Chemical characterization of microbial-dominated soil organic matter in the Garwood Valley, Antarctica     

Xiaojuan Feng  André J. Simpson  Bo Elberling  David W. Hopkins  Ashley D. Sparrow  Lawrence G. Greenfield 《Geochimica et cosmochimica acta》2010,74(22):6485-6498

Despite its harsh environmental conditions, terrestrial Antarctica contains a relatively large microbial biomass. Natural abundance carbon and nitrogen stable isotope signatures of organic materials in the dry valleys indicate mixed provenance of the soil organic matter (SOM) with varying proportions of contributions from lichens, mosses, lake-derived algae and cyanobacteria. Here we employed two complementary analytical techniques, biomarker measurements by gas chromatography/mass spectrometry and solution-state ¹H nuclear magnetic resonance spectroscopy, to provide further information at a molecular-level about the composition and possible source of SOM in the Garwood Valley, Antarctica. The predominance of branched alkanes and short-chain lipids in the solvent extracts indicates that the primary contribution to the SOM was microbial-derived. Chemical structures in the NaOH extracts from soils were also dominated by amide, peptides, and a CH₃-dominating aliphatic region that were characteristic of microbial signatures. Furthermore, the SOM in the Garwood Valley contained compounds that were different from those in the cyanobacteria-dominated mat from a nearby lake (including monoethyl alkanes and enriched side-chain protons). This observation suggests that easily degradable carbon sources from the nearby lake did not dominate the SOM, which is consistent with a fast turnover of the mat-derived organic matter found in the valley. This study highlights the important role of native soil microbes in the carbon transformation and biogeochemistry in terrestrial Antarctica.  相似文献   

The evolution of the near-infrared galaxy luminosity function and colour bimodality up to z≃ 2 from the UKIDSS Ultra Deep Survey Early Data Release     

M. Cirasuolo  R. J. McLure  J. S. Dunlop  O. Almaini  S. Foucaud  Ian Smail  K. Sekiguchi  C. Simpson  S. Eales  S. Dye  M. G. Watson  M. J. Page  P. Hirst 《Monthly notices of the Royal Astronomical Society》2007,380(2):585-595

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The braided Milk River, northern Montana, fails the Leopold–Wolman discharge-gradient test     

Chris J. Simpson  Derald G. Smith 《Geomorphology》2001,41(4):2900

The Milk River, the northernmost tributary to the Missouri–Mississippi River system, exhibits an anomalous sand-bed braiding reach in an otherwise meandering system. Shortly after leaving Alberta and entering Montana the river suddenly changes to braiding and maintains this pattern for 47 km before entering Fresno Reservoir. Measured stream gradient and bankfull discharge in the braiding reach severely fail the Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] slope–discharge test for differentiating channel patterns. While channel slope has long been regarded as one of the primary variables associated with braiding, our data from the sand-bed Milk River do not support this relationship. Instead, the data show that the braiding reach has a lower channel slope (0.00047) than the meandering reach (0.00055). Coupled with a constant discharge the unit length stream power is comparable between the two reaches. At the morphologic transition between meandering and braiding, a dramatic reduction in channel bank strength occurs where the sampled silt–clay content declines from 65% in the meandering reach to 18% in the braiding. This enables channel widening which is reflected in a 60% reduction in unit area stream power in the braiding reach. Thus, sediment transport capacity declines and channel bars are deposited. During waning flows, these bars are dissected, producing a braiding morphology. We suggest that for sand-bed braiding rivers the silt–clay percentage in the channel banks may be more important than slope. A review of the original Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] dataset, and many subsequent analyses, reveals that most braided rivers studied were gravel-bed. As a result, causal variables associated with braiding in sand-bed environments may need a thorough evaluation.  相似文献   

Abundance of marine resources in relation to dissolved oxygen in Long Island Sound     

