Reconnaissance-Scale Prospectivity Analysis for Gold Mineralisation in the Southern Uplands-Down-Longford Terrane, Northern Ireland

The Southern Uplands-Down-Longford Terrane in southeast Northern Ireland is prospective for Caledonian-age, turbidite-hosted orogenic gold mineralisation with important deposits at Clontibret in the Republic of Ireland and in Scotland. Geochemical and geophysical data from the DETI-funded Tellus project have been used, in conjunction with other spatial geoscience datasets, to map the distribution of prospectivity for this style of mineralisation over this terrane. A knowledge-based fuzzy logic modelling methodology using Arc Spatial Data modeller was utilised. The prospectivity analysis has identified several areas prospective for turbidite-hosted gold mineralisation, comparable to that at Clontibret and gold occurrences in the Southern Uplands of Scotland. A number of these either coincide with known bedrock gold occurrences or with areas considered prospective and targeted by previous exploration work, validating the predictive capability of the exploration model devised and its translation into a GIS-based prospectivity model. The results of the modelling suggest that as in other parts of the Southern Uplands the coincidence of regional strike-parallel structures and intersecting transverse faults are highly prospective, as these are likely to create zones of anomalous stress for fluid flow and deposit formation. Those areas in which there are no known gold occurrences are considered to be favourable targets for further exploration and should be followed up.     

AN INVESTIGATION OF THE ACCURACY OF PARALLAX BAR HEIGHTING BY MEANS OF NUMERICAL SIMULATION

Previous assessments of the accuracy of parallax bar heighting were handicapped by small sample sizes. Numerical simulation was avoided due to the difficulty of obtaining co-ordinates for the conjugate principal points. This problem has been overcome, however, and all required photograph coordinates have been rigorously computed following the generation of fictitious data for ground points and exposure stations. The parallax bar heighting process has additionally been simulated in order to investigate both external factors, such as image errors, tilts and ground relief, and also influences intrinsic in the heighting process, for example estimation of flying height, baselining, control configuration and error surface model.     

Stocks and dynamics of dissolved and particulate organic matter in the southern Ross Sea, Antarctica

Dissolved and particulate organic matter was measured during six cruises to the southern Ross Sea. The cruises were conducted during late austral winter to autumn from 1994 to 1997 and included coverage of various stages of the seasonal phytoplankton bloom. The data from the various years are compiled into a representative seasonal cycle in order to assess general patterns of dissolved organic matter (DOM) and particulate organic matter (POM) dynamics in the southern Ross Sea. Dissolved organic carbon (DOC) and particulate organic carbon (POC) were at background concentrations of approximately 42 and 3 μM C, respectively, during the late winter conditions in October. As the spring phytoplankton bloom progressed, organic matter increased, and by January DOC and POC reached as high as 30 and 107 μM C, respectively, in excess of initial wintertime conditions. Stocks and concentrations of DOC and POC returned to near background values by autumn (April). Approximately 90% of the accumulated organic matter was partitioned into POM, with modest net accumulation of DOM stocks despite large net organic matter production and the dominance of Phaeocystis antarctica. Changes in NO₃ concentration from wintertime values were used to calculate the equivalent biological drawdown of dissolved inorganic carbon (DIC_equiv). The fraction of DIC_equiv drawdown resulting in net DOC production was relatively constant (ca. 11%), despite large temporal and spatial variability in DIC_equiv drawdown. The C : N (molar ratio) of the seasonally produced DOM had a geometric mean of 6.2 and was nitrogen-rich compared to background DOM. The DOM stocks that accumulate in excess of deep refractory background stocks are often referred to as “semi-labile” DOM. The “semi-labile” pool in the Ross Sea turns over on timescales of about 6 months. As a result of the modest net DOM production and its lability, the role DOM plays in export to the deep sea is small in this region.     

