Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones   总被引：6，自引：0，他引：6  

Luke D Nothdurft  Gregory E Webb 《Geochimica et cosmochimica acta》2004,68(2):263-283

Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements (n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd_SN/Yb_SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement (n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd_SN/Yb_SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modern seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans.  相似文献   

Ground-water surface-water interactions and long-term change in riverine riparian vegetation in the southwestern United States   总被引：1，自引：0，他引：1  

Robert H. Webb  Stanley A. Leake 《Journal of Hydrology》2006,320(3-4):302-323

Riverine riparian vegetation has changed throughout the southwestern United States, prompting concern about losses of habitat and biodiversity. Woody riparian vegetation grows in a variety of geomorphic settings ranging from bedrock-lined channels to perennial streams crossing deep alluvium and is dependent on interaction between ground-water and surface-water resources. Historically, few reaches in Arizona, southern Utah, or eastern California below 1530 m elevation had closed gallery forests of cottonwood and willow; instead, many alluvial reaches that now support riparian gallery forests once had marshy grasslands and most bedrock canyons were essentially barren. Repeat photography using more than 3000 historical images of rivers indicates that riparian vegetation has increased over much of the region. These increases appear to be related to several factors, notably the reduction in beaver populations by trappers in the 19th century, downcutting of arroyos that drained alluvial aquifers between 1880 and 1910, the frequent recurrence of winter floods during discrete periods of the 20th century, an increased growing season, and stable ground-water levels. Reductions in riparian vegetation result from agricultural clearing, excessive ground-water use, complete flow diversion, and impoundment of reservoirs. Elimination of riparian vegetation occurs either where high ground-water use lowers the water table below the rooting depth of riparian species, where base flow is completely diverted, or both. We illustrate regional changes using case histories of the San Pedro and Santa Cruz Rivers, which are adjacent watersheds in southern Arizona with long histories of water development and different trajectories of change in riparian vegetation.  相似文献   

Geochemistry of Early–Middle Palaeozoic basalts in the Hodgkinson Province: a key to tectono-magmatic evolution of the Tasman Fold Belt System in northeastern Queensland, Australia     

I. M. A. Vos  F. P. Bierlein  J. Webb 《International Journal of Earth Sciences》2006,95(4):569-585

The Palaeozoic Hodgkinson Province in northeastern Queensland, Australia, is host to Late Ordovician to Devonian rock assemblages that contain tholeiitic to calc-alkaline basalts. These basalts occur as massive fault-bounded units interspersed with marine sedimentary rocks and limestones that are metamorphosed to lower greenschist facies in the Ordovician Mulgrave, Silurian Chillagoe and Devonian Hodgkinson formations, respectively. The petrogenetic and Sm–Nd isotope characteristics of these mafic volcanic rocks were investigated to constrain the tectonic setting in which they erupted. Major, trace and rare earth element analyses were carried out on samples from these formations and intrusive dolerites. The mafic rocks can be classified as basalts and basaltic andesites with distinct MORB characteristics. Furthermore, the basalts are characterized by a slight to moderate enrichment in Th and concomitant depletion in Nb, both of which become less pronounced with basalt evolution through time. These features are consistent with decreasing volcanic arc affinity of Silurian and Devonian MORB-type basalts in the Hodgkinson Province. Sm–Nd isotope characteristics of these basalts indicate a change in source region from dominantly sub-continental lithospheric mantle in the Silurian to asthenospheric input in the Devonian. Collectively, the geochemical and isotopic characteristics of the Hodgkinson Province basalts are interpreted to reflect deposition in an evolving back-arc basin setting. The onset of basin extension was initiated in the Silurian. Accelerated basin subsidence occurred throughout the Devonian and was halted by basin inversion in the Late Devonian. Basin evolution was controlled by an eastward stepping subduction zone outboard of the Australian Craton.  相似文献   

Quantifying uncertainty in changes in extreme event frequency in response to doubled CO2 using a large ensemble of GCM simulations   总被引：2，自引：1，他引：2  