Penelope Howell  David Simpson 《Estuaries and Coasts》1994,17(2):394-402

This study was designed to examine effects of low dissolved oxygen on finfish, lobster, and squid under field conditions in western Long Island Sound. The relationship between bottom dissolved oxygen (DO) and catch was examined for effects on abundance, numbers of species, and mean length, for trawl sites throughout the sound. Examination of mean catch per tow, and species number per tow, showed that both abundance and diversity decreased markedly with bottom DO, with dramatic declines at sites with DO<2 mg l^?1. Of 18 species examined, 15 were found to occur with greater frequency at sites with DO>3 mg l^?1 compared to sites with <2 mg l^?1, and three of these species occurred significantly less frequency at a DO of 2–3 mg l^?1. Trawl samples taken in the Narrows west of Greenwich, an area which chronically experiences summer hypoxia (DO<3 mg l^?1) consistently yielded below average species number during hypoxic events. However, when DO was>3 mg l^?1, 13 of 14 tows had above average species number. Abundance of five common species at sites>3 mg l^?1 DO were significantly higher than at sites <3 mg l^?1 DO in this area. Lobster was the only species not showing this DO threshold. Patterns in abundance vs DO for squid, bluefish, and butterfish suggest that these species are among the most sensitive to hypoxia. Only one of the four examined species, winter flounder, showed a decrease in mean length with DO. Scup, bluefish, and lobster lengths did not vary significantly with respect to dissolved oxygen.  相似文献   

Seismicity changes associated with reservoir loading   总被引：12，自引：0，他引：12  

David W. Simpson 《Engineering Geology》1976,10(2-4):123-150

Changes in seismic activity have been related to the filling of large reservoirs in over thirty cases. These changes range from variations in the level of micro-earthquake activity detectable only with instruments of high sensitivity to destructive earthquakes with magnitudes greater than 6. On the other hand, the filling of many other large reservoirs has not been accompanied by increased seismicity.

A number of factors may contribute to the generation or absence of post-impounding seismicity. Increased vertical stress due to the load of the reservoir and decreased effective stress due to increased pore pressure can modify the stress regime in the reservoir region. Whether or not these stress changes are sufficient to generate earthquake activity will depend on a complex interaction of the induced stress with the state of pre-existing stress near the reservoir, and on the geologic and hydrologic conditions at the site. The combined effect of increased vertical load and increased pore pressure will have the greatest tendency to increase activity in regions where the maximum compressive stress is vertical (normal faulting). In regions where the minimum compressive stress is vertical (thrust faulting) increased stress due to a vertical load should have a minimum effect. For all of the larger reservoir-induced earthquakes the stress system determined from fault plane solutions is in agreement with the pre-existing stress field in the region of the reservoir. These earthquakes are all of strike-slip or normal type, there being no reported cases of large induced earthquakes with thrusting mechanisms.

The potential for major changes in seismicity may be highest in regions of moderate strain accumulation (low to moderate natural seismicity). In areas of high strain accumulation and high levels of natural seismicity, the stress changes induced by the reservoir will be small compared to natural variations. In aseismic areas, with low strain accumulation, the reservoir-induced stresses may be insufficient to raise the stress level to a state of failure.  相似文献   

The composition of spherules and other features on shatter cone surfaces from the Vredefort structure,South Africa     

N.C. Gay  N.R. Comins  C. Simpson 《Earth and Planetary Science Letters》1978,41(3):372-380

Minute spherules and other glassy objects have been found on the surfaces of shatter cones in the sedimentary rocks forming the collar around the Vredefort ring structure. The composition of twenty-five of these spherules was determined using an energy-dispersive X-ray analysis system attached to a scanning electron microscope. These results show that spherules can be classified into a Si-, Al-rich variety, an Fe-rich variety and an intermediate variety.At first, the origin of the spherules appeared enigmatic: we considered that they could have been of organic origin; that they may represent dust or fly ash particles due to atmospheric pollution; or that they may result from processes operating during shatter cone formation. For analytical and logical reasons, we argue that the first two possible modes of origin can be discounted. Instead, it is suggested that the spherules result from melts generated by friction on the host rock surfaces during the formation of shatter cones. The variations in composition of the individual spherules may indicate selective melting of different parts of the rock surface or that the melts may have been affected by liquid immiscibility processes. This argument is reinforced by the striking analogy between the particles discussed here and those found in lunar samples.  相似文献   

