Reductive transformation of iron and sulfur in schwertmannite-rich accumulations associated with acidified coastal lowlands   总被引：1，自引：0，他引：1  

Edward D. Burton  Richard T. Bush  David R.G. Mitchell 《Geochimica et cosmochimica acta》2007,71(18):4456-4473

We examined the transformations of Fe and S associated with schwertmannite (Fe₈O₈(OH)₆SO₄) reduction in acidified coastal lowlands. This was achieved by conducting a 91 day diffusive-flux column experiment, which involved waterlogging of natural schwertmannite- and organic-rich soil material. This experiment was complemented by short-term batch experiments utilizing synthetic schwertmannite. Waterlogging readily induced bacterial reduction of schwertmannite-derived Fe(III), producing abundant pore-water Fe^II, SO₄ and alkalinity. Production of alkalinity increased pH from pH 3.4 to pH ∼6.5 within the initial 14 days, facilitating the precipitation of siderite (FeCO₃). Interactions between schwertmannite and Fe^II at pH ∼6.5 were found, for the first time, to catalyse the transformation of schwertmannite to goethite (αFeOOH). Thermodynamic calculations indicate that this Fe^II-catalysed transformation shifted the biogeochemical regime from an initial dominance of Fe(III)-reduction to a subsequent co-occurrence of both Fe(III)- and SO₄-reduction. This lead firstly to the formation of elemental S via H₂S oxidation by goethite, and later also to formation of nanoparticulate mackinawite (FeS) via H₂S precipitation with Fe^II. Pyrite (FeS₂) was a quantitatively insignificant product of reductive Fe and S mineralization. This study provides important new insights into Fe and S geochemistry in settings where schwertmannite is subjected to reducing conditions.  相似文献   

Uranium-Series Dating of Carbonate (Tufa) Deposits Associated with Quaternary Fluctuations of Pyramid Lake, Nevada   总被引：1，自引：0，他引：1  

Barney J. Szabo  Charles A. Bush  Larry V. Benson 《Quaternary Research》1996,45(3):271-281

Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by²³⁰Th/²³⁴U ages of the cave samples,²³⁰Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous¹⁴C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported¹⁴C ages.  相似文献   

Temperature depression in the lowland tropics in glacial times     

Paul A. Colinvaux  Kam-Biu Liu  Paulo de Oliveira  Mark B. Bush  Michael C. Miller  Mirriam Steinitz Kannan 《Climatic change》1996,32(1):19-33

Equatorial air temperatures at low elevations in the New World tropics are shown by pollen and other data to have been significantly lowered in long intervals of the last glaciation. These new data show that long recognized evidence for cooling at high elevations in the tropics were symptomatic of general tropical cooling and that they did not require appeal to altered lapse rates or other special mechanisms to be made to conform with conclusions that equatorial sea surface temperatures (SSTs) were scarcely changed in glacial times. The new data should be read in conjunction with recent findings that Caribbean (SSTs) were lowered in the order of 5 ° C, contrary to previous interpretations. Thus these accumulating data show that low latitudes as well as high were cooled in glaciations. In part the earlier failure to find evidence of low elevation cooling in the lowland tropics resulted from the data being masked by strong signals for aridity given by old lake levels in parts of Africa and elsewhere. Global circulation models used to predict future effects of greenhouse warming must also be able to simulate the significant cooling of the large tropical land masses at glacial times with reduced greenhouse gas concentrations. Plants and animals of the Amazon forest and similar ecosystems are able to survive in wide ranges of temperatures, CO₂ concentrations, and disturbance, though associations change constantly.  相似文献   

Space and time variations in the fine structure of the upper atmosphere according to acoustic sounding data     

S. N. Kulichkov  I. P. Chunchuzov  G. A. Bush  A. A. Mishenin  E. V. Golikova 《Izvestiya Atmospheric and Oceanic Physics》2016,52(2):176-187

