Geomorphological characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia, Mars

We describe and interpret the surface terrain types associated with a widely-reported ∼4 km long, mid-latitude martian viscous flow feature (VFF). The feature is located in the southern hemisphere, on the poleward-facing rim of a ∼60 km-diameter crater in eastern Hellas Planitia. High Resolution Imaging Science Experiment (HiRISE) images, analysed in both 2D and 3D, reveal that the upper margin of the feature is bounded by steep (∼30°) headwalls, typically some tens of metres high, that are formed from unconsolidated material and characterised by a series of slope-parallel linear incisions. Below these incised headwalls, the feature flows at a general angle of ∼10° from a broad upper basin to a confined lower tongue that is bounded by a nested sequence of elongate raised ridges. These characteristics are typical of several VFFs in the region and are strikingly similar to moraine-bounded valley glaciers on Earth, and we sub-classify this feature as a ‘glacier-like form’ (GLF)¹. The GLF comprises five distinctive surface terrain types that contrast sharply with surface characteristics outside its bounding moraines. Four of these terrains (scaly terrain, polygonized terrain, linear terrain and mound-and-tail terrain) are located within the GLF’s innermost bounding moraine, while the fifth (rectilinear-ridge terrain) is located between its frontal moraines. These terrains are mapped, characterised and associated with possible mechanisms of formation to draw inferences about the GLF’s glaciology and glacial history. This analysis suggests that the GLF reached its maximal extent in the geologically-recent past, and that it may have been partially wet-based at that time. Subsequent to this phase, the GLF experienced an extended period of general recession that has been punctuated by several episodes of still-stand or advance. Currently, the GLF’s basin appears to be composed of a lower zone that is dominated by an exposed former glacier bed and an upper zone that may still contain a now-degraded and dust-mantled viscous mass, similar to many partially-glacierized basins on Earth.     

Methylation of Hg downstream from the Bonanza Hg mine,Oregon

Speciation of Hg and conversion to methyl-Hg were evaluated in stream sediment, stream water, and aquatic snails collected downstream from the Bonanza Hg mine, Oregon. Total production from the Bonanza mine was >1360 t of Hg, during mining from the late 1800s to 1960, ranking it as an intermediate sized Hg mine on an international scale. The primary objective of this study was to evaluate the distribution, transport, and methylation of Hg downstream from a Hg mine in a coastal temperate climatic zone. Data shown here for methyl-Hg, a neurotoxin hazardous to humans, are the first reported for sediment and water from this area. Stream sediment collected from Foster Creek flowing downstream from the Bonanza mine contained elevated Hg concentrations that ranged from 590 to 71,000 ng/g, all of which (except the most distal sample) exceeded the probable effect concentration (PEC) of 1060 ng/g, the Hg concentration above which harmful effects are likely to be observed in sediment-dwelling organisms. Concentrations of methyl-Hg in stream sediment collected from Foster Creek varied from 11 to 62 ng/g and were highly elevated compared to regional baseline concentrations (0.11–0.82 ng/g) established in this study. Methyl-Hg concentrations in stream sediment collected in this study showed a significant correlation with total organic C (TOC, R² = 0.62), generally indicating increased methyl-Hg formation with increasing TOC in sediment. Isotopic-tracer methods indicated that several samples of Foster Creek sediment exhibited high rates of Hg-methylation. Concentrations of Hg in water collected downstream from the mine varied from 17 to 270 ng/L and were also elevated compared to baselines, but all were below the 770 ng/L Hg standard recommended by the USEPA to protect against chronic effects to aquatic wildlife. Concentrations of methyl-Hg in the water collected from Foster Creek ranged from 0.17 to 1.8 ng/L, which were elevated compared to regional baseline sites upstream and downstream from the mine that varied from <0.02 to 0.22 ng/L. Aquatic snails collected downstream from the mine were elevated in Hg indicating significant bioavailability and uptake of Hg by these snails. Results for sediment and water indicated significant methyl-Hg formation in the ecosystem downstream from the Bonanza mine, which is enhanced by the temperate climate, high precipitation in the area, and high organic matter.     

The Trænadjupet Slide: a large slope failure affecting the continental margin of Norway 4,000 years ago

¹⁴C AMS-dated gravity cores reveal that the Trænadjupet Slide offshore Norway occurred about 4,000 ¹⁴C years B.P. (ca. 4,000 cal. years ago). From 4,000 to 3,000 years B.P., minor areas of the newly formed slide scar were probably eroded, the result of smaller episodes of mass wasting caused by delayed collapse of part of the western, upper sidewall or by bottom currents. From about 3,000 years B.P. to the present, sediments were derived from alongslope-flowing, north-eastward-oriented ocean currents carrying sediments in suspension. These results demonstrate that large-scale mass wasting during sea-level highstand is rather common on passive continental margins.     

