Carbonate platforms as recorders of high-amplitude eustatic sea-level fluctuations: the late Albian appenninica-event     

JÜRGEN Grötsch  ROLF Schroeder†  SIBYLLE No釠 ERIK Flugel§ 《Basin Research》1993,5(4):197-212

The mid-Cretaceous was a time in which rapid vertical carbonate accumulation alternated with intervals of world-wide crisis on carbonate platforms. Next to the well-known mid-Valanginian, mid-Aptian and Cenomanian/Turonian event, the late Albian Rotalipora appennirjica-7.one is a period of severe platform crisis which is reflected differently on different platforms in the Tethys, the Pacific and the Atlantic Ocean. On the Dinaric Platform in western Slovenia (Tethys) karstification and subsequent temporary drowning of parts of the platform is recorded in sediment infillings of a cave system in a reef along the northern edge of the open ocean platform. Biostratigraphic analysis indicates that vertical aggradation, karstification, short-term pelagic influx and subsequent shallow-water carbonate production all took place in the late Albian. Simultaneous, but complete drowning can be observed in the north-west Pacific which is an area where the demise of reefs is most obviously expressed by numerous sunken atolls. New sedimentological and palaeontological data, as well as seabeam and seismic data from several cruises, suggest a dramatic fall of sea level prior to a rise of even higher amplitude in the Rotalipora appenninica-zone. This sea-level rise led to the deposition of a drowning succession and the formation of terminally drowned barrier reefs, rimming the top of many guyots. Karstification and drowning during the same biozone is also recorded on an isolated, shelf-attached platform in the Basco-Cantabrian Basin (North Atlantic) which shows similar cavity systems, infilling sediments and a thin drowning succession. Another example of late Albian platform drowning is the Maracaibo Platform in north-west Venezuela. Thus, carbonate platforms can be used as recorders of high-amplitude sea-level changes, providing quantitative data on magnitude of fluctuations and their nature (relative vs. eustatic). In the case of the appenninica-extnt, they also provide evidence that even during Cretaceous greenhouse climate period(s), dramatic disturbance of climatic equilibrium linked with short-term high-amplitude regressive-transgressive cycle(s) occurred. The reason for this eustatic sea-level fluctuation is yet unclear but could ultimately be triggered by volcanotectonic processes.  相似文献   

The origin of olivine-plagioclase coronas in metagabbros from the Adirondack Mountains, New York   总被引：1，自引：1，他引：1  

C. D. JOHNSON  W. D. CARLSON 《Journal of Metamorphic Geology》1990,8(6):697-717

Olivine-plagioclase coronas in metagabbros from the Adirondack Mountains, New York (USA) are spatially well-organized reaction textures consisting most commonly of sequential layers of orthopyroxene, clinopyroxene, plagioclase, and garnet; the textures are characteristic of diffusion-controlled reaction kinetics. Although similar coronas have been interpreted by previous workers in terms of an isochemical steady-state diffusion model, petrographical relations and material-balance calculations establish that coronas in the Adirondack metagabbros cannot be treated as isochemical and do not form in a single-stage steady-state process; instead they evolve through time in a complex open-system reaction. In this study, the isochemical diffusion model is modified to account for elemental fluxes across the outer boundaries of the coronal reaction band, thereby approximating the open-system behaviour of the coronas. The sequence and relative proportions of product minerals calculated by the open-system steady-state model correspond closely to those observed in coronas of the Adirondacks, over a wide range of values for the relative diffusivities of chemical components involved in the reaction, regardless of the particular method used to determine material balance in the reaction texture. Despite this correspondence, petrographical evidence for successive replacement of coronal product layers reveals that the Adirondack coronas evolved through one or more transient states, rather than forming in a single-stage steady-state process. There is no evidence that the successive replacement of coronal product layers resulted from changes in pressure or temperature, but there is petrographical evidence that these changes resulted from modification of the composition of reactant plagioclase as the corona-forming reaction proceeded. This is confirmed by the fact that the evolution of the coronas over time can be replicated with the open-system diffusion model by simulating the effect of the gradual exhaustion of plagioclase as a source of the Ca and Si components required for reaction. These simulations suggest that successive stages in the evolution of the coronas are characterized by these product sequences: (i) orthopyroxene-clinopyroxene-plagioclase-garnet; (ii) orthopyroxene-clinopyroxene-garnet; and (iii) orthopyroxene-garnet. All of these stages, and the transitions between them, are observed petrographically. Coronas in Adirondack metagabbros appear, therefore, to have originated in a complex, open-system, diffusion-controlled reaction in which the product assemblages changed as the reaction progressed.  相似文献   

