Synchronous changes in seawater strontium isotope composition and global climate

Abstract

THE ⁸⁷Sr/⁸⁶Sr ratio of sea water has increased gradually over the past 40 Myr, suggesting a concomitant increase in global chemical weathering rates^1–6. Recently, Dia et al.⁷ analysed a 250-kyr ⁸⁷Sr/⁸⁶Sr record, and found superimposed on this gradual increase higher-frequency ⁸⁷Sr/⁸⁶Sr variations which appeared to follow a 100-kyr cycle; this periodicity corresponds to one of the prominent cycles in the Earth's orbital parameters, which are known to modulate the patterns of solar insolation and hence climate^8–10. The resolution of this record was, however, insufficient to establish the phase relationship between the ⁸⁷Sr/⁸⁶Sr variations and global climate cycles. Here we present a high-resolution seawater ⁸⁷Sr/⁸⁶Sr record spanning the past 450 kyr. We find that maxima and minima in ⁸⁷Sr/⁸⁶Sr coincide with minima and maxima, respectively, in continental ice volume (from the SPECMAP oxygen isotope record²⁰), apparently suggesting that there was less chemical weathering in arid glacial periods than in the more humid interglacials. During glacial–interglacial transitions, however, seawater ⁸⁷Sr/⁸⁶Sr changes at a rate of ∼ 1 p.p.m. kyr⁻¹, approximately three times that evaluated by Dia et al.⁷. Mass-balance calculations illustrate that simple changes in modern chemical weathering regimes cannot fully account for such rapid changes, suggesting that we need to revise current ideas about strontium reservoirs and the mechanisms for exchange between them.