Precise Measurement of Growth Rate Variations in Atlantic, Indian and Pacific Ferromanganese Crusts, Derived from 10Be/9Be Ratios

Abstract

Ferromanganese crusts provide an important record of palaeoceanographic conditions through their trace element and isotopic signatures. The unlocking of information contained within crust growth layers requires reliable and precise dating. We will present a synthesis of dating results so far produced from ferromanganese crusts using l~ ratios measured on the Oxford ISOLAB120. Alternative attempts to date crusts have included the comparison 87Sr/86Sr profiles with the global seawater STSr/86Sr curve, (Burton et al., 1997; Ling et al., 1997) but with only one consistent data set providing meaningful age information having so far been generated (Burton et aL, 1997). These data, however, disagree with the l~ dating approach on the same crust. 23~ and 23]pa profiles have been employed but they only yield information on growth rates over the last 300 and 150 kyrs, respectively. A Co constant flux model, which is based on an empirical relationship between Co content and radiometrically derived growth rate, has proved to be a valid approach for Co-rich equatorial Pacific seamount crusts (Puteanus and Halbach 1998), but does not appear to be generally applicable to crusts from other parts of the world's oceans. Direct measurement of the l~ ratio of crust surfaces has shown that each ocean basin has a distinct signature (yon Blanckenburg et al., 1996) that is assumed to have remained constant over the period of growth of the crusts studied. Using a l~ half-life of 1.5 Ma, Ling et al. (1997) analysed three crusts from the Pacific and gained a consistent data set of the Ndand Pb-isotopic evolution of the Pacific Ocean. l~ dating of crusts has also been applied successfully to crusts from the Atlantic and Indian oceans, implying that l~ ratios provide a robust dating tool for element or isotope time series data in ferromanganese crusts from the world,s oceans. Direct measurement of the l~ ratio removes the problem of variations caused by dilution effects which are encountered in dating approaches which only utilise measurements of a single isotope. The sampling resolution, which on average corresponds to 0.5 Ma, integrates over variations in the global l~ flux caused by changes in production rate of