A pulse in the delivery of ice-rafted debris at site 704 in the southeast Atlantic during glacial Termination V     

Sharon L. Kanfoush 《Marine Geophysical Researches》2016,37(1):37-48

Termination V, the transition from glacial marine isotope stage 12 to interglacial stage 11–425 ka, is the largest deglaciation of the late Pleistocene and culminated with temperatures potentially warmer than present. Coastal geomorphic and stratigraphic evidence provides estimates of a sea-level high-stand 20 m above present at the time (Hearty et al. in Geology 27(4):375–378, 1999). Such sea-level rise would require disintegration of the Greenland Ice Sheet and West Antarctic Ice Sheet as well as part of the East Antarctic Ice Sheet (Raynaud et al. in Earth’s climate and orbital eccentricity: the marine isotope stage 11 question. Geophysical monograph 137. American Geophysical Union, Washington, 2003). Lithic fragments in deep-sea sediments >150 μm at Site 704 in the South Atlantic Ocean were quantified. A large multipronged peak in concentration of this ice-rafted debris consisting of clear minerals, rose-colored transparent minerals, and ash punctuates glacial Termination V. It coincides with a brief two-pronged 1 ‰ reversal to heavier isotopic values from ~2.4 to ~3.4 ‰ at ~416 ka interpreted to reflect cooling resulting from influx of a large number of icebergs. The peak in ice-rafted debris also coincides with a 1 ‰ decrease in carbon isotopic ratios interrupting the ~2 ‰ increase in carbon isotope values across the entirety of Termination V. This is interpreted to reflect a reduction or shutdown in North Atlantic Deep Water formation and attendant Circumpolar Deep Water upwelling at the site and is also consistent with a shift in storage of carbon and carbonate from the deep sea to continental shelves resulting from a dramatic sea-level high-stand. Consequently, the lithic record at Site 704 lends support for the upper end of sea-level estimates based upon land-based evidence that requires a substantial contribution from the East Antarctic Ice Sheet. However, caution is warranted as differences with lithic records from Site 1089, 1090 and 1094 suggest sea-surface temperatures may have also affected lithic concentration through controls on iceberg trajectories and decay.  相似文献   

Spatial reconstruction of semi-quantitative precipitation fields over Africa during the nineteenth century from documentary evidence and gauge data     

Sharon E. Nicholson  Douglas Klotter  Amin K. Dezfuli 《Quaternary Research》2012,78(1):13-23

The article presents a newly created precipitation data set for the African continent and describes the methodology used in its creation. It is based on a combination of proxy data and rain gauge records. The data set is semi-quantitative, with a “wetness” index of ? 3 to + 3 to describe the quality of the rainy season. It covers the period AD 1801 to 1900 and includes data for 90 geographical regions of the continent. The results underscore a multi-decadal period of aridity early in the nineteenth century.  相似文献   

A comparative study of the molecular composition of a grassland soil with adjacent unforested and afforested moorland ecosystems     

Sharon L. Mason  Timothy R. FilleyGeoffrey D. Abbott 《Organic Geochemistry》2012,42(12):1519-1528

On-line thermally assisted hydrolysis and methylation (THM) in the presence of both unlabelled and ¹³C-labelled tetramethylammonium hydroxide (TMAH) was used to assess the relative contributions of phenolics (lignin, demethylated lignin and non-lignin phenolics) in a peaty gley soil profile beneath an unimproved grassland (LL), from a study site located at Harwood (Northumberland, northeast England, UK). This site also includes an unforested moorland (ML) and a second rotation Sitka spruce stand (SS). The common lignin proxies have been corrected for contributions of non-lignin phenols and demethylated lignin in the LL ecosystem and then compared with those from the ML and SS ecosystems. The phenolic compositions from the contributory vegetation inputs (i.e. grasses, heather and Sitka spruce) to all three soils (LL, ML and SS) were also analysed. By using ¹³C-labelled TMAH it was possible to show that the chemical composition of soil organic matter (SOM) reflected the different vegetation inputs in each of the L/F layers but these characteristics were lost from the deeper organic and mineral layers. Similar changes in the yield of lignin monomers (Λ) with increasing soil depth were displayed in the LL soil profile as reported previously in the ML soil in that no maxima were observed in these amount-depth profiles. The tannin input to the LL soil is low and as a consequence, unlike the ML and SS soils, there is no progressive decrease in the amounts of these non-lignin phenolics with increasing depth. Finally the methylated carbohydrate derivatives (MC) become more abundant relative to the phenolics with increasing soil depth in all three ecosystems (LL, ML and SS).  相似文献   

