Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts

The direct measurement of the ratio of cosmogenic ¹⁰Be (T12= 1.5 Ma">T12= 1.5 Ma) to stable terrigenously sourced ⁹Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10Be9Be">10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10Be9Be">10Be9Be ratios, time-corrected for radioactive decay of cosmogenic ¹⁰Be using 234U238U">234U238U, are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4–0.5 × 10⁻⁷). In the Southwest Atlantic ratios increase up to 1 × 10⁻⁷, they vary between 0.7 and 1.0 × 10⁻⁷ in Indian Ocean samples, and have a near constant value of 1.1 ± 0.2 × 10⁻⁷ for all Pacific samples.

If the residence time of ¹⁰Be (τ10_Be) in deep water is constant globally, then the observed variations in 10Be9Be">10Be9Be ratios could be caused by accumulation of ¹⁰Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both ¹⁰Be and ⁹Be reach local steady-state concentration in Pacific deep water and the global τ10Be≌ 600a">≌τ10Be≌ 600a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ10_Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10Be≌ 230a">≌τ10Be≌ 230a in the Atlantic.

Mass balance considerations indicate that hydrothermal additions of ⁹Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of ⁹Be appears insufficient to provide the dissolved ⁹Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater ⁹Be content. If true, ocean margins would be the sites for ⁹Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10Be9Be">10Be9Be, and establishment of a relatively constant ⁹Be concentration.

As τ10_Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10Be9Be">10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10Be9Be">10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10Be9Be">10Be9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.