ABSTRACTThis paper provides a review of the classification of travertines with emphasis on their morphology. Three criteria are used to describe them: geochemistry, microfabric and morphology. Geochemically, travertines may be divided into two groups, the meteogene travertines, where the carrier carbon dioxide originates in the soil and epigean atmosphere, and the thermal (thermogene) travertines where the carbon dioxide comes from thermally generated sources. Many travertine fabrics are influenced by bacteria and plants. These include 'stromatolitic' forms, many oncoids, shrubs, tufts, mats and moss travertines. Morphologically, travertines are conveniently divided into autochthonous (spring mounds and ridges, cascades, barrages, fluvial and lacustrine crusts, paludal deposits and cemented rudites) and the allochthonous or clastic travertines (valley-fills, back-barrage deposits, alluvial cones). Travertine deposits often include a wide range of fabrics and morphologies in one system. They are influenced locally by discharge, slope, vegetation, climate and human activity. Intergradations occur, both within travertine types but also with other freshwater deposits, e.g. calcrete and lake chalk. The influence of travertine deposition on the local hydrology and geomorphology is also discussed. The review emphasises the significance of scale and hydrology and aims to provide a unified scheme of travertine classification.
Over much of Europe, it has been postulated that in the late Holocene (since c.2500 BP) there was a marked decline in the deposition of tufa. There has been considerable debate about the causes of this phenomenon, with some authors postulating the importance of natural climatic changes, and others asserting that miscellaneous human influences have been crucial. This article discusses the role that various changes, both natural and anthropogenic, may have played, and suggests ways in which the large number of possible hypotheses can be tested.
Cyanobacteria are the major oxygenic photosynthetic microorganisms of hot spring travertines. This review describes the known cyanobacterium flora of travertine-depositing waters > 37 °C. The communities develop either upon (epilithon) or within (endolithon) the travertine surface, where they may influence the travertine fabric by providing nucleation sites for calcium carbonate. Mat photosynthesis locally increases the amount of travertine deposited, but the importance of photosynthesis in travertine deposition is rarely significant because the main loss of CO2is by atmospheric evasion of the hot, CO2-rich waters. The Oscillatoriales are the most important group of cyanobacteria in terms of their biomass, but the taxonomy of the order is in a state of chaos. Molecular methods will ultimately disclose the true affinities of the organisms but, at present, the use of form-taxa based upon morphological characters provides a practical alternative. Identification of fossilized cyanobacteria is problematic since few of the key characters survive as fossils. The known cyanobacterium flora is tabulated and an analytical key provided to identify the form-taxa of hot spring travertines.
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