On the formation of metal-rich deposits at ridge crests

Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO₂ crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).     

Stratigraphy,paleocurrents and stromatolites of Hadrynian (Upper Precambrian) rocks of Victoria Island,Arctic Archipelago,Canada

The Hadrynian (Upper Precambrian) rocks of Victoria Island in the western part of the Canadian Arctic Archipelago comprise a sedimentary sequence some 12,000 ft. (4,000 m) thick, capped by a 1,000 ft. (330 m)-thick basic volcanic and pyroclastic unit. These rocks outcrop in an elongate, topographic high called the Minto Arch, and in smaller inliers in Lower Paleozoic rocks in the south part of the island (Duke of York and Wellington highs).Stratigraphic and sedimentological investigation of these rocks in the southern part of the Wellington high near Cambridge Bay suggest that they were deposited by rivers flowing from a land mass that lay to the east. Clasts in the Hadrynian sedimentary rocks attest to the existence of at least two earlier periods of sedimentation and one phase of tectonic compression in the basement rocks.Study of the lower part of the Hadrynian succession (Glenelg and Reynolds Point Formations) in the northeastern part of the Minto Arch reveals a general thickening to the west. Rock types present include dolomite, limestone, siltstone, sandstone and shale deposited in a dominantly shallow marine environment together with some marine deltaic units and possibly distal fluvial deposits. Cross bedding and ripple-mark orientations suggest a paleoslope to the northwest. However, the pattern of cross-bedding distribution is complex with modes not only in the northwest quadrant, but also in the northeast and southwest quadrants, the latter possibly reflecting longshore currents in a generally shallow marine environment.Stromatolites are abundant in both formations studied. They are almost exclusively laterally linked shallow-dipping domes in the cherty dolomites of the Glenelg Formation. However the capping stromatolitic dolomite of this formation is composed of columnar stromatolites, as also are the abundant stromatolitic banks and reefs of the overlying Reynolds Point Formation. Elongate domes in the lower part of the Glenelg Formation have a preferred orientation in a northwest-southeast direction, perhaps reflecting tidal currents. The topmost stromatolitic unit of the Glenelg Formation in more easterly areas has large elongate mounds on its upper surface. Elongation of these mounds has a preferred orientation in a northeast-southwest direction. Possibly this orientation is related to longshore currents flowing along the depositional strike.The Hadrynian sediments of Victoria Island and areas to the south and west may have been deposited in a shallow embayment of the Upper Precambrian Sea, open to the northwest and in continuity with a wedge of sediment that thickened westward and extended along the length of the North American continent. There is some evidence to support the existence of three such embayments in the southern part of the Canadian Arctic Archipelago, separated and defined by northerly salients of older rocks of the Canadian Shield. These salients reflect an east-west tectonic stress on a very large scale.     

Assimilation of Paleoseismic Data for Earthquake Simulation

— Simulation of the complete earthquake generation process requires assimilation of observational data on long-term behavior of faults over multiple rupture cycles. Paleoseismology, the study of pre-instrumental earthquakes from geological field investigations, provides the only observations of long-term fault behavior. Paleoseismic data present challenges for assimilation into models because data sets are small, sparse, analog, and contain qualitative uncertainties. Observations can be categorized as primary measurements from field data, or non-primary extrapolations or interpretations of primary data. Assimilation of non-primary data could yield biased simulation results. We present a primary data set for the San Andreas and Imperial faults for comparison with non-primary data in existing databases, and propose standard formats and structure for paleoseismic data assimilation into numerical simulation models .     

The Misis–Andırın Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis–Andırın Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), Andırın (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments.The Misis–Andırın Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time.Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE–SW linking segment (Andırın) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid–Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis–Andırın Complex.     

