Geochemische Untersuchungen an Sinterkalken (Travertinen)

Abstract

Formation of travertines from continental waters requires very special conditions. Their porous, cellular and concretionary appearance is often due to calcite incrustations on plants. Therefore the high porosity beside a very low concentration of detrital silicates must be related to a high rate of calcite accumulation (0.1 to 1 mm/year). A high rate of sedimentation from high Ca-concentrations (>0.01%) does not occur in normal continental waters equilibrated with the atmospheric carbon dioxide. Formation of travertines is probably not an indicator of special climatic conditions as often assumed but is related to a special chemical composition of fresh waters. Creeks with continuous travertine deposition, investigated by us, contain about 200 ppm Ca. This is more than ten times the normal concentration of average continental waters. Bicarbonate concentration in these waters is five times that of equilibrium with atmospheric carbondioxide. Many of the productive waters are characterized by high sulfate concentrations and Sr/Ca-ratios (1000 Sr/Ca ranges from 9 to 22; seawater: 20). Except one from the Schwabische Alb 15 samples (Leine-valley, Teutoburger Wald, Thuringen) are high in sulfate and Sr/Ca-ratio to be accumulated in waters with remarkable concentrations in the respective elements. The Sr/Ca-ratios cannot be explained from dissolved limestones underlying the exposed areas. The assumption of dissolution of gypsum beds very well explains our observations and data. From the S32/S34 ratio an origin from Zechstein (Upper Permian), Muschelkalk (Middle Triassic) or Keuper (Upper Triassic) deposits respectively can be assumed. Like other freshwater limestones the travertines under investigation differ from marine carbonates by their light carbon (mean δC13 −8,1) from decomposed organic matter and their light oxygen (mean δO18 −6,7) from exchange with freshwaters. The discrimination freshwater- and marine limestones in oxygen isotopes is only valid for those of low age (Upper Mesozoic to Recent). Hence the special conditions of travertine formation are related to bicarbonate spring waters which have circulated on faults where they could dissolve gypsum (and/or calcite) from subsurface beds. Additional examples of travertine deposits from Czechoslovakia (Zyka, 1958), Luneburger Heide (NW Germany), Italy (Tivoli near Rom) and Jugoslavia can be explained as formations of either sulfate-bicarbonate or of bicarbonate waters.