Tropical Storm and Hurricane wind effects on water level, salinity, and sediment transport in the river-influenced Atchafalaya-Vermilion Bay System, Louisiana, USA

Changes in circulation, water level, salinity, suspended sediments, and sediment flux resulted from Tropical Storm Frances and Hurricane Georges in the Vermilion-Atchafalaya Bay region during September 1998. Tropical Storm Frances made landfall near Port Aransas, Texas, 400 km west of the study area, and yet the strong and long-lived southeasterly winds resulted in the highest water levels and salinity values of the year at one station in West Cote Blanche Bay. Water levels were abnormally high across this coastal bay system, although salinity impacts varied spatially. Over 24 h, salinity increased from 5 to 20 psu at Site 1 on the east side of West Cote Blanche Bay. Abnormally high salinities were recorded in Atchafalaya Bay but not at stations in Vermilion Bay. On September 28, 1998, Hurricane Georges made landfall near Biloxi, Mississippi, 240 km east of the study area. On the west side of the storm, wind stress was from the north and maximum winds locally reached 14 m s⁻¹. The wind forcing and physical responses of the bay system were analogous to those experienced during a winter cold-front passage. During the strong, north wind stress period, coastal water levels fell, salinity decreased, and sediment-laden bay water was transported onto the inner shelf. As the north wind stress subsided, a pulse of relatively saline water entered Vermilion Bay through Southwest Pass increasing salinity from 5 to 20 psu over a 24-h period. National Oceanic and Atmospheric Administration (NOAA)-14 reflectance imagery revealed the regional impacts of wind-wave resuspension and the bay-shelf exchange of waters. During both storm events, suspended solid concentrations increased by an order of magnitude from 75 to over 750 mg l⁻¹. The measurements demonstrated that even remote storm systems can have marked impacts on the physical processes that affect ecological processes in shallow coastal bay systems.     

Sources of nutrient pollution to coastal waters in the United States: Implications for achieving coastal water quality goals

Some 60% of coastal rivers and bays in the U.S. have been moderately to severely degraded by nutrient pollution. Both nitrogen (N) and phosphorus (P) contribute to the problem, although for most coastal systems N additions cause more damage. Globally, human activity has increased the flux of N and P from land to the oceans by 2-fold and 3-fold, respectively. For N, much of this increase has occurred over the past 40 years, with the increase varying by region. Human activity has increased the flux of N in the Mississippi River basin by 4-fold, in the rivers of the northeastern U.S. by 8-fold, and in the rivers draining to the North Sea by more than 10-fold. The sources of nutrients to the coast vary. For some estuaries, sewage treatment plants are the largest single input; for most systems nonpoint sources of nutrients are now of relatively greater importance, both because of improved point source treatment and control (particularly for P) and because of increases in the total magnitude of nonpoint sources (particularly for N) over the past three decades. For P, agricultural activities dominate nonpoint source fluxes. Agriculture is also the major source of N in many systems, including the flux of N down the Mississippi River, which has contributed to the large hypoxic zone in the Gulf of Mexico. For both P and N, agriculture contributes to nonpoint source pollution both through losses at the field scale, as soils erode away and fertilizer is leached to surface and ground waters, and from losses from animal feedlot operations. In the U.S. N from animal wastes that leaks directly to surface waters or is volatilized to the atmosphere as ammonia may be the single largest source of N that moves from agricultural operations into coastal waters. In some regions, including the northeastern U.S., atmospheric deposition of oxidized N from fossil-fuel combustion is the major flux from nonpoint sources. This atmospheric component of the N flux into estuaries has often been underestimated, particularly with respect to deposition onto the terrestrial landscape with subsequent export downstream. Because the relative importance of these nutrient sources varies among regions and sites, so too must appropriate and effective mitigation strategies. The regional nature and variability of nutrient sources require that nutrient management efforts address large geographic areas.     

Mangrove Range Expansion Rapidly Increases Coastal Wetland Carbon Storage

The climate change-induced expansion of mangroves into salt marshes could significantly alter the carbon (C) storage capacity of coastal wetlands, which have the highest average C storage per land area among unmanaged terrestrial ecosystems. Mangrove range expansion is occurring globally, but little is known about how these rapid climate-driven shifts may alter ecosystem C storage. Here, we quantify current C stocks in ecotonal wetlands across gradients of marsh- to mangrove-dominance, and use unique chronological maps of vegetation cover to estimate C stock changes from 2003 to 2010 in a 567-km² wildlife refuge in the mangrove-salt marsh ecotone. We report that over the 7-yr. period, total wetland C stocks increased 22 % due to mangrove encroachment into salt marshes. Newly established mangrove stands stored twice as much C on a per area basis as salt marsh primarily due to differences in aboveground biomass, and mangrove cover increased by 69 % during this short time interval. Wetland C storage within the wildlife refuge increased at a rate of 2.7 Mg C ha^?1 yr.^?1, more than doubling the naturally high coastal wetland C sequestration rates. Mangrove expansion could account for a globally significant increase of terrestrial C storage, which may exert a considerable negative feedback on warming.     

