MARINE BARITE AS A MONITOR OF SEAWATER STRONTIUM ISOTOPE COMPOSITION

THE strontium isotope ratio in sea water is influenced by climate, tec tonics, weathering and hydrothermal activity at ocean ridges1-4. Its e volution through time, determined primarily by measuring the strontium isotope composition of marine carbonates, holds information about var iations in these processes, and is also useful for stratigraphic corre lation and dating5-7. Carbonates are absent from some marine sediments such as siliceous oozes and red clays, and can be significantly diage netically altered in others, especially in Eocene and older sediments. Here we show that marine barite is an effective alternative monitor o f seawater Sr-87/Sr-86. We find that microcrystals of marine barite se parated from Holocene Pacific, Atlantic and Indian Ocean sediments all record the modern seawater Sr-87/Sr-86 value. Moreover, the Sr-87/Sr- 86 of barite from 25 sediment samples spanning the past 35 Myr falls w ithin the range of published data for carbonates over this time period . We conclude that marine barite reliably records both present and pas t variations in seawater strontium isotope composition.