Vincent de Medwecki

Bio: Vincent de Medwecki is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Holocene & Shore. The author has an hindex of 2, co-authored 2 publications receiving 94 citations.

Causes and Emission of Luminescence in Calcite and Dolomite

Abstract: Luminescence in calcite and dolomite is governed by physical phenomena that are common to all oxygen-dominated crystalline substances, including other carbonates and silicates. Absorption of excitation energy, energy transfer, and emission involve predictable transitions between electronic energy levels. Strong emission in various colors is always caused by impurities which function as activators of luminescence. Visible luminescence is not expected from pure, undistorted insulators, including carbonates. However, a faint blue 'intrinsic' luminescence, with a broad emission peak (band) around 400 nm, presumably caused by lattice defects, occurs in pure calcite and dolomite, and even in some samples containing impurities. The most important activators in carbonates are transition elements and rare earth elements. Luminescence spectra can be used for activator identification. These spectra are largely independent of the type of excitation, e. g., electron beam (cathodoluminescence = CL), photon (photoluminescence = PL), X-Ray (radioluminescence = RL) excitation, and others. Emission intensities depend on activator, sensitizer, and quencher concentrations, and on the method of excitation. At a given activator concentration, the luminescence intensity generally increases with an increase in excitation energy from PL (relatively weak) to CL (strong). Changes in visual luminescence color between different excitation methods are caused by relative changes in emission peak heights. Mn2* appears to be the most abundant and important activator in natural calcite and dolomite. Substituting for calcium in both minerals, its emission is orange-red to orange-yellow, with a fairly broad band between 570-640 nm (maximum between 590-620 nm). The emission band maximum of Mn2* substituting for Mg2* (in dolomite) is located around 640-680 nm. As little as 10-20 ppm Mn2* in solid solution are sufficient to produce visually detectable luminescence, if total Fe contents are below about 150 ppm. Sm3* activated luminescence can be visually indistinguishable from that activated by Mn2*. The spectrum of Sm3* emission, however, is quite distinct from that of Mn2* and consists of three narrow bands at 562 nm, 604 nm, and 652 nm. Tb3+ and Dy3+ activate green and cream-white luminescence, respectively. The main emission of Tb3* is at 546 nm. The emission of Dy3t consists of three bands, located at 484 nm, 578 nm, and 670 nm. Emission from Eu-containing calcite is red or blue. Narrow spectral bands of 590 nm, 614 nm, and 656 nm are caused by Eu3* and correspond to the red emission. A broad emission spanning a large range of shorter wavelengths is caused by Eu2* and corresponds to the blue emission. As in the case of Sm3*-activated luminescence, the red Eu3* luminescence can be mistaken visually for Mn2*-activated luminescence. Visual luminescence detection limits for rare earths are on the order of 10 ppm. Pb2* is an activator, with an emission band around 480 nm, but it also is a sensitizer of Mn2*-activated luminescence in carbonates. Another recognized sensitizer for Mn2* in carbonates is Ce3*. Sensitizers appear to be effective at concentrations as low as 10 ppm in calcite. Quenchers of Mn2*-activated luminescence in carbonates are Fe2*, Co2*, Ni2*, and Fe3*. The concentrations at which quenchers appear to be effective may vary from element to element and with host mineralogy. Effective minimum concentrations as low as 30-35 ppm have been reported for calcite. The interplay of Mn2* and Fe2*, commonly regarded to be the most important activator and quencher, respectively, in determining the luminescence characteristics of natural carbonates is not well understood because the available data are partially inconsistent. The Mn/Fe ratio may exert a control on luminescence intensity. Mn and Fe concentrations at which 'bright' CL changes to 'dull' can be determined only semi-quantitatively. The available data on the concentration of Mn2* at which quenching starts are partially inconsistent Consequently, the Mn2* concentration at which concentration extinction occurs has not been determined unequivocally. The data presented and summarized in this paper can be used as a basis for the interpretation of luminescence of geological materials. Li particular, knowledge of the possibilities and complexities of activation, sensitization, and quenching has great potential for the interpretation of diagenetic carbonate cements.

Holocene vegetation dynamics in the northeastern Rub' al-Khali desert, Arabian Peninsula: a phytolith, pollen and carbon isotope study

Abstract: The Holocene vegetation history of the Arabian Peninsula is poorly understood, with few palaeobotanical studies to date. At Awafi, Ras al-Khaimah, UAE, a 3.3 m lake sediment sequence records the vegetation development for the period 8500 cal. yr BP to � 3000 cal. yr BP. 13 C isotope, pollen and phytolith analyses indicate that C3 Pooid grassland with a strong woody element existed during the early Holocene (between 8500 and 6000 cal. yr BP) and became replaced by mixed C3 and C4 grasses with a strong C4 Panicoid tall grass element between 5900 and 5400 cal. yr BP. An intense, arid event occurred at 4100 cal. yr BP when the lake desiccated and was infilled by Aeolian sand. From 4100 cal. yr BP the vegetation was dominated by C4 Chloridoid types and Cyperaceae, suggesting an incomplete vegetation cover and Aeolian dune reactivation owing to increased regio- nal aridity. These data outline the ecosystem dynamics and carbon cycling in response to palaeomon- soon and north-westerly variability during the Holocene. Copyright 2004 John Wiley & Sons, Ltd.

Climate Changes during the Holocene and their Impact on Hydrological Systems

Abstract: Preface Acknowledgements 1. Climate changes in the Levant during the Late Quaternary 2. Climate changes during the Holocene in Europe 3. Climate changes during the Holocene in East Asia (China, Korea and Japan) 4. Climate changes during the Holocene in Africa 5. Climate changes over Western USA and Mexico during the Holocene 6. General conclusions References.