Geochemical evidence for the formation of a large miocene "travertine" mound at a sublacustrine spring in a Soda Lake (Wallerstein Castle Rock, Nordlinger Ries, Germany)

"Travertines" (tufa pinnacles) of the Miocene Riescrater basin have been in vestigated to test whether carbon, oxygen and strontium isotopes can be use d for the recognition of fossil subaquatic spring deposits in high-alkalini ty settings. The Ries basin "travertines" have so far been interpreted as a product of subaerial to sublacustrine artesian springs discharging calcare ous groundwater into a freshwater or slightly saline lake. However, recent studies on microfacies and fabric development propose a formation at Ca2+-s upplying sublacustrine springs of a soda lake. Geochemical analysis of "tra vertines" of the castle rock Wallerstein, including "sickle-cell" limestone s, thrombolites, non-skeletal stromatolites, and speleothems, now support t he latter interpretation. High Sr contents surpassing that of the contemporaneously formed dolomitic algal bioherms of the lake shore point to an aragonitic composition of prim ary precipitates. The delta C-13 values of diagenetically moderately to wea kly altered "travertine" facies types are in the same range of the impact-b recciated Upper Jurassic limestones, thus, are inconsistent with a mixture of soil-derived CO2 and CO32- from the Jurassic limestones. In addition, th e delta O-18 values are too high to support a significant contribution of C O32- by meteoric waters seeping through marine Jurassic limestones. Instead the delta C-13 and delta O-18 values indicate an origin of the CO32- from a lake water body characterized by evaporation. This is consistent with a s odium-rich lake water as indicated by high sodium contents of aragonitic al gal bioherms of the lake shore. The Sr-87/Sr-86 isotope ratio of the "trave rtine" mound carbonates are consistent with calculated mixing of spring wat ers discharging from the crystalline basement and lake water high in dissol ved inorganic carbon. This points to an origin of the divalent cations from sublacustrine spring waters. In turn, Sr-87/Sr-86 isotope ratios of green algal reef carbonates of the take shore are closer to that of the Upper Jur assic carbonates, due to surface run-off from surrounding limestone uplands .