Eolian Deposition and Soil Fertility in a Prehistoric Agricultural Complex in Central Arizona,USA     

Dana K. Nakase  Anthony S. Hartshorn  Katherine A. Spielmann  Sharon J. Hall 《Geoarchaeology》2014,29(2):79-97

Prehistoric farmers in arid and semiarid ecosystems commonly used rock alignments to concentrate water and sediments on their fields. Previous research has emphasized the importance of runoff from organic matter‐rich uplands as a mechanism for soil nutrient replenishment. However, eolian inputs to these dryland ecosystems might also contribute substantially to mineral‐derived nutrient pools. We explored the relative importance of eolian deposition, prehistoric agriculture, and the presence of rock alignments on soil properties in a semiarid grassland in Arizona. Subsurface soils behind natural rock alignments are finer in texture than soils unbound by rock alignments, while subsurface soils behind agricultural rock alignments coarsen relative to unbound soils. Neither rock alignments nor estimated crop yields had detectable effects on mineral‐derived nutrient pools. In contrast, eolian deposition is an important source of soil mass and nutrients to modern soils. While dust deposition likely reduced soil heterogeneity across this landscape, it could have also contributed to the sustainability of prehistoric agriculture.  相似文献   

The signatures of voids — II. In front of the last scattering surface     

Sharon L. Vadas 《Monthly notices of the Royal Astronomical Society》1998,299(1):285-298

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A GIS-based Spatial Pattern Analysis Model for eco-region mapping and characterization     

Yingchun Zhou  Sunil Narumalani  William J. Waltman  Sharon W. Waltman  Michael A. Palecki 《International journal of geographical information science》2013,27(5):445-462

Growing concerns about global climate change, biodiversity maintenance, natural resources conservation, and long-term ecosystem sustainability have been responsible for the transformation of traditional single resource management approaches into integrated ecosystem management models. Eco-regions are large ecosystems of regional extent that contain smaller ecosystems of similar response potential and resource production capabilities. They can be used as a geographical framework for organizing and reporting resource information, setting bioecological recovery criteria, extrapolating site-level management, and monitoring global change. The objective of this research is to develop a quantitative, multivariate regionalization model that is capable of delineating eco-regions at multiple levels from remotely sensed information and other environmental and natural resources spatial data. The Spatial Pattern Analysis Model developed in this study uses a region-growing algorithm to generate spatially contiguous regions from primitive polygonal land units. The algorithm merges the most similar pair of neighbouring units at each iteration, based on satisfying certain similarity criteria until all units are grouped into one. This model was utilized to develop an eco-region map of Nebraska with three hierarchical levels. In the mapping process, the STATSGO data set was used to build the primitive map units. Environmental parameters included in the model were multi-temporal AVHRR data, soil rooting depth, organic matter content, available water capacity, and long-term annual averages of water balance and growing degree day totals. Development of the model provides a new and useful approach to eco-region mapping for resource managers and researchers. The method is automated and efficient, reduces the judgement biases and uncertainty of manual analyses, and can be replicated for other regions or for the regionalization of other themes.  相似文献   

Vegetation structure controls on snow and soil moisture in restored ponderosa pine forests     

Frances C. O'Donnell  Jonathon Donager  Temuulen Sankey  Sharon Masek Lopez  Abraham E. Springer 《水文研究》2021,35(11):e14432

Thinning of semi-arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent (SWE) and the spatial and temporal variation in soil moisture at four experimental paired treatment-control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3-dimensional lidar data to determine the relationship between vegetation structure, snow and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8–11 years before this study. However, we did not find evidence that SWE was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R² < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties.  相似文献   

The X-ray luminosity function of AGN at z∼ 3     

J. Aird  K. Nandra  A. Georgakakis  E. S. Laird  C. C. Steidel  C. Sharon 《Monthly notices of the Royal Astronomical Society》2008,387(2):883-896

We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg² to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at   z ∼ 3  . Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at   z = 1  , we find no evidence that the faint slope of the XLF flattens at high z , but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity   L _*  . Our data therefore support models of luminosity-dependent density evolution between   z = 1  and   z = 3  . A sharp upturn in the the XLF is seen at the very lowest luminosities  ( L _X≲ 10^42.5 erg s⁻¹)  , most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.  相似文献   

The climatic and physiographic controls of the eastern Mediterranean over the late Pleistocene climates in the southern Levant and its neighboring deserts   总被引：1，自引：0，他引：1  

Yehouda Enzel  Rivka Amit  Uri Dayan  Onn Crouvi  Ron Kahana  Baruch Ziv  David Sharon   《Global and Planetary Change》2008,60(3-4):165-192

