Geochemical evidence for the formation of a large miocene “travertine” mound at a sublacustrine spring in a soda lake (Wallerstein castle rock, nördlinger ries, Germany)

Abstract

Travertines” (tufa pinnacles) of the Miocene Riescrater basin have been investigated to test whether carbon, oxygen and strontium isotopes can be used for the recognition of fossil subaquatic spring deposits in high-alkalinity settings. The Ries basin “travertines” have so far been interpreted as a product of subaerial to sublacustrine artesian springs discharging calcareous groundwater into a freshwater or slightly saline lake. However, recent studies on microfacies and fabric development propose a formation at Ca2+-supplying sublacustrine springs of a soda lake. Geochemical analysis of “travertines” of the castle rock Wallerstein, including “sickle-cell” limestones, thrombolites, non-skeletal stromatolites, and speleothems, now support the latter interpretation. High Sr contents surpassing that of the contemporaneously formed dolomitic algal biocherms of the lake shore point to an aragonitic composition of primary precipitates. the δ13C values of diagenetically moderately to weakly altered “travertine” facies types are in the same range of the impact-brecciated Upper Jurassic limestones, thus, are inconsistent with a mixture of soil-derived CO2 and CO32− from the Jurassic limestones. In addition, the δ18O values are too high to support a significant contribution of CO32− by meteoric waters seeping through marine Jurassic limestones. Instead the δ13C and δ18O values indicate an origin of the CO32− from a lake water body characterized by evaporation. This is consistent with a sodium-rich lake water as indicated by high sodium contents of aragonitic algal bioherms of the lake shore. The87Sr/86Sr isotope ratio of the “travertine” mound carbonates are consistent with calculated mixing of spring waters discharging from the crystalline basement and lake water high in dissolved inorganic carbon. This points to an origin of the divalent cations from sublacustrine spring waters. In turn,87Sr/86Sr isotope ratios of green algal reef carbonates of the lake shore are closer to that of the Upper Jurassic carbonates, due to surface run-off from surrounding limestone uplands.