Textural and geochemical discrimination between xenotime of different origin in the Archaean Witwatersrand Basin, South Africa

Xenotime (YPO₄) of detrital, diagenetic, and hydrothermal origin within siliciclastic rocks of the Archaean Witwatersrand Basin, South Africa, has been identified on the basis of petrography and in situ ion microprobe (SHRIMP) age data. The chemical composition of xenotime, determined by in situ electron microprobe analysis, can be correlated with its origin. This allows the origin of any xenotime grain to be assessed by a non-destructive microanalytical method prior to ion microprobe geochronology. The main chemical discriminators are MREE-HREE abundance, normalised HREE slope and Eu anomaly, and, in some cases, U and Th contents. Igneous-detrital xenotime (> 2800 Ma) is distinguished from diagenetic (∼2780 Ma) and hydrothermal (< 2780 Ma) xenotime in having lower Eu, Dy, and Gd concentrations and a distinctively lower Gd/Yb ratio. Hydrothermal xenotime has distinctively lower U and Th concentrations when compared to igneous-detrital and diagenetic xenotime. Three separate hydrothermal fluid events and episodes of post-diagenetic xenotime growth are recognised in the geochemical and geochronological data, which correspond in time to the extrusion of the Ventersdorp lavas at ∼2720 Ma, the emplacement of the Bushveld Igneous Complex at ∼2061 Ma, and an event of unknown affinity at ∼2210 Ma. Although geochemical discrimination of the xenotime types from the Witwatersrand Basin, in combination with careful petrography, appears achievable, universal application of these discriminators to xenotime in other sedimentary basins remains untested.     

Abyssal current influence on the southwest Bermuda Rise and surrounding region

Echograms (3.5 kHz) and bottom photographs reveal that the northward flowing Antarctic Bottom Water (AABW) has strongly influenced the modern depositional regime on the southwest Bermuda Rise. The spatial distribution of echo character types, the orientation and nature of current-controlled structures, and limited current meter data show that AABW flows with varying intensities along three primary pathways around and over the southwest Bermuda Rise. The main core of AABW flows clockwise around the eastern and western flanks of the southern Bermuda Rise, roughly parallel to the 5400 m isobath. This current bifurcates at 28°30′N, 69°W where a portion flows northeast over the southwest Bermuda Rise and the remainder continues north along the physiographic boundary between the southwest Bermuda Rise and the Hatteras Abyssal Plain. Secondary ribbons of AABW branch off the main core of AABW during its southerly journey along the southeastern Bermuda Rise, and flow west through fracture zones. Finally, a diffuse, northward flowing AABW sweeps the entire southwest Bermuda Rise.

A progression of current-controlled bedforms occurs beneath the main path of the AABW reflecting the spatially varying current velocities and sediment supply. The main core of AABW flows west through the narrow Vema Gap creating erosional furrows along the border between the southwest Bermuda Rise and the Vema Gap. Current velocities greater than 20 cm s⁻¹ are inferred from the bedforms in this region. Farther north along the southwestern edge of the Bermuda Rise, sediment waves become more prevalent. This transition from erosional to more depositional bedforms results from diminished current velocities (5–15 cm s⁻¹) and increased sediment supply. Although some of these bedforms on the southwest Bermuda Rise appear to be relict, their orientation is consistent with current meter data and abyssal current direction inferred from bottom photographs.     

Time Variability of EUV Brightenings in Coronal Loops Observed with TRACE

Solar Physics - We analyze coronal loops in active region 8272, observed with TRACE on 23 July 1998 during a 70-min interval with a cadence of 1.5&;nbsp;min, in the temperature range of...     

Water quality, nutrients and the water framework directive in an agricultural region: The lower Humber Rivers, northern England

The water quality of rivers in the eastern part of the Humber Basin, north-eastern England is described from a baseflow survey (11–13 August 2006) of a wide range of water quality determinants, and long-term nutrient records of the Environment Agency of England and Wales (EA). The baseflow survey shows that the rivers are oversaturated with respect to dissolved CO₂ and calcite. They are sodium, potassium, lithium, boron, chloride, sulphate and fluoride bearing from a combination of atmospheric, within-catchment, agricultural and sewage effluent sources. Nitrate concentrations are uniform for rivers draining permeable bedrock but decrease for clay drainage areas. Soluble reactive phosphorus (SRP) concentrations are variable across the catchments reflecting the importance and variability of point sources and within-river processing. The EA data show annual oscillations for both NO₃ and SRP concentrations. Average NO₃ concentrations vary between 3.3 and 18.8 mgN/l and concentrations are low during the summer months. Average SRP concentrations vary between 23 and 1959 μg/l. Highest SRP concentrations generally occur when there is effluent input from sewage treatment works and agricultural point sources (e.g. overflow from slurry tanks, farmyard washings). Despite many of the rivers being nutrient rich, they are generally of good biological quality when point source inputs are not important.     