The results of studying variations in the fine layered structure of the upper atmosphere (heights of 20–140 km) according to data obtained from acoustic sounding within the range of infrasonic waves are given. The sources of infrasounds were surface explosions equivalent to 10 kg to 70 t of TNT. These explosions were set off in different seasons in different regions of Russia. Experimental data obtained in 1981–2011 have been analyzed. It has been found that the fine structure in the form of vertically distributed layered formations occurs in the upper atmosphere in all seasons. Moreover, the vertical distribution of both air-temperature and wind-velocity inhomogeneities in the upper atmosphere may be invariable over a time interval of no less than several hours. It has also been found that, throughout the entire atmospheric thickness from the stratopause to the lower thermosphere heights (up to 140 km), the instantaneous height distribution of layered air-temperature and wind-velocity inhomogeneities may remain almost unchanged during a time interval of no less than 20 min.  相似文献   

Observables Processing for the Helioseismic and Magnetic Imager Instrument on the Solar Dynamics Observatory     

S. Couvidat  J. Schou  J. T. Hoeksema  R. S. Bogart  R. I. Bush  T. L. Duvall  Y. Liu  A. A. Norton  P. H. Scherrer 《Solar physics》2016,291(7):1887-1938

  相似文献   

Microdiamond grade as a regionalised variable – some basic requirements for successful local microdiamond resource estimation of kimberlites     

Stiefenhofer  Johann  Thurston  Malcolm L.  Bush  David E. 《Mineralogy and Petrology》2018,112(2):673-684

Microdiamonds offer several advantages as a resource estimation tool, such as access to deeper parts of a deposit which may be beyond the reach of large diameter drilling (LDD) techniques, the recovery of the total diamond content in the kimberlite, and a cost benefit due to the cheaper treatment cost compared to large diameter samples. In this paper we take the first step towards local estimation by showing that micro-diamond samples can be treated as a regionalised variable suitable for use in geostatistical applications and we show examples of such output. Examples of microdiamond variograms are presented, the variance-support relationship for microdiamonds is demonstrated and consistency of the diamond size frequency distribution (SFD) is shown with the aid of real datasets. The focus therefore is on why local microdiamond estimation should be possible, not how to generate such estimates. Data from our case studies and examples demonstrate a positive correlation between micro- and macrodiamond sample grades as well as block estimates. This relationship can be demonstrated repeatedly across multiple mining operations. The smaller sample support size for microdiamond samples is a key difference between micro- and macrodiamond estimates and this aspect must be taken into account during the estimation process. We discuss three methods which can be used to validate or reconcile the estimates against macrodiamond data, either as estimates or in the form of production grades: (i) reconcilliation using production data, (ii) by comparing LDD-based grade estimates against microdiamond-based estimates and (iii) using simulation techniques.

  相似文献   

Paleoecological perspectives on human adaptation in central Panama. II the Holocene     

Dolores R. Piperno  Mark B. Bush  Paul A. Colinvuax 《Geoarchaeology》1991,6(3):227-250

A 17.5 m sedimentary core from Lake La Yeguada, central Panama provides the first complete Holocene sequence for the Pacific watershed of Lower Central America. Phytolith, pollen, and charcoal records document a long lowland history of foraging and food production. Human modification of tropical forests can be described as systematic during the early Holocene and was possibly associated with small-scale (garden?) horticulture involving native tubers. The incorporation of seed crops like maize and development of slash and burn techiques for larger-scale field systems is indicated by the increase of secondary forest taxa and removal of primary trees between 7000 and 4000 years ago. After this time, agricultural intensification in an essentially deforested landscape proceeded to the point where, by the time of Christ, agricultural abandonment of the lake watershed may have occurred as a result of loss of soil fertility. Events such as this in the interfluve forests may have, in part, precipitated the coalescence of population and settlement around the river valleys and the emergence of sedentary village life.  相似文献   

Paleoecological perspectives on human adaptation in central Panama. I. The Pleistocene     

Dolores R. Piperno  Mark B. Bush  Paul A. Colinvaux 《Geoarchaeology》1991,6(3):201-226

