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

Abstract

δ88/86Sr 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 88Sr/86Sr 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 88Sr-enrichment in rivers. For this, stable and radiogenic Sr isotopes (88Sr, 87Sr and 86Sr) 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 88Sr-depleted relative to their precipitating waters, placing them as one of the most 88Sr-depleted reservoirs on earth. The average difference in δ88/86Sr values between waters and solid CaCO3 (tufas or speleothems) that they precipitate is Δcarb-water=−0.218±0.014‰ (1SD). An even larger fractionation (εcarb-water=−0.285±0.02‰) 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 88Sr-enrichment in the residual dissolved Sr (δSr88/86=0.656‰). Results also suggest that pedogenic carbonate precipitation in soil profile removes 50–85% of the Sr from the recharging soil–water, thereby increasing the δ88/86Sr value of the soil-water from ∼0.18‰ to 0.3‰–0.6‰. Similar 88Sr-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 (δSr88/86=0.32‰) relative to their rock sources when using the fractionation factor derived in this study, Δcarb-water=−0.218‰, and the published δ88/86Sr composition of marine carbonates of 0.16‰. This surprisingly large flux requires that ∼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 88Sr-enrichment. Alternatively, the average δ88/86Sr value of marine carbonate rock sections undergoing weathering is higher than suggested in current literature. It is concluded that additional δ88/86Sr data on marine carbonate rock sections should be collected to resolve the enigma of the high value of riverine δ88/86Sr.