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  • 1
    Publication Date: 2012-04-15
    Description: Numerical modelling by finite element methods provides two significant insights into the formation of the giant amethyst geodes of the Paraná volcanic province: the conditions needed to open the cavities and the conditions that control their size and shape. Giant amethyst geodes were formed in the Cretaceous (135 Ma) in altered volcanic rocks by water vapour pressure (Δp) at about 0.5 MPa under an altered basalt cover of 5–20 m. Only rocks with Young’s modulus values (E) in the range 1–2 GPa can sustain ballooning, which is the growth of a cavity in a ductile medium by the pressure of water and its vapour. The size of the proto-geode is dependent on the water vapour pressure, which is directly related to thickness of the overlying basalt. Varying the yield points causes the formation of either prolate or oblate cavities. A low transition point (smaller than 0.18 MPa) generates a prolate-shaped cavity, whereas a high transition point (larger than 0.18 MPa) generates oblate proto-geodes. Proto-geodes are smaller when Young’s modulus is higher (rock is less altered) or when water vapour pressure is lower (because of thinner overburden of basalt). The calculations are an indication that the processes operative in the altered basalts led to the opening of giant cavities by ballooning.
    Print ISSN: 1468-8115
    Electronic ISSN: 1468-8123
    Topics: Geosciences
    Published by Wiley
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  • 2
    Publication Date: 2017-06-01
    Description: Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters that becomes superimposed by additional inputs further downstream. Major drivers of carbon dioxide partial pressures ( p CO 2 ) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average p CO 2 of 3,100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (〉500 km 2 ). Because of their large proportion in global river networks (〉96 % of the total number of streams), headwaters contribute large – but still poorly quantified – amounts of CO 2 to the atmosphere. Conservative estimates imply that globally 36 % (i.e. 0.93 Pg C yr -1 ) of total CO 2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO 2 sources, concentrations and fluxes. To overcome uncertainties of CO 2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO 2 outgassing isotope approaches and multi-scale geophysical imaging techniques.
    Print ISSN: 8755-1209
    Topics: Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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