Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes   总被引：1，自引：0，他引：1  

Robert C Aller  Neal E Blair 《Geochimica et cosmochimica acta》2004,68(8):1815-1825

Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr⁻¹ particulate terrestrial organic carbon with initial particle C_org loading ∼0.7 mg m⁻². C_org loading is reduced to ∼0.3 mg m⁻² in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1-4 cm yr⁻¹. Carbon isotopic analyses (δ¹³C, Δ¹⁴C) of ΣCO₂ and C_org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ¹³C <−28.6) and marine C_org (δ¹³C ∼−20.5) ranging in average age from modern (bomb) (Δ¹⁴C ∼60) to ∼1000 yr (Δ¹⁴C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C_org often preferentially dominates pore water ΣCO₂ relative to bulk C_org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO₂ reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C_org from particles in the topset-upper foreset zone (i.e. recycled marine C_org leaves little residue). Preferential utilization of C_org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C_org composition can produce changes in ¹⁴C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ¹⁴C of pore water ΣCO₂ averaged ∼ 300‰ greater than C_org sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C_org compromises simple application of ¹⁴C for determination of sediment accumulation rates in diagenetically reactive deposits.  相似文献   

Internal structure of mixed-sand-and-gravel beach deposits revealed using ground-penetrating radar   总被引：4，自引：1，他引：4  

Adrian Neal  Nigel I. Pontee†  Ken Pye‡  Julie Richards‡ 《Sedimentology》2002,49(4):789-804

ABSTRACT Mixed‐sand‐and‐gravel beaches are a distinctive type of coarse‐clastic beach. Ground‐penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed‐beach deposits at Aldeburgh in Suffolk, south‐east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward‐dipping bounding surfaces that are gently onlapped by seaward‐dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach‐profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm‐wave activity. Beach‐ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm‐ridge, overtop and overwash deposits. Relict berm‐ridge deposits, separated by seaward‐dipping bounding surfaces, form the main depositional element beneath the beach‐ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm‐ridge deposits. Consequently, beach‐ridge development in this progradational, mixed‐beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.  相似文献   

内蒙古乌拉山地区大桦背岩体SHRIMP锆石U—Pb年代学研究   总被引：12，自引：0，他引：12       下载免费PDF全文

苗来成  Yumin QIU  关康  Neal McNAUGHTON  裘有守  罗镇宽  David GROVES 《地质论评》2001,47(2):169-174

大桦背花岗岩体位于内蒙古乌拉山地区,东距哈达门沟大型钾长石—石英金矿床约5km。前人对该岩体曾做过不少同位素年代学研究,所得结果相差较大,但是多数研究者认为大桦背岩体与区内金矿化具有成因联系。本文采用先进的高灵敏、高分辨率的离子探针(SHRIMP)对大桦背岩体进行了锆石U-Ph定点定年研究。结果表明,大桦背岩体的形成年龄为353±7Ma,应为海西旋回早期产物。此年龄比已得到的乌拉山金矿区成矿年龄大至少70Ma,因而推测大桦背岩体与金矿化之间不存在直接的成因联系。  相似文献   

Archaean Au−Ag mineralisation at Racetrack,near Kalgoorlie,Western Australia: a high crustal-level expression of the Archaean composite lode-gold system     

M. Gebre-Mariam  D. I. Groves  N. J. McNaughton  E. J. Mikucki  J. R. Vearncombe 《Mineralium Deposita》1993,28(6):375-387

The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO₂ content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, f_{O 2} between 10^−38.8 and 10^−39.6 bars, and m_Σs between 10^−3.2 and 10^−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.  相似文献   

The source of lead in Archaean lode gold deposits of the Menzies-Kalgoorlie-Kambalda region,Yilgarn Block,Western Australia     

N. J. McNaughton  D. I. Groves  W. K. Witt 《Mineralium Deposita》1993,28(6):495-502

The Archaean greenstone terrane between Menzies and Kambalda exhibits a coherent, although deformed, stratigraphic sequence intruded by granitoids and bounded by major NNW-trending shear and/or fault zones. The greenstone terrane hosts a large number of lode gold prospects and deposits, including the giant Kalgoorlie deposits. The initial Pb isotope compositions of lode gold deposits, as determined from ore related galena and pyrite, vary systematically in a linear trend on a²⁰⁷Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb diagram which reflects crustal heterogeneity at the time of mineralisation. Deposits hosted within a 90 km section of the Menzies-Boorara Shear Zone have a uniform, radiogenic initial Pb isotope composition irrespective of temperature of mineralisation and proximity to granitoid-gneiss in plan view. The Pb in these deposits is considered to be derived largely from older felsic crust underlying the greenstone belt and was accessed via this major shear-zone system. Deposits in a transect unrelated to a major shear zone show a systematic correlation between initial Pb isotope compositions and proximity to granitoid-gneiss and/or to mineralisation temperature. These compositions are less radiogenic than those within the Menzies-Boorara Shear Zone, but trend on a²⁰⁷Pb/²⁰⁴Pb versus⁶⁰⁶Pb/²⁰⁴Pb diagram between this isotope signature and the uniform Pb isotope signature which characterises the >100 km greenstone transect from the Mt Pleasant area through Kalgoorlie to Kambalda. These data are interpreted to reflect Pb derivation from discrete crustal segments within and below the greenstones, and require that mineralisation was related to crustal-scale hydrothermal systems that accessed both sialic mid- to lower-crust and the greenstone succession.  相似文献   