Modern-day synoptic-scale eastern Mediterranean climatology provides a useful context to synthesize the diverse late Pleistocene (60–12 ka) paleohydrologic and paleoenvironmental indicators of past climatic conditions in the Levant and the deserts to its south and east. We first critically evaluate, extract, and summarize paleoenvironmental and paleohydrologic records. Then, we propose a framework of eastern Mediterranean atmospheric circulation features interacting with the morphology and location of the southeast Mediterranean coast. Together they strongly control the spatial distribution of rainfall and wind pattern. This cyclone–physiography interaction enforces the observed rainfall patterns by hampering rainfall generation south and southeast of the latitude of the north Sinai coast, currently at 31°15′.The proposed framework explains the much-increased rains in Lebanon and northern Israel and Jordan as deduced from pollen, rise and maintenance of Lake Lisan, and speleothem formation in areas currently arid and semiarid. The proposed framework also accounts for the southward and eastward transition into semiarid, arid, and hyperarid deserts as expressed in thick loess accumulation at the deserts' margins, dune migration from west to east in the Sinai and the western Negev, and the formation of hyperarid (< 80 mm yr^− 1) gypsic–salic soils in the southern Negev and Sinai. Our climatic synthesis explains the hyperarid condition in the southern Negev, located only 200–250 km south of the much-increased rains in the north, probably reflecting a steeper rainfall gradient than the present-day gradient from the wetter Levant into its bordering southern and eastern deserts.At present, the rainiest winter seasons in Lebanon and northern and central Israel are associated with more frequent (+ 20%), deeper Cyprus Lows traversing the eastern Mediterranean at approximately the latitude of southern Turkey. Even these wettest years in northern Israel do not yield above average annual rainfall amounts in the hyperarid southern Negev. This region is mainly influenced by the Active Red Sea Troughs that produce only localized rains. The eastern Mediterranean Cyprus Lows also produce more dust storms and transport higher amounts of suspended dust to the loess area than any other atmospheric pattern. Concurrent rainfall and dust are essential to the late Pleistocene formation of the elongated thick loess zone along the desert northern margin. Even with existing dust storms, the lack of rain and very sparse vegetation account for the absence of late Pleistocene loess sequences from the southern Negev and the formation of hyperarid soils.When the north Sinai coast shifted 30–70 km northwest due to last glacial global sea level lowering, the newly exposed coastal areas supplied the sand and dust to these active eastern Mediterranean cyclones. This enforced the latitude of the northern boundary of the loess zone to be directly due east of the LGM shoreline. This shift of coast to the northwest inhibited rainfall in the southern Levant deserts and maintained their hyperaridity. Concurrently, frequent deep eastern Mediterranean Cyprus Lows were funneled along the northern Mediterranean increasing (probably doubling) the rains in central and northern Israel, Lebanon, southwestern Syria and northern Jordan. These storms and rains formed lakes, forests, and speleothems only a short distance north of the deserts in the southern Levant.  相似文献   

Submarine eruption-fed and resedimented pumice-rich facies: the Dogashima Formation (Izu Peninsula,Japan)     

Martin Jutzeler  Jocelyn McPhie  Sharon R. Allen 《Bulletin of Volcanology》2014,76(10):1-29

In the Izu Peninsula (Japan), the Pliocene pumice-rich Dogashima Formation (4.55?±?0.87 Ma) displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave-base marine setting. It includes high-concentration density current deposits that contain clasts that were emplaced hot, indicating an eruption-fed origin. The lower part of the Dogashima 2 unit consists of a very thick sequence (<12 m) of massive grey andesite breccia restricted to the base of a submarine channel, gradationally overlain by pumice breccia, which is widespread but much thinner and finer in the overbank setting. These two breccias share similar mineralogy and crystal composition and are considered to be co-magmatic and derived from the destruction of a submarine dome by an explosive, pumice-forming eruption. The two breccias were deposited from a single, explosive eruption-fed, sustained, sea floor-hugging, water-supported, high-concentration density current in which the clasts were sorted according to their density. At the rim of the channel, localised good hydraulic sorting of clasts and stratification in the pumice breccia are interpreted to reflect local current expansion and unsteadiness rather than to be the result of hydraulic sorting of clasts during fall from a submarine eruption column and/or umbrella plume. A bimodal coarse (>1 m) pumice- and ash-rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension. In Dogashima 1 and 2, thick cross- and planar-bedded facies composed of sub-rounded pumice clasts are intercalated with eruption-fed facies, implying inter-eruptive mass-wasting on the flank of a submarine volcano, and reworking and resedimentation by high-energy tractional currents in a below wave-base environment.  相似文献