56 YEARS OF SOLIFLUCTION MEASUREMENTS IN THE ABISKO MOUNTAINS, NORTHERN SWEDEN – ANALYSIS OF TEMPORAL AND SPATIAL VARIATIONS OF SLOW SOIL SURFACE MOVEMENT     

BY HANNA RIDEFELT  JONAS ÅKERMAN  ACHIM A BEYLICH  JAN BOELHOUWERS  ELSE KOLSTRUP  ROLF NYBERG 《Geografiska Annaler: Series A, Physical Geography》2009,91(3):215-232

Solifluction movement rates from 1952 to 2008 for the Abisko region, northern Sweden, have been compiled and analysed through correlation tests and multiple regression. The temporal analysis is based on two datasets ( Lobe11 & gridAB and Line B ) from Kärkevagge. The dataset Lobe11 & gridAB show a strong correlation between movement rates and mean annual air temperature (MAAT) and MAAT is also identified as one of the significant contributing parameters in the multiple regression model. No significant correlations were found for the Line B dataset. The spatial analysis indicates generally higher movement rates in the western part of the region and at lower altitudes mainly between 700 and 900 m a.s.l., but the spatial variability is high. To reduce the influence of the temporal variation the data for the correlation tests of the spatial variations were divided into two parts: 1957 to 1980 and 1981 to 2008. The correlation analysis of the dataset 1957 to 1980 shows a significant negative correlation between annual average movement rates and permafrost probability and altitude. The dataset 1981 to 2008 shows a positive correlation between movement rates and wetness index. It is concluded that movement rates may increase with higher MAAT in the western part of the region (Kärkevagge), the spatial variability of movement rates within the region is very high and that altitude (and/or permafrost) together with wetness index are the main controls on the regional spatial variation. The study highlights the limitations in establishing statistical relationships between movement rates and climate using data from different field empirical studies.  相似文献   

Numerical simulation of diffusion‐controlled nucleation and growth of porphyroblasts     

R. A. KETCHAM  W. D. CARLSON 《Journal of Metamorphic Geology》2012,30(5):489-512

This article presents the conceptual basis for a new numerical model of diffusion‐controlled nucleation and growth of porphyroblasts, describes its implementation, and illustrates the range of crystallization behaviours encompassed by it. The model differs from previous efforts principally in its ability to track explicitly the evolution in time and space of the chemical affinity for reaction in the intergranular medium, which provides a more accurate characterization of nucleation suppression in the vicinity of pre‐existing crystals and of growth suppression due to competition for nutrients among neighbouring crystals. Critical new features of these numerical simulations include: maintenance of local equilibrium for fluid in contact with reactants or products; persistence of reactants until they are eliminated by dissolution due to reaction progress and local diffusive flux; nucleation rates that vary as the local reaction affinity evolves; complex initial distributions of reactants if desired; and the flexibility to encompass any porphyroblast‐forming reaction for which changes in free energy as a function of time and temperature are specified. Model results reveal that radial growth rates remain proportional to the square‐root of time in diffusion systems buffered by persistent reactants; they document the interchangeable effects of diffusivity, porosity, and solubility on material fluxes and thus growth rates; and they illustrate the offsetting textural effects of rates of diffusion, nucleation, and heating. The initial distribution of reactants is found to exert a first‐order effect on crystal size distributions, confirming their limited utility for diagnosing crystallization mechanisms. These numerical simulations yield novel and rigorous confirmation of the textural effects of nucleation‐site saturation and variation in interconnected porosity, and reproduce with high fidelity much of the textural and chemical information gathered from natural specimens.  相似文献   

Holocene climatic variations and postglacial sea-level rise geochemically recorded in the sediments of the brackish karst lake An Loch Mor, western Ireland     

