SYNCHRONOUS CHANGES IN SEAWATER STRONTIUM ISOTOPE COMPOSITION AND GLOBAL CLIMATE

THE Sr-87/Sr-86 ratio of sea water has increased gradually over the Past 40 Myr, suggesting a concomitant increase in global chemical weathering rates1-6. Recently, Dia et al.7 analysed a 250-kyr Sr-87/Sr-86 record, and found superimposed on this gradual increase higher-frequency Sr-87/Sr-86 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 climate8-10. The resolution of this record was, however, insufficient to establish the phase relationship between the Sr-87/Sr-86 variations and global climate cycles. Here we present a high-resolution seawater Sr-87/Sr-86 record spanning the past 450 kyr. We rind that maxima and minima in Sr-87/Sr-86 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 Sr-87/Sr-86 changes at a rate of approximately 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.