Climatic significance of natric horizons in Permian (Asselian) palaeosols of north-central Kansas, USA     

THOMAS J. McCAHON  KEITH B. MILLER 《Sedimentology》1997,44(1):113-125

Columnar structured horizons have been recognized in ancient coastal palaeosols of several Lower Permian (Asselian) stratigraphic units of north-central Kansas. These strongly developed columnar, polygonal-shaped peds are characteristic of sodium-influenced (natric) argillic horizons, and are commonly indicative of semi-arid to arid environments. Evaporite features above and below these palaeosols support the conclusion for a dry palaeoclimate. The columnar peds are typically 3–15 cm in diameter and exhibit domed tops. Fine clay fills the cracks between the columnar peds, and is generally of a darker colour than the peds. Each natric horizon has a low value and chroma colour, apparently the result of carbonate accumulation. The natric horizons in these Permian palaeosols appear to have been partially influenced by sodium-rich groundwaters. Root traces and root moulds are found between peds in all natric horizons, indicating plant succession after columnar ped formation. These sodium-influenced palaeosol profiles occur as part of a spectrum of palaeosol types that indicate cyclical climate change associated with glacioeustatic sea-level fluctuations.  相似文献   

Nd, Pb and Sr Isotopic Compositions of East African Carbonatites: Evidence for Mantle Mixing and Plume Inhomogeneity   总被引：14，自引：8，他引：14  

BELL  KEITH; TILTON  G. R. 《Journal of Petrology》2001,42(10):1927-1945

New Pb isotopic data are presented for 10 young Mesozoic toCenozoic (0–116 Ma) carbonatites from a 1400 km long segmentof the East African Rift. Patterns observed in Pb vs Pb, Srvs Pb and Nd vs Pb isotope diagrams define unusual, nearly linear,trends that are interpreted as mixing between two componentsthat are broadly similar to the two mantle end-member components,HIMU and EM1, which were first recognized from ocean-islandbasalts. The two plutons with isotope signatures closest toHIMU and EM1 crop out within 140 km of each other. From thesedata, EM1 and HIMU are now known to occur in both continentaland oceanic settings that are associated with plumes or rifts.Moreover, these isotopic signatures tend to occur in regionswhere seismic tomography indicates prominent low-velocity zonesin the lower mantle. For these reasons, we favour a model forthe origin of the East African Rift carbonatites that involvesmelting and mixing of HIMU and EM1 components contained withinan isotopically heterogeneous mantle plume. We consider theHIMU and EM1 sources to be stored within the deep (lower 1000km) mantle, possibly the core–mantle boundary. The rolethat continental lithosphere plays in carbonatite generationis probably one of concentrating volatiles at the upper levelsof an ascending mantle plume. KEY WORDS: carbonatites; isotopes; rifts; plumes; FOZO  相似文献   

Petrology of the Great Abitibi Dyke, Superior Province, Canada     

ERNST  RICHARD E.; BELL  KEITH 《Journal of Petrology》1992,33(2):423-469

The Great Abitibi Dyke (GAD) which can be traced northeast,for >> 700 km, across the Abitibi Belt in the southeasternSuperior Province of the Canadian Shield, is composed of olivinegabbro to monzodiorite, weakly saturated to undersaturated insilica. All rocks of the GAD can be derived by mainly plagioclase andolivine fractionation from a parental magma corresponding incomposition to chilled margin samples. Two units can be distinguished,a marginal unit (Unit 1) representing 0–50% crystallizationand a central unit (Unit 2), found over about half of the dykelength, representing 50–70% crystallization. Modelling,using Pearce elemental ratio analysis, quantifies the fractionationhistory and allows mass–balance calculations over thepresent exposure level of the dyke. The approximate balancebetween the amounts of cumulate and fractionated rocks suggeststhat Unit 1 differentiated essentially in situ as a closed system.In contrast, Unit 2 rocks were formed by loss of substantialplagioclase and olivine from the parental magma. This fractionatemust have either been lost to depth or left behind in an externalchamber. Unit 2 rocks show depletion in plagioclase and enrichmentin mafic minerals along strike towards the southwest (deeperexposure level?), a trend explained by density stratification.Regional variation in Unit 1 chemistry is interpreted in termsof lateral magma injection towards the northeast from the locusof Keweenawan rift volcanism. Feldspar, olivine, and augitecompositions are linearly correlated with equilibrium temperatureand extent of magma evolution. The parent magma had a trace element chemistry correspondingto a ‘within–plate’ setting and was probablyderived from an incompatible–element enriched mantle similarto the source for ocean island basalts (OIBs).  相似文献   

Nitrogen fixation amplifies the ocean biogeochemical response to decadal timescale variations in mineral dust deposition   总被引：1，自引：0，他引：1  

J. KEITH MOORE  SCOTT C. DONEY  KEITH LINDSAY  NATALIE MAHOWALD  ANTHONY F. MICHAELS 《Tellus. Series B, Chemical and physical meteorology》2006,58(5):560-572

