Spatial distribution, morphometry and activity of La Puebla de Alfindén sinkhole field in the Ebro river valley (NE Spain): applied aspects for hazard zonation

A highly active collapse sinkhole field in the evaporitic mantled karst of the Ebro river valley is studied (NE Spain). The subsidence is controlled by a NW-SE trending joint system and accelerated by the discharge of waste water from a nearby industrial state. The morphometry, spatial distribution and temporal evolution of the sinkholes have been analysed. The volume of the sinkholes yields a minimum estimate of average lowering of the surface by collapse subsidence of 46 cm. The clustering of the sinkholes and the tendency to form elongated uvalas and linear belts, in a NW–SE direction have a predictive utility and allow the establishment of criteria for a hazard zonation. With the precipitation record supplied by a pluviograph and periodic cartographic and photographic surveys the influence of heavy rainfall events on the triggering of collapses has been studied.     

Subsidence hazard avoidance based on geomorphological mapping in the Ebro River valley mantled evaporite karst terrain (NE Spain)

In the valley of the Ebro River to the southeast of the city of Zaragoza (NE Spain), the dissolution of evaporite sediments (gypsum, halite and Na-sulphates) which underlie alluvial deposits gives rise to numerous sinkholes. These sinkholes are a potential hazard to human safety, particularly where they develop in a catastrophic way. Even slow-developing sinkholes are problematic, as they damage urban and agricultural infrastructure, necessitating costly repairs and vigilant maintenance. To assist in developing avoidance strategies for these hazards, the factors controlling sinkhole occurrence have been assessed using geomorphological maps produced from aerial photographs for 1956 and 1981. Important controls on sinkhole development are found to include underlying geological structure (manifest in preferred orientations of sinkholes on the azimuths N130-150E and N30-40E), and the presence of glauberite in the groundwater flow path, which apparently promotes accelerated gypsum dissolution. Perhaps surprisingly, alluvium thickness does not appear to significantly correlate with the density of sinkholes on the floodplain in this area. The maps for 1956 and 1981 reveal that both human activity and natural processes can serve to obscure the true density of sinkhole development. For instance, a large number of sinkholes which were conspicuous in 1956 have since been back-filled by farmers. In the most fluvially active zone of the Ebro valley (the meander belt), the relatively low density of sinkholes compared with adjoining zones suggests that subsidence is being masked by morpho-sedimentary dynamic processes (aggradation and erosion). Careful geomorphological mapping for different time periods yields a much more accurate impression of the frequency of sinkhole development than would be gained from surveying currently visible sinkholes in the area of interest.     

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day, includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment, while extensional deformations are active in the outer swell of the trench.     

Sources and Transport of Urban and Biomass Burning Aerosol Black Carbon at the South–West Atlantic Coast

The total extent of the atmospheric impacts associated to the aerosol black carbon (BC) emissions from South America is not completed described. This work presents results of BC monitored during three scientific expeditions (2002, 2003 and 2004) on board of a Brazilian oceanographic vessel Ary Rongel that covered the South–West Atlantic coast between 22–62°S. This latitudinal band encloses major urban regions of South America and the outflow region of the SACZ (South Atlantic Convergent Zone), which is an important mechanism of advective transport of heat, moisture, minor gases and aerosols from the South America continental land to the Southern Atlantic Ocean. Our results showed that aerosol BC enhanced concentrations from urban/industrial origin can be transported to the South–West Atlantic Ocean due to the migration of sub-polar fronts that frequently reach tropical/subtropical regions. Despite the decrease of aerosol BC concentrations southwards (from ∼1,200 ng m⁻³ at latitude 22°S to ∼10 ng m⁻³ at latitude 62°S), several observed peak events were attributed to regional urban activities. Most of such events could be explained by the use of air mass back trajectories analysis. In addition, a global model simulation is presented (Goddard Institute for Space Studies – GISS GCM BC simulation) to explore the origins of aerosol BC in the South–West Atlantic. The model allowed isolating the biomass emissions from South America and Africa and industrial (non-biomass) pollution from other regions of the globe. This model suggests that the apportionment of about half of the aerosol BC at the South–West Atlantic may derive from South American biomass burning.     

Measurement of air temperature in the presence of a large radiant flux: an assessment of passively ventilated thermometer screens

The energy balance of small temperature sensors was modelled to illustrate the effects of sensor characteristics, particularly size, on the accuracy of readings in the presence of strong shortwave or longwave radiant loads. For all but extremely small sensors, radiant exchange may lead to unacceptable errors. The common practice of using passively ventilated instrument screens was evaluated in a series of comparative measurements. The differences resulting from the use of different models of shields may be an order of magnitude greater than the error resulting from sensor calibration. In the absence of technological innovation capable of reducing the error due to radiant exchange to negligible proportions, it is suggested that a standard methodology for calibrating and labelling the error resulting from the characteristics of the screens be adopted, to allow comparison of new data with long-established records.     