GEORG SCHETTLER  ROLF L. ROMER  MICHAEL O'CONNELL  KAREN MOLLOY 《Boreas: An International Journal of Quaternary Research》2006,35(4):674-693

Palaeohydrology of the brackish karst lake An Loch Mór (Inis Oirr) was reconstructed based on its 11 600 year sedimentation record. Low calcareous early Holocene sediments, characterized by high TOC contents and deposited at generally low accumulation rates, show a gradual decrease upwards in the concentration of the siliciclastic sediment until c . 7000 cal. yr BP. From c . 9000 cal. yr BP, sediment accumulation became increasingly dominated by the deposition of autochthonous calcite and organic matter. The deposition of autochthonous calcite and its chemical composition (Ca/Sr, Ca/Mg) are determined by the subsurface inflow of freshwater from the catchment of the lake and by the balance between freshwater and seawater influx. During the early Holocene, the lake mainly received input of sea salt by sea-spray. The sediments document the further development from seasonal towards the onset of diurnal infiltration of seawater at c . 5100 cal. yr BP. At that time, the relative sea level must have risen nearly to its present level in Galway Bay. Diurnal seawater infiltration during high tides was controlled by freshwater input. Freshwater inflow from the catchment gradually increased with human impact on local vegetation (EVPT decrease). Permanent diurnal seawater infiltration in concert with this general freshwater increase made the geochemical sedimentation record highly sensitive to changes in annual precipitation. We use information from the palynological record to interpret geochemically inferred freshwater inflow variations in terms of dry and wet climatic periods and document distinct century-scale successions between wet and dry climatic episodes for western Ireland.  相似文献   

Lipid composition of phytoplankton from the Barents Sea and environmental influences on the distribution pattern of carbon among photosynthetic end products     

R. JAMES HENDERSON  ROLF E. OLSEN  HANS C. EILERTSEN 《Polar research》1991,10(1):229-238

The colonial algae Phaeocystis pouchetii and Dinobryon pellucidum dominated the phytoplankton crop at three stations in the Polar Front area of the Barents Sea.
Lipid extracted from the seawater containing the phytoplankton was dominated by neutral lipid classes, particularly triacylglycerols, and phospholipids were more abundant than galactolipids at all stations. Polyunsaturated fatty acids comprised between 15 and 26% of fatty acids of total lipid.
Of the carbon assimilated into lipid over 24 hours, 40% was located in the neutral lipid fraction. Phospholipids contained a smaller proportion of fixed carbon than galactolipids.
No defiinte relationships were observed between the distribution of fixed carbon in photosynthetic end products and the temperature or irradiance at which the phytoplankton was incubated. At a constant irradiance of 8.5 μmol m⁻²s⁻¹, the highest proportion of fixed carbon was recovered in protein at 4.5°C, but at −1.5°C most radioactivity was present in low molecular weight compounds. Regardless of incubation conditions, lipid always contained less than 30% of total assimilated carbon.  相似文献   

Origin and mechanical significance of honeycomb garnet in high-pressure metasedimentary rocks from the Tauern Window, Eastern Alps     

A. T. HAWKINS  J. SELVERSTONE  A. J. BREARLEY  R. J. BEANE  R. A. KETCHAM  W. D. CARLSON 《Journal of Metamorphic Geology》2007,25(5):565-583

High-pressure schists (2–2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core–rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz–quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.  相似文献   

The 'common vole' in Svalbard identified as Microtus epiroticus by chromosome analysis     

KARL FREDGA  MAARIT JAAROLA  ROLF ANKER IMS  HARALD STEEN  NIGEL G. YOCCOZ 《Polar research》1990,8(2):283-290

The chromosomes were studied in six individuals from a population of Microtus from Grumantbyen, Svalbard, and in six Microtur arualis (Pallas 1778) from Lauwersee, Holland. It was shown that the voles from Svalbard did not belong, as earlier supposed, to the species M. arualis (2n = 46) but to M . epiroticus (Ondrias, 1966) (2n = 54). We suggest that the Svalbard voles were introduced by man between 1920 and 1960 together with hay on Russian ships from the vicinity of Leningrad, USSR.  相似文献   

