The neodymium isotopic composition of manganese nodules from the Southern and Indian oceans, the global oceanic neodymium budget, and their bearing on deep ocean circulation

Abstract

Manganese nodules from the Indian and Southern oceans have been analyzed for their Nd isotope composition. The nodules of hydrogenous origin with > 100 ppm Nd have ϵ Nd values in the range −11 to −6.5. The data show that the Nd isotope ratios form two geographical isotopic domains. The samples with the least radiogenic Nd (lower ϵ Nd values) are found around the southern tip of Africa where the influence of the North-Atlantic Deep Water (NADW) dominates. From the Macquarie Ridge south of Australia to the Drake Passage at South America, the Antarctic Circumpolar Current (ACC) seems to be isotopically well mixed with more radiogenic ϵ Nd values in the range −6.5 to −8. These values extend across the Drake Passage and the Argentine Basin, although diagenetic nodules with < 100 ppm in this region show a further increase of the ϵ Nd values to −4 to −6. The Indian Ocean data east of the Madagascar Ridge display a rather small variation with ϵ Nd values similar to the same sector of the ACC (−6.2 to −8.0) and confirm that Nd from deep Circum-Antarctic waters dominate the Indian Ocean. Despite the small variation, slightly higher Nd isotope ratios in the eastern relative to the western Indian Ocean indicate a component of Nd from deep Pacific waters flowing into the Indian Ocean. The isotopic composition of Nd in Mn nodules is usually similar to that of their ambient seawater, and the global patterns mimic the broad features of present-day deep ocean circulation. Because the Mn nodules as sampled reflect the average Nd in the deep water masses over periods of 10 5–10 6 years, the consistency with present-day seawater indicates that the present-day patterns of deep ocean circulation predominated throughout the Pleistocene. An exception is in the Drake Passage and Argentine Basin, where more nodules are more radiogenic than the ambient seawater and reflect either a weakening of NADW during glacial stages or, alternatively, a local source which is clearly present in diagenetic Mn nodules from this region. The global Nd budget in the Southern Ocean is consistent with models of geostrophic transport: Nd in the ACC can be accounted for by a mixture of unradiogenic Nd from Atlantic and radiogenic Nd from Pacific sources. The radiogenic component released by volcanogenic particles in the nepheloid layer, erupted from of the peri-Pacific rim volcanoes, exerts a major influence on Pacific seawater. The residence time of Nd in the ACC is estimated to be 70 years. Mixing therefore requires more than one revolution around Antarctica.