Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia

The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km², enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Niño). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen.Superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fi ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S-macroband deposition (chert-carbonate-silicate BIF, with shale), gradually returning to the dominant hematite-magnetite-chert BIF as the volcanic input waned. During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). S Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone. Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub-photic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorus, to surface waters increased the organic contribution despite lower light values, leading to an almost total absence of ferric iron oxides in the S macrobands (i.e. no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “olcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or diagenesis, resulted in overgrowths of magnetite on hematite, and eventually in the substantial conversion of hematite to magnetite, where higher temperatures were achieved during low-grade regional metamorphism.Changes in sea-level to explain facies changes in BIF are not required in this model, but are not excluded. The preferred conditions are for a very low oxygen to anoxic atmosphere, a much higher level of MOR activity than at present, the presence of photosynthetic plankton, the absence of si silica-secreting organisms, and a deep sea-water temperature higher than 20°C. However, none of these conditions is essential to the model.A narrow carbonate bank is postulated for part of the Fortescue River Valley area during Marra Mamba Iron Formation times (basal Hamersley Group), with BIF precipitation on either side. The reef is postulated to have grown northward becoming a major shallow-water carbonate platform on the Pilbara continent during upper Marra Mamba Iron Formation and Wittenoom Dolomite times, but ceased to play an important role in subsequent periods.     

A combined boundary-profile and automated data-reduction and analysis system

A portable boundary-layer meteorological data-acquisition and analysis system is described which employs a small tethered balloon and a programmable calculator. The system is capable of measuring pressure, wet- and dry-bulb temperature, wind speed, and temperature fluctuations as a function of height and time. Other quantities, which can be calculated in terms of these, can also be made available in real time. All quantities, measured and calculated, can be printed, plotted, and stored on magnetic tape in the field, during the data-acquisition phase of an experiment.The National Center for Atmospheric Research is Sponsored by the National Science Foundation.     

Acute toxicity of kepone to selected freshwater fishes

The acute toxicity of Kepone in freshwater was determined with three fish species,Ictalurus punctatus (channel catfish),Lepomis macrochirus (bluegills), andAnguilla rostrata (American eel). Elvers ofA. rostrata were most sensitive with a 96 h lethal concentration for 50% of the animals tested (LC50) of 35 μg per. 1. Bluegills were slightly less sensitive with a 96 h LC50 of 50 μg per 1. Catfish were most tolerant with a 96 h LC50 of 514 μg per l. Bluegills and catfish exposed to comparable concentrations of Kepone accumulated equivalent amounts in 96 h. This observation in conjunction with the markedly different 96 h LC50's for these species suggest a difference in the ways these fish cope with Kepone.     

Electron paramagnetic resonance study of the site preferences of Gd3+ and Eu2+ in polycrystalline silicate and aluminate minerals

Electron paramagnetic resonance (EPR) measurements were made on Gd³⁺ and Eu²⁺ ions in polycrystalline samples to determine the nature of the sites occupied by those ions in mineral structures. Both Gd³⁺ and Eu²⁺ ions were incorporated at Ca²⁺ structural sites in β-Ca₂SiO₄, pseudo-CaSiO₃, CaMgSiO₄, CaMgSi₂O₆, hex-CaAl₂Si₂O₈, CaAl₂O₄, and Ca₃Al₂O₆. For tri-CaAl₂Si₂O₈, Eu²⁺ was incorporated at a Ca²⁺ site and Gd³⁺ was incorporated at a site where the crystalline electric field was disordered. That difference in behavior may contribute to the anomalous behavior of Eu in plagioclase feldspar. Both Gd³⁺ and Eu²⁺ were incorporated as aggregates or clusters of those ions in Mg₂SiO₄ and clino-MgSiO₃.     

