GLOBAL DISTRIBUTION OF BERYLLIUM ISOTOPES IN DEEP-OCEAN WATER AS DERIVED FROM FE-MN CRUSTS

The direct measurement of the ratio of cosmogenic Be-10 (T-1/2 = 1.5 M a) to stable terrigenously sourced Be-9 in deep seawater or marine dep osits can be used to trace water mass movements acid to quantify the i ncorporation of trace metals into the deep sea. In this study a SIMS-b ased technique has been used to determine the Be-10/Be-9 ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the w orlds oceans. Be-10/Be-9 ratios, time-corrected for radioactive decay of cosmogenic Be-10 using U-234/U-238, 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 X 10(-7)). In the Sout hwest Atlantic ratios increase up to 1 X 10(-7), they vary between 0.7 and 1.0 X 10(-7) in Indian Ocean samples, and have a near constant va lue of 1.1 +/- 0.2 x 10(-7) for all Pacific samples. If the residence time of Be-10 (tau 10(Be)) in deep water is constant globally, then th e observed variations in Be-10/Be-9 ratios could be caused by accumula tion of Be-10 in deep water as it flows and ages along the conveyor, f ollowing a transient depletion upon its formation in the Northern Atla ntic. In this view both Be-10 and Be-9 reach local steady-state concen tration in Pacific deep water and the global tau 10(Be) congruent to 6 00 a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario tau 10(Be) would va ry according to local particle concentration and would congruent to 60 0 a in the central Pacific, but tau 10(Be) congruent to 230 a in the A tlantic. Mass balance considerations indicate that hydrothermal additi ons of Be-9 to the oceans are negligible and that the dissolved riveri ne source is also small. Furthermore, aeolian dust input of Be-9 appea rs insufficient to provide the dissolved Be-9 inventory. The dissoluti on of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater Be-9 content. If true, ocea n margins would be the sites for Be-9 addition. Due to the particle-re active nature of Be, these would also be the primary sites of Be remov al. A possible net result of horizontal water masses passing through t hese marginal areas might be a decrease in seawater Be-10/Be-9, and es tablishment of a relatively constant Be-9 concentration. As tau 10(Be) (similar to 600 a) is less than the apparent age of deep water in the Pacific (similar to 1500 a), the Pacific record of Be-10/Be-9 is not expected to show secular variations due to changes in deep-water flow, despite the large variations in Be-10/Be-9 between different water ma sses. Because of this insensitivity to deep-water flow, however, it is suggested that the Be-10/Be-9 ratio, determined in the authigenic pha se of marine sediments or hydrogenetic precipitates, should be a suita ble tool for monitoring changes in continental input or cosmic ray int ensity on longer time scales.