Paleoecological data from two sites in central Pacific Panama have allowed the reconstruction of Late Pleistocene climate and vegetation in lowland areas, the timing of important environmental changes, and the generation of predictions concerning Paleoindian settlement and subsistence. The last 9000 years of the glacial period, from 20,000 B.P. until 11,050 B.P. were marked by climates cooler and drier than today's. The period from ca. 16,000 B.P. until 11,000 B.P. appears to have been the coolest and driest. We postulate that the major effects of these conditions were to have brought montane vegetation 900 m lower than its present range and to create tracts of open landscape along the Pacific coast. Forests, however, were widespread in extent and many lowland forest taxa apparently persisted, creating the basis for the rejection of refugial theory as an explanatory model for early human occupation. Neither the admittedly limited archaeological or paleoecological data indicated the presence of pre-Clovis populations in Panama, and we chose to view Clovis as the first human expression on the Isthmus. Correlation of reconstructed environmental setting with Paleoindian site location suggests that Clovis adaptations were fluid and flexible, and utilized a wide variety of vegetation types. Occupation and modification of tropical forest appear to have been integral parts of Clovis subsistence and settlement strategies.  相似文献   

On-Orbit Performance of the <Emphasis Type="Italic">Helioseismic and Magnetic Imager</Emphasis> Instrument onboard the <Emphasis Type="Italic">Solar Dynamics Observatory</Emphasis>     

J. T. Hoeksema  C. S. Baldner  R. I. Bush  J. Schou  P. H. Scherrer 《Solar physics》2018,293(3):45

The Helioseismic and Magnetic Imager (HMI) instrument is a major component of NASA's Solar Dynamics Observatory (SDO) spacecraft. Since commencement of full regular science operations on 1 May 2010, HMI has operated with remarkable continuity, e.g. during the more than five years of the SDO prime mission that ended 30 September 2015, HMI collected 98.4% of all possible 45-second velocity maps; minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology. HMI velocity, intensity, and magnetic-field measurements are used in numerous investigations, so understanding the quality of the data is important. This article describes the calibration measurements used to track the performance of the HMI instrument, and it details trends in important instrument parameters during the prime mission. Regular calibration sequences provide information used to improve and update the calibration of HMI data. The set-point temperature of the instrument front window and optical bench is adjusted regularly to maintain instrument focus, and changes in the temperature-control scheme have been made to improve stability in the observable quantities. The exposure time has been changed to compensate for a 20% decrease in instrument throughput. Measurements of the performance of the shutter and tuning mechanisms show that they are aging as expected and continue to perform according to specification. Parameters of the tunable optical-filter elements are regularly adjusted to account for drifts in the central wavelength. Frequent measurements of changing CCD-camera characteristics, such as gain and flat field, are used to calibrate the observations. Infrequent expected events such as eclipses, transits, and spacecraft off-points interrupt regular instrument operations and provide the opportunity to perform additional calibration. Onboard instrument anomalies are rare and seem to occur quite uniformly in time. The instrument continues to perform very well.  相似文献   

Changes in water quality following tidal inundation of coastal lowland acid sulfate soil landscapes     

Scott G. Johnston  Richard T. Bush  Leigh A. Sullivan  Edward D. Burton  Douglas Smith  Michelle A. Martens  Angus E. McElnea  Col R. Ahern  Bernard Powell  Luisa P. Stephens  Steve T. Wilbraham  Simon van Heel 《Estuarine, Coastal and Shelf Science》2009

This study examines the remediation of surface water quality in a severely degraded coastal acid sulfate soil landscape. The remediation strategy consisted of partial restoration of marine tidal exchange within estuarine creeks and incremental tidal inundation of acidified soils, plus strategic liming of drainage waters. Time-series water quality and climatic data collected over 5 years were analysed to assess changes in water quality due to this remediation strategy. A time-weighted rainfall function (TWR) was generated from daily rainfall data to integrate the effects of antecedent rainfall on shallow groundwater levels in a way that was relevant to acid export dynamics. Significant increases in mean pH were evident over time at multiple monitoring sites. Regression analysis at multiple sites revealed a temporal progression of change in significant relationships between mean daily electrical conductivity (EC) vs. mean daily pH, and TWR vs. mean daily pH. These data demonstrate a substantial decrease over time in the magnitude of creek acidification per given quantity of antecedent rainfall. Data also show considerable increase in soil pH (2–3 units) in formerly acidified areas subject to tidal inundation. This coincides with a decrease in soil pe, indicating stronger reducing conditions. These observations suggest a fundamental shift has occurred in sediment geochemistry in favour of proton-consuming reductive processes. Combined, these data highlight the potential effectiveness of marine tidal inundation as a landscape-scale acid sulfate soil remediation strategy.  相似文献