Setting seagrass depth, coverage, and light targets for the Indian River Lagoon system, Florida

Seagrass protection and restoration in Florida’s Indian River Lagoon system (IRLS) is a mutual goal of state and federal programs. These programs require, the establishment of management targets indicative of seagrass recovery and health. We used three metrics related to seagrass distribution: areal coverage, depth limit, and light requirement. In order to account for the IRLS’s spatial heterogeneity and temporal variability, we developed coverage and depth limit targets for each of its 19 segments. Our method consisted of two steps: mapping the union of seagrass coverages from all availabe mapping years (1943, 1986, 1989, 1992, 1994, 1996, and 1999) to delineate wherever seagrass had been mapped and determining the distribution of depth limits based on 5,615 depth measurements collected on or very near the deep-edge boundary of the union coverage. The frequency distribution of depth limits derived from the union coverage, along with the median (50th percentile) and maximum (95th percentile) depth limits, serve as the seagrass depth targets for each segment. The median and maximum depth targets for the IRLS vary among segments from 0.8 to 1.8 and 1.2 to 2.8 m, respectively.Halodule wrightii is typically the dominant seagrass species at the deep-edge of IRLS grass beds. We set light requirement targets by using a 10-yr record of light data (1990–1999) and the union coverage depth limit distributions from the most temporally stable seagrass segments. The average annual light requirement, based on the medians of the depth limit distributions, is 33 ± 17% of the subsurface light. The minimum annual light requirement, based on of the 95th percentile of the depth distributions, is 20 ± 14%; the minimum growing season light requirement (March to mid September) is essentially the same (20 ± 13%). Variation in depth limits and light requirements, is probably due to factors other than light that influence the depth limit of seagrasses (e.g., competition, physical disturbance). The methods used in this study are robust when applied to large or long-term data sets and can be applied to other estuaries where grass beds are routinely monitored and mapped.     

Exhumation of the Pyrenean orogen: implications for sediment discharge

Apatite fission track analyses of 21 samples from the central and eastern Pyrenees are modelled to generate time–temperature plots for the post 110±10 °C cooling history over the 40–10 Ma time interval. Modelled thermal histories have been converted into exhumation plots through the application of the present-day geothermal gradient in the Pyrenees. The documented geology of the Pyrenees allows us to assume no significant extensional unroofing and subvertical exhumation trajectories, thus enabling exhumation to be translated into erosional denudation. Maps of denudation have been constructed for six, 5-Myr time intervals. Denudation varied with a 20–50-km length scale, and does not appear to have been related to the major structural zones of the mountain belt. Spatially averaged denudation rates for the six time intervals ranged from 34 to 61 mm kyr^?1 assuming the present-day geothermal gradient. Maximum rates of 240 mm kyr^?1 occurred in the interval 35–30 Ma, in the region of the Querigut-Millas massif. Assuming the denudation resulted primarily from erosion, the denudation maps can be used to calculate sediment discharge through time to the neighbouring foreland basins. Using a series of rectangular drainage basins with a 2:1 aspect ratio (based on modern linear mountain belts) and a location of the main drainage divide based on the mean present-day position, it is possible to evaluate the potential for spatial and temporal variations in sediment discharge as a function of denudation. The results show along-strike variations in sediment discharge between drainage basins of 500%, and temporal variations from individual basins of >300%. A comparison of total sediment discharge per year to the Ebro and Aquitaine basins, assuming a fixed drainage divide, shows that the discharge to the south is likely to have been between 1.5 and 2.8 times greater than to the north.     

Estimates of future impacts of degrading streams in the deep loess soil region of western Iowa on private and public infrastructure costs

In the beginning of the 20th century, many streams in western Iowa were channelized to reduce flooding and to open swamp land to cultivation. Channel straightening accomplished its goal. However, it resulted in greater streamflow velocities, causing stream channels to degrade. This degradation has resulted in significant loss of land and damage to transportation and communications infrastructure in western Iowa and in several states in the United States. Baumel et al. (1994; Impact of Degrading Western Iowa Streams on Private and Public Infrastructure Costs. Final Report Iowa DOT HR-352, Stream Stabilization in Western Iowa) estimated the historical cost of this degradation on land loss and damage to transportation and communications infrastructure in western Iowa. The purpose of this paper is to extend the Baumel et al. analysis to estimate future degradation costs on 141 streams in western Iowa. It also presents two types of degradation cost estimates. One is a time neutral cost that does not consider the dates on which the degradation costs are incurred. The second is a time value cost which considers the dates on which the costs are incurred and then discounts these costs back to 1992 dollars. The time value costs are the more accurate estimates of the cost of future degradation in 1992 dollars and should be used to evaluate stream stabilization project proposals.