Observed thermal pollution and post-development simulations of low-temperature geothermal systems in Winnipeg, Canada

Two issues arise in the long-term use of groundwater for thermal purposes: (1) the sustainability of an individual system; and (2) the effect of neighbouring systems on each other. Both of these effects are observed in an area of the Carbonate Rock Aquifer beneath Winnipeg in Manitoba, Canada, where groundwater has been exploited in thermal applications since 1965. In this area, there are four systems that utilize groundwater for cooling purposes that are closely spaced. The current temperatures observed in this area of the Carbonate Rock Aquifer and the results of the numerical modeling conducted in this study confirm that in each system, temperatures at the production well have risen as a result of breakthrough of injected water. The results of numerical modeling also indicate that interference effects are present in three of the four systems examined in this study. The influence of these systems on each other implies that these systems have a spacing that is smaller than the optimum spacing for such systems, and indicates that there is a limit to the density of development that can occur in a given aquifer.Contaminación termal y simulaciones post-desarrollo de sistemas geotermales de baja temperatura en Winnipeg, Canadá     

On a Non-local Parameterisation for Shear Turbulence and the Uniqueness of its Solutions

A non-local parameterisation of shear turbulence is proposed, which includes a dimensionless multiplicative constant as the sole tunable parameter. Analytical and numerical solutions in the case of plane Couette flow exhibit sheared velocity profiles with logarithmic behaviour near the boundaries, and the classical logarithmic flow profile is reproduced for a semi-infinite domain. We also prove that the families of analytical solutions obtained are locally unique: if the velocity is a strictly-increasing function of the distance from the boundary, a small perturbation of the velocity profile must be of the same functional form as the basic flow.     

The Effect of Orography on Evaporation

The effect of orography on the spatial variability of surface evaporation is studied numerically for two hill heights and two prescribed forms of surface resistance. A numerical model of the planetary boundary layer carrying scalars of temperature and specific humidity, and with a surface energy balance scheme, is employed. It is found that the difference in mean evaporation between model runs with hills and analogous runs for flat terrain may be explained primarily by an increase in surface area and adiabatic cooling at the hill surface.     

Firmgrounds – key surfaces in the recognition of parasequences in the Aptian Lower Greensand Group, Isle of Wight (southern England)

Aptian Lower Greensand Group exposures in the cliffs of the Isle of Wight (southern England) display a consistent coarsening-up cyclicity on the scale of centimetres to tens of metres that reflects the bed, bed-set, parasequence, parasequence set and sequence hierarchy. These coarsening-up cycles are most commonly recognized at the scale of parasequences (20 cm to 10 m thick), genetically related groups of which form parasequence sets. Both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development. Numerous episodes of erosion, deposition and colonization are recorded, reflecting multiple erosion/bypass events. The increase in mean grain-size through each cycle is reflected by changes in physical sedimentary structures; ichnofauna or bioturbational fabric; fossil fauna and diagenesis. Interbedded mudstones, siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low-angle, hummocky, or tabular cross-strata, sandstone-filled erosional gutters and planar lamination. The cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble/plant debris-filled scours preserved. Bioturbational fabrics change upward through each cycle from small, subhorizontal, mud- or sandstone-filled burrows to large, branching, clay-filled or cemented burrow systems. The top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions. Concentrations of bivalves, brachiopods, bryozoa, crinoids and corals are preferentially cemented by iron oxide, carbonate or phosphate. Such cements were early and thus utilized by firm or hard substrate dwellers. This fossiliferous, cemented sandstone is overlain by a flooding surface marked by the mudstone and silt-rich sandstones at the base of the next cycle. Together, the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions, culminating in the diverse firmground fauna preserved at the top of each cycle. The fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding. High species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces. Increasing faunal maturity allows recognition of a hierarchy of hiatuses. This hierarchy is analogous to the parasequence–parasequence set division. The stratigraphic condensation of firmgrounds can be used to empirically define the condensed section, the thickness of sediment between firmgrounds being a function of sediment supply and water depth (accommodation space).     

The effect of open-marsh water management on summer bird populations on Topsail Island, North Carolina

Open-marsh water management (OMWM) manipulates selected areas of an estuarine marsh by constructing narrow, deep ditches that harbor resident populations of predators of mosquito larvae. It does not, however, alter the surrounding vegetation by deposition of spoil mounds. We censused the summer (breeding and nonbreeding) bird populations in two adjacent salt marsh sites on Topsail Island, North Carolina, for two summers before OMWM ditching occurred and for four seasons postditching. A total of 53 nonbreeding species and five breeding species of birds were encountered on the study sites. None of the interannual changes in bird populations can be attributed to OMWM.