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Coherent slumping of early postglacial lake sediments at Hall Lake, Ontario, Canada     

KEITH DAVID BENNETT 《Boreas: An International Journal of Quaternary Research》1986,15(3):209-215

Cores of organic postglacial sediment from Hall Lake, southern Ontario, Canada, are described. The laminated, calcareous sediments deposited between 9,000 and 8,000 B.P. have coherently slumped into the deep part of the lake, causing overturning and replication of parts of the sequence. This process may be considered as a mechanism for sediment focusing, since it involves reworking of marginal sediments into deeper areas. Unlike previously described mechanisms, it may be a source of error in the interpretation of diagrams of pollen percentages as well as accumulation rates. The slumping has only been recognized through the distortion of the laminae. It might not be possible to recognize slumping in unstratified sequences.  相似文献   

Convection forced by a descending dry layer and low-level moist convergence     

ANDREW RUSSELL  GERAINT VAUGHAN  EMILY G. NORTON  HUGO M. A. RICKETTS  CYRIL J. MORCRETTE  TIM J. HEWISON  KEITH. A. BROWNING  ALAN M. BLYTH 《地球，A辑:动力气象学与海洋学》2009,61(2):250-263

A narrow line of convective showers was observed over southern England on 18 July 2005 during the Convective Storm Initiation Project (CSIP). The showers formed behind a cold front (CF), beneath two apparently descending dry layers (i.e. sloping so that they descended relative to the instruments observing them). The lowermost dry layer was associated with a tropopause fold from a depression, which formed 2 d earlier from a breaking Rossby wave, located northwest of the UK. The uppermost dry layer had fragmented from the original streamer due to rotation around the depression (This rotation was also responsible for the observations of apparent descent—ascent would otherwise be seen behind a CF). The lowermost dry layer descended over the UK and overran higher  θ _w   air beneath it, resulting in potential instability. Combined with a surface convergence line (which triggered the convection but had less impact on the convective available potential energy than the potential instability), convection was forced up to 5.5 km where the uppermost dry layer capped it. The period when convection was possible was very short, thus explaining the narrowness of the shower band. Convective Storm Initiation Project observations and model data are presented to illustrate the unique processes in this case.  相似文献   

Vertical profiles of biospheric and fossil fuel-derived CO₂ and fossil fuel CO₂ : CO ratios from airborne measurements of Δ¹⁴C, CO₂ and CO above Colorado, USA     

By HEATHER D. GRAVEN †  BRITTON B. STEPHENS  THOMAS P. GUILDERSON  TERESA L. CAMPOS  DAVID S. SCHIMEL  J. ELLIOTT CAMPBELL  RALPH F. KEELING 《Tellus. Series B, Chemical and physical meteorology》2009,61(3):536-546

Measurements of  Δ¹⁴C  in atmospheric CO₂ are an effective method of separating CO₂ additions from fossil fuel and biospheric sources or sinks of CO₂. We illustrate this technique with vertical profiles of CO₂ and  Δ¹⁴C  analysed in whole air flask samples collected above Colorado, USA in May and July 2004. Comparison of lower tropospheric composition to cleaner air at higher altitudes (>5 km) revealed considerable additions from respiration in the morning in both urban and rural locations. Afternoon concentrations were mainly governed by fossil fuel emissions and boundary layer depth, also showing net biospheric CO₂ uptake in some cases. We estimate local industrial CO₂:CO emission ratios using in situ measurements of CO concentration. Ratios are found to vary by 100% and average 57 mole CO₂:1 mole CO, higher than expected from emissions inventories. Uncertainty in CO₂ from different sources was ±1.1 to ±4.1 ppm for addition or uptake of −4.6 to 55.8 ppm, limited by  Δ¹⁴C  measurement precision and uncertainty in background  Δ¹⁴C  and CO₂ levels.  相似文献   

Dutch Migrants in the Economy of New Zealand     

KEITH W. THOMSON 《New Zealand geographer》1967,23(2):95-105

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Comparison of sources and nature of the tropical aerosol with the summer high Arctic aerosol   总被引：1，自引：0，他引：1  

CAROLINE LECK  E. KEITH BIGG † 《Tellus. Series B, Chemical and physical meteorology》2008,60(1):118-126

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Tailor–made geology 1: General introduction and Salthill Quarry, Lancashire     

KEITH DUFF 《Geology Today》1985,1(2):55-57

The protection and long-term safeguarding of geological localities are fundamental to the continuation of fieldwork, both now and in the future. The responsibility for these lies with the Nature Conservancy Council, the government agency charged with protecting geological and biological sites. Part of the work of the Council's Geology Branch is concerned with creating or re-exposing geological localities which are of value for teaching or collecting; and in the article which follows, Keith Duff explains the background and methods of this work, using the recently established geological trail at Salthill Quarry in Lancashire as as example. This is the first of a series of articles which will give details of British sites developed in this way.  相似文献