Current controlled sediment deposition from the shelf to the deep ocean: the cenozoic evolution of circulation through the SW pacific gateway     

R. M. Carter  L. Carter  I. N. McCave 《International Journal of Earth Sciences》1996,85(3):438-451

 The circulation of cold, deep water is one of the controlling factors of the Earth's climate. Forty percent of this water enters the world ocean through the Southwest Pacific as a deep western boundary current (DWBC) flowing northwards at bathyal to abyssal depths, east of the New Zealand microcontinent. South of latitude 50°S, the DWBC is intimately linked with the Antarctic circumpolar current (ACC), which is the prominent force for the shallow-water circulation. The Pacific DWBC is presently the largest single contributor of deep ocean water, and deciphering its evolution is of fundamental importance to understanding ocean and climate history, and global ocean hydrography. The evolution of the DWBC system, and of related circum-Antarctic currents, has taken place since 30–25 Ma when plate movements created the first oceanic gaps south of Australia and South America. The stratigraphic record preserved in sediment drifts of the Southwest Pacific, in eastern New Zealand, is the best available for deciphering the Neogene history of Southern Ocean water masses, and of the circulation of the ACC, DWBC and their precursor systems. Major current activity commenced on the New Zealand margin in the late Eocene or early Oligocene (Hoiho Drift; early ACC) and was widespread by the mid-late Oligocene (Marshall Paraconformity and Weka Pass Limestone drift; ACC). During the Neogene the eastern South Island continental shelf built seawards by accretion at its outer edge of large Miocene current drifts up to tens of kilometres long and hundreds of metres thick (Canterbury drifts). Also commencing in the mid-Cenozoic, but in depths >2000 m, the DWBC emplaced large deep-water sediment drifts. Rates of drift deposition accelerated considerably in the late Neogene, when climatic change (and particularly glacial sea-level falls) caused the delivery of large volumes of turbiditic sediment into the path of the DWBC via the Bounty and Hikurangi channels. Received: 9 August 1995 / Accepted: 15 January 1996  相似文献   

Determination of Pb/Pb ages on zircon and monazite by laser-ablation ICPMS and application to a study of sedimentary provenance and metamorphism in southeastern Brazil     

N. Machado  G. Gauthier 《Geochimica et cosmochimica acta》1996,60(24):5063-5073

The recently developed method of laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) allows the determination of ²⁰⁷Pb/²⁰⁶Pb ages of single zircon grains. The main advantages of the method are minimal sample preparation, low cost, and high throughput. In this work we present an analytical routine for geochronological analyses of zircon and monazite by LA-ICPMS and its application to the Ribeira Belt of the Brazilian Orogen in southeastern Brazil. The ²⁰⁷Pb/²⁰⁶Pb ages of one hundred and thirty-seven detrital zircons from amphibolite facies quartzites from three lithotectonic domains in the central Ribeira Belt indicate that they are derived mainly from Paleoproterozoic crust of Transamazonian age (2.0−2.3 Ga). A small number of zircons originated in 2.6−2.9 Ga Archean crust. These results are coherent with 2.1−2.2 Ga and 2.6−3.0 Ga U---Pb ages previously obtained for basement gneisses. The viability of the method to date monazite is also assessed. Monazites from the same quartzite samples yield ages between 2.1 Ga and 0.57 Ga. indicating variable resetting of the U---Pb system during amphibolite facies metamorphism. In contrast, monazite from a basement migmatite and syn-metamorphic granitoids yields ages in the 500–700 Ma range, in general agreement with U-Pb ages of 590-565 Ma for the main metamorphic event.

