Neogene fjordal sedimentation on the western margin of the Lambert Graben, East Antarctica

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo‐Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet‐based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice‐transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice‐rafted clasts. The laminites represent less proximal, mainly ice‐free fjordal sediments, resulting either from tidal‐current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast‐flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts.     

U–Pb Geochronology of VMS mineralization in the Iberian Pyrite Belt

A geochronology study using U-Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76ǂ.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded ²⁰⁷Pb/²⁰⁶Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26ǂ.81 Ma, slightly younger and outside of the 2C error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with ²⁰⁷Pb/²⁰⁶Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U-Pb zircon age of 352.9ǃ.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with ²⁰⁷Pb/²⁰⁶Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21ǂ.73 and 356.4ǂ.8 Ma, for footwall and hanging wall samples, respectively. The hanging wall sample has two zircon analyses with ²⁰⁷Pb/²⁰⁶Pb ages of 464 and 466 Ma, indicating inheritance from an Ordovician or older source. The age for an altered dacite tuff sample from the hanging wall of the Las Cruces deposit is 353.97ǂ.69 Ma. One zircon analysis from the Las Cruces sample has a ²⁰⁷Pb/²⁰⁶Pb age of 1048 Ma, indicating inheritance from a Neoproterozoic source. These U-Pb ages refine the IPB geochronology provided by previous studies, and they suggest that either volcanism progressed toward the center of the IPB, or that volcanism was broadly static and the strata were progressively rifted to the margins during transtensional basin formation. The zircon inheritance provides direct evidence for Proterozoic to Ordovician sources, reflecting either basement rocks beneath the Phyllite-Quartzite Group during VMS formation in late Tournaisian times, or a Proterozoic to Ordovician detrital component in Phyllite-Quartzite Group source rocks. The presence of an older crustal component is consistent with VMS formation during rift development at a continental margin.     

Textural variation in the pyrite-rich ore deposits of the Røros district, Trondheim Region, Norway: implications for pyrite deformation mechanisms

The Røros district is a pyrite-rich polymetallic sulfide orefield in the southeastern part of the Trondheim region, Central Norwegian Caledonides. All of the ore deposits at Røros are hosted within a Cambrian to Silurian succession that was deformed and metamorphosed at lower greenschist to lower amphibolite facies conditions during the Caledonian orogeny. Samples from five individual deposits across the orefield have been analyzed using a combination of reflected light petrographic observation, orientation contrast imaging, and electron backscatter diffraction. Results indicate that, whereas samples from each ore deposit have a variety of different textures, all of them preserve plastic deformation in pyrite grains that occurred at peak metamorphic conditions characterized by the development of internal lattice misorientation within pyrite grains and low-angle (～2°) dislocation walls. These observations indicate that the principal deformation mechanisms at peak metamorphic conditions were dislocation glide and creep. The preservation of brittle fracturing represents later overprinting events.     

Atmospheric and Oceanic Variability Associated with Growing Season Droughts and Pluvials on the Canadian Prairies

Abstract

The importance of measurements of the vertical distribution of odd nitrogen in studies of ozone chemistry and climate change has long being recognized. In this paper, we use the optimal estimation method developed by Rodgers (1976, 1990) to retrieve NO₂ vertical profiles from slant column observations made with a portable ultraviolet (UV)‐visible zenith‐sky spectrometer operated on the ground during the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) balloon campaign carried out at Vanscoy, Saskatchewan, Canada (52°N, 107°W), from 18 to 25 August 1998. Late summer was chosen for the campaign because the stratospheric zonal wind velocity changes sign at that time. Under such conditions the stratospheric winds are at a minimum, leaving the stratosphere in a dynamically quiescent state and closer to photochemical control (Fahey et al., 2001; Fioletov and Shepherd, 2003). The NO₂ profile retrieved from the ground‐based observations is compared with the co‐located and simultaneous NO₂ profile measured by a balloon‐borne UV‐visible spectrometer during sunrise on 24 August. Good agreement is observed, giving us confidence in the retrieval technique adopted. The retrieved NO₂ profiles are also compared with the output of the Model for Evaluating oZONe trends (MEZON) 3D stratospheric chemical transport model. It is observed that, for altitudes below the peak concentration, the model underestimates the NO₂ amount, and at the altitude of peak concentration, the model values lie between the values measured from the balloon and those retrieved from the ground‐based measurements. Nevertheless, the model reproduces the general shape of the retrieved profiles, including the altitude of the NO₂ maximum, for both sunrise and sunset conditions.     

Large-scale sedimentary bedforms and sediment dynamics on a glaciated tectonic continental shelf: Examples from the Pacific margin of Canada

The Pacific margin of Canada has been subjected to tectonism, dramatic sea level change and vigorous storm and tidal energy since glacial times resulting in a complex seafloor. Extensive multibeam mapping of this shelf has provided an opportunity to understand how these processes have impacted sedimentology and morphology. Bathymetric restriction of the tidally dominated flow between the inland seas and the open Pacific has resulted in the development of very large subaqueous dune fields and terrace moats. For example, in the southern Strait of Georgia nearly symmetrical dunes with wavelengths between 100 and 300 m, dune heights up to 28 m, cover the seafloor in 170–210 m water depth. In northern Hecate Strait a 72 km² area of large 2D dunes occurs at the transition with Dixon Entrance which opens to the Pacific Ocean and steep (>10°) wave-cut terraces and drowned spits, a result of sea level changes during the Holocene, are now being undercut to generate moats 7 m deep, in a narrowing shelf trough. Currents, with velocities ranging between 0.2 and 2.2 m s^?1, are dominated by semi-diurnal tidal streams that are continually modified by wind and estuarine circulation. There appears to be a clear association of grain size, water depth and flow velocity controlling the size of the subaqueous dunes.     

An experimental investigation of Arctic haze at Alert,N.W.T., March 1985

Abstract

Arctic haze has been attributed to industrial pollution released at mid‐latitudes. Our current understanding has been pieced together from routine meteorological data, ground‐based air chemistry observations and limited aircraft measurements. This study investigates the relationship between synoptic boundary‐layer meteorology and the composition of the near‐surface atmosphere during the polar sunrise at Alert, N.W.T. A secondary objective is to characterize the influence of local activity on the atmospheric composition at a site for a new baseline monitoring station and at a location where aerosol chemistry and grab‐flask samples for CO₂ have been made for many years. Detailed measurements of the vertical distribution of aerosols were obtained from an upward‐looking lidar to complement the ground‐based measurements. Meteorological profiles of the near‐surface boundary layer were obtained from both free‐flying and tethered balloons. Near‐surface measurements were made of aerosol physical and chemical properties, O₃, NO₂, NO/NO_x, Peroxyacetylnitrate (PAN) and hydrocarbons.

The study period was characterized by prolonged periods with strong surface inversions, which were broken up occasionally by intrusions of cold air into the warmer air aloft. Lidar observations showed that ice crystals often accompanied aerosols and were responsible for reducing visibility below 30–40 km. There was a strong correlation between aerosol mass in the diameter size range 0.15 to 1.5 μm and total SO₄ ⁼ . PAN found at concentrations of about 200 ppt(v) was the main carrier of atmospheric nitrogen. Aerosol trace elements were divisible into anthropogenic soil, mixed soil/anthropogenic sea salt and halogens. Vertical transport in the surface boundary layer, as regulated by the strength of the surface radiation inversion, may play an important role in influencing the chemical composition of the air at the ground. The location of the new baseline monitoring laboratory was found to be generally windier and warmer than the lower altitude weather station, and the influence of local activity was found to be minimal.