Stratigraphy and Mesozoic–Cenozoic tectonic history of northern Sierra Los Ajos and adjacent areas,Sonora, Mexico

Geologic mapping in the northern Sierra Los Ajos reveals new stratigraphic and structural data relevant to deciphering the Mesozoic–Cenozoic tectonic evolution of the range. The northern Sierra Los Ajos is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata, equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic–Paleozoic sequence is mantled by Upper Cretaceous rocks partly equivalent to the Fort Crittenden and Salero Formations in Arizona, and the Cabullona Group in Sonora, Mexico.Absence of the Upper Jurassic–Lower Cretaceous Bisbee Group below the Upper Cretaceous rocks and above the Proterozoic–Paleozoic rocks indicates that the Sierra Los Ajos was part of the Cananea high, a topographic highland during the Late Jurassic and Early Cretaceous. Deposition of Upper Cretaceous rocks directly on Paleozoic and Proterozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Laramide Cabullona basin during Late Cretaceous time. Basal beds of the Upper Cretaceous sequence are clast-supported conglomerate composed locally of basement (Paleozoic) clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Cabullona basin.The present-day Sierra Los Ajos reaches elevations of greater than 2600 m, and was uplifted during Tertiary basin-and-range extension. Upper Cretaceous rocks are exposed at higher elevations in the northern Sierra Los Ajos and represent an uplifted part of the inverted Cabullona basin. Tertiary uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north-to-northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the headwaters of the San Pedro River basin, an important bi-national water resource in the US-Mexico border region.     

How Do Surficial Lithic Assemblages Weather in Arid Environments? A Case Study from the Atacama Desert,Northern Chile

Archaeological sites composed only of surficial lithics are widespread in arid environments. Numerical dating of such sites is challenging, however, and even establishing a relative chronology can be daunting. One potentially helpful method for assigning relative chronologies is to use lithic weathering, on the assumption that the most weathered artifacts are also the oldest. Yet, few studies have systematically assessed how local environmental processes affect weathering of surficial lithics. Using macroscopic analyses, we compared the weathering of surficial lithic assemblages from seven mid‐to‐late Holocene archaeological sites sampled from four different microenvironments in the Atacama Desert of northern Chile. Changes in polish, texture, shine, and color were used to establish significant differences in weathering between two kinds of locations: interfluves and canyon sites. Lithics from interfluve sites were moderately to highly weathered by wind and possessed a dark coating, whereas canyon lithics were mildly weathered despite greater exposure to moisture, often lacked indications of eolian abrasion, and lacked dark coatings. Our results show that lithic weathering can be used as a proxy for relative age, but only after considering local environmental factors. The power of such chronologies can be improved by combining archaeological, paleoenvironmental, geomorphological, and taphonomic data.     

Furrows in the southern Scan Basin, Antarctica: interplay between tectonic and oceanographic influences

Multibeam echosounder data and TOPAS seismic reflection profiles collected during the AntPac 1997, Scan 2004, and Scan 2008 cruises aboard the RV Hespérides reveal a host of surficial geomorphological features as yet poorly investigated in the Scan Basin, south-central Scotia Sea. This area represents one of the deep gateways between the Weddell Sea and the Scotia Sea, since it enables the northward flow of a branch of the Weddell Sea Deep Water (WSDW). Analysis of the data identifies numerous elongated depressions interpreted as furrows in the southernmost sector of the basin. These furrows show two main trends, i.e., either N?CNNW parallel to, or NE oblique to regional bathymetric contours. These trends plausibly reflect a tectonic influence on the bottom-flow distribution, conditioned by a set of recent, conjugate strike-slip faults that developed on the seafloor under dominant NNE?CSSW compression and orthogonal extension. The furrows exhibit distinct geomorphological patterns at either side of the basin, which can be related to west?Ceast asymmetry in the WSDW flow direction. Consistent with existing knowledge of regional WSDW dynamics, northward WSDW overflows would be channeled along the western part of the basin at higher bottom-current velocities, thereby generating more erosional-type furrows that are straighter, more elongated, and have more abrupt sidewalls than their eastern counterparts. In contrast, weaker southward WSDW would flow along the eastern part of the basin, resulting in more depositional-type furrows that are more curved, less elongated, and have gentler sidewalls.     

Impact of pulsed Atlantic water inflow into the Alboran Basin at the time of the Zanclean flooding

The study of more than 500 single- and multichannel seismic records enabled the generation of a detailed palaeo-bathymetric map of the Messinian surface over most of the Alboran Basin, Western Mediterranean. This regional surface is characterized by several erosional features (channels, terraces and canyons) and topographic highs (structural, volcanic and diapiric in origin). The most prominent feature is the incised Zanclean Channel crossing the entire basin, its entrenchment having been associated with the opening of the Strait of Gibraltar and subsequent inflow of Atlantic waters. The incision depth of the channel is variable, suggesting local variations in the erosive capacity of the Atlantic inflow, conditioned mainly by the regional basin topography and the local presence of topographic highs. Adjacent to this channel along the Spanish and Moroccan margins, and near the Strait of Gibraltar, several submarine terraces developed at different depths suggest a pulsed flooding of the Alboran Basin. There could have been two major inflow phases of Atlantic water, one shortly before and another during the Zanclean flooding, the latter accompanied by periods of relative sea-level stillstands that enabled terrace development. Alternatively, these features were all generated during the main flooding evident and subsequent pulsed infilling of the basin.