The LA-ICPMS ²⁰⁷Pb/²⁰⁶Pb ages are coherent and agree with expected results based on previous U---Pb geochronology, and show that the method has immediate applicability. At present, the most significant limitations of the method are the inability to yield reliable U/Pb values, analytical precision in the 1–10% range, and the requirement of grains larger than 80 gmm The method may be advantageous for provenance studies of Precambrian detrital sequences.  相似文献   

Calculating surface water PCO₂ from foraminiferal organic δC     

M.A. Maslin  M.A. Hall  N.J. Shackleton  E. Thomas 《Geochimica et cosmochimica acta》1996,60(24):5089-5100

The δ¹³C of organic matter bound within the crystal lattice of foraminiferal calcite tests may provide a potential tracer of the isotopic composition of the surface water primary photosynthate. Using δ¹³C of the organic matter extracted from the crystal lattice and the calcite test, it is theoretically possible to estimate the paleo-surface water pCO₂. We have tailored this technique initially for the subpolar planktonic foraminifera species Globigerina bulloides. Initial surface water pCO₂ estimates from deep-sea core BOFS 5K (50°41.3′N, 21°51.9′W, water depth 3547 m) indicate that the northeast Atlantic Ocean may have been a greater sink for CO₂ during the last glacial than during the Holocene. Greatly reduced benthic foraminifera abundances, especially phytodetritus feeders, in BOFS 5K during the last glacial indicates low surface productivity. This rules out a productivity-driven CO₂ sink. The enhanced glacial CO₂ sink must, therefore, have results from a southwards shift of the centre of deep water formation.  相似文献   

Sedimentary model and high-frequency cyclicity in a Mediterranean, shallow-shelf, temperate-carbonate environment (uppermost Miocene, Agua Amarga Basin, Southern Spain)   总被引：1，自引：0，他引：1  

JOSÉ M. MARTÍN  JUAN C. BRAGA  CHRISTIAN BETZLER†  THOMAS BRACHERT‡ 《Sedimentology》1996,43(2):263-277

Uppermost Tortonian to lower Messinian temperate carbonates crop out in the Agua Amarga Basin (SE Spain). They consist of four units. The lower three units can be tentatively assigned to the lowstand systems tract of a fourth-order sequence, constituting in turn the lowstand (‘megatrough unit’), transgressive (‘breccia unit’) and highstand (‘bedded unit’) stages of a higher-order cycle. All these materials were deposited in a small pull-apart basin related to the sinistral Carboneras strike-slip fault system. The best represented is the bedded unit (up to 25 m thick), which consists of bioclastic, bryozoan/bivalve-dominated calcarenites/calcirudites with abundant fragments of echinoids, barnacles, benthic foraminifers, coralline algae, brachiopods and solitary corals. Facies trends within this unit are roughly arranged in an E-W direction, with the coastline to the north of the basin. The depositional model is that of a gentle ramp with prograding beaches and shoals in its higher parts. Seaward of the shoals was the ‘factory area’, where most organisms lived and maximum carbonate production took place. From this area some of the skeletons were washed landwards by waves and/or currents during storms and incorporated into the shoals and beaches, and others moved downslope along the ramp as mass-flows, accumulating to form the ‘fan-bedded zone’. The factory-area and fan-bedded sediments intercalate five well-defined, thick beds of calcarenites/fine-grained calcirudites. They show bar morphologies (single or amalgamated), or make up sand-waves with very consistent tabular cross-bedding pointing landwards. These beds formed in a very shallow, wave/current-influenced, coastal environment. The bars and sand waves in the fan-bedded zone developed during lowstands, while those located higher up in the ramp interbedded with the factory facies are related to transgressive stages. Prograding beaches, shoals, factory facies and fan-bedded layers developed during the highstands. Net skeletal production occurred mainly during the highstands. Sediment-accretion values of these sediments are similar to those of present and ancient shallow-marine, temperate carbonates considering that the whole bedded unit was deposited in a 100 000-year interval (equivalent to the short eccentricity cycle). The five cycles inside the bedded unit would therefore correspond to the c. 20 000-year precession cycles of the Milankovitch band.  相似文献   

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia   总被引：9，自引：0，他引：9  

N. V. Sobolev  F. V. Kaminsky  W. L. Griffin  E. S. Yefimova  T. T. Win  C. G. Ryan  A. I. Botkunov 《Lithos》1997,39(3-4):135-157

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.  相似文献   

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil   总被引：1，自引：0，他引：1  

S. A. Gibson  R. N. Thompson  R. K. Weska  A. P. Dickin  O. H. Leonardos 《Contributions to Mineralogy and Petrology》1997,126(3):303-314

