Enrichment of 88Sr in continental waters due to calcium carbonate precipitation

delta Sr-88/86 data published over the last few years suggest that continental waters are enriched with 88Sr as compared to the rocks in their drainage basins. In an attempt to understand this phenomenon, this study established the fractionation in the Sr-88/Sr-86 ratio during precipitation of continental carbonates (i.e., carbonates precipitated on land from surface, pedogenic, or ground waters), and evaluated the contribution of this process to the Sr-88-enrichment in rivers. For this, stable and radiogenic Sr isotopes (Sr-88, Sr-87 and Sr-86) were measured in calcite samples and their precipitating waters collected in various continental environments, such as soil, cave, streams and groundwater. The results indicate that continental carbonates are Sr-88-depleted relative to their precipitating waters, placing them as one of the most 88Sr-depleted reservoirs on earth. The average difference in delta Sr-88/86 values between waters and solid CaCO3 (tufas or speleothems) that they precipitate is Delta(carb-water) = -0.218 +/- 0.014%0 (1SD). An even larger fractionation (epsilon(carb-water) = -0.285 +/- 0.02 parts per thousand) was measured in groundwater with particularly high carbonate-alkalinity and high carbonate precipitation rate that depleted 65% of the Sr in the groundwater, resulting in substantial Sr-88-enrichment in the residual dissolved Sr (delta Sr-88/86 = 0.656 parts per thousand). Results also suggest that pedogenic carbonate precipitation in soil profile removes 50-85% of the Sr from the recharging soil-water, thereby increasing the delta Sr-88/86 value of the soil-water from -0.18 parts per thousand to 0.3 parts per thousand-0.6 parts per thousand. Similar Sr-88-enrichment was observed in drip water from a karst cave. A maximum removal flux of Sr into continental carbonates of about 20 Gmol((sr)).y(-1) is required to yield the reported 88Sr-enrichment in global rivers (delta Sr-88/86 = 0.32 parts per thousand) relative to their rock sources when using the fractionation factor derived in this study, Delta(carb-water) = -.218 parts per thousand, and the published delta Sr-88/86 composition of marine carbonates of 0.16 parts per thousand. This surprisingly large flux requires that similar to 40% of the originally weathered Sr should co-precipitate with continental carbonates. This calculation may suggest that CaCO3 precipitation is a significant mechanism in the continental Sr cycle. It is possible however that other mechanisms such as Sr uptake by plants or incongruent weathering of silicates, could also contribute to the riverine Sr-88-enrichment. Alternatively, the average delta Sr-88/86 value of marine carbonate rock sections undergoing weathering is higher than suggested in current literature. It is concluded that additional delta Sr-88/86 data on marine carbonate rock sections should be collected to resolve the enigma of the high value of riverine delta Sr-88/88. (C) 2016 Elsevier B.V. All rights reserved.