Seasonal reorganization of subglacial drainage inferred from measurements in boreholes

The effect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of Haut Glacier d'Arolla, Switzerland, was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters. Electrical conductivity profiles were also measured within borehole water columns to identify the water sources driving water level changes, and to determine patterns of water circulation in boreholes. Prior to channel formation, boreholes showed idiosyncratic and poorly coordinated behaviour. Diurnal water level fluctuations were small and driven by supraglacial/englacial water inputs, even when boreholes were connected to a subglacial drainage system. This system appeared to consist of hydraulically impermeable patches interspersed with storage spaces, and transmitted a very low water flux. Drainage reorganization, which occurred around 31 July, 1993, in response to rapidly rising meltwater and rainfall inputs, seems to have involved the creation of a connection between an incipient channel and a well-established channelized system located further down-glacier. Once a major channel existed within the area of the borehole array, borehole water level fluctuations were forced by discharge-related changes in channel water pressure, although a diversity of responses was observed. These included (i) synchronous, (ii) damped and lagged, (iii) inverse, and (iv) alternating inverse/lagged responses. Synchronous responses occurred in boreholes connected directly to the channel, while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system. Pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed, producing inverse and alternating patterns of water level response. © 1998 John Wiley & Sons, Ltd.     

Flat-pebble conglomerate: its multiple origins and relationship to metre-scale depositional cycles

Carbonate flat‐pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat‐pebble conglomerate beds in shallow‐water cyclic and non‐cyclic strata. Several origins of flat‐pebble conglomerate are inferred for these strata. In one case, all stages of development of flat‐pebble conglomerate are captured within storm‐dominated shoreface deposits of hummocky cross‐stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat‐pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre‐scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm‐ to dm‐scale clasts. This transition was accompanied by the addition of sand‐sized echinoderm‐rich debris from local sources, which slightly lowered friction by reducing clast–clast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat‐pebble conglomerate bed occurs at the top of upward‐coarsening, metre‐scale cycles. The flat‐pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron‐rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat‐pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third type of flat‐pebble conglomerate bed occurs within unusual metre‐scale, shale‐dominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat‐pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds. These deeper water cycles can be interpreted as either upward‐shoaling or ‐deepening cycles. The flat‐pebble conglomerate beds record winnowing and reworking of shale and carbonate nodules to lags, during either lowstand or the first stages of transgression.     

Impact of a rock avalanche on a moraine‐dammed proglacial lake: Laguna Safuna Alta,Cordillera Blanca,Peru

Moraines that dam proglacial lakes pose an increasing hazard to communities in the Andes and other mountain ranges. The moraines are prone to failure through collapse, overtopping by lake waters or the effect of displacement waves resulting from ice and rock avalanches. Resulting floods have led to the loss of thousands of lives in the Cordillera Blanca mountains of Peru alone in the last 100 years. On 22 April 2002 a rock avalanche occurred immediately to the south‐west of Laguna Safuna Alta, in the Cordillera Blanca. The geomorphic evidence for the nature, magnitude and consequences of this event was investigated in August 2002. Field mapping indicated that the avalanche deposited 8–20 × 10⁶ m³ of rock into the lake and onto the surface of the frontal region of Glaciar Pucajirca, which flows into the lake. Repeated bathymetric surveying indicated that ～5 × 10⁶ m³ of this material was deposited directly into the lake. The immediate effect of this event was to create a displacement wave that gained in height as it travelled along the lake basin, overtopping the impounding moraine at the lake's northern end. To achieve overtopping, the maximum wave height must have been greater than 100 m. This, and subsequent seiche waves, caused extensive erosion of both the proximal and distal faces of the impounding terminal moraine. Further deep gullying of the distal face of this moraine resulted from the supply of pressurized water to the face via a relief overflow tunnel constructed in 1978. Two‐dimensional, steady‐state analysis of the stability of the post‐avalanche moraine rampart indicates that its proximal face remains susceptible to major large‐scale rotational failure. Copyright © 2005 John Wiley & Sons, Ltd.     

Biofilm Formation and Effectiveness of Biosand Filtration Systems with Typical and Innovative Filter Media

Geotechnical and Geological Engineering - Clean drinking water is a necessity and recycling stormwater and greywater has been more appealing in the recent decades to provide filtered water for...     

Diurnal fluctuations in glacier ice deformation: Haut Glacier d'Arolla,Switzerland

Measurements of surface velocity, ice deformation (at 42 and 89% ice depth) and proglacial stream discharge were made at Haut Glacier d'Arolla, Switzerland, to determine diurnal patterns of variation in each. Data are analysed in order to understand better the relationship between hydraulically induced basal motion and glacier ice deformation over short timescales. The data suggest that hydraulically induced localized basal ‘slippery’ spots are created over diurnal cycles, causing enhanced basal motion and spatially variable glacier speed‐up. Our data indicate that daily glacier speed‐up is associated with reduced internal deformation over areas previously identified as slippery spots and increased deformation in areas located adjacent to or down‐glacier from slippery spots. We interpret this pattern in terms of a transfer of mechanical support for basal shear stress away from slippery spots to adjacent sticky areas, where the resulting stronger ice–bed coupling causes increased ice deformation near the bed. These patterns indicate that basal ice is subjected to stress regimes that are variable at a high spatial and temporal resolution. Such variations may be central to the creation of anomalous vertical velocity profiles measured above and down‐glacier of basal slippery zones, which have shown evidence for ‘plug flow’ and extrusion flow over annual timescales. Copyright © 2007 John Wiley & Sons, Ltd.