Marine barite as a monitor of seawater strontium isotope composition

THE strontium isotope ratio in sea water is influenced by climate, tectonics, weathering and hydrothermal activity at ocean ridges^1-4. Its evolution through time, determined primarily by measuring the strontium isotope composition of marine carbonates, holds information about variations in these processes, and is also useful for stratigraphic correlation and dating^5-7. Carbonates are absent from some marine sediments such as siliceous oozes and red clays, and can be significantly diagenetically altered in others, especially in Eocene and older sediments. Here we show that marine barite is an effective alternative monitor of seawater ⁸⁷Sr/ ⁸⁶Sr. We find that microcrystals of marine barite separated from Holocene Pacific, Atlantic and Indian Ocean sediments all record the modern seawater ⁸⁷Sr/⁸⁶Sr value. Moreover, the ⁸⁷Sr/⁸⁶Sr of barite from 25 sediment samples spanning the past 35 Myr falls within the range of published data for carbonates over this time period. We conclude that marine barite reliably records both present and past variations in seawater strontium isotope composition.