Tufas and travertines of the Mediterranean region: a testing ground for freshwater carbonate concepts and developments

Actively forming travertine and tufa sites are widespread in temperate and warm climates, such as the Mediterranean region, but also develop in other climatic regimes from the Arctic to equatorial latitudes. The terms travertine and tufa are sometimes applied indiscriminately today; however, each has a specific definition. Most studied deposits are active or of Quaternary (especially Holocene) age. There are fewer Tertiary and even less late Mesozoic examples on record. In part, this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants. Physico‐chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition. Rapid cooling of waters further aids the process in thermal (travertine) sites. However, biomediation of calcium carbonate, associated with prokaryote–microphyte biofilms, is now recognized to be equally important in slow flowing and near static sites. Much of this precipitation occurs in close association with cyanobacterial, heterotrophic bacterial and diatoms. The Mediterranean region figures significantly in the development of travertine and tufa research. Consequently, this article employs key literature on Mediterranean examples to illustrate developments and concepts in freshwater carbonate research. Modern work started in the 1950s and defined many previously unknown outcrops. Many such sites became the targets for early radiocarbon dating studies. Since the late 1970s, a number of laboratory‐based and field‐based classification schemes have been proposed. Some schemes were based on associated macrovegetation types; others were based on petrological characteristics or on geometry and sedimentary facies characteristics. Of late, the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits. Two distinct research strands were developed during the 1990s, both aimed at a better understanding of environmental change. The first considered biocomponents. The research was multifaceted and ranged from exploring the role of micro‐organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains; these provided proxy environmental information which has helped to define past climate variability in the region. The second research strand focussed on geochemical signatures as a proxy tool for defining environmental change. This definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics. This improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies. Lithofacies modelling has also brought to light important early post‐depositional neomorphic changes, which obscure primary fabrics, and complex diagenetic histories have been revealed. Stable isotope signatures of carbon and oxygen have also been used to define palaeo‐water temperature. Fine‐tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium/thorium.