Strain and shape-fabric variations associated with ductile shear zones     

Carol Simpson 《Journal of Structural Geology》1983,5(1):61-72

The foliated and compositionally-banded granitic orthogneisses in the central core of the Maggia Nappe, a Lower Pennine basement nappe of the Central Swiss Alps, are shown to be the sheared equivalent of late-Hercynian age granitic intrusions. These ductile shear zones show mineral assemblages in amphibolite facies, are Alpine in age and form an anastomosing network enclosing remnant lozenge-shaped pods of relatively undeformed rock.The foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional, through ‘tear-drop’ shapes, to ribbon-like aggregates. These shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation-induced planar surfaces.  相似文献   

Combining Sample and Census Data in Small Area Estimates: Iterative Proportional Fitting with Standard Software*   总被引：1，自引：0，他引：1  

Ludi Simpson  Mark Tranmer 《The Professional geographer》2005,57(2):222-234

The combination of detailed sample data with less detailed but fully enumerated marginal subtotals is the focus of a wide range of research. In this article we advocate careful modeling of sample data, followed by Iterative Proportional Fitting (IPF). The modeling aims to estimate accurately the interaction or odds ratios of complex tables, which is information not contained in the marginal subtotals. IPF ensures consistency with the subtotals. We advance this work in three practical ways. First, we show that detailed small‐area estimates of both counts and proportional distributions usually gain accuracy by combining data for larger areas containing the small areas, and we illustrate the multilevel framework to achieve these estimates. Second, we find that a general classification or socioeconomic typology of the small areas is even more associated with the within‐area interactions than is membership of the larger area. Third, we show how the Statistical Package for the Social Sciences (SPSS) can be used for IPF in any number of dimensions and with any structure of constraining marginal subtotals. Throughout, we use an example taken from the 1991 U.K. Census. These data allow us to evaluate various methods combining 100 percent tabulations and the Samples of Anonymised Records.  相似文献   

Role of the Land Plume in the Transport of Ozone Over the Ocean During Indoex (1999)     

M. D. Simpson  S. Raman 《Boundary-Layer Meteorology》2004,111(1):133-152

Observations from aircraft, an island station, and tworesearch vessels are used to investigate the development of an elevated mixed layeror land plume over the Arabian sea during the Indian Ocean Experiment Intensive FieldPhase 1999 (INDOEX) through air mass modification. Much of the transport of aerosolsand gases occurs in this plume located above a well-mixed convective marine boundary layerwith a depth of 800–1000 m. The depth of the land plume isapproximately 2000 m with the peak ozone concentrations occurring near the centre of this land plume.Significant latitudinal variations in the concentration of ozone occur in the marineboundary layer and in the plume. Mean ozone concentrations in the land plume decreasedwith distance from the Indian coastline.  相似文献   

Development and propagation of a pollution gradient in the marine boundary layer during INDOEX (1999)     

Matthew Simpson  Sethu Raman 《Journal of Earth System Science》2005,114(1):3-16

The development and propagation of a pollution gradient in the marine boundary layer over the Arabian Sea during the Intensive Field Phase of the Indian Ocean Experiment (1999) is investigated. A hypothesis for the generation of the pollution gradient is presented. Infrared satellite images show the formation of the pollution gradient as the leading edge of a polluted air mass in the marine boundary layer and also its propagation over the Arabian Sea and the northern Indian Ocean. Aerosol data measured from two research vessels over the Arabian Sea show a variation in the concentrations caused by the passage of this pollution gradient. Depth of the pollution gradient was found to be about 800 m. A numerical model was used to simulate the development of this gradient and its propagation over the ocean. Results show that its formation and structure are significantly influenced by the diurnal cycle of coastal sea-land breeze circulations along India’s west coast. Transport of aerosols and gases over the Arabian Sea in the lower troposphere from land sources appears to be through this mechanism with the other being the elevated land plume.  相似文献