Sr isotope fractionation in a karst river: case study of Krka, Croatia

Precipitation of calcite from water fractionates strontium (Sr) isotopes because of preferential incorporation of light (⁸⁶Sr) isotopes into the solid phase, making continental carbonates one of the most ⁸⁸Sr depleted reservoirs. It was suggested that carbonate precipitation is the most likely process controlling δ^88/86Sr composition of karst water. Therefore, the ⁸⁸Sr enrichment of river water could be used for the estimation of Sr and carbonate precipitation at catchment scale.

In the present study, we report on trace element partitioning and Sr isotope fractionation between tufa and water in the groundwater fed karst river Krka (Croatia). Water and tufa along with samples of bedrock and soil as the main contributors of dissolved and particulate Sr at seven main waterfalls and cascades along a 33 km section of the river were analysed for trace element and Sr isotope composition (δ^88/86Sr).

The highest δ^88/86Sr values were measured in soils and in siliciclastic rocks, while in limestone, the δ^88/86Sr values were similar to those of old tufa precipitated in the period between 96 and 141 ky BP. Recent tufa, however, was considerably depleted in ⁸⁸Sr. The isotope fractionation between water and recent tufa varied a lot and was inversely correlated with Mg and Sr partitioning coefficients, while correlations with precipitation rates and temperature were rather weak. The δ^88/86Sr of recent tufa was strongly correlated with the stable isotope composition of organic carbon, which indicates that apart from hydrochemical, hydraulic parameters and temperature, plants and microbial communities that knowingly stimulate the tufa formation also affect the isotope fractionation of Sr.