David N. Barnett  Simon J. Brown  James M. Murphy  David M. H. Sexton  Mark J. Webb 《Climate Dynamics》2006,26(5):489-511

We discuss equilibrium changes in daily extreme surface air temperature and precipitation events in response to doubled atmospheric CO₂, simulated in an ensemble of 53 versions of HadSM3, consisting of the HadAM3 atmospheric general circulation model (GCM) coupled to a mixed layer ocean. By virtue of its size and design, the ensemble, which samples uncertainty arising from the parameterisation of atmospheric physical processes and the effects of natural variability, provides a first opportunity to quantify the robustness of predictions of changes in extremes obtained from GCM simulations. Changes in extremes are quantified by calculating the frequency of exceedance of a fixed threshold in the 2 × CO₂ simulation relative to the 1 × CO₂ simulation. The ensemble-mean value of this relative frequency provides a best estimate of the expected change while the range of values across the ensemble provides a measure of the associated uncertainty. For example, when the extreme threshold is defined as the 99th percentile of the 1 × CO₂ distribution, the global-mean ensemble-mean relative frequency of extremely warm days is found to be 20 in January, and 28 in July, implying that events occurring on one day per hundred under present day conditions would typically occur on 20–30 days per hundred under 2 × CO₂ conditons. However the ensemble range in the relative frequency is of similar magnitude to the ensemble-mean value, indicating considerable uncertainty in the magnitude of the increase. The relative frequencies in response to doubled CO₂ become smaller as the threshold used to define the extreme event is reduced. For one variable (July maximum daily temperature) we investigate this simulated variation with threshold, showing that it can be quite well reproduced by assuming the response to doubling CO₂ to be characterised simply as a uniform shift of a Gaussian distribution. Nevertheless, doubling CO₂ does lead to changes in the shape of the daily distributions for both temperature and precipitation, but the effect of these changes on the relative frequency of extreme events is generally larger for precipitation. For example, around one-fifth of the globe exhibits ensemble-mean decreases in time-averaged precipitation accompanied by increases in the frequency of extremely wet days. The ensemble range of changes in precipitation extremes (relative to the ensemble mean of the changes) is typically larger than for temperature extremes, indicating greater uncertainty in the precipitation changes. In the global average, extremely wet days are predicted to become twice as common under 2 × CO₂ conditions. We also consider changes in extreme seasons, finding that simulated increases in the frequency of extremely warm or wet seasons under 2 × CO₂ are almost everywhere greater than the corresponding increase in daily extremes. The smaller increases in the frequency of daily extremes is explained by the influence of day-to-day weather variability which inflates the variance of daily distributions compared to their seasonal counterparts.  相似文献   

GeoPT2. International Proficiency Test for Analytical Geochemistry Laboratories - Report on Round 2   总被引：1，自引：1，他引：0  

Michael Thompson  Philip J. Potts  Jean S. Kane  Peter C. Webb  John S. Watson 《Geostandards and Geoanalytical Research》1998,22(1):127-156

Results from sixty laboratories participating in GeoPT2, the international proficiency testing programme for analytical geochemistry laboratories, are reported. Compared with 71.3% in the last round, 74.6% of the reported data complied with the laboratories' selected fitness for purpose criteria.  相似文献   

Book reviews     

Lluis Cabera  Patrick De Deckker  Robert S. Webb 《Journal of Paleolimnology》1995,14(2):225-230

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Rates of sediment accumulation in pollen cores from small lakes and mires of eastern North America     

Robert Stabler Webb  Thompson Webb  III 《Quaternary Research》1988,30(3)

