Quantifying the Strontium Budget of the Oceans, past and present, using coupled Radiogenic and Stable Strontium Isotopes

The chemical evolution of the oceans is controlled by a range of biological and sedimentary processes, many of which are influenced by tectonic and climatic change. Of these, consumption of atmospheric CO2 through chemical weathering of the continents is thought to play a fundamental role in regulating the Earth's temperature. Therefore, records of ancient seawater chemistry potentially provide a means of determining the importance of weathering on the global carbon cycle and its affect on the Earth's climate. Many natural radiogenic isotopes in seawater are sensitive to changes in the balance of input from continental weathering, and sedimentary archives preserve a record of changes in chemical weathering through geological time. For over twenty five years the rubidium-strontium (Rb-Sr) radiogenic isotope system has been amongst the most commonly used and variations in seawater 87Sr/86Sr ratios on both long and short timescales reflect changing continental input through time. However, changes in the 87Sr/86Sr record cannot be used to reconstruct past CO2 consumption rates because it is unlikely that the composition of the continental source (in particular that delivered by rivers) remains constant through time. Indeed, coupled Sr/Ca and 87Sr/86Sr records indicate significant changes in both the chemical weathering flux and composition of weathered material delivered to seawater over the Cenozoic. Remarkably the published 87Sr/86Sr seawater record may itself be compromised because recent data suggests that there are significant variations in the stable isotopes of Sr. This is because the stable 88Sr/86Sr ratio is traditionally considered to be a constant value and used to correct instrumental mass fractionation during measurement of the radiogenic ratio. Consequently, variations in the stable isotope composition may dramatically alter the measured 87Sr/86Sr ratios in marine records relative to their true value. Our own preliminary data for diverse continental rock types and rivers indicates a total variation in the 88Sr/86Sr ratio of at least 0.9 per mil. While such a variation might be considered small compared to other lighter or redox sensitive elements, it results in a variation of 0.45 per mil (450 ppm) in the 87Sr/86Sr ratio (some 50-100 times greater than current analytical uncertainties). If this continental 88Sr/86Sr variation is imparted to seawater through chemical weathering then this may, in turn, significantly alter the 87Sr/86Sr seawater record. For example, some 20% of the change in Cenozoic seawater 87Sr/86Sr could simply be due to variations in 88Sr/86Sr rather than any actual variation in 87Sr/86Sr. Our own preliminary stable Sr isotope data for a 2.3 Ma record obtained from planktonic foraminifera from the Labrador Sea indicates a significant shift (>100 ppm) in the 88Sr/86Sr composition of seawater over this interval, altering both the pattern and magnitude of change seen in the marine 87Sr/86Sr record. Despite the complexities introduced by variations in 88Sr/86Sr, the coupled measurement of both radiogenic and stable isotope ratios offers a means of determining the true 87Sr/86Sr value of seawater. The corrected the corrected 87Sr/86Sr record can then be combined with Sr/Ca data to deconvolve changes in the global average continental flux from changes in global average composition. Where concomitant variations in 88Sr/86Sr may themselves reveal information on the nature of those changes in composition, for example, whether they result from changes in the continental source, hydrothermal exchange or the precipitation of marine carbonate. Overall these results will thus serve to provide a better understanding of the relationship between chemical weathering, its regulation of the atmospheric partial pressure of carbon dioxide, and thus influence on the greenhouse effect and global climate.