Models of Earth's main magnetic field incorporating flux and radial vorticity constraints

We describe a new technique for implementing the constraints on magnetic fields arising from two hypotheses about the fluid core of the Earth, namely the frozen-flux hypothesis and the hypothesis that the core is in magnetostrophic force balance with negligible leakage of current into the mantle. These hypotheses lead to time-independence of the integrated flux through certain 'null-flux patches' on the core surface, and to time-independence of their radial vorticity. Although the frozen-flux hypothesis has received attention before, constraining the radial vorticity has not previously been attempted. We describe a parametrization and an algorithm for preserving topology of radial magnetic fields at the core surface while allowing morphological changes. The parametrization is a spherical triangle tesselation of the core surface. Topology with respect to a reference model (based on data from the Oersted satellite) is preserved as models at different epochs are perturbed to optimize the fit to the data; the topology preservation is achieved by the imposition of inequality constraints on the model, and the optimization at each iteration is cast as a bounded value least-squares problem. For epochs 2000, 1980, 1945, 1915 and 1882 we are able to produce models of the core field which are consistent with flux and radial vorticity conservation, thus providing no observational evidence for the failure of the underlying assumptions. These models are a step towards the production of models which are optimal for the retrieval of frozen-flux velocity fields at the core surface.     

The chemical and physical characteristics of heavy metals in humus and till in the vicinity of the base metal smelter at Flin Flon, Manitoba, Canada

 Trace element geochemistry of humus (<0.425 mm) and till (<0.002 mm) collected in the Flin Flon-Snow Lake area, northern Manitoba and Saskatchewan, provides a regional context for assessing smelter contamination in the environment. The area includes a Cu-Zn smelter known to discharge As, Cd, Cu, Fe, Hg, Pb, and Zn. In this study, sequential extraction analyses, scanning electron microscopy and x-ray diffraction analyses were used on a suite of samples to determine: (1) the chemical and physical characteristics of heavy metals in surficial sediments related to distance from the smelter, (2) criteria for assessing the relative contribution of these metals from natural and anthropogenic sources, and (3) the potential of these metals for remobilization in the environment. Humus geochemistry reflects the anthropogenic and natural component of heavy metal concentrations. Smelter-related elements show anomalously high values adjacent to the smelter, decreasing with distance until background values are reached at 70–104 km, depending on the element. In humus, Zn is associated primarily with labile phases; Hg with non-labile phases. Adjacent to the smelter, high proportions and concentrations of Zn and Hg in non-labile phases, indicative of smelter-derived particulates, are confirmed by SEM examination. The particles occur as spheres, irregular grains, and with organics. With increasing distance from the smelter, the geochemical response to bedrock composition is more obvious than the anthropogenic input. Till geochemistry reflects the natural variation imposed by bedrock composition. At highly contaminated sites (<3 km from the smelter), increased percentages of smelter-related elements in labile phases suggests heavy metals are leached from humus to the underlying sediment. Received: 5 November 1996 · Accepted: 31 March 1997     

Ab initio crystal structure and elasticity of tuite, γ-Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>, with implications for trace element partitioning in the lower mantle

Tuite forms by the breakdown of apatite at high pressure and is thus expected to play a role in extending the phosphorus cycle beyond the stability field of apatite and into the lower mantle. With its large, high-coordination cation sites, tuite is thought to be able to dissolve large quantities of incompatible elements such as rare earth elements, Sr, Th, and U, and is potentially an important mantle reservoir for these elements. In this paper, ab initio calculations of the structure and elasticity of tuite to lower mantle pressure are presented and used to probe trace element incorporation. The calculated zero-pressure volumes of the M1 and M2 cation sites were 50.23 and 36.61 Å³, while the corresponding bulk moduli K ₀ are 116.1 and 94.2 GPa, significantly lower than the 234.1 GPa calculated for the M site of CaSiO₃ perovskite (cpv), another likely host for incompatible elements in the mantle. The partitioning of impurities between tuite and cpv is investigated using a lattice strain model, parameterized by the ab initio calculations, to calculate isovalent substitution energies across a range of pressures and impurity sizes. Additionally, energies of strontium and barium defects in tuite are compared with those of equivalent defects in cpv, and it is found that both elements will partition strongly from cpv into tuite.