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.  相似文献   

Nitrogen isotope indicators of seasonal source variability to groundwater   总被引：5，自引：0，他引：5  

M. Z. Iqbal  N. C. Krothe  R. F. Spalding 《Environmental Geology》1997,32(3):210-218

 A nitrogen isotope study of soil water and groundwater in southern Indiana, USA, in 1991–1992 demonstrated considerable variations in nitrate degradation processes compared to an earlier investigation in 1986–1987. Although N-fertilizers were applied in May 1991, the δ¹⁵N values in soil water decreased in February 1992, indicating its delayed release into the system after substantial rainfall. The δ¹⁵N values of groundwater decreased from +12.3‰ in November 1991 to +11.3‰ in February 1992, and to +7.5‰ in March 1992. The increased residence time of nitrate in the soil resulted in increased denitrification, ammonia volatilization and plant uptake, and reduced threat to the groundwater quality. The 1986–1987 study in the area reported that excessive rainfall during the summer rapidly transported the nitrate to deeper horizons and drastically reduced volatilization and microbial reduction of nitrate, thus increasing the immediate threat to the groundwater quality in the area. The present study demonstrated that nitrogen isotopic signatures can be used to determine the effects of local soil type, rainfall, and land-use practices on the fate of nitrate in the subsurface. Received: 18 February 1997 · Accepted: 17 June 1997  相似文献   

An application of factor analysis for the study of the hydrogeological conditions in Plio-Pleistocene aquifers of NW Achaia (NW peloponnesus, Greece)   总被引：4，自引：0，他引：4  

K. S. Voudouris  N. J. Lambrakis  G. Papatheothorou  P. Daskalaki 《Mathematical Geology》1997,29(1):43-59

R- andQ-mode factor analysis is applied to 51 groundwater samples collected from wells drilled in the Plio-Pleistocene aquifers of NW Achaia, Greece. The purpose ofR- andQ-mode factor analysis application is to identify (i) the regional groundwater flow pattern, and (ii) the deterioration of groundwater quality. Sixteen hydrogeological parameters are used in order to examine their importance and to provide significant insight into their correlations. In theR-mode factor analysis, a six-factor model is suggested which can explain more than 77.5% of the total variance. The contribution of each factor at every site (factor scores) also is computed. Maps are constructed showing the geographical distribution of the factor scores. From these maps, the high salinity areas are delineated (seawater intrusion, possible appearance of halite layers) and the areas with elevated contribution of karastic-water are defined. Using theQ-mode correspondence analysis the meaning of the electrical conductivity as the most important variable in groundwater quality characterization is demonstrated.  相似文献   

Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa     

J. Gutzmer  N. J. Beukes 《Mineralium Deposita》1997,32(3):250-256

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO₂ and SiO₂ from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness. Received: 28 May 1996 / Accepted: 22 January 1997  相似文献   

Conservation tillage and input use   总被引：1，自引：0，他引：1  

N. D. Uri 《Environmental Geology》1997,29(3-4):188-201

 There continues to be a question as to the overall effectiveness of conservation tillage practices in reducing the impact of agricultural production on the environment. While it is generally recognized that water runoff and soil erosion will decline further, as tillage and mulch tillage systems are not used more extensively on cropland, what will happen to pesticide and fertilizer use remains uncertain. To gain some insight into this, the conservation tillage adoption decision is modelled. On the assumption that the decision to adopt conservation tillage is a two-step procedure, the first decision is whether or not to adopt a conservation tillage production system and the second concerns the extent to which conservation tillage should be used – appropriate models of the Cragg and Heckman (dominance) type are estimated. Based on farm-level data on corn production in the United States for 1987, the profile of a farm on which conservation tillage was adopted is that the cropland had above-average slope and experienced above-average rainfall, the farm was a cash grain enterprise, and it had an above-average expenditure on pesticides and a below-average expenditure on fuel and custom pesticide applications. Additionally, for a farm adopting a no-tillage production practice, an above-average expenditure was made on fertilizer. Received: 18 September 1995 · Accepted: 6 December 1995  相似文献