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 rati os, one of the parameters required is the Sr-87/Sr-86 ratio of the str eamwater. 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, th e Sr-87/Sr-86 ratio was measured in streamwater samples taken at low a nd 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 Sr-87/Sr-86 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 show ed no relationship to flow rate. Weathering of non-radiogenic plagiocl ase feldspar in the soils controls the isotope signature of the stream water and the constancy of the ratio with flow rate indicates that (1) more radiogenic minerals such as K-feldspar and mica do not contribut e any more significantly during high flow than during low flow, and (2 ) from the available information, it is not necessary to take flow rat e into account when using Sr-87/Sr-86 ratios in weathering studies in this particular catchment, although it must be emphasized that this co nclusion may not apply to other catchments. The concentration of stron tium in the streamwater was inversely proportional to the flowrate. (C ) 1998 Elsevier Science B.V. All rights reserved.