIX can very accurately describe the modal particle size and proportions of the major composite populations within a particle‐size distribution. However MIX has difficulty in identifying small populations (those contributing <10 per cent of a total particle‐size distribution), particularly when they are located in the central sections of particle‐size distributions, overlain by larger populations, or when positioned in the fine tails of distributions. Despite these minor shortcomings, MIX is a valuable tool for the examination and interpretation of particle‐size data. Copyright © 2005 John Wiley & Sons, Ltd.