Other
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996e%26psl.141..213v&link_type=abstract
Earth and Planetary Science Letters, vol. 141, Issue 1-4, pp.213-226
Other
29
Be-10/Be-9, Residence Time, Deep-Ocean Environment, Ferromanganese Composition, Secondary Ion Mass Spectroscopy, Elsevier: Be-10/(Be-9), Residence Time, Deep-Ocean Environment, Ferromanganese Composition, Secondary Ion Mass Spectroscopy
Scientific paper
The direct measurement of the ratio of cosmogenic 10 Be (T1/2 = 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 ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10 Be/ 9 Be ratios, time-corrected for radioactive decay of cosmogenic 10 Be using 234 U/ 238 U, 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 ( 10 Be ) in deep water is constant globally, then the observed variations in 10 Be/ 9 Be 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 10 Be 600 a. An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario 10 Be would vary according to local particle concentration and would 600 a in the central Pacific, but 10 Be 230 a 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, and establishment of a relatively constant 9 Be concentration. As 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 is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10 Be/ 9 Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10 Be/ 9 Be 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.
Belshaw Nick S.
Gibb Andy
Hein James R.
Keith O'Nions R.
von Blanckenburg Friedhelm
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