Data from 291 small lakes and mires in eastern North America provide information on the natural variability of rates of sediment accumulation in these environments over the last 18,000 yr. Accumulation rates were calculated by linear interpolation between radiocarbon and biostratigraphic dates from sediment cores taken for pollen analysis. Within the data set, the rates were lognormally distributed with a mean accumulation rate of 91 cm/10³ yr, and a range from less than 1 to over 3500 cm/10³ yr. The accumulation rate data were divided into five subsets that were temporally or spatially distinct and therefore represent different geomorphic and climatic conditions at the time of deposition. Sediments deposited in basins north of 50°N, south of 40°N, and before 10,000 yr B.P. accumulated at much slower rates than sediments accumulating in midlatitude basins (between 40° and 50°N) between 10,000 and 330 yr B.P. Sediment accumulation over the last 330 yr has, on average, been at rates four to five times faster than any time previously. Inorganic sediments that could be radiocarbon-dated have accumulated at significantly lower rates than organic sediments, reflecting differences in depositional processes. For midlatitude basins during the Holocene, the most likely rate of continuous sediment accumulation within our data set is 65 cm/10³ yr. Rates below 10 cm/10³ yr are likely to be associated with nonconstant processes of sediment accumulation.  相似文献   

Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes   总被引：4，自引：0，他引：4  

Donald B. Dingwell  Sharon L. Webb 《Physics and Chemistry of Minerals》1989,16(5):508-516

The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, , consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log₁₀ units of time above . In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; _S = _S/G (where _S is the shear relaxation time, G is the shear modulus at infinite frequency and _S is the zero frequency shear viscosity). The value of G known for SiO₂ and several other silicate glasses. The shear modulus, G , and the bulk modulus, K , are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to _S, G may be considered a constant (independent of composition and temperature) and the value of _S, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known.For silicate melts, the relaxation times calculated from the Maxwell relationship agree well with available data for the onset of the frequency-dependence (dispersion) of acoustic velocities, the onset of non-Newtonian viscosities, the scan-rate dependence of the calorimetric glass transition, with the timescale of an oxygen diffusive jump and with the Si-O bond exchange frequency obtained from ²⁹Si NMR studies.  相似文献   

Measurement of solar UVB radiation in the English Midlands     

A. Webb  M. D. Steven 《Meteorology and Atmospheric Physics》1984,35(3):221-231

Summary Measurements of solar radiation in the ultraviolet B band have been made at Sutton Bonington in the English East Midlands and relationships with more routinely measured meteorological variables have been established. On clear days, linear relationships have been found between the logarithm of irradiance and airmass and between the ratio of UVB to visible irradiance and the cosine of solar zenith angle. The ratio of diffuse to global radiation (D/I) in the UVB band is always greater than 0.5 and on clear days there is a linear relationship with (D/I) for the visible and full solar wave-bands, suggesting an effect of atmospheric turbidity. Such relationships may enable UVB exposures to be estimated without the need for special instrumentation.

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Messung der UVB-Sonnenstrahlung in Mittelengland

Zusammenfassung In Sutton Bonington in den East Midlands von England wurden Messungen der ultravioletten Sonnenstrahlung im B-Spektrum vorgenommen. Es wurden dabei Beziehungen zu routinemäßig gemessenen meteorologischen Variablen festgestellt.An klaren Tagen wurden lineare Beziehungen zwischen dem Logarithmus der Bestrahlung und der Luftmasse und zwischen dem Verhältnis von UVB zur sichtbaren Strahlung und dem Cosinus des Zenitwinkels der Sonne festgestellt. Das Verhältnis Himmelsstrahlung zu Globalstrahlung (D/I) im UVB-Spektrum ist immer größer als 0,5. An klaren Tagen besteht eine lineare Beziehung zu (D/I) für den sichtbaren und den vollen Wellenbereich des Sonnenlichtes, die den Eindruck einer atmosphärischen Trübung erzeugt. Solche Beziehungen machen es möglich, UVB-Bestrahlung ohne spezielle Instrumente abzuschätzen.

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With 6 Figures  相似文献   

On the absence of shear mode softening in single-crystal fayalite Fe2SiO4 at high pressure and room temperature     

S. L. Webb  I. Jackson  H. Takei 《Physics and Chemistry of Minerals》1984,11(4):167-171

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