Inferences on Atlantic Deep Water Circulation from Pb and Nd Isotopes in Atlantic Fe-Mn Deposits

A. Simonetti Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

W. Abouchami Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

S. L. Goldstein Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

Pb isotopes in Circum-Antarctic Mn deposits show systematic geographical variations related to present-day oceanic circulation (Abouchami and Goldstein, 1995). Circum-Antarctic Mn nodules show two distinct lead domains. Pb isotope values systematically decrease eastward in the Pacific sector. In contrast, the eastern Atlantic and Indian sectors of the Circum-Antarctic are influenced by addition of water from the Atlantic whose low Nd isotope ratios reflect an old continental source. High Pb isotope ratios in these sectors, also indicating an old continental source, were attributed to mixing of North Atlantic Deep Water (NADW) with Circumpolar water from the Pacific. We report Pb and Nd isotope ratios of Atlantic Mn nodules and crusts from the flow path of NADW. Our objectives are: 1- to characterise Pb isotopes in the deep Atlantic; 2- follow its evolution along the approximate flow path of NADW;
3- determine if they trace deep water circulation; and
4- determine the suitability of NADW as a source of Circum-Antarctic Pb.

The northernmost Atlantic sample, from a formation region of NADW northeast of the Gibbs Fracture zone, displays the highest Pb (206Pb/204Pb ~19.1) and lowest Nd (eNd ~- 12) values, indicating an old continental source. We suggest this approximates the initial isotope ratios
of NADW, and this "initial" NADW signal remains strong southward through the northwestern Atlantic. Samples
from the southeastern Atlantic have higher Nd isotope ratios (eNd = -10 to -8) and lower 206Pb/204Pb ratios (18.80 to 18.90) compared to those from the north Atlantic. In contrast, Mn deposits from the southwestern Atlantic have the highest Nd isotope ratios (eNd > -8) and lowest 206Pb/204Pb ratios (<18.80). These characteristics are like those from Pacific-derived Circumpolar water entering the southwestern Atlantic from the Drake Passage. In addition, Nd and Pb isotope ratios of Mn nodules from the southwestern and north Atlantic basins are correlated, and we interpret this as binary mixing between a component with north Atlantic characteristics and one with characteristics of the East Pacific-West Atlantic sectors of the Circum-Antarctic. Nd and Pb isotope ratios of Mn nodules from the southeastern Atlantic are distinct from those of the southwestern Atlantic, however, they appear to contain the same north Atlantic and East Pacific sector Circum-Antarctic components.

The NADW signal is distinct from the Pb in the Indian sector of the Circum-Antarctic ("A-Indian" trend: Abouchami and Goldstein, 1995), which has higher 208/204Pb for the same 206/204Pb. In Pb-Pb isotope plots, the data define linear arrays which indicate two distinct binary mixtures. Both contain a common end-member with Pb isotope ratios of Circum-Antarctic water near the Drake Passage. Abouchami and Goldstein (1995) suggested that the
"A-Indian" trend reflects mixing of NADW and Circumpolar water in the east Atlantic and Indian sectors of the Circum-Antarctic. Pb isotope ratios for samples from the Atlantic overlap mainly with the Mn samples from north of the Circum-Antarctic and suggests the presence of any NADW signal is in the samples from north of the Circum-Antarctic.

We conclude that we have identified the Pb isotopic signature of NADW. It appears to be acquired in the northernmost Atlantic and can be traced into the southern Atlantic. Pb and Nd isotopes for Fe-Mn deposits from the southwestern and southeastern Atlantic basins can be explained by binary mixing between a NADW and a Circum-Antarctic component, which enters the southern Atlantic through the Drake passage. This is consistent with deep water circulation patterns for the south Atlantic. In the Atlantic, as in
the Circum-Antarctic, Pb isotopes in Mn deposits record systematic geographical variations reflecting deep ocean circulation. This indicates that, despite short-term changes in deep ocean circulation patterns over the Pleistocene, on average, element sources and ocean circulation have been similar to the present day.


Abouchami, W. & Goldstein, S.L., Geochim. Cosmochim. Acta 59, 1809-1820 (1995).