Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts
The direct measurement of the ratio of cosmogenic 10 Be ( T 1 2 = 1.5 Ma "> T12= 1.5 Ma ) to stable terrigenously sourced 9 Be 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 10 Be 9 Be "> 10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10 Be 9 Be "> 10Be9Be ratios, time-corrected for radioactive decay of cosmogenic 10 Be using 234 U 238 U "> 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 10 Be ( τ 10 Be ) in deep water is constant globally, then the observed variations in 10 Be 9 Be "> 10Be9Be ratios could be caused by accumulation of 10 Be 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 10 Be and 9 Be reach local steady-state concentration in Pacific deep water and the global τ10 Be ≌ 600 a ">≌ τ10Be≌ 600a . An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ 10 Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10 Be ≌ 230 a ">≌ τ10Be≌ 230a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9 Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9 Be appears insufficient to provide the dissolved 9 Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater 9 Be content. If true, ocean margins would be the sites for 9 Be 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 10 Be 9 Be "> 10Be9Be , and establishment of a relatively constant 9 Be concentration. As τ 10 Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10 Be 9 Be "> 10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10 Be 9 Be "> 10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10 Be 9 Be "> 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.