40K–40Ca isotopic constraints on the oceanic calcium cycle

The contributions of crustal silicates to the oceanic calcium cycle are investigated using high-precision 40K–40Ca measurements in Archean and Proterozoic carbonates, river waters, and a series of terrestrial and extraterrestrial mafic/ultramafic samples. Using a multidynamic data collection scheme with strict controls on instrumental mass fractionation, we show that a reproducibility of 0.35 ε-units (2σ) is obtained for the 40Ca/44Ca ratio using the ThermoFinnigan Triton. This represents an improvement by a factor of 3 compared with previous generations of data. Well-defined excesses of 40Ca from 40K decay were found in river waters draining the Himalayas, but not in the Mississippi and Columbia Rivers. All marine carbonate samples ranging in age from Archean to recent show no discernable effects of 40K decay to within the limit of ± 0.35 ε-units (2σ) in 40Ca/44Ca. Our results, therefore, indicate that the Ca isotopic composition of seawater has remained constant and indistinguishable from that of the mantle for the past 3.5 Ga, despite the influx of radiogenic calcium delivered by weathering of the high K/Ca components of the continental crust. Thus, over most of geologic time, the contributions of the high K/Ca sources from the continents are far below the contributions of hydrothermal sources. Mass balance constraints indicate that unless the present-day contribution of continental silicates is much less than 8% of the river Ca flux, the total hydrothermal flux of calcium must exceed the input from high-temperature vents at ridge axes by at least one order of magnitude. The present-day oceanic mass balance requires a high input of calcium [(4–15) × 1012 mol Ca/yr] at low-temperature hydrothermal sites.