Intercomparison of fog water samplers     

DIETER SCHELL  HANS-WALTER GEORGII  ROLF MASER  WOLFGANG JAESCHKE  BEATE G. ARENDS  GERARD P. A. KOS  PETER WINKLER  THOMAS SCHNEIDER  AXEL BERNER  CHRISTIAN KRUISZ 《Tellus. Series B, Chemical and physical meteorology》1992,44(5):612-631

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Genesis of Ultramafic Lamprophyres and Carbonatites at Aillik Bay, Labrador: a Consequence of Incipient Lithospheric Thinning beneath the North Atlantic Craton   总被引：7，自引：0，他引：7  

TAPPE  SEBASTIAN; FOLEY  STEPHEN F.; JENNER  GEORGE A.; HEAMAN  LARRY M.; KJARSGAARD  BRUCE A.; ROMER  ROLF L.; STRACKE  ANDREAS; JOYCE  NANCY; HOEFS  JOCHEN 《Journal of Petrology》2006,47(7):1261-1315

Numerous dykes of ultramafic lamprophyre (aillikite, mela-aillikite,damtjernite) and subordinate dolomite-bearing carbonatite withU–Pb perovskite emplacement ages of 590–555 Ma occurin the vicinity of Aillik Bay, coastal Labrador. The ultramaficlamprophyres principally consist of olivine and phlogopite phenocrystsin a carbonate- or clinopyroxene-dominated groundmass. Ti-richprimary garnet (kimzeyite and Ti-andradite) typically occursat the aillikite type locality and is considered diagnosticfor ultramafic lamprophyre–carbonatite suites. Titanianaluminous phlogopite and clinopyroxene, as well as comparativelyAl-enriched but Cr–Mg-poor spinel (Cr-number < 0.85),are compositionally distinct from analogous minerals in kimberlites,orangeites and olivine lamproites, indicating different magmageneses. The Aillik Bay ultramafic lamprophyres and carbonatiteshave variable but overlapping ⁸⁷Sr/⁸⁶Sr_i ratios (0·70369–0·70662)and show a narrow range in initial _Nd (+0·1 to +1·9)implying that they are related to a common type of parentalmagma with variable isotopic characteristics. Aillikite is closestto this primary magma composition in terms of MgO (15–20wt %) and Ni (200–574 ppm) content; the abundant groundmasscarbonate has ¹³C_PDB between –5·7 and –5,similar to primary mantle-derived carbonates, and ¹⁸O_SMOW from9·4 to 11·6. Extensive melting of a garnet peridotitesource region containing carbonate- and phlogopite-rich veinsat 4–7 GPa triggered by enhanced lithospheric extensioncan account for the volatile-bearing, potassic, incompatibleelement enriched and MgO-rich nature of the proto-aillikitemagma. It is argued that low-degree potassic silicate to carbonatiticmelts from upwelling asthenosphere infiltrated the cold baseof the stretched lithosphere and solidified as veins, therebycrystallizing calcite and phlogopite that were not in equilibriumwith peridotite. Continued Late Neoproterozoic lithosphericthinning, with progressive upwelling of the asthenosphere beneatha developing rift branch in this part of the North Atlanticcraton, caused further veining and successive remelting of veinsplus volatile-fluxed melting of the host fertile garnet peridotite,giving rise to long-lasting hybrid ultramafic lamprophyre magmaproduction in conjunction with the break-up of the Rodinia supercontinent.Proto-aillikite magma reached the surface only after coatingthe uppermost mantle conduits with glimmeritic material, whichcaused minor alkali loss. At intrusion level, carbonate separationfrom this aillikite magma resulted in fractionated dolomite-bearingcarbonatites (¹³C_PDB –3·7 to –2·7)and carbonate-poor mela-aillikite residues. Damtjernites maybe explained by liquid exsolution from alkali-rich proto-aillikitemagma batches that moved through previously reaction-lined conduitsat uppermost mantle depths. KEY WORDS: liquid immiscibility; mantle-derived magmas; metasomatism, Sr–Nd isotopes; U–Pb geochronology  相似文献