Geoarchaeology of stratified paleoindian deposits at the Big Eddy site,Southwest Missouri,U.S.A.

The Big Eddy site (23CE426) in the Sac River valley of southwest Missouri is a rare recorded example of distinctly stratified Early through Late Paleoindian cultural deposits. Early point types recovered from the site include Gainey, Sedgwick, Dalton (fluted and unfluted), San Patrice, Wilson, and Packard. The Paleoindian record at Big Eddy represents only a fraction of the site's prehistoric cultural record; stratified cultural deposits in alluvium above the Paleoindian components span the entire known prehistoric sequence, and terminal Pleistocene alluvium may contain pre‐Early Paleoindian cultural deposits. This study focused on the paleogeomorphic setting, stratigraphy, depositional environments, pedology, geochronology, and history of landscape evolution of the late Pleistocene and early Holocene alluvium at the site. The Paleoindian sequence is associated with a complex buried soil 2.85 m below the modern surface (T1a) of the first terrace of the Sac River valley in the site vicinity. This soil formed at the top of the early submember of the Rodgers Shelter Member (underlying the T1c paleogeomorphic surface) and contains at least 70 cm of stratified Paleoindian cultural deposits, all in floodplain and upper point‐bar facies. A suite of 36 radiocarbon ages indicates that the alluvium hosting the Paleoindian sequence aggraded between ca. 13,250 and 11,870 cal yr B.P. (11,380 and 10,180 14C yr B.P.). Underlying deposits accumulated between ca. 15,300 and 13,250 cal yr B.P. (12,950 and 11,380 14C yr B.P.). By ca. 11,250 cal yr B.P. (9,840 14C yr B.P.) the T1c paleogeomorphic surface was buried by the earliest increment of a thick sequence of overbank sheetflood facies, ultimately resulting in deep burial and preservation of the Paleoindian record. The landform‐sediment assemblage that hosts the Paleoindian and possibly earlier cultural deposits at Big Eddy is both widespread and well preserved in the lower Sac River valley. Moreover, the terminal Pleistocene and early Holocene depositional environments were favorable for the preservation of the archaeological record. © 2007 Wiley Periodicals, Inc.     

Observing Global Surface Water Flood Dynamics

Flood waves moving along river systems are both a key determinant of globally important biogeochemical and ecological processes and, at particular times and particular places, a major environmental hazard. In developed countries, sophisticated observing networks and ancillary data, such as channel bathymetry and floodplain terrain, exist with which to understand and model floods. However, at global scales, satellite data currently provide the only means of undertaking such studies. At present, there is no satellite mission dedicated to observing surface water dynamics and, therefore, surface water scientists make use of a range of sensors developed for other purposes that are distinctly sub-optimal for the task in hand. Nevertheless, by careful combination of the data available from topographic mapping, oceanographic, cryospheric and geodetic satellites, progress in understanding some of the world’s major river, floodplain and wetland systems can be made. This paper reviews the surface water data sets available to hydrologists on a global scale and the recent progress made in the field. Further, the paper looks forward to the proposed NASA/CNES Surface Water Ocean Topography satellite mission that may for the first time provide an instrument that meets the needs of the hydrology community.     

When does spatial resolution become spurious in probabilistic flood inundation predictions?

Advances in remote sensing have enabled hydraulic models to run at fine scale resolutions, producing precise flood inundation predictions. However, running models at finer resolutions increase their computational expense, reducing the feasibility of running the multiple model realizations required to undertake uncertainty analysis. Furthermore, it is possible that precision gained by running fine scale models is smoothed out when treating models probabilistically. The aim of this paper is to determine the level of spatial complexity that is required when making probabilistic flood inundation predictions. The Imera basin, Sicily is used as a case study to assess how changing the spatial resolution of the hydraulic model LISFLOOD‐FP impacts on the skill of conditional probabilistic flood inundation maps given model parameter and boundary condition uncertainties. We find that model performance deteriorates at resolutions coarser than 50 m. This is predominantly caused by changes in flow pathways at coarser resolutions which lead to non‐stationarity in the optimum model parameters at different spatial resolutions. However, although it is still possible to produce probabilistic flood maps that contain a coherent outline of the flood extent at coarser resolutions, the reliability of these maps deteriorates at resolutions coarser than 100 m. Additionally, although the rejection of non‐behavioural models reduces the uncertainty in probabilistic flood maps the reliability of these maps is also reduced. Models with resolutions finer than 50 m offer little gain in performance yet are more than an order of magnitude computationally expensive which can become infeasible when undertaking probabilistic analysis. Furthermore, we show that using deterministic, high‐resolution flood maps can lead to a spurious precision that would be misleading and not representative of the overall uncertainties that are inherent in making inundation predictions. Copyright © 2015 The Authors Hydrological Processes Published by John Wiley & Sons Ltd.