High Precision Pb Isotopes in Fe-Mn crusts from the NE Atlantic: A Proxy for Palaeosources and Palaeocean Circulation

Abstract

Mixing in the oceans is traced using conservative properties (salinity, temperature....) of water masses where mixing is reflected as linear arrays (Broecker and Peng, 1982). The potential of Pb isotopes as tracers of ocean circulation was first demonstrated by the consistency of Pb isotopic variations in CircumAntarctic Fe-Mn deposits with the present-day ocean bottom water circulation in this region (Abouchami and Goldstein, 1995). Although Pb itself is a nonconservative element, binary mixing between water masses or sources will still result in straight linear arrays when plotted in Pb isotope space. However, resolution of such mixing lines depends on the accuracy with which Pb isotope ratios can be measured. In recent studies, palaeoseawater records of Pb and Nd isotopes were obtained using dated depth-profiles from hydrogenous Fe-Mn crusts, and have shown that long-term Pb and Nd isotopic changes in the Pacific and Atlantic appear to be related to deep water circulation changes (Abouchami et al., 1997; Burton et al., 1997; Christensen et al., 1997). So far, because of limitations in analytical precision, only variations in Z~176 ratios have been resolvable. We report high precision Pb isotopic data (2~ext. ~< 100 ppm, a factor of 10 better than conventional analyses) obtained using a Pb triple spike (Galer and Abouchami, This volume), together with Nd isotopic composition on depth profiles from two Fe-Mn crusts from the eastern Atlantic basin. Crust 121DK was dredged from Tropic Seamount which lies off West Africa (24~ 21~ 2000 m), while crust 65GTV comes from the Lion Seamount, west of the Straits of Gibraltar (35~ 15~ 1500 m). These crusts grew in the cores of NADW and MOW at rates of 3 miniMa and 4.5 mm/Ma, respectively, as infen'ed from 1~ data (Koschinsky et al., 1996). The profiles provide a 12 Ma record of changes in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean Outflow Water (MOW). Max-Planck Institut far Chemic, Postfach 3060, 55020 Mainz, Germany