Strontium isotope ratios in streams and the effect of flow rate in relation to weathering in catchments

In calculating catchment weathering rates using strontium isotope ratios, one of the parameters required is the 87Sr/86Sr ratio of the streamwater. The effect of flow rate on this ratio is not well understood with conflicting reports in the literature. To provide more detailed information on the effect of flow rate on strontium isotope ratios, the 87Sr/86Sr ratio was measured in streamwater samples taken at low and high flow rates and also in samples taken at four and eight hourly intervals during two rain events in a small catchment (area 90 ha) in andesitic lavas. The 87Sr/86Sr ratio varied over a narrow range of 0.70797 to 0.70852 for a flow rate range of 6.1 to 41.51 s−1 and showed no relationship to flow rate. Weathering of non-radiogenic plagioclase feldspar in the soils controls the isotope signature of the streamwater and the constancy of the ratio with flow rate indicates that (1) more radiogenic minerals such as K-feldspar and mica do not contribute any more significantly during high flow than during low flow, and (2) from the available information, it is not necessary to take flow rate into account when using 87Sr/86Sr ratios in weathering studies in this particular catchment, although it must be emphasized that this conclusion may not apply to other catchments. The concentration of strontium in the streamwater was inversely proportional to the flowrate.