Sea Level change: Environmental and socio-economic impacts     

H. Jesse Walker 《GeoJournal》1992,26(4):511-520

The coast was among the first of the earth’s environments to be subjected to human modification; modification that was both direct and indirect, as it is today. During the last 4.000 to 5.000 years, human impacts on the coast were at what was basically a stable sea level. Because many of the modifications were made to protect against unusual events such as typhoons and tsunami, humans developed a variety of techniques for protecting shorelines. Recent calculations that a rapid sea level rise (in response to greenhouse warming) is imminent, has prompted concerned efforts at meeting the socio-economic impacts that are likely to occur. These impacts, rather than being localized and temporary as has been true in the recent past, are likely to be universal and long-lasting. This thought, penned by Blaise Pascal, was accompanied by an artistic interpretation by Corita Kent inNewsweek, June 14,1976, p. 87.  相似文献   

The age and origin of younger granitic plutons of the Shaw Batholith in the Archaean Pilbara Block,Western Australia   总被引：1，自引：0，他引：1  

M. J. Bickle  L. F. Bettenay  H. J. Chapman  D. I. Groves  N. J. McNaughton  I. H. Campbell  J. R. de Laeter 《Contributions to Mineralogy and Petrology》1989,101(3):361-376

The whole-rock Pb-Pb method has been used to date four of the younger, mainly adamellite, late-tectonic plutonic phases within the ca. 3.5 Ga Shaw Batholith of the Archaean east Pilbara Block. Three suites give ages within error of 2966 Ma (Porphyritic Granites at 2948±50 Ma, Leuco-adamellites at 2943±46 Ma and Garden Creek Adamellite at 3007±48 Ma). The post-tectonic Cooglegong Adamellite gives an age of 2847±34 Ma. The Sm-Nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca. 3200 and 3600 Ma in age. The sources for these four younger plutonic phases were heterogeneous and, although exhibiting some isotopic characteristics of the older (3.5–3.3 Ga) calc-alkali plutonic suites, were more depleted in the LIL elements Rb, U and Th. In addition, the Garden Creek Adamellite and the Cooglegong Adamellite lack the very fractionated and HREE-depleted REE patterns characteristic of both the older calc-alkali plutonic rocks and the Porphyritic Granites and Leuco-adamellites. The crust underlying the Shaw Batholith at ca. 2950 Ma must have been both markedly heterogeneous and variably depleted, a conclusion consistent with the complex tectonic and plutonic evolution of this region.  相似文献   

Hydrogen generation from mantle source rocks in Oman     

C. Neal  G. Stanger 《Earth and Planetary Science Letters》1983

Hydrogen gas, associated with Ca²⁺ OH-rich alkaline groundwaters (pH = 10–12), is at present emanating from ultramafic rocks of the Oman ophiolite. Isotopic and chemical evidence indicates that hydrogen is formed by low-temperature redox reactions in a closed groundwater environment. This is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in Oman where atmospheric oxygen is totally assimilated. It is suggested that hydrogen generation in mantle source rocks at depth and in the early Archaean surface environment may be more widespread than has hitherto been realised.  相似文献   

An experimental appraisal of the terms in the heat and moisture flux equations for local advection     

A. R. G. Lang  K. G. McNaughton  Chen Fazu  E. F. Bradley  E. Ohtaki 《Boundary-Layer Meteorology》1983,25(1):89-102

Terms in the heat and vapour flux equations, appropriate to the atmospheric surface layer with horizontal heterogeneity in one direction, have been evaluated empirically. The experimental site was a flooded rice field, which was bounded to windward by a semi-arid region. Local conditions over the rice were always stable, but the vertical fluxes of heat and water vapour were large. All terms in the flux equations were either measured directly, or, if sufficiently small, estimated, except the term containing fluctuating pressure, which was obtained by difference. The relative magnitudes of the major terms (production and pressure covariance) were similar to those reported previously for horizontally homogeneous flow with similar stabilities. Current parameterizations of the pressure covariance terms sometimes gave values which differed by factors of five or ten from the experimental results, and the interrelationships depended upon the stability.  相似文献   

Can we assess the impacts of climatic changes?     

Jesse H. Ausubel 《